200_Seeley's Essentials of Anatomy and Physiology, 9th Ed-690

Urinary system and fluid Balance 523

3 4
Actions Reactions

Kidney: Effectors Respond:
The distal convoluted tubules decrease
H+ secretion into the urine and Fewer H+ are removed from the
decrease HCO3– reabsorption into the blood, and fewer HH+C. O3– are
blood. available to bind

Lungs: Increased blood CO2 reacts with
The respiratory control center in the water to produce carbonic acid,
brain decreases the rate and depth of which dissociates to increase H+.
respiration, which increases blood CO2. H2O + CO2 H2CO3 H+ + HCO3–
Buffers:
Buffers release H+. The number of H+ in the blood increases.
H2O + CO2 H2CO3 H+ + HCO3–

2 Homeostasis Disturbed: 5 Homeostasis Restored:
Blood pH increases (H+ decreases). Blood pH decreases (H+ increases).

Blood pH
(normal range)

Blood pH
(normal range)
1 Start here 6

Homeostasis Disturbed: Homeostasis Restored:
Blood pH decreases (H+ increases). Blood pH increases (H+ decreases).

Actions Reactions

Buffers: Effectors Respond:
Buffers bind H+.
H2O + CO2 H2CO3 H+ + HCO3– The number of H+ in the blood
decreases.
Lungs:
The respiratory control center in the aDreceaidccr,tewwahsitiehcdhHbCdleoOco3rd–eatCosOefo2srcmHa+ucisanerbtshoeHn+ibcltoood.
brain increases the rate and depth of
respiration, which decreases blood H2O + CO2 H2CO3 H+ + HCO3–
CO2.
More H+ are removed from the blood,
Kidney: and more HCO3– are available to
The distal convoluted tubules increase bind H+.
H+ secretion into the urine, and increase
HCO3– reabsorption into the blood.

Urinary

Homeostasis Figure 18.22 Regulation of Acid-Base Balance

(1) Blood ph is in its normal range. (2) Blood ph increases outside the normal range, which causes homeostasis to be disturbed. (3) the blood ph control
centers respond to the change in blood ph. (4) the control centers cause decreased h+ secretion from the blood and increased carbonic acid production,
which increases blood h+ concentration. (5) these changes cause blood ph to decrease. (6) Blood ph returns to its normal range, and homeostasis is restored.
observe the responses to a decrease in blood ph outside its normal range by following the red arrows.

524 Chapter 18

systems PaTHOLOGY

NGaemndee: r: roger acute Renal Failure
age: male
52 background Information

tdresaesdhratpcereouwlurptowimcreunreegikaiarneandne.nssatedresifyr’henoodosy-ovilrfcsutosuintdhtsmtoburoeaieeiorlnorv+vonsnieeelhonctasarrodofvlaiyldiuannoatlprmlnylcCmohhAsurmdeaweodiiomsaesnlnltsemaeeubafditseulvsctgctrlreurmeoetshrahgrtsbveri,orerie.inteeeeserenhdaiBtseelonlcgehitspuesy.dstonotaereiecsdryecnbdnaabsiiuacrntsl,.onuoosuetteccCavsosrtoscnrefuehtnhoedeuioodrcddilfaristnlrtepneuuibsmoigoectonrelrlgeramonrnatndarees,rosel,ovosarsdhddterubgitsoaotwienrelueeoewonboa.tr,ngse’hrussashievs,siil.essr, for approximately 7 days, roger required renal dialysis to maintain
his blood volume and ion concentrations within normal ranges.
After about 3 weeks, his kidney functions slowly began to improve,
although many months passed before they returned to normal. in

roger’s case, the events 24 hours after his accident are consistent
with acute renal failure caused by prolonged low blood pressure and
lack of blood flow to the kidneys. the reduced blood flow to the
kidneys was severe enough to cause damage to the epithelial lining

of the kidney tubules. the period of reduced urine volume resulted
from tubular damage. dead and damaged tubular cells sloughed
off into the tubules and blocked them, so that filtrate could not
flow through. in addition, the filtrate leaked from the blocked

or partially blocked tubules back into the interstitial spaces
and therefore back into the circulatory system. As a result, the
amount of filtrate that became urine was markedly reduced.

Blood levels of urea and of creatine usually increase due
to reduced filtrate formation and reduced function of the
tubular epithelium. A small amount of urine is produced
that has a high na+ concentration, although the osmolality

is usually close to the concentration of the body fluids. the
kidney is not able to reabsorb na+, nor can it effectively
concentrate urine.

Treatments for Renal Failure

Hemodialysis (he¯′mo¯-d¯ı -al′i-sis) is used when a person is
suffering from severe acute or chronic kidney failure. the
procedure substitutes for the excretory functions of the

From an artery Blood
Blood pump
Diffusion
Bubble of waste
trap products,
such as
urea

To a vein Dialysis Diffusion of
membrane waste products
across the dialysis
membrane

Urinary Compressed Fresh Constant Used Dialysis fluid
CO2 and air dialysis temperature dialysis

Figure 18A fluid bath fluid

during hemodialysis, blood flows through a system of tubes composed of a selectively permeable membrane. dialysis fluid, which has a composition similar to that
of normal blood (except that the concentration of waste products is very low), flows in the opposite direction on the outside of the dialysis tubes. Waste products,
such as urea, diffuse from the blood into the dialysis fluid. other substances, such as na+, K+, and glucose, can diffuse from the blood into the dialysis fluid if they
are present in higher than normal concentrations, because these substances are present in the dialysis fluid at the same concentrations found in normal blood.

SKeLeTaL Urinary system and fluid Balance 525

INTeGUMeNTaRY Bone resorption can result MUSCULaR
because of excessive and chronic
Anemia causes pallor, and bruising results from loss of Ca2+ in the urine. Vitamin d
clotting proteins lacking in the blood because levels may be reduced.
they are lost in the urine. Accumulation of
urinary pigments changes skin tone. high acute Renal neuromuscular irritability results from the
urea gives a yellow cast to light-skinned Failure toxic effect of metabolic wastes on the central
people, and white crystals of urea, called nervous system and ionic imbalances, such
uremic frost, may appear on areas of the as elevated blood K+ levels. involuntary jerking
skin where there is heavy perspiration. and twitching can occur as neuromuscular
irritability develops.
DIGeSTIVe
NeRVOUS
decreased appetite, mouth infections,
nausea, and vomiting result from altered Symptoms elevated blood K+ levels and the toxic
digestive tract functions due to the effects • Decreased urine effects of metabolic wastes result in
of ionic imbalances on the nervous system. volume depolarization of neurons. slowing of
• Increased Na+ action potential conduction, burning
ReSPIRaTORY in urine sensations, pain, numbness, or tingling
• Decreased urine results. Also, decreased mental acuity,
early during acute renal failure, the depth of osmolality reduced ability to concentrate, apathy,
breathing increases and becomes labored and lethargy occur. or in severe cases,
as acidosis develops because the kidneys confusion and coma occur.
are not able to secrete h+. Pulmonary
edema often develops because of water Treatment eNDOCRINe
and na+ retention as a result of reduced • Hemodialysis
• Kidney transplant
urine production. the likelihood of infection
increases as a result of pulmonary edema. CaRDIOVaSCULaR major hormone deficiencies
include vitamin d deficiency.
LYMPHaTIC Water and na+ retention cause edema in peripheral in addition, the secretion of
tissues and in the lungs, leading to increased blood reproductive hormones decreases
there are no major direct effects on the pressure and congestive heart failure. elevated blood due to the effects of metabolic
lymphatic system, except that increased K+ levels result in dysrhythmias and can cause cardiac wastes and ionic imbalances on
lymph flow occurs as a result of edema. arrest. Anemia due to decreased erythropoietin the hypothalamus.
production by the damaged kidney exists.

kidney. hemodialysis is based on blood flow through tubes composed Kidney transplants are sometimes performed on people who Urinary
of a selectively permeable membrane. Blood is usually taken from an have severe renal failure. often, the donor suffered an accidental
artery, passed through the tubes of the dialysis machine, and then death and had granted permission to have his or her kidneys used for
returned to a vein (figure 18A). on the outside of the dialysis tubes is transplantation. the major cause of kidney transplant failure is rejec-
a fluid, called dialysis fluid, which contains the same concentration of tion by the recipient’s immune system. Physicians therefore attempt
solutes as normal plasma, except for the metabolic waste products. to match the immune characteristics of the donor and recipient to
As a consequence, the metabolic wastes diffuse from the blood to the reduce the tendency for rejection. even with careful matching, recipi-
dialysis fluid. the dialysis membrane has pores that are too small to ents have to take medication for the rest of their lives to suppress
allow plasma proteins to pass through them, and because the dialy- their immune reactions. But in most cases, the transplanted kidney
sis fluid contains the same beneficial solutes as the plasma, the net functions well, and the tendency of the recipient’s immune system to
movement of these substances is zero. reject the transplanted kidney can be controlled.

Peritoneal (per′i-to¯-ne¯′a˘l) dialysis is sometimes used to treat Predict 9
kidney failure. the principles by which peritoneal dialysis works are the Nine days after the accident, Roger began to appear pale,
same as for hemodialysis, but the dialysis fluid flows through a tube became dizzy on standing, and was very weak and lethargic.
inserted into the peritoneal cavity. the visceral and parietal peritonea His hematocrit was elevated, and his heart was arrhythmic.
act as the dialysis membrane. Waste products diffuse from the blood Explain these manifestations.
vessels beneath the peritoneum, across the peritoneum, and into the
dialysis fluid.

526 Chapter 18

DISEASES AND DISORDERS:  Urinary System

CONDITION DESCRIPTION

Inflammation of the Kidneys Inflammation of the filtration membrane within the renal corpuscle, causing increased membrane permeability;
Glomerulonephritis plasma proteins and blood cells enter the filtrate, causing increased urine volume due to increased osmotic
(glo¯-ma¯r′u¯-lo¯-nef-r¯ı ′tis) concentration of the filtrate
Often occurs 1–3 weeks after a severe bacterial infection, such as strep throat; normally subsides after several days
Acute glomerulonephritis Long-term and progressive process whereby the filtration membrane thickens and is eventually replaced by
Chronic glomerulonephritis connective tissue and the kidneys become nonfunctional

Renal Failure Can result from any condition that interferes with kidney function
Acute renal failure Occurs when damage to the kidney is rapid and extensive; leads to accumulation of wastes in the blood; can lead to
death in 1–2 weeks if renal failure is complete
Chronic renal failure Results from permanent damage to so many nephrons that the remaining nephrons are inadequate for normal
kidney function; can be caused by chronic glomerulonephritis, trauma to the kidneys, tumors, or kidney stones

ANSWER TO Learn to Predict condition can produce as much as 20–30 L of urine each day.
Because they lose so much water, they are continually in danger
First we learn that Sadie is unable to retain water and is urinat- of severe dehydration. Even though their urine is dilute, producing
ing excessively. This helps us to narrow our focus to kidney func- such a large volume of urine leads to the loss of Na+, Ca2+, and
tion. Next we learn that Sadie has normal levels of ADH. Excessive other ions. The resulting ionic imbalances cause the nervous sys-
urination falls under the category of diabetes (derived from the tem and cardiac muscle to function abnormally. Because Sadie’s
Greek for “siphon”); however, diabetes insipidus is a condition in- levels of ADH are normal, we can conclude that she has the type
volving ADH abnormality. There are two main causes of diabetes of diabetes insipidus involving abnormal receptors. Treatments for
insipidus: The posterior pituitary fails to secrete ADH, or the kid- Sadie include making sure she drinks plenty of water and giving
ney tubules have abnormal receptors for ADH and do not respond her a sodium-sparing diuretic so that her kidneys retain sodium.
to the presence of ADH. In people suffering from diabetes insipi-
dus, much of the filtrate entering both the proximal convoluted Answers to the rest of this chapter’s Predict questions are in Appendix E.
and the distal convoluted tubules becomes urine. People with this

Urinary Summary 3. Each renal pyramid has a base located at the boundary between the
cortex and the medulla. The tip of the renal pyramid extends toward
The urinary system consists of two kidneys, two ureters, the urinary the center of the kidney and is surrounded by a calyx.
bladder, and the urethra.
4. Calyces are extensions of the renal pelvis, which is the expanded
18.1 Functions of the Urinary System   (p. 499) end of the ureter within the renal sinus.

1. The kidneys excrete waste products. 5. The functional unit of the kidney is the nephron. The parts of a
2. The kidneys control blood volume and blood pressure by regulating nephron are the renal corpuscle, the proximal convoluted tubule,
the loop of Henle, and the distal convoluted tubule.
the volume of urine produced.
3. The kidneys help regulate the concentration of major ions in the 6. The filtration membrane is formed by the glomerular capillaries,
the basement membrane, and the podocytes of the Bowman capsule.
body fluids.
4. The kidneys help regulate the pH of the extracellular fluid. Arteries and Veins
5. The kidneys regulate the concentration of red blood cells in the blood.
6. The kidneys participate, with the skin and liver, in regulating 1. Renal arteries give rise to branches that lead to afferent arterioles.
2. Afferent arterioles supply the glomeruli.
vitamin D synthesis. 3. Efferent arterioles carry blood from the glomeruli to the

18.2 Anatomy of the Kidneys   (p. 500) peritubular capillaries.
4. Blood from the peritubular capillaries flows to the renal veins.
1. Each kidney is behind the peritoneum and surrounded by a renal
capsule and adipose tissue.

2. The kidney is divided into an outer cortex and an inner medulla.

Urinary System and Fluid Balance 527

18.3 Urine Production   (p. 505) 18.6 Body Fluid Compartments   (p. 518) Urinary

Urine is produced by filtration, tubular reabsorption, and tubular secretion. 1. Water and the ions dissolved in the water are distributed in the
intracellular and extracellular fluid compartments.
Filtration
1. The renal filtrate passes from the glomerulus into the Bowman 2. Approximately two-thirds of the total body water is found within cells.
3. Approximately one-third of the total body water is found outside
capsule and contains no blood cells and few blood proteins.
2. Filtration pressure is responsible for filtrate formation. cells, mainly in interstitial fluid, blood plasma, and lymph.
3. Increased sympathetic activity decreases blood flow to the kidney,
Composition of the Fluid in the Body Fluid Compartments
decreases filtrate formation, and decreases urine production. 1. Intracellular fluid contains more K+, Mg2+, PO43−, SO42−, and protein
Decreased sympathetic activity has the opposite effect.
than does extracellular fluid.
Tubular Reabsorption 2. Extracellular fluid contains more Na+, Ca2+, Cl−, and HCO3− than
1. About 99% of the filtrate volume is reabsorbed; 1% becomes urine.
2. Among the substances reabsorbed are proteins, amino acids, does intracellular fluid.

glucose, fructose, Na+, K+, Ca2+, HCO3−, and Cl−. Exchange Between Body Fluid Compartments
3. About 65% of the filtrate volume is reabsorbed in the proximal Water moves continuously between compartments in response to hydrostatic
pressure differences and osmotic differences between the compartments.
convoluted tubule, 15% is reabsorbed in the descending limb of
the loop of Henle, and another 19% is reabsorbed in the distal 18.7 Regulation of Extracellular Fluid
convoluted tubule and collecting duct. Composition  (p. 519)

Tubular Secretion The total amount of water and electrolytes in the body does not change
Hydrogen ions, some by-products of metabolism, and some drugs are unless the person is growing, gaining weight, or losing weight.
actively secreted into the nephron.
Thirst Regulation
18.4 Regulation of Urine Concentration The sensation of thirst increases if extracellular fluid becomes more
and Volume   (p. 510) concentrated or if blood pressure decreases.

Hormonal Mechanisms Ion Concentration Regulation
1. Renin is secreted from the kidney when the blood pressure 1. Sodium ions are the dominant extracellular ions. Aldosterone increases

decreases. Renin converts angiotensinogen to angiotensin I, which Na+ reabsorption from the filtrate, ADH increases water reabsorption
is then converted to angiotensin II by angiotensin-converting from the nephron, and ANH increases Na+ loss in the urine.
enzyme. Angiotensin II stimulates aldosterone secretion, and 2. Aldosterone increases K+ secretion in the urine. Increased blood
aldosterone increases the rate of Na+ and Cl− reabsorption from levels of K+ stimulate, and decreased blood levels of K+ inhibit,
the nephron. aldosterone secretion.
2. ADH is secreted from the posterior pituitary when the concentration 3. Parathyroid hormone secreted from the parathyroid glands increases
of blood increases or when blood pressure decreases. ADH increases extracellular Ca2+ levels by causing bone resorption and increased
the permeability to water of the distal convoluted tubule and Ca2+ uptake in the kidney. Parathyroid hormone increases vitamin
collecting duct. It increases water reabsorption by the kidney. D synthesis. Calcitonin, secreted by the thyroid gland, inhibits bone
3. Atrial natriuretic hormone, secreted from the right atrium in resorption and lowers blood Ca2+ levels when they are too high.
response to increases in blood pressure, acts on the kidney to 4. When PO43− and SO42− levels in the filtrate are low, nearly all
increase Na+ and water loss in the urine. PO43− and SO42− are reabsorbed. When levels are high, excess is
lost in the urine.
18.5 Urine Movement   (p. 514)
18.8 Regulation of Acid-Base Balance   (p. 521)
Anatomy and Histology of the Ureters, Urinary Bladder,
and Urethra Buffers
1. Each ureter carries urine from a renal pelvis to the urinary bladder. Three principal classes of buffers in the body fluids resist changes in
2. The urethra carries urine from the urinary bladder to the outside of the pH: proteins, the phosphate buffer system, and the bicarbonate
buffer system.
the body.
3. The ureters and urinary bladder are lined with transitional Respiratory System
The respiratory system rapidly regulates pH. An increased respiratory
epithelium and have smooth muscle in their walls. rate raises the pH because the rate of CO2 elimination is increased,
4. The external urinary sphincter regulates the flow of urine through and a reduced respiratory rate reduces the pH because the rate of CO2
elimination is reduced.
the urethra.
Kidneys
Micturition Reflex The kidneys excrete H+ in response to a decreasing blood pH, and they
1. Increased volume in the urinary bladder stretches its wall and reabsorb H+ in response to an increasing blood pH.

activates the micturition reflex. Acidosis and Alkalosis
2. Parasympathetic action potentials cause the urinary bladder to 1. Acidosis occurs when the pH of the blood falls below 7.35. The two

contract. Reduced somatic motor action potentials cause the major types are respiratory acidosis and metabolic acidosis.
external urinary sphincter to relax. 2. Alkalosis occurs when the pH of the blood increases above 7.45. The
3. Higher brain centers control the micturition reflex. Stretching of the
urinary bladder stimulates sensory neurons that carry impulses to two major types are respiratory alkalosis and metabolic alkalosis.
the brain and inform the brain of the need to urinate.

528 Chapter 18

Review and Comprehension

1. Name the structures that make up the urinary system. List the 15. Explain how renin controls the synthesis of angiotensin I. What
functions of the urinary system. enzyme regulates the conversion of angiotensin I to angiotensin II?

2. What structures surround the kidney? 16. Describe the effect of angiotensin II on aldosterone secretion.
3. Describe the relationships of the renal pyramids, calyces, renal 17. Where is aldosterone produced, and what effect does it have on

pelvis, and ureter. urine volume? What factors stimulate aldosterone secretion?
4. What is the functional unit of the kidney? Name its parts. 1 8. Where is atrial natriuretic hormone produced, and what effect does
5. Describe the blood supply to the kidney.
6. Name the three general processes involved in the production of urine. it have on urine production?
7. Describe the filtration membrane. What substances do not pass 1 9. What are the functions of the ureters, urinary bladder, and urethra?

through it? Describe their structure.
8. How do changes in blood pressure in the glomerulus affect the 20. Describe the micturition reflex. How is voluntary control of

volume of filtrate produced? micturition accomplished?
9. What effect does sympathetic stimulation have on the kidneys? 2 1. What stimuli result in an increased sensation of thirst?
10. What substances are reabsorbed in the nephron? What happens to 22. Describe how Na+ levels are regulated in the body fluids.
23. Describe how K+ levels are regulated in the body fluids.
most of the filtrate volume that enters the nephron? 24. Describe how Ca2+ levels are regulated in the body fluids.
11. In what parts of the nephron are large volumes of filtrate 2 5. Explain how buffers respond to changes in the pH of body fluids.
2 6. Explain how the respiratory system and the kidneys respond to
reabsorbed? In what part of the nephron is no filtrate reabsorbed?
12. In general, what substances are secreted into the nephron? changes in the pH of body fluids.
13. What effect does ADH have on urine volume? Name the factors that 27. Define respiratory acidosis, metabolic acidosis, respiratory

cause an increase in ADH secretion. alkalosis, and metabolic alkalosis.
14. Where is renin produced, and what stimulates its secretion?

Critical Thinking Questions

1. Mucho McPhee decided to do an experiment after reading the urinary 5. Propose as many ways as you can to decrease the rate at which
system chapter in his favorite anatomy and physiology textbook. He filtrate enters the Bowman capsule.
drank 2 L of water in 15 min and then monitored his rate of urine
production and urine concentration over the next 2 hours. What did 6. Swifty Trotts has an enteropathogenic Escherichia coli infection that
he observe? Explain the major mechanism involved. causes severe diarrhea. Diarrhea produces a large volume of mucus
that contains high concentrations of pHHC,Oan3−d. How would this diarrhea
2. A student ate a full bag of salty (NaCl) potato chips but drank no affect this patient’s blood pH, urine respiration rate?
liquids. What effect did this have on urine concentration and the
rate of urine production? Explain the mechanisms involved. 7. Spanky and his mother went to a grocery store, where Spanky eyed
some candy he wanted. His mother refused to buy it, so Spanky
3. During severe exertion in a hot environment, a person can lose became angry. He held his breath for 2 min. What effect did this
up to 4 L of sweat per hour (sweat is less concentrated than have on his body fluid pH? After the 2 min, what mechanisms were
extracellular fluid in the body). What effect would this loss most important in reestablishing the normal body fluid pH?
have on urine concentration and rate of production? Explain
the mechanisms involved. 8. Martha suffered from severe nausea for 2 days. She vomited
frequently and was so nauseated that she could not tolerate to eat
4. Which of the following symptoms are consistent with reduced secretion or drink anything. Explain how each of the following levels had
of aldosterone: excessive urine production, low blood pressure, high changed in her body by the second day: blood pH, blood ADH,
plasma potassium levels, high plasma sodium levels? Explain. blood aldosterone, and urine pH.

Answers in Appendix D

Urinary

19c H a p t e r Reproductive System

learn TO preDict

chase and christina were studying for their last anatomy
and physiology exam of the semester. after reviewing
his notes on meiosis, chase said, “Well, since meiosis is
the same in males and females, i don’t need to study
the meiosis notes from the female reproductive system
lecture.” christina quickly pointed out that he was mis-
taken and that if he reviewed his notes he would see
that meiosis is very different in males and females. after
reading chapter 19, explain the major differences be-
tween meiosis in males and meiosis in females. predict
any advantages to the differences between male and
female meiosis.

19.1 Functions oF tHe color-enhanced scanning electron micrograph of a human oocyte.
reproDuctive system
Module 14 reproductive system
Learning Outcome After reading this section, you should be able to
nourishes sperm until they are mature and are deposited
A. list the functions of the male and female reproductive in the female reproductive tract by the penis. The female
systems. reproductive system receives the male’s sperm and
transports them to the fertilization site.
The human species could not survive without functional male and 3. Development and nourishment of a new individual. The
female reproductive systems. The reproductive systems play essential female reproductive system nurtures the development of
roles in the development of the structural and functional differences a new individual in the uterus until birth and provides
between males and females, influence human behavior, and produce nourishment (milk) after birth.
offspring. However, a reproductive system, unlike other organ 4. Production of reproductive hormones. Hormones produced
systems, is not necessary for the survival of an individual human. by the reproductive system control its development and the
development of the gender-specific body form. These
Most of the body’s organ systems show little difference hormones are also essential for the normal function of
between males and females, but the male and female reproductive the reproductive system and for reproductive behavior.
systems exhibit striking differences. On the other hand, they have
a number of similarities. For example, many reproductive organs 529
of males and females are derived from the same embryological
structures (see chapter 20), and some hormones are the same in
males and females, even though they produce different responses.
The reproductive system performs the following functions:

1. Production of gametes. The reproductive system produces
gametes: sperm cells in the testes of males and oocytes
(eggs) in the ovaries of females.

2. Fertilization. The reproductive system enhances fertilization
of the oocyte by the sperm. The duct system in males

530 Chapter 19

19.2 Formation of Gametes with part of another chromatid from the other chromosome. This
event, called crossing over, allows the exchange of genetic material
Learning Outcome After reading this section, you should be able to between chromosomes.
The chromosomes align along the center of the cell (figure 19.2,
A. Describe the function of meiosis in the formation of sperm step 3), and then the pairs of chromosomes are separated to each
cells and oocytes. side of the cell (figure 19.2, step 4). As a consequence, when
meiosis I is complete, each daughter cell has 1 chromosome from
The testes in males and the ovaries in females (figure 19.1) pro- each of the pairs (figure 19.2, step 5), or 23 chromosomes. Each of
duce gametes (gam′ ētz), or sex cells. The formation of gametes in the 23 chromosomes in each daughter cell consists of 2 chromatids
males and females occurs by a type of cell division called meiosis joined by a centromere.
(mı̄-ō′ sis; a lessening) (see chapter 3). It is during the first meiotic division that the chromosome
Meiosis occurs only in the testis and ovary. During meiosis, one number is reduced from 46 (23 pairs) to 23 chromosomes. The
cell undergoes two consecutive cell divisions to produce four daugh- first meiotic division is therefore called a reduction division.
ter cells, each having half as many chromosomes as the ­parent cell. The second meiotic division is similar to mitosis. The chro-
The two divisions of meiosis are called meiosis I and meiosis II. mosomes, each consisting of 2 chromatids (figure 19.2, steps 6
Like mitosis, each division of meiosis has prophase, metaphase, and 7), align along the center of the cell. Then the chromatids
anaphase, and telophase. However, there are distinct differences separate at the centromere, and each daughter cell receives 1 of the
between meiosis and mitosis. chromatids from each chromosome (figure 19.2, steps 8 and 9).
Before meiosis begins, all the chromosomes are duplicated. At When the centromere separates, each of the chromatids is called a
the beginning of meiosis, each of the 46 chromosomes consists of chromosome. Consequently, each of the 4 daughter cells produced
2 chromatids connected by a centromere (figure 19.2, step 1). The by meiosis contains 23 chromosomes.
chromosomes align as pairs in a process called synapsis (si-nap′ sis; During fertilization, the zygote receives 1 set of chromosomes
a connection) (figure 19.2, step 2). Because each chromosome (23) from each parent. Although half the genetic material of a
consists of 2 chromatids, the pairing of the chromosomes brings zygote comes from each parent, the genetic makeup of each zygote
2 chromatids of each chromosome close together. Occasionally, is unique.
part of a chromatid of 1 chromosome breaks off and is exchanged

Ductus Seminal Mammary
deferens vesicle gland
Prostate (in breast)
gland
Uterine
tube
Ovary
Uterus

Epididymis Penis Vagina
Testis
(b)
(a)

Figure 19.1    Major Organs of the Reproductive System

Reproductive (a) The male reproductive system: testes, epididymides, ducta deferentia, seminal vesicles, prostate gland, and penis. (b) The female reproductive system:
ovaries, uterine tubes, uterus, vagina, and mammary glands.

Reproductive System 531

First Meiotic Division (Meiosis I) Second Meiotic Division (Meiosis II)
(continued from the bottom of previous column)
1 Early prophase I Centromere Chromosome
The duplicated Nucleus 6 Prophase II
chromosomes Each chromosome
become visible consists of two
chromatids. chromatids.

Chromatids Centrioles

2 Middle prophase I Pair of chromosomes
Pairs of chromosomes Spindle
synapse. Crossing fibers
over may occur at
this stage.

3 Metaphase I Equatorial 7 Metaphase II
Pairs of chromosomes align plane Chromosomes
along the center of the cell. align along the
Random assortment center of the cell.
of chromosomes occurs.

4 Anaphase I 8 Anaphase II
Chromosomes move Chromatids
apart to opposite separate, and each
sides of the cell. is now called a
chromosome.
5 Telophase I Cleavage
New nuclei form, furrow 9 Telophase II
and the cell divides. New nuclei form
Each cell now has around the
two sets of half the chromosomes.
chromosomes. The cells divide to
form four daughter
cells with half as
many chromosomes
as the parent cell.

Prophase II (top of next column)

PROCESS Figure 19.2    Meiosis

Reproductive

532 Chapter 19

19.3 Male Reproductive System Scrotum

Learning Outcomes After reading this section, you should be able to The scrotum (skrō′ tum) is a saclike structure containing the testes. It
is divided into right and left internal compartments by an incom-
A. Describe the scrotum and its role in regulating the plete connective tissue septum. Externally, the scrotum consists of
temperature of the testes. skin. Beneath the skin are a layer of loose connective tissue and a
layer of smooth muscle called the dartos (dar′ tōs) muscle.
B. Describe the structure of the testes, the specialized cells In cold temperatures, the dartos muscle contracts, causing the
of the testes, and the process of spermatogenesis. skin of the scrotum to become firm and wrinkled and reducing
the overall size of the scrotum. At the same time, extensions of
C. Describe the ducts of the male reproductive system and abdominal muscles into the scrotum, called cremaster (krē-mas′ ter)
their functions. muscles, contract (see figure 19.6). Consequently, the testes are
pulled nearer the body, and their temperature is elevated. During
D. Describe the structure of the penis, seminal vesicles, warm weather or exercise, the dartos and cremaster muscles
bulbourethral glands, and prostate gland, and explain relax, the skin of the scrotum becomes loose and thin, and the tes-
their functions. tes descend away from the body, which lowers their temperature.
The response of the dartos and cremaster muscles is important in
The male reproductive system consists of the testes (sing. testis), regulating the temperature in the testes. If the testes become too
a series of ducts, accessory glands, and supporting structures. warm or too cold, normal sperm cell development does not occur.
The ducts include the epididymides (sing. epididymis), the ducta
deferentia (sing. ductus deferens; also, vas deferens), and the ure- Testes
thra. Accessory glands include the seminal vesicles, the prostate
gland, and the bulbourethral glands. Supporting structures include The testes (tes′ tēz), or male gonads (gō′ nădz; gone¯, seed), are
the scrotum and the penis (figure 19.3). The sperm cells are very oval organs, each about 4–5 cm long, within the scrotum (see
heat-sensitive and must develop at a temperature slightly less than figure 19.3). The outer part of each testis consists of a thick, white
normal body temperature. The testes, in which the sperm cells connective tissue capsule. Extensions of the capsule project into
develop, are located outside the body cavity in the scrotum, where the interior of the testis and divide each testis into about 250 cone-
the temperature is lower. Sperm cells travel from each testis to the shaped lobules (figure 19.4a). The lobules contain seminiferous
prostate gland and then empty into the urethra within the prostate
gland. The urethra exits the pelvis, passes through the penis, and
opens to the outside of the body.

Ureter

Urinary bladder Seminal vesicle

Urethra Ejaculatory duct
Penis Rectum
Prostate gland
Bulbourethral gland

Anus

Glans penis Ductus deferens
Prepuce Epididymis

Testis
Scrotum

Medial view

Reproductive Figure 19.3    Male Reproductive Structures

Medial view of the male pelvis, showing the male reproductive structures.

reproductive system 533

CLINICAL IMPACT Descent of the Testes

the testes develop in the Failure of the testes to descend into the after the testes descend, the ingui-
abdominopelvic cavity. they move from scrotal sac is called cryptorchidism (krip- nal canals narrow permanently, but they
the abdominopelvic cavity through the to¯r′ki-dizm; crypto, concealed + orchis, remain as weak spots in the abdominal
inguinal (ing′gwi-na˘l; inguen, groin) testis). it results in sterility because of wall. an inguinal canal that enlarges or
canal to the scrotum. the inguinal canals the inhibiting effect of normal body ruptures can result in an inguinal hernia
and the internal layers of the scro- temperature on sperm cell development. (her′ne¯-a˘; rupture), through which a loop
tum originate as outpocketings of the in addition, about 10% of testicular of intestine can protrude. this herniation
abdominal cavity along the lateral, supe- cancer cases occur in men with a history can be quite painful and even very dan-
rior margin of the pubis. the descent of cryptorchidism. Fortunately, if the tes- gerous, especially if the inguinal canal
of the testes occurs during the seventh tis has not descended into the scrotum compresses the intestine and cuts off its
or eighth month of fetal development by the age of 4 months, it can be surgi- blood supply. Fortunately, inguinal hernias
or, in some cases, shortly after birth. cally repaired. can be repaired surgically.

(sem′ ı̆-nif′er-ŭs) tubules, in which sperm cells develop. Delicate spermatogenesis
connective tissue surrounding the seminiferous tubules contains
clusters of endocrine cells called interstitial (in-ter-stish′ăl) cells, Spermatogenesis (sper′mă-tō-jen′ĕ-sis) is the formation of sperm
or Leydig (lı̄′dig) cells, which secrete testosterone. The seminifer- cells. Before puberty, the testes remain relatively simple and
ous tubules contain germ cells and sustentacular cells, or Sertoli unchanged from the time of their initial development. The interstitial
(ser-tō′lē) cells (figure 19.4b). Sustentacular cells are large and
extend from the periphery to the lumen of the seminiferous tubule. Basement
They nourish the germ cells and produce a number of hormones. membrane

Interstitial cells Sustentacular cell
(Leydig cells)

Rete testis Testis Seminiferous tubule Spermatogonia
Efferent
ductules Primary and
secondary
Epididymis (b) spermatocytes
Duct of Spermatids
epididymis Sperm cells

Ductus Septa Head Acrosome
deferens Lobules
(a) with coiled Midpiece Nucleus
seminiferous Tail Centriole
tubules Mitochondria

(c)

Figure 19.4 Structure of the Testis and Sperm Cell

(a) Gross anatomy of the testis with a section cut away to reveal internal structures. the Tail Reproductive
epididymis is also shown. (b) cross section of a seminiferous tubule. spermatogonia are near (d)
the periphery, and mature sperm cells are near the lumen of the seminiferous tubule. (c) the
head, midpiece, and tail of a sperm cell. (d) enlargement of the head and midpiece of a sperm
cell shows the nucleus and acrosome in the head and the mitochondria in the midpiece.

534 Chapter 19

cells are not prominent, and the seminiferous tubules are small A primary spermatocyte contains 46 chromosomes, each con-
and not yet functional. At the time of puberty, the interstitial cells sisting of 2 chromatids. Each primary spermatocyte passes through
increase in number and size, the seminiferous tubules enlarge, and the first meiotic division to produce 2 secondary spermatocytes.
spermatogenesis begins. Each secondary spermatocyte undergoes the second meiotic division
Germ cells are partially embedded in the sustentacular cells. to produce 2 smaller cells called spermatids (sper′ mă-tidz), each
The most peripheral germ cells are spermatogonia (sper′ mă-tō- having 23 chromosomes. After the second meiotic division, the
gō′ nē-ă; undeveloped sperm cells), which divide through mitosis spermatids undergo major structural changes to form sperm cells
(figure 19.5). Some daughter cells produced from these mitotic divi- (figure 19.5; see figure 19.4b,c). Much of the cytoplasm of the sper-
sions remain as spermatogonia and continue to divide by mitosis. matids is eliminated, and each spermatid develops a head, midpiece,
Other daughter cells form primary spermatocytes (sper′ mă-tō- and flagellum (tail) to become a sperm cell, or spermatozoon
sı̄tz; sperm cells), which divide by meiosis and become sperm cells. (figure 19.5; see figure 19.4b–d). The nucleus of the sperm cell is
located in the head of the sperm cell. Just anterior to the nucleus is
1 Spermatogonia 1 46 a vesicle called the acrosome (ak′ rō-sōm), which contains enzymes
are the cells from Spermatogonium that are released during the process of fertilization and are necessary
which sperm (germ cell) for the sperm cell to penetrate the oocyte, or egg cell.
cells arise. The At the end of spermatogenesis, the developing sperm cells are
spermatogonia 46 Mitotic division located around the lumen of the seminiferous tubules, with their
divide by mitosis. heads directed toward the surrounding sustentacular cells and their
One daughter 46 Daughter cell tails directed toward the center of the lumen (figure 19.5; see fig-
cell remains a ure 19.4b). Finally, sperm cells are released into the lumen of the
spermatogonium Primary Sustentacular seminiferous tubules.
that can divide spermatocyte cell
again by mitosis. First meiotic 2 Ducts
One daughter division 46
cell becomes a After their production, sperm cells are transported through the semi-
primary Secondary 23 23 niferous tubules and a series of ducts to the exterior of the body.
spermatocyte. spermatocyte
Epididymis
2 The primary 3 Second meiotic
spermatocyte division The seminiferous tubules of each testis empty into a tubular net-
divides by work called the rete (rē′ tē; net) testis (see figure 19.4a). The rete
meiosis to form Spermatid 23 23 23 23 testis empties into 15–20 tubules called the efferent ductules
secondary (ef′ er-ent dŭk′ toolz). The efferent ductules carry sperm cells from
spermatocytes. the testis to a tightly coiled series of threadlike tubules that form
a comma-shaped structure on the posterior side of the testis called
3 The secondary the epididymis (ep-i-did′ i-mis) (figure 19.6; see figures 19.3 and
spermatocytes 19.4). The sperm cells continue to mature within the epididymis,
divide by meiosis developing the capacity to swim and the ability to bind to the
to form oocyte. Sperm cells taken directly from the testes are not capable
spermatids. of fertilizing oocytes, but after maturing for several days in the
epididymis, the sperm cells develop the capacity to function as
4 The spermatids 4 Spermatid 23 23 23 23 gametes. Final changes in sperm cells, called capacitation (kă-
differentiate to becoming a pas′ i-tă′ shun), occur after ejaculation of semen into the vagina
form sperm cells. sperm cell and prior to fertilization.

23 23 23 23 Ductus Deferens
Lumen of Sperm
seminiferous cells The ductus deferens (dŭk′ tŭs def′ er-enz), or vas deferens, emerges
tubule from the epididymis and ascends along the posterior side of the
testis to become associated with the blood vessels and nerves
Process Figure 19.5    Spermatogenesis that supply the testis. These structures form the spermatic cord
(figure 19.6a). Each spermatic cord consists of the ductus defer-
A section of the seminiferous tubule illustrates the process of meiosis ens, testicular artery and veins, lymphatic vessels, and ­testicular
nerve. It is surrounded by the cremaster muscle and two connective
Reproductive and sperm cell formation. The number in each cell indicates the total tissue sheaths.
Each ductus deferens extends, in the spermatic cord, through
chromosome number. the abdominal wall by way of the inguinal canal. Each ductus def-
erens then crosses the lateral wall of the pelvic cavity and loops
behind the posterior surface of the urinary bladder to approach the
prostate gland (figure 19.6a; see figure 19.3). The total length of

Reproductive System 535

Ureter Ampulla of
Urinary bladder ductus deferens

Prostate gland Ductus deferens
Prostatic Seminal vesicle
urethra Duct of seminal vesicle
Membranous Ejaculatory duct
urethra
Bulbourethral Inguinal canal
gland
Ductus deferens Spermatic
cord
Testicular artery
Testicular veins
Testicular nerve

Penis Corpus cavernosum External connective Coverings of
tissue spermatic cord
Corpus spongiosum Cremaster muscle
Spongy
urethra Internal connective
tissue

Ductus deferens

External urethral
orifice

Epididymis Head
Body
Tail

Ductus deferens Dartos Scrotum
(arises from tail of muscle
epididymis) Glans penis Skin

Testis Anterior view
(rotated so the epididymis on the
posterior of the testis can be seen)
(a)

Dorsal surface Dorsal vein
Dorsal artery
Figure 19.6    Male Reproductive Organs Dorsal nerve
Connective
(a) Frontal view of a testis, an epididymis, and a ductus deferens, along with tissue

the penis and glands of the male reproductive system. The testis is viewed Corpora
cavernosa
in the scrotal sac with the smooth muscle and cremaster muscles on one
Corpus
side. The ductus deferens extends from the epididymis in the scrotal sac and spongiosum

passes through the inguinal canal and pelvic cavity to the prostate gland. Spongy urethra

Note that the ductus deferens and the testicular artery, vein, and nerves

are surrounded by the cremaster muscle and connective tissue to form the

spermatic cord. (b) A cross section of the penis illustrates the two dorsal

corpora cavernosa and the ventral corpus spongiosum. Connective tissue

sheaths and skin cover the three erectile bodies. Blood vessels, including (b) Ventral surface Reproductive
the dorsal artery and vein, and the dorsal nerve of the penis are visible.

536 Chapter 19

the ductus deferens is about 45 cm. Just before reaching the pros- Each is about 5 cm long and tapers into a short duct that joins the
tate gland, the ductus deferens increases in diameter to become ampulla of the ductus deferens to form the ejaculatory duct, as
the ampulla of the ductus deferens (figure 19.6a). The wall of previously mentioned.
the ductus deferens contains smooth muscle, which contracts in The prostate (pros′ tāt) gland consists of both glandular
peristaltic waves to propel the sperm cells from the epididymis and muscular tissue and is about the size and shape of a walnut
through the ductus deferens. (figure 19.6a; see figure 19.3). The prostate gland surrounds the
urethra and the two ejaculatory ducts. It consists of a capsule and
Reproductive Seminal Vesicle and Ejaculatory Duct numerous partitions. The cells lining the partitions secrete prostatic
fluid. There are 10–20 short ducts (not seen in figure 19.6a) that
Near the ampulla of each ductus deferens is a sac-shaped gland called carry secretions of the prostate gland to the prostatic urethra.
the seminal vesicle (sem′ i-năl ves′ i-kl). A short duct extends from the
seminal vesicle to the ampulla of the ductus deferens. The ducts from A Case in Point
the seminal vesicle and the ampulla of the ductus deferens join at
the prostate gland to form the ejaculatory (ē-jak′ ū-lă-tōr-ē) duct. Prostate Cancer
Each ejaculatory duct extends into the prostate gland and ends by
joining the urethra within the prostate gland (figure 19.6a). Sixty-five-year-old Chance O’Prostancer has a wellness checkup
every year. Ten years ago, a prostate-specific antigen (PSA)
Urethra test indicated that Chance’s PSA levels were higher than the
results from his previous tests. His physician reported moderate
The male urethra (ū-rē′ thră) extends from the urinary bladder to enlargement of the prostate gland but detected no obvious
the distal end of the penis (figure 19.6a; see figure 19.3). The ure- tumorlike structures in a physical examination. Because of the
thra can be divided into three parts: the prostatic urethra, which increasing PSA levels, Chance’s physician recommended a needle
passes through the prostate gland; the membranous urethra, biopsy of the prostate gland through the rectum. The pathology
which passes through the floor of the pelvis and is surrounded report described suspicious cells consistent with prostate cancer
by the external urinary sphincter; and the spongy urethra, which in one of the tissue samples. Chance’s physician had the biopsy
extends the length of the penis and opens at its end. The urethra samples examined by another pathology laboratory, which
is a passageway for both urine and male reproductive fluids. did not confirm the first pathology report. As a consequence,
However, urine and the reproductive fluids do not exit the ure- Chance’s physician explained that one option was to do nothing
thra at the same time. While male reproductive fluids are passing and continue having regular checkups because prostate cancer
through the urethra, a sympathetic reflex causes the internal uri- typically develops slowly. This is what Chance chose to do.
nary sphincter to contract, which keeps semen from passing into Eight years later, a PSA test showed another substantial
the urinary bladder and prevents urine from entering the urethra. increase in Chance’s PSA levels, although no tumor could be
detected by a physical exam, and Chance had no complaints,
Penis such as difficulty in urinating. This time, a needle biopsy of the
prostate gland revealed cancer cells in two of the six biopsy
The penis (pe′ nis) is the male organ of copulation and func- samples. Chance’s physician explained that so far the cancer had
tions in the transfer of sperm cells from the male to the female. not metastasized (spread) to areas outside the prostate gland.
The penis contains three columns of erectile tissue (figure 19.6). Therefore, Chance could choose to do nothing, have his prostate
Engorgement of this erectile tissue with blood causes the penis to gland surgically removed, or treat the cancer with radiation
enlarge and become firm, a process called erection (ē-rek′ shŭn). therapy, hormonal therapy, or chemotherapy. Statistics indicate
Two columns of erectile tissue form the dorsal portion and the that surgery and radiation therapy have similar success rates for
sides of the penis and are called the corpora cavernosa (kōr′ pōr- small, localized tumors like Chance’s. The trauma of surgery and
ă kav-er-nōs′ ă). The third, smaller erectile column occupies the the higher probability of erectile dysfunction following surgery
ventral portion of the penis and is called the corpus spongiosum convinced Chance that radiation therapy, which focuses radiation
(kōr′ pŭs spŭn-gē-ō′ sŭm). It expands over the distal end of the on the prostate gland to kill the cancer cells, was preferable.
penis to form a cap, the glans (glanz) penis. The spongy urethra Chance’s physician indicated that doing nothing is a reasonable
passes through the corpus spongiosum, including the glans penis, option for men who are significantly older than Chance because
and opens to the exterior as the external urethral orifice. older men diagnosed with prostate cancer often die of other
The shaft of the penis is covered by skin that is loosely conditions before they succumb to prostate cancer. Chance’s
attached to the connective tissue surrounding the penis. The skin physician explained that, for patients like him, approximately 85%
is firmly attached at the base of the glans penis, and a thinner layer are cancer free after 5 years. Chance was grateful that he had
of skin tightly covers the glans penis. The skin of the penis, espe- had annual checkups. Prostate cancer represents 29% of cancers
cially the glans penis, is well supplied with sensory receptors. A in males in the United States and 14% of the deaths due to cancer.
loose fold of skin, called the prepuce (prē′ pūs), or foreskin, covers Only lung cancer results in more cancer deaths in men.
the glans penis (see figure 19.3).
Predict 2
Glands
Changes in the size and texture of the prostate gland can be
The seminal vesicles are glands consisting of many saclike an indication of developing prostate cancer. Suggest a way that
structures located next to the ampulla of the ductus deferens the size and texture of the prostate gland can be examined by
(figure 19.6a; see figure 19.3). There are two seminal vesicles. palpation without surgical techniques (see figure 19.3).

reproductive system 537

CLINICAL IMPACT Circumcision

Circumcision (ser-k˘um-sizh′k˘un) sion. uncircumcised males have a higher in which the prepuce is “too tight” to be
is the surgical removal of the prepuce, incidence of penile cancer, but the under- moved over the glans penis, circumcision
usually shortly after birth. there are few lying causes seem related to chronic infec- can be necessary to avoid chronic infec-
compelling medical reasons for circumci- tions and poor hygiene. in those few cases tions and maintain normal circulation.

The bulbourethral (bul′bō-ū-rē′thrăl) glands, or Cowper The male reproductive system depends on both hormonal and neu- Reproductive
glands, are a pair of small, mucus-secreting glands located near the ral mechanisms to function normally. Hormones control the devel-
base of the penis (figure 19.6a; see figure 19.3). In young adults, opment of reproductive structures, the development of secondary
each is about the size of a pea, but they decrease in size with age. A sexual characteristics, spermatogenesis, and some aspects of sexual
single duct from each gland enters the urethra. behavior. The mature neural mechanisms are primarily involved in
controlling the sexual act and in the expression of sexual behavior.
secretions
regulation of reproductive Hormone
Semen (sē′men) is a mixture of sperm cells and secretions from the secretion
male reproductive glands. The seminal vesicles produce about 60%
of the fluid, the prostate gland contributes approximately 30%, the The hypothalamus of the brain, the anterior pituitary gland, and
testes contribute 5%, and the bulbourethral glands contribute 5%. the testes (figure 19.7) produce hormones that influence the male
reproductive system. Gonadotropin-releasing (gō′nad-ō-trō′pin)
The bulbourethral glands and the mucous glands in the urethra hormone (GnRH) is released from neurons in the hypothalamus
produce a mucous secretion, which lubricates the urethra, helps and passes to the anterior pituitary gland (table 19.1). GnRH
neutralize the contents of the normally acidic urethra, provides a causes cells in the anterior pituitary gland to secrete two hor-
small amount of lubrication during intercourse, and helps reduce mones, luteinizing (loo′tē-ı̆-nı̄z-ing) hormone (LH) and follicle-
acidity in the vagina. stimulating hormone (FSH), into the blood. LH and FSH are
named for their functions in females, but they are also essential
Testicular secretions include sperm cells and a small amount of reproductive hormones in males.
fluid. The thick, mucuslike secretion of the seminal vesicles contains
the sugar fructose and other nutrients that nourish sperm cells. The LH binds to the interstitial cells in the testes and causes them
seminal vesicle secretions also contain proteins that weakly coagulate to secrete testosterone. FSH binds primarily to sustentacular cells
after ejaculation and enzymes that are thought to help destroy abnor- in the seminiferous tubules and promotes sperm cell development.
mal sperm cells. Prostaglandins, which stimulate smooth muscle It also increases the secretion of a hormone called inhibin (in-
contractions, are present in high concentrations in the secretions of hib′in; to inhibit).
the seminal vesicles and can cause contractions of the female repro-
ductive tract, which help transport sperm cells through the tract. The blood levels of reproductive hormones are under negative-
feedback control. Testosterone has a negative-feedback effect on
The thin, milky secretions of the prostate have an alkaline the secretion of GnRH from the hypothalamus and the secretion
pH and help neutralize the acidic urethra, as well as the acidic of LH and FSH from the anterior pituitary gland. Inhibin has a
secretions of the testes, the seminal vesicles, and the vagina. The negative-feedback effect on the secretion of FSH from the anterior
increased pH is important for normal sperm cell function. The pituitary gland.
movement of sperm cells is not optimal until the pH is increased
to between 6.0 and 6.5. In contrast, the pH of vaginal secretions is For GnRH to stimulate LH and FSH release, the pituitary
between 3.5 and 4.0. Prostatic secretions also contain proteolytic gland must be exposed to a series of brief increases and decreases
enzymes that break down the coagulated proteins of the seminal in GnRH. If GnRH is maintained at a high level in the blood for
vesicles and make the semen more liquid. The normal volume of days or weeks, the anterior pituitary cells become insensitive to it.
semen is 2–5 milliliters (mL), with each milliliter of semen typi- GnRH can be produced synthetically and is useful in treating some
cally containing about 100 million sperm cells. people who are infertile. Synthetic GnRH must be administered
in small amounts in frequent pulses or surges. GnRH can also
19.4 pHysioloGy oF male inhibit reproduction because long-term administration of GnRH
reproDuction can sufficiently reduce LH and FSH levels to prevent sperm cell
production in males or ovulation in females.
Learning Outcomes After reading this section, you should be able to
puberty in males
A. list the hormones that influence the male reproductive
system, and describe their functions. Puberty (pū′ber-tē) is the sequence of events by which a child is
transformed into a young adult. The reproductive system matures
B. Describe the changes that occur in males during puberty. and assumes its adult functions, and the structural differences
C. explain the events that occur during the male sexual act.

538 Chapter 19

1 Gonadotropin-releasing GnRH
hormone (GnRH) from the
hypothalamus stimulates the Hypothalamus
secretion of luteinizing hormone Stimulatory
(LH) and follicle-stimulating Inhibitory
hormone (FSH) from the anterior
pituitary. LH, FSH 1 Anterior pituitary

2 LH stimulates testosterone 5
secretion from the interstitial
cells. LH FSH Inhibin
6
3 FSH stimulates sustentacular 23
cells of the seminiferous
tubules to increase Interstitial Sustentacular
spermatogenesis and to cells of cells of
secrete inhibin. testis seminiferous
Testosterone tubules
4 Testosterone has a
stimulatory effect on the 4 Spermatogenesis
sustentacular cells of the Development of
seminiferous tubules, as well as on reproductive
the development of reproductive organs and
organs and secondary sexual secondary sexual
characteristics. characteristics

5 Testosterone has a
negative-feedback effect on the
hypothalamus and pituitary to
reduce GnRH, LH, and FSH
secretion.

6 Inhibin has a negative-feedback
effect on the anterior pituitary to
reduce FSH secretion.

PROCESS Figure 19.7 Regulation of Reproductive Hormone Secretion in Males

Reproductive between adult males and females become more apparent. In boys, Secondary sexual characteristics in males include hair distribution
puberty commonly begins between the ages of 12 and 14 and is and growth, skin texture, fat distribution, skeletal muscle growth,
largely completed by age 18. Before puberty, small amounts of and changes in the larynx. After puberty, testosterone maintains
testosterone, secreted by the testes and the adrenal cortex, inhibit the adult structure of the male genitals, reproductive ducts, and
GnRH, LH, and FSH secretion. Beginning just before puberty secondary sexual characteristics.
and continuing throughout puberty, developmental changes in the
hypothalamus cause the hypothalamus and the anterior pituitary male sexual Behavior and the male sex act
gland to become much less sensitive to the inhibitory effect of tes-
tosterone, and the rate of GnRH, LH, and FSH secretion increases. Testosterone is required for normal sexual behavior. Testosterone
Consequently, elevated FSH levels promote spermatogenesis, and enters certain cells within the brain, especially within the hypo-
elevated LH levels cause the interstitial cells to secrete larger thalamus, and influences their functions. The blood levels of
amounts of testosterone. Testosterone still has a negative-feedback testosterone remain relatively constant throughout the lifetime
effect on the hypothalamus and anterior pituitary gland, but GnRH, of a male, from puberty until about 40 years of age. Thereafter,
LH, and FSH secretion occurs at substantially higher levels. the levels slowly decline to approximately 20% of this value by
80 years of age, causing a slow decrease in sex drive and fertility.
effects of testosterone
Predict 3
Testosterone (tes′tos′tĕ-rōn) is the major male hormone secreted
by the testes. Testosterone influences reproductive organs and Predict the effect on secondary sexual characteristics, external
nonreproductive structures (table 19.2). During puberty, testoster- genitalia, and sexual behavior if the testes fail to produce normal
one causes the enlargement and differentiation of the male genitals amounts of testosterone at puberty.
and the reproductive duct system. It is necessary for spermatogen-
esis and for the development of male secondary sexual character- The male sex act is a complex series of reflexes that result in
istics. The secondary sexual characteristics are those structural erection of the penis, secretion of mucus into the urethra, emission,
and behavioral changes, other than in the reproductive organs, and ejaculation. Emission (ē-mish′ŭn) is the movement of sperm
that develop at puberty and distinguish males from females. cells, mucus, prostatic secretions, and seminal vesicle secretions
into the prostatic, membranous, and spongy urethra. Ejaculation
(ē-jak′ū-lā′shŭn) is the forceful expulsion of the secretions that

Reproductive System 539

Table 19.1 Major Reproductive Hormones in Males and Females

Hormone Source Target Tissue Response

Male Reproductive System Hypothalamus Anterior pituitary Stimulates secretion of LH and FSH
Gonadotropin-releasing
hormone (GnRH) Anterior pituitary Interstitial cells of the testes Stimulates synthesis and secretion of testosterone
Luteinizing hormone (LH) Anterior pituitary Seminiferous tubules
Follicle-stimulating hormone (FSH) Interstitial cells of testes (sustentacular cells) Supports spermatogenesis and inhibin secretion
Testosterone Testes; body tissues
Sustentacular cells Development and maintenance of reproductive
Inhibin Anterior pituitary and organs; supports spermatogenesis and causes
Female Reproductive System hypothalamus the development and maintenance of secondary
Gonadotropin-releasing hormone Anterior pituitary sexual characteristics
(GnRH)
Luteinizing hormone (LH) Inhibits GnRH, LH, and FSH secretion through
negative feedback
Follicle-stimulating hormone (FSH)
Estrogen Inhibits FSH secretion through negative feedback

Progesterone Hypothalamus Anterior pituitary Stimulates secretion of LH and FSH

Anterior pituitary Ovaries Causes follicles to complete maturation and
Anterior pituitary undergo ovulation; causes ovulation; causes the
Oxytocin Follicles of ovaries and Ovaries ovulated follicle to become the corpus luteum
corpus luteum Uterus
Human chorionic gonadotropin Causes follicles to begin development
Corpus luteum of ovaries
Proliferation of endometrial cells
Posterior pituitary
Placenta Breasts Development of mammary glands (especially
Anterior pituitary and duct systems)
hypothalamus
Positive feedback before ovulation, resulting in
Other tissues increased LH and FSH secretion; negative
Uterus feedback with progesterone on the
hypothalamus and anterior pituitary after
Breasts ovulation, resulting in decreased LH and
Anterior pituitary FSH secretion

Other tissues Development and maintenance of secondary
Uterus and mammary glands sexual characteristics

Corpus luteum of ovaries Enlargement of endometrial cells and secretion
of fluid from uterine glands; maintenance of
pregnant state

Development of mammary glands (especially alveoli)

Negative feedback, with estrogen, on the
hypothalamus and anterior pituitary after
ovulation, resulting in decreased LH and
FSH secretion

Secondary sexual characteristics

Contraction of uterine smooth muscle and
contraction of cells in the breast, resulting
in milk letdown in lactating women

Maintains corpus luteum and increases its rate
of progesterone secretion during the first
one-third (first trimester) of pregnancy;
increases testosterone production in testes
of male fetuses

Reproductive

540 Chapter 19

CLINICAL IMPACT Anabolic Steroids

some athletes, especially those mass. many of the synthetic androgens and sterility can develop. other side effects
who depend on muscle strength, may are structurally different from testosterone. of large doses of synthetic androgens
either ingest or inject synthetic androgens their effect on muscle is greater than their include kidney and liver damage, heart
(an′dro¯-jenz; andros, male), which are hor- effect on the reproductive organs. However, attack, and stroke. in addition, synthetic
mones that have testosterone-like effects, when taken in large amounts, they can androgens cause abrupt mood swings, usu-
such as stimulating the development of influence the reproductive system. large ally toward intense anger and rage. taking
male sexual characteristics. the synthetic doses of synthetic androgens have a nega- synthetic androgens is highly discouraged
androgens are commonly called anabolic tive-feedback effect on the hypothalamus by the medical profession, violates the rules
steroids, or simply steroids, and they are and pituitary, reducing GnrH, lH, and FsH of most athletic organizations, and is illegal
used in an attempt to increase muscle levels. as a result, the testes can atrophy, without a prescription.

TabLe 19.2 effects of Testosterone on Target Tissues

Target Tissue Response

penis and scrotum enlargement and differentiation
Hair follicles
Hair growth and coarser hair in the pubic area, legs, chest, axillary region, face, and occasionally back; male pattern baldness
skin on the head if the person has the appropriate genetic makeup

larynx coarser texture of skin; increased rate of secretion of sebaceous glands, frequently resulting in acne at the time of puberty;
most tissues increased secretion of sweat glands in axillary regions
red blood cells
kidneys enlargement of larynx and deeper masculine voice
skeletal muscle
Bone increased rate of metabolism

increased rate of red blood cell production; a red blood cell count increase by about 20% as a result of increased erythropoietin secretion

retention of sodium and water to a small degree, resulting in increased extracellular fluid volume

a skeletal muscle mass increase at puberty; average increase is greater in males than in females

rapid bone growth, resulting in increased rate of growth and early cessation of bone growth; males who mature sexually at a later
age do not exhibit a rapid period of growth, but they grow for a longer time and can become taller than men who mature earlier

Reproductive have accumulated in the urethra to the exterior. Sensations, normally Psychological stimuli, such as sight, sound, odor, or thoughts,
interpreted as pleasurable, occur during the male sex act and result have a major effect on male sexual reflexes. Ejaculation while sleep-
in an intense sensation called an orgasm (ōr′gazm), or climax. ing (nocturnal emission) is a relatively common event in young
In males, orgasm is closely associated with ejaculation, although they males and is thought to be triggered by psychological stimuli associ-
are separate functions and do not always occur simultaneously. A ated with dreaming.
phase called resolution occurs after ejaculation. During resolution,
the penis becomes flaccid, an overall feeling of satisfaction exists, Erection, Emission, and Ejaculation
and the male is unable to achieve erection and a second ejaculation.
Erection is the first major component of the male sex act.
Sensory Impulses and Integration Parasympathetic action potentials from the sacral region of the
spinal cord cause the arteries that supply blood to the erectile tissues
Sensory action potentials from the genitals are carried to the sacral to dilate. Blood then fills small venous sinuses called sinusoids
region of the spinal cord, where reflexes that result in the male sex in the erectile tissue and compresses the veins, which reduces
act are integrated. Action potentials also travel from the spinal cord blood flow from the penis. The increased blood pressure in the
to the cerebrum to produce conscious sexual sensations. sinusoids causes the erectile tissue to become inflated and rigid.
Parasympathetic action potentials also cause the mucous glands
Rhythmic massage of the penis, especially the glans, and sur- within the urethra and the bulbourethral glands to secrete mucus.
rounding tissues, such as the scrotal, anal, and pubic regions, pro-
vide important sources of sensory action potentials. Engorgement of Failure to achieve erections, or erectile dysfunction (ED),
the prostate gland and seminal vesicles with secretions or irritation sometimes called impotence, can be a major source of frustration.
of the urethra, urinary bladder, ducta deferentia, and testes can also The inability to achieve erections can be due to reduced testoster-
cause sexual sensations. one secretion resulting from hypothalamic, pituitary, or testicular

reproductive system 541

CLINICAL IMPACT Male Pattern Baldness

some men have a genetic ten- Baldness usually reaches its maximum rate ness. it is most effective in those who are
dency called male pattern baldness, which of development when the individual is in young and just starting to show evidence of
develops in response to testosterone and the third or fourth decade of life. minoxidil baldness. minoxidil causes blood vessels to
other androgens. When testosterone levels (mi-noks′si-dil; rogaine) is a drug that effec- dilate, including those close to hair follicles,
increase at puberty, the density of hair tively prevents a decrease in hair growth in which may explain how it works. However,
on top of the head begins to decrease. many men who exhibit male pattern bald- the mechanism has not been confirmed.

complications. In other cases, ED can be due to defective stimula- motility also results in infertility. A major cause of reduced sperm Reproductive
tion of the erectile tissue by nerve fibers or reduced response of the cell motility is the presence of antisperm antibodies, which are
blood vessels to neural stimulation. Some men can achieve erections produced by the immune system and bind to sperm cells.
by taking oral medications, such as sildenafil (Viagra), tadalafil
(Cialis), or verdenafil (Livitra), or by having specific drugs injected In cases of infertility due to low sperm cell count or reduced
into the base of the penis. These drugs increase blood flow into the motility, fertility can sometimes be achieved by collecting several
erectile tissue of the penis, resulting in erection for many minutes. ejaculations, concentrating the sperm cells, and inserting them
into the female’s reproductive tract, a process called artificial
Before ejaculation, the ductus deferens begins to contract insemination (in-sem-i-nā′shŭn).
rhythmically, propelling sperm cells and testicular fluid from
the epididymis through the ductus deferens. Contractions of the 19.5 Female reproDuctive system
ductus deferens, seminal vesicles, and ejaculatory ducts cause the
sperm cells, testicular secretions, and seminal fluid to move into Learning Outcomes After reading this section, you should be able to
the urethra, where they mix with prostatic secretions released by
contraction of the prostate. A. name the organs of the female reproductive system, and
describe their structure.
Emission is stimulated by sympathetic action potentials that
originate in the lumbar region of the spinal cord. Action potentials B. Describe the anatomy and histology of the ovaries.
cause the reproductive ducts to contract and stimulate the seminal C. Discuss the development of the oocyte and the follicle, and
vesicles and the prostate gland to release secretions. Consequently,
semen accumulates in the urethra. describe ovulation and fertilization.
D. Describe the structure of the uterine tubes, uterus, vagina,
Ejaculation results from the contraction of smooth muscle in
the wall of the urethra and skeletal muscles surrounding the base external genitalia, and mammary glands.
of the penis. Just before ejaculation, action potentials are sent to
the skeletal muscles that surround the base of the penis. Rhythmic The female reproductive organs consist of the ovaries, the uter-
contractions are produced that force the semen out of the urethra, ine tubes (or fallopian tubes), the uterus, the vagina, the external
resulting in ejaculation. In addition, muscle tension increases genitalia, and the mammary glands (see figure 19.1b). The internal
throughout the body. reproductive organs of the female are located within the pelvis,
between the urinary bladder and the rectum (figure 19.8). The
infertility in males uterus and the vagina are in the midline, with an ovary to each side
of the uterus (figure 19.9). The internal reproductive organs are
Infertility (in-fer-til′i-tē) is reduced or diminished fertility. The held in place within the pelvis by a group of ligaments. The most
most common cause of infertility in males is a low sperm cell count. conspicuous is the broad ligament, which spreads out on both
If the sperm cell count drops to below 20 million sperm cells per sides of the uterus and attaches to the ovaries and uterine tubes.
milliliter, the male is usually sterile.
ovaries
The sperm cell count can decrease because of damage to
the testes as a result of trauma, radiation, cryptorchidism (See The two ovaries (ō′vă-rēz; ovum, egg) are small organs suspended
Clinical Impact “Descent of the Testes”), or infections, such as in the pelvic cavity by ligaments. The suspensory ligament
mumps, which block the ducts in the epididymis. Reduced sperm extends from each ovary to the lateral body wall, and the ovarian
cell counts can also result from inadequate secretion of luteinizing ligament attaches the ovary to the superior margin of the uterus
hormone and follicle-stimulating hormone, which can be caused (figure 19.9). In addition, the ovaries are attached to the poste-
by hypothyroidism, trauma to the hypothalamus, infarctions of the rior surface of the broad ligament by folds of peritoneum called
hypothalamus or anterior pituitary gland, or tumors. Decreased the mesovarium (mez′ō-vā′rē-ŭm). The ovarian arteries, veins,
testosterone secretion reduces the sperm cell count as well. and nerves traverse the suspensory ligament and enter the ovary
through the mesovarium.
Even when the sperm cell count is normal, fertility can be
reduced if sperm cell structure is abnormal, as occurs due to chromo- A layer of visceral peritoneum covers the surface of the ovary.
somal abnormalities caused by genetic factors. Reduced sperm cell The outer part of the ovary is composed of dense connective tissue
and contains ovarian follicles (figure 19.10). Each of the ovarian

542 Chapter 19
Uterine tube
Ovary Vertebral
column

Anterior Uterus Cervix Posterior
of uterus
Urinary Rectum
bladder
Vagina
Pubic
symphysis
Mons pubis
Urethra
Clitoris
Urethral orifice
Vaginal orifice
Labia minora
Labia majora

Medial view

Figure 19.8    Female Pelvis

The female reproductive tract, including the uterus, the vagina, and the surrounding pelvic structures, is shown in a medial view of the female pelvis.

Note that the female reproductive and urinary tracts open separately to the exterior.

Uterine tube Fundus Uterine cavity Suspensory
ligament
Mesovarium Fimbria
Ovary Ovary Ampulla
Broad ligament of uterine
tube
Uterine
tube

Broad Uterus Body Ovarian ligament
ligament Round ligament
Endometrium
Cervix Myometrium (muscular layer)
Perimetrium (serous layer)
Cervical canal
Opening of cervix Broad ligament (cut)

Vagina (cut)

Anterior view

Figure 19.9    Female Reproductive Organs

Reproductive Anterior view of the uterus, uterine tubes, and associated ligaments. The uterus and uterine tubes are cut in section (on the left side), and the vagina is cut to

show the internal anatomy. The inset shows the relationships among the ovary, the uterine tube, and the ligaments that suspend them in the pelvic cavity.

Reproductive System 543

Degenerated corpus luteum Mesovarium
Visceral peritoneum Blood vessels
Outer part of ovary Primordial follicles
Inner part of ovary Primary oocyte
Primary follicles
Corpus luteum Granulosa cells
Zona pellucida
Mature, or
graafian, follicle Degenerating follicle

Secondary follicle
Vesicles
Zona pellucida
Theca

Zona pellucida
Primary oocyte
Cumulus cells
Antrum
Theca

Figure 19.10    Structure of the Ovary and Ovarian Follicles

The ovary is sectioned to illustrate its internal structure (inset shows plane of section). Represented are ovarian follicles from each major stage of development,
as well as a corpus luteum.

follicles contains an oocyte (ō′ ō-sı̄t; o¯on, egg), the female sex around 300,000 to 400,000; of these, only about 400 will complete Reproductive
cell. Loose connective tissue makes up the inner part of the ovary, development and be released from the ovaries. Nearly all others
where blood vessels, lymphatic vessels, and nerves are located. degenerate after partial development.
Ovulation is the release of an oocyte from an ovary (figure 19.11,
Oogenesis and Fertilization step 8). Just before ovulation, the primary oocyte completes the first
meiotic division to produce a secondary oocyte and a polar body.
The formation of female gametes begins during fetal development, Unlike meiosis in males, cytoplasm is not split evenly between the
even before the female is born. By the fourth month of develop- two cells. Most of the cytoplasm of the primary oocyte remains with
ment, the ovaries contain 5 million oogonia (ō-ō-gō′ nē-ă), the the secondary oocyte. The polar body either degenerates or divides
cells from which oocytes develop (figure 19.11). By the time of to form two polar bodies. The secondary oocyte begins the second
birth, many of the oogonia have degenerated, and the remaining meiotic division but stops in metaphase II.
ones have begun meiosis. Also, some data indicate that oogonia After ovulation, the secondary oocyte may be fertilized by a
can form after birth from stem cells, but the extent to which this sperm cell (figure 19.11, step 9). Fertilization (fer′ til-i-zā′ shŭn)
occurs, and how long it occurs, is not clear. As in meiosis in males, begins when a sperm cell penetrates the cytoplasm of a secondary
the genetic material is duplicated, and two cell divisions occur (see oocyte. Subsequently, the secondary oocyte completes the second
“Formation of Gametes” earlier in this chapter). Meiosis stops, meiotic division to form 2 cells, each containing 23 chromosomes.
however, during the first meiotic division at prophase I. The cell One of these cells has very little cytoplasm and is another polar
at this stage is called a primary oocyte, and at birth there are body that degenerates. In the other, larger cell, the 23 chromo-
about 2 million of them. From birth to puberty, many primary somes from the sperm join with the 23 from the female gamete to
oocytes degenerate. The number of primary oocytes decreases to

544 Chapter 19 Oocyte Maturation Follicle Maturation Age
Oogonia Before
1 Oogonia give rise to oocytes. birth
Before birth, oogonia multiply by 46 46
mitosis. During development of Mitotic division Birth to
the fetus, many oogonia begin Daughter 46 1 puberty
meiosis, but stop in prophase I cell
and are now called primary Primary
oocytes. They remain in this state oocyte
until puberty.

2

2 Before birth, the primary oocytes 46 First meiotic Primary oocyte Primordial
become surrounded by a single division begins Granulosa follicle
layer of granulosa cells, creating a cells
primordial follicle. These are and then stops
present until puberty.

Number of primary oocytes
decreases to 300,000

3 After puberty, primordial follicles 3 Primary Puberty to
develop into primary follicles Primary oocyte follicles menopause
when the granulosa cells enlarge
and increase in number.

Granulosa
cells

4 Secondary follicles form when Zona pellucida
fluid-filled vesicles develop and
theca cells arise on the outside Primary 4 Secondary
of the follicle. oocyte follicle
Zona
5 Mature follicles form when the pellucida Fluid-filled vesicles
vesicles create a single antrum. Granulosa
cells Theca

5

6 Just before ovulation, the primary First meiotic Mature (graafian)
oocyte completes meiosis I, division follicle
creating a secondary oocyte and 6 completed just
a nonviable polar body. Zona pellucida
Cumulus cells
before ovulation Antrum
Theca
7 The secondary oocyte begins Secondary 23 First polar body
meiosis II, but stops at oocyte 23 (may divide to Granulosa cells
metaphase II. being converted
form two polar to corpus luteum
Second meiotic bodies) Ovulation cells
8 During ovulation, the secondary division begins 7 Secondary
oocyte 10
oocyte is released from the ovary. and then stops
Zona Corpus
9 The secondary oocyte only Secondary 23 8 pellucida luteum
completes meiosis II if it is oocyte Cumulus
fertilized by a sperm cell. The cells 11
completion of meiosis II forms an Sperm cell 9 Second Corpus
oocyte and a second polar body. unites with 23 polar albicans
Fertilization is complete when the secondary 23 body
oocyte nucleus and the sperm oocyte
cell nucleus unite, creating a Zygote
zygote. 23 46

10 Following ovulation, the granulosa Second meiotic division
cells divide rapidly and enlarge to completed after sperm
form the corpus luteum. cell unites with the
secondary oocyte
11 The corpus luteum degenerates
to form a scar, or corpus albicans. Fertilization

Reproductive Process Figure 19.11    Maturation of the Oocyte and Follicle

The events leading to oocyte and follicle maturation are closely linked. Once these structures are mature, fertilization can result. The numbers written in the

cells are total numbers of chromosomes.

reproductive system 545

CLINICAL IMPACT Cancer of the Cervix

Cancer of the cervix is rela- developed that targets types 16 and 18, as are smeared on a glass slide and later
tively common in women. in the united well as 2 types (6 and 11) that are linked stained and examined microscopically for
states, approximately 11,000 women are to 90% of genital warts cases. Fortunately, signs of cancer. early in the development
diagnosed with cervical cancer annually, cervical cancer can be detected and treat- of cervical cancer, the cells of the cervix
and about 3700 women die from it each ed. a Pap smear is a diagnostic test to change in a characteristic way. cells that
year. it has been estimated that 70% of all determine if a woman has cancer of the are cancerous appear less mature than the
cases of cervical cancer can be linked to cervix. By inserting a swab through the characteristic epithelial cells of the cervix
infection with 2 of the over 100 types of the vagina, a physician obtains a sample of or vaginal wall.
human papillomavirus (Hpv, types 16 and epithelial cells from the area of the cervix
18). an immunization (Gardisil) has been and the wall of the vagina. these cells

form a zygote (zı̄′gōt) and complete fertilization. The zygote has uterine tubes Reproductive
23 pairs of chromosomes (a total of 46 chromosomes). All cells of
the human body contain 23 pairs of chromosomes, except for the A uterine tube, also called a fallopian (fa-lō′pē-an) tube or oviduct
male and female gametes. The zygote divides by mitosis to form (ō′vi-dŭct), is associated with each ovary. The uterine tubes extend
2 cells, which divide to form 4 cells, and so on. The mass of cells from the area of the ovaries to the uterus. They open directly into
formed may eventually implant in, or attach to, the uterine wall the peritoneal cavity near each ovary and receive the secondary
and develop into a new individual (see chapter 20). oocyte. The opening of each uterine tube is surrounded by long,
thin processes called fimbriae (fim′brē-ē; fringes) (see figure 19.9).
Follicle Development
The fimbriae nearly surround the surface of the ovary. As a
As we discussed, when a female is in her mother’s uterus, her ovaries result, as soon as the secondary oocyte is ovulated, it comes into
have already begun oocyte formation. The primary oocytes present at contact with the surface of the fimbriae. Cilia on the fimbriae
birth are surrounded by a primordial follicle. A primordial follicle surface sweep the oocyte into the uterine tube. Fertilization usu-
is a primary oocyte surrounded by a single layer of flat cells, called ally occurs in the part of the uterine tube near the ovary, called
granulosa cells (figure 19.11). Once puberty begins, some of the pri- the ampulla (am-pul′lă). The fertilized oocyte then travels to
mordial follicles are converted to primary follicles when the oocyte the uterus, where it embeds in the uterine wall in a process
enlarges and the single layer of granulosa cells becomes enlarged called implantation.
and cuboidal. Subsequently, several layers of granulosa cells form,
and a layer of clear material called the zona pellucida (zō′nă pel- uterus
lū′sid-dă) is deposited around the primary oocyte.
The uterus (ū′ter-ŭs; womb) is as big as a medium-sized pear
Approximately every 28 days, hormonal changes stimulate (see figures 19.8 and 19.9). It is oriented in the pelvic cavity with
some of the primary follicles to continue to develop (figure 19.11). the larger, rounded part directed superiorly. The part of the uterus
The primary follicle becomes a secondary follicle as fluid-filled superior to the entrance of the uterine tubes is called the fundus
spaces called vesicles form among the granulosa cells, and a cap- (fŭn′dŭs). The main part of the uterus is called the body, and the
sule called the theca (thē′kă; a box) forms around the follicle. narrower part, the cervix (ser′viks; neck), is directed inferiorly.
Internally, the uterine cavity in the fundus and uterine body
The secondary follicle continues to enlarge, and when the fluid- continues through the cervix as the cervical canal, which opens
filled vesicles fuse to form a single, fluid-filled chamber called the into the vagina. The cervical canal is lined by mucous glands.
antrum (an′trŭm), the follicle is called the mature follicle, or graaf-
ian (graf′ē-ăn) follicle. The primary oocyte is pushed off to one side The uterine wall is composed of three layers: a serous layer,
and lies in a mass of granulosa cells called the cumulus cells. a muscular layer, and a layer of endometrium (see figure 19.9).
The outer layer, called the perimetrium (per-i-mē′trē-ŭm), or serous
The mature follicle forms a lump on the surface of the ovary. layer, of the uterus is formed from visceral peritoneum. The mid-
During ovulation, the mature follicle ruptures, forcing a small dle layer, called the myometrium (mı̄′ō-mē′trē-ŭm), or muscular
amount of blood, follicular fluid, and the secondary oocyte, surround- layer, consists of smooth muscle, is quite thick, and accounts for
ed by the cumulus cells, into the peritoneal cavity. In most cases, the bulk of the uterine wall. The innermost layer of the uterus is
only one of the follicles that begin to develop forms a mature follicle the endometrium (en′dō-mē′trē-ŭm), which consists of simple
and undergoes ovulation. The other follicles degenerate. After ovu- columnar epithelial cells with an underlying connective tissue
lation, the remaining cells of the ruptured follicle are transformed layer. Simple tubular glands, called spiral glands, are formed by
into a glandular structure called the corpus luteum (kōr′pŭs, body; folds of the endometrium. The superficial part of the endometrium
loo′tē-ŭm, yellow). If pregnancy occurs, the corpus luteum enlarges is sloughed off during menstruation.
in response to a hormone secreted by the placenta called human
chorionic gonadotropin hormone (hCG) (kō-rē-on′ik gō′nad-o- The uterus is supported by the broad ligament and the round
trō′pin) (see table 19.1). If pregnancy does not occur, the corpus ligament. In addition to these ligaments, much support is provided
luteum lasts for 10–12 days and then begins to degenerate. inferiorly to the uterus by skeletal muscles of the pelvic floor. If

546 Chapter 19

ligaments that support the uterus or muscles of the pelvic floor are ing structures (figure 19.12). The vestibule (ves′ ti-bool) is the
weakened, as may occur due to childbirth, the uterus can extend space into which the vagina and urethra open. The urethra opens
inferiorly into the vagina, a condition called a prolapsed uterus. just anterior to the vagina. The vestibule is bordered by a pair of
Severe cases require surgical correction. thin, longitudinal skin folds called the labia minora (lā′ bē-ă, lips;
mı̄-nō′ ră, small). A small, erectile structure called the clitoris
Vagina (klit′ ŏ-ris, klı̄′ tŏ-ris) is located in the anterior margin of the vesti-
bule. The two labia minora unite over the clitoris to form a fold of
The vagina (vă-jı̄′ nă) is the female organ of copulation; it receives skin called the prepuce.
the penis during intercourse. It also allows menstrual flow and The clitoris (see figure 19.8) consists of a shaft and a dis-
childbirth. The vagina extends from the uterus to the outside of tal glans. Like the glans penis, the clitoris is well supplied with
the body (see figures 19.8 and 19.9). The superior portion of the sensory receptors, and it is made up of erectile tissue. Additional
vagina is attached to the sides of the cervix, so that a part of the erectile tissue is located on each side of the vaginal opening.
cervix extends into the vagina. On each side of the vestibule, between the vaginal opening
The wall of the vagina consists of an outer muscular layer and and the labia minora, are openings of the greater vestibular
an inner mucous membrane. The muscular layer is smooth muscle glands. These glands produce a lubricating fluid that helps main-
and contains many elastic fibers. Thus, the vagina can increase tain the moistness of the vestibule.
in size to accommodate the penis during intercourse, and it can Lateral to the labia minora are two prominent, rounded folds
stretch greatly during childbirth. The mucous membrane is moist of skin called the labia majora (mă-jō′ ră; large). The two labia
stratified squamous epithelium that forms a protective surface majora unite anteriorly at an elevation of tissue over the pubic
layer. Lubricating fluid passes through the vaginal epithelium into symphysis called the mons pubis (monz pū′ bis) (figure 19.12).
the vagina. The lateral surfaces of the labia majora and the surface of the
In young females, the vaginal opening is covered by a thin mons pubis are covered with coarse hair. The medial surfaces of
mucous membrane called the hymen (hı̄′ men; membrane). In the labia majora are covered with numerous sebaceous and sweat
rare cases, the hymen may completely close the vaginal orifice glands. The space between the labia majora is called the pudendal
and it must be removed to allow menstrual flow. More commonly, cleft. Most of the time, the labia majora are in contact with each
the hymen is perforated by one or several holes. The openings other across the midline, closing the pudendal cleft and covering
in the hymen are usually greatly enlarged during the first sexual the deeper structures within the vestibule.
intercourse. The hymen can also be perforated or torn earlier in a The region between the vagina and the anus is the clinical
young female’s life during a variety of activities, including strenu- perineum (per′ i-nē′ um; area between the thighs). The skin and
ous exercise. The condition of the hymen is therefore an unreliable muscle of this region can tear during childbirth. To prevent such
indicator of virginity. tearing, an incision called an episiotomy (e-piz-ē-ot′ ō-mē) is
sometimes made in the clinical perineum. Traditionally, this clean,
External Genitalia straight incision has been thought to result in less injury, less
trouble in healing, and less pain. However, many studies report
The external female genitalia, also called the vulva (vŭl′ vă) or less injury and pain when no episiotomy is performed.
pudendum (pū-den′ dŭm), consist of the vestibule and its surround-
Mammary Glands
Mons Prepuce
pubis Clitoris The mammary (mam′ ă-rē; relating to breasts) glands are the
Urethra organs of milk production and are located in the breasts (fig-
Labia Vagina ure 19.13). The mammary glands are modified sweat glands.
majora Externally, each of the breasts of both males and females has a
Labia Anus raised nipple surrounded by a circular, pigmented area called the
minora areola (ă-rē′ ō-lă).
Vestibule In prepubescent children, the general structure of the male
Pudendal and female breasts is similar, and both males and females possess
cleft a rudimentary duct system. The female breasts begin to enlarge
during puberty, under the influence of estrogen and progesterone.
Clinical Some males also experience a minor and temporary enlargement
perineum of the breasts at puberty. Occasionally, the breasts of a male can
become permanently enlarged, a condition called gynecomastia
Reproductive Figure 19.12    Female External Genitalia (gı̄′ nĕ-kō-mas′ tē-ă). Causes of gynecomastia include hormonal
imbalances and the abuse of anabolic steroids.
Each adult female breast contains mammary glands con-
sisting of usually 15–20 glandular lobes covered by adipose
tissue (figure 19.13a,b). It is primarily this superficial adipose
tissue that gives the breast its form. Each lobe possesses a single
lactiferous duct that opens independently to the surface of the

reproductive system 547

CLINICAL IMPACT Cancer of the Breast

Cancer of the breast is a seri- often allows tumors to be identified even the tumor is benign or malignant. most
ous, often fatal disease that most often before they can be detected by palpation. tumors of the mammary glands are benign,
occurs in women. regular self-examination mammography uses low-intensity x-rays to but those that are malignant can spread
of the breast can lead to early detection of detect tumors in the soft tissue of the breast. to other areas of the body and ultimately
breast cancer and effective treatment. in once a tumor is identified, a biopsy is nor- lead to death.
addition, mammography (ma-mog′ra˘ fe) mally performed to determine whether

Lactiferous duct Alveoli Fascia
(a) Rib
Lobe Pectoralis
Lobule major

Adipose tissue Epithelium
Suspensory
ligaments Nonlactating

Lactiferous Myoepithelial Epithelium
ducts cell Alveoli
(b) Nonlactating breast
(c) Lactating breast Lactating
Nipple

Areola
Lobule
Lobe

Figure 19.13 Anatomy of the Breast

(a) each lactiferous duct of the mammary gland branches. at the end of each branch is one or more alveoli. (b) the nonlactating breast has a duct system that is not
extensively developed. the branches of the lactiferous ducts end as small, tubelike structures. (c) the lactating breast has a well-developed duct system with many
branches. the branches of the lactiferous duct end with well-developed alveoli. adipose tissue is abundant in both the nonlactating and the lactating breast.

nipple. The duct of each lobe is formed as several smaller ducts, The breasts are supported by suspensory ligaments that Reproductive
which originate from lobules, converge. Within a lobule, the extend from the fascia over the pectoralis major muscles to the
ducts branch and become even smaller. In the milk-producing, or skin over the breasts (figure 19.13b).
lactating, mammary gland, the ends of these small ducts expand
to form secretory sacs called alveoli. Myoepithelial cells sur- The nipples are very sensitive to tactile stimulation and
round the alveoli and contract to expel milk from the alveoli contain smooth muscle. When the smooth muscle contracts in
(figure 19.13c). response to stimuli, such as touch, cold, and sexual arousal, the
nipple becomes erect.

548 Chapter 19

Reproductive 19.6 Physiology of Female stimulated by estrogen. Menses typically lasts 4 or 5 days and
Reproduction can be accompanied by strong uterine contractions, called men-
strual cramps, that are extremely uncomfortable in some women.
Learning Outcomes After reading this section, you should be able to Menstrual cramps are the result of forceful myometrial contractions
that occur before and during menstruation. The cramps can result
A. Describe the changes that occur in females during puberty from excessive secretion of prostaglandins. As the endometrium of
and the changes in the ovary and uterus that occur during the uterus sloughs off, it becomes inflamed, and prostaglandins are
the menstrual cycle. produced as part of the inflammation. Many women can alleviate
painful cramps by taking medications, such as aspirin-like drugs,
B. List the hormones of the female reproductive system, and that inhibit prostaglandin biosynthesis just before the onset of
explain how their secretion is regulated. menstruation. These medications, however, are not effective in
treating all painful menstruation, especially when the cause of
C. Explain the events that occur during the female sexual act. the pain, such as that experienced by women who have tumors of the
D. Define menopause, and describe the changes that result from it. myometrium, is not due to the inflammatory response.
Ovulation occurs on about day 14 of the menstrual cycle,
As in the male, female reproduction is controlled by hormonal and although the timing of ovulation varies from individual to individual
nervous system mechanisms. and can vary within an individual from one menstrual cycle to the
next. To avoid or optimize contraception, it is critical to predict
Puberty in Females ovulation; however, there is no single reliable method that can pre-
dict its exact timing. The simplest method of predicting ovulation is
The initial change that results in puberty is most likely matura- looking for a drop in basal body temperature preceding ovulation,
tion of the hypothalamus. In girls, puberty, which typically begins but it is the least reliable method.
between ages 11 and 13 and is largely completed by age 16, is Between the end of menses and ovulation is the proliferative
marked by the first episode of menstrual bleeding, which is called phase, which refers to proliferation of the endometrium. During the
menarche (me-nar′ kē; me¯n, month + arche¯, beginning). During proliferative phase, the secondary follicles in the ovary mature; as
puberty, the vagina, uterus, uterine tubes, and external genitalia they do so, they secrete increasing amounts of estrogen. Estrogen
begin to enlarge. Adipose tissue is deposited in the breasts and acts on the uterus and causes the epithelial cells of the endometrium
around the hips, causing them to enlarge and assume an adult to divide rapidly. The endometrium thickens, and spiral glands form.
form. In addition, pubic and axillary hair grows. The development The sustained increase of estrogen secreted by the developing
of sexual drive is also associated with puberty. follicles stimulates GnRH secretion from the hypothalamus. GnRH,
The changes associated with puberty primarily result from in turn, triggers FSH and LH secretion from the anterior pituitary
the increasing rate of estrogen and progesterone secretion by the gland. FSH stimulates estrogen secretion at an increasing rate from
ovaries. Before puberty, estrogen and progesterone are secreted the developing follicles. This positive-feedback loop produces a
in very small amounts. At puberty, the cyclical adult pattern of series of larger and larger surges of FSH and LH secretion. Ovulation
hormone secretion is gradually established. occurs in response to the large increases in LH levels that normally
Before puberty, the rate of GnRH secretion from the hypo- occur on about day 14 of the menstrual cycle. This large increase in
thalamus and the rate of LH and FSH secretion from the anterior LH is also responsible for the development of the corpus luteum.
pituitary are very low. Estrogen and progesterone from the ovaries Following ovulation, the corpus luteum begins to secrete pro-
have a strong negative-feedback effect on the hypothalamus and gesterone and smaller amounts of estrogen. Progesterone acts on
pituitary. After the onset of puberty, the hypothalamus and anterior the uterus, causing the cells of the endometrium to become larger
pituitary secrete larger amounts of GnRH, LH, and FSH. Estrogen and to secrete a small amount of fluid. Together, progesterone and
and progesterone have less of a negative-feedback effect on the estrogen act on the hypothalamus and anterior pituitary gland to
hypothalamus and pituitary, and a sustained increase in estrogen inhibit GnRH, LH, and FSH secretion. Thus, LH and FSH levels
concentration has a positive-feedback effect. The normal cyclical decline after ovulation.
pattern of reproductive hormone secretion that occurs during the Between ovulation and the next menses is the secretory
menstrual cycle becomes established. phase of the menstrual cycle, called this because of the small
amount of fluid secreted by the cells of the endometrium.
Menstrual Cycle During the secretory phase, the lining of the uterus reaches its
greatest degree of development.
The term menstrual (men′ stroo-ăl) cycle refers to the series of If fertilization occurs, the zygote undergoes several cell divi-
changes that occur in sexually mature, nonpregnant females and sions to produce a collection of cells called the blastocyst (blas′ tō-
that culminate in menses. Menses (men′ sēz; month) is a period sist). The blastocyst passes through the uterine tube and arrives
of mild hemorrhage, during which part of the endometrium is in the uterus by 7 or 8 days after ovulation. The endometrium is
sloughed and expelled from the uterus. Typically, the menstrual prepared to receive the blastocyst, which becomes implanted in
cycle is about 28 days long, although it can be as short as 18 days the endometrium, where it continues to develop. If the secondary
or as long as 40 days (figure 19.14 and table 19.3). The menstrual oocyte is not fertilized, the endometrium sloughs away as a result of
cycle results from the cyclical changes that occur in the endome- declining blood progesterone levels. Unless the secondary oocyte is
trium of the uterus. These changes, in turn, result from the cyclical fertilized, the corpus luteum begins to produce less progesterone by
changes that occur in the ovary and are controlled by the secretions
of FSH and LH from the anterior pituitary gland.
The first day of menstrual bleeding (menses), when the endo-
metrium sloughs off, is considered day 1 of the menstrual cycle.
Sloughing of the endometrium is inhibited by progesterone but

Hypothalamus reproductive system 549
GnRH

LH Pituitary gland Figure 19.14
FSH
Pituitary Menstrual Cycle
gland Ovulation
over approximately 30 days,
Degenerating fluctuations occur in the levels of
corpus follicle-stimulating hormone (FsH)
luteum and luteinizing hormone (lH)
secretion from the anterior pituitary
Developing follicles Corpus gland and in the levels of estrogen
Estrogen luteum and progesterone secretion from
the ovary. in addition, changes
in the ovary and changes in the
endometrium of the uterus are
correlated with the changes in
hormone secretion throughout the
menstrual cycle. ovulation occurs
on about day 14.

Ovary

Progesterone

Endometrium

Uterus

24 6 8 10 12 14 16 18 20 22 24 26 28 days
Menses
Proliferative Secretory Menses
phase phase

day 24 or 25 of the menstrual cycle. By day 28, the declining pro- section. However, maternal mortality rates for abdominal pregnancies Reproductive
gesterone causes the endometrium to slough away to begin menses are significantly higher than for fallopian tube ectopic pregnancies.
and the next menstrual cycle. The declining progesterone secretion
results in a small increase in FSH secretion at the beginning of the menopause
next menses, which triggers more follicles to mature.
When a woman is 40–50 years old, the menstrual cycles become
Predict 4 less regular, and ovulation does not occur consistently during each
cycle. Eventually, the cycles stop completely. The cessation of
Predict the effect of administering a relatively large amount of menstrual cycles is called menopause (men′ō-pawz; me¯n, month
progesterone and estrogen just before the increase in LH that + pausis, cessation), and the whole time period from the onset of
precedes ovulation. irregular cycles to their complete cessation is called the female
climacteric (klı̄-mak′ter-ik).
An ectopic pregnancy results if implantation occurs anywhere
other than in the uterine cavity. The most common site of ectopic The major cause of menopause is age-related changes in
pregnancy is the uterine tube. Implantation in the uterine tube is even- the ovaries. The number of follicles remaining in the ovaries of
tually fatal to the fetus and can cause the tube to rupture. In some rare menopausal women is small. In addition, the follicles that remain
cases, implantation occurs in the mesenteries of the abdominal cav- become less sensitive to stimulation by FSH and LH, and therefore
ity; the fetus can develop normally but must be delivered by cesarean fewer mature follicles and corpora lutea are produced.

550 Chapter 19

CLINICAL IMPACT Amenorrhea

the absence of a menstrual cannot occur. many female athletes and in addition, secondary amenorrhea
cycle is called amenorrhea (a˘-men-¯o-re¯′a˘; ballet dancers who have rigorous train- can occur due to a lack of normal hormone
without menses). if the pituitary gland ing schedules have secondary amenor- secretion from the ovaries, which can result
does not function properly because of rhea. the physical stress that can be from autoimmune diseases that attack the
abnormal development, a woman does coupled with inadequate food intake ovary or occur due to polycystic ovarian
not begin to menstruate at puberty. this also results in very low GnrH secretion. disease, in which cysts in the ovary pro-
condition is called primary amenorrhea. increased food intake for anorexic women duce large amounts of androgen, which
in contrast, if a woman has had normal or reduced training for dancers and ath- is converted to estrogen by other body
menstrual cycles and later stops menstru- letes generally restores normal hormone tissues. the increased estrogen prevents
ating, the condition is called secondary secretion and normal menstrual cycles. the normal cycle of FsH and lH secre-
amenorrhea. one cause of secondary tion required for ovulation to occur. other
amenorrhea is anorexia, in which a lack secondary amenorrhea can also hormone-secreting tumors of the ovary can
of food causes the hypothalamus of the be the result of a pituitary tumor that also disrupt the normal menstrual cycle
brain to decrease GnrH secretion to decreases FsH and lH secretion or a lack and result in amenorrhea.
levels so low that the menstrual cycle of GnrH secretion from the hypothalamus
due to head trauma or a tumor.

TabLe 19.3 events During the Menstrual Cycle

Menses (day 1 to day 4 or 5 of the menstrual cycle)

pituitary gland the rate of FsH and lH secretion is low, but the rate of FsH secretion increases as progesterone levels decline.

ovary the rate of estrogen and progesterone secretion is low after degeneration of the corpus luteum produced during the previous menstrual cycle.

uterus in response to declining progesterone levels, the endometrial lining of the uterus sloughs off, resulting in menses followed by repair of
the endometrium.

Proliferative Phase (from day 4 or 5 until ovulation on about day 14)

pituitary gland the rate of FsH and lH secretion is only slightly elevated during most of the proliferative phase; FsH and lH secretions increase near
the end of the proliferative phase in response to increasing estrogen secretion from the ovaries.

ovary Developing follicles secrete increasing amounts of estrogen, especially near the end of the proliferative phase; increasing FsH and
lH cause additional estrogen secretion from the ovaries near the end of the proliferative phase.

uterus estrogen causes endometrial cells of the uterus to divide. the endometrium of the uterus thickens, and tubelike glands form. estrogen
causes the cells of the uterus to be more sensitive to progesterone by increasing the number of progesterone receptors in uterine tissues.

Ovulation (about day 14)

pituitary gland the rate of FsH and lH secretion increases rapidly just before ovulation in response to increasing estrogen levels. increasing FsH and
lH levels stimulate estrogen secretion, resulting in a positive-feedback cycle.

ovary lH causes final maturation of a mature follicle and initiates the process of ovulation. FsH acts on immature follicles and causes several
of them to begin to enlarge.

uterus the endometrium continues to divide in response to estrogen.

Secretory Phase (from about day 14 to day 28)

pituitary gland estrogen and progesterone reach levels high enough to inhibit FsH and lH secretion from the pituitary gland.

ovary after ovulation, the follicle is converted to the corpus luteum; the corpus luteum secretes large amounts of progesterone and smaller
amounts of estrogen from shortly after ovulation until about day 24 or 25. if fertilization does not occur, the corpus luteum degenerates
after about day 25, and the rate of progesterone secretion rapidly declines to low levels.

uterus in response to progesterone, the endometrial cells enlarge, the endometrial layer thickens, and the glands of the endometrium reach
their greatest degree of development; the endometrial cells secrete a small amount of fluid. after progesterone levels decline, the
endometrium begins to degenerate.

Menses (day 1 to day 4 or 5 of the next menstrual cycle)

pituitary gland the rate of lH remains low, and the rate of FsH secretion increases as progesterone levels decline.

ovary the rate of estrogen and progesterone secretion is low.

Reproductive uterus in response to declining progesterone levels, the endometrial lining of the uterus sloughs off, resulting in menses followed by repair of
the endometrium.

Reproductive System 551

Older women experience gradual changes in response to the necessary for fertilization to occur. Ovulation results from hormonal
reduced amount of estrogen and progesterone produced by the stimuli and is not dependent on the female sex act.
ovaries (table 19.4). For example, during the climacteric, some
women experience sudden episodes of uncomfortable sweating A Case in Point
(hot flashes), fatigue, anxiety, temporary decrease in libido, and
occasionally emotional disturbances. Many of these symptoms can Endometriosis
be treated successfully with hormone replacement therapy (HRT),
which usually consists of small amounts of estrogen and progester- Helen Hurtz is in her mid-twenties and has been married for 4 years.
one. HRT has been linked to a slightly increased risk of developing She has become very frustrated because she experiences pain
breast cancer, uterine cancer, heart attack, stroke, or blood clots. On during and after sexual intercourse, and the pain has become worse
the positive side, HRT slows the decrease in bone density that can over the past 2 years. She also has an increasing, persistent pain in
become severe in some women after menopause, and it decreases her pelvic region that is especially uncomfortable before and during
the risk of developing colorectal cancer. menstruation and becomes more intense during urination and bowel
movements. In addition, she recently developed periodic bouts of
Female Sexual Behavior and the Female Sex Act diarrhea. She reported all these symptoms to her physician.
Helen’s physician suspects endometriosis (en′do¯-me¯-tre¯-o¯′sis),
Sexual drive in females, like sexual drive in males, is dependent on a condition in which endometrial tissue migrates from the lining
hormones. Testosterone-like hormones, and possibly estrogen, affect of the uterus into the peritoneal cavity, where it attaches to the
brain cells (especially in the area of the hypothalamus) and influence surface of organs. Common sites of attachment are the ovaries and
sexual behavior. Testosterone-like hormones are produced primarily the pelvic peritoneum. Other possible sites are the intestines, uterus,
in the adrenal cortex. Psychological factors also play a role in sexual urinary bladder, and vagina. If the attached endometrial tissue has
behavior. The sensory and motor neural pathways involved in control- an adequate blood supply, it proliferates, breaks down, and bleeds
ling female sexual responses are similar to those found in the male. in response to the hormones produced during the menstrual cycle.
During sexual excitement, erectile tissue within the clitoris Unlike the normal endometrium, which is shed each month during
and around the vaginal opening becomes engorged with blood. The menstruation, endometrial tissue attached outside of the uterus
mucous glands within the vestibule, especially the greater vestibular causes lesions or tumors to develop, resulting in internal bleeding, scar
glands, secrete small amounts of mucus. Larger amounts of mucus- tissue formation, inflammation, and pain. Other major complications
like fluid are also extruded into the vagina through its wall. These of endometriosis are infertility and ovarian cyst formation. Between
secretions provide lubrication to allow easy entry and movement of 40% and 50% of infertile women have endometriosis.
the penis in the vagina during intercourse. Tactile stimulation of the Helen’s physician explained that the most accurate method
female’s genitals during sexual intercourse and psychological stim- of confirming the diagnosis is by laparoscopy, a procedure that
uli normally trigger an orgasm, or climax. The vaginal and uterine allows the physician to visually observe the abdominopelvic cavity.
smooth muscle, as well as the surrounding skeletal muscles, con- After a few weeks of thinking about the procedure, Helen agreed to
tract rhythmically, and muscle tension increases throughout much the laparoscopic examination. During the procedure, the physician
of the body. After the sex act, there is a period of resolution, which observed several lesions characteristic of endometriosis and removed
is characterized by an overall sense of satisfaction and relaxation. them with a laser instrument that vaporizes them.
Females are sometimes receptive to further immediate stimulation, Helen’s physician explained to her that there is no cure for
however, and can experience successive orgasms. Orgasm is not endometriosis, but the condition can be managed by administering
medication and removing the endometrial lesions periodically.

Table 19.4 Possible Changes in Postmenopausal Women Caused by Decreased Ovarian
Hormone Secretion

Changes

Menstrual cycle Five to seven years before menopause, the cycle becomes irregular; the number of cycles in which ovulation does not occur
and in which corpora lutea do not develop increases.

Uterus Gradual increase in irregular menstruations is followed by no menstruation; the endometrium finally atrophies, and the uterus
becomes smaller.

Vagina and external The epithelial lining becomes thinner; the external genitalia become thinner and less elastic; the labia majora become smaller;
genitalia the pubic hair decreases; reduced secretion leads to dryness; the vagina is more easily inflamed and infected.

Skin The epidermis becomes thinner.

Cardiovascular system Hypertension and atherosclerosis occur more frequently.

Vasomotor instability Hot flashes and increased sweating are correlated with vasodilation of cutaneous blood vessels; the hot flashes are related to
decreased estrogen levels.

Libido Temporary changes, usually a decrease in libido, are associated with the onset of menopause.

Fertility Fertility begins to decline about 10 years before the onset of menopause; by age 50, almost all the oocytes and follicles have degenerated.

Pituitary function Low levels of estrogen and progesterone produced by the ovaries cause the pituitary gland to secrete larger than normal amounts Reproductive
of LH and FSH; increased levels of these hormones have little effect on the postmenopausal ovaries.

552 Chapter 19

CLINICAL IMPACT Control of Pregnancy

Reproductive many methods are used to pre- lactation to prevent pregnancy after the when they have an iuD implanted. iuDs
vent pregnancy, either by preventing fertiliza- first 6 months postdelivery is not effective. also alter the endometrium, which may
tion (contraception) or by preventing implan- prevent implantation of an embryo. iuDs are
tation of the developing embryo. many of barrier Methods 99.99% effective in preventing pregnancy.
these techniques are quite effective when
used perfectly and consistently. But most of a male condom (kon′dom) is a sheath made Chemical Methods
these methods also have disadvantages, and of animal membrane, rubber, or plastic
the use of some of them is controversial. (figure 19aa). When placed over the erect synthetic estrogen and progesterone in
penis, a condom is a barrier device because oral contraceptives (birth control pills) (fig-
behavioral Methods it collects the semen instead of allowing it ure 19ae) are among the most effective con-
to be released into the vagina. condoms traceptives, providing 99.9% effectiveness.
abstinence, or refraining from sexual inter- also provide some protection against sexu- the synthetic steroids can have more than
course, is 100% effective in preventing preg- ally transmitted diseases. condoms alone one action, but they reduce lH and FsH
nancy when it is practiced consistently. it are 98% effective when used correctly, 99% release from the anterior pituitary. estrogen
is not an effective method when used only effective when used with spermicide. and progesterone are present in high enough
occasionally. concentrations to have a negative-feedback
a vaginal condom (or female con- effect on the pituitary, which prevents the
Coitus interruptus (ko¯′i-t˘us int-e˘- dom) also acts as a barrier. a woman large increase in lH and FsH secretion
r˘up′t˘us), or withdrawal, is removal of the can place the vaginal condom into the that triggers ovulation. over the years, the
penis from the vagina just before ejacula- vagina before sexual intercourse. Female dose of estrogen and progesterone in birth
tion. this is a very unreliable method of condoms are 95% effective. using spermi- control pills has been reduced. the current
preventing pregnancy because it requires cide further increases their effectiveness. lower dose of birth control pills has fewer
perfect awareness and willingness to side effects than earlier dosages. However,
withdraw the penis at the correct time. methods to prevent sperm cells from the risk of heart attack or stroke increases
statistically, about 23 women out of reaching the oocyte once they are in the in female users of oral contraceptives who
100 become pregnant while relying on vagina include a diaphragm, spermicidal smoke or who have a history of hypertension
this method. the withdrawal method also agents, and a vaginal sponge. the dia- or coagulation disorders. For most females,
ignores the fact that some sperm cells are phragm and the cervical cap (figure 19ab) the pill is effective and has a minimum
present in preejaculatory emissions. are flexible latex domes that are placed frequency of complications, until at least
over the cervix within the vagina, where age 35. the mini-pill is an oral contraceptive
the calendar method requires abstain- they prevent sperm cells from passing from that contains only synthetic progesterone. it
ing from sexual intercourse near the time of the vagina through the cervical canal of the reduces and thickens the mucus of the cer-
ovulation. a major factor in the success of this uterus. the diaphragm is a larger, shallow vix, which prevents sperm cells from reach-
method is the ability to predict accurately latex cup, and the cervical cap is a smaller, ing the oocyte. it also prevents blastocysts
the time of ovulation. although the calendar thimble-shaped latex cup. Diaphragms are from implanting in the uterus.
method provides some protection against 94% effective, whereas cervical cap effec-
becoming pregnant, it has a relatively high tiveness ranges from 71% in a woman who progesterone-like chemicals, such as
failure rate because of both the inability to has previously been pregnant to 86% in a medroxyprogesterone (med-rok′-se¯-pro¯-
predict the time of ovulation and the failure woman who has never been pregnant. the jes′ter-o¯n) (Depo-provera), which are inject-
to abstain from intercourse around that time. most commonly used spermicidal agents ed intramuscularly and slowly released into
about 9 women out of 100 become pregnant are foams or creams that kill sperm cells the circulatory system, can act as effective
while using the calendar method. (figure 19ac). they are inserted into the contraceptives. injected progesterone-like
vagina before sexual intercourse, often in chemicals can protect against pregnancy
continuous breastfeeding, or lacta- conjunction with diaphragm or condom for approximately 1 month, depending on
tion (lak-ta¯′sh˘un) (also known as lactation use. alone, spermicidal agents are only the amount injected, and are 99.9% effec-
amenorrhea, or lam), often stops the men- about 85% effective. tive. the patch (ortho evra) is an adhesive
strual cycle for up to the first 6 months after skin patch containing synthetic estrogen
childbirth, as long as the baby is exclusively Intrauterine (in′tra¯-yu¯′ter-in) devices and progesterone. it is worn on the lower
breastfed and the mother does not resume (IUDs) (figure 19ad) are inserted into the abdomen, buttocks, or upper body and is
menstruation while lactating. this method uterus through the cervix. the two types 99.9% effective. the vaginal contraceptive
is 99% effective. continuous breastfeed- of iuDs now available in the united states ring (nuva ring) is inserted into the vagina,
ing works because action potentials sent are the copper-containing paraGard and where it releases synthetic estrogen and
to the hypothalamus in response to infant the progestin hormone–coated mirena. the progesterone; it is 99.9% effective.
suckling inhibit GnrH release from the paraGard may be left in place for 12 years,
hypothalamus. reduced GnrH reduces lH, whereas the mirena may be left in place a drug called RU486, or mifepristone
which prevents ovulation. eventually, the for 5 years. Both types of iuDs thicken cer- (mif′pris-t¯on), blocks the action of progester-
menstrual cycle resumes. Because ovulation vical mucus, which bars sperm from enter- one, causing the endometrium of the uterus
normally precedes menstruation, relying on ing the uterus. some women stop ovulating

reproductive system 553

CLINICAL IMPACT (continued)

to slough off as it does at the time of men- tion, alter the rate at which the fertilized tomy has little effect on the volume of the
struation. it can therefore be used to induce oocyte is transported through the uterine ejaculated semen. the sequestered sperm
menstruation and reduce the possibility of tube to the uterus, or inhibit implantation. cells are reabsorbed in the epididymis. only
implantation when sexual intercourse has 1–4 in 1000 surgeries of this type fail.
occurred near the time of ovulation. it can Surgical Methods
also be used to terminate pregnancies. a common method of permanent
Vasectomy (va-sek′to¯-me¯) is a common birth control in females is tubal ligation
Morning-after pills, similar in compo- method used to render males permanently (l¯ı-ga¯′sh˘un), in which the uterine tubes are
sition to birth control pills, are available. infertile without affecting the performance tied and cut or clamped by means of an inci-
alternatively, doubling the number of birth of the sex act. vasectomy is a surgical proce- sion through the wall of the abdomen (figure
control pills after sexual intercourse within dure in which the ductus deferens from each 19ag). this procedure closes off the path
3 days and again after 12 more hours is testis is cut and tied off within the scrotal sac between the sperm cells and the oocyte.
sometimes recommended. these techniques (figure 19af ). this procedure prevents sperm commonly, a technique called laparoscopy
can be used after intercourse, but they are cells from passing through the ductus def- (lap-a˘-ros′k˘o-p¯e) is used, in which an instru-
only about 75% effective. the elevated blood erens and becoming part of the ejaculate. ment is inserted into the abdomen through
levels of estrogen and progesterone may Because such a small volume of ejaculate a small incision, so that only small openings
inhibit the increase in lH that causes ovula- comes from the testis and epididymis, vasec- need to be made to perform the operation.

(a) (b) (c)
Ductus deferens within
(d) spermatic cord
Uterine tube
cut and tied Ductus deferens
(g) (e) (vas deferens)

(f ) cut and tied

Ovary
Uterus

Figure 19A Reproductive

contraceptive devices and techniques include (a) condom, (b) cervical
cap and diaphragm used with spermicidal jelly, (c) spermicidal foam,
(d) intrauterine device (iuD), (e) oral contraceptives, (f ) vasectomy,
and (g) tubal ligation.

systems PaTHOLOGY

NGaemndee: r: molly benign Uterine Tumors
age: Female
43 background Information

saeaplfvnmpoeoepararafrotefltaoncdpopesrhlodtreafpnroemdnecvtlosariaedmorcoimernelyinrogndtostasetipemuvoipodlaneoedlnifteundnapssgmntoggehosrltDueeoualef(vws&iwrccsolitusycuhhiectmtetCrheamhfee,aeasotysocrxbwtshDnhsroiouhamtnoeeadeacmhl&s2mlrcvtterymmgiciewicoaomnmrimephrhnmveostadei)h.aohiaenelnsciiliyocesnd.dtaynnats4iafart’snigosdilsfchc3tttofadniaearhsitlu.gauabu,e,ndtestlrnretaairnemadhnit.edsctoormdechitutuonhrnouliethsmlteunndihnlael.oyenddgoopatalrfwsltmirrimtpyhnlunteietcarehmgthpeensamueisyeestddadtctosrsoneoieoiautclucrclisceeenynimteatrsscgohv.nearixrd. Leiomyomas (l¯ı ′o¯-m¯ı -o¯′ma˘z), also called uterine fibroids, are
fibroid tumors of the uterus (figure 19B). they are one of the most
common disorders of the uterus, and the most frequent tumor in
women, affecting one of every four. However, three-fourths of the

women with this condition experience no symptoms. the enlarged
masses that originate from smooth muscle tissue compress the
uterine lining (endometrium), resulting in ischemia and inflam-
mation. the increased inflammation, which shares some char-

acteristics with menstruation, results in frequent and severe
menses, with associated abdominal cramping due to strong
uterine contractions. constant menstruation is a frequent man-
ifestation of these tumors, and it is one of the most common

reasons women elect to have the uterus removed, a procedure
called a hysterectomy (his-ter-ek′to¯-me¯).

Interstitial leiomyoma

Submucous Uterus
leiomyoma Vagina

Subserous
leiomyoma

Figure 19B

leiomyomas, or fibroid tumors, are enlarged masses of smooth
muscle. they are located near the mucosa (submucous), within
the myometrium (interstitial), or near the serosa (subserous).

Reproductive infertility in Females Interruption of implantation can result from uterine tumors
or conditions causing abnormal ovarian hormone secretion. For
Causes of infertility in females include malfunctions of the uterine example, premature degeneration of the corpus luteum causes
tubes, reduced hormone secretion from the pituitary gland or the progesterone levels to decline and menses to occur. If the corpus
ovaries, and interruption of implantation. Uterine tube malfunc- luteum degenerates before the placenta begins to secrete proges-
tion can occur when infections result in pelvic inflammatory terone, the endometrium and the developing embryonic mass will
disease (PID), which causes adhesions to form in one or both of degenerate and be eliminated from the uterus. The conditions that
the uterine tubes. Inadequate secretion of LH and FSH can occur result in secondary amenorrhea also reduce fertility.
for a variety of reasons, including hypothyroidism or a tumor in or
trauma to the hypothalamus or anterior pituitary. Decreased secre-
tion of LH and FSH interrupts ovulation.

554

SKeLeTaL

the rate of red blood cell
synthesis in the red bone
marrow increases.

INTeGUMeNTaRY benign MUSCULaR
Uterine Tumors
if anemia develops, the skin can if anemia develops and is severe, muscle
appear pale because of the reduced Symptoms weakness results because of the reduced
• None in 75% of cases ability of the cardiovascular system to
hemoglobin in red blood cells. • Frequent and deliver adequate oxygen to muscles.
severe menses
URINaRY • Strong menstrual CaRDIOVaSCULaR
uterine cramping
the kidney increases erythropoietin a chronic loss of blood, as occurs in
secretion in response to the reduced Treatment prolonged menstruation over many
oxygen-carrying capacity of the blood. • Hysterectomy months to years, frequently results in
the erythropoietin increases red blood iron-deficiency anemia. manifestations
cell synthesis in red bone marrow. an ReSPIRaTORY of anemia include pale skin, reduced
enlarged uterine tumor can put pressure hematocrit, reduced hemoglobin
Because of anemia, the oxygen-carrying concentration, smaller than normal
on the urinary bladder, resulting in capacity of the blood is reduced. increased red blood cells (microcytic anemia),
frequent and painful urination. respiration during physical exertion and rapid and increased heart rate.
fatigue are likely to occur if anemia develops.

Predict 5

When discussing her condition with her mother, Molly discovered that
her mother had experienced frequent menses that were irregular and
prolonged when she was in her late 40s. Molly’s mother did not have
a hysterectomy, and in a few years, the frequency gradually began to
subside. Explain.

19.7 eFFects oF aGinG on In men, benign prostatic enlargement is common after Reproductive
tHe reproDuctive system 50 years of age. A major consequence of prostatic enlargement
is blockage of the prostatic urethra. Although benign pros-
Learning Outcome After reading this section, you should be able to tatic enlargement is not preventable, treatments are available to
reverse its negative effects. The frequency of prostate cancer also
A. Describe the major age-related changes in the increases as men age and is a significant cause of death in men.
reproductive system. In addition, the tendency for erectile dysfunction increases as men
age. However, less than 15% of men age 60 or under experience
Aging affects the reproductive system in both men and women abnormal erectile dysfunction. Keeping physically healthy can
in several ways. Sexual activity is often maintained in men and minimize some factors leading to abnormal erectile dysfunction,
women as they age, but the frequency of sexual intercourse usu- and medical treatments are available.
ally decreases gradually.
555

556 Chapter 19

In women, the most significant age-related change is meno- Approximately 10% of all women will develop breast cancer.
pause. By age 50, the amount of estrogen and progesterone The incidence of breast cancer is greatest between 45 and 65 years
produced by the ovaries has decreased. The uterus decreases of age and is greater for women who have a family history of breast
in size, and the endometrium decreases in thickness. The times cancer. The single most important measure to guard against death
between menses become irregular and longer until menstruation from breast cancer is early detection through breast self-exams
stops. The vaginal wall becomes thinner and less elastic, and and yearly mammograms after age 40. The incidences of uterine
there is less lubricant in the vagina, resulting in an increased cancer, ovarian cancer, and cervical cancer all increase between
tendency for vaginal yeast infections. However, wearing cotton 50 and 65 years of age. Annual medical checkups, including Pap
underwear and loose clothing reduces this possibility. In the event smears for cervical cancer, are important in order to detect cancer
of infections, very effective medical treatments are available. at early stages, when it can be easily treated.

DISEASES AND DISORDERS:  Reproductive System

CONDITION DESCRIPTION

Infectious Diseases Bacterial infection of the female pelvic organs; commonly caused by a vaginal or uterine infection with the bacteria
Pelvic inflammatory disease (PID) gonorrhea or chlamydia; early symptoms include increased vaginal discharge and pelvic pain; antibiotics are effective;
if untreated, can lead to sterility or be life-threatening
Sexually Transmitted Diseases
Nongonococcal urethritis Commonly known as STDs; spread by intimate sexual contact
(non-gon′o¯-kok′a˘l u-r¯e-thr¯ı ′tis)
Inflammation of the urethra that is not caused by gonorrhea; can be caused by trauma, insertion of a nonsterile
Trichomoniasis catheter, or sexual contact; usually due to infection with the bacterium Chlamydia trachomatis (kla-mid′e¯-a˘ tra-ko¯′ma˘-
(trik-o¯-mo¯-n¯ı ′a˘-sis) tis); may go unnoticed and result in pelvic inflammatory disease or sterility; antibiotics are effective treatment
Gonorrhea (gon-o¯-re¯′a˘)
Caused by Trichomonas (trik′o¯-mo¯′nas), a protozoan commonly found in the vagina of women and in the urethra of
Genital herpes (her′pe¯z) men; results in a greenish-yellow discharge with a foul odor; more common in women than in men

Genital warts Caused by the bacterium Neisseria gonorrhoeae (n¯ı -se¯′re¯-a˘ gon-o¯-re¯′a˘), which attaches to the epithelial cells of the
vagina or male urethra and causes pus to form; pain and discharge from the penis occur in men; asymptomatic in
Syphilis (sif′i-lis) women in the early stages; can lead to sterility in men and pelvic inflammatory disease in women

Acquired immunodeficiency Caused by herpes simplex 2 virus; characterized by lesions on the genitals that progress into blisterlike areas, making
syndrome (AIDS) urination, sitting, and walking painful; antiviral drugs can be effective

Caused by a viral infection; very contagious; warts vary from separate, small growths to large, cauliflower-like clusters; lesions
are not painful, but sexual intercourse with lesions is; treatments include topical medicines and surgery to remove the lesions

Caused by the bacterium Treponema pallidum (trep-o¯-ne¯′ma˘ pal′i-d˘um); can be spread by sexual contact; multiple
disease stages occur; children born to infected mothers may be developmentally delayed; antibiotics are effective

Caused by the human immunodeficiency virus (HIV), which ultimately destroys the immune system (see chapter 14);
transmitted through intimate sexual contact or by allowing infected body fluids into the interior of another person

Reproductive ANSWER TO Learn to Predict before the ovulation of each oocyte, the primary oocyte that will
be ovulated completes the first meiotic division to produce two
In this chapter, we learned that meiosis is cell division that produc- different-sized cells: one large secondary oocyte and one small
es haploid cells. When comparing meiosis in males and females, polar body. The secondary oocyte begins the second meiotic divi-
we find that the processes differ in several ways: the stage in an sion but will complete the process only if fertilized by a sperm cell.
individual’s life when meiosis begins, the types of cells produced, In the case of fertilization, the secondary oocyte divides unevenly
the number of functional cells produced with each cell division, and to form two cells. The smaller cell is another polar body and de-
the stage of life when meiosis ceases to occur. generates. In the larger cell, the haploid sperm nucleus combines
with the haploid oocyte nucleus to form a zygote. So, in females,
In males, meiosis begins at puberty in the seminiferous each primary oocyte produces only one large functional cell. The
tubules. Spermatogonia give rise to primary spermatocytes, advantage of the size difference lies in the fact that sperm cells
which will undergo the process of meiosis. During this process, contribute only their DNA to the zygote; it is the oocyte that con-
each primary spermatocyte eventually gives rise to four equal- tributes cytoplasm and all the organelles to the zygote.
sized mature sperm cells.
In addition, one final difference between male and female
Meiosis in females is more complex. The process actually begins meiosis is that in males, meiosis can continue until death and for
before a female is even born. During fetal development, many of females, the process of meiosis stops at menopause.
the oogonia in the ovaries degenerate. The remaining oogonia
actually begin meiosis I and are called primary oocytes. At birth, Answers to the rest of this chapter’s Predict questions are in Appendix E.
the existing primary oocytes stop meiosis. After puberty and just

Reproductive System 557

Summary Secretions Reproductive

19.1 Functions of the Reproductive System   1. Semen is a mixture of sperm cells and gland secretions.
2. The bulbourethral glands and the urethral mucous glands produce
(p. 529)
mucus that neutralizes the acidic pH of the urethra.
The reproductive system produces male and female gametes, enhances 3. The testicular secretions contain sperm cells.
fertilization of an oocyte by a sperm, nurtures the new individual until 4. The seminal vesicle fluid contains nutrients, prostaglandins, and
birth (in the female), and produces reproductive hormones.
proteins that coagulate.
19.2 Formation of Gametes   (p. 530) 5. The prostate fluid contains nutrients and proteolytic enzymes, and it

The reproductive organs in males and females produce gametes by meiosis. neutralizes the pH of the vagina.
1. Two consecutive cell divisions halve the chromosome number from
19.4 Physiology of Male Reproduction   (p. 537)
46 total chromosomes to 23 total chromosomes.
2. Meiosis forms male and female gametes. Regulation of Reproductive Hormone Secretion

19.3 Male Reproductive System   (p. 532) 1. GnRH is produced in the hypothalamus and released in surges.
2. GnRH stimulates release of LH and FSH from the anterior pituitary.
Scrotum 3. LH stimulates the interstitial cells to produce testosterone.
1. The scrotum is a sac containing the testes. 4. FSH binds to sustentacular cells and stimulates spermatogenesis
2. The dartos and cremaster muscles help regulate testes temperature.
and secretion of inhibin.
Testes 5. Testosterone has a negative-feedback effect on GnRH, LH, and
The testes are divided into lobules containing the seminiferous tubules
and interstitial cells. FSH secretion.
6. Inhibin has a negative-feedback effect on FSH secretion.
Spermatogenesis
1. Spermatogenesis begins in the seminiferous tubules at the time Puberty in Males

of puberty. 1. Before puberty, small amounts of testosterone inhibit GnRH
2. Sustentacular cells nourish the sperm cells and produce small release.

amounts of hormones. 2. During puberty, testosterone does not completely suppress
3. Spermatogonia divide (mitosis) to form primary spermatocytes. GnRH release, resulting in increased production of FSH, LH,
4. Primary spermatocytes divide by meiosis to produce first secondary and testosterone.

spermatocytes and then spermatids. The spermatids then mature to Effects of Testosterone
form sperm cells.
5. A spermatid develops a head, midpiece, and flagellum to become 1. Testosterone causes enlargement of the genitals and is necessary for
a sperm cell. The head contains the acrosome and the nucleus. spermatogenesis.

Ducts 2. Testosterone is responsible for the development of secondary sexual
1. The epididymis, a coiled tube system, is located on the testis and is characteristics.

the site of sperm maturation. Final changes, called capacitation of Male Sexual Behavior and the Male Sex Act
sperm cells, occur after ejaculation.
2. The seminiferous tubules lead to the rete testis, which opens into 1. Testosterone is required for normal sex drive.
the efferent ductules that extend to the epididymis. 2. Stimulation of the sex act can be tactile or psychological.
3. The ductus deferens passes from the epididymis into the abdominal 3. Sensory impulses pass to the sacral region of the spinal cord.
cavity. 4. Motor stimulation causes erection, mucus production, emission,
4. The ejaculatory duct is formed by the joining of the ductus deferens
and the duct from the seminal vesicle. and ejaculation.
5. The ejaculatory ducts join the prostatic urethra within the prostate
gland. Infertility in Males
6. The urethra extends from the urinary bladder through the penis to
the outside of the body. The most common cause of male infertility is a low sperm cell count.

Penis 19.5 Female Reproductive System   (p. 541)
1. The penis consists of erectile tissue.
2. The two corpora cavernosa form the dorsum and the sides. Ovaries, Oogenesis, and Fertilization
3. The corpus spongiosum forms the ventral portion and the glans
1. By the fourth month of development, the ovaries contain 5 million
penis, and it encloses the spongy urethra. The prepuce covers the oogonia.
glans penis.
2. By birth, many oogonia have degenerated, and for the remaining
Glands oogonia meiosis has stopped in prophase I, causing them to become
1. The seminal vesicles empty into the ejaculatory duct. primary oocytes.
2. The prostate gland consists of glandular and muscular tissue and
3. By puberty, 300,000 to 400,000 primary oocytes remain, of which
empties into the urethra. about 400 will be released from the ovaries.
3. The bulbourethral glands empty into the urethra.
4. Ovulation is the release of an oocyte from an ovary. The first
meiotic division is completed, and a secondary oocyte is released.

5. A sperm cell penetrates the secondary oocyte, the second meiotic
division is completed, and the nuclei of the oocyte and sperm cell
are united to complete fertilization.

6. A primordial follicle is a primary oocyte surrounded by a single
layer of flat granulosa cells.

558 Chapter 19

7. In primary follicles, the oocyte enlarges, and granulosa cells become Menstrual Cycle
cuboidal and form more than one layer. A zona pellucida is present.
1. The cyclical changes in the uterus are controlled by estrogen and
8. In a secondary follicle, fluid-filled vesicles appear, and a theca progesterone produced by the ovary.
forms around the follicle.
2. Menses (from day 1 to day 4 or 5): Menstrual fluid is produced by
9. In a mature follicle, vesicles fuse to form an antrum, and the degeneration of the endometrium.
primary oocyte is surrounded by cumulus cells.
3. Proliferative phase (from day 5 to day of ovulation): Epithelial cells
10. During ovulation, the mature follicle ruptures, releasing the multiply and form glands.
secondary oocyte, surrounded by cumulus cells, into the peritoneal
cavity. 4. Secretory phase (from day of ovulation to day 28): The endometrium
becomes thicker, and endometrial glands secrete. The uterus is
11. The remaining granulosa cells in the follicle develop into the prepared for implantation of the developing blastocyst by day 21.
corpus luteum.
5. Estrogen stimulates proliferation of the endometrium, and
12. If fertilization occurs, the corpus luteum persists. If there is no progesterone causes thickening of the endometrium. Decreased
fertilization, it degenerates. progesterone causes menses.

Uterine Tubes 6. FSH initiates the development of the follicles.
7. Estrogen produced by the follicles stimulates GnRH, FSH, and LH
1. The ovarian end of the uterine tube is surrounded by fimbriae.
2. Cilia on the fimbriae move the oocyte into the uterine tube. secretion, and FSH and LH stimulate more estrogen secretion. This
3. Fertilization usually occurs in the ampulla of the uterine tube, which positive-feedback mechanism causes FSH and LH levels to increase
near the time of ovulation.
is near the ovary. 8. LH stimulates ovulation and formation of the corpus luteum.
9. Estrogen and progesterone inhibit LH and FSH secretion following
Uterus ovulation.
1 0. If fertilization does not occur, progesterone secretion by the corpus
1. The uterus is a pear-shaped organ. The uterine cavity and the luteum decreases and menses begins.
cervical canal are the spaces formed by the uterus. 1 1. If fertilization does occur, the corpus luteum continues to secrete
progesterone and menses does not occur.
2. The wall of the uterus consists of the perimetrium, or serous layer;
the myometrium (smooth muscle); and the endometrium. Menopause

Vagina The cessation of the menstrual cycle is called menopause.

1. The vagina connects the uterus (cervix) to the vestibule. Female Sexual Behavior and the Female Sex Act
2. The vagina consists of a layer of smooth muscle and an inner lining
1. Female sex drive is partially influenced by testosterone-like hormones
of moist stratified squamous epithelium. produced by the adrenal cortex and estrogen produced by the ovary.
3. The wall of the vagina produces lubricating fluid.
4. The hymen covers the vestibular opening of the vagina in young 2. Autonomic nerves cause erectile tissue to become engorged with
blood, the vestibular glands to secrete mucus, and the vagina to
females. produce a lubricating fluid.

External Genitalia Infertility in Females

1. The vestibule is a space into which the vagina and the urethra open. Causes of infertility in females include malfunctions of the uterine tubes,
2. The clitoris is composed of erectile tissue and contains many reduced hormone secretion from the pituitary or ovary, and interruption
of implantation.
sensory receptors important in detecting sexual stimuli.
3. The labia minora are folds that cover the vestibule and form 19.7 Effects of Aging on the Reproductive
System  (p. 555)
the prepuce.
4. The greater vestibular glands produce a mucous fluid. 1. Benign prostatic enlargement affects men as they age, and it blocks
5. The labia majora cover the labia minora, and the pudendal cleft urine flow through the prostatic urethra.

is a space between the labia majora. 2. Prostate cancer is more common in elderly men.
6. The mons pubis is an elevated area superior to the labia majora. 3. Menopause is the most common age-related change in females.
4. Cancers of the breast, the cervix, and the ovaries increase in
Mammary Glands
elderly women.
1. Mammary glands are the organs of milk production. 5. Early detection is key to the successful treatment of most cancers.
2. The mammary glands are modified sweat glands that consist of

glandular lobes and adipose tissue.
3. The lobes connect to the nipple through ducts. The nipple is

surrounded by the areola.
4. The female breast enlarges during puberty under the influence of

estrogen and progesterone.

19.6 Physiology of Female Reproduction   (p. 548)

Puberty in Females

1. Puberty begins with the first menstrual bleeding (menarche).
2. Puberty begins when GnRH, LH, and FSH levels increase.

Reproductive

Reproductive System 559

Review and Comprehension

1. List the functions of the male and female reproductive systems. 13. Describe the process of follicle development and ovulation.
2. What is the scrotum? Explain the function of the dartos and 1 4. What is the corpus luteum? What happens to the corpus luteum

cremaster muscles. if fertilization occurs? If fertilization does not occur?
3. Where, specifically, are sperm cells produced in the testes? 15. Describe the normal pathway the oocyte follows after ovulation.

Describe the process of spermatogenesis. Where does fertilization usually take place?
4. Name the ducts the sperm cells traverse from their site of 16. Describe the relationship among the uterus, vagina, vestibule, and

production to the outside of the body. external genitalia.
5. In which duct do sperm cells develop their ability to fertilize? 17. Describe the labia minora, the prepuce, the labia majora, the
6. Name the erectile tissues of the penis, and describe how erectile
pudendal cleft, and the mons pubis.
tissue becomes erect. 18. What are the effects of estrogen and progesterone on the uterus?
7. State where the seminal vesicles, prostate gland, and bulbourethral 1 9. Describe the hormonal changes that result in ovulation. Explain

glands empty into the male reproductive duct system. the sequence of events during each phase of the menstrual cycle.
8. Define emission and ejaculation. 20. Define menopause and female climacteric. What causes these
9. Define semen. What structures give rise to secretions that make up
changes?
the semen? Describe the composition of the secretions of each gland. 21. List the major age-related diseases and conditions that occur in
10. Describe where GnRH, FSH, LH, and testosterone are produced,
the male reproductive system.
and explain how their secretion is regulated. 22. List the major age-related changes that occur in the female
11. Describe the effects of testosterone on males during puberty and
reproductive system.
in adulthood. 2 3. List the major age-related types of cancer that occur in the female
12. Describe the regulation of the male sex act.
reproductive system.

Critical Thinking c. blood levels of progesterone to be near their maximum.
d. a and b
1. If an adult male were castrated by having his testes removed, what e. a, b, and c
would happen to the levels of GnRH, FSH, LH, and testosterone in 5. On day 15 of the menstrual cycle, you would normally expect
his blood? a. decreasing blood levels of LH.
b. decreasing blood levels of estrogen.
2. Birth control pills for women contain estrogen and progesterone c. increasing blood levels of progesterone.
compounds. Explain how these hormones can prevent pregnancy. d. a and b
e. a, b, and c
3. During the secretory phase of the menstrual cycle, you would 6. Predict the consequences if a drug that blocks the effect of progesterone
normally expect is taken by a woman 2 or 3 days following ovulation or by a woman
a. the highest levels of progesterone that occur during the who is pregnant.
menstrual cycle. 7. During menopause, which reproductive hormones are reduced in
b. a follicle present in the ovary that is ready to undergo ovulation. the blood and which are increased?
c. the endometrium to reach its greatest degree of development.
d. a and b Answers in Appendix D
e. a and c

4. During approximately days 12–14 of the menstrual cycle, you
would normally expect
a. increasing blood levels of estrogen.
b. increasing blood levels of LH.

Reproductive

20C H A P t E r Development, Heredity,
and aging

LEArn TO PrEDiCt

After reading about the benefits of breast milk, Ming
was determined to breastfeed her baby until he was
a year old. Unfortunately, 2 weeks after her son’s
birth, Ming developed a serious urinary tract infec-
tion. Her physician prescribed some medication and
explained that she must not breastfeed her son while
taking it. After reading “Lactation” in this chapter,
explain the effect on Ming if she stops breastfeeding
for an extended time, and propose a strategy that
would allow her to cease breastfeeding for a while
but resume after she has finished the antibiotic.

20.1 PrEnAtAL DEVELoPMEnt Module 14 reproductive system

learning Outcomes After reading this section, you should be able to The prenatal (prē-nā′tăl; before birth) period, the period from
conception to birth, can be divided into three parts: (1) During
A. List the prenatal periods, and state the major developmental approximately the first 2 weeks of development, the primitive
events associated with each. germ layers are formed; (2) from about the second to the eighth
week of development, the major organ systems come into
B. Describe the process of fertilization. existence; and (3) during the last 7 months of the prenatal period, the
C. Describe the blastocyst, the process of implantation, and organ systems grow and become more mature. Between the time of
fertilization and 8 weeks of development, the developing human is
placental formation. called an embryo (em′brē-ō). From 8 weeks to birth, the develop-
D. Describe the maternal hormonal changes that occur during ing human is called a fetus (fē′tus; offspring).

pregnancy. To calculate the clinical age of an unborn child, the medi-
E. List the three germ layers, describe their formation, and list cal community uses the mother’s last menstrual (men′stroo-ăl)
period (LMP). An embryo or a fetus is therefore considered to be
the adult derivatives of each layer. a certain number of days post-LMP. Most embryologists, on the
F. Describe the formation of the neural tube and the neural other hand, use developmental age, which begins with fertilization,

crest cells.
G. Describe the formation of the limbs, the face, and the

digestive tract.
H. Explain how the single heart tube is divided into four

chambers.

The human lifespan is usually considered the period between
birth and death; however, the 9 months before birth are a critical
part of existence. What happens in these 9 months profoundly
affects the rest of a person’s life. Although most people develop
normally and are born without defects, approximately 3 of every
100 people are born with a birth defect so severe that it requires
medical attention during the first year of life. Later in life, many
more people discover previously unknown problems, such as the
tendency to develop asthma, certain brain disorders, or cancer.

560

Development, Heredity, and Aging 561

to describe the timing of developmental events. Because fertiliza- the nucleus of the sperm cell. Each of these nuclei has 23 chromo-
tion is assumed to occur approximately 14 days after LMP, the somes. Their fusion, which completes the process of fertilization,
developmental age is 14 days less than the clinical age. The times produces a zygote (zı̄′ gōt; having a yoke) that has 46 chromosomes
presented in this chapter are based on developmental age. (figure 20.1, step 3). The zygote develops into the embryo.

Fertilization Early Cell Division

Fertilization is the union of a sperm cell and an oocyte, along About 18–36 hours after fertilization, the zygote divides to form
with their genetic material (chromosomes), to produce a new two cells (figure 20.2). Those two cells divide to form four cells,
individual. After sperm cells are ejaculated into the vagina, they which divide to form eight, and so on. Even though the number of
are transported through the cervix and the body of the uterus to cells increases, the size of each cell decreases, so that the total mass
the uterine tubes, where fertilization occurs. The swimming abil- of cells remains about the same size as the zygote. These cells have
ity of the sperm cells and the muscular contractions of the uterus the ability to develop into a wide range of tissues. As a result, the
and uterine tubes are responsible for the movement of sperm cells total number of cells can be decreased, increased, or reorganized
through the female reproductive tract. Both oxytocin released by during this period without affecting normal development.
the female posterior pituitary and prostaglandins within the semen
stimulate contractions in the uterus and uterine tubes. A Case in Point
While passing through the uterus and the uterine tubes, the
sperm cells undergo capacitation. Capacitation (kă-pas′ i-tā′ shŭn) Twins
makes the sperm cells capable of releasing the concentrated
enzymes contained in the acrosome, a region of the sperm cell The parents of twin boys named the babies Juan and Hamall
head. The enzymes digest a pathway through the cumulus cells because, at birth, they looked so much alike that, if you’ve seen
and the zona pellucida of the secondary oocyte (ō′ ō-sı̄t; egg cell). Juan, you’ve seen Hamall. As the twins grew older, they continued to
One sperm cell attaches to the oocyte cell membrane and enters look very much alike but not exactly the same. Their parents wanted
the oocyte (figure 20.1, step 1). to know if they were identical or fraternal twins, so they had an
The secondary oocyte is capable of being fertilized for up to expensive genetic analysis performed. The results indicated that the
about 1 day after ovulation, and some sperm cells remain viable twins were identical, despite their slight differences in appearance.
in the female reproductive tract for up to 6 days, although most of In rare cases, following early cell divisions, the cells separate
them degenerate before that time. and form two individuals, called “identical twins,” or monozygotic
(mon-o¯-z¯ı -got′ik) twins. Identical twins therefore have identical
Predict 2 genetic information in their cells. Identical twins can also occur by
other mechanisms a little later in development.
During what days of the menstrual cycle is sexual intercourse most Occasionally, a woman can ovulate two or more secondary
likely to result in pregnancy? oocytes at the same time. Fertilization of multiple oocytes by different
sperm cells results in “fraternal twins,” or dizygotic (d¯ı′z¯ı-got′ik) twins.
Hundreds of sperm cells reach the secondary oocyte, but nor- Even though who we are and what we look like are determined
mally a change in the oocyte cell membrane prevents more than in large part by genes, identical twins—although they have the same
one sperm cell from entering the secondary oocyte. The secondary genes—may not look exactly alike. In fact, identical twins often look
oocyte undergoes the second meiotic division only after a sperm more like mirror images of each other. Genes interact with minute
cell enters it (figure 20.1, step 2). After the second meiotic division, environmental cues in the embryo and in the child to determine the
the oocyte nucleus moves to the center of the cell, where it meets final form of the ­individual.

Cumulus Head of
cells sperm cell
2 The head of the sperm, carrying
Sperm cell the genetic material, enters the
contacting oocyte, which completes the
oocyte cell second meiotic division.
membrane

Oocyte
nucleus

1 Many sperm cells come in contact with Single
the cumulus cells of the secondary nucleus
oocyte, but only one sperm cell will 3 The two nuclei fuse to form a single
penetrate the oocyte's cell membrane. nucleus. Fertilization is complete,
and a zygote results.

PROCESS Figure 20.1    Fertilization Reproductive

562 Chapter 20

(b) (c) (d)
2 cells 4 cells
(day 1) (day 2)

Morula Trophoblast (e)
(day 5)
Blastocele

Cells that Inner
become the Blastocyst cell mass
embryo proper (day 6)
(a) Fertilization
Ovulation

Implantation

Figure 20.2    Development of the Blastocyst and Implantation

Successive cell divisions produce a multicellular morula by day 5, which becomes a hollow blastocyst on day 6. In the figure of the blastocyst, green cells
are trophoblastic and orange cells are embryonic. (a) Zygote (120 μM in diameter with two polar bodies attached). (b-e) During the early cell divisions, the
embryo divides into more and more cells, but the total size of the embryo remains relatively constant.

Reproductive Blastocyst Implantation of the Blastocyst and
Development of the Placenta
After fertilization, multiple cell divisions have produced an embry-
onic mass of about 12–16 cells, called a morula (mōr′ ū-lă; mul- All these early events, from the first cell division to formation of
berry) (figure 20.2). Most of the cells of the morula will not form the the blastocele, occur as the embryonic mass moves from the site of
embryo proper but will form support structures, such as the placenta. fertilization in the uterine tube to the site of implantation (im-plan-
When a cavity begins to appear within the mass of cells, the tā′ shun) in the uterus. By 7 or 8 days after ovulation, the endome-
whole structure is called a blastocyst (blas′ tō-sist) (figure 20.2). trium of the uterus is prepared for implantation. About 7 days after
The fluid-filled cavity is called the blastocele (blas′ tō-sēl). Most fertilization, the blastocyst attaches itself to the uterine wall and
of the blastocele is surrounded by a single layer of cells, but at one begins the process of implantation. The trophoblast cells of the blas-
end of the blastocyst, the cells are several layers thick. This thick- tocyst digest the uterine tissues as the blastocyst burrows into the
ened area is called the inner cell mass. Not all cells of the blasto- uterine wall. Before implantation and for a short time afterward, the
cyst give rise to the new individual. The embryo proper, which will embryo is insensitive to environmental toxins. During the first few
become the new individual, will develop from only a few cells of days of development, each cell has enough yolk to supply its own
the inner cell mass. These cells are commonly referred to as stem energy needs and requires few external nutrients. Furthermore,
cells because they give rise to all the cell types within the body. during the first couple of weeks of development, large numbers of
The remaining cells of the blastocyst are called the trophoblast cells can die, yet the embryo can fully recover.
(trof′ ō-blast, trō′ fō-blast), which forms the embryonic part of the As the blastocyst burrows into the uterine wall, trophoblast
placenta and the membranes (chorion and amnion) surrounding cells, called the chorion (kō′ rē-on), form the embryonic portion
the embryo. of the placenta (plă-sen′ tă), the organ of nutrient and waste

Development, Heredity, and Aging 563

product exchange between the embryo and the mother. Fingerlike The estrogen and progesterone secreted by the corpus luteum
projections, called chorionic villi, protrude into cavities formed (see chapter 19) are essential for maintaining the endometrium for the
within the maternal endometrium. Those cavities, called lacunae first 3 months of pregnancy. After the placenta forms, it also begins to
(lă-koo′ nē), are filled with maternal blood (figure 20.3). In the secrete estrogen and progesterone. By the third month of pregnancy,
mature placenta, the embryonic blood supply is separated from the the placenta has become an endocrine gland that secretes sufficient
maternal blood supply by the embryonic capillary wall, a basement quantities of estrogen and progesterone to maintain pregnancy, and
membrane, and a thin layer of chorion. As a result, the embryonic the corpus luteum is no longer needed. Estrogen and progesterone
blood and maternal blood do not mix. Nutrients and waste products levels increase in the mother’s blood throughout pregnancy.
must cross this semipermeable barrier between the two circulations.
Initially, the embryo is attached to the placenta by a connecting Formation of the Germ Layers
stalk. As the embryo matures, the connecting stalk elongates and
becomes the umbilical (ŭm-bil′ i-kăl; navel) cord (figure 20.3b). After implantation, a new cavity, called the amniotic (am-nē-ot′ ik)
Within the umbilical cord, blood vessels carry blood from the cavity, forms inside the inner cell mass and causes the part of the
embryo to the placenta and from the placenta to the embryo. inner cell mass nearest the blastocele to separate as a flat disk of
tissue called the embryonic disk (figure 20.5). The amniotic cavity
Maternal Hormonal Changes is bounded by a membrane called the amnion and is filled with
amniotic fluid. The embryo will grow in the amniotic cavity,
The chorion secretes human chorionic gonadotropin (gō′ nad-ō- where the amniotic fluid forms a protective cushion. The embryonic
trō′ pin) (hCG), which travels in the blood to the maternal ovary and disk at first is composed of two layers of cells: an epiblast adja-
causes the corpus luteum to remain functional. The secretion of hCG cent to the amniotic cavity and a hypoblast on the side of the disk
begins shortly after implantation, increases rapidly, and reaches a opposite the amniotic cavity. A third cavity, the yolk sac, forms
peak about 8 or 9 weeks after fertilization. Subsequently, hCG levels inside the blastocele from the hypoblast.
decline to a lower level, where they are maintained throughout the At about 14 days after fertilization, the embryonic disk has
remainder of the pregnancy (figure 20.4). Most pregnancy tests are become a slightly elongated, oval structure. Some of the epiblast
designed to detect hCG in either urine or blood. cells migrate toward the center of the disk, forming a thickened

Maternal arteriole Maternal venule Maternal blood
Endometrium in lacuna
Maternal blood Chorionic villi Chorion
in lacuna Umbilical vein Basement
Fetal arteriole Umbilical arteries membrane
Fetal venule Embryonic blood
in capillary
(b)
Separation between
Placenta maternal and embryonic
blood
(a) (c)

Umbilical cord

Figure 20.3  Interface Between Maternal and Fetal Circulation Reproductive

(a) Location of the placenta and umbilical cord in relation to the fetus. (b) As the chorion
encounters maternal blood vessels, lacunae (cavities) form and become filled with maternal
blood. In the mature placenta, the embryonic blood vessels and other tissue form chorionic villi,
and nutrients are exchanged between embryonic and maternal blood. Note that the umbilical
arteries and fetal arterioles contain deoxygenated blood and that the umbilical vein and fetal
venules contain oxygenated blood. (c) Under normal conditions, the maternal and fetal blood
are separated by the chorion and a basement membrane and do not mix.

564 Chapter 20

1 Human chorionic gonadotropin 2 Progesterone (blue line) continues 3 Estrogen (green line) increases
slowly throughout pregnancy but
(red line) (hCG) increases until it to increase until it levels off near increases more rapidly as the end
of pregnancy approaches. Early
reaches a maximum concentration the end of pregnancy. Early in in pregnancy, estrogen is produced
only in the ovary; by the second
near the end of the first 3 months pregnancy, progesterone is trimester, production shifts to the
placenta.
of pregnancy and then decreases produced by the corpus luteum

to a low level thereafter. in the ovary; by the second

trimester, production shifts to the

placenta.

First trimester Ovary Second trimester Ovary Third trimester
Placenta Placenta Placenta
hCG hCG Ovary

hCG

Hormone Progesterone Progesterone Progesterone
concentration Estrogen Estrogen Estrogen

1 2 3
Progesterone
hCG Estrogen

First trimester Second trimester Third trimester
(first 3 months) (second 3 months) (third 3 months)

PROCESS Figure 20.4  Changes in Hormone Concentration During Pregnancy

During pregnancy, hCG, progesterone, and estrogen are secreted. The placenta secretes hCG. Early in pregnancy, the ovary secretes estrogen and
progesterone. During midpregnancy, there is a shift toward estrogen and progesterone secretion by the placenta.

Lacuna Amnion

Chorion Amniotic cavity Develops
Connecting stalk from the
Epiblast Embryonic inner cell
Endometrium Hypoblast disk mass
of uterus
Yolk sac

Blastocele

Uterine epithelium

Reproductive Figure 20.5  Early Embryo and Surrounding Structures in the Placenta

The embryonic disk consists of epiblast and hypoblast surrounded by the amniotic cavity and yolk sac. The connecting stalk, which attaches the embryo to the
uterus, becomes part of the umbilical cord.

Development, Heredity, and Aging 565

line called the primitive streak (figure 20.6). The formation of the is called gastrulation. The embryo is now three-layered, having
primitive streak establishes the central axis of the embryo. Some ectoderm, mesoderm, and endoderm (figure 20.6). All the tissues of
of the epiblast cells migrate through the primitive streak. Of these an adult can be traced to these three germ layers (table 20.1).
migrating cells, some displace the hypoblast to form the endoderm From about day 14 until about day 35, the embryo is at maxi-
(en′ dō-derm; inside layer) while others emerge between the epi- mum risk from environmental toxins and drugs that can cause birth
blast and endoderm as a new germ layer, called the mesoderm defects. The causes of birth defects are a major unsolved issue in
(mez′ ō-derm; middle layer). Those epiblast cells that do not biology at present. However, it appears that oxidative damage to
migrate form the ectoderm (ek′ tō-derm; outside layer). This pro- key molecules in certain developing cells and/or changes in the
cess of cell migration and the formation of three distinct germ layers DNA function within those cells may be involved.

1 Cells in the surface epiblast move Primitive streak Notochord
toward the primitive streak and migrate Connecting
through the streak (blue arrow tails). stalk Amnion
Cephalic (towards the head)
2 Cells of the epiblast that migrate Caudal
through the primitive streak become (towards the tail) 1 3 Ectoderm Embryonic
endodermal and mesodermal cells 2 Mesoderm disk
(red arrows).
Endoderm
3 The mesoderm (pink) lies between the
ectoderm (blue) and the endoderm
(yellow).

Yolk sac

PROCESS Figure 20.6  Primitive Streak and Germ Layers

The head of the embryo develops over the notochord.

Table 20.1 Tissues Derived from Each Germ Layer

Endoderm Ectoderm Mesoderm

Lining of digestive tract Epidermis of skin Dermis of skin
Lining of lungs Tooth enamel Heart and blood vessels
Lining of hepatic, pancreatic, and other exocrine ducts Lens and cornea of eye Parenchyma (substance) of glands
Kidney ducts and urinary bladder Nasal cavity Kidneys
Thymus Anterior pituitary Gonads
Thyroid gland Neuroectoderm Muscle
Parathyroid glands Brain and spinal cord Bones (except facial)
Tonsils Somatic motor neurons Microglia
Preganglionic autonomic neurons
Posterior pituitary Reproductive
Neuroglial cells (except microglia)
Neural crest cells
Melanocytes
Sensory neurons
Postganglionic autonomic neurons
Adrenal medulla
Facial bones
Teeth (dentin, pulp, and cementum) and gingiva
A few skeletal muscles in head

566 Chapter 20

CLINICAL IMPACT In Vitro Fertilization and Embryo Transfer

in a small number of women, For in vitro fertilization and embryo several hours after the embryos have been
normal pregnancy is not possible because transfer to be accomplished, a woman is first introduced into the uterus to prevent possible
of an anatomical or physiological condition. injected with a substance similar to luteiniz- expulsion before implantation can occur. it
in 87% of these cases, the uterine tubes are ing hormone (LH), which causes more than is not fully understood why such expulsion
incapable of transporting the zygote to the one follicle to ovulate at a time. Just before does not occur in natural fertilization and
uterus or allowing sperm cells to reach the the follicles rupture, the secondary oocytes implantation. implantation and subsequent
oocyte. since 1978, in vitro fertilization and are surgically removed from the ovary. the development then proceed in the uterus as
embryo transfer have made pregnancy pos- oocytes are then incubated in a dish and they would for natural implantation.
sible in hundreds of such women. In vitro maintained at body temperature for 6 hours.
fertilization (IVF) involves removing sec- then, sperm cells are added to the dish. the success rate of embryo transfer var-
ondary oocytes from a woman, placing the Different techniques may then be utilized ies from clinic to clinic, with the age of the
oocytes into a petri dish, and adding sperm that enhance sperm entry into the oocyte. embryo at the time of transfer, and with
cells to the dish, where fertilization and the age of the patient. the current success
early development occur in vitro, which After 24–48 hours, several of the embryos rate for achieving pregnancy following iVF
means “in glass.” embryo transfer involves are transferred to the uterus. several embryos is 31%. of these pregnancies, 83% result in
removing the developing embryo from the are introduced into the uterus to increase the live births. Multiple births have occurred fre-
petri dish and introducing the embryo into success rate as much as possible, because quently following embryo transfer because
the uterus of a recipient woman. only a few of them survive. the woman of the practice of introducing more than
is usually required to lie perfectly still for one embryo into the uterus.

Reproductive A specialized group of cells at the cephalic (towards the head) of the limbs or the bowels and bladder, depending on where the
end of the primitive streak moves from one end of the primitive defect occurs. More severe forms of spina bifida result from
streak to the other and, in some yet unknown way, organizes the failure of the neural tube in the area of the spinal cord to close.
embryo. A cordlike structure called the notochord (nō′tō-kōrd) It has been demonstrated that adequate amounts of the B vitamin
is formed by these cells as they move down the primitive streak. folate, more commonly referred to as folic acid, in the diet during
The notochord marks the central axis of the developing embryo pregnancy can reduce the risk of such defects.
(figure 20.6).
As the neural folds come together and fuse, a population of
Predict 3 cells breaks away from the neuroectoderm all along the crests
of the folds. Most of these neural crest cells become part of the
Predict the result if two primitive streaks form in one embryonic peripheral nervous system or become melanocytes in the skin. In
disk. What if the two primitive streaks are touching each other? the head, neural crest cells also contribute to the skull, the dentin
of teeth, blood vessels, and general connective tissue.
neural tube and neural Crest Formation
Formation of the General Body structure
At about 18 days after fertilization, the ectoderm overlying the
notochord thickens to form the neural plate. The lateral edges Arms and legs first appear at about 28 days after fertilization as
of the plate begin to rise like two ocean waves coming together. limb buds (figure 20.8) and quickly begin to elongate. At about 35
These edges are called the neural folds, and between them lies days, expansions called hand and foot plates form at the ends of
a neural groove. The neural folds begin to meet in the midline the limb buds. Zones of cell death between the future fingers and
and fuse into a neural tube (figure 20.7). The cells of the neural toes of the hand and foot plates help sculpt the fingers and toes.
tube are called neuroectoderm (noor-ō-ek′tō-derm) (table 20.1).
Neuroectoderm becomes the brain, the spinal cord, and parts of The face develops by fusion of five growing masses of tis-
the peripheral nervous system. The neural tube becomes com- sue, called processes (figure 20.9). One, the frontonasal pro-
pletely closed by day 26. If the neural tube fails to close, major cess, forms the forehead, nose, and center of the upper jaw and
defects of the central nervous system can result. lip. Two maxillary processes form the maxillae (upper lip and
jaw), and two mandibular processes form the mandible (lower
Anencephaly (an′en-sef′ă-lē; no brain) is a birth defect lip and jaw).
wherein much of the brain fails to form because the neural tube
did not close in the region of the head. A baby born with anen- The nose begins as two structures, one on each side of the
cephaly cannot survive. Spina bifida (spı̄′nă bif′i-dă; split spine) forehead mass. As the brain enlarges and the face matures, the
is a general term describing defects of the spinal cord or vertebral two parts of the nose approach each other in the midline and
column. Spina bifida can range from a simple defect with one or fuse (figure 20.9). The two masses forming the upper jaw expand
more vertebral spinous processes split or missing but no clinical toward the midline and fuse with part of the nose to form the
manifestation to a more severe defect that can result in paralysis upper jaw and lip. A cleft lip results from failure of these struc-
tures to fuse. Cleft lips usually do not occur in the midline, but

Development, Heredity, and Aging 567

1 1

Neural groove Neural
Neural fold plate
Notochord Frontonasal
process
2 Neural groove 1 28 days after fertilization Maxillary
Crest of the neural fold The face develops from five process
processes: frontonasal (blue), Mandibular
Neural fold two maxillary (yellow), and process
two mandibular (orange;
already fused).

3 2

Crest of the neural fold Eye
Neural crest cells Frontonasal
process
4 2 33 days after fertilization
Skin Nasal placodes, areas of Nose
thickening, appear in beginning
(a) Neural crest cells the frontonasal process. to form
Maxillary
(b) Neural tube process
Notochord

1 The neural plate forms from ectoderm. 3
2 Neural folds form as parallel ridges along the embryo.
3 Neural crest cells begin to form from the crest of the neural folds. 3 40 days after fertilization Nasal
4 The neural folds meet at the midline to form the neural tube, and Maxillary processes extend placode
toward the midline. The nasal
neural crest cells separate from the neural folds.

PROCESS Figure 20.7  Formation of the Neural Tube placodes also move toward Maxillary
the midline and fuse with the process
The neural folds, which consist of neuroectoderm, come together at the maxillary processes to form the
midline and fuse to form a neural tube. This fusion begins in the center jaw and lip.
and moves both cranially and caudally. (a) The embryo shown is about
21 days after fertilization. (b) These cross sections represent progressive
closure of the neural tube.

4 48 days after fertilization 4
Continued growth brings Eye
structures more toward the Nose
midline. Upper
lip

Eye

Heart 5
Upper
Umbilical limb bud
cord
Lower 5 14 weeks after fertilization Nose
limb bud Colors show the contributions
of each process to the adult Upper lip
face. and jaw
Lower lip
and jaw

Figure 20.8  Human Embryo 35 Days After Fertilization Process Figure 20.9  Development of the Face Reproductive

568 Chapter 20

to one side (or both sides). The cleft can vary in severity from Development of the Organ Systems
a slight indentation in the lip to a fissure that extends from the
mouth to the nares (nostril). The major organ systems appear and begin to develop during the
The roof of the mouth, or palate, begins to form as vertical embryonic period (second to eighth week of development). This
shelves of tissue that grow on the inside of the maxillary masses. period is therefore also called the period of organogenesis (ōr′ gă-
These shelves swing to a horizontal position and begin to fuse nō-jen′ ĕ-sis). The individual organ systems are listed in table 20.2;
with each other at about 56 days of development. If the palate only general comments about a few select systems are presented
does not fuse, a midline cleft in the roof of the mouth called a in the text.
cleft palate results. A cleft palate can range in severity from a While the neural tube is forming (18–26 days), the remainder
slight cleft of the uvula to a fissure extending the entire length of of the embryo is folding to form a tube along the upper part of the
the palate. A cleft lip and cleft palate can occur together, forming yolk sac (figure 20.10). The developing digestive tract pinches off
a continuous fissure. from the yolk sac as a tube but remains attached in the center to the
yolk sac by a yolk stalk.

Table 20.2 Development of the Organ Systems

General Features Age (Days Since Fertilization)

1–5 6–10 11–15 16–20 21–25 26–30

Fertilization, Blastocyst implants. Primitive streak, Neural plate Neural tube closed Limb buds and
blastocyst other “buds”
three germ layers Melanocytes appear.
form from
Integumentary Ectoderm, neural crest. Limb buds
System mesoderm Neural crest cells
(form facial Somites are
Skeletal System Mesoderm bones) all formed.

Muscular System Mesoderm Somites (body Neural tube Lens begins
Nervous System Ectoderm segments) complete; to form.
begin to form. neural crest
forms; eyes Parathyroid glands
Neural plate and ears begin and pancreas
to form. appear.

Endocrine System Ectoderm, Thyroid begins
mesoderm, to develop.
endoderm

Cardiovascular Mesoderm Blood islands form; Single-tubed heart Interatrial septum
System two-tubed begins to beat. forms.
heart forms.

Lymphatic System Mesoderm Thymus appears.

Respiratory System Mesoderm, Diaphragm begins Trachea, lung buds
Digestive System endoderm to form. appear.
Mesoderm,
Urinary System endoderm Tooth dentin forms; Liver and pancreas
Reproductive foregut and appear as
System Mesoderm, hindgut form. buds; tongue
endoderm bud appears.
Mesoderm, Embryonic kidneys
endoderm appear. Embryonic kidneys
elongate.
Primordial germ
cells form on Male reproductive
yolk sac. ducts appear;
external genital
Reproductive structures begin
to form.

Development, Heredity, and Aging 569

A considerable number of outpocketings appear at about The major chambers of the heart, the atrium and ventricle,
28 days after fertilization along the entire length of the digestive expand rapidly. The single ventricle is subdivided into two chambers
tract (figure 20.11). A surprisingly large number of important by the development of an interventricular (in-ter-ven-trik′ ŭ-lăr)
internal organs develop from those outpocketings, including the septum (figure 20.12, steps 3 and 4). If the interventricular sep-
auditory tubes, tonsils, thymus, anterior pituitary gland, thyroid tum does not grow enough to completely separate the ventricles, a
gland, parathyroid glands, lungs, liver, pancreas, and urinary bladder. ventricular septal defect results.
The heart develops from two blood vessels, which lie side by An interatrial (in-ter-ā′ trē-ăl) septum forms to separate the
side in the early embryo and fuse about 21 days after fertilization two atria (figure 20.12, steps 3–5). An opening in the interatrial
into a single, midline heart (figure 20.12, steps 1 and 2). At about septum called the foramen ovale (ō-val′ ē) connects the two atria
this time, the primitive heart begins to beat. Blood vessels form from and allows blood to flow from the right to the left atrium in the
“blood islands” on the surface of the yolk sac and inside the embryo. fetus. Because of the foramen ovale, some blood in the fetus
These islands expand and fuse to form the circulatory system. passes from the right atrium to the left atrium and bypasses the

Age (Days Since Fertilization)

31–35 36–40 41–45 46–50 51–55 56–60

Hand and foot plates Fingers and toes External ear forming; Embryo 25 mm Limbs elongate to Face is distinctly human
on limbs appear; lips form; embryo 20 mm adult proportions; in appearance.
embryo 15 mm embryo 35 mm
Sensory receptors
appear in skin. Collagen fibers are Extensive sensory nerve
clearly present endings in skin
in skin.
Mesoderm Cartilage in site of Cartilage in site of Ossification begins in
condensation in future humerus Cartilage in site of future hand and clavicle and then in
areas of future bone future ulna and fingers other bones.
radius
Muscle precursor cells Functional muscle Nearly all muscles
enter limb buds. appearing in an
adult form
Nerve processes enter External ear forming; Semicircular canals
limb buds. olfactory nerves in inner ear are Eyelids form; cochlea
begin to form. complete. in inner ear is
complete.

Pituitary gland appears Gonads begin to form; Pineal body appears. Thyroid gland in adult Anterior pituitary gland
as evaginations adrenal glands position loses its connection
from brain and form. Interatrial septum is to mouth.
mouth. complete but
Interventricular septum foramen ovale
Interventricular septum is complete. remains until birth.
begins to form.
Palate begins to form; Tracheal cartilage
Large lymphatic vessels Spleen appears. tooth buds begin begins to form. Adult lymph pattern
form in neck. to form. is formed.
Tertiary bronchi to
Secondary bronchi to lobules form. Palate begins to fuse
lobes form. (fusion complete by
90 days); anus opens.
Mouth opens to outside.

Adult kidneys begin Embryonic kidneys
to develop. degenerate.

Gonads begin to form. Primordial germ cells Female reproductive Uterus is forming;
enter gonads. ducts appear. external genitalia
begin to
differentiate in Reproductive
male and female.

570 Chapter 20
Embryo
Amniotic Developing
cavity digestive tract
Amniotic
Neural tube cavity

Yolk Neural tube
sac
Neural crest

Ectoderm Yolk sac

Mesoderm Yolk stalk Developing
Endoderm digestive
Yolk sac tract
(b)
Yolk sac
(a)

Figure 20.10  Development of the Digestive Tract

The digestive tract develops along the dorsal side of the yolk sac (yellow) as the body folds into a tube (blue arrows). The figures in back are shown in sagittal
section. The figures in front are shown in cross section. The dashed line on the figures in back shows the plane of section in the figures in front. (a) An early
embryo (about 24 days). (b) A slightly older embryo (about 28 days).

Thymus and Tonsil The kidneys develop from mesoderm located along the lateral
parathyroid Pharynx wall of the body cavity (see figure 20.11). The embryonic kidneys
glands Auditory tube are much more extensive than the adult kidneys, extending the
Lung Anterior entire length of the body cavity. They are closely associated with
pituitary internal reproductive organs, such as the ovaries or testes, and
Embryonic Stomach reproductive ducts, such as the uterine tubes or ductus deferens.
kidney Liver Most of the embryonic kidneys degenerate, with only a very small
Pancreas part forming the adult kidney.

Yolk stalk Growth of the Fetus
Intestine
The embryo becomes a fetus about 8 weeks after fertilization
Urinary bladder (figure 20.13). The beginning of the fetal period is marked by the
beginning of bone ossification. In the embryo, most of the organ
Rectum systems are developing, whereas in the fetus the organs are
present. During the fetal period, the organ systems enlarge and
Figure 20.11  Embryonic Digestive and Urinary Systems mature. The fetus grows from about 3 cm and 2.5 g (0.09 oz)
at 8 weeks to 50 cm and 3300 g (7 lb, 4 oz) at the end of preg-
Outpocketings of the digestive tract (yellow), form many adult structures, nancy. The growth during the fetal period represents more than
such as the lungs and glands. The embryonic kidney is also shown (purple). a 15-fold increase in length and a 1400-fold increase in weight.
The amniotic fluid contains toxic waste products from the
Reproductive right ventricle and the lungs. The foramen ovale normally closes fetus’s digestive tract and kidneys. Fine, soft hair called lanugo
off at the time of birth, and blood then circulates through the right (lă-noo′ gō) covers the fetus, and a waxy coat of loose epithelial
ventricle and the lungs. If this does not occur, an interatrial septal cells called vernix caseosa (ver′ niks kā′ sē-ō′ să) forms a protec-
defect occurs. An interatrial septal defect or a ventricular septal tive layer between the fetus and the amniotic fluid.
defect usually results in a heart murmur. Subcutaneous adipose tissue that accumulates in the fetus
provides a nutrient reserve, helps insulate, and aids the newborn
in sucking by strengthening and supporting the cheeks, so that a
small vacuum can be developed in the oral cavity.

Development, Heredity, and Aging 571

1 20 days after fertilization Fusing heart
At this age, the heart consists of tube
two parallel tubes that will fuse into
a single, midline heart. 1

2 22 days after fertilization Unfused heart
The two parallel tubes have fused to tubes
form one tube. This tube bends as it
elongates (blue arrows suggest the 2 Ventricle
direction of bending) within the
confined space of the pericardium. Atrium
Interatrial septum
3 31 days after fertilization 3
The interatrial septum (green) and the Right atrium Left atrium
interventricular septum grow toward Left ventricle
the center of the heart. Canals between Interventricular
atria and ventricles septum
Right ventricle Interatrial septum
Foramen
4 35 days after fertilization 4
The interventricular septum is nearly Interventricular Interatrial septum
complete. A foramen, which will Left atrium
become the left side of the foramen septum Left ventricle
ovale, opens in the left side of the
interatrial septum (green) as the right
side of the interatrial septum begins to
form (blue).

5 Final embryonic condition 5
of the interatrial septum
A foramen remains in the right side Right atrium
of the interatrial septum (blue), Foramen ovale
which forms the right part of the Right ventricle
foramen ovale. Blood from the right
atrium can flow through the foramen Reproductive
ovale into the left atrium. After birth,
as blood begins to flow in the other
direction, the left side of the
interatrial septum is forced against
the right side, closing the foramen
ovale.

Process Figure 20.12  Formation of the Heart

572 Chapter 20

Reproductive (a) Peak body growth occurs late in gestation, but as the placenta
reaches its maximum size, the oxygen and nutrient supply to the
(b) fetus becomes limited. Growth of the placenta essentially stops at
about 35 weeks, limiting fetal growth.
(c) At approximately 38 weeks of development, the fetus is ready
to be delivered. The average weight at this point is 3250 g (7 lb, 2 oz)
Figure 20.13  Late Embryo and Fetus for a female fetus and 3300 g (7 lb, 4 oz) for a male fetus.

(a) Embryo at 50 days of development. (b) Fetus at 3 months of development. 20.2  Parturition
(c) Fetus at 4 months after fertilization.
Learning Outcome After reading this section, you should be able to

A. Explain the events that occur during parturition.

Physicians usually calculate the gestation (jes-tā′ shŭn) period
(length of pregnancy) as 280 days (40 weeks, or 10 lunar months)
from the LMP to the date of delivery of the fetus. Parturition (par-
toor-ish′ ŭn) is the process by which the baby is born (figure 20.14).
Near the end of pregnancy, the uterus becomes progressively more
excitable and usually exhibits occasional contractions that become
stronger and more frequent until parturition is initiated. The cervix
gradually dilates, and strong uterine contractions help expel the
fetus from the uterus through the vagina.
Labor is the period during which uterine contractions occur
that result in expulsion of the fetus. Although labor may differ
greatly from woman to woman and from one pregnancy to another
for the same woman, it can usually be divided into three stages.

1. The first stage of labor, often called the dilation phase,
begins with the onset of regular uterine contractions and
extends until the cervix dilates to a diameter about the size
of the fetus’s head (10 cm) (figure 20.14). This stage takes
approximately 24 hours, but it may be as short as a few
minutes in some women who have had more than one
child. During this phase, the amnion surrounding the fetus
ruptures, and amniotic fluid flows through the vagina to
the exterior. This event is commonly referred to as the
“water breaking” and usually occurs naturally, but the
amnion may need to be ruptured artificially.

2. The second stage of labor, often called the expulsion phase,
lasts from the time of maximum cervical dilation until the
time the baby exits the vagina. This stage may last from
1 minute to 1 hour or more. During this stage, contraction
of the woman’s abdominal muscles assists the uterine
contractions.

3. The third stage of labor, often called the placental stage,
involves the expulsion of the placenta from the uterus.
Contractions of the uterus cause the placenta to tear away
from the wall of the uterus. Some bleeding from the uterine
wall occurs because of the intimate contact between the
placenta and the uterus. However, bleeding is normally
limited because uterine smooth muscle contractions
compress the blood vessels.

Predict 4

Compare and contrast clinical age and developmental age for
fertilization, implantation, the beginning of the fetal period,
and parturition.