Laser Scissors and Tweezers - Cellular Biophotonics and ...

Laser Scissors and Tweezers

Researchers are using lasers to grasp single cells and tinier components
in vises of light while delicately altering the held structures.

These lasers offer new ways to investigate and manipulate cells

by Michael W. Berns

T he intense, pure beams of light nucleus, such as chromosomes and the py is actually the result of a change in
known as lasers are now stan- mitotic spindle that segregates chromo- refractive index rather than a complete
dard components of such com- somes during cell division. Lasers also physical removal of material—the laser
monplace objects as compact-disc play- facilitated studies of cytoplasmic con- is changing the chemical and physical
ers and printers. The everyday presence stituents—namely, mitochondria (the properties of the chromosome without
of lasers does not mean, however, that energy factories of cells) and such struc- totally destroying it.
they have been reduced to performing tures as microfilaments, microtubules
only pedestrian tasks. Imagine focusing and centrosomes—involved in maintain- The cell membrane can likewise be
a beam specifically onto an organelle, a ing cellular architecture and transport- studied, via a gentle perturbation of its
structure within a living cell. Consider ing molecules within cells. fluidity. The membrane can even be in-
further that the beam can actually grasp cised, the laser cutting a micron-size hole
that minuscule entity and hold it in place. Putting Lasers to Work that seals within a fraction of a second.
Now imagine that while this “micro- Through this technique, called optopo-
beam” acts as tweezers, a second beam Although we do not always know ex- ration (pore production through optical
serves as scalpel or scissors to conduct actly how lasers produce the spe- means), molecules can be inserted into
delicate surgery on the organelle. cific changes they make in cellular com- a cell when the pores are open without
ponents, we can nonetheless generate permanently damaging the membrane.
Even in a world accustomed to lasers, certain alterations reproducibly and
such musings have the ring of science without compromising the target’s struc- Optoporation may be especially suit-
fiction. Nevertheless, much as medical ture or environment. For example, the able for genetic manipulation of plants,
surgeons guide micromachined twee- traditional biological tools of light and which have rigid cell walls that are rela-
zers and scissors through endoscopes to electron microscopy show that laser tively impenetrable compared with the
perform minimally invasive surgery on scissors can produce a particular change supple membranes of animal cells. At
organs, the cell biologist can now use in a chromosome, deep within a cell. the University of California at Irvine, my
“laser tweezers” and “laser scissors” to Early work by our group demonstrated colleague Hong Liang and I have taken
perform minimally invasive manipula- that scissors can inactivate a selected advantage of optoporation to insert
tions on living cells and their organelles. part of a chromosome in dividing cells— genes into single rice cells; these geneti-
specifically, a region containing genes cally modified cells gave rise to whole
Laser scissors came first. Almost three that control construction of a nuclear plants in which every cell carried and
decades ago Donald E. Rounds and I, organelle known as a nucleolus. What is expressed the introduced genes. This
while at the Pasadena Foundation for more, the alteration persisted in the work, when considered with the inacti-
Medical Research, suggested lasers cloned progeny of those cells, all of vation of nucleolus genes, demonstrates
might be wielded to probe the structure which possessed inactive versions of the that laser scissors can be employed ei-
and function of cells and organelles [see genes in that same region. ther to insert or to delete genes.
“Cell Surgery by Laser,” by Michael W.
Berns and Donald E. Rounds; Scien- The alteration to the chromosome—a In Europe, laser-scissors manipulation
tific American, February 1970]. Our lesion less than a micron in size—ap- of gametes (sperm and egg) has been ap-
early work focused on defining the pa- pears as a lightened region when the live plied recently in the human clinic, as part
rameters of our lasers (such as wave- cell is viewed under a phase-contrast of a procedure called assisted hatching.
lengths of light and durations of expo- light microscope. Careful transmission The scissors thin or remove a small area
sure) and on determining which organ- electron microscopy, capable of 10,000 of the protective zona pellucida of eggs
elles could be successfully manipulated to 100,000 magnification, reveals that that have been fertilized in a laboratory
with light beams that could alter intra- same region to be a cleanly defined struc- dish. The very early embryos are then
cellular regions as small as 0.25 micron tural alteration, with the chromosomal placed in the womb, where the thinning
in diameter. (The diameter of an aver- material on either side of the lesion, as of the zona appears to abet implanta-
age human hair is about 100 microns.) well as the cytoplasm surrounding the tion. Thinning can also be accomplished
chromosome, apparently unaffected. by more conventional techniques, but
During the intervening years, my col- The “lightening” seen in light microsco- the laser method works without toxic
leagues and I found that laser scissors chemicals that can damage the embryo.
could be used to study organelles of the
The most extensive human study to

62 Scientific American April 1998 Copyright 1998 Scientific American, Inc. Laser Scissors and Tweezers

TOMO NARASHIMA

COMBINATIONS of laser scissors and
tweezers can make it possible to perform
subtle subcellular manipulations. In a proce-
dure that should be feasible within a decade,
two tweezers beams (pink) hold a cell firmly in
place. One scissors beam (lighter blue) penetrates
the cell to delete a faulty gene (red). A second scis-
sors beam (dark blue) cuts a hole in the cell mem-
brane through which a competent genetic sequence
(black dots) can pass. Clones of the genetically al-
tered cell could then be produced and transplanted
into the body for therapeutic use.

date, performed by Severino Antinori’s tion of this microplasma generates me- controlled alteration of the target while
group at the Associated Research Insti- chanical stresses that, in turn, can rup- leaving the surroundings unaffected.
tute for Human Reproduction in Rome, ture the cell. (Physicians exploit a simi-
reported a greater than 50 percent in- lar effect on a macroscale to pulverize Achieving precision is relatively
crease in pregnancy rates in more than kidney stones and some cataracts via a straightforward, thanks to the high lev-
200 women whose embryos had under- laser-initiated shock wave; see “Laser el of optical elements in today’s micro-
gone laser zona thinning compared with Surgery,” by Michael W. Berns; Scien- scopes. Objectives, or lenses, are precise-
women who had not undergone the tific American, June 1991.) ly machined and chromatically correct-
procedure. The first multicenter trials of ed throughout the visible spectrum so
laser zona thinning have just gotten un- By positioning a tiny glass capillary that all wavelengths focus at nearly the
der way in the U.S. Similar studies are tube just above the cell, we are able to identical point in space; researchers us-
in progress in Australia and await gov- collect its contents and analyze them, ing multiple laser beams of different
ernment approval in Israel. producing a snapshot of the cell’s bio- wavelengths thus can confidently focus
chemistry at that moment. This tech- those beams to the same point.
In collaboration with Nancy L. All- nique has the potential for major appli-
britton’s group at Irvine, my laboratory cations in single-cell analytical chemistry. With a state-of-the-art, oil-immersion
is using laser scissors in another way as One goal is to determine the exact iden- microscope objective at 100 magnifica-
well—to open a single cell so that its tities and concentrations of proteins im- tion, the physics governing the size of a
chemical components can be analyzed portant in cancer at the single-cell level. laser focal spot result in that spot’s be-
at any given moment. We take advan- ing slightly smaller than the laser wave-
tage of what can sometimes be an un- Expanding Understanding of Scissors length. A neodymium yttrium alumi-
wanted side effect of laser scissors: de- num garnet (Nd:YAG) laser operating
velopment of a minuscule cloud of ion- In all applications of laser scissors, in- at a wavelength of 532 nanometers can
ized gas called a microplasma that forms vestigators need to achieve precision produce a focused spot of 499 nanome-
when laser light is focused on or above and selectivity. Precision refers to the ters, or 0.499 micron. Precision, how-
the glass microscope slide on which the targeting of the laser beam exactly to the ever, can be better even than it first ap-
cell rests. The expansion and contrac- correct point; selectivity pertains to the pears. Our laser of choice produces a
Gaussian distribution of energy—that

Laser Scissors and Tweezers Copyright 1998 Scientific American, Inc. Scientific American April 1998 63

gered by the absorption of single pho-

tons or by the nearly simultaneous ab-

sorption of multiple photons.

Single photon absorption may simply

heat the target; it can also initiate chem-

ical reactions that produce free radicals

or other cell-damaging products. And

photons of high energy (such as in the

ultraviolet) may even break molecular

bonds, thus tearing molecules apart, in

a process called photoablation. Further-

more, multiple photons can be absorbed

MICHAEL W. BERNS so closely in time as to be equivalent to
MICHAEL W. BERNS
a single, high-energy photon. Such mul-

tiphoton absorption can drive chemical

reactions or lead to the kind of molecu-

FEATS ACCOMPLISHED by laser scissors include etching of grooves (left) in the zona lar dissociation seen when single ultra-
pellucida around a fertilized mouse egg (inset) to facilitate implantation. They also in- violet photons are absorbed. Any of
clude removing a V-shaped wedge from a chromosome (right) in a living cell from a these scenarios, or any combination,
kangaroo rat. At its widest, the wedge is about one micron across. may operate in a submicron cell target.

In addition to the absorption proper-

ties of the target, another key factor that

is, the energy forming the focal spot can applications. Therefore, as might be ex- determines the laser’s effect is the irradi-

be characterized by a bell-shaped curve. pected, scientists are working intensive- ance of the impinging light (the energy

Because only the peak of the curve may ly to clarify the complex interactions reaching a target’s surface in a given time

have sufficient energy to alter a particu- between laser and living cell. period, measured in watts per square

lar target organelle, the effective spot Several factors make defining exact la- centimeter). With pulsed lasers that ex-

can be significantly smaller than the di- ser effects difficult. Measuring and re- pose the target to light in periods rang-

ameter of the measured focal spot. cording events in submicron regions of a ing from microseconds down to femto-

Selectivity—controlled alteration— single living cell represents a formidable seconds (10–15 second), the irradiance

can also be achieved as is evident from challenge, as does controlling the ener- can be enormous. Laser effects follow-

applications that have been developed gy of the laser beam within such small ing absorption in tissue exposed for rel-

so far. Nevertheless, investigators do volumes. Despite these obstacles, we atively long periods (hundredths of sec-

not yet know how to guarantee selec- can honestly say we are not completely onds to minutes) are well known and

tivity in many new applications (other in the dark. From macroscopic studies, include heating—which can cause mo-

than by empirically noting what works) we understand that various physical and lecular denaturation, coagulation or va-

because of incomplete understanding of chemical processes can potentially come porization—as well as chemical reactions

the interactions between laser light and into play when lasers interact with or- that can create deleterious photoprod-

biological targets. Such a circumstance ganic tissue. These processes can be trig- ucts, such as free radicals. We are less

is not unusual in science: people

took aspirin for a century be-

fore researchers unraveled how 1015 CREATION OF MECHANICAL STRESS WAVES
it worked at the molecular lev- Allows scissors to rupture cells
el. Still, knowing exactly how a so that their components can be
drug or a technology operates collected and analyzed
IRRADIANCE (WATTS PER SQUARE CENTIMETER )21,000,0001J,/0c0m0

HAMID GHANADAN1 J/cm

cannot help but lead to better 1012

HEATING

PLASMA 2 Allows scissors to
109 GENERATION microdissect small
ENERGY DELIVERED to cells 2 regions of chromosomes
(energy dose), which is measured Allows scissors for genetic analysis
in joules per square centimeter J/cm
( J/cm2), depends on the irradi- to perform opto- TRAPPING
ance of the lasers themselves and 106 poration (creation of Allows tweezers
on how long a cell is exposed to to grasp sperm
temporary pores in

103 cell membranes)

the light. (Diagonal dotted lines 100 BREAKING OF INDUCTION OF
represent constant energy doses.) 10–3 MOLECULAR BONDS PHOTOCHEMISTRY
Some of the effects that can be Allows scissors to thin Enables formation of
achieved in cells at different com- the zona pellucida chemical bonds, such as
binations of irradiance and time around eggs DNA cross-
are highlighted. At least some (to aid fertility) linking
heating (red) is common over a

wide range of energies. Dark red 10–15 10–12 10–9 10– 6 10–3 100 103
is where heat is the dominant

mechanism. EXPOSURE TIME (SECONDS)

64 Scientific American April 1998 Copyright 1998 Scientific American, Inc. Laser Scissors and Tweezers

certain, though, of the effects of the high measure or calibrate makes sense. But A small enough object that is relatively
irradiances in the volumes in which we the idea of light creating a force that transparent to laser light of a particular
work intracellularly—a sphere one mi- can hold and move an object may seem frequency refracts incident laser beams,
cron in diameter has a volume of less as fanciful as a Star Trek tractor beam. bending the light. As a result of this re-
than a femtoliter. Still, light has momentum that can be fraction, momentum is transferred from
imparted to a target. The resultant Lil- the light to the target. When the geome-
A major challenge, therefore, is to de- liputian forces fall far below our senso- try of the arrangement of light beams
termine the boundaries at which in- ry awareness when, for example, the and target is correct, the momentum
creases in irradiance will change the ef- sun’s light shines on, and imperceptibly imparted to the target pulls the target in
fect on the target—for example, discov- pushes against, us. But these forces can the direction of the incident laser beam,
ering the level at which the laser’s effect be large enough to influence biological and the beam can thus hold the target
switches from heating of the target to processes at the subcellular level, where in place [see illustration below]. By mov-
the generation of microplasma-induced the masses of objects are infinitesimal. ing the beam, the laser operator can pull
shock waves. Irradiance effects and the target from place to place.
boundaries are still incompletely defined What Tweezers Can Do
but are certainly affected by the dura- The beams of scissors and tweezers
tion of the laser pulse and by the absor- In the mid-1980s Arthur Ashkin of differ significantly in duration and in-
bance properties of both the target and AT&T Bell Laboratories discovered tensity. Whereas the scissors employ
its physical environment. that a continuous-wave, low-power (un- short pulses of high irradiance, twee-
der one watt) laser beam could “optical- zers make use of continuous, low-irra-
Laser-irradiance boundary studies ly trap” individual bacteria and proto- diance beams. The target must be trans-
represent a dynamic area of research, zoa. He and his co-workers demonstrat- parent to tweezers so that the beam will
with our group especially concentrating ed—first with a blue-green argon ion pass through the object without signifi-
on the submicron realm. The next few laser and then with the infrared Nd:YAG cant energy being absorbed and con-
years should see great progress in the laser (to which cells are more transpar- verted to destructive heat or even gener-
characterization of laser-target interac- ent)—that whole cells and their organ- ating damaging photochemistry.
tions, which will create the opportunity elles could be grasped and moved about.
for ever more delicate applications of A single beam can grasp a cell or or-
lasers in subcellular domains. Steven Chu, a 1997 Nobelist in phys- ganelle, but that object may still be able
ics, and his colleagues at Stanford Uni- to wag; a second beam can lock the tar-
Despite the unknowns, extremely ac- versity later showed that laser tweezers get in place, in a vise of light. Laser-
curate laser ablation, in which we de- could also grip molecules. They conju- tweezers beams typically use light at
stroy or inactivate specific regions of tar- gated transparent polystyrene beads to wavelengths between 0.7 and 1.06 mi-
gets, can be performed in virtually any the ends of coiled, naked DNA and crons and of 25 to 500 milliwatts of
cell component that can be visualized used optical trapping forces to pull the power in a focal spot between 0.5 and
through the light microscope. As eagle- beads and stretch the DNA molecule to 1.0 microns in diameter. Such a beam
eyed baseball great Ted Williams might its full length [see “Laser Trapping of generates forces in the piconewton
have put it, “If I can see it, I can hit it.” Neutral Particles,” by Steven Chu; Sci- range, more than sufficient for trapping
entific American, February 1992]. cells and moving organelles inside and
In the near future we should be able Steven M. Block of Princeton Universi- outside of cells.
to surpass even that visual limitation. ty and Michael P. Sheetz of Duke Uni-
For example, we might employ light- versity have employed optical trapping Laser tweezers make new kinds of ex-
absorbing molecules (LAMs) capable of molecules to study kinesin motors, periments possible. My Irvine colleagues
of finding and binding to a sequence of the proteinaceous cellular structures that Michael Cahalan, Bruce J. Tromberg,
DNA in a gene we wish to manipulate. drive the whiplike action of a flagellum
This sequence could be too small to see, or a sperm tail and move intracellular AB
or its location within a chromosome particles and organelles.
might be unknown. Bathing the entire LASER BEAM
cell in light of the correct wavelength LASER AS TRACTOR BEAM can hold
would energize the LAMs, which in turn an object in its grasp because of basic MICROSCOPE LENS
would inactivate the bound gene. (The physical principles, such as the conserva-
process could also potentially activate tion of momentum. The refraction of any TARGET F HAMID GHANADAN; SOURCE: ARTHUR ASHKIN AT&T Bell Laboratories
that gene or other genes, by transferring pair of symmetric laser light rays within Fb Fa
energy to initiate a cascade of chemical the beam produces forces. A typical set
reactions or by simply inactivating a sup- (rays A and B) produces forces Fa and Fb. BA
pressor gene.) The rest of the cell mere- These result from the response by the tar-
ly would be bathed in light briefly and get to the change in momentum of the
be none the worse after the exposure. light rays. If the focal point lies before the
center of the target, the sum of any pair of
Laser scissors, with their short, intense such forces is a total force (vertical arrow
pulses of light, led the way in our at- F) pulling the target toward the beam. A
tempts to perform microscopic surgery focal point after the center of the target
on cells and molecules. More recently, would push the target. Focal points to the
they have been joined by the equivalent left or right of the center would move the
of the forceps—laser tweezers. target left or right.

To the nonphysicist, the use of light
to trap and move an object is counter-
intuitive. That light can heat or burn,

Laser Scissors and Tweezers Copyright 1998 Scientific American, Inc. Scientific American April 1998 65

XUNBIN WEI Beckman Laser Institute responding to signals received by recep- rises in irradiated regions of cells. They
tors on their front ends. bound a heat-sensitive molecule, Laur-
POLARITY OF T CELLS is borne out in dan, to either artificial cell membranes
studies made possible by laser tweezers. B Laser tweezers can trap even highly (liposomes) or Chinese hamster ovary
cells, which provoke calcium release by T motile cells. As my Irvine co-workers cells in culture and measured changes
cells, were carefully positioned alongside Yona Tadir, Gregory J. Sonek and Wil- in fluorescence intensity and in emission
T cells using tweezers. Positioning of the liam H. Wright (now at SRI Interna- wavelengths.
B cell at one end of a quiescent T cell eli- tional in Menlo Park, Calif.) first dem-
cited no change; a fluorescent red stain in onstrated, tweezers can grasp individu- This cellular “microthermometer” de-
the T cell remained red (top). But when al human sperm cells and manipulate tected a rise of 1.15 to 1.45 degrees Cel-
the B cell touched the other end of the T them at will. We took advantage of be- sius per 100 milliwatts of laser power in
cell, calcium was released, signaled by yel- ing able to study the swimming forces the focal spot. Tweezers may produce
low fluorescence (bottom). of sperm, gradually reducing the trap- 10 times that power; if heat dissipation
ping power to determine the level at is inefficient, deleterious changes in the
Xunbin Wei, Tatiana Krasieva and Paul which the sperm could escape. Investi- rates of biochemical reactions, inactiva-
Negulescu recently employed them to gation of this “relative escape force” has tion of heat-sensitive proteins (includ-
analyze the relation of form to the func- provided a method to examine the rela- ing enzymes) and even cell death be-
tion of the immune system’s T cells. Pre- tion between swimming force, velocity come serious concerns. These Laurdan-
sentation of foreign molecules, or an- and swimming pattern. One exciting dependent measurements represent an
tigens, by the immune system’s B cells discovery—sperm that swim in a zigzag important contribution to the growing
starts a cascade of reactions that in- pattern swim with greater force than database of laser-target interactions.
cludes an increase in calcium ion con- straight swimmers—may explain clinical
centration in the T cells. This rise in observations suggesting that men with Putting It All Together
turn leads to the T cell specialization a higher proportion of tacking sperm
and proliferation so vital in immunity. are more fertile than their counterparts Laser scissors and tweezers are each
who produce straight swimmers. powerful tools in their own right,
T cells have a polarized appearance, but techniques making use of both, ei-
defined by shape and the direction in Another finding from laser-tweezers ther sequentially or in combination, al-
which they crawl. Titanium sapphire la- studies shows that sperm surgically re- low for even more sophisticated and
ser tweezers trapped B cells and placed moved directly from the epididymus creative manipulation and alteration of
them at different locations along the T (where they mature and are stored pri- cells. The first application of both laser
cell surface. With the B cell placed at or to ejaculation) of men incapable of scissors and tweezers was conducted by
the back end of the T cell, no response ejaculation swim with only one third the my former postdoctoral fellow Romy
occurred, and the B cell detached from force of normally functioning sperm. Wiegand-Steubing. She used an infrared
the T cell within two minutes. But place- (Full swimming force appears to require Nd:YAG laser tweezers to move togeth-
ment of the B cell at the leading edge of the complete maturation that occurs er two human myeloma cells and a
the same T cell quickly elicited a rise in during passage through the epididymus.) pulsed ultraviolet nitrogen laser scissors
intracellular calcium ion concentration, This result helps to explain why fertiliza- to cut the adjoining cell membranes; the
meaning that the response cascade was tion is optimized when these patients’ two cells merged into a single hybrid cell
in progress [see illustration above]. This sperm are physically injected into eggs, containing the genomes of both. Fused
finding fits nicely with the notion that T as opposed to simply having them at- cells can combine valuable attributes
cells and other white blood cells mi- tempt to fertilize eggs in a petri dish, within one entity, such as one cell’s abil-
grate in specific directions, in part by where success may be a consequence of ity to produce a useful product with an-
swimming force. Evaluation of relative other’s ability to divide indefinitely.
escape force should therefore be a valu-
able screening tool to determine if low Furthering the sperm-trapping work,
fertility is caused by poor motility and a European group led by Karin Schütze
could also test treatments aimed at im- of the Harlaching Center for Laser Ap-
provement of motility. Thus, laser twee- plications in Munich has used infrared
zers should find a role both in clinical tweezers to grasp and guide individual
management of infertility and in sperm bovine sperm into eggs by way of a hole
motility research. in the zona that was produced with
scissors. This technique resulted in fer-
Application of tweezers in those roles tilization in a small percentage of cases,
must be achieved delicately, because ex- as did a comparable effort with mouse
posure to tweezers may hamper motility, sperm and eggs.
probably because of unavoidable ther-
mal or photochemical effects. Heat is A similar approach, if shown to be
actually more of an issue with tweezers safe and effective, could provide an al-
than with scissors because of the con- ternative to existing gamete-manipula-
tinuous beam. tion procedures in humans and almost
certainly will find a role in animal hus-
Measurements of tweezers-generated bandry. Although caution—as well as
heat have been conducted by Tromberg, strict adherence to federal governmen-
Sonek and Yagang Liu (now at Beck- tal policies—must clearly guide new and
man Instruments in Fullerton, Calif.), experimental procedures, researchers in
who were able to record temperature

66 Scientific American April 1998 Copyright 1998 Scientific American, Inc. Laser Scissors and Tweezers

MICROBEAM SYSTEM for laser scis-
sors incorporates a pulsed laser and a
phase contrast microscope. The pulses,
lasting 10 nanoseconds, are visible as the
discrete light bursts along the line of the
laser path. The system shown is the im-
mediate predecessor to the confocal abla-
tion trapping system, which has enclosed
beams that are not visible.

MICHAEL W. BERNS or organelles through the confocal mi-

croscope during and after the period

when tweezers and scissors are in oper-

ation. This joystick-controlled confocal

ablation trapping system (CATS) is

assisted reproduction have shown con- duced chromosomal fragments, from available for a wide range of cellular
siderable interest in laser scissors and entering the spindle. Because mitosis car-
tweezers; two companies have already ries a cell’s genetic material to the next and subcellular studies, including ones
begun to market multipurpose laser generation, it makes sense that evolu-
workstations for eventual use in fertility tion developed a system that apparently that improve procedures necessary for
clinics. Micromanipulations of sperm blocks undesirable material from the
and eggs using laser systems should be spindle area. DNA sequencing—deciphering of the
faster and more efficient than tradition-
al techniques. These tools are being more fully ex- order of base pairs, or codes, carried by
ploited by Rieder and his colleagues,
The combination of laser scissors and who have since been able to demonstrate a stretch of DNA.
tweezers makes possible unprecedented that laser tweezers can inhibit the move-
manipulations of organelles as well as ment of laser-scissored chromosome My Irvine colleagues Barbara Ham-
whole cells. In collaboration with Ed- fragments already within the spindle.
ward D. Salmon of the University of These techniques finally make possible kalo and Al Jasinskas are using this tech-
North Carolina at Chapel Hill and Con- noninvasive studies of forces within the
ly L. Rieder of the Wadsworth Center mitotic spindles of single cells. Because nology to cut out the chromosome’s cen-
for Laboratories and Research in Alba- the spindle is a key player in cell divi-
ny, N.Y., we initiated studies at the or- sion, unraveling its workings can lead tromere, where the spindle microtubules
ganelle level that used laser scissors to to more detailed understanding of dis-
cut chromosomes, which were in the eases related to cell division, such as attach. Investigators disagree over
midst of mitosis (cell division). Then, cancer and birth defects.
with laser tweezers, we moved the frag- whether this region is likely to be genet-
ments within the cell, our goal being As part of the National Institutes of
the study of forces exerted by the mitot- Health Laser Microbeam and Medical ically active, but the centromere has
ic spindle, the cellular machinery that Program Biotechnology Resource, we
pulls replicated chromosomes to oppo- have now built a microscope worksta- been extremely difficult to isolate and
site ends of a cell as it divides. tion that incorporates two laser-twee-
zers beams and one laser-scissors beam analyze for unique active gene sequenc-
Unexpectedly, we were able to freely into a confocal laser fluorescence micro-
move fragments that were outside the scope. The lasers are tunable—that is, es. Laser scissors and tweezers should
spindle but were unable to move frag- they can be set to any desired wave-
ments from the cytoplasm into the spin- length. Having all these capabilities in a help settle this controversy.
dle. These studies confirmed the exis- single, all-purpose workstation meets
tence of a spindle “cage” that blocks multiple needs of the cell biologist. A More than 80 years have passed since
foreign material, such as the newly pro- researcher can observe fluorescing cells
Albert Einstein laid the theoretical foun-

dation for the existence of lasers. By the

early 1960s, thanks to the work of

American and Russian physicists, laser

light became a reality. Now optical tools

incorporating lasers enable biologists to

become cell surgeons, probing and ma-

nipulating cells and organelles. The ap-

plication of these technologies has far-

reaching implications for medicine, de-

velopmental biology, the study of cell

structure and function, and the unravel-

ing and manipulation of the human ge-

nome. The future of laser-based biolog-

ical research and applications should be

very bright indeed. SA

The Author Further Reading

MICHAEL W. BERNS is the Arnold and Mabel Laser Microbeam as a Tool in Cell Biology. M. W. Berns, W. H. Wright and R.
Beckman Professor at the University of California, Wiegand-Steubing in International Review of Cytology, Vol. 129, pages 1–44; 1991.
Irvine. He is also president and co-founder (with Fertilization of Bovine Oocytes Induced Solely with Combined Laser Mi-
his mentor, industrialist Arnold O. Beckman) of crobeam and Optical Tweezers. A. Clement-Sengewald et al. in Journal of As-
the university’s Beckman Laser Institute and Med- sisted Reproduction and Genetics, Vol. 13, No. 3, pages 260–265; March 1996.
ical Clinic. His research interests include genetics, A Synergy of Technologies: Combining Laser Microsurgery with Green
cell motility, laser-tissue interactions and clinical Fluorescent Protein Tagging. A. Khodjakov, R. W. Cole and C. L. Rieder in Cell
applications of lasers in cancer, ophthalmology Motility and the Cytoskeleton, Vol. 38, No. 4, pages 311–317; 1997.
and reproductive medicine. He dedicates this arti- Laser Tweezers in Cell Biology. Edited by Michael P. Sheetz. Methods in Cell Bi-
cle to Beckman, who is 98 years old this month. ology series, Vol. 55. Academic Press, 1998.

Laser Scissors and Tweezers Copyright 1998 Scientific American, Inc. Scientific American April 1998 67