Skin cells reveal they have hairy origins About every 2 weeks, people literally

shed their skin. That’s how long it takes for the outer layers, also known as the epidermis, to turn over. Dead cells on top slough off, exposing younger cells below.

The ultimate source of fresh skin may not be the skin itself. According to one group of biologists, hair follicles are the natural birthplaces of skin cells.

“There may be a very slow and very steady trafficking of cells, in the nor- mal adult, from hair follicles to the epi-

The bulge contains cells that replenish the hair follicle and the epidermis.

dermis,” contends Robert M. Lavker of the University of Pennsylvania School of Medicine in Philadelphia. In the Aug. 18 CELL, a team led by Lavker and his longtime collaborator Tung-Tien Sun of the New York University School of Med- icine offers evidence supporting this hypothesis.

The two investigators specialize in teasing out the origins of tissues. In the past, they identified the eye’s reservoir of unspecialized cells, so-called stem cells, that replenish corneal tissue. A decade ago, Lavker and Sun proposed that a small bulge about halfway up the side of hair follicles harbors stem cells that could give rise to the specialized cells of both the hair follicle and the surrounding epidermis.

That cells from hair follicles can pro- duce skin under extreme circumstances is well known. In people with thirddegree burns, for example, any regenerating skin appears as a growing island with a hair follicle at its center.

In their recent work, Lavker and Sun la- beled cells in mouse hair follicles so they could follow the fate of those cells. When they marked bulge cells in the hair folli- cles of newborn mice, the investigators found that some offspring of these cells migrated downward to form the lower part of the follicles.

In an attempt to see where skin arises in newborn mice, the scientists tagged progeny of bulge cells that had moved in- to the upper portion of follicles and fol-

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lowed their fate. “All of a sudden we found [these labeled] cells appearing in the epidermis. They couldn’t have come from anywhere other than migration up the follicle,” says Lavker.

To discern where new skin comes from during wound repair, the scientists creat- ed a small wound on the backs of mice and injected the animals with the mark- ers that label cells in the upper follicles. Over time, the number of labeled cells in the injured epidermal areas rose, while the tally of such cells in the upper folli- cles fell. This implies that follicular cells play a crucial role in normal skin repair, says Lavker.

It’ll be tricky to prove bulge cells are the key to the skin’s natural ability to turn over in the adults. Epidermal cells proliferate so readily that normally there

may not be a need for a large flow of stem cells up from the bulge to the skin, Lavker notes.

Some researchers point out that sever- al areas of skin, such as the palms, con- tain no hair follicles at all. That leads them to suspect that there’s a population of skin stem cells residing in a lower layer of the epidermis.

“That the hair follicles serve as a source of epidermal cells does not rule out the possibility that there is a resident stem cell in the epidermis as well. Both camps are probably correct,” says Kurt Stenn, director of skin biology at the RW. Johnson Pharmaceutical Research Institute in Skillman, N.J.

Lavker and Sun plan to continue studying hair follicle cells, in part to ex- plain why laboratory-induced skin tu- mors in mice usually seem to originate in the upper follicle but not the bulge itself. -J. Travis

Nanotechnologists get a squirt gun, almost It’s anything but a Super Soaker. According to computer simulations by

physicists at the Georgia Institute of Tech- nology in Atlanta, a fantastically tiny squirt gun that can spit liquids a few hun- dred nanometers ought to work-if it can be built.

Michael F. Moseler and Uzi Landman developed the model of the miniature device to investigate a possible new tech- nology and to confront a challenge facing scientists at the forefront of the much- anticipated nanotechnology revolution (SN: 3/1/97, p. S14). In the nanoworld, dif-

Propane molecules {blue) squeeze through a gold nozzle some 20 molecular diameters wide. Some molecules stick to the nozzle or boil away but most shoot off as a liquid jet.

ferent forces prevail, compared with our everyday, macroscopic domain. Not only do intuitions of scientists and engineers fail at the nanometer scale, so do their equations.

Computer models of molecular behav- ior offer a research alternative. On com- puters, researchers depict thousands to millions of individual molecules pro- grammed to interact according to physics rules. By putting those tiny players through their paces in these so-called mo- leculardynamics simulations, scientists can predict nanometer-scale phenomena that are often counterintuitive.

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Researchers welcome the simulations, but they also need equations, which are simpler and apply more generally, to de- scribe submicroscopic happenings.

Moseler and Landman have done more than just simulate the movements of mol- ecules in nanojets-liquids forced to spurt through nozzles with orifices small- er than viruses. They also created a so- called continuum description of the jets, which ignores the molecular details. They derived this description by adding the effects of random, thermal fluctuations to existing hydrodynamic equations. This modification of equations that were de- veloped to describe macroscopic flows yielded results in line with the molecular simulations. The theorists report their findings in the Aug. 18 SCIENCE.

The newfound agreement of the two a p proaches means “that the powerful mathe- matical tools developed to solve the hy- drodynamic equations can be put to use in the nanoworld,” comments Jens Eggers of the University of &sen, Germany.

On the practical side, Moseler and Landman predict that nanojets may make possible tiny fuel injectors whose smaller, more uniform droplets would burn clean- er in engines. Other minuscule machines might use nanojets to insert genes into

Snapshots of hypothetical jets show thinning before breakup. Contours Fmm moleculardynamics simulation {top) and modified hydrodynamic equation {bottom) closely match. Unmodified equation (middle) yields a much different profile.

AUGUST 19,2000

cells or to lay down wires only a few nanometers wide in electronic circuits. "The major technological challenge will be to actually build nozzles of the re- quired size," Eggers remarks.

The new study shows that nanojets ought to behave differently from macro- scopic ones. For instance, because ther- mal fluctuations add instability to the nanojets, the tiny spurts break into droplets after going only about half as far as large-scale jets would go under corre- sponding conditions, Moseler says. That's why water nanojets are unlikely to be- come the nanoworld's answer to Super Soaker squirt guns.

In another example of peculiar nano- scale effects, the spurting liquid becomes a plug. When the researchers simulated liquid propane gushing from a gold nanonozzle, they found that the fuel tend- ed to stick to the outside of the nozzle, shutting off further flow.

By simulating heating of the nozzle or the presence of a slick exterior coating, the researchers fixed that problem. Still, the unexpected effect highlights the unfa- miliar territory of the nanoworld, Land- man says. There, surface forces, such as the attractive tug between the gold atoms and the propane molecules, become much more important than they are in the macroscopic world. Next to come in the nanojet investiga-

tions are water, molten metals, silicon, and polymers, and liquid biomolecules. No one yet knows how those materials might behave. Landman notes. 4 Weiss

Telescope takes close-ups of distant star New images from an array of radio tel-

escopes are providing the first detailed view of activity near a star other than our sun.

Although the elderly star, TX Camel@ pardalis, lies 1,000 light-years away, re- searchers were able to examine radio- emitting gas flung just a few hundred million kilometers-equivalent to about twice Earth's distance from the sun. As- tronomers obtained the images by using the Very Large Baseline Array, a network of 10 radio telescopes stretching from the US. Virgin lslands to Hawaii.

Silicon monoxide gas spewing from the star generates intense radio emis- sions that the telescope array record- ed. Visible-light telescopes can't detect these gaseous expulsions because ma- terial surrounding the star absorbs and scatters visible light.

At least 3 billion years old, TX Camelo- pardalis is classified as a Mira variable, an old, pulsating star that pe- riodically brightens and dims. When it contracts, the star draws in near- by gas, and when it ex- pands, it blows out an even larger amount. At this age, the star expels

The pictures, recorded over 80 weeks, reveal for the first time how the gas flows out from a star during this late stage, which lasts only about 10,000 years, notes Athol J. Kemball of the Na- tional Radio Astronomy Observatory in Socorro, N.M. Exactly how such stars lose mass remains a mystery, but "this monitoring campaign has allowed a movie of the gas motions to be pro- duced, which provides new insights into how this might occur," he notes. Instead of ejecting the gas equally in all direc- tions, the star expels the material asym- metrically, just as the sun does during localized storms.

Kemball's collaborator, Philip Diamond of the University of Manchester in Eng- land, unveiled the images Aug. 15 at a meeting of the International Astronomi- cal Union in Manchester. -R. Cowen

about an Earth's mass of Bright spots in these radio-wavelength images show gas gas each year. expelled from a dying star. Circle indicates stark surface.

FeatEilered fossil stil l stirs debate Pity poor little &ud@feryx, an animal

that undoubtedly would be suffering an identity crisis if it were still around.

Researchers who first described the 1 2 O . m l l U ~ a l d fossils of the turkey-

ancestors were meat-eatlng dinosaurs @N 6/27/98, p. 404). Even though Caudi- preryx obviously had feathers on its short tail and forelimbs, it lacked many other typlcal features of birds. Other sci- entists vehemently argued that the spedes was neither a dinosaur nor a link between dinosaurs and birds. Instead, they claimed, it was a flightless descerr dent d m o m c i e n t Mrds. More than 2 years later, the debate still

rages about where to place the animal in life's famUy tree. A new analysis present- edIntheAug. 17Nm~~e,however,seems to bolster the view that Caudipfetyv was aMrd haabeen and not a bird wanna-be.

A team of researchers led by Terry D. Jones, now a paleobiologist at Stephen F. A u s t h State University in Nacog- doches, Texas, measured fossils and used previously published data to calcu- late the ratio of leg length to body length for 38 types of bipedal dinosaurs. They

slzlespedesIn1~claimedtheanids

did the same for 24 types of flightless birds, including living species, such as roadrunners and ostriches, and extinct ones, such as moas and elephant birds.

Jones' team found that, as a group, flightless birds had significantly longer legs than bipedal dinosaurs with the same body length. When the resear- chers examined Caudipferyx, they found that its leg proportions matched those of flightless birds and were distinctly different from those of the bipedal dinosaurs.

Other clues also suggest that Caudi- pferyx was a flightless bird, Jones says. Almost all bipedal dinosaurs have a long tail to counterbalance the weight of their upper body, and they typically walk by swinging the entire leg about the hip joint.

On the other hand, Jones says, &ud@feryx has a very short tail-possi- bly shorter than that of any bipedal di- nosaur. As a result, the animal's center of gravity was located forward of the hips, a typical body structure of flight- less birds, he notes. When such birds stride, they typically swing only the por- tion of the leg below the knee while

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UJGUST 19,2000 SCIENCE NEWS, VOL. 158

holding their upper legs motionless. Jones also points out that Caudipferyx's leg bones lack indications that the u p per leg muscles were major sources of locomotion.

Jones contends it's unlikely that Caudipteryx's ancestors would have abandoned a body structure and style of walking that proved successful among all other bipedal dinosaurs. Instead, he says, the most likely explanation for the animal's distinct body proportions Is that Caudipferyx is a bird that for some reason lost the ability to fly. Says Jones: "It's almost ludicrous to think that this animal reorganized its body so that it could run like a bird."

Thomas R Holtz Jr., a vertebrate pale- ontologist at the University of Maryland at College Park, describes the analysis as "interesting but somewhat less than convincing."

"The body proportions, short tall, and other features of the upper leg bones show only that Caudipferyx was a transi- tional animal with a very 'birdy' gait," Holtz says. It will be interesting, he adds, to see whether recently discovered fos- sils of animals related to Gzudipreryx also show the relatively longer legs that Jones assochteswithBightlessbirds.sperkins

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