Caltech Geophysicist Offers Evidence For New View of Earth's Inner Workings

SAN FRANCISCO—In two closely related presentations today at the annual American Geophysical Union conference, Caltech geophysicist Don Anderson will describe work suggesting a radical new interpretation of how Earth operates inside. The work is based on recently declassified satellite imagery as well as a revisiting of the issue of primordial helium (the 3He isotope) within Earth.

"It's becoming more and more clear that Earth is driven from above by motion of the lithosphere (the cold outer shell of Earth) and cold 'fingers' sticking down under continents into the mantle," Anderson said in an interview prior to the conference. "So rather than Earth being like a pot on a stove that gets heated from below and boils, it's more like a glass of iced tea where ice cubes cause cold downwellings in the liquid beneath it from thermal convection, and cause cracking in the 'lid' that permits volcanism."

Anderson's presentation of two lectures on the topic is appropriate because he has been working at the problem for the last eight months from two directions. The first, the satellite imagery evidence, is based on highly accurate global satellite gravity data compiled by David Sandwell of the Scripps Institution of Oceanography and Walter Smith of the National Oceanic and Atmospheric Administration.

These maps show that many hot spot tracks (chains of volcanoes) exist along preexisting cracks on the plate. Others exist where new cracks are forming as the oceanic plate bends to go under other plates—for example, at the Chile Trench and near Samoa.

According to Anderson's analysis of the maps, the evidence is compelling that there are five regions in the Pacific Ocean where hot spots of underwater volcanic activity can be associated with new fractures in Earth's crust. Two of the hot spots are located a few hundred miles to the west of Chile, the third is near Samoa, the fourth is on the Easter Microplate near Easter Island, and the fifth is near the Galapagos Islands. Many other "hot spot tracks" are along ancient fractures.

All of the hot spots were previously thought to be random outcroppings, Anderson says. In the old interpretation, these hot spots are caused by the molten mantle burning through the cooler mantle and the crust from a boundary near the core.

However, the nature of the data supplied by the satellite imagery allows the fabric of the crust at those points to be inferred. The structure of the seafloor suggests that these hot spots show fracturing of the crust. The conclusion, then, is that bending of the crust and locations of previous crustal boundaries (faults) are creating weak spots in the lithosphere, which in turn make the hot spots inevitable because hot mantle can penetrate and break through the plate. The fault lines are pressure-release valves.

Another helpful analogy Anderson offers is the polar oceans. In the Arctic and Antarctic, the oceans are not driven by heating from below, but by winds blowing across the surface and icebergs cooling parts of the surface. Earth's mantle, like the Arctic ocean, is driven from above, not below. Even though the ultimate source of energy is from primordial heat and radioactivity, the continents and tectonic plates form a surface template that controls the shape of convection and the locations of hot upwellings.

Anderson's other line of reasoning is based on a reinterpretation of the ratio of primordial helium, or 3He, in Earth's mantle to 4He, which is created by the decay of uranium and thorium. Primordial helium, having two protons and one neutron, was created in the early stages of the universe and is conventionally thought to have sat more or less unperturbed within Earth for billions of years. The 4He isotope, by contrast, has two protons and two neutrons, and has been created far more recently within Earth by the radioactive decay of uranium and thorium.

The previous geophysical interpretation was that a high ratio of primordial helium to 4He in basalt was evidence that the lava is an upgushing from the molten magma thousands of kilometers below the surface. Thus, the primordial helium ratio was thought to support the view that a volcanic eruption was solely the result of an upsurge of the lava from the primordial mantle, because the primordial store of helium could be envisioned as existing only at great depths.

It has always been a mystery, Anderson explains, how any part of Earth could have survived the violent impacts during accretion, or planet-building, without melting or vaporizing. That is, it's difficult to see how any part of Earth could have remained "primordial."

However, Anderson's new interpretation suggests that the ratio may not presuppose a relatively high amount of primordial helium, but rather a relatively low amount of 4He. This is in keeping with evidence that Earth's mantle just below the crust, the lithosphere, is low in uranium and thorium. Cracking of the lithospheric plate allows access to helium-rich "bubbles."

The new interpretation suggests that geological processes cause tectonic plates to stretch and break, and that volcanic lava from an eruption utilizes these weak zones rather than being the result of deep narrow hoselike upwellings of magma. The strange chemistry of hot-spot lavas, such as at Hawaii and Iceland, is the result of near-surface contamination (crustal, lithosphere, and ocean), rather than a property of the deepest mantle.

The heat of the magma is involved in the convection processes, Anderson notes, but the old idea of a chute of molten material blowing all the way from the deepest mantle to the upper surface can no longer be supported.

"Volcanoes with high ratios of helium isotopes can be interpreted as new cracks in the lithosphere," Anderson concludes, noting that the helium evidence further supports the "top-down" evidence from the new satellite imagery. "Volcanoes are more like grass growing through cracks in the sidewalk rather than tree roots which break the pavement.

"This turns everything upside-down."

Writer: 
Robert Tindol
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Question of the Week: Why Didn't the Ice Found at the South Pole of the Moon Sublimate Away, Like Ice Cubes Do in a Refridgerator?

Submitted by Dean Bessette, Huntington Beach, Calif., and answered by Dave Stevenson, George Van Osdol Professor of Planetary Science, California Institute of Technology

As everyone with a refrigerator knows, ice cubes tend to shrink over time. And if you have an old-style refrigerator, you may have observed that the ice molecules go directly from the ice cubes to the walls of the freezer compartment without ever becoming liquid water. This is the process of sublimation, and your question about its relevance to the water found on the moon is a good one.

The answer is that it takes more energy than is available in a crater at the south pole of the moon to make the molecules fly off the ice in significant numbers. The refrigerator analogy is appropriate to a point, but there is far more energy in your typical freezing compartment than in a shadowy area on the moon where the sun never shines. And since there is not much internal heat coming from within the moon, there simply isn't enough energy available to sublimate this very cold ice.

It has been estimated that if you have water ice sitting in a vacuum, you need to keep the temperature at about 150 degrees Kelvin—or minus 123 degrees Celsius—for sublimation to be significant over geological time. And since the temperature in a shadowy crater of the moon could be much lower than this, there simply hasn't been enough time for the ice to sublimate, assuming there was a reasonable amount of ice to begin with. So the ice the space probe Clementine detected could have been there for billions of years. How's that for a refrigerator?

Neuroscientists Single Out Brain Enzyme Essential To Memory and Learning

PASADENA— Researchers have singled out a brain enzyme that seems to be essential in memory retention and learning.

The enzyme is endothelial nitric oxide synthase (eNOS), and is found in microscopic quantities near the synapses, or nerve junctions. In today's issue of Science, California Institute of Technology neuroscientist Erin Schuman, her colleague Norman Davidson, and their six coauthors write that the gas nitric oxide (NO) produced by eNOS has been demonstrated in rat brains to be crucial for "long-term potentiation," which is the enhancement of communication between neurons that may make memory and learning possible.

"This study shows how memory may be stored by changing the way neurons talk to one another," says Schuman, who has worked for years on the role of chemical messengers in learning and memory.

In short, the chemical signals interchanged between neurons during memory formation somehow make future signal transmissions occur more readily. Whatever the precise chemical nature of the exchange, Schuman says that there is a feedback mechanism at the basis of long-term potentiation—a "retrograde messenger" likely to be NO—and that this messenger is what makes learning and long-term memory possible.

Scientists have known for some time that the gas nitric oxide is important in certain physiological processes, says Schuman. Further, her own work in the last couple of years has shown that long-term potentiation can occur even when neurons are not directly connected to one another, presumably because NO is a gas that can diffuse between neurons. Evidence has pointed to nitric oxide as a component in this mechanism despite the fact that rats with a defective gene for manufacturing a closely related form of nitric oxide synthase known as nNOS have no problems with long-term potentiation.

The new study shows that eNOS, however, is crucial in the mediation of signals between neurons. The authors demonstrated this by manipulating a common virus in such a way that it performed like a "Trojan horse." The region of the virus responsible for illness was eliminated, and the gene inserted into the virus was chosen for its action on brain chemistry. The virus infected the neurons and forced the cells to manufacture the protein encoded for by the inserted gene.

One viral construct blocked the function of eNOS in the hippocampus of the rodents, while another restored the eNOS function. The end results showed that eNOS is crucial for long-term potentiation.

Schuman says that while there is no immediate application for the finding, the greater molecular understanding of how brain cells change their properties is an important basic result in itself. Too, the use of viral vectors in understanding brain chemistry is a new approach, and somewhere down the line might be considered as a strategy for gene therapy.

"This gives us a good idea of a model for how brain cells change during learning," Schuman says.

Also involved in the work are Caltech neuroscientists David B. Kantor, Markus Lanzrein, Gisela M. Sandoval, W. Bryan Smith, S. Jennifer Stary, and Brian M. Sullivan.

Writer: 
Robert Tindol
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Question of the Week: How Do We Know That a Rock Found In The Ice In Antarctica Came From Mars?

Question of the Month Submitted by: Audra Martin, La Puente, Calif., and answered by: Bill Bottke, Postdoctoral Fellow, Division of Geological and Planetary Sciences, Caltech.

Meteorites are rocks that fall to Earth from space. Most are thought to be asteroid fragments that have survived fiery entry through Earth's atmosphere. Twelve of the thousand or so meteorites held in worldwide collections, however, are thought to come from Mars. Eleven of these meteorites, formed roughly 1.3 billion years ago, were named the SNC meteorites after the sites where they were found: Shergotty (India), Nakhla (Egypt), and Chassigny (France).

The 12th Martian meteorite is much older and different from the rest. It is called ALH 84001, named for the year it was discovered, 1984, and the location where it was found, Allen Hills, Antarctica. It was formed 4.5 billion years ago, when Mars had a much thicker atmosphere and liquid water on its surface. Recently, scientists have suggested that ALH 84001 might even contain fossil evidence of ancient Martian life.

How do we know that these meteorites are from Mars, when people have never been there and no rocks have been collected on its surface? In 1976, two NASA spacecraft named Viking 1 and 2 landed on Mars and analyzed its atmosphere and surface. These spacecraft examined soil and air samples using onboard instruments, making careful measurements and radioing their data back to scientists on Earth. After careful study, it was determined that the atmosphere of Mars was very different from Earth's atmosphere or any other combination of gases found in the solar system. Then, by analyzing small traces of gas trapped in the interior of these 12 meteorites, scientists were able to identify the characteristic "fingerprint" of the Martian atmosphere, proof that these rocks were blasted off the surface of Mars at some time in the past.

Even before gas was discovered in these meteorites, scientists were suspicious that they might have originated on Mars. The SNC meteorites have young formation ages, and all 12 Martian meteorites have complex chemical compositions that set them apart from other known meteorite classes. Moreover, the abundance of oxygen isotopes (different kinds of oxygen) in the meteorites are inconsistent with oxygen isotopes found in Earth rocks.

Question of the Month: We Hear of Humans Going to Distant Planets in the Future. But if Some Planets Are Light-years Away, How Could An Astronaut Live Long Enough To Get There?

Question of the Month Submitted by: Richard Scott, Whittier, Calif., and answered by: Teviet Creighton, Caltech doctoral student in theoretical physics.

At first, it might seem unlikely. After all, even the nearest star (after the Sun) is over four light-years away — that is, it would take more than four years for light, traveling at 300,000 kilometers per second, to make the trip. Most points of interest are much farther; the center of our galaxy, for instance, is roughly 30,000 light-years away. And as you may know, Einstein's theory of relativity tells us that nothing can accelerate past the speed of light. How could anyone hope to live through a flight lasting over 30,000 years?

Fortunately, the same theory which imposes this universal speed limit also provides a sneaky way of getting around it, at least for the people making the trip. In the theory of relativity, the time measured by a person moving very quickly (near the speed of light) is not the same as the time measured by someone standing still. In particular, if someone (call her Alice) accelerates to very near the speed of light and then decelerates to a stop, the time that she measures for the trip will be less than the time measured by her brother Bert, who was standing still. And the closer she comes to the speed of light (without, of course, ever going faster than light), the bigger the difference in times. This effect is called "time dilation."

Now let us imagine Alice taking a trip to a distant star. Her starship is designed to accelerate at "one gee," which means that the acceleration of the ship pushes her as hard as the force of gravity would on Earth. The farther she goes, the more speed she builds up along the way, and the more time she saves because of time dilation. In this way even the most distant galaxies can be reached within her lifetime! Here are her travel times to a few interesting destinations:

Nearest star (4.3 light-years away) ................... 3.6 years Center of our galaxy (30,000 light-years away) ......... 21 years Andromeda galaxy (2 million light-years away) .......... 29 years Edge of known universe (15 billion light-years away) ... 47 years

If she returns to Earth, the return trip will take an equal amount of time. However, people on Earth will have continued to age at the normal, undilated rate. So while Alice might be able to travel to Andromeda and back in only 60 years (as she sees it), the Earth and its people will have aged 4 million years!

Neural Research Shows That the Nose Needs Time To Smell

PASADENA— New research from the California Institute of Technology shows that it literally takes some time to smell the roses.

In the current issue of Nature, Caltech neuroscientists Michael Wehr and Gilles Laurent present work demonstrating that information about odors is contained in the temporal activity patterns of groups of neurons over an interval of time.

"Perfumers sometimes speak of 'top notes' and 'medium notes' in a bouquet," says Laurent, associate professor of biology and computation and neural systems. "These refer to early and late perceptions that unfold over time during long sniffs or successive sniffs. Our new research suggests that the brain is actually representing odors by making a neural melody of its own."

A helpful analogy Laurent offers for the research is the musical notes that make up a tune. A listener can perceive one note in an instant, but must listen for a time before he or she can recognize the tune. Therefore, the specific manner in which the notes follow one another is the very thing that gives a song its individual character.

"It is the order in which specific neurons are activated that appears to contain useful information about the identity of the odor," says Laurent, adding that different odors cause different neural "melodies."

Laurent and Wehr, a graduate student in computation and neural systems, did their research by analyzing the brain waves of locusts. When an odor was wafted by the olfactory organ of the locust, the collective response of neurons in the olfactory brain was such that specificity in the responses arose from considerations of their temporal characteristics. And because olfactory systems are very similar among most animals, the researchers think that these coding principles may be common to most, including humans.

What happens in the brain during the act of smelling is not well understood, but has been known for a long time to involve neural synchronization and oscillations of the EEG. The function of oscillations, which are observed also in all other sensory areas of the brain, remains totally speculative.

"If our hypothesis is right, oscillations are a kind of clock for the temporal codes we observe," Laurent says. In a parallel study by Laurent and Caltech behavioral biology graduate student Katrina MacLeod in the November 8 issue of Science, the authors described a method by which the neurons representing odors can be simply desynchronized, thereby eliminating the clock signal for the temporal codes. This result will now allow the researchers to directly test, in future experiments, whether these temporal codes are essential for odor perception.

The conclusions of the researchers is that animals as primitive as snails and as complex as humans do some mental "data crunching" each time they pick up a smell. This allows the neurons to separate a certain odor from the background, provided a window of time is available.

In a manner of speaking, the research shows that time is of the essence, and vice versa.

Writer: 
Robert Tindol
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Caltech Scientists Offer Theory of Ganymede's Oxygen and Ozone

Tucson, Arizona — When Galileo discovered Ganymede four centuries ago, little did he suspect that the third satellite from Jupiter might be glazed over with the very substance he was breathing.

It took modern astronomical instruments and chemical knowledge for scientists to detect the oxygen and ozone that coat Ganymede. Now, two planetary scientists affiliated with the California Institute of Technology have developed a theory to account for the presence of the substances, as well as the mechanism by which their concentrations are maintained.

Dr. Yuk Yung and Dr. Ming-Taun Leu, in a presentation at the annual Division for Planetary Science (DPS) meeting of the American Astronomical Society, today provide an explanation for the amount of oxygen and ozone detected by ground-based telescopes and the Hubble Space Telescope. According to Yung, the oxygen and ozone are remnants of the primordial water that became part of Ganymede when the solar system was formed 4.5 billion years ago.

Yung, professor of planetary science at Caltech, theorizes that the water ice on Ganymede has since time immemorial been attacked by two sources: ultraviolet light from the sun, and ions thrown off by the volcanic activity of the sister Jovian satellite Io. Both sources of disturbance have the effect of blasting the water molecules apart. Once the hydrogen and oxygen of the water are separated, the lighter, energetic hydrogen ions blast away from the light gravity of Ganymede into outer space, while the heavier oxygen molecules settle back onto the surface.

Yung also offers a theory to account for the relatively high concentration of molecular oxygen and ozone on Ganymede's surface. While each is present at about one-thousandth its concentration on Earth, both are far and away more prevalent than on any other body in the solar system except Mars, which by coincidence has roughly the same concentrations.

"So far, Earth, Mars, and Ganymede are the only bodies in the solar system with ozone," said Yung in an interview prior to the DPS conference.

Yung's theory of oxygen and ozone concentrations on Ganymede assumes the presence of a vast structure of tiny surface cracks in the ice where the frozen oxygen and ozone can reside. To test the hypothesis that such cracks indeed exist, Yung's colleague, Ming-Taun Leu of Caltech's Jet Propulsion Laboratory, devised an experiment in which the conditions of Ganymede could be simulated.

The results showed that such cracks could indeed occur in the ice matrix, and that even a very thin surface coating of ice could harbor the high concentrations of oxygen and ozone seen on Ganymede.

As for the destruction of the water molecules, Yung explains that the manner in which the molecules are blown apart can account for the ratio of oxygen to ozone, as well as the net amount of oxygen atoms on the surface. Light energy from the sun, for example, tends to split a molecule of oxygen (O2) into two oxygen atoms, and a molecule of ozone (O3) into a molecule of oxygen and a single oxygen atom.

An oxygen ion that finds its way from Io's volcanoes to Ganymede, however, tends to turn a molecule of oxygen it hits into a molecule of ozone. Or, if it hits an existing molecule of ozone, the particle from Io tends to combine with the three atoms to make two molecules of O2. By this scenario, the energy expended and consumed in these reactions accounts for the amount of oxygen as compared to the amount of ozone.

Yung says that his work is very basic in nature and has little to do with the present amount of oxygen on Earth, because our own planet is dependent on living processes for most of its oxygen. Nonetheless, he says that the study of Ganymede can perhaps lead to answers about how oxygen might have arisen on Earth and Mars before life began.

 

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Robert Tindol
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Caltech Seismo Lab Gets Location Data on October 3 Meteor

PASADENA— Caltech seismologist Kate Hutton has some data that shows where the October 3 meteor may have landed. She's providing the information publicly to help anyone and everyone who wants to try for the $5,000 reward UCLA is offering.

According to Hutton, any larger chunks from the meteor that lit up the Western skies on the night of October 3 may have landed in the Rose Valley area near Little Lake. Hutton figured this out by analyzing data from 31 of the seismic stations belonging to the Southern California Seismographic Network (operated by Caltech and the U.S. Geological Survey).

"As it fell, the atmospheric drag caused the meteoroid to explode in midair at least twice," Hutton says. "The explosions generated sound waves in the air similar to a sonic boom, which were detected by the seismographs.

"Using a procedure that is very similar to the one used to locate earthquakes underground, I used the arrival times of the sound waves at the various seismic stations to estimate where the explosions occurred."

Two of the explosions were well located, Hutton adds. Both were 20 to 30 miles above the Fivemile Canyon area in the eastern Sierra foothills. The explosions were separated by about 25 seconds, and the second was about five miles lower and about a mile further eastward than the first one.

Based on this data and on eyewitness accounts provided by John Wasson of UCLA and Caltech alumnus Mark Boslough of Sandia National Laboratory in New Mexico, Hutton thinks that any larger fragments that survived the fiery entry into Earth's atmosphere would have landed to the east-northeast of the explosions, perhaps in the Rose Valley area near Little Lake. Smaller fragments may have fallen more or less straight down from where the explosions occurred.

The Little Lake area would probably be the more seductive area to search, and for a very good reason. UCLA has offered a $5,000 reward for the first fragment that weighs at least four ounces.

Hutton says the seismographic instruments didn't pick up a meteorite impact on Earth, but this is not surprising, since a single fragment would probably have to weigh several tons in order for its impact to be detected.

The term "meteorite," by the way, refers to chunks of extraterrestrial debris that survive the entry into the atmosphere and end up on the ground. "Meteoroids" are chunks that travel through space, while "meteor" is the proper designation for the light show produced by a rock from outer space slowing down in Earth's atmosphere.

Any surviving meteorite fragments would probably have a fresh black matte crust. If the meteorite struck something on the ground, part of the crust might have chipped off to reveal a lighter interior. If anyone finds a meteorite fragment weighing at least four ounces, he or she should get in touch with Dr. John Wasson at UCLA. Wasson's e-mail address is wasson@igpp.ucla.edu.

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Robert Tindol
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Thundercloud Photos, Theory Suggest That Jupiter Is "Wet" After All

PASADENA— The Galileo probe that dropped into Jupiter's atmosphere last December detected a surprisingly small amount of water. But scientists at the California Institute of Technology have new thundercloud photographs and a theory to suggest that the solar system's largest planet may be "wet" after all.

According to Andrew Ingersoll, a professor of planetary science at Caltech, the probe didn't find much water because it dropped into a cloud-free area of the Jovian atmosphere. Because water would likely be manifested in clouds, Ingersoll thinks the probe might have detected the predicted amount of water had it fallen into another region. As it stands, the probe data suggest that water is about 10 times more scarce than most scientists once thought.

"This was the most cloud-free area on the planet," Ingersoll says, "so the probe went into an anomalous region. A low cloud abundance means that there's less of the condensable substances, including water."

At the 28th annual meeting of the Division for Planetary Sciences of the American Astronomical Society, Ingersoll and colleagues are offering photographic evidence and a theory to back up their assertion that water on Jupiter was merely hidden from the probe. The photographic evidence is a series of near-infrared images taken by the Galileo orbiter of the Great Red Spot. The images were released as part of a report just published in Science by the Galileo imaging team under the leadership of Dr. Michael Belton of Kitt Peak National Observatory near Tucson, Arizona.

The infrared images show several dozen thunderclouds to the northeast of the Great Red Spot. These round clouds jut about 20 miles above the cloud cover, and are of similar diameter.

The researchers are not sure yet whether the clouds are made of water. But they do know that they are high and that they evolve rapidly, like terrestrial thunderstorms. Ingersoll and Caltech graduate student Ashwin Vasavada documented how the features changed over a 70-minute interval.

"Water is the most likely cause of these explosive convective events," says Ingersoll. "Other gases present in Jupiter's atmosphere just don't have the energy."

The new theory, which Ingersoll developed jointly with graduate student Adam Showman, argues that the cloudless region the probe dropped into is part of a huge downdraft that extends deep into Jupiter's fluid interior.

Large-scale downdrafts, like those over Earth's deserts, are dry because the moisture falls out in the neighboring updraft regions, Ingersoll explains. "The trick here is to keep the downdraft going to the level where the Galileo probe failed, which is 40 miles below the cloud base. You have to keep the dry descending air from mixing with the moist air from Jupiter's interior.

"That can happen if the descending air is less dense (lighter) than the surroundings, but that violates another rule that says less dense air wants to rise.

"We are saying that the region where the probe went in is a heat engine running backward, like a refrigerator. It takes energy from the convection at neighboring latitudes, and uses it to stuff the less dense air downward. The energy released in the neighboring latitudes is not dissipated, so it has to go somewhere."

Thus, the researchers believe that the Galileo probe could have missed the water, and that Jupiter can still be viewed as a planet of violent thunderstorms, lightning, hurricanes, and rain.

"But if they get a good sample of different places on the planet and still fail to find water," Ingersoll says, "then I'll throw in the towel."

Writer: 
Robert Tindol
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Caltech Biologists Identify Gene Thought to Initiate Neural Development

PASADENA— Biologists have identified a gene that determines whether a given cell in a human or animal embryo will become a neuron rather than some other kind of cell.

In an article appearing in today's journal Cell , California Institute of Technology Professor of Biology David Anderson and his colleagues announce that the gene encodes neurogenin, a member of the basic-helix-loop-helix (bHLH) family of proteins, which in turn control the activity of other genes. When neurogenin RNA appears in cells of the early embryo, the research shows, a genetic chain-reaction begins that turns the cell into a neuron.

According to Anderson, the discovery provides an important piece of information about how embryonic cells develop into cells with specific functions and locales within an organism. Up to a certain stage, all cells in an early embryo look alike in a microscope. But forces are at work that determine whether a specific cell will be a neuron, a muscle cell, a germ cell for sexual reproduction, or any of the other types of cell that make up an organism.

"It's been clear for decades that cells are different in an invisible way long before we are able to see them as being visibly different," says Anderson. "The idea has been that there must be specific genes that confer this invisible predisposition on particular cells."

To demonstrate that neurogenin indeed fulfills such a function, Anderson's coauthor Chris Kintner of the Salk Institute injected tiny amounts of neurogenin RNA in the embryo of a toad. Kintner performed the procedure on the left side of toad embryos at the two-cell stage, so that the effect of the injection could be traced as early in development as possible. The right side was left untouched so that it could serve as a "control."

As each embryo continued to grow by means of cell division, the side that had been flooded with neurogenin RNA became filled with neurons, while the right side developed in a normal manner. This indicated to the researchers that neurogenin is the substance that begins the cascade of genetic steps that turn an undifferentiated cell into a neuron.

Importantly, Anderson, Kintner and colleague Qiufu Ma (a research fellow in biology at Caltech) showed that, once cells make neurogenin, they inhibit their neighbors from becoming neurons by inhibiting their production of neurogenin. Thus, uncommitted embryonic cells are engaged in a winner-take-all competition to become neurons, the winner being decided by the cell that makes the highest level of neurogenin.

The research also showed that other genes suspected of being the initiators of neural development, such as neuroD, actually come into play later after the process has been started by neurogenin. Moreover, the fact that mouse neurogenin RNA can also be successfully used to artificially activate neurogenesis in frog embryos suggests that little difference exists in the gene from species to species.

"The neurogenin (used in the research) is probably about 80 percent identical in its 'business end' to that in humans," Anderson said.

The research builds on earlier work done by J. E. Lee and the late Harold Weintraub at the Fred Hutchinson Cancer Research Center in Seattle, Washington, as well as work done by others on the early development of nerve cells in fruit flies.

Writer: 
Robert Tindol
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