Studying how zebrafish neural crest cells differentiate

David Raible Photo by Jordan Rehm

In a warm, humid facility on the second floor of the Health Sciences Center, Dr. David Raible peers into a tank at a dozen or so of the more than 8,000 zebrafish in the bubbling room and explains how to tell the difference between the males and females.

"You just have to know what you're looking for," he says.

Male zebrafish are slightly redder and the females have slightly bigger bellies, he explains.

Indeed, considering that he studies them from the earliest stages of development through maturity, there are few things Raible does not know how to look for in these zebrafish, particularly when it comes to the neural crest.

The neural crest is a group of initially homogenous cells that branch out to all different parts of the body during development. In zebrafish, neural crest cells differentiate into cartilage, connective tissue, sensory neurons and pigment cells. In humans, neural crest cells become sensory neurons, pigment cells, and cells important to the formation of bone in our necks and faces.

Raible said the latter suggests that certain neural crest cells may play a role in the development of malformations such as cleft lip or cleft palate, but that much more fundamental questions need to be answered before those problems are solved.

What determines the fate of these seemingly homogenous neural crest cells? Why does one cell migrate from the neural crest to the tail in a developing zebrafish, for instance, while another migrates elsewhere?

Raible will discuss issues surrounding these questions on Friday, Feb. 20, from noon to 1 p.m. in room T-625 of the Health Sciences Center. The lecture, fifth in this year's Science in Medicine Lecture Series, is titled "Cell Fate Determination in Zebrafish Neural Crest."

Under a microscope, this migration is easier to see, literally; three days into its development, one can make out individual cells under the skin of a zebrafish. In fact, throughout much of its early life, the zebrafish is translucent. "Because they are translucent they make an excellent model," Raible said.

The translucency of the zebrafish allows researchers to dye a neural crest cell in a developing embryo and optically follow the cell's migration as the fish grows. Along the way, they look for the things that determine the cell's fate.

In the same way that both culture and genes form a person as he or she grows up, Raible said, cell fate decisions result from an interplay between intrinsic factors and signals from the surrounding environment.

In a recent study, researchers took early-migrating neural crest cells, which grow into neurons, and transplanted them to a host so that they migrated late. In the different environment, the cells still produced neurons. The result, Raible said, suggests that early-migrating neural crest cells have an intrinsic bias toward producing neurons.

On the other hand, when the same early-migrating cells were cut out from a developing zebrafish embryo, researchers observed that late-migrating cells gained the ability to produce neurons. Normally, late-migrating cells produce only non-neural derivatives. In other words, without having their early-migrating partners around, late-migrating cells generated cells they never normally produce. The change in the environment caused a change in cell fate.

In another brightly lit tank, this interplay between environmental cues and intrinsic factors is on display. A number of zebrafish that have lost their stripes swim languidly about.

These zebrafish have been genetically altered. Yet, the mutation that has resulted is not what's most important; the key here is the hundreds of eggs the female will lay in the bottom of the tank. By studying the progeny of the mutant parents from their earliest stages through maturity, researchers hope to find the specific genes regulating cell fate decisions in the neural crest.

Once those genes are identified, researchers will then be able to search for matches in the human genome; finding a match is the first step toward fixing mutations that develop in the neural crest.

At present, however, there are still more questions than answers. So far, researchers have identified only a small portion of the zebrafish genome. "It's a work in progress," says Raible.

With 8,000 zebrafish to work with, at least Raible and researchers in his lab know they are in the right environment.

Raible received a bachelor's degree in biology from Cornell University in 1983 and a Ph.D. from the University of Pennsylvania in 1989. He joined the UW faculty in 1995. ¶

Will Morton

For more on Raible's work, see http://www.biostr.washington.edu/html/raible.html