The fine art of brain development pg. 2
As primitive nerve cells become neurons, they develop extensions from their cell bodies—many short projections called dendrites that receive incoming signals, and a single, long axon that transmits those signals to the next neuron.
Glial cells, though they have many similar features to neurons, do not develop these specialized appendages. Instead, some of them go on to form the protective sheath called myelin that wraps and insulates the axons of many neurons and enhances the speed with which nerve impulses can travel from cell to cell.
Bruce Appel, Ph.D., associate professor of Biological Sciences at Vanderbilt, is studying the development and specification of oligodendrocytes, the glial cells that form myelin in the central nervous system (CNS), which includes the brain and spinal cord.
In humans, myelination begins shortly before birth and continues into adolescence. In Appel’s research subject, the zebrafish, myelination starts around the third day after fertilization.
The zebrafish is a great model system for studying nervous system development, Appel says, because the embryo is transparent and develops entirely outside the mother. And it develops in two days. By comparison, the mouse embryo takes about 10 times longer to mature.
By engineering certain zebrafish genes to glow green, Appel can easily view specific sets of neural progenitor cells—immature nerve cells—and in particular, the cells that go on to produce oligodendrocytes.
“We’ve found that oligodendrocytes, which were always considered to be really boring cells, actually turn out to be incredibly dynamic,” he says. The cells send out fine processes, called filopodia, and appear to use these membranous “arms” to explore their surroundings, sampling the environment.
“They zip around, all over, until they finally arrive at their target axons. They continue to explore their area and move around and settle into a fairly regular distribution. That’s really fascinating to me, and we don’t understand it at all.”