December 15, 2009 — Glial cells, which help neurons communicate with each other, can leave the central nervous system and cross into the peripheral nervous system to compensate for missing cells, according to a new study in the Dec. 2 issue of The Journal of Neuroscience.
The study, led by Sarah Kucenas, an assistant professor of biology at the University of Virginia, contributes to basic understanding of how the two nervous systems develop and are maintained, which is essential for the effective treatment of diseases such as multiple sclerosis. She conducted the research as a postdoctoral fellow at Vanderbilt University.
The nervous system is divided into the central nervous system (the brain and spinal cord) and the peripheral nervous system (sensory organs, muscles and glands). A major difference between the systems is that each has its own type of glial cells. In a healthy body, glial cells are tightly segregated and aren't known to travel between the two systems. The peripheral nervous system also regenerates more readily than the central nervous system, due in part to its glial cells – a characteristic that, if better understood, might be used to improve the regenerative capabilities of the central nervous system.
Glial cells serve nerve cells by insulating them with layers of fats and proteins called myelin. Myelin coatings are necessary for nerve signals to be transmitted normally; when the protective sheaths are lost, disorders involving impairment in sensation, movement and cognition, such as multiple sclerosis or amyotrophic lateral sclerosis, develop.
Glial cells named oligodendrocytes produce myelin around nerves of the central nervous system, while those named Schwann cells make myelin that insulates peripheral nerves.
"Not unlike borders between countries, cell movement across the border between the central nervous system and peripheral nervous system is tightly controlled," Kucenas said. "Our study shows that in the absence of Schwann cells, oligodendrocytes exit the central nervous system and myelinate peripheral nerves, indicating that glial cell movement across the border is controlled by a self-policing mechanism and reveals a previously unappreciated compensatory mechanism."
Kucenas and her Vanderbilt colleagues used time-lapse video of mutant zebrafish to study the glial cell movement. Movies of translucent live zebrafish that lacked Schwann cells showed that oligodendrocytes left the central nervous system to wrap peripheral nerves with myelin – effectively attempting to compensate for the missing Schwann cells.
Kucenas, who continues this research at U.Va., said future investigations will help determine how different glial cells communicate to restrict their movements between nervous systems, whether oligodendrocyte myelin can fully substitute for Schwann cell myelin on motor nerves, and whether mammals have similar mechanisms of border control to restrict movements of glial cells.