Myelin sheath is essential to normal conduction in nerves and is altered in multiple sclerosis and Guillain-Barre diseases. Undestanding how myelin is formed and repaired requires basic studies of the diffentiation of myelin-forming cells both in vitro ad in vivo. To this aim, we are culturing myelin-forming cells in isolaiton, obtaining enriched populations, and studying the differentiation of these cells. Oligodendrocytes cultures without neurons develop on schedule a complex phenotype similar to their in vivo counterpart. Although devoid of intermediate filaments, they display specific motile activities which may function in vivo to find the target-axon and transport myelin components at the site of myelin assembly. Moreover, these cultured cells express MBP, MAG, and pLP in a predetermined sequence and timing, independent of continuous neuronal influenes. MBP specific mRNA is found in the cell body and is immediately followed by the appearance of MBP, suggesting a control of gene expression at the level of transcription. In vivo, the developmental expression of MBP message in different areas of the CNS closely follows the emergency of MBP protein a detected by immunocytochemical method. During demyelination caused by a coronavirus in mice, the in situ hybridization method is used to detect new synthesis of myelin specific messages around the lesion and appears to be the ideal tool to analyze the reactivity of oligodendrocytes to the disease process. In order to elucidate mechanisms of myelin formation, defects in myelin assembly are being analyzed in a rat genetic mutant.