Our radioautographic results obtained with light and electron microscopy confirmed that the oligodendrocytes are the only cells involved in the formation of the myelin sheath in the developing primary visual system of the rat. The oligodendrocyte, whose differentiation requires the intervention of an axonal signal, causes myelination gliosis, allowing, among other things, the nervous influx to acquire its definitive speed. Radioautographic and freeze-fracture analyses of Wallerian degeneration in the optic nerve after enucleation showed the proliferative modalities of cells issued from the glioblastic precursors involved in reactive gliosis. The microglial cell participated in the process of phagocytosis (resorption gliosis) aided by the astrocytes that formed the glial scar (substitution gliosis). In this context of dynamic neurobiology, it is now possible to study the role of glio-axonal interactions in the regeneration of the CNS using the transplanted nerve technique.