Multiplication and differentiation of glial cells in the optic nerve of the postnatal rat. A reassessment.

In order to establish a firm basis for our studies on cell reactivity during Wallerian degeneration in the optic nerve of the rat, gliogenesis in this fiber tract was reassessed. Rats aged 2, 5, 8 and 20 days (key-stages) received a single injection of tritiated thymidine and were sacrificed after a survival period of 1, 3, 5, 10 and 20 days. Before the 5th postnatal day, glioblasts and astrocytes are the only cell types identifiable in the optic nerve. Most of astrocytes undergo their last mitosis during this period. Oligodendrocytes are first seen after the 5th postnatal day, and their maturation proceeds through a regular sequence of light, medium and dark cells. Detailed analysis of this time-course reveals that those precursors of oligodendrocytes that undergo last mitosis at the 5th postnatal day are retarded in their differentiation as compared with those undergoing their last mitosis during the 6-8 days period. When considering glio-axonal interactions, the onset of oligodendrocyte differentiation could proceed in two phases, especially at the 5 day key-stage, with an initial signal triggering the cessation of mitosis of glioblasts and a second signal inducing their maturation, conditioned by the type of surrounding tissue.