Rat optic nerve oligodendrocytes develop in the absence of viable retinal ganglion cell axons

Retinal ganglion cell axons and axonal electrical activity have been consid ered essential for migration, proliferation, and survival of oligodendrocyt e lineage cells in the optic nerve. To define axonal requirements during ol igodendrogenesis, the developmental appearance of oligodendrocyte progenito rs and oligodendrocytes were compared between normal and transected optic n erves. In the absence of viable axons, oligodendrocyte precursors migrated along the length of the nerve and subsequently multiplied and differentiate d into myelin basic protein-positive oligodendrocytes at similar densities and with similar temporal and spatial patterns as in control nerves. Since transected optic nerves failed to grow radially, the number of oligodendroc yte lineage cells was reduced compared with control nerves. However, the mi totic indices of progenitors and the percentage of oligodendrocytes undergo ing programmed cell death were similar in control and transected optic nerv es. Oligodendrocytes lacked their normal longitudinal orientation, develope d fewer, shorter processes, and failed to form myelin in the transected ner ves. These data indicate that normal densities of oligodendrocytes can deve lop in the absence of viable retinal ganglion axons, and support the possib ility that axons assure their own myelination by regulating the number of m yelin internodes formed by individual oligodendrocytes.