In most mammals, retinal ganglion cell axons are unmyelinated in the retina. The same axons become myelinated in the optic nerve. Various studies suggest that retinal ganglion cell axons are also in principle, myelination competent intraretinally and that non-neuronal factors at the retinal end of the optic nerve prevent the migration of oligodendrocyte progenitor cells into the retina. To test this hypothesis directly, we injected oligodendrocyte progenitor cells into the retina of young postnatal rats. We observed massive myelination of ganglion cell axons in the retina 1 month after cell transplantation. Electron microscopic analysis revealed that intraretinal segments of ganglion cell axons were surrounded by central nervous system myelin sheaths with a normal morphology. Our results thus provide direct evidence for the myelination competence of the intraretinal part of rat retinal ganglion cell axons.
