Postnatal gliogenesis in the nerve fiber layer of the rabbit retina: an autoradiographic study.

Cell proliferation was studied in the retina of rabbits at various postnatal stages. Autoradiography was performed with animals that received a single injection of 3H-thymidine and were sacrificed 1 hour later. This short survival time allowed the determination of the position of a cell undergoing DNA synthesis at that moment. Between birth and day 6, cells engaged in DNA synthesis were seen in the inner nuclear layer (INL) of the entire retina. Cytogenesis ceases in this layer after the first postnatal week. Few labeled cells were detectable in the INL at day 9; these were found close to the ora serrata. Thus neurogenesis, which is known to occur in this layer of the retina, ceases by that time. In the nerve fiber layer (NFL), labeled cells were found at all ages between birth and day 27, which was the oldest stage examined in this study. By using horizontal sections through the NFL of entire retinae, it was observed that almost all labeled cells were confined to the medullary ray region, which is the neuroglia (astrocyte and oligodendrocyte)-bearing part of the NFL. Microglial cells, the only cellular elements present in the NFL outside the medullary ray region, were rarely labeled, and thus do not play a major role in gliogenesis occurring in the NFL. In addition to neuroglia, some endothelial cells were labeled after day 9. It is concluded that gliogenesis taking place in the NFL persists after the cessation of neurogenesis, suggesting that both processes occur independently.