Cessation of DNA synthesis in retinal ganglion cells correlated with the time of specification of their central connections

Although no causal relationship has been shown, there appears to be a correlation between the cessation of DNA synthesis in ganglion cell neuroblasts at stages 28–29, and the specification of the central connections of theganglion cells at stages 29–31.

Abstract

A series ofXenopus larvae (stages 26–45) either were given a single injection of thymidine-3H and killed after an interval of 1–24 hours, or were given several injections at intervals extending over a period of 1–10 days. Autoradiographs were made of serial sections through the eyes. Injections at stages 26–28 resulted in uniform labeling of 100% of the nuclei of the optic vesicle within 12 hours. If injections were commenced at stage 29, less than 20% of the ganglion cells, more than 80% of nuclei of the inner nuclear layer, and 100% of receptor nuclei were labeled. All the ganglion cells, except at the periphery, remained unlabeled if thymidine-3H was injected at stage 30. The results show that before stage 29 the optic vesicle is composed of cells which all replicate DNA, and whose cycle lasts about 12 hours. At stage 29 a mantle layer of neuroblasts, which have ceased DNA synthesis, is formed external to the cells which continue DNA synthesis. These neuroblasts differentiate into ganglion cells, for if cumulative labeling is commenced at stage 29 and continued until the retina is clearly differentiated, only the ganglion cells remain unlabeled in many cases. Therefore, at stage 29, within less than 10 hours, DNA replication ceases in all the neuroblasts which later differentiate into ganglion cells, with the exception of a small percentage at the periphery of the retina. Although no causal relationship has been shown, there appears to be a correlation between the cessation of DNA synthesis in ganglion cell neuroblasts at stages 28–29, and the specification of the central connections of the ganglion cells at stages 29–31.