CELL BIRTHDAYS IN XENOPUS-LAEVIS RETINA

Using an in vitro differentiation system, we reevaluated the stages of development during which retinal neurons become postmitotic in Xenopus laevis. We also examined whether retinal detachment and removal of the pigment epithelium stimulated proliferation of previously postmitotic retinal cells. Retinas with and without an adherent pigment epithelium (stages 24-40 and stage 33/34, respectively) were removed from X. laevis embryos, placed in culture and allowed to differentiate in the presence of H-3-thymidine. After 2 days, eyes were fixed and processed for autoradiography. The proportion of labeled to unlabeled nuclei in the posterior pole of the retina was determined for each of the three cell layers. Early in development, most unlabeled cells were found in the ganglion cell layer; at stage 24, 53% of the cells showed no labeling, but by stage 32-33/34 all of the cells present were unlabeled. Within the outer nuclear layer, 17% of the cells failed to incorporate the H-3-thymidine at stage 24 and by stage 33/34, 100% of the cells were unlabeled. Within the inner nuclear layer, 13.5% of the cells failed to show labeling at stage 24, whereas at stage 40, none of the cells were labeled. There was no difference in the proportion of labeled to unlabeled nuclei in any of the cell layers when retinas were allowed to differentiate either with or without an adherent pigment epithelium. These results indicate that as early as stage 24, some cells that will become positioned in each of the nuclear strata fail to incorporate 3H-thymidine, suggesting that these cells become postmitotic very early in neurogenesis. In addition, removal of the pigment epithelium does not stimulate mitosis in retinal cells that have previously left the mitotic cycle.