Diffusible rod-promoting signals in the developing rat retina.

We previously developed a reaggregate cell culture system in which embryonic rat retinal neuroepithelial cells proliferate and give rise to opsin-expressing rod photoreceptor cells (rods) on the same schedule in vitro as they do in vivo. We showed that the proportion of neuroepithelial cells in the embryonic day 15 (E15) retina that differentiated into opsin+ rods after 5-6 days in such cultures increased by approximately 40-fold when the E15 cells were cultured in the presence of an excess of postnatal day 1 (P1) neural retinal cells. In the present study, we have further analyzed this rod-promoting activity of neonatal neural retinal cells. We show that the activity is mediated by a diffusible signal(s) that seems to act over a relatively short distance. Whereas neonatal (P1-P3) neural retina has rod-promoting activity, E15 and adult neural retina, neonatal thymus, cerebrum and cerebellum do not. Finally, we show that neonatal neural retina promotes rod but not amacrine cell development.