9- cis-Retinoic acid in combination with retinal pigment epithelium induces apoptosis in cultured retinal explants only during early postnatal development

Retinoic acid is one of the active metabolites of vitamin A and has profound effects on the development of the CNS including retina. Previously, we have shown that rod-specific apoptosis is induced in retinal explants from neonatal mice by exposure to 9- cis-retinoic acid (9CRA) when the retinal pigment epithelium (RPE) is present. In explants lacking RPE, it instead has a differentiation-promoting effect seen as an accelerated opsin expression on postnatal day 3. To investigate the long-term effect of 9CRA exposure, we have explanted retinas from neonatal C3H mice with or without RPE attached and placed in organ culture. After 19 or 48 h in culture or 7, 8 or 13 days in culture, the explants were either fixed for histochemical examination or frozen for assay of DEVDase activity. We found that long-term exposure to 9CRA caused a decrease in the number of cell layers in the outer nuclear layer (ONL) only in explants with the RPE attached. When explants with RPE attached were exposed to 9CRA only during the second postnatal week, neither an increase in DEVDase activity, TUNEL-positive cells, nor a decrease in cell layers of the ONL could be demonstrated, indicating that the retina was insensitive to the apoptosis-inducing effect of 9CRA after the first postnatal week. The absence of RPE in control explants resulted in a higher number of rosettes and the extrusion of cells into the subretinal space.