Exocytic clearing of degraded membrane material from pigment epithelial cells in frog retina.

Posterior eye cups of the frog Rana ridibunda were prefixed in the presence of tannic acid, osmicated and stained en bloc with uranyl acetate. Electron microscopy revealed the following features of membrane traffic in cells of the retinal pigment epithelium (RPE): (1) intracellular membranes with increased electron density undergo degradation within phagolysosomes and also outside lysosomes and are committed to be exocytized; (2) exocytized degraded membranes, organized as lamellar arrays, are evacuated through the basolateral extracellular space, through Bruch's membrane and into the choriocapillaris. Comparison of exocytic activity in animals kept either with a light cycle or in constant darkness suggests that exocytized lamellar arrays derive from membranes degraded through both lysosomal and non-lysosomal pathways. Autophagocytosis may contribute a substantial amount of material to exocytosis. On the other hand, exocytosis of heterophagosomes, if it takes place at all, involves only residual bodies at a final stage of degradation following slow proteolysis which is achieved within the RPE cells. Rapid transcytosis of heterophagosomes does not occur. Taken together, our observations suggest that, in the frog, exocytosis and subsequent evacuation of residual bodies is an efficient mechanism for clearing from worn-out organelles the metabolically highly active, non-dividing RPE cells.