Pigment epithelium-photoreceptor interactions in the normal and dystrophic rat retina.

The pigment epithelium of normal and dystrophic rat retinas phagocytized carbon particles that were injected into the extra-cellular space between photoreceptors and pigment epithelium. The epithelium of both infant and adult rats displayed phagocytic activity. Radioactively-labeled outer segment fragments from normal rats were usually not phagocytized by the pigment epithelium of either normal or dystrophic rats, although in rare cases some phagocytosis was observed in both the normal and dystrophic retinas. It is suggested that, prior to phagocytosis in normal animals, the pigment epithelium recognizes specific sites on the plasma membrane of discarded outer segment discs. It is further suggested that these sites are frequently removed in artificially prepared outer segment fragments either through the washing away of soluble components or removal of the enveloping plasma membrane. Although the pigment epithelium of the dystrophic rat retina will not phagocytize its endogenous outer segment debris, we now know that it is capable of phagocytosis when presented with the appropriate material. Our results also imply that modification of normal outer segment fragments can lead to their rejection by the normal pigment epithelium.