Phagocytosis by pigment epithelium of human retinal cones

Abstract

AUTORADIOGRAPHIC studies of vertebrate retinae indicate that rod outer segment disks are continuously manufactured at the base of the outer segment^1,2. The disks are slowly displaced towards the tip of the outer segment, where they are shed and phagocytosed in groups by the apical processes of pigment epithelial cells^3,4. Identical autoradiographic studies indicate that cone disks are not continuously manufactured at the base of the outer segment, leading to the conclusion that they are not phagocytosed⁵. The observation that phagosomes occur within the pigment epithelial cells of the rod-free region of human foveola, however, suggested that outer segment disks from foveal cones are phagocytosed⁶. Are human foveal cones uniquely rod-like in this respect, just as they are rod-like in their form and in their extension to the pigment epithelial surface, or are the disks of all human cones phagocytosed by the pigment epithelium? Our observation of a close association between cone outer segments of cat retina and pigment epithelial cell processes⁷ suggested an answer. As with human extrafoveal cones, those in cat do not reach the pigment epithelial surface; instead, long leaf-like processes extend from the pigment epithelium to the outer segments. These cell processes form a multilaminar sheath around the external one-third of the outer segment. If these processes engage in phagocytosis, phagosomes containing groups of cone disks should be observed within their cytoplasm and in good sections there should be very little chance of confusing them with phagocytosed rod disks. In human retinae, where cones are more plentiful than in the cat, the extrafoveal cone outer segments are also closely associated with long pigment epithelial processes^8–11. We report here that these processes phagocytose groups of disks from the cone outer segements.