MAMMALIAN CONES - DISK SHEDDING, PHAGOCYTOSIS, AND RENEWAL

Abstract

During the past several years a variety of different mammalian retinas was examined for ultrastructural evidence of cone disc shedding and RPE [retinal pigment epithelium] phagocytosis. Previously published evidence from squirrel and human retinas was reviewed and new evidence of cone disc shedding in rhesus monkey and cat was presented. All these species showed definitive evidence for the sheeding of discs from cone outer segments and the phagocytosis of shed discs by apical processes of the RPE; both of these events closely resembled those described for mammalian rods. The occurrence of cone disc shedding led directly to the conclusion that new membrane must be added to the cone outer segment to maintain its length. The successive evaginations, observed at the bases of cone outer segments, were considered to be indirect evidence for the addition of new discs. A model for the structural organization of mammalian cone outer segments was proposed.