Mammalian cones: disc shedding, phagocytosis, and renewal

During the past several years we have examined a variety of different mammalian retinas for ultrastructural evidence of cone disc shedding and RPE phagocytosis. In this paper we review our previously published evidence from squirrel and human retinas as well as present new evidence of cone disc shedding in rhesus monkey and cat. All these species show definitive evidence for the shedding of discs from cone outer segments and the phagocytosis of shed discs by apical processes of the RPE; both of these events closely resemble those described for mammalian rods. The occurrence of cone disc shedding leads directly to the conclusion that new membrane must be added to the cone outer segment in order to maintain its length. The successive evaginations, which are observed at the bases of cone outer segments, we consider to be indirect evidence for the addition of new discs. Finally, we propose a model for the structural organization of mammalian cone outer segments.