Biogenesis and renewal of visual cell outer segment membranes

Abstract

Visual cell outer segment membranes are formed by the growth of existing membrane. Membrane components are synthesized at one or more sites outside the membrane system, and are then transported to it for membrane assembly. Protein and carbohydrate are produced in the myoid portion of the visual cell inner segment, and are delivered to the outer segment by way of the connecting cilium. The same may be true of phospholipid, but this is currently less certain, and the possibility of multiple synthetic sites has not been excluded. The pigment epithelium appears to play a significant role in lipid metabolism of outer segment membranes. Whether membrane constituents are combined prior to insertion in the growing membrane, or remain separate until the stage of membrane assembly is not known. There are two mechanisms by which outer segment membranes are renewed. Renewal by membrane replacement takes place only in rods, and involves assembly of new membrane at the base of the outer segment, coupled with shedding of old membrane from the apex of that structure. Renewal by molecular replacement occurs in both classes of visual cells. The rate of membrane renewal by molecular replacement differs between rods and cones and varies depending upon the molecule.