Shedding of discs from rod outer segments in the rhesus monkey

Abstract

Renewal of the photosensitive outer segment of rod visual cells involves the continued assembly of new membranous discs accompanied by a balanced loss and destruction of old disc material. The process by which the old discs are detached from the rod outer segments in the retina of the rhesus monkey has been studied by electron microscopy. Discs are shed from the end of the cylindrical segment in groups generally containing 8–30 discs. The endmost discs curl up at the edge, displacing a small amount of cytoplasm into the space thereby created. The outer rod membrane then folds into the zone separating the deformed discs from the remainder of the outer segment. This serves to dissect away the terminal discs while reforming the membrane over the tip of the cell. Next, the shed discs are surrounded by cytoplasmic extensions from the pigment epithelium and withdrawn into the body of that cell within a phagosome.