The discovery of disc protein renewal in rod outer segments, in 1960s, was followed by the
observation that old discs were ingested by the retinal pigment epithelium. This process
occurs in both rods and cones and is crucial for their survival. Photoreceptors completely
degenerate in the Royal College of Surgeons mutant rat, whose pigment epithelium cannot
ingest old discs. The complete renewal process includes the following sequential steps
involving both photoreceptor and pigment epithelium activity: new disc assembly and old …