The photoreceptor outer segment: Development and renewal.

Vision is one of our most essential senses allowing us to see and interact with the world around us. It is dependent on the absorption and transduction of light by retinal photoreceptor cells. Each photoreceptor cell contains a highly-modified cilium that forms the photoreceptor outer segment (OS) and serves as an antenna for photons. The OS contains hundreds of disk membranes that form by an expansion of the ciliary plasma membrane. The disks are studded with an extremely high concentration of the visual pigment, thus maximizing their ability to capture photons of light. The OS forms during photoreceptor development but undergoes continuous renewal throughout the lifetime of the organism: new disks continue to form at the base of the OS and older disks are phagocytized from the OS tip by the adjacent retinal pigment epithelium. These anabolic and catabolic phases ensure that the OS can maintain its capacity to detect light, thus preserving our sense of vision.