fotoreceptoroutersegments

Fotoreceptor outer segments are the distal parts of retinal photoreceptor cells, located in the outer retina and adjacent to the retinal pigment epithelium. They are connected to the inner segments by the outer segment connecting cilium and are the primary site of phototransduction. Rod and cone outer segments differ in size and disc organization, with rod outer segments typically 25–30 μm long and cone outer segments shorter, and with rod discs enclosed within the outer segment and cone discs often remaining continuous with the plasma membrane.

Structure: They consist of stacked membranous discs rich in photopigments. Rods contain rhodopsin; cones contain cone opsins. Photopigments absorb light and initiate the G-protein mediated cascade that hyperpolarizes the photoreceptor.

Renewal: Outer segments undergo daily turnover. New discs are formed at the base; older discs are shed at the tip and phagocytosed by the retinal pigment epithelium.

Development and variation: Outer segments develop from the connecting cilium; they adapt in length and composition in response to light exposure and metabolic state.

Clinical relevance: Disruption of outer segment structure or phagocytosis is associated with retinal degenerations such as retinitis pigmentosa and cone-rod dystrophies; efficient outer segment renewal is critical for sustained vision.