retinal rod

Major photoreceptor cell of vertebrate retina (about 125 million in a human eye). Columnar cells (about 40 m m long, 1 m m diameter) having three distinct regions: a region adjacent to, and synapsed with, the neural layer of the retina contains the nucleus and other cytoplasmic organelles, below this is the inner segment, rich in mitochondria, that is connected through a thin ‘neck’ (in which is located a ciliary body) to the outer segment. The outer segment largely consists of a stack of discs (membrane infoldings that are incompletely separated in cones) that are continually replenished near the inner segment and that are shed from the distal end and phagocytosed by the pigmented epithelium. The membranes of the discs are rich in rhodopsin, the pigment that absorbs light.