The rod-shaped outer segment of a rod cell, which is the light-sensitive part, consists largely of a stack of flat and parallel unit membranes, at right angles to the cell length. This develops embryologically from a cilium, and retains its characteristic 9+2 pattern of fibrils. The response to light depends on the pigment rhodopsin, also known as visual purple.
There are about 120 million rods in one retina of a human eye – about 20 times the number of cones. Rods, however, are not found at all in the fovea.
Related categories• ANATOMY AND PHYSIOLOGY
• CELL BIOLOGY
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