The daily rhythm of shedding and degradation of rod and cone outer segment membranes in the chick retina.

Newly hatched chickens were maintained on a daily light cycle of 12 hr of light and 12 hr of darkness for 12 days. The pigment epithelium was then examined by electron microscopy at different times of day. Shortly after the beginning of the light period, the rods discarded groups of outer segment membranes. During the remainder of the light period, the membranes were degraded by the pigment epithelium. Early in the dark period, the cones shed membranes, which were digested by the pigment epithelial cells during the subsequent hours of darkness. Available evidence suggests that at least some of the chemical activities of the visual cells and pigment epithelium oscillate with a daily rhythm, which is synchronized with the daily fluctuation of light in the environment.