The renewal of photoreceptor cell outer segments

The renewal of protein in retinal rods and cones

The renewal of protein in retinal rods and cones has been analyzed by quantitative electron microscope radioautography in adult frogs injected with a mixture of radioactive amino acids. Protein synthesis occurs predominantly in the ergastoplasm, localized in the myoid region of the photoreceptor cells. Much of the newly formed protein next flows through the Golgi complex. In rods, a large proportion of the protein then moves past the mitochondria of the ellipsoid segment, passes through the connecting cilium into the outer segment, and is there assembled into membranous discs at the base of that structure. Discs are formed at the rate of 36 per day in red rods and 25 per day in green rods at 22.5 degrees C ambient temperature. In cones, a small proportion of the protein is similarly displaced to the outer segment. However, no new discs are formed. Instead, the protein becomes diffusely distributed throughout the cone outer segment. Low levels of radioactivity have been detected, shortly after injection, in the mitochondria, nucleus, and synaptic bodies of rods and cones. Nevertheless, in these organelles, the renewal process also appears to involve the utilization of protein formed in the ergastoplasm of the myoid.

Visual pigment renewal in the mature frog retina

It has been demonstrated by autoradiography that radioactive amino acids serve as precursors for proteins which are subsequently incorporated into retinal rod outer segment discs in mature animals. By the isolation and purification of visual pigment from retinas of adult frogs after injection of tritiated leucine and phenylalanine, it has been shown that at least part of this labeled protein consists of visual pigment (rhodopsin).