The photoreceptors and pigment epithelium of the adult Xenopus retina: morphology and outer segment renewal

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Abstract
Outer segment renewal and the fine structure of photoreceptors and pigment epithelium (PE) were studied in the adult Xenopus retina by light microscopic autoradiography and EM. Following the injection of [3H]leucine, the pattern of labeling observed in receptor outer segments was typical of that reported in other adult retinae: only diffuse labeling was found in cones, but in rods a discrete band of label accumulated at the base of the outer segment and migrated sclerally with time. The rate of band displacement and thus disk addition in Xenopus rods was 1.86 .mu.m/day (or 78 disks/day), which is more than twice that reported for red rods in Rana under similar experimental conditions, although these species have similar metabolic rates. Average rod outer segment (ROS) length did not change, demonstrating a balance between disk addition and shedding. ROS renewal time was about 24 days, corresponding to the time when labeled phagosomes were first found in the PE. Ultrastructurally, 1 kind of (red) rod and 1 kind of cone were found whose outer segments differed in membrane topology. Although microfilaments were found in the apical processes of the PE and its cytoplasm contained pigment granules and myeloid bodies, pigment granules did not migrate into these processes during light adaptation. In addition to possible morphological evidence for phagosomes of cone origin, large and small rod phagosomes were observed in the PE. The latter appeared to represent small stacks of partial disks shed from individual ROS scallops.