V. Its Form and Differentiation in Pigment Epithelial Cells of the Frog Retina

Pigment epithelial cells of the frog's retina have been examined by methods of electron microscopy with special attention focused on the fine structure of the endoplasmic reticulum and the myeloid bodies. These cells, as reported previously, send apical prolongations into the spaces between the rod outer segments, and within these extensions, pigment migrates in response to light stimulation. The cytoplasm of these cells is filled with a compact lattice of membrane-limited tubules, the surfaces of which are smooth or particle-free. In this respect, the endoplasmic reticulum here resembles that encountered in cells which produce lipid-rich secretions. The myeloid bodies comprise paired membranes arranged in stacks shaped like biconvex lenses. At their margins the membranes are continuous with elements of the ER and in consequence of this the myeloid body is referred to as a differentiation of the reticulum. The paired membranes resemble in their thickness and spacings those which make up the outer segments; they are therefore regarded as intracellular photoreceptors of possible significance in the activation of pigment migration and other physiologic functions of these cells. The fuscin granules are enclosed in membranes which are also continuous with those of the ER. The granules seem to move independently of the prolongations in which they are contained. The report also describes the fine structure of the terminal bar apparatus, the fibrous layer intervening between the epithelium and the choroid blood vessels, and comments on the functions of the organelles depicted. I N T R O D U C T I O N A few years ago an unusual development of the endoplasmic reticulum was encountered in cells of the pigment epithelium of the frog's eye. This had the form of a close lattice of tubules limited by smooth, particle-free membranes. It was observed, further, that at scattered points this reticulum showed remarkable differentiations consisting of closely stacked, paired membranes resembling the structure encountered in the outer segments of the retinal rods and cones. The disposition of the Study supported in part by Research Grant No. G-5826 Received for publicalion, December 3, 1959. paired membranes was highly ordered and gave the impression that one was viewing a '~crystalline" body (1). These structures were regarded as equivalent to the "myeloid bodies" described by Kiihne (2) and by other histologists of the last century. Subsequently these observations were confirmed and extended to a variety of species, particularly among the lower vertebrates (3). There were in these observations on the fine structure of the pigment epithelium a number of from the National Science Foundation. 181 on A ril 4, 2017 D ow nladed fom Published September 1, 1960