VERTEBRATE RETINAL CELLS AND THEIR ORGANIZATION

Summary 1. Electron microscopy has added numerous details emphasizing the kinship of the retina with the central nervous system. The space between the receptors and pigment epithelium resembles neural tube lumen in that all cells facing this lumen possess terminal bars at their margins. The receptors, glial cells of Muller and cells of the pigment epithelium all appear to be derivatives of ependymal cell varieties. As is the neural tube and mature central nervous system, the entire optic cup derivative in the eye is jacketed in a basement membrane. Blood vessels run through glial tunnels lined by this membrane. The choroid capillaries and pigment epithelium resemble the choroid plexus. 2. The choroid capillaries have endothelial cells which possess numerous focal attenuations or perhaps pores where they oppose the cells of the pigment epithelium. The latter show deep membranal infoldings and an extensive endoplasmic reticulum. Some species possess spindle-shaped aggregations composed of stacked pairs of membranes which are continuous with the reticulum. The melanin pigment granules lie within microvilli which drape the receptors. 3. The receptors are derived from ciliated ependymal cells. Their outer segments contain stacks of flattened membranous sacs which are apparently formed by ingrowth or infolding of the plasma membrane. The connexions of the resultant sacs to the membrane may be very narrow or perhaps absent in most rod sacs, but are generally persistent in cone sacs. Rod sacs possess one to many incisures, depending on the group of animals examined. Details of the molecular architecture of these membranes are highly speculative since they are based upon the appearance of the membranes after various techniques involving empirical fixation procedures and little fundamental research. 4. Receptors are in intimate relation with processes of cells of the pigment epithelium and microvilli of the glial cell of Muller. The apex of the receptor inner segments may be reflected, calyx-like, over the base of the outer segments. 5. Receptor inner segments are joined to the outer by narrow stalks which contain nine pairs of filaments or microtubules like those of cilia, except for the absence of a central pair. The filaments end in relation to a centriole and a second centriole is also present. Rootlets run from the centrioles through the inner segments. 6. The ellipsoid of the inner segment is a cluster of mitochondria. The paraboloid, when present, is seen to be an ellipsoidal configuration of expanded tubules of the endoplasmic reticulum with clusters of granules dispersed in the cytoplasm outside the tubules. In double cones, the inner segments and nuclear areas of two independent cells are closely applied to one another. Golgi regions overlie the receptor nuclei. 7. The receptor synaptic area is highly complex. Processes, presumably from bipolar cells, end in pits in the receptor base. Above these pits specialized organelles are found in the receptor cytoplasm. Other processes terminate superficially. These may include processes from certain receptors ending on others, a situation possibly analogous to that existing between receptors of Limulus. 8. The neural retina as a whole exhibits a paucity of extracellular space, with estimates of intercellular distances made precarious by our ignorance of the molecular composition of the light areas intervening between the osmiophilic lines partially representing the cell membranes but probably not the cell surfaces! However, more extracellular space must exist in areas where cell processes are small and numerous owing to geometric factors. Receptors and retinal neurons and their processes may have broad surfaces of contact, but glial processes, particularly of the cells of Muller, are rather pervasive, and fill in spaces not otherwise occupied.