Histochemical observations on rods and cones in retinas of vertebrates

Retinas containing varied proportions of rods and cones, including those of frog, Amphiuma, turtle, chick, kitten and monkey, have been studied by histochemical means.

In all visual cells ribonucleic acid occurs exclusively in the myoids. These structures also sometimes contain glycogen. The paraboloids, whenever present, contain glycogen and nothing else. The ellipsoids are colored by mitochondrial stains and by sudan black B, and are rich in a mildly acidophilic protein. The outer segments of the rods and cones contain an acidophilic protein and lipids which are characterized by sudanophilia, the Ashbel-Seligman carbonyl reaction, and autofluorescence. The outer segments of the rods also contain a complex carbohydrate and react strongly for protein-bound sulfhydryl groups. The carbohydrate and the sudanophilic lipid are present in the outer segments of immature photoreceptor cells before the onset of vision and are considered to be structural components; the sulfhydryl reaction, on the other hand, occurs only in outer segments of mature rods and may represent opsin. Retinene, the carotenoid component of the visual pigments, is a fatty aldehyde which we have not identified histochemically. Inconstant, faint metachromasia appears between the outer segments of the photoreceptor cells and the processes of the pigmented epithelial cells. The basal cytoplasm of the pigmented epithelium contains acidophilic protein, sulfhydryl groups, ribonucleic acid, lipids demonstrable by their sudanophilia, carbonyl staining, and autofluorescence, and stains for the enzymes acetylcholinesterase and lipase.

The ellipsoids and outer segments of the visual cells, and the basal cytoplasm of the pigmented epithelium show similar histochemical reactions, whereas the myoids stain quite differently. Differences in histochemical properties of rods and cones in certain species are described.