Rod receptors in the retina of Tupaia belangeri.

The retina of Tupaia belangeri was studied by transmission electron microscopy. - Among a vast majority of cones a few typical rods have been found as a distinctly different class of photoreceptors. The outer segments of the rods measure 10 microns X 1.5 microns versus 6 microns X 2 microns in the cones. The repeating unit of the rod discs is 18 nm versus 22 nm in the cone discs. Rod discs tend to lose their continuity with the ciliary membrane, while cone discs do not. Rods lack the highly specialized giant mitochondria of the cone inner segment of Tupaia. The rods of Tupaia show other distinctive characters of mammalian rods as well. However, the rods of Tupaia are relatively short and do not develop a rod fiber.