The existence of cones in the rat retina is controversial. Although the rat has been thought to possess an exclusive rod retina (Lashley, 1932; Detwiler, 1943), Walls (1934) reported the existence of “typical cones” essentially similar to those of the primate. More recent light and electron microscope studies have also reported on the presence of cones in the rat retina (Leure-duPree, 1974; LaVail, 1976). A quantitative analysis of their incidence and fine structure has not previously been reported.
The results of the present investigation demonstrate that although the rat photoreceptor layer is rod-dominant, approximately 17% of its cells would, on morphological grounds, qualify as “cone-type” receptors. These, however, are not typical cones, as their inner and outer segments do not show the characteristic conical taper and appear morphologically similar to the slender cylindrical-shaped rod inner and outer segment. Apart from this, the rat cone-type receptor cell does resemble the cone typically observed in other species.