A search for the photoreceptor in the photosensitive irises of normally pigmented and albino hamsters (Mesocricetus auratus).

Hamsters are among the mammalian species whose irises are directly sensitive to light, responding with a constriction even when completely isolated from the rest of the hamster. Examination of the ultrastructure of the irises of both normally pigmented and albino hamsters disclosed a typical vertebrate iris, containing a sphincter and dilator pupillae, and anterior stromal layer, and two layers of pigment epithelial cells. No structures obviously specialized for photoreception were found. We examined the possibility that the pigment granules in the cells of the sphincter pupillae are photosensitive, and that their electron-dense melanin masks some photosensitive structure. However, this was ruled out by the observation that albino irises, which are photosensitive, are devoid of even rudimentary pigment granules. These results are consistent with the hypothesis that the sarcolemmas of the smooth muscle cells of the sphincter pupillae are photosensitive in this species.