Photoreceptor cells in the Xenopus retina

This review summarizes our present state of knowledge about spectrally different photoreceptor cell types in the Xenopus retina. The classification of the photoreceptors was based on morphology, combined with immunolabelling with various anti-visual pigment antibodies and other molecular probes on semithin sections and retinal wholemounts. The majority of photoreceptors is represented by rods. Altogether 97-98% of the total rod population consists of the principal ("red") rods that are selectively labeled by N-terminal specific anti-bovine rhodopsin monoclonal antibodies (mAbs) and are maximally sensitive to green light. The other, rare, blue-sensitive rod type ("green rod") is thinner, not stained by these antibodies but binds C-terminal specific anti-rhodopsin mAbs. The major representatives of the cones are red-sensitive and consist of a morphologically heterogeneous group comprising both (principal and accessory) members of double cones, as well as large single cones. Outer segments in this group are selectively labeled by mAb COS-1, specific to the L/M group of cone visual pigments. Another, relatively rare cone type is similar in size, but slightly smaller than the large single cone and is not labeled by mAb COS-1. This cone type is assumed to have a blue-sensitive cone visual pigment. The third, least abundant, and immunocytochemically distinct cone type is a small single (miniature) cone, which binds mAb OS-2 relatively strongly, and anti-rhodopsin mAbs 4B4 and 1D4 weakly. By exclusion, this small single cone may be identical with the UV-sensitive cone. Further studies are needed, however, to identify the color sensitivity of the latter two cone types.