Cone differentiation with no photopigment coexpression

PURPOSE. TO decide whether the transitory coexpression of cone visual pigme nts described in the developing rat and gerbil retina is a universal featur e of dichromatic mammalian species. METHODS. The rabbit, a species widely used in eye research, was selected fo r the study and a search made for the presence of cones that bound more tha n one cone antibody during the first postnatal week. To plot the densities of individual cone types and to colocalize the two visual pigments, immunoc ytochemistry on retinal wholemounts and consecutive tangential sections, re spectively, were used. RESULTS. The sequence in which the visual pigments began to be expressed wa s the same as that observed in other mammals: first, rhodopsin; second, blu e pigment; and last, green pigment. The striking increase in blue cone dens ity numbers observed in the rat, however, did not occur in the rabbit. Inst ead, some days after the first blue cones appeared, the green cones also st arted to express their visual pigment, and this cone type soon outnumbered the blue cones. Within the limits of the immunocytochemical method, it was established that unlike the developing rat, the presence of double-labeled cones was not a character of the rabbit retina. CONCLUSIONS. Visual pigment coexpression is an interesting phenomenon of re tinal development, however, it is not the exclusive scenario of photorecept or differentiation. Each species must be carefully studied before deciding whether its retinal cones synthesize both pigments during retinal developme nt.