DEVELOPMENT OF PHOTORECEPTOR MOSAICS IN THE PRIMATE RETINA

The mosaic of rods and cones in the primate retina is the neuronal arr ay where the visual world is first mapped onto the central nervous sys tem. Rods, which mediate scotopic vision, and cones, which mediate pho topic and color vision, are found in all vertebrate species. However, regional differences in the topographic arrangement and ratio of rods to cones vary dramatically among species, including different primates . Furthermore, the proportion and distribution of the wavelength-sensi tive cone subtypes vary considerably between primates that occupy diff erent visual habitats. What genetic or environmentally regulated mecha nisms specify the position, phenotype, and ratios of photoreceptor sub types? Available data suggest that the transient appearance of early-d ifferentiating cones may serve to delineate basic species-specific ret inal coordinates and determine the opsin phenotype of local assemblies of cones in the fetal photoreceptor mosaic. This article will summari ze presently available data and our ideas of how the photoreceptor mos aic is organized in the adult primate retina, the features of these mo saics, which vary between primate species, and the developmental mecha nisms, which may account for the emergence of photoreceptor position a nd specification of their phenotypes in the primate retina.