An array of early differentiating cones precedes the emergence of the photoreceptor mosaic in the fetal monkey retina.

We previously have demonstrated that approximately 10% of cones in the fetal monkey retina precociously express the red/green opsin. These data suggested the possibility that a subset of cones differentiates prior to their nascent cone neighbors. To further assess this early cone differentiation in the fetal monkey retina, we used monoclonal antibodies proven to be important developmental markers of photoreceptor phenotypes and synaptogenesis (XAP-1, specific to photoreceptor membranes; SV2, specific to synaptic vesicle protein). Although these two antibodies recognize functionally distinct antigens, our analyses revealed that both identify a subset of precociously immunoreactive cones. Further, XAP-1- and SV2-positive cones are distributed in the same pattern as precocious red/green-sensitive cones in immature regions of the fetal monkey retina. These results support the hypothesis that the primate retina possesses a spatially organized protomap that may induce the emergence of the photoreceptor mosaic and trigger the formation of color-specific pathways that include horizontal, bipolar, and retinal ganglion cells.