Early emergence of photoreceptor mosaicism in the primate retina revealed by a novel cone‐specific monoclonal antibody

Abstract

We have examined the emergence of the photoreceptor mosaic in fetal macaque monkeys by using a novel monoclonal antibody, 7G6, that recognizes all cones in the adult primate retina. In the fetal retina, however, between embryonic (E) day 80 and E130, some opsin-positive cones were not labeled by 7G6. Double-labeling experiments revealed that though long and middle wavelength-sensitive fetal cones are 7G6-positive, a subset of short wavelength-sensitive cones are delayed in their acquisition of 7G6 immunoreactivity. Heterogeneity in cone labeling with 7G6 was evident at all fetal ages. The onset of 7G6 immunoreactivity, surprisingly, precedes both the expression of the cone opsins and the formation of synaptic contacts in the outer plexiform layer. Specifically, a population of cones was labeled in the periphery of the E65 retina, concomitant with ongoing cone genesis. Moreover, early-differentiating 7G6-positive cones are organized into a regular array in immature, peripheral regions of the fetal retina indicating that the spatial arrangement of cones is initiated either during active cone proliferation or the initial differentiation of these cells. These results suggest that the periodic spacing of cones in the primate retina emerges autonomously within the photoreceptor layer, prior to the formation of synaptic connections within the retina or with the brain. J. Comp. Neurol. 377:500–508, 1997. © 1997 Wiley-Liss, Inc.