Video-enhanced differential interference contrast optics was used to determine the number and distribution of photoreceptors across the entire retinal surface of 9 eyes obtained from 7 adult rhesus monkeys. We found that the retina of this primate contains an average of 3,100,000 cones (+/- 130,000) and 61,000,000 rods (+/- 7,500,000). Variation among animals in the number of rods and cones cannot be accounted for by differences in sex, age, or retinal surface area, nor is there a correlation between the number of rods and cones (a retina with a high number of rods does not typically have a high number of cones). Cone density peaks at 141,000 cones/mm2 in the foveola and decreases about 100-fold toward the periphery. Rod density in a central annulus around the fovea is 130,000/mm2 and decreases 6-8-fold toward the periphery. In all 9 retinae, we found that an area 4-5 mm dorsal to the fovea had the highest rod density at 184,000 rods/mm2. The functional significance of this area, which we term the dorsal rod peak (DRP), may be related to high sensitivity vision under scotopic conditions. Outside of the DRP, rod density is symmetrical around the major axes of the retina, whereas cone density is elevated in nasal retina. Among animals, both rods and cones display a 2-fold individual difference in receptor density at any given eccentricity. Although rods and cones differ in absolute number, the location and magnitude of their peak densities, and their central to peripheral density gradients, the ratio of the density of rods to cones (15-30:1) is remarkably stable from 3 mm to 15 mm eccentricity. The relative consistency in the proportion of rods and cones in extrafoveal retina may be related to mechanisms of retinal development and functional interactions between scotopic and photopic systems.

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In both human and Old World primates visual information is conveyed by two parallel pathways: the magnocellular (M) and parvocellular (P) streams that project to separate layers of the lateral geniculate nucleus and are involved primarily in motion and color͞form discrimination. The present study provides evidence that retinal ganglion cells in the macaque monkey embryo diverge into M and P subtypes soon after their last mitotic division and that optic axons project directly and selectively to either the M or P moieties of the developing lateral geniculate nucleus. Thus, initial M projections from the eyes overlap only in prospective layers 1 and 2, whereas initial P projections overlap within prospective layers 3-6. We suggest that the divergence of the M and P pathways requires developmental mechanisms different from those underlying competition-driven segregation of initially intermixed eye-specific domains in the primate visual system.The segregation of motion and feature vision is a pervasive attribute of primate brain organization at all levels of the neuraxis, from the retina to the frontal lobe (1, 2). In Old World primates motion and color͞form discrimination is carried out by separate, magnocellular (M) and parvocellular (P) neuronal pathways that originate in the retina and project to separate M-and P-dedicated layers of the lateral geniculate nucleus (LGN) situated in the thalamus (refs. 3 and 4; Fig. 1). This segregated information then is transmitted from the LGN to M-and P-related sublayers and modules in the visual cortex (5-8). Although the details of the physiology have not been agreed on, it is generally accepted that neurons belonging to these separate streams can be distinguished on the basis of their time of origin (9), morphology (10, 11), connectivity (12, 13), biochemistry (14, 15), and different signaling molecules (16,17).In spite of the significance of the separation of M and P systems for understanding the functional anatomy of the human brain, surprisingly little is known about their development. In contrast, the development of ocular dominance pathways, which transmit information from the eyes via separate synaptic chains to the cerebral cortex, has been extensively studied and used as a premier model system for understanding principles governing the segregation of neuronal connections in the mammalian brain (18-21). A major step in understanding the development of this system was made in the 1970s when the injection of radioactive axonal tracers into the eyes of macaque embryos revealed that visual connections from the eyes initially overlap in the LGN (22, 23). The subsequent discovery that optic axons are more numerous during this early period of overlap than in adults (24), and that prenatal enucleation results in the maintenance of a widespread projection from the remaining eye (25), demonstrated that competitive interactions play an important role in the segregation of initially overlapping retinal projections. These findings, as well as the seminal studies of Hubel, ...

The retina of diurnal primates, including humans, contains a reiterative mosaic of red-, green- and blue-sensitive cones whose visual pigments are maximally sensitive to long, middle or short wavelengths, respectively. Although the distribution of the cone subtypes in the adult rhesus monkey has been quantified using opsin-specific antisera, the mechanism for the phenotypic specification of the cone subtypes and the establishment of their ratios in the retinal mosaic remain unknown. Here we present immunocytochemical evidence that a subset of cones (about 10%) express their cell-specific opsin two to three weeks before the surrounding cones. Remarkably, these precocious cones are evenly stationed throughout undifferentiated regions of the retinal surface from several weeks after their last mitotic division, and at least one month before the formation of their synapses with bipolar and horizontal cells. Use of confocal laser microscopy reveals that the inner segments of immunolabelled and surrounding unlabelled cones are transiently in apposition with one another, enabling surface mediated interactions to occur during this period. We suggest that the early maturing cones induce neighbouring undifferentiated cones to express an appropriate opsin phenotype, and therefore constitute a 'protomap' for the emergence of the species-specific retinal mosaic.