Photoreceptor mosaic: number and distribution of rods and cones in the rhesus monkey retina.

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.