Count and density of human retinal photoreceptors.

This investigation was directed at determining the count and regional distribution of photoreceptors in the eyes of 21 human cornea donors aged between 2 and 90 years. Mean count of rods was 60,123,000 +/- 12,907,000, and mean cone count was 3,173,000 +/- 555,000. Determined 40 microns away from the foveola, cone density measured 125,500 cones/mm2. Extrapolating the distribution curve, cone concentration in the foveal center can be assumed to be about 150,000 cells/mm2 to 180,000 cones/mm2. Towards the retinal periphery, cone density decreased from 6000 cones/mm2 at a distance of 1.5 mm from the fovea to 2500 cells/mm2 close to the ora serrata. Comparing different fundus regions, cone concentration was significantly highest in the nasal region. Cone diameter increased from the center towards the periphery. At a distance of 40 microns away from the foveola, it measured about 3.3 microns, and in the outer retinal regions about 10 microns. Rod density was highest in a ring-like area at a distance of about 3-5 mm from the foveola with a mean of 72,246 +/- 17,295 cells/mm2. Rod density peaked at 150,000 rods/mm2. It decreased towards the retinal periphery to 30,000-40,000 rods/mm2. Rod diameter increased from 3 microns at the area with the highest rod density to 5.5 microns in the periphery. The hexagonal rod and cone inner segments were regularly arranged in a honey-comb fashion.