Distribution and development of short-wavelength cones differ between Macaca monkey and human fovea

Macaca monkey and humans have three cone types containing either long-wavel ength (L), medium-wavelength (M), or short-wavelength (S)-specific opsin. T he highest cone density is found in the fovea, which mediates high visual a cuity. Most studies agree that the adult human fovea has a small S cone-fre e area, but data are conflicting concerning S-cone numbers in the adult Mac aca monkey fovea, and little evidence exists for how either primate fovea d evelops its characteristic cone pattern. Single- and double-label in situ h ybridization and immunocytochemistry have been used to determine the patter n of foveal S cones in both the fetal and adult Macaca and human. Both labe ls find a clear difference at all ages between monkey and human. Adult huma ns have a distinct but variable central zone about 100 mu m wide that lacks S cones and is surrounded by a ring in which the S-cone density is 8%. Thi s S cone-free zone is detectable at fetal week 15.5 (Fwk15.5), shortly afte r S opsin is expressed, and is similar to the adult by Fwk20.5. Adult monke y foveas have an overall S-cone foveal density of 10%, with several areas l acking a few S cones that are not coincident with the area of highest cone density. A surrounding zone at 200-mu m eccentricity has an S-cone density averaging 25%, but, by 800 mu m, this has decreased to 11%. Fetal day 77-13 5 monkeys all have a distribution and density of foveal S cones similar to adults, although the high-density ring is not obvious in fetal retinas. Est imates of the numbers of S cones missing in the fetal human fovea range fro m 234 to 328, whereas no more than 40 are missing in the fetal monkey. Thes e results show that, in these two trichromatic primates, S-cone distributio n and the developmental mechanisms determining S-cone topography are marked ly different from the time that S cones are first detected. (C) 1999 Wiley- Liss, Inc.