Distribution of Carbonic Anhydrase Among Human Photoreceptors

The distribution of carbonic anhydrase (CA) among human photoreceptors has been a topic of dispute.In our experiments, by modifying an established enzyme histochemical technique, reproducible stain-ing was observed. Of the cones in the peripheral retina, 91% were positive for CA. The CA-negative(CA—) cones were absent within approximately 8 arc min of the foveal center and their density peakedat 2 arc deg. No CA activity was found in the rods. Morphologic differences were noted between theCA-positive (CA+) and CA— cones. Compared to the CA+ cones, the CA— cones had longer and morenearly columnar inner segments, more nearly spherical nuclei, and more generous amounts of peri-karyal cytoplasm. In the peripheral retina, the distance between CA+ to CA+ nearest neighbors werelarger compared to CA— to CA+ nearest neighbors (P < 0.0001). The frequency distribution andmorphology of the CA— cones suggest that they are the blue-sensitive cones. As such, this studydemonstrates a biochemical similarity between blue cones and rods that may provide insight into thefunction and phylogeny of the blue cones. Invest Ophthalmol Vis Sci 31:1451-1458,1990