Primate cones maximally sensitive to short wavelength light (blue cones) have been previously identified by using indirect methods. We stained 7 wholemounted human retinas obtained from 6 female donors, using an affinity purified antibody to a 19 amino acid peptide sequence at the N-terminus of blue opsin (Lerea et al., '89: Neuron 3:367-376), standard PAP immunocytochemistry, and controls. Cones were counted where all outer segments could be traced to inner segments and were measured where cells were well aligned vertically.We find that: (1) 7% of cones within 4 mm of the foveal center are labeled by antiblue opsin; (2) compared to neighboring red/green cones, blue cone inner segments are 10% taller, have a larger cross-sectional diameter near the junction with the outer segment, and a smaller diameter near the external limiting membrane, resulting in a more cylindrical shape, (3) foveal blue cones are sparse, irregularly spaced, and missing in a zone about 100-mu-m (0.35-degrees) in diameter near the site of peak cone density, (4) the highest densities of blue cones (> 2,000 cells/mm2) are found in a ring at 0.1-0.3 mm eccentricity, and (5) the shortest distances between neighboring cones are between blue and red/green cones, and the blue and red/green mosaics are statistically independent.These findings are consistent with psychophysical reports of foveal tritanopia and maximum sensitivity to blue light at 1-degrees eccentricity. Blue cone spacing may limit resolution of the blue channel out to 20-30-degrees eccentricity. The blue and red/green mosaics appear to be formed by separate processes.

Phototransduction in vertebrate rod and cone photoreceptor cells involves G protein-mediated light stimulation of cGMP hydrolysis. Enzymes of the cGMP hydrolysis cascades of rods and cones are products of different genes. Three different classes of cones in the human retina are maximally sensitive to either blue, green, or red light. Distinct opsin genes are expressed in each type of cone. The distribution of cone types in human retina was determined using anti-peptide antibodies that recognize specific amino acid sequences in green/red opsin and blue opsin. These antibodies together with an anti-peptide antibody against Tcα were used in double labeling experiments to demonstrate the presence of the Tcα peptide in all types of cones. cDNA clones corresponding to human rod and cone transducin α subunit (Tcα and Tcα) genes were isolated. Southern blot analyses of human genomic DNA suggest that there is only one rod Tα gene but more than one cone Tα gene. The multiple Tcα genes could be closely related genes or different Tcα alleles, or one could be a pseudogene.

Transducin is a guanyl nucleotide-binding protein that couples rhodopsin photolysis to hydrolysis of guanosine 3′,5′-monophosphate in rod photoreceptor cells of vertebrate retinas. Several complementary DNA clones encoding transducin subunits have recently been characterized. One clone, isolated from a bovine retina complementary DNA library, encodes a previously unidentified polypeptide with an amino acid sequence 78% identical to the sequence of the α subunit of bovine rod outer segment transducin. Antibodies to a synthetic peptide with amino acid sequence derived specifically from this novel polypeptide recognize a 41-kilodalton polypeptide in homogenates of bovine retina. Localization of this polypeptide in bovine retina by indirect immunofluorescence demonstrates that it is expressed only in cone outer segments. Antibodies to specific sequences found only in the rod transducin α subunit recognize a polypeptide localized only in the rod outer segment. Therefore, bovine rod and cone cells each express structurally related yet significantly different forms of transducin.