Morphology of horizontal cells in the retina of the capuchin monkey, Cebus apella: how many horizontal cell classes are found in dichromatic primates?

The morphology of horizontal cells was studied in the retina of dichromatic capuchin monkeys, Cebus apella. The cells were labeled with the carbocyanine dye, 1,1',dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI), and the labeling was then photoconverted to a stable product by using a diaminobenzidine reaction. The sizes of cell body, dendritic field, and axon terminal, as well as the number of dendritic clusters and cone convergence, were measured at increasing distance from the fovea. Three distinct morphological classes of horizontal cells were identified. Their dendritic and axonal morphology resembles those of H1, H2, and H3 cells described in trichromatic primates. The size of the cell bodies, dendritic fields, and axon terminals of all cell classes increases towards retinal periphery. H3 cells have larger dendritic fields and more dendritic clusters than H1 cells. All labeled horizontal cells located in selected patches of retina were further analyzed to quantify the differences between H1 and H3 cells. H1 cells have smaller dendritic field area, smaller total length of primary dendrites, more dendritic branching points, and larger fractal dimension than H3 cells. We have distinguished H1 and H3 cells based solely in morphological criteria. Their physiology should be further analyzed with detail, but their presence in both dichromatic and trichromatic primates suggests that neither of them have a specialized role in the red-green color opponent channel of color vision.