Cone connections of the horizontal cells of the rhesus monkey's retina.

The presence in the rhesus monkey's retina of a second morphological type of horizontal cell (H2), described by Kolb et al. (1980), is confirmed. Both types of cell are here further described. Their cone connections are quantified and compared with those of mammals and other vertebrates. The dendrites and axons of the H2 type of cell contact only cones as do the dendrites of the H1 cell (originally described by Polyak (1941)) which has an axon contacting only rods. The dendrites of foveal H2 cells contact between 11 and 14 cones; those of H1 contact 7. The number of cones that each type of cell contacts increases with increasing distance from the fovea, so that, by 5-6 mm eccentricity, H2-type cells synapse with between 20 and 30 cones, and the H1 cells with 12-15. The qualitatively estimated coverage factors of each are 3 or 4; every cone synapses with more than one of both types. Neither type of horizontal cell makes chromatically specific connections that are anatomically recognizable, unlike the situation in some teleostean and turtle retinae. Individual horizontal cells, particularly those connected to foveal cones, may have different ratios of chromatic input. At equivalent eccentricities, up to about 6 mm from the fovea, the dendritic fields of H2 horizontal cells are about twice the size of H1 cells and contact about twice the number of cones. These relative differences are closely similar to those of the cat's horizontal cells and it is suggested that they are a basic feature of most placental mammals. The organization of foveal cone fibres within Henle's layer is described. The distribution of primate cone telodendria, gap junctions and synapses in the outer plexiform layer are briefly reviewed and compared with those of other vertebrate retinae.