The mosaic of horizontal cells in the macaque monkey retina: With a comment on biplexiform ganglion cells

To further characterize the H1 and H2 horizontal cell populations in macaqu e monkey retinae, cells were injected with the tracer Neurobiotin following intracellular recordings. Tracer coupling between cells of the same type r evealed all H1 or H2 cells in small patches around the injected cell. The m osaics of their cell bodies and the tiling of the retina with their dendrit es were analyzed. Morphological differences between the H1 and H2 cells obs ervable in Neurobiotin-labeled patches made it possible to recognize H1 and HZ cells in retinae immunolabeled for the calcium-binding proteins parvalb umin and calbindin, and thus to study their relative spatial densities acro ss the retina. These data, together with the intracellularly stained patche s, show that H1 cells outnumber H2 cells at all eccentricities. There is, h owever, a change in the relative proportions of H1 and H2 cells with eccent ricity: close to the fovea the ratio of H1 to H2 cells is similar to 4 to 1 , in midperipheral retina similar to 3 to 1, and in peripheral retina simil ar to 2 to 1. In both the Neurobiotin-stained and the immunostained retinae , about 3-5% of the H2 cells were obviously misplaced into the ganglion cel l layer. Several features of the morphology of the misplaced H2 cells sugge st that they represent the so-called "biplexiform ganglion cells" previousl y described in Golgi studies of primate retina.