The morphology of horizontal cells in ox, sheep, and pig retinae as observed after Lucifer Yellow injections are described and compared with the descriptions of Golgi-stained cells by Ramón y Cajal (1893). Horizontal cells in the retinae of less domesticated species, wild pig, fallow and sika deer, mouflon, and aurochs were also examined. All these retinae have two types of horizontal cell; their morphologies are in common, although with some familial differences. Their basic appearance is as Cajal described; except in one important respect, a single axon-like process could not be identified on the external horizontal cells. It is concluded that external horizontal cells of artiodactyls correspond to the axonless (A-type) cells of other mammals. Cajal's internal horizontal cells have a single axon which contacts rods. This type corresponds to the B-type cells of other mammalian retinae. Artiodactyl A- and B-type horizontal cells differ from those of many other mammals in that the B-type dendritic tree is robust and the A-type dendritic tree is delicate. Historically, this morphological difference between orders of mammals has led to some confusion. The comparisons presented here suggest that the morphological types of primate horizontal cells can be integrated into a general mammalian classification.

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PubMedDate not supplied

Cone bipolar cells and cone synapses in the primate retina.

Primate retinal bipolar cells synapsing with two adjacent cones (2C bipolars) are further described. Their synaptic contacts are either as the central (invaginating) component of the cone triads or as basal (flat) contacts on the membrane of the cone pedicle base. Correspondingly, their axons end either in the b (inner half or in the a (outer) half of the inner plexiform layer. The shape and size of the axon terminals of 2C bipolars are indistinguishable from those of adjacent midget bipolars. Therefore 2C bipolars, like midget bipolars, probably synapse with midget ganglion cells. Two C bipolars have not been identified as connected to foveal cones. But they are not restricted to the retinal periphery, as has previously been supposed, since they occur, mixed with midget (single cone) bipolars, throughout all parts of the retina from about 2.5 mm to at least 10.0 mm from the fovea. It is likely that 2C bipolars are a variant of the midget bipolars; and that they contact some members of the same population of cones, instead of the midgets. This paper briefly reviews, and raises some new, problems concerning our current understanding of the synaptic connectivity patterns of the midget, 2C, and diffuse cone bipolar cells.

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PubMedDate not supplied

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 macaque monkey retinae, cells were injected with the tracer Neurobiotin following intracellular recordings. Tracer coupling between cells of the same type revealed all H1 or H2 cells in small patches around the injected cell. The mosaics of their cell bodies and the tiling of the retina with their dendrites were analyzed. Morphological differences between the H1 and H2 cells observable in Neurobiotin-labeled patches made it possible to recognize H1 and H2 cells in retinae immunolabeled for the calcium-binding proteins parvalbumin and calbindin, and thus to study their relative spatial densities across the retina. These data, together with the intracellularly stained patches, show that H1 cells outnumber H2 cells at all eccentricities. There is, however, a change in the relative proportions of H1 and H2 cells with eccentricity: close to the fovea the ratio of H1 to H2 cells is approximately 4 to 1, in midperipheral retina approximately 3 to 1, and in peripheral retina approximately 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 cell layer. Several features of the morphology of the misplaced H2 cells suggest that they represent the so-called "biplexiform ganglion cells" previously described in Golgi studies of primate retina.

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