The rod dominated rodent retina is the preferred tissue for in vitro studies of mammalian retinal physiology and pharmacology. The rod pathway through the rat retina was investigated, therefore, in order to find out whether its organization follows the mammalian "plan." AII-amacrine cells of the rat retina were injected with Lucifer Yellow to characterize the morphology of this bistratified interneuron of the rod pathway. When sections or whole mounts of the rat retina were stained with antibodies against the calcium binding protein parvalbumin (PV), two different amacrine cell types were labeled: the AII-amacrine cell and a widefield amacrine cell. They occur at a ratio of 12:1. Weak label was also observed in ganglion cells. The density of PV-labeled AII-cells decreases from approximately 7,000 cells/mm2 in upper central retina to 2,000 cells/mm2 in peripheral retina. Their cell bodies form a regular mosaic, and the dendritic arbors of three neighbouring AII-amacrine cells overlap (coverage of 3).

No abstract

It has long been contentious whether the large representation of the fovea in the primate visual cortex (V1) indicates a selective magnification of this part of the retina, or whether it merely reflects the density of retinal ganglion cells. The measurement of the retinal ganglion-cell density is complicated by lateral displacements of cells around the fovea and the presence of displaced amacrine cells in the ganglion cell layer. We have now identified displaced amacrine cells by GABA immunohistochemistry and by retrograde degeneration of ganglion cells. By reconstructing the fovea from serial sections, we were able to compare the densities of cones, cone pedicles and ganglion cells; in this way we found that there are more than three ganglion cells per foveal cone. Between the central and the peripheral retina, the ganglion cell density changes by a factor of 1,000-2,000, which is within the range of estimates of the cortical magnification factor. There is therefore no need to postulate a selective magnification of the fovea in the geniculate and/or the visual cortex.

Horizontal cells of the macaque monkey retina were quantified and the number of cones converging onto an individual horizontal cell as well as the number of horizontal cells contacting a single cone were determined. This was done by combining data from individual horizontal cells stained by the Golgi method with the results of immunocytochemical staining described in the preceding paper (Röhrenbeck et al., 1989). The observation (Boycott et al., 1987) that all horizontal cells contact all cones in their dendritic field irrespective of cone type was confirmed. The particular cones contacted by the terminal aggregates of each horizontal cell were found. The dendritic fields of H1 and H2 cells increase with increasing eccentricity; close to the fovea H1 cells are smaller than H2 cells, at 6 mm eccentricity they are about the same size and in peripheral retina H1 cells are much larger than H2 cells. The density gradients of the two cell types balance their denritic field changes so that throughout the retina each and every cone synapses with 3 - 5 horizontal cells of each type. Horizontal cells of both cat (Wässle et al., 1978) and monkey retina follow the general rule that all cones in the dendritic fields are contacted, their perikarya form a regular mosaic and the boundaries of their dendritic fields are marked by the perikarya of their homologous neighbours.