Parasol and midget ganglion cells of the primate retina

Abstract

We have intracellularly filled the dendritic arbors of 996 midget and parasol ganglion cells with horseradish peroxidase (HRP) in macaque and baboon retinas. Only minor differences in the properties of these cell groups were found between species. Ninety of these cells were cut from their retinas, embedded in methacrylate, and transversely sectioned. According to their depth of stratification, there are two types of parasol cells (termed a‐parasol and b‐parasol), and two types of midget ganglion cells (a‐midget and b‐midget). Each of these four types stratifies at a different level within the IPL. The dendritic fields of midget ganglion cells lie either near the border of the ganglion cell layer (GCL) or near the border of the amacrine cell layer (ACL). The dendrites of the two types of parasol cells stratify closer to the center of the IPL, where they divide it into three approximately equal parts. There was no vertical overlap in the dendritic fields of a‐parasols and b‐parasols; they were always separated by at least 1 μm. The border between the a‐ and b‐ sublaminae of the IPL, defined in terms of this narrow gap between the stratification of the two parasol cell types, lies approximately at the center of the IPL. The dendritic‐field thickness for each of these types, on average, is no greater than 30% of the IPL thickness. At a similar location, there is no significant difference between the dendritic‐field diameters of the two parasol types or between those of the two midget types. As previously reported (Perry et al.: Neuroscience 12:1101–1123, '84) the dendritic fields of both parasol and midget ganglion cells are smaller in the nasal retina than at a position in the temporal retina equidistant from the fovea. Because dendritic‐field diameters prove to depend upon local ganglioncell density, the scatter in these diameters as a function of retinal eccentricity is due in part to the asymmetric distribution of ganglion cells. We have devised a measure, termed equivalent eccentricity, that allows data points of cells from regions having the same local ganglion‐cell density to be plotted at the same position on this scale. The use of this measure, rather than eccentricity per se, significantly reduces the scatter of dendritic‐field diameters. The dendriticfield diameters of parasol cells within the nasal quadrant of the retina are not fully brought into line with those of cells lying elsewhere in the retina. Consideration of the various factors involved leads to the suggestion that the ratio of parasol to midget ganglion cells may be higher in the nasal quadrant of the retina than in other portions. The residual scatter in the dendritic‐field diameters of parasols increases with equivalent eccentricity, in an approximately proportional manner. By using the upper and lower quartiles as a measure of scatter, the variation in the dendritic‐field diameter of parasol cells at any retinal location is about 21% of the median diamter of parasol cells in the local area. The dendritic fields of both midget and parasol ganglion cells are often elongated, and we determined the best‐fitting ellipse for each dendritic feild. Although all orientations of this ellipse were observed, there was a weak but significant tendency for the major axis of macaque midget or parasol cells to be directed toward a position near the fovea, as in the human retina (Rodieck et al.: J. Comp. Neurol. 233:115–132, '85). By contrast in the baboon retina, no preference for any retinal location was found for the dendritic‐field orientations of midget or parasol cells.