Alpha and delta ganglion cells in the rat retina.

In the rat retina a distinctive class of large ganglion cell was demonstrated by intracellular staining with Lucifer Yellow and with reduced silver staining. They are referred to as alpha cells because they resemble the alpha cells of other mammalian retinae. A second class, called delta cells, is also described. Both classes belong to the type I group defined by Perry (Proc. R. Soc. Lond. [Biol.] 204:363-375, '79). The dendritic trees of both classes stratify in either an inner or outer lamina of the inner plexiform layer which presumably corresponds to an on/off dichotomy in the response to light. Rat alpha cells constitute 2-4% of all ganglion cells, and their density, size, and detailed morphological appearance change with retinal location. Inner and outer stratifying alpha cells of the rat show significant differences compared to those of other mammals. In central retina (at the large cell density maximum) the densities and dendritic field sizes of inner and outer alpha cells are approximately equal. However, in peripheral retina outer alpha cells are up to three times more numerous and have dendritic field areas only one-third the size of those of the inner alpha cells. The maximal density is about 110 alpha cells/mm2; peripheral densities are about 30/mm2. The smallest central dendritic field diameters are 220 microns. Peripheral dendritic field diameters are 350-550 microns for outer and 570-790 microns for inner alpha cells. Each subpopulation is distributed in a regular mosaic, and the territorial arrangement of the dendritic fields provides a homogeneous coverage of the retina. The dendritic coverage is three- to 3.6-fold for each subpopulation, irrespective of their other quantitative differences. Eccentricity-dependent receptive field sizes of the alpha cells are predicted from the morphological data.