Morphological comparisons between outer and inner ramifying alpha cells of the albino rat retina

The somato-dendritic morphologies of large ganglion cells were studied by intracellular injections of Lucifer yellow in perfused in vitro preparations of the albino rat retina. The ganglion cells were prelabeled with retrogradely transported granular blue or labeled with acridine orange dropped into the perfusate of in vitro preparations. After the dye injection, somato-dendritic morphologies were successfully studied for 210 cells, the majority of which had a large soma more than 20 microns in diameter and were identified as alpha cells. According to the level of dendritic extensions within the inner plexiform layer (IPL) these alpha cells were further classified into inner ramifying (inner) and outer ramifying (outer) cells. Both qualitative and quantitative observations led us to conclude the following: 1) The outer cells have a spherical soma with relatively few primary dendrites, while inner cells have a large polygonal soma with more primary dendrites. 2) The dendritic field of inner cells was always larger than that of outer cells at every retinal location. The dendritic field diameter tended to increase as a function of retinal eccentricity from the optic disk, the tendency being more clear among inner cells. 3) The dendrites of outer cells branch more frequently in the proximal part of the dendritic field while those of inner cells branch more distally. 4) Total dendritic length of outer cells increases linearly with eccentricity whereas that of the inner cells does not change much irrespective of retinal location.