Morphology of bipolar cells labeled by DAPI in the rabbit retina

The morphology, distribution, and coverage of certain cone bipolar cell types were investigated in rabbit retina. Brief in vitro incubation of isolated rabbit retina in the fluorescent dye 4,6-diamino-2-phenylindole labeled only a few cell types in the inner nuclear layer. Intracellular injection of Lucifer Yellow into these types showed them to be horizontal cells and cone bipolar cells. All stained bipolar cells ramified in sublamina a of the inner plexiform layer (IPL) and formed three classes. Two types ranged from 20 to 60 microns in diameter in both plexiform layers; the other large bipolar cell was 40-70 microns in diameter in the outer plexiform layer (OPL) and up to 150 microns in diameter in the IPL. The brightest type was narrowly stratified in the outer portion of sublamina a. Its density increased from about 500 cells/mm2 in the periphery to about 2,500 cells/mm2 in the visual streak. Staining of neighboring cells of this type showed that processes in the IPL rarely crossed, but often converged at a common site so as to impart a "honeycomb" appearance to a single sublayer of retina. The other small bipolar cell was similar in density and coverage, but stratified diffusely throughout sublamina a. The large bipolar cell stratified narrowly in the distal portion of sublamina a and was more sparsely distributed. Whether determined by staining adjacent cells or by density vs. area calculations, coverage in the OPL approached 1 for each type, as did coverage in the IPL for the two types with narrow fields.