Morphological Classification of Bipolar Cells of the Primate Retina.

Bipolar cells were studied in Golgi - Colonnier-stained whole mounts of macaque monkey retinae. A piece of retina, at 6 - 7 mm eccentricity, was particularly well stained for the analysis of the different bipolar cell types. Many midget bipolar cells were encountered and the dichotomy into flat and invaginating midget bipolars was confirmed. Six types of diffuse cone bipolar cell are distinguished. They differ in their dendritic branching pattern, in the number of cones contacted-usually between five and ten-and in the shape and branching level of their axons. The size, shape and stratification of the axons were found to be the most reliable distinguishing features for classifying diffuse cone bipolar cells. The stratification of the axons in the inner plexiform layer (IPL), whether closer to the amacrine or ganglion cells, was used to name diffuse cone bipolar cells in the order DB1 to DB6. Blue cone and rod bipolar cells were confirmed as distinct types. Axon terminals of diffuse cone bipolars were found to tile their sublamina of the IPL in a territorial manner. From this the density of each type could be estimated, and it is shown that a single cone is likely to be in contact with as many as 15 individual diffuse bipolar cells, as well as two midget bipolars. The diffuse bipolar cells observed contact all the cone pedicles in their dendritic fields. It is, therefore, unlikely that they carry a chromatic signal into the inner retina. The presence of many midget bipolar cells, which make contact with one cone pedicle only, suggests that midget bipolars provide chromatic input to ganglion cells in peripheral retina as well as in the fovea. The data show that the P- and M-cell pathways of the primate visual system are, to a significant extent, already anatomically discrete at the photoreceptor synapse.