Horizontal cells and cone photoreceptors in primate retina: a Golgi-light microscopic study of spectral connectivity.

The relationship of primate horizontal cells (HC) to cone pedicles was assessed by superimposing the cone inner segment mosaic upon Golgi-impregnated HC dendritic terminal clusters in a light microscope (LM) study. The HI, HII, and HIII types of HC were identified, hand-drawn, photographed, and analyzed by computer graphics methods. Blue cone (B-cones) inner segments and their projected pedicles were distinguished from red (R-cones) and green (G-cones) cones on morphological criteria. Thus the inclusion or avoidance of B-cone pedicles by the various HC types' dendritic terminal clusters establishes whether there is any color specificity to their connections. In addition, we made counts of the number of dendritic terminals in the clusters going to cone pedicles in the various HCs' dendritic fields and plotted these against distances the cone pedicles lay from the cell body. In this way we could evaluate the weighting of spectral type of cone input. In general, the three HC types made the majority of their dendritic contacts with cones lying closest to their cell bodies at the center of their dendritic fields. However, HI and HIII cells, with their distinct terminal clusters, did not contact all the centrally located cones uniformly. They either avoided completely (HIII cells) or made only sparse dendritic connections (HI cells) with certain cones. The avoided or sparsely innervated cones were identified as B-cones. HII cells, on the other hand, with their more profuse and diffusely branched dendrites, appeared to contact all overlying cone pedicles and, in contrast to HI and HIII cells, directed a relatively larger number of dendrites to B-cone positions. Axon terminals of HII cells appeared to contact B-cones exclusively.