Starburst amacrine cells of the primate retina.

A group of readily recognized amacrine cells were observed in Golgi-impregnated and flat-mounted macaque, baboon, and human retinas. These cells had roughly-circular or oval dendritic fields that were narrowly stratified within the inner plexiform layer (IPL). Most of these cells stratified in the inner half (sublamina b) of the IPL, and they had their somata in the ganglion-cell layer; a few stratified in the outer half (sublamina a) of the IPL and had their somata in the amacrine-cell layer. Typically, a single dendrite issued from the soma, and, after passing for 10 microns or so, gave rise to five or more radiate processes. As these processes neared the edge of the dendritic field they branched, turned, and became varicose. Most showed no evidence of an axon, although a few had a short process extending inward, toward the optic-fiber layer. Dendritic-field diameters were about 100 microns near the fovea and increased to about 350 microns in the peripheral retina. Mean somal diameter also increased slightly from near the fovea (7.8 microns) to the periphery (8.7 microns). Although the primate cells are smaller, and there are some minor differences in the form of the dendritic fields, these cells appear to be morphologically equivalent to the starburst amacrines of the rabbit retina, whose counterparts have also been observed in the retinas of rats and cats. Presuming that these cells correspond to the choline acetyltransferase immunoreactive primate cells described by Mariani and Hersh (J. Comp. Neurol. 267:269-280, '87), their overlap factor is about ten for the type whose somata lay in the ganglion-cell layer and about 0.25 for those whose somata lay in the amacrine-cell layer.