(3H) glycine‐accumulating neurons of the human retina

Isolated human retinas were incubated in physiological saline containing micromolar (3H) glycine. The types, distributions, and synaptologies of glycine‐accumulating neurons were determined by light and electron microscope autoradiography. Two types of amacrine cells were discriminated on the bases of number of processes descending into the inner plexiform layer, density of label in light‐microscope autoradiographs, size, and synaptic features: (1) Gly 1 amacrine cells have moderate labeling, several oblique dendrites arising from the soma, and electron lucent synaptic terminals containing large presynaptic specializations, and (2) Gly 2 amacrine cells have dense labeling, a single proximal dendrite, and moderately electron‐dense terminals with small presynaptic specializations. Gly 1 amacrine cells constitute ∼ 15% and Gly 2 amacrine cells ∼ 38% of all cells in the amacrine cell layer. The laminar distribution of label in the inner plexiform layer was measured by scanning microdensitometry, which provided a format for categorizing types of synaptic contacts. Many features of glycine‐accumulating amacrine cell contacts were similar to those of cat AII/Gly 2 amacrine cells: a diffuse yet bisublaminar distribution of label, concentration of synaptic output in sublamina a , rod bipolar cell input in sublamina b and gap junctions in mid‐inner plexiform layer involving labeled cells. The evidence seems to indicate that human Gly 2 amacrine cells and cat AII/Gly 2 amacrine cells are homologous cell types. Finally, some cone bipolar cells were labeled with (3H) glycine.