MORPHOLOGY OF HUMAN RETINAL GANGLION-CELLS WITH INTRARETINAL AXON COLLATERALS

Ganglion cells with intraretinal axon collaterals have been described in monkey (Usai et al., 1991), cat (Dacey, 1985), and turtle (Gardiner st Dacey, 1988) retina. Using intracellular injection of horseradish peroxidase and Neurobiotin in in vitro whole-mount preparations of hum an retina, we filled over 1000 ganglion cells, 19 of which had intrare tinal axon collaterals and wide-field, spiny dendritic trees stratifyi ng in the inner half of the inner plexiform layer. The axons were smoo th and thin (similar to 2 mu m) and gave off thin (<1 mu m), bout-on-s tudded terminal collaterals that extended vertically to terminate in t he outer half of the inner plexiform layer. Terminal collaterals were typically 3-300 mu m in length, though sometimes as ions as 700 mu m: anc. were present in clusters: or as single branched or unbranched var icose processes with round or somewhat flattened lobular terminal bout ons 1-2 mu m in diameter. Some cells had a single axon whereas other c ells had a primary axon that gave rise to 2-4 axon branches. Axons wer e located either in the optic fiber layer or just beneath it in the ga nglion cell layer, or near the border of the ganglion cell layer and t he inner plexiform layer. This study shows that in the human retina, i ntraretinal axon collaterals are associated with a morphologically dis tinct ganglion cell type. The synaptic connections and functional role of these cells are not yet known. Since distinct ganglion cell types with intraretinal axon collaterals have also been found in monkey, cat , and turtle, this cell type may be common to all vertebrate retinas.