Wide-spreading terminal axons in the inner plexiform layer of the cat's retina: evidence for intrinsic axon collaterals of ganglion cells.

Extracellular, iontophoretic injections of horseradish peroxidase (HRP) were made directly into the cat's retina. The retinas were processed with the cobalt-enhanced diaminobenzidine method and prepared as whole-mounts. These retinas reveal HRP-filled axons that extend widely and terminate within the inner plexiform layer. The axons are morphologically distinct from ganglion and amacrine cell dendritic trees that were retrogradely labelled from the same injection sites. The axons are long and straight, approximately 1 micron in diameter, and in some cases can be traced for several millimeters in the inner plexiform layer. Each axon gives rise to many short, terminal branches that extend, on average, 100 microns from the parent axon and bear clusters of boutons. The terminal branches are widely spaced so that the bouton clusters are distributed in small, isolated patches along the length of the axon. Bouton clusters vary in size and contain from two to over 100 loosely arranged boutons. Single boutons are frequently large, up to 3 microns in diameter. In one case a terminal axon was traced to its origin from the parent axon of an HRP-filled ganglion cell. It is suggested, therefore, that these axons are intrinsic to the retina and originate as primary collaterals of ganglion cells.