The presence of a large population of some 730,000 displaced amacrines is confirmed in the ganglion cell layer of the cat retina. These cells correspond to the microneurons of Hughes and Wieniawa-Narkiewicz (Nature 284:468–470,'80) and the bar-cells of Hughes (J. Comp. Neurol. 197:303–339, '81): a population of profiles of which the majority had previously been presumed to be glia (Stone: J. Comp. Neurol 12:337–352, '65; J. Comp. Neurol 180:753–772, '78; Hughes: J. Comp. Neurol 163:107–128, '75).
A sample of such nonganglion cells was identified by Nissl criteria in an area of retina subsequently subjected to serial sectioning and electron microscopy. Such cells form synapses with other processes in the inner plexiform layer. Members of each morphological subclass were found to bear synapses. In some instances, synapses occurred both onto and from the soma and processes of a cell, which is strong evidence for their being displaced amacrines, or preferably, “amacrines of the ganglion cell layer.”
In confirmation of their amacrine nature, it was established that the microneurons and bar-cells survive optic nerve section for up to 2.5 years. Ganglion cells underwent retrograde degeneration and completely disappeared in a much shorter time. Injection of kainic acid, a neurotoxin, into an eye whose optic nerve had been cut over 2 years previously resulted in the pyknosis of all morphologically classified microneurons and bar-cells without influence on conventional glial cells. These results further support the conclusion that microneurons and bar-cells are neurons and that they collectively form the displaced amacrine population of the cat ganglion cell layer.
The topographic distribution of the displaced amacrines resembles that of the ganglion cells in form; their density peaks at 4,500–5,000 cells mm−2 in the area centralis and falls to less than 1,000 mm−2 in peripheral retina. A ganglion cell distribution map based on the latest morphological criteria derived from this study confirms that there are 170,000 ganglion cells in the cat retina. Displaced amacrines form some 80% of the total neuron population of the cat ganglion cell layer.
The large population magnitude of these confirmed displaced amacrines implies their nonectopic origin and now provides a fresh insight into the ontogeny of the cat retinal ganglion cell layer.