Number, distribution, and morphology of retinal ganglion cells with axons regenerated into peripheral nerve graft in adult cats

The optic nerve of adult cats was transected and an autologous peripheral nerve was transplanted to the stump. Two months later retinal ganglion cells (RGCs) that had regenerated their axons were retrogradely labeled with fluorescent dye injected into the graft. The more than 4000 labeled cells, which were 3.0–4.3% of total ganglion cell number in the intact retina, were distributed in a high-density area elongating horizontally, in a pattern similar to that observed in the intact retina. The dendritic morphology of RGCs with regenerated axons was examined after intracellular injections of HRP or Lucifer yellow in vitro. The morphology of alpha- and gamma-cells appeared comparable to normal, whereas the dendrites of some beta-cells seemed to degenerate. Some “unclassified” cells could not be identified as alpha-, beta-, or gamma-cells. The main dendrites of most unclassified cells were thin and less ramified than alpha- and beta-cells. Some unclassified cells had a vacant area in their dendritic fields, suggesting degenerative change in dendrites. The distal dendrites of some unclassified and gamma-cells had many fine processes that probably regrew during axonal regeneration. The proportion of alpha-, beta-, and other cells including gamma- and unclassified cells in the regenerating cell population was evaluated by examining all of the Lucifer yellow- injected cells in local patches. The average proportion of alpha-cells was 23.9%, about six times higher than in intact retinas. The average proportion of beta-cells was 50.3%, slightly higher than in intact retinas. The remaining cells (25.7%) could not be categorized as alpha or beta; this proportion is about half that found in intact retinas. These findings suggest that alpha-cells have the highest capacity to regenerate their axons into nerve grafts, rather than all types equally regenerating their axons.