Displaced retinal ganglion cells in normal frogs and those with regenerated optic nerves

We have analysed the number and spatial distribution of displaced retinal ganglion cells in the frog Litoria (Hyla) moorei. A series of normal animals was compared with one in which the optic nerve was crushed and allowed to regenerate. Ganglion cells were labelled with horseradish peroxidase (HRP) applied to the optic nerve, and retinae were examined as sections or whole mounts. We analysed separately ganglion cells with somata displaced to the inner nuclear (Dogiel cells, DGCs) and to the inner plexiform layer (IPLGCs). These findings were related to data for the orthotopic ganglion cells (OGCs). The mean number of DGCs in the normal series was 2,550 (+/- 281) and fell to 1,630 (+/- 321) after regeneration, representing a mean loss of 36%. This reduction was not significantly different from the mean loss of 43% from the OGC population in which mean values fell from 474,700 (+/- 47,136) to 268,700 (+/- 54,395). In both the normal and the regenerate series, DGCs were estimated to represent means of only 0.6% of the OGC population. Densities of DGCs were highest in the naso-ventral and temporo-dorsal peripheries; densities of both DGCs and OGCs were lower after optic nerve regeneration. We conclude that the factors which affect ganglion cell death during optic nerve regeneration, do so to similar extents amongst the DGC and the OGC populations. The IPLGCs were very rare in normal animals with a mean of 420 (+/- 95). However, their numbers increased after regeneration to a mean of 3,350 (+/- 690), estimated to be 1.2% of the OGC population. These cells normally favoured peripheral retina but became pan-retinal after regeneration. The primary dendrites of the majority of IPLGCs were oriented in the same direction as those of OGCs. We conclude that most IPLGCs were OGCs which had relocated their somata to the inner plexiform layer.