Prevention of optic nerve regeneration in the frog Hyla moorei transiently delays the death of some ganglion cells.

We have previously reported that a proportion of ganglion cells die during optic nerve regeneration in the adult frog Hyla moorei (Humphrey and Beazley, '85). Here we assess the effect of preventing optic nerve regeneration on this cell loss. The optic nerve was crushed unilaterally and regeneration was allowed to progress unimpeded in one experimental series but was prevented by ligating or capping the nerve in another. We estimated total cell numbers in the ganglion cell layer from cresyl-stained wholemounts, comparing each experimental retina with its unoperated partner. At 70-78 days postcrush, mean cell numbers had fallen by 31.5% for frogs with unimpeded regeneration (N = 9), a significantly greater reduction than the 21.5% (N = 8) loss for the impeded regeneration series (p less than 0.001). Thereafter, cell numbers were stable for frogs with unimpeded regeneration. Cell death continued in the series with impeded regeneration, and losses exceeded those of frogs with unimpeded regeneration from 110 days postcrush. When regeneration was impeded, ganglion cell somas underwent an intense cell soma reaction and became arranged in rows radiating from the optic nerve head. Our findings indicate that some ganglion cells are transiently spared when regeneration of their axons is prevented. The abnormally extensive contacts formed between somas may delay ganglion cell loss. However, the eventual death of most ganglion cells shows them to be target-independent in the long term.