Loss and displacement of ganglion cells after optic nerve regeneration in adult Rana pipiens

After studying pathway selection in the brain of Rana pipiens during unilateral optic nerve regeneration, several frogs were allowed to survive for lengthy periods for use in the present investigation. Retina flat-mounts were prepared from both eyes at 42-50 weeks postoperation. In some cases, HRP was infiltrated into both optic nerves prior to sacrifice to assist in identifying retinal ganglion cells. All specimens showed reduced cell-densities in the ganglion cell layer of the eye that had sustained the nerve regeneration. In addition, many ganglion cells were displaced, abnormally, into the inner plexiform layer, and the normally-situated cells formed irregular bands and islands in some parts of the retina. Cell-counts showed an apparently time-related change in neuron number ranging from a loss of 41% compared with the unaffected eye at 42 weeks, to losses as great as 71% at 50 weeks. The probable number of displaced amacrine cells in the ganglion cell layer, assumed to be unaffected by the experiment, was estimated at a maximum of 16%. Possible factors underlying the loss and displacement of ganglion cells are discussed.