Characterization of the sprouting response of axon-like processes from retinal ganglion cells after axotomy in adult hamsters: A model using intravitreal implantation of a peripheral nerve

Abstract

Peripheral nerves provide a favourable environment for damaged CNS axons to sprout and regenerate. It has also been demonstrated that retinal ganglion cells respond to a peripheral nerve segment grafted to the retina by emitting axon-like processes from the somatodendritic compartment into the graft. The factors influencing the pattern of sprouting of axotomized retinal ganglion cells were explored in this study by implanting a short segment of peripheral nerve, which did not come into contact with the retina, into the vitreous body of an eye whose optic nerve was concurrently crushed. Silver staining was used to assess the morphology of the retinal ganglion cells which underwent sprouting. Some retinal ganglion cells were induced to sprout axon-like processes; these emerged primarily from dendrites and less frequently from the soma or intraretinal axon. Implantation of a nonviable graft (freezed-thawed) elicited only minimal sprouting. These results suggest that diffusible factors secreted by cells in the graft are a possible stimulus to sprouting in axotomized retinal ganglion cells. Examination of the pattern of dendritic sprouting indicates that sprouting was most intense (in terms of number of sprouts per cell) at early times post-axotomy. Moreover, a differential pattern of development of sprouts arising from individual primary dendrites of the same cell was observed; sprouts tend to arise from all primary dendrites initially but as the post-axotomy time increased, retraction of sprouts from some primary dendrites occurred. Concomitant with this retraction, however, there was an increase in the number of sprouts on those primary dendrites which were still in the active phase of sprouting. Selective stabilization of sprouts by extrinsic factors may account for this phenomenon. Changes in the area and outline (irregularity) of the somata of retinal ganglion cells with sprouts from two weeks to two months after optic nerve crush could be correlated temporally with the intensity of sprouting from the dendritic tree, suggesting that during sprouting, intrinsic mechanisms coordinate the responses of different cellular compartments. In contrast to extensive ectopic sprouting of axotomized retinal ganglion cells in the presence of an intravitreal graft, when a long peripheral nerve segment is grafted to the cut optic nerve, there is extensive axonal regeneration into the graft from retinal ganglion cells, most of which did not exhibit ectopic sprouting. Thus, a hierarchy of sprouting sites within a neuron seems to exist, with the damaged axonal tip being the most favoured site, followed by the dendrites, and then the intraretinal axon. The soma appears to be the least preferred compartment for sprout emission.