Research on Regeneration of the Optic Nerve and Functional Recovery of Retinal Ganglion Cells with Regenerated Axons

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Transected axons of retinal ganglion cells elongate a long distance along the autografted sciatic nerve in adult mammals. Morphological and physiological properties of retinal ganglion cells (RGCs) were examined in adult cats in order to compare the results with knowledge reported.The optic nerve of adult cats was transected and an autologous peripheral nerve was transplanted to the stump. Two months later RGCs that had regenerated their axons were retrogradely labeled with fluorescent dye injected into the graft. The more than 4,000 labeled cells, which were 3.0% to 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 a and g cells appeared comparable to normal, whereas the dendrites of some beta cells seemed … More to degenerate. Some "unclassified" cells could not be identified as alpha, beta, or gamma cells. The proportion of alpha,beta and other cells including gamma and unclassified in the regenerating cell population was evaluated by examining all the Lucifer yellow injected cells in local patches. The average proportion of alpha cells was 23.9%, about 6 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 axon into nerve grafts, rather than all the types equally regenerate their axons.Regenerated axons were labeled anterogradely with biocytin two months after transplanting the sciatic nerve to the axotomized optic stump, and myelin formation by Schwann cells was examined electron microscopically. Among 511 labeled axons in three cross-sectioned grafts, 96 fibers were myelinated. Proportions of myelinated fibers varied from 2.8% to 76%. Mean diameter. of myelinated fibers was 1.28 mum (range 0.71-2.47) and that of unmyelinated fibers was 0.76 mum (range 0.18-2.46). Mean ratio of inner versus outer diameters of myelin sheath (g-value) was 0.72 which was a little smaller than the value (0.8) in the optic axons of intact adult cats.A total of 102 units recorded from teased fibers of a nerve graft in one cat responded to stimulation of a restricted visual field. 28 units had large receptive fields and brisk phasic responses characteristic of Y cells ; 51 units had small receptive fields and brisk phasic-tonic responses characteristic of X cells ; 10 units had sluggish phasic or tonic responses characteristic of W cells.Some examples of each category had suppressive or antagonistic surrounds. 8 units could not be classified as Y,X or W cells. The proportions of Y (28%), X(50%) and others (22%) were similar to the proportions of alpha, beta and other obtained in the morphological study. Less