Electron microscopic study of the interaction of axons and glia at the site of anastomosis between the optic nerve and cellular or acellular sciatic nerve grafts

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Abstract
The interactions between retinal ganglion cell (RGC) axons and glia at the site of optic nerve section and at the junctional zone between optic nerve and cellular or acellular peripheral nerve (PN) grafts have been studied electron microscopically. After transection, RGC axons, accompanied by processes of astrocyte cytoplasm, grew out from the proximal optic nerve stump into the scar tissue that developed between proximal and distal stumps. However, axons failed to cross the scar, and none entered the distal stump. By 3 days post lesion (DPL), bundles of RGC axons, accompanied by astrocytes and oligodendrocytes, grew out from the proximal optic nerve stump into the junctional zone between optic nerve and either type of PN graft. The bundles of RGC axons and growth cones that grew towards acellular PN grafts degenerated within 10–20 DPL; by 30 DPL a small number of axons persisted within the end of the proximal optic nerve stump. No axons were seen within the acellular PN grafts. These results suggest that reactive axonal sprouting, axon outgrowth and glial migration from the proximal optic nerve stump are events that occur during an acute response to injury, and that they are independent of the presence of Schwann cells. However, it would appear that few axons entered either scar or junctional zone unless accompanied by glia. There was little evidence that axon outgrowth was laminin-dependent. The bundles that grew towards cellular PN grafts encountered cells that we have identified as Schwann cells within the junctional zone: the axons in these bundles survived and entered the cellular grafts. Schwann cells migrated into the junctional zone from the cellular PN graft. It is probable that Schwann cells facilitated RGC axon entry into the graft directly by both cell contact and the secretion of neuronotrophic factors, and indirectly by modifying the CNS glia in the junctional zone.