Neural repair of the injured spinal cord by grafting: comparison between peripheral nerve segments and embryonic homologous structures as a conduit of CNS axons.

To study the differences between peripheral nerve (PN) and embryonic homologous CNS structures as a conduit for CNS axons, spinal cord segments in neonatal rats were removed at the mid-thoracic level and PN or embryonic spinal cord (ESC) segments were grafted into the vacancy. Neural connections across the graft were examined by the anterograde and retrograde tracing methods. Anterogradely labeled pyramidal tract fibers entered the PN segment meanderingly and were dispersed; most stopped at the caudal end of the graft. Although a small fraction of fibers re-entered the host spinal cord, they terminated near the graft-host interface without further extension. By contrast, similarly labeled fibers entering the ESC segments crossed the graft and extended further to reach the lumbar segments. The fibers were defasciculated in the graft but became fasciculated and oriented dorsally near the caudal end of the graft, and descended in the normal path. Consistent with these findings, the retrograde tracing study revealed that in animals with ESC segment grafts but not in those with PN segment grafts, many neurons in the upper brain structures were labeled with Fast Blue that was injected into the lumbar enlargement. The difference between the two kinds of graft as a conduit for CNS axons is likely to be due to differences in matching growing axons and their guidance cues through the graft-host interface.