Motoneurons of the injured spinal cord of the adult dog can grow lengthy axons into an autologous peripheral nerve graft. A retrograde axonal tracing study.

To our knowledge, the capacity of injured spinal neurons to regenerate axons into peripheral nerve autografts has not yet been documented with axonal tracing methods in large adult mammals such as dogs. In the present study, one end of an autologous peripheral nerve graft (PNG), 10-15 cm long, was introduced dorsally into the lumbar (L4) spinal cord of six adult beagle dogs, thus producing a small focal lesion. The other end of the PNG was driven outside the spinal cord, then crushed and tied to nearby peripheral tissue with non-absorbable suture. Clinical examination of the operated animals was performed throughout the postoperative period. In five animals (out of six), the neurological deficit induced by the grafting procedure disappeared within five days. Four months after surgery, application of horse radish peroxidase (HRP) to the transected peripheral tip of the PNG led to the retrograde axonal labelling of about 30 lumbar neurons. The labelled cells, which had extended lengthy (up to 10 cm) axons into the PNGs, were mainly located in the vicinity of the intraspinal tip of the grafted nerve. Upon specific criteria, most of them were characterized as motoneurons. As the surgical procedure probably left the original stem axon of these neurons uninjured, it is suggested that axonal production from the labelled motoneurons might have arisen either from collateral axonal sprouts or even directly from the neuronal soma.