The use of embryonic spinal cord grafts to replace identified motoneuron pools depleted by a neurotoxic lectin, volkensin.

The viability of solid pieces of embryonic spinal cord grafted into adult spinal cords that had been depleted of motoneurons by a neurotoxic lectin, volkensin, was investigated. The possibility that the grafted neurons will extend their axons via the degenerated ventral roots and reinnervate the denervated muscles was also studied. Cells in the embryonic spinal cord were prelabeled with bromodeoxyuridine and grafted into the host cord 2 or 3 weeks after volkensin treatment. Six to 24 weeks later the fluorescent dyes Fast Blue and Diamidino Yellow were applied to the peripheral nerve or muscles which were expected to be reinnervated by neurons from the graft. The grafted cells were visualized by bromodeoxyuridine immunocytochemistry. Although the transplants survived, no neurons of graft origin reinnervated the muscles via the ventral roots of the host spinal cord. Therefore, in another series of experiments axons from grafted and host neurons were guided toward an implanted extensor hallucis longus (EHL) muscle via its motor nerve. The EHL muscle was removed from either healthy or volkensin-affected hindlimb of the host, placed paravertebrally, and the proximal end of its motor nerve inserted into the spinal cord at the site of the graft. In these experiments, neurons of graft origin reinnervated the paravertebrally placed muscle implant via the muscle's own nerve. However, significantly fewer neurons were observed when volkensin-affected nerve-muscle implants were used. To establish whether grafted spinal cord neurons are necessary for reinnervation of skeletal muscles, in separate experiments embryonic neocortical tissue was transplanted into the motoneuron-depleted spinal cord. These grafts were connected to healthy nerve-muscle implants. In this case little reinnervation of the muscle occurred. These results suggest that, though volkensin-affected peripheral nerve is not a good conduit for regenerating axons, other inhibitory effects located in the CNS may be responsible for the failure of growing embryonic neurons to reach their peripheral targets via the existing anatomical routes. Experiments with grafted neocortical tissue indicate that good reinnervation of implanted muscles can only be achieved by using homologous embryonic tissue.