Factors influencing survival of transplanted embryonic motoneurones in the spinal cord of adult rats.

The survival of transplanted embryonic motoneurones in the initially intact spinal cord of adult rats was studied and compared to that previously observed in the motoneurone-depleted cord. Embryonic (ED 11-12) spinal grafts prelabeled with 5-bromo-2'-deoxyuridine (BrDUr) were placed in the intact lumbar cord of the hosts. To provide a target for grafted embryonic motoneurones and to guide their axons to it, the contralateral extensor hallucis longus (EHL) muscle with its nerve attached was transferred paravertebrally. The nerve stump was implanted in the cord at the site of transplantation. Eight to 14 weeks later BrDUr-labeled motoneurone-like cells had migrated outside the grafts into the host's neuropil, preferentially into the anterior horn. Following injection of HRP into the implanted EHL muscle 6-17 weeks after transplantation a few retrogradely labeled motoneurones were seen in the host's anterior horn around the grafts. The lumbar cord of the rats with neuromuscular implants but without embryonic grafts had no retrogradely labeled cells. However, most animals, both with and without embryonic grafts, had retrogradely labeled motoneurones in the thoracic cord, which may contribute to the reinnervation of the implanted muscle. Thus, although some embryonic motoneurones can survive, migrate into the proper location, and probably innervate a host muscle when transplanted into the intact spinal cord, their number was significantly fewer than that in the motoneurone-depleted cord. The results show that reduction of the host's motoneurone pool increases chances of their survival.