The ability of developing spinal neurons to reinnervate a muscle through a peripheral nerve conduit is enhanced by cografted embryonic spinal cord.

The ability of neurons in the spinal cord of rats aged 5-12 days to reinnervate a muscle via a peripheral nerve bridge was examined and the possible influence of the cografted ED-12 embryonic spinal cord was tested. The soleus muscle was transferred paravertebrally and connected to the contralateral L4-L5 hemicord by its nerve. In some experiments embryonic spinal cord was grafted at the same level. Six to 12 weeks later fast blue and diamidino yellow were injected into the muscle or applied on the cut nerve bridge. The animals were perfused after 3-4 days and their spinal cords were examined using fluorescent microscopy, but retrogradely labeled neurons were only rarely seen. The embryonic spinal cord grafts survived well but had no influence on the outcome of these experiments. However, when neuromuscular implants from adult immunocompatible rats were used instead of the immature autologous ones, a variety of neurons including motoneurons extended their axons into the implants. The numbers of retrogradely labeled neurons were significantly higher in the spinal cords with embryonic grafts. These retrogradely labeled neurons were in the host's grey matter and only exceptionally in the grafts. Thus, the developing neurons can extend their axons outside the spinal cord into the implants of adult soleus muscle and nerve, but immature nerve-muscle implants fail to attract and/or support axonal outgrowth. The reinnervation potential of the host's spinal neurons was enhanced by cografting of embryonic spinal cord.