Long axon growth from embryonic neurons transplanted into myelinated tracts of the adult rat spinal cord.

Suspensions of embryonic mouse hippocampal neurons were micro-transplanted into the high cervical region of the corticospinal tracts and the ascending dorsal columns of immunosuppressed adult rat hosts. The mouse specific marker, M6, showed that the transplanted neurons grew long, straight, and uniform axons which passed both rostrally and caudally in a narrow beam within the tract occupied by the transplant. The donor axons did not fasciculate with each other, but were intermingled singly with the host myelinated axons, parallel to them and to the host interfascicular glial rows. The donor axons extended at about 0.5 mm per day for the first 6-10 days, after which they slowed down, gradually reaching a maximum distance of around 8 mm from the grafts by 6-7 weeks. Beyond this time M6 ceased to be an effective marker. The presence of the donor axonal projections was confirmed by retrograde cellular labelling of the donor cells from injections of horseradish peroxidase caudal to the transplants in the same host tract. To test whether the donor embryonic hippocampal axons would be attracted to neuropil containing appropriate postsynaptic sites, embryonic rat hippocampal grafts were placed caudal to the mouse grafts. When placed where they could make direct contact with the projection, the rat grafts became M6 positive, suggesting that they had been innervated by mouse axons. But even when placed directly within the beam of donor projections, the rat grafts did not prevent large numbers of mouse axons from continuing caudally in the host spinal tracts. When the rat grafts were placed to one side of the beam of donor projection, the long projecting mouse axons did not deviate from their course. Thus the long ascending and descending myelinated fibre tracts of the adult spinal cord provide a permissive environment for the rapid growth of axons from embryonic donor neurons from a region not normally projecting to the spinal cord. The route taken by these axons is determined by the internal structure of the host tracts. Donor interfascicular axons are able to pass, without deviation, through or beside tissue containing appropriately matched postsynaptic targets.