Axon sprouting into segments of rat spinal cord adjacent to the site of a previous transection.

Transection of the spinal cord is adult Long-Evans Hooded rats is followed by the formation of a connective tissue matrix in the lesion site and the rapid erosion of the neural elements above and below this zone particularly within the dorsal white columns. In the period between 15--45 days after operation two significant events begin to occur. First the injured surfaces of the divided cord become invested by a glial limiting membrane (glia limitans) and, concomitantly, large numbers of axons ensheathed by Schwann cells sprout into the scar matrix and along the eroded dorsal column region. The injured surface of the spinal cord is highly irregular with deep, collagen-filled rifts into which the sprouting axons may probe and penetrate into the adjacent normal neuropil. Electron microscopic examination generally reveals the interposed glia limitans and that these fibres are usually restricted to the peripheral environment. However, as some axons approach the reconstituting glia limitans, they are enveloped by an astrocytic cytoplasmic process which may either displace the Schwann cell or encompass it together with the enclosed axons. This last phenomenon appears to precede the entry of some axons into the neuropil and suggests that the glia limitans may not necessarily represent an impenetrable barrier to the passage of regenerating axons into the CNS. Apparent maintenance of most of these fibres for periods of up to 3 months may suggest that viable, functional synapses were established upon available neuronal elements, but clear evidence of this could not be documented.