PATTERNS OF SCHWANN-CELL MYELINATION OF AXONS WITHIN THE SPINAL-CORD

Patterns of Schwann cell myelination of long-projecting axons in the spinal cord were studied. The goal was to determine if such axons arising from neurons whose somata and processes are normally confined to the central nervous system can interact effectively with Schwann cells, the myelinating cells of the peripheral nervous system. In one paradigm Schwann cells develop in the dorsal funiculi of the lumbar spinal cord subsequent to radiation-induced alterations in development of the glial populations. Light and electron microscopic evaluations were made in the region of the corticospinal tracts (CSTs), which in the rat occupy the base of the dorsal funiculi. At 90 days following irradiation, larger axons of these tracts (> 1.5 mum in diameter) were myelinated by Schwann cells, and smaller axons were ensheathed by them. In the second paradigm cultured Schwann cells were injected into the medial portions of the ventral funiculi at 13 days post-irradiation when the glial population was markedly reduced. Earlier investigations from this laboratory demonstrated that Schwann cells do not develop in the irradiated ventral funiculi, as they do dorsally. When placed in proximity to long-projecting axons in the medial portion of the ventral funiculi, the Schwann cells either formed compact myelin sheaths or ensheathed axons, depending upon their diameter. Fasciculation and presence of collagen were characteristic of this paradigm but were absent from the Schwann cell-occupied regions of the CSTs. This probably relates to the presence of fibroblasts in the injected cultures. These studies demonstrate that long-projecting axons of the spinal cord are capable of interacting with Schwann cells, irrespective of the mechanism by which these cells gain access to the central nervous system.