Studies on the control of myelinogenesis. 3. Signalling of oligodendrocyte myelination by regenerating peripheral axons.

Continuing from earlier work which demonstrated the peripheral axonal regulation of Schwann cell myelination, this study has investigated the possibility that a peripheral axon can stimulate oligodendrocyte myelination. To test this hypothesis, regenerating PNS axons were allowed to interact with uncommitted oligodendrocytes by transecting a rat peroneal nerve and inserting a segment of the autologous optic nerve between the cut ends. Grafts were maintained for 4-28 weeks and then examined by light and electron microscopy. A few regenerating peripheral myelinated nerve fibers penetrated the optic nerve graft. Some axons penetrated the outer margin of the graft, were myelinated by Schwann cells, and surrounded by astrocyte processes bordered by basal lamina. More centrally in the optic nerve graft, regenerating peripheral axons displayed myelin of CNS type. The outer myelin lamella abutted directly on the plasmalemma surface of surrounding astrocytic processes and was expanded focally to form a glial tongue. These observations demonstrate the experimental induction of central myelination by regenerating peripheral axons and suggest the existence of a common neuronal mechanism to stimulate myelin formation by both the Schwann cell and the oligodendrocyte.