Studies on the control of myelinogenesis. I. Myelination of regenerating axons after entry into a foreign unmyelinated nerve.

The proximal stump of a predominantly myelinated nerve (sternohyoid) was anastomosed in a tube to the distal stump of a largely unmyelinated nerve (cervical sympathetic) in order to explore the role of the axon in activating Schwann cells to produce myelin. The two nerves were examined after being united for periods between 3 and 15 weeks. At these times, regenerating myelinated and nonmyelinated fibres, originating from the proximal stump, were seen within the original fascicle in the distal stump of the sympathetic trunk. There was a significant (P less than 0.01) increase in the mean number of myelinated fibres in the anastomosed sympathetic nerves from the mean number of myelinated fibres in the normal sympathetic nerve. The histological and ultrastructural features of the anastomosed nerves are described and the differences between the morphology of the intratubal and extratubal regions are highlighted. The results of this study may indicate that the axon instructs the Schwann cell to produce myelin, but before this conclusion is reached, the origin of the myelinating cells has to be determined.