Polyaxonal myelination in developing dystrophic and normal mouse nerves.

Myelin-forming Schwann cells in the peripheral nervous system characteristically surround and myelinate only single axons. Polyaxonal myelination is an anomaly of this one-to-one relationship whereby one normal-appearing Schwann cell myelinates multiple axons. We examined the ventral roots and the proximal sciatic and posterior tibial nerves of developing normal mice and of dy2J/dy2J dystrophic mice with proximal failure of myelination. Polyaxonal myelination was a rare feature in normal nerves. Examples of polyaxonal myelination were observed six times more often in dystrophic than in normal mice and were most abundant in proximal sciatic nerves. Polyaxonal myelination could result from either an axonal or a Schwann-cell abnormality, or it may be the nonspecific response of uncommitted Schwann cells to an early failure of myelination.