How do axons control myelin formation? The model of 6-aminonicotinamide neuropathy.

Injection of 6-aminonicotinamide into young rats produces a peculiar neuropathy characterized by selective swelling and disruption of the layer of Schwann cell cytoplasm lining the inner surface of the myelin sheath. This layer increases greatly in volume, compressing the axon and distending the myelin sheath. Morphometry of such swollen fibers discloses that the amount of myelin in the distended sheaths is considerably greater than would correspond to the size of the axons, even if axonal compression is accounted for. The data favor the concept that sheath growth is stimulated by non-specific distension of the myelin sheath from inside.