Adaptation of the myelin sheath during axonal atrophy.

Distal axonal atrophy was produced by proximal constriction of the tibial nerve in the rabbit. Single fibres were teased from the distal tibial nerve, and a selected internode from each fibre was measured and then cut transversely for electron microscopy, so that axon area and perimeter, myelin lamellar number and periodicity, and myelin spiral length could be related to the length of the internode which had been sectioned. When sections from atrophic and control internodes of similar length were compared, there was no difference in the mean number of myelin lamellae or their periodicity in the two groups, in spite of a mean reduction in axon cross-sectional area of 60% in the atrophic group. Mean values for axon perimeter and myelin spiral length were reduced by 14%-15% in atrophic fibres, compared with controls. The reduction in mean myelin spiral length might seem to imply that myelin had been lost from the atrophic internodes. However, there was also an increase in the length of individual lamellae in the long axis of the internode, due to the irregular folding of the sheath, and it is uncertain whether any overall change in internodal myelin volume had occurred.