The role of Schmidt-Lanterman incisures in Wallerian degeneration. I. A quantitative teased fibre study.

It is conventionally accepted that during the early stages of Wallerian degeneration of myelinated peripheral nerve fibres Schmidt-Lanterman incisures represent the sites at which the myelin sheath, together with enclosed axoplasm, is segmented into myelin ovoids. This mechanism is considered by some authors to be facilitated by the progressive intercalation of additional incisures in order to allow the later division of primary ovoids. We have demonstrated that this reported increase in the number of incisures is a misinterpretation of the changes occurring. By 36 h after crush of the rat sural nerve most myelinated fibres showed segmentation at incisures to form myelin ovoids. At 12 h and 24 h after crush, however, no ovoids were apparent and the number of incisures present was determined from teased fibres by light microscopy using oil immersion. There was no increase in the number of incisures either internodally or paranodally at 12 h and 24 h compared with a normal control population of fibres. However at 12 h, and to a greater extent at 24 h, incisures were more readily apparent than in normal fibres. It is likely, therefore, that previous reports have confused an increase in the number of incisures with an increase in their perceptibility resulting from their progressive dilatation.