The role of Schmidt-Lanterman incisures in Wallerian degeneration. II. An electron microscopic study.

The electron microscopy of changes at Schmidt-Lanterman incisures in Wallerian degeneration has been described only briefly previously. We have demonstrated that the changes up to 36 h after nerve crush are chiefly peri-incisural. At 12 h and 24 h 'incisural dilatation' consisted of an intraperiod line separation of peri-incisural myelin lamellae, which began among inner (adaxonal) lamellae extending later to outer (abaxonal) lamellae. The incisure itself showed little or no change. At 36 h, ovoid formation was apparent in most fibres. The sites of fibre cleavage to form ovoids occurred adjacent to incisures at the focal regions of myelin lamellae separation. Even within ovoids the incisures themselves remained intact at 36 h. The fine structural changes at incisures following nerve crush provide an understanding of the increased perceptibility of incisures by light microscopy during early Wallerian degeneration.