Incisures of Schmidt Lanterman
With the light microscope oblique clefts can often be seen in the myelin sheath (Fig. 1.17). These clefts are called the Schmidt Lanterman clefts. Many workers have considered these clefts to be artefacts. However, EM studies show the clefts to be areas where adjoining layers of Schwann cell plasma membrane (forming the myelin sheath) have failed to fuse (Fig. 1.18) leaving (i) a layer of Schwann cell cytoplasm that passes spirally around the axon in the position of the period line; and (ii) a spiral space through which the perineural space communicates with the periaxonal space in the position of the intraperiod line. This space provides a path for passage of substances into the myelin sheath and axon, from the space around the nerve fibre. The clefts enlarge greatly when a nerve fibre undergoes Wallerian degeneration.
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