Schmidt-Lantermann cleft

Schmidt-Lantermann clefts are structures where the cytoplasmic surfaces of the myelin sheath have not compacted to form the major dense line and therefore contain Schwann or glial cell cytoplasm (Fig. 4-9). They are common in peripheral myelin but rare in the CNS. These inclusions of cytoplasm are present in each layer of myelin. The clefts can be visualized in the unrolled myelin sheet as tubes of cytoplasm similar to the tubes making up the lateral loops but in the middle regions of the sheet, rather than at the edges (Fig. 4-9). 

Figure 30.4. Myelinating Schwann cell of PNS. The same Schwann cell is shown unwrapped (top), in longitudinal section (middle), and in cross section (bottom). Note the channels of cytoplasm (Schmidt-Lantermann clefts) and the large expanses of compacted cell membranes (myelin). (These drawings are not to scale.) (From Raine, Morphology of myelin and myelination. In P. Morell (Ed.). Myelin, 2nd ed., Plenum Press, New York, 1984.)...

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