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In the peripheral nervous system (PNS) axons can be either myelinated or unmyelinated. Myelination means that the axon is insulated by surrounding layers of fatty membrane (myelin) produced by Schwann cells. These layers are continuous (like rolling up rope), but, due to the way they are formed, they often include small amounts of Schwann cellcytoplasm not displaced to the periphery during Schwann cell formation of the myelin. These are the so-called Myelin incisures or Schmidt-Lanterman clefts (a.k.a. Schmidt-Lanterman incisures, clefts of Schmidt-Lanterman, segments of Lanterman, medullary segments), which subdivide the myelinated axon into irregular portions.
They are histological evidence of the small amount of cytoplasm that remains in the inner layer of the myelin sheath created by Schwann cells wrapping tightly around a nerve.