Studies on the ‘incisures’ of schmidt and lanterman

ABSTRACT Since its initial description over a century ago the ‘incisure’ of Schmidt-Lanterman has usually been regarded as a histological artifact, or a post-mortem change. With the establish ment of in vivo discrete zones in the myelin sheath, an ultrastructural study was undertaken and correlation attempted between their electron-microscopical appearances and their range of states seen in vivo and produced in vitro. It was demonstrated that under conditions of hypotonicity the incisure dilates and further that it closes in hypertonic solutions. Such behaviour is reversible within certain limits. Ultrastructurally, the incisural region appears as a highly complex zone in which the repeat characteristics of the compact myelin sheath are drastically altered. At the incisure, the major dense line of the myelin splits to enclose a granular Schwann cytoplasm which frequently contains a single central helical microtubule, dense bodies and small vesicles. The intraperiod line also splits, dilatation of the incisure involving further widening of this intraperiod line gap beyond the normal confines of the incisure. Desmosomoid stacks are a common feature, particularly of the outer layers of incisures of both mature and immature fibres. Surface invaginations, vesicles, longitudinal microtubules, filaments, mitochondria and granular endoplasmic reticulum are typical constituents of peri-incisural Schwann cytoplasm. The incisure may be related to fibre growth, remodelling and degeneration; to dynamic changes of internodal contour—possibly peristaltic; or may operate as a fluid-satellite cell-neuron interchange pathway.