Fine structure of the central myelin sheath in the myelin deficient mutant Shiverer mouse, with special reference to the pattern of myelin formation by oligodendroglia.

The central myelin sheaths of the recessive autosomal mutant mouse, Shiverer, were investigated by electron microscopy with special reference to the pattern of myelin formation. The results obtained may be summarized as follows: (1) the major dense line of myelin lamellae generally failed to be formed, but the axons were enclosed by tri-lamellar structures of about 14 nm in width, which were composed of one central membrane formed by fusion of outer leaflets of adjacent cytoplasmic sheets of oligodendroglia, corresponding to the intraperiod line, and two outer membranes which were identical with inner leaflets of adjacent cytoplasmic sheets and became highly electron-dense. However, major dense lines were sporadically observed; (2) the myelin sheath was not generally formed by spiralling of one cytoplasmic process of the oligodendroglia, but by enclosing an axon with several stratified layers of cytoplasmic sheets, some of which were certainly formed by the splitting of one process; and (3) aberrant myelination frequently occurred around the soma of oligodendroglia, not related to any axons.