Myelin deficiency (mld) is an autosomal recessive mutation in mice and is considered to be allelic to the shiverer (shi) mutation. Mld mice are characterized by hypomyelination of the central nervous system (CNS). They show typical symptoms such as tremor, tonic convulsion and ataxic movement. Subcellular fractionation of the CNS revealed that the MBP bands were greatly decreased in the P2A (myelin) fraction and the total content of myelin basic protein (MBP) was much lower than that in the control in all parts of the CNS. Sections from mld mice were examined by immunohistochemical tests with MBP antiserum, and a mosaic expression of MBP was found in the myelin of the mld mice. Since the major dense line is considered to be composed mainly of MBP, we investigated the myelin of mld mice by electron microscopy and found that there were 3 types of myelin: (1) a normal type compact myelin with a major dense line, (2) a shiverer-type myelin with no major dense line, and (3) a mixed-type myelin, in which within a myelin lamella the major dense line abruptly changes to cytoplasm of oligodendrocytes.
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