Abnormal axoglial junctions in the myelin-deficient rat mutant

Although the myelin-deficient rat displays a gross deficiency of myelin in the CNS, occasional myelin segments of moderate thickness can be found. The typical lamellar pattern, consisting of alternating major dense and intermediate lines, is present in some regions of such segments, but the pattern is abnormal elsewhere. Redundant folds are common, and astrocyte processes occur frequently between the myelin sheath and axolemma or within the sheath. In the paranodal region, myelin lamellae occasionally form a palisade of 'terminal loops' against the axolemma, but discrete transverse bands occur only rarely and regular arrays of transverse bands over an extended length have not been seen. 'Reversed' paranodal junctions occur more often. Here the outermost layer of myelin, instead of being closest to the node, is furthest from it, and successive layers form terminal loops that approach the node progressively. These loops face away from the axon and do not contact it. At paranodal junctions of this kind only the innermost loop, or a small number of inner loops, adjoins the axolemma and, as a result, the size of the paranodal axoglial junction is markedly restricted. These defects in the paranodal junction may underlie the intrusion of astrocyte processes from either end of a myelin segment into the internodal periaxonal space and between myelin lamellae. Thus, one of the normal functions of the paranodal junction may be to restrict extension of astrocyte processes into and beneath myelin segments. The myelin-deficient rat also exhibits node-like specializations of the axolemma in association with glial cell processes.