MYELIN STRUCTURE IN PROTEOLIPID PROTEIN (PLP)-NULL MOUSE SPINAL-CORD

Fixed preparations of proteolipid protein (PLP)-null mouse spinal cord show myelin sheaths which in some regions consist of typical alternat ing major dense lines (MDLs) and intermediate lines (ILs) with a repea t period of 10.3 nm. More commonly, the lamellar structure consists of what appears to be a single population of dense lines, having a repea t period of 5.2 nm. These apparently equivalent lines are, however, so metimes distinguishable as MDLs or ILs based on continuity with cytopl asmic or extracellular regions. Focal separations of lamellae at the i ntermediate line are common. MDLs too may be replaced focally by cytop lasmic pockets, sometimes in the same quadrant over several lamellae, resembling Schmidt-Lanterman clefts. Occasional densities reminiscent of the ''radial component'' can be seen. Otherwise; this structure, wh ich is prominent in wild-type myelin, is conspicuously absent. Redunda nt folding of some lamellae but not others may occur in the same sheat h. These observations conform to those made previously on the isolated myelin segments that occur in the myelin-deficient rat central nervou s system (CNS), which also lacks PLP. Thus, a compact lamellar structu re can be seen in fixed PLP-null myelin, but defects in the apposition of both the extracellular and the cytoplasmic surfaces of the myelin membranes are common. The abnormalities seen suggest a lack of firm in termembrane bonding, resulting in structural instability. PLP-null mye lin may therefore be more susceptible than normal myelin to disruption by mechanical or osmotic stresses. Although PLP is not essential for the formation of either major dense lines or intermediate lines, it ma y play a role in stabilizing the compact structure. (C) 1996 Wiley-Lis s, Inc.