Myelin: Dynamic or Stable?

The concept of a spirally wrapped cell membrane as the mode of origin of the myelin sheath has made it difficult to explain (a) myelin sheath adjustment to growth of the axon in diameter and length, (b) the herniation and implied flexibility of the myelin sheath observed in normal fibers (1), (c) the ability of myelinated axons in tissue culture to lose and regain myelin in relatively rapid response to the application and withdrawal of noxious stimuli (2), and (d) the varied responses of myelin in pathologic situations (3). Potential and actual spaces have been demonstrated between the outer surfaces of the myelin lamellae (intraperiod line) by exposure to hypotonic solutions (4) and by penetration of l an thanum solutions? Cytoplasmic trapping is often observed between the inner surfaces (5). Separation of myelin layers has been induced by irradiation damage to Schwann cells in vitro (6). While recent evidence suggests that preferred orientation and positioning of Schwann or glial cytoplasmic microtubles are influential in the over-all maintenance of a flexibly stable sheath form (7), the above observations are consistent with a picture of a stable myelin sheath adaptable to necessary form changes by temporary separation, slippage, and retightening of the spirally wrapped constituent lamellae (3). O u r view of the structural form of myelin regards the electron opaque loci of the outer and inner surfaces of the cell membrane not as "leaflets" but simply as regions of charge distribution to which osmium, dichromate ions, permanganate ions, etc. m ay be selectively bound. 2 This, in agreement with prevailing ideas about the myelin sheath and cell membranes, supports the thesis that a structural order prevails, as demonstrated by polarization optics and Xray diffraction (8). I n this general scheme, liquid-crystalline smectic arrays of bimolecular lipid layers containing the phospholipid and cholesterol molecules alternate with protein a n d / o r polysaccharide components. The ability of the cell surface to respond quickly to change in shape and volume, as well as the necessity during myelin formation for the Schwann (or glial)