Structure and Function of Myelin, an Extended and Biochemically Modified Cell Surface Membrane

The formation of myelin involves the extension and biochemical modification of plasma membranes of oligodendrocytes or Schwann cells (Morell, et al., 1994). Although there is a continuous surface membrane from the perikaryon of the myelin-forming cell to compact myelin, there are many specializations within the membrane which exhibit variability in terms of structure, function and biochemical composition. In addition to the compact myelin itself, the specialized domains include the surface membranes of the cell body and processes, surface and periaxonal membranes of the myelin sheath, paranodal loops, and incisures. When considering function or pathogenic mechanisms involving myelin sheaths, it is important to distinguish the constituents of compact myelin from those of these other myelin-related membranes. Many of the myelin-related membranes are adjacent to cytoplasmic pockets in the sheaths where dynamic processes such as trans-membrane signaling and ion flux are likely to occur. The first part of this article consists of a brief overview the principal proteins in the compact myelin and myelin-related membranes, respectively. Since glycoproteins are important components of plasma membranes in general, it is not surprising that many of the proteins in both the compact myelin and the myelin-related membranes are glycosylated. This article will consider plasma membrane proteins which are specific for myelin-forming cells or have been studied primarily in the context of myelination, but it should be remembered that these myelin-related membranes probably contain many other components which are common to plasma membranes of most cells in the body. Glycosphingolipids are another characteristic component of cell surface membranes, and the second part of this article will describe recent research in our laboratory concerning the expression and possible function of gangliosides in differentiating oligodendrocytes.