Topical Review Is the Myelin Membrane Abnormal in Multiple Sclerosis?

Myelin, a multilamellar membrane surrounding nerve fibers of both the central and peripheral nervous systems, is derived from the plasma membrane of the oligodendrocyte (CNS) and Schwann cell (PNS). The chemical composition of myelin differs markedly from that of its progenitor membrane. In addition, the chemical composition of CNS myelin differs from that of PNS myelin. This review will be concerned only with CNS myelin. Myelin lipid, which accounts for approximately 75% of the dry weight (20% for most cell membranes), consists of cholesterol, phospholipids and galactolipids in a ratio of 4:3.5:2. Myelin has less protein than plasma membranes (25% as compared to 75%) and has fewer types. There are two major myelin proteins, the extrinsic or basic protein (MBP) and the intrinsic or Folch-Lees proteolipid apoprotein. Lipophilin has been purified from human proteolipid fraction and constitutes about 50% of this fraction. Although the lipids of myelin are considered to exist predominantly in a bilayer, the arrangement of the proteins has not been fully elucidated. Evidence to date suggests that the chemical -architecture of the bilayer will be similar to that of most plasma membranes. In man, when destruction and loss of myelin occurs but the nerve fiber remains intact, a ' `demyelinating disease" termed multiple sclerosis (MS), is acquired. Since the disease was first described, a number of etiologic theories has been proposed, the most important of which are (a) an infectious or environmental agent, or (b) an abnormal immune mechanism is responsible for disease induction. A large body of knowledge has been accumulated, but 10,