Myelin Composition in Developing Brain

Myelin accounts for more than 25% of the weight of the mature brain. Myelination is thus a major event in brain development. Myelin is composed mainly of lipids, proteins, and water, with small amounts of inorganic salts. About 40% of the wet weight of myelin is water. Of the material, 75-80% is lipids, and most of the remainder is proteins. The most extensive studies on myelin composition have been confined to lipids, although attention is now being given to proteins.

1. Myelin Lipids and Proteins

The most prominent changes that take place in the lipid composition of the CNS myelin during development are in relative mole proportions of cholesterol, phospholipids, and galactolipids, which attain a final value of approximately 2:2:1 as opposed to about 8:10:1 in the whole brain.

The content of protein in myelin is much less than that usually found in typical cell-surface membranes. The protein composition of myelin is unique. Most typical membranes contain many different kinds of proteins, none of which supercede the others in quantitative terms. On the other hand, myelin, not only contains less protein than other membranes but also has fewer (four or five) types of proteins. These four or five proteins have been separated by polyacry-lamide gel electrophoresis. Of these, three are found to be highly specific for myelin: myelin basic protein (so named because of its relative richness in basic amino acids), proteolipid protein (so named because it is soluble in neutral organic solvents), and the so-called intermediate protein (lies intermediate between basic and proteolipid protein in an electric field). The relative proportions of these and other minor proteins change during myelin maturation.

2. Lipid-Protein Associations in Developing Myelin

At the beginning of myelination, as the flattened surface membrane of the myelin-forming glial cell wraps the axon for the first time, a double-membrane structure, the mesaxon, is produced. This doublemembrane mesaxon, in the course of its differentiation into myelin, seems to serve as a "template" (proto-myelin lattice) for the accumulation of myelin-specific lipids and proteins leading to the assembly of the myelin structure. The synthesis and deposition of the basic proteins and the lipids is thought to represent an early step in the differentiation process, which is advanced further by the accumulation of large amounts of more lipids and the proteolipid protein. Myelin basic proteins are synthesized on free polysomes in glial processes, whereas the proteo-lipid protein is synthesized on bound polysomes

(endoplasmic reticulum) in the glial cell body. The basic proteins are more rapidly incorporated into the developing CNS myelin sheath (within 2 min after synthesis), whereas the proteolipid protein appears in the myelin sheath after about 30 min. This assembly of myelin (incorporation of finished products into myelin sheath) is thought to be regulated by axonal signals.

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