abstract Myelination in the human central nervous system (CNS) is a complex but orderly process, occurring in predictable topographical and chronological sequences. CNS myelination begins as early as 12-14 weeks of gestation in the spinal cord and continues well into the fourth decade of life. The most significant period of CNS myelination, however, occurs between midgestation and the second postnatal year. Myelination can be studied by various methods, including myelin-stained histopathological brain sections and, more recently, magnetic resonance (MR) imaging. The latter constitutes the focus of this chapter.
Myelin is a wrapping of surface membrane of oligodendrocytes (Schwann cells in the peripheral nervous system) around the axons, with little or no cytoplasm in between (Everett, 1971). The process of myelin formation occurs in two, partially overlapping stages. Initially, oligodendrocytes proliferate and differentiate. Subsequently, myelin is synthesized. Although the precise chemical structure has been only partially elucidated (Kinney et al., 1994), myelin, like other membranes, is composed of a lipid bilayer with several large proteins. Proteolipid protein (PLP) and myelin basic protein (MBP) are two such proteins, both of which are necessary for membrane compaction and span the bilayer. The outer layer of the membrane is composed mainly of cholesterol and glycolipids while the inner portion of the lipid bilayer is composed mainly of phospholipids (Braun,1984).
Myelination in the human central nervous system (CNS) is a complex but orderly process, occurring in predictable topographical and chronological sequences. The sequence of myelination in the human brain has been carefully defined by histochemical (Kinney et al., 1994; Yakovlev and Lecours, 1967) as well as imaging (Barkovich et al., 1988; Bird et al., 1989; Martin et al., 1991; Nakagawa et al., 1998; Staudt et al., 1993; van der Knaap and Valk, 1990) methods. Histochemi-cally, the CNS myelination begins as early as 12-14
ricardo c. sampaio and Charles l. truwit Department of Radiology, University of Minnesota, Minneapolis, Minnesota.
weeks of gestation in the spinal cord and continues well into the third and fourth decades of life in the intra-cortical fibers of the cerebral cortex; but the most important and dramatic changes occur between mid-gestation and the end of the second postnatal year, with myelination accounting in large part for the large gain in brain weight, which more than triples during this period. The process slows markedly after 2 years of age (Yakovlev and Lecours, 1967).
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