Myelination

Schwann cells myelinate the peripheral nerves. These cells originate from neural crest, migrate peripherally, and wrap themselves around axons, forming the neurilemma sheath (Fig. 19.12). Beginning at the fourth month of fetal life, many nerve fibers take on a whitish appearance as a result of deposition of myelin, which is formed by repeated coiling of the Schwann cell membrane around the axon (Fig. 19.12C).

The myelin sheath surrounding nerve fibers in the spinal cord has a completely different origin, the oligodendroglial cells (Fig. 19.12, B and C). Although myelination of nerve fibers in the spinal cord begins in approximately the fourth month of intrauterine life, some of the motor fibers descending from higher brain centers to the spinal cord do not become myelinated until the first year of postnatal life. Tracts in the nervous system become myelinated at about the time they start to function.

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Figure 19.12 A. Motor horn cell with naked rootlet. B. In the spinal cord oligoden-droglia cells surround the ventral rootlet; outside the spinal cord, Schwann cells begin to surround the rootlet. C. In the spinal cord the myelin sheath is formed by oligoden-droglia cells; outside the spinal cord the sheath is formed by Schwann cells.

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Figure 19.12 A. Motor horn cell with naked rootlet. B. In the spinal cord oligoden-droglia cells surround the ventral rootlet; outside the spinal cord, Schwann cells begin to surround the rootlet. C. In the spinal cord the myelin sheath is formed by oligoden-droglia cells; outside the spinal cord the sheath is formed by Schwann cells.

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