Figure 1111

Diagram showing successive stages in the formation of myelin by a Schwann cell.

a. The axon is initially surrounded by a Schwann cell. b. The Schwann cell then wraps around the axon, forming multiple Schwann cell layers, c. During the wrapping process the cytoplasm is extruded from between the two apposing plasma membranes of the Schwann cell, which then fuse, inner leaflet to inner leaflet, to form the major dense line in the myelin (arrows). The outer opposing leaflets also fuse to form the less conspicuous in-traperiod line. The outer mesaxon represents invaginated plasma membrane extending from the outer surface of the Schwann cell to the myelin. The inner mesaxon extends from the inner surface of the Schwann cell (the part facing the axon) to the myelin. (From Ban ML, Klernan JA. The Human Neivous System. New York: Harper & Row, 1983.)

plasm is left in the first few concentric layers (Fig. 11 .lib). Similarly, the outermost layer contains some cytoplasm as well as the Schwann cell nucleus (Fig. 11.11c). The apposition of the plasma membrane of the last layer to itself as it closes the ring produces the outer mesaxon, the narrow intercellular space adjacent to the external lamina.

Myelin is rich in lipid because as the Schwann cell winds around the axon, its cytoplasm, as noted, is extruded from between the opposing layers of the plasma

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