Schwann Cells and the Myelin Sheath

Axons in the peripheral nervous system are described as myelinated or unmyelinated

Myelinated axons are surrounded by a lipid-rich layer called the myelin sheath.

External to, and contiguous with, the myelin sheath is a thin layer of Schwann cell cytoplasm called the sheath of Schwann, or the neurilemma (Fig. 11.10). This layer contains the nucleus and most of the organelles of the Schwann cell. Surrounding the Schwann cell is a basal or external lamina.

Functionally, the myelin sheath with its external lamina and the neurilemma isolates the axon from the surrounding extracellular compartment. The axon hillock and the terminal arborizations where the axon synapses with its target cells do not have a myelin sheath.

The myelin sheath is composed of multiple layers of Schwann cell membrane wrapped concentrically around the axon

To produce the myelin sheath, each Schwann cell wraps in a spiral around a short (0.08 to 0.1 mm) segment of the axon. During the wrapping of the axon, cytoplasm is squeezed out from between the membrane of the concentric layers of the Schwann cell. The inner leaflets of the plasma membrane then fuse. With the TEM, these fused inner leaflets are electron opaque, appearing as the major dense lines in the TEM image of the myelin. These concentric dense lamellae alternate with the slightly less dense intraperiod lines that are formed by fusion of the outer membrane leaflets (Fig. Tl.ll).

The myelin sheath is segmented because it is formed by numerous Schwann cells arrayed sequentially along the axon. The junction where two adjacent Schwann cells meet is devoid of myelin. This site is called the node of Ranvier. Therefore, the myelin between two sequential nodes of Ranvier is called an internodal segment.

During formation of the myelin sheath, the axon initially lies in a groove on the surface of the Schwann cell (Fig. 11.11a). Fusion of the edges of the groove to enclose the axon produces the inner mesaxon, the narrow intercellular space of the innermost rings. The first few lamellae are not compactly arranged; i.e., some cyto-

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