Myelination is an example of cell-to-cell communication in which the axon interacts with the Schwann cell. Experimental studies show that the number of layers of myelin is determined by the axon and not by the Schwann cell. Myelin sheath thickness is regulated by a growth factor called neuregulin (Ngr1) that acts on Schwann cells. Ngr1 is a transmembrane protein expressed on the axolemma (cell membrane) of the axon.
The node of Ranvier represents the junction between two adjacent Schwann cells.
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 (Plate 28, page 392).
Myelin is composed of about 80% lipids because, as the Schwann cell membrane winds around the axon, the cytoplasm of the Schwann cell, as noted, is extruded from between the opposing layers of the plasma membranes. Electron micrographs, however, typically show small amounts of cytoplasm in several locations (Figs. 12.11 and 12.12): The inner collar of Schwann cell cytoplasm, between the axon and the myelin; the Schmidt-Lanterman clefts, small islands within successive lamellae of the myelin; perinodal cytoplasm, at the node of Ranvier; and the outer collar of perinuclear cytoplasm, around the myelin (Fig. 12.13). These areas of cytoplasm are what light microscopists identified as the Schwann sheath. If one conceptually unrolls the Schwann cell process, as shown in Figure 12.14, its full extent can be appreciated, and the inner collar of Schwann cell cytoplasm can be seen to be continuous with the body of the Schwann cell through the Schmidt-Lanterman clefts and through the perinodal cytoplasm. Cytoplasm of the clefts contains lysosomes and occasional mitochondria and microtubules, as well as cytoplasmic inclusions, or dense bodies. The number of Schmidt-Lanterman clefts correlates with the diameter of the axon; larger axons have more clefts.
Unmyelinated axons in the peripheral nervous system are enveloped by Schwann cells and their external lamina.
The nerves in the PNS that are described as unmyelinated are nevertheless enveloped by Schwann cell cytoplasm as shown in Figure 12.15. The Schwann cells are elongated in parallel to the long axis of the axons, and the axons fit into grooves in the surface of the cell. The lips of the groove may be open, exposing a portion of the axolemma of the axon to the adjacent external lamina of the Schwann cell, or the lips may be closed, forming a mesaxon.
A single axon or a group of axons may be enclosed in a single invagination of the Schwann cell surface. Large Schwann cells in the PNS may have 20 or more grooves, each containing one or more axons. In the ANS, it is common for bundles of unmyelinated axons to occupy a single groove.
The neuronal cell bodies of ganglia are surrounded by a layer of small cuboidal cells called satellite cells. Although they form a complete layer around the cell body, only their nuclei are typically visible in routine H&E preparations (Fig. 12.16, a and b). In paravertebral and peripheral ganglia, neural cell