The oligodendrocyte is the cell responsible for producing CNS myelin. The myelin sheath in the CNS is formed by concentric layers of oligodendrocyte plasma membrane. The formation of the sheath in the CNS is more complex, however, than the simple wrapping of Schwann cell mesaxon membranes that occurs in the PNS (page 364).
Oligodendrocytes appear in specially stained light microscopic preparations as small cells with relatively few processes compared with astrocytes. They are often aligned in rows between axons. Each oligodendrocyte gives off several tonguelike processes that find their way to the axons, where each process wraps itself around a portion of an axon, forming an internodal segment of myelin. The multiple processes of a single oligodendrocyte may myelinate one axon or several nearby axons (Fig. 12.20). The nucleus-containing region of the oligodendrocyte may be at some distance from the axons it myelinates.
Because a single oligodendrocyte may myelinate several nearby axons simultaneously, the cell cannot embed multiple axons in its cytoplasm and allow the mesaxon membrane to spiral around each axon. Instead, each tonguelike process appears to spiral around the axon, always staying in proximity to it, until the myelin sheath is formed.
The myelin sheath in the CNS differs from that in the PNS.
There are several other important differences between the myelin sheaths in the CNS and those in the PNS. Oligodendrocytes in the CNS express different myelin-specific proteins during myelination than those expressed by Schwann cells in the PNS. Instead of P0 and PMP-22, which are expressed only in myelin of the PNS, other proteins, including proteolipid protein (PLP), myelin oligodendrocyte glyco protein (MOG), and oligodendrocyte myelin glycoprotein (OMgp), perform similar functions in CNS myelin.
Deficiencies in the expression of these proteins appear to be important in the pathogenesis of several autoimmune demyelinating  diseases of the CNS.
On the microscopic level, myelin in the CNS exhibits fewer Schmidt-Lanterman clefts because the astrocytes provide metabolic support for CNS neurons. Unlike Schwann cells of the PNS, oligodendrocytes do not have an external lamina. Furthermore, because of the manner in which oligodendrocytes form CNS myelin, little or no cytoplasm may be present in the outermost layer of the myelin sheath, and with the absence of external lamina, the myelin of adjacent axons may come into contact. Thus, where myelin sheaths of adjacent axons touch, they may share an intraperiod line. Finally, the nodes of Ranvier in the CNS are larger than those in the PNS. The larger areas of exposed
FIGURE 12.15 • Electron micrograph of unmyelinated nerve fbers. The individual fibers or axons (A) are engulfed by the cytoplasm of a Schwann cell. The arrows indicate the site of mesaxons. In effect, each axon is enclosed by the Schwann cell cytoplasm, except for the intercellular space of the mesaxon. Other features evident in the Schwann cell are its nucleus (N), the Golgi apparatus (G), and the surrounding basal (external) lamina (BL). In the upper part of the micrograph, myelin (M) of two myelinated nerves is also evident. 27,000. Inset. Schematic diagram showing the relationship of axons engulfed by the Schwann cell. (Reprinted with permission from Barr ML, Kiernan JA. The Human Nervous System. New York: Harper & Row, 1983.) axolemma thus make saltatory conduction (see below) even more efficient in the CNS.