Protoplasmic astrocyte in the gray matter of the brain. This schematic drawing shows the foot processes of the protoplasmic astrocyte terminating on a blood vessel and the axonal process of a nerve cell. The foot processes terminating on the blood vessel contribute to the blood-brain barrier. The bare regions of the vessel as shown in the drawing would be covered by processes of neighboring astrocytes, thus forming the overall barrier.
limitans, a relatively impermeable barrier surrounding the CNS (Fig. 11.21).
Oligodendrocytes produce and maintain the myelin sheath in the CNS
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 that occurs in the PNS.
Oligodendrocytes appear in specially stained preparations in the light microscope as small cells with relatively few processes compared with astrocytes. They are often aligned in rows between axons. Each oligodendrocyte gives off several tongue-like 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. 11.22). The nucleus-containing region of the oligodendrocyte may be at some distance from the axons it myelinates.
The precise manner by which oligodendrocyte plasma membrane becomes concentrically wrapped around a portion of a CNS neuron is not as clearly understood as the parallel process in the PNS. Because a single oligodendrocyte may myelinate several nearby axons simultaneously, the cell cannot spiral around each axon in an independent manner, as the Schwann cell does in the PNS. Instead, each tongue-like process appears to spiral around the axon, always staying in proximity to it, until the myelin sheath is formed. Thus, PNS myelin can be described as forming centrifugally by the movement of the Schwann cell around the outer surface of the newly formed myelin. CNS myelin, on the other hand, can be described as forming cen-tripetally by the continued insinuation of the leading edge of the growing process between the inner surface of the newly formed myelin and the axon.
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. 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 axolemma thus make saltatory conduction (see below) even more efficient in the CNS.
Another difference between the CNS and the PNS in regard to the relationships between supporting cells and neurons is that unmyelinated neurons in the CNS are often found to be bare; i.e., they are not embedded in glial cell processes. The lack of supporting cells around unmyelinated axons as well as the absence of basal lamina material and connective tissue within the substance of the CNS help to distinguish the CNS from the PNS in histologic sections and in TEM specimens.
Ependymal cells form the simple epithelial lining of the fluid-filled cavities of the CNS. They are cuboidal-to-columnar cells that have the morphologic and physiologic characteristics of fluid-transporting cells (Fig. 11.23). They are tightly bound by junctional complexes located at the apical surfaces. Unlike a typical epithelium, ependymal
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