Within the CNS, supporting cells are designated neuroglia, or glial cells. The four types of glial cells are

• Oligodendrocytes, small cells that are active in the formation and maintenance of myelin in the CNS

• Astrocytes, morphologically heterogeneous cells that provide physical and metabolic support for the neurons of the CNS

• Microglia, inconspicuous cells with small, dark, elongated nuclei that possess phagocytotic properties

• Ependymal cells, column-shaped cells that line the ventricles of the brain and the central canal of the spinal cord

Only the nuclei of glial cells are seen in routine histologic preparations of the CNS. Heavy-metal staining or immunocytochemical methods are necessary to demonstrate the shape of the entire glial cell.

Although glial cells have long been described as supporting cells of nerve tissue in the purely physical sense, current concepts emphasize the functional interdependence of neuroglial cells and neurons. The most obvious example of physical support occurs during development. The brain and spinal cord develop from the embryonic neural tube. In the head region, the neural tube undergoes remarkable

In general, demyelinating diseases are characterized by preferential damage to the myelin sheath. Clinical symptoms of these diseases are related to decreased or lost ability to transmit electrical impulses along nerve fibers. Several immune-mediated diseases affect the myelin sheath.

Cuillain-Barre syndrome, known also as acute inflammatory demyelinating polyradiculoneuropathy, is one of the most common life-threatening diseases of the PNS. Microscopic examination of nerve fibers obtained from patients affected by this disease shows a large accumulation of lymphocytes, macrophages, and plasma cells around nerve fibers within nerve fascicles. Large segments of the myelin sheath are damaged, leaving the axons exposed to the extracellular matrix. These findings are consistent with a T cell-mediated Immune response directed against myelin, which causes its destruction and slows or blocks nerve conduction. Patients exhibit symptoms of ascending muscle paralysis, loss of muscle coordination, and loss of cutaneous sensation.

Multiple sclerosis (MS) is a disease that attacks myelin in the CNS. MS is also characterized by preferential damage to myelin, which becomes detached from the axon and is eventually destroyed. In addition, destruction of oligodendroglia, which are responsible for the synthesis and maintenance of myelin, is also evident. Chemical changes in the lipid and protein constituents of myelin produce irregular, multiple plaques throughout the white matter of the brain. Symptoms of MS depend on the area in the CNS where myelin is damaged. MS is usually characterized by distinct episodes of neurologic deficits such as unilateral vision impairment, loss of cutaneous sensation, lack of muscle coordination and movement, and loss of bladder and bowel control.

Treatment of both diseases is related to diminishing the causative immune response by immunomodulatory therapy with interferon as well as administrating adrenal steroids. For more severe, progressive forms, immunosuppressive drugs may be used.

thickening and folding, leading ultimately to the final structure, the brain. During the early stages of the process, embryonic glial cells extend through the entire thickness of the neural tube in a radial manner. These radial glial cells serve as the physical scaffolding that directs the migration of neurons to their appropriate position in the brain.

Microglia possess phagocytotic properties

Microglia are phagocytotic cells. They are normally present only in small numbers in the adult CNS but proliferate and become actively phagocytotic in regions of injury and disease. They are considered part of the mononuclear phagocytotic system (see page 144) and are believed to originate in the bone marrow. Microglia cells enter the CNS parenchyma from the vascular system. Evidence also sug-


Electron micrograph of unmyelinated nerve fibers. 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, x27,000. Inset. Schematic diagram showing the relationship of axons engulfed by the Schwann cell. (Barr ML, Kiernan JA. The Human Nervous System. New York: Harper & Row, 1983.)

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