Info

Characteristics

Functions

Star-shaped cells between neurons and blood vessels

Structural support, formation of scar tissue, transport of substances between blood vessels and neurons, communicate with one another and with neurons, mop up excess ions and neurotransmitters, induce synapse formation Form myelin sheaths within the brain and spinal cord, produce nerve growth factors

Type

Characteristics

Functions

Astrocytes

Oligodendrocytes

Star-shaped cells between neurons and blood vessels

Shaped like astrocytes, but with fewer cellular processes, occur in rows along axons

Structural support, formation of scar tissue, transport of substances between blood vessels and neurons, communicate with one another and with neurons, mop up excess ions and neurotransmitters, induce synapse formation Form myelin sheaths within the brain and spinal cord, produce nerve growth factors

Microglia Ependyma

Small cells with few cellular processes and found throughout the CNS Cuboidal and columnar cells in the inner lining of the ventricles of the brain and the central canal of the spinal cord

Structural support and phagocytosis (immune protection)

Form a porous layer through which substances diffuse between the interstitial fluid of the brain and spinal cord and the cerebrospinal fluid axons, and they form myelin in the brain and spinal cord.

Unlike the Schwann cells of the peripheral nervous system, oligodendrocytes can send out a number of processes, each of which forms a myelin sheath around a nearby axon. In this way, a single oligodendrocyte may provide myelin for many axons. However, these cells do not form neurilemmal sheaths.

3. Microglia. Microglial cells are small and have fewer processes than other types of neuroglia. These cells are scattered throughout the central nervous system, where they help support neurons and phagocytize bacterial cells and cellular debris. They usually increase in number whenever the brain or spinal cord is inflamed because of injury or disease.

4. Ependyma. Ependymal cells are cuboidal or columnar in shape and may have cilia. They form the inner lining of the central canal that extends downward through the spinal cord. Ependymal cells also form a one-cell-thick epithelial-like membrane that covers the inside of spaces within the brain called ventricles (see chapter 11, p. 398). Throughout the ventricles, gap junctions join ependymal cells to one another. They form a porous layer through which substances diffuse freely between the interstitial fluid of the brain tissues and the fluid (cerebrospinal fluid) within the ventricles.

Ependymal cells also cover the specialized capillaries called choroid plexuses that are associated with the ventricles of the brain. Here they help regulate the composition of the cerebrospinal fluid.

Neuroglia form more than half of the volume of the brain. Table 10.2 summarizes characteristics of neu-roglial cells.

Abnormal neuroglia are associated with certain disorders. Most brain tumors, for example, consist of neuroglia that divide too often. Gene therapy is being tested to treat such tumors. Researchers add genes that instruct the cancerous glia to bear cell surface proteins that attract the immune system or render them more sensitive to cancer-fighting drugs.

Another experimental medical approach is to construct implants consisting of certain types of neuroglia that secrete substances that may

• replace neurochemicals whose absence causes degenerative diseases of the nervous system, such as Parkinson disease, Alzheimer disease, multiple sclerosis, and amyotrophic lateral sclerosis,

• repair damaged spinal cords,

• halt damage to delicate nervous tissues caused by AIDS or cancer chemotherapy.

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