Glial Cells and Brain Tumors

A tumor consists of a mass of rapidly dividing cells. Mature neurons, however, have little capacity for mitosis and seldom form tumors. Some brain tumors arise from the meninges (protective membranes of the CNS) or arise by metastasis from tumors elsewhere, such as malignant melanoma and colon cancer. Most adult brain tumors, however, are composed of glial cells, which are mitotically active throughout life. Such tumors are called gliomas.21 Gliomas usually grow rapidly and are highly malignant. Because of the blood-brain barrier (see chapter 14), brain tumors usually do not yield to chemotherapy and must be treated with radiation or surgery.

21glia = glial cells + oma = tumor

Myelin

The myelin (MY-eh-lin) sheath is an insulating layer around a nerve fiber, somewhat like the rubber insulation on a wire. It is formed by oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system. Since it consists of the plasma membranes of these glial cells, its composition is like that of plasma membranes in general. It is about 20% protein and 80% lipid, the latter including phospholipids, gly-colipids, and cholesterol. Myelination of the nervous system begins in the fourteenth week of fetal development, yet hardly any myelin exists in the brain at the time of birth. Myelination proceeds rapidly in infancy

Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition

12. Nervous Tissue

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452 Part Three Integration and Control and isn't completed until late adolescence. Since myelin has such a high lipid content, dietary fat is important to early nervous system development. Well-meaning parents can do their children significant harm by giving them the sort of low-fat diets (skimmed milk, etc.) that may be beneficial to an adult.

In the CNS, each oligodendrocyte reaches out to several nerve fibers in its immediate vicinity. The armlike process of the oligodendrocyte spirals repeatedly around the nerve fiber, laying down many compact layers of its own membrane with almost no cytoplasm between the membranes. These layers constitute the myelin sheath. A

nerve fiber is much longer than the reach of a single oligo-dendrocyte, so it requires many oligodendrocytes to cover one nerve fiber.

In the PNS, a Schwann cell spirals around a single nerve fiber, putting down as many as a hundred layers of membrane (fig. 12.7). External to the myelin sheath is the neurilemma22 (noor-ih-LEM-ah), the outermost coil of the Schwann cell. Here, the bulging body of the Schwann cell contains its nucleus and most of its cytoplasm. To neuri = nerve + lemma = husk, peel, sheath

Medullated Nerve Schwann Cell

Unmyelinated axon

Figure 12.7 Schwann Cells and Myelin. (a) The repetitive wrapping of a Schwann cell around an axon, forming the multilayered myelin sheath. (b) A myelinated axon (top) and unmyelinated axon (bottom) (TEM).

Unmyelinated axon

Figure 12.7 Schwann Cells and Myelin. (a) The repetitive wrapping of a Schwann cell around an axon, forming the multilayered myelin sheath. (b) A myelinated axon (top) and unmyelinated axon (bottom) (TEM).

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Chapter 12 Nervous Tissue 453

visualize this, imagine that you wrapped an almost-empty tube of toothpaste tightly around a pencil. The pencil represents the axon, and the spiral layers of toothpaste tube (with the toothpaste squeezed out) represent the myelin. The toothpaste would be forced to one end of the tube, which would form a bulge on the external surface of the wrapping, like the body of the Schwann cell.

External to the neurilemma is a basal lamina and then a thin sleeve of fibrous connective tissue called the endoneurium. Nerve fibers of the CNS have no neurilemma or endoneurium.

Since each glial cell (Schwann cell or oligodendro-cyte) myelinates only part of an axon, the myelin sheath is segmented. The gaps between the segments of myelin are nodes of Ranvier23 (RON-vee-AY), and the myelin-covered segments from one gap to the next are called internodes (see fig. 12.4). The internodes are about 0.2 to 1.0 mm long in the PNS. The short section of nerve fiber between the axon hillock and the first glial cell is called the initial segment. Since the axon hillock and initial segment play an important role in initiating a nerve signal, they are collectively called the trigger zone.

23L. A. Ranvier (1835-1922), French histologist and pathologist

Insight 12.2 Clinical Application

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