Epithelial Design of Nervous Tissue

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As stated, nervous tissue is a specialized epithelial tissue. Epithelia are sheets of cells that cover surfaces or line cavities of the body and in places grow into subjacent connective tissue to form hair follicles, glands, or other derivatives. Features of epithelia include close aggregation of functionally related cells, small amounts of intercellular substance, a basement membrane, a free surface, and utter avascularity. Nervous tissue, wherever encountered clearly shows the first two attributes and the free surface and basement membrane in the CNS as well. The presence of blood vessels might seem to depart from epithelial design, yet they are not part of the nervous tissue but only pass through it, ensheathed by basal lamina.

Nervous tissue meets all structural criteria of epithelial tissue. It arises from epithelium: the embryonic neural plate. As epithelial duties include sensory reception, protection of the body, and secretion of hormones, nervous tissue performs these functions to a fault, offering general body responsiveness, integration, and chemical transmission. It is thus properly and usefully classified as epithelial tissue. Where the resemblance of nervous tissue to epithelium ends is in the configurations of its cells. They have far more complex shapes than the squamous, cuboidal, columnar, and domelike cells of other epithelia. With its many wavy processes, a neuron looks like a medusa.

Figure 18 Gray and white matter: light micrographs (LMs) of the CNS fail to show its epithelial nature. Myelinated axons in the white matter resemble closely packed strands of spaghetti; nerve cell bodies in the gray seem to float in a watery matrix. At first glance in hematoxylin and eosin preparations, the CNS looks much like loose areolar connective tissue. From A. W. Ham, Histology, 6th ed., J. B. Lippincott, 1969 (illustration by Steven J. Harrison, from Dorothy Chubb).

Figure 18 Gray and white matter: light micrographs (LMs) of the CNS fail to show its epithelial nature. Myelinated axons in the white matter resemble closely packed strands of spaghetti; nerve cell bodies in the gray seem to float in a watery matrix. At first glance in hematoxylin and eosin preparations, the CNS looks much like loose areolar connective tissue. From A. W. Ham, Histology, 6th ed., J. B. Lippincott, 1969 (illustration by Steven J. Harrison, from Dorothy Chubb).

Light micrographs of gray and white matter in the CNS (Fig. 18) convey a misleading impression, failing to show the epithelial design of the nervous system. Nerve fibers in the white matter resemble strands of spaghetti, and nerve cell bodies in the gray float here and there amid processes in a watery substance, like the fibroblasts and collagen fibers in the fluid extracellular matrix of connective tissue.

Yet nervous tissue is epithelial throughout. Little intercellular matrix exists, mainly cells and their processes. A schematic drawing (Fig. 19) shows this epithelial character. Two of the three meninges covering the CNS, the arachnoid and pia mater, separated by the subarachnoid space, lie externally. Beneath them, the CNS is bounded by a basement membrane (basal lamina). Such membranes separate each of the four basic tissues of the body, preventing contact and unregulated commerce between them and thereby serving tissue homeostasis. An excellent example is the basal lamina between axon terminals and muscle fibers at the intimate nerve-muscle synapse.

Internal to the basement membrane are three elements seen in CNS tissue: neurons, neuroglia, and blood vessels. As noted, epithelia are avascular. For oxygen and nutrients, they depend on vessels in nearby connective tissue. But in the CNS superepithelium, the volume of tissue and the never-ending demand for oxygen and glucose mandate their proximity. The blood vessels entering and exiting the CNS and their interposed capillary networks (so dense that neurons lie no more than 100 mm away) are extrinsic as stated, separated from nervous tissue by the basal lamina of the CNS and that of the vessels. Substances from the blood must traverse both laminae to enter the CNS and then an interposed astrocytic process before reaching a neuron.

The inner free surface of the CNS epithelium comprises a single layer of ciliated ependymal cells lining a brain ventricle. Nearby, in attenuated regions of the brain where the pia and ependyma are apposed, modified ependymal cells form the choroid epithelium, which produces the cerebrospinal fluid (CSF). Its cells are specialized for secretion, with basal infoldings, free surface microvilli, many mitochondria, and tight junctions to limit the passage of substances larger than ions.

Epithelial design is striking in electron micrographs of the CNS. Figure 20 shows the soma of a spinal motor neuron, replete with nucleus, nucleolus, and surrounding organelles. Proximal parts of three dendrites are visible, as well as other dendritic, axonal, and glial processes around the cell and two capillaries. The

Tessuto Epiteliale Disegno

Figure 19 Epithelial nature of the CNS: neurons (N) and glial cells (G) lie within a basement lamina (basal lamina), like the diverse cells of conventional epithelia. Unlike other epithelia, however, is the presence of blood vessels; the volume of tissue and unceasing demand for oxygen and glucose mandate them, but they are separated from CNS parenchyma by their own basal lamina and that of the CNS. Blood-borne gases and substances must traverse both laminae to enter or exit the CNS. FromC. R. NobackandR. J. Demarest, The Human Nervous System. Basic Principles of Neurobiology, 2nd ed., McGraw-Hill, New York, 1975 (illustration by Robert J. Demarest).

Figure 19 Epithelial nature of the CNS: neurons (N) and glial cells (G) lie within a basement lamina (basal lamina), like the diverse cells of conventional epithelia. Unlike other epithelia, however, is the presence of blood vessels; the volume of tissue and unceasing demand for oxygen and glucose mandate them, but they are separated from CNS parenchyma by their own basal lamina and that of the CNS. Blood-borne gases and substances must traverse both laminae to enter or exit the CNS. FromC. R. NobackandR. J. Demarest, The Human Nervous System. Basic Principles of Neurobiology, 2nd ed., McGraw-Hill, New York, 1975 (illustration by Robert J. Demarest).

key point: everywhere the tissue is epithelial. Every The capillary basal lamina is barely apparent. The nook and cranny is occupied, with the 2-4 nm of darkly encircled fibers in the field are small, myelinated extracellular space invisible at this low magnification. axons of arriving input.

Figure 20 Neuronal cell body: low-magnification EM of a motor neuron. Nucleus (Nuc) is pale and contains a nucleolus (ncl). Perikaryon shows Nissl bodies (NB), Golgi apparatus (G), mitochondria (mit), and a few lipofuscin granules (Lf). Extending from the perikaryon are three dendrites. Other dendrites, as well as axonal and glial processes and two capillaries, lie nearby. See also text. From The Fine Structure of the Nervous System: Neurons and Their Supporting Cells, 3rd ed., by Alan Peters, Sanford L. Palay, and Henry de F. Webster, copyright 1990 by Alan Peters. Used by permission of Oxford University Press, Inc. (EM from collections of authors).

Figure 20 Neuronal cell body: low-magnification EM of a motor neuron. Nucleus (Nuc) is pale and contains a nucleolus (ncl). Perikaryon shows Nissl bodies (NB), Golgi apparatus (G), mitochondria (mit), and a few lipofuscin granules (Lf). Extending from the perikaryon are three dendrites. Other dendrites, as well as axonal and glial processes and two capillaries, lie nearby. See also text. From The Fine Structure of the Nervous System: Neurons and Their Supporting Cells, 3rd ed., by Alan Peters, Sanford L. Palay, and Henry de F. Webster, copyright 1990 by Alan Peters. Used by permission of Oxford University Press, Inc. (EM from collections of authors).

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  • eyob bisrat
    Which nerve resemble spaghetti?
    2 years ago

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