Figure 1117

Photomicrograph of a nerve ganglion, a. Photomicrograph showing a ganglion stained by the Mallory-Azan method. Note the large nerve cell bodies (arrows) and nerve fibers (NF) in the ganglion. Satellite cells are represented by the very small nuclei at the periphery of the neuronal cell bodies. The ganglion is surrounded by a dense irregular

connective tissue capsule (CT) that is comparable to, and continuous with, the epineurium of the nerve. x200. b. Higher magnification of the ganglion, showing individual axons and a few neuronal cell bodies with their satellite cells (arrows). The nuclei in the region of the axons are mostly Schwann cell nuclei. x640.

gests that they remove debris of cells that die during development of the nervous system.

Microglia are the smallest of the neuroglial cells and have relatively small, elongated nuclei (Fig. 11.18.) When stained with heavy metals, microglia exhibit short, twisted processes. Both the processes and the cell body are covered with numerous spikes. The spikes may be the equivalent of the ruffled border seen on other phagocytotic cells. The TEM reveals numerous lysosomes, inclusions, and vesicles. However, microglia contain little rER and few microtubules or actin filaments.

Astrocytes are of two types, protoplasmic and fibrous

Astrocytes are the largest of the neuroglial cells. Two kinds of astrocytes are identified:

• Protoplasmic astrocytes, which are more prevalent in gray matter. These astrocytes have numerous, short, branching cytoplasmic processes (Fig. 11.19).

• Fibrous astrocytes, which are more common in white matter. These astrocytes have fewer processes, and they are relatively straight (Fig. 11.20).

Both types of astrocytes contain prominent bundles of intermediate filaments composed of glial fibrillary acidic protein (GFAP). The filaments are much more numerous in the fibrous astrocytes, however, hence the name. Antibodies to GFAP are used as specific stains to identify astrocytes in sections and tissue cultures (see Fig. 11.20b). Tumors arising from fibrous astrocytes, fibrous astrocytomas, account for about 80% of adult primary brain tumors. They are also characterized by microscopic examination and GFAP specificity.

Astrocyte processes extend between the blood vessels and neurons. The ends of the processes expand, forming end feet that cover large areas of the outer surface of the vessel or of the axolemma. It is now thought that astrocytes play a role in the movement of metabolites and wastes to and from neurons and regulate ionic concentrations in the intercellular compartment, thus maintaining the microenvironment of the neurons. They also have a role in maintaining tight junctions of the capillaries that form the blood-brain barrier (page 313).

In addition, astrocytes provide a covering for the "bare areas" of myelinated axons, e.g., at the nodes of Ranvier and at synapses. They may confine neurotransmitters to the synaptic cleft and may remove excess neurotransmitters by pinocytosis. Protoplasmic astrocytes at the brain and spinal cord surfaces extend their processes (subpial feet) to the basal lamina of the pia mater to form the glia

Microglial cell in the gray matter of the brain, a. This diagram shows the shape and characteristics of a microglial cell. Note the elongated nucleus and relatively few processes emanating from the body, b. Photomicrograph of microglial cells (arrows) showing their characteristic elongated nuclei. The specimen was obtained from an individ-

Microglial cell in the gray matter of the brain, a. This diagram shows the shape and characteristics of a microglial cell. Note the elongated nucleus and relatively few processes emanating from the body, b. Photomicrograph of microglial cells (arrows) showing their characteristic elongated nuclei. The specimen was obtained from an individ-

b ual with diffuse microgliosis. In this condition, the microglial cells are present in large numbers and are readily visible in a routine H&E preparation. x420. (From Fuller GN, Burger PC. Central nervous system. In: Sternberg SS, ed. Histology for Pathologists. Philadelphia: Lippincott-Raven, 1997.)

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