Astrocytes

The role of astrocytes begins during brain development, when the radial astrocytes guide the migrating neurons. After birth, the astrocytes are important in a number of physiologic processes: They provide support and nutrients to the neurons, protect them from excitotoxic neurotransmitters, contribute to the blood-brain barrier, and maintain homeostasis in the extracellular compartment.

Astrocytes are star-shaped. In HE-stained section, only the nucleus is visible; it is medium-sized, round or oval, with a moderate amount of chromatin and one or two nucleoli. Astrocytic processes and fibrils are visualized with specific stains. A longer process is attached to the capillary walls or the pial surface by a foot-plate. The fibrils contain glial fibrillary acidic protein (GFAP), an important astrocytic marker (Fig. 2.5).

The two types of astrocytes are protoplasmic and fibrillary. Protoplasmic astrocytes, predominantly in the cortex and subcortical gray structures, display shorter processes and less fibrils. Fibrillary astrocytes, predominantly in the white matter, contain abundant fibrils. In diseases, protoplasmic astrocytes may convert to fibrillary.

Pathology

The pathology of astrocytes is summarized in Table 2.5. Swelling is an early reaction to a variety of injuries,

FIGURE 2.5

Astrocytes. A. Protoplasmic astrocytes. Cerebral cortex shows oval and round astrocytic nuclei with moderate amount of chromatin and prominent nucleoli (Cresyl violet). B. Fibrillary astrocytes display numerous short, fine processes and one long process attached to the capillary wall with a foot plate (Cajal gold stain). C. Positive immunostaining for glial fibrillary acidic protein (GFAP).

TABLE 2.5.

Pathology of Glial Cells

Astrocytes

Oligodendrocytes

Swelling

Hypertrophy/hyperplasia

Gemistocytic astrocytes Fibrillary gliosis Alzheimer type 1 and 2 glia Rosenthal fibers Corpora amylacea Inclusions Argyrophilic Viral

Neoplastic transformation

Swelling Satellitosis Inclusions Argyrophilic Viral

Neoplastic transformation

Ependymal glia

Microglia

Atrophy-tearing Granular ependymitis Viral inclusions Neoplastic transformation

Rod-shaped activated microglia Multinucleated giant cell Macrophage

chiefly hypoxia, and results from an influx of fluid and electrolytes into the cytoplasm (cellular edema). In histologic preparations, the swollen cytoplasm appears clear, and the nucleus is prominent.

Hypertrophy and hyperplasia. In acute and chronic diseases, astrocytes commonly increase in size and number. A gemistocytic or hypertrophied astrocyte displays a large, round cytoplasm with short fibrillated processes and a vesicular, eccentrically displaced nucleus. The cytoplasm stains intensely with eosin and immunoreacts strongly with GFAP (Fig. 2.6).

Fibrillary gliosis. Astrocytes are capable of replacing destroyed tissue by producing more and more fibrils and ultimately forming a dense fibrillary gliosis or glial scar. A glial scar is isomorphic when the astrocytic fibers conform to the pattern of the original structures, or is anisomorphic when the pattern is haphazard (see Fig. 2.6).

Alzheimer's type 1 and type 2 astrocytes. Alzheimer's type 2 astrocytes have large, round or lobulated vesicu lar nuclei with scanty chromatin condensed at the margin of well-defined nuclear membranes (see Fig. 2.6). They are characteristic findings in hepatic and other metabolic encephalopathies and Wilson disease, and are commonly found in the cerebral cortex, palli-dum, and dentate nucleus. Alzheimer's type 1 astro-cytes, also found in metabolic encephalopathies, have large lobulated nuclei and large slightly granular cytoplasms.

Rosenthal fibers. These alterations in astrocytic processes appear as homogenous oval, round, elongated, or carrot-shaped eosinophilic structures. They are found in the walls of cystic cavities, fibrillary gliosis, and astrocytic tumors, and are the diagnostic hallmark of Alexander's leukodystrophy (see Fig. 2.6).

Corpora amylacea. Corpora amylacea are the degenerative products of astrocytic processes. They are round, basophilic and argyrophilic structures, 20 to 50 microns in diameter. They commonly occur beneath the pia mater and around the ventricles and blood vessels. They contain polyglucosans, are PAS positive, and immuno-react for ubiquitin. Corpora amylacea are found in variable amounts in brains after the age of 40 to 45 years, and are particularly abundant in chronic degenerative diseases (see Fig. 2.6).

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