Astrocytes

Although two major types exist, astrocytes show extreme variations in size, shape, and relationships with other structures. The Bergmann astrocyte or candelabra cell of Golgi is extreme in all three respects. Protoplasmic astrocytes, found mainly in gray matter, have a stellate cell body with many branching processes (Fig. 15, PA). Some of these terminate on blood vessels as end feet. Sometimes the soma itself lies against the vessel. Astrocytes near the surface of the neuraxis send similar processes to the pia mater, beneath which their end feet are apposed, fastened by puncta adherentia and displaying gap junctions. Taken together with the overlying pia, this barrier is the pia-glial membrane.

Protoplasmic astrocytes, as their name implies, have abundant cytoplasm with a generous number of organelles and a large, pale nucleus that helps to distinguish them from other glial cells and nerve cells in dye-stained preparations. Smaller examples lie close to neuronal cell bodies as satellite cells. Velate astrocytes (Fig. 21C) extend thin veils between neurons and their processes and in the case of the Bergmann astrocytes, partition input to neurons.

Fibrous astrocytes are found mainly in white matter, but some regions of the periventricular gray matter also show them (Fig. 15, FA). They have the familiar euchromatic nucleus and a soma with relatively few organelles: long, thin, smooth processes that branch infrequently and extend to blood vessels and the pia with end feet like the protoplasmic forms. As noted, CNS blood vessels and the subpial border of the neuraxis are surrounded by basal lamina. Protoplasmic and fibrous astrocytes may be a single cell type, varying in form in different locales, or two distinct subtypes; more study is necessary. The former expand into whatever space is available, and this may account for their irregular outline.

The shapes of astrocytes are shown, not in dye-stained material but by special metallic impregnation techniques, along with cytoplasmic fibrils, which are numerous in the fibrous type, less so in the protoplasmic type, and highly variable in number within the cell and regionally. In the EM, glial fibrils are resolved as bundles of slender intermediate filaments. Unlike neurofibrils, they are thinner (8 nm) and packed more closely. Chemically they are distinct, consisting of glial fibrillar acidic protein of 51,000 Da molecular weight. GFAP is specific to astrocytes, and antibodies to GFAP selectively stain and identify these cells in tissue preparations and cell cultures. The high content of astrocytic fibrils accords with their supportive functions.

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