Page 361 - Atlas of Histology with Functional Correlations

FIGURE 9.20 ■ Microglia of the brain. Stain: Hortega method. Medium
magnification.

FUNCTIONAL CORRELATIONS 9.3 ■ Neuroglia

There are four types of neuroglial cells in the CNS: astrocytes,
oligodendrocytes, microglia, and ependymal cells.

Astrocytes are the largest and most abundant neuroglia cells in the gray

matter and consist of two types: fibrous astrocytes and protoplasmic
astrocytes. In the CNS, both types of astrocytes attach to the surfaces of
capillaries and neurons. Their perivascular endfeet surround the capillary
basement membrane, form the tight junctions around the capillaries, and

form part of the blood–brain barrier. The blood–brain barrier is a
physiologic barrier that regulates the passage of substances from blood to
brain. This allows for a stable and balanced ionic composition in the
interstitial neuronal environment and protects the cells from potentially

harmful substances. The branched processes of astrocytes also extend to the
basal lamina of the pia mater to form an impermeable barrier, the glia
limitans, or glial limiting membrane, which surrounds the brain and spinal
cord. They support metabolic exchange between the neurons and capillaries

of the CNS. In addition, the astrocytes control the chemical environment
around neurons by clearing intercellular spaces of increased potassium ions

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