Microglial cells, derived from bone marrow monocytes, enter the nervous tissue during early brain development. They proliferate, migrate, and mature into resident or resting microglia. A major function of microglia is the surveillance of and participation in immunologic processes.

Resident microglia are distributed in gray and white matter around neurons as satellite cells and around blood vessels. Their small, round, or elongated nuclei are not readily recognizable in HE preparation. Silver stain discloses a triangular perikaryon with two or more branching processes (Fig. 2.9). Pathologic states rapidly activate the microglial cells, which then are easily identifiable.


The pathology of microglial cells is summarized in Table 2.5.

Activated microglia. Rod cells, prominent in viral diseases and parenchymal neurosyphilis, are distinguished by conspicuously hypertrophied rod-shaped nuclei. They are diffusely distributed in gray and white matter and are almost perpendicularly oriented to the cortical surface (see Fig. 2.9). Rod-, crescent-, and kidney-shaped microglial cells occur around neuritic plaques in Alzheimer's disease.

Microglial nodules. Nodules commonly are found in viral diseases, in which they cluster around infected neurons, invade and digest them (neuronophagia), and eventually replace them with residual nodules. Loose collections of microglial cells (shrubs) are occasionally found in the molecular layer of the cerebellar cortex.

Multinucleated giant cells. These giant cells, derived from microglia and macrophages, are distinctive features of HIV encephalitis.

Macrophages. Macrophages with phagocytic activity are scavengers of the neural tissue. They are prominent in vascular infarcts, acute demyelinating diseases, leu-kodystrophies, hemorrhages, and traumatic lesions. Distinguished by large, rounded, foamy cytoplasms and small eccentric nuclei, the macrophages engulf and remove degraded myelin, necrotic tissue debris, and hemosiderin pigments (see Fig. 2.9).

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