Morphology and Subtypes

"Brain macrophage'' is a general term that comprises several subtypes of cells based on morphology, localization, surface antigen markers, and function. Perivascular microglia, with an elongate shape, are located around blood vessels in adult tissue. They are thought to be regularly replenished by peripheral monocytes that infiltrate the CNS through the he-moencephalic or blood-brain barrier. The surface antigen profile is similar to that of circulating mono-cytes. In vivo, perivascular microglia are considered the most important antigen presenting cells, given their diversified, anatomical location. Intraparenchymal microglia, or resident microglia, are more numerous in gray matter than in white matter. They are maintained as a pool with a low turnover rate in normal adult brain. Parenchymal microglia can be subdivided into two populations. The first group includes ramified or resting microglial cells, which are highly branched cells with a small amount of perinuclear cytoplasm and a small, dense, and hetero-chromatic nucleus. The second population consists of ameboid microglial cells, which display migratory capacity and phagocytosis. These cells are particularly present during embryonic development and after CNS injury. In fact, ramified and retracted microglial processes can become ameboid-like, forming reactive microglia during brain injury. The best defined functions of microglial cells are related to microglial activation during pathological processes and include antigen presentation, cytotoxicity, neurovasculariza-tion, and phagocytosis. Some of these functions are also important for the normal physiology of the adult brain as well as the developing brain.



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