Macrophages and the Nervous System

This chapter reviews the distribution and specialized differentiation of macrophages in the nervous system, in the normal state and following injury. Macrophages are hematopoietic cells that form a long-lived resident population of cells within tissues. Macrophages are generated in the bone marrow, enter the circulation as monocytes, and then migrate into almost all tissues of the body. Mature macrophages are highly responsive to their microenvironment, differing in their morphology and expression of cell surface receptors, the repertoire of substances they secrete, and their functional capabilities. These phenotypic differences of resident macrophages depend on their interactions with other tissue cells and on the surface or substrate to which they are bound. Under the influence of unidentified factors within the central nervous system (CNS), they develop into the morphologically and phenotypically distinct microglia. Following injury to the CNS, resident and recruited mononuclear phagocytes are prominent components of the cellular response. The techniques available for studying cells in situ bring new insights to the cellular and molecular interactions among macrophages and microglia and the other components of the nervous system.