Macrophages: functions, tissue locations, and origins
THE CONTEXT
Macrophages are professional phagocytes with roles in host defense, inflammation, and repair. Their circulating counterparts are monocytes.
Functions and morphology
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The primary phagocytic function of macrophages is ingestion and destruction of particulate matter, microbes, and dead cells.
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Macrophages secrete cytokines and growth factors and activate other cells, notably T lymphocytes.
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Macrophages are normally scattered throughout most connective tissues.
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Blood monocytes have bean-shaped nuclei and finely granular cytoplasm. Tissue macrophages have abundant cytoplasm with phagocytic vacuoles, often containing ingested material, along with lysosomes and other organelles.
Mononuclear phagocyte system
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Tissue macrophages include Kupffer cells in the liver, sinus histiocytes in the spleen and lymph nodes, microglial cells in the central nervous system, and alveolar macrophages in the lungs.
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These cells collectively constitute the mononuclear phagocyte system.
Origins and recruitment
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Tissue macrophages arise from bone marrow hematopoietic stem cells and from embryonic yolk sac and fetal liver progenitors.
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During inflammation, bone marrow progenitors produce monocytes that enter the blood, migrate into tissues, and differentiate into macrophages.
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Monocyte entry into tissues is governed by factors also involved in neutrophil emigration, including adhesion molecules and chemokines.
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Because macrophages survive longer in tissues than other leukocytes, they often become the dominant inflammatory cell population within 48 hours of onset.
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Resident macrophages such as microglia and Kupffer cells arise early in embryogenesis, persist for long periods, and are replenished mainly by proliferation of resident cells.