Macrophages have highly developed phagocytic ability and specialize in turnover of protein fibers and removal of apoptotic cells, tissue debris, or other particulate material, being especially abundant at sites of inflammation. Size and shape vary considerably, corresponding to their state of functional activity. A typical macrophage measures between 10 and 30 μm in diameter and has an eccentrically located, oval or kidney-shaped nucleus. Macrophages are present in the connective tissue of most organs and are sometimes referred to by pathologists as “histiocytes.”
In the TEM, macrophages are shown to have a characteristic irregular surface with pleats, protrusions, and indentations, features related to their active pinocytotic and phagocytic activities (Figure1). They generally have well-developed Golgi complexes and many lysosomes.
Fig1. Macrophage ultrastructure.
Macrophages derive from precursor cells called monocytes circulating in the blood. Monocytes cross the epithelial wall of small venules to enter connective tissue, where they differentiate, mature, and acquire the morphologic features of macrophages. Monocytes formed in the yolk sac during early embryonic development circulate and become resident in developing organs throughout the body, comprising a group of related cells called the mononuclear phagocyte system. Many of these macrophage-like cells with prominent functions in various organs have specialized names (Table1). All are long-living cells, surviving with relative inactivity in tissues for months or years. During inflammation and tissue repair which follow organ damage, macro phages become activated and play a very important role. Under such conditions these cells increase in number, mainly in the connective tissue stroma, both by proliferation and by recruiting additional monocytes formed in the bone marrow. The transformation from monocytes to macrophages in connective tissue involves increases in cell size, increased protein synthesis, and increases in the number of Golgi complexes and lysosomes. In addition to debris removal, macrophages secrete growth factors important for tissue repair and also function in the uptake, processing, and presentation of antigens for lymphocyte activation, a role dis cussed later with the immune system.
Table1. Distribution and main functions of the cells of the mononuclear phagocyte system.
