The monocyte lineage comprises a variety of phagocytic cells, related by origin and function, which include the blood monocyte, alveolar (lung) macrophages, peritoneal macrophages, Kupffer cells in the liver, free and fixed macrophages of the bone marrow (osteoclasts), and lymphatic tissue, and histiocytes in tissues.
It is generally believed that all of these cells are derived from bone marrow monoblasts and promonocytes, that they enter the bloodstream as monocytes, and later the tissue to develop into macrophages. The precursor cell of the monocyte in the bone marrow is unknown, but most probably, it derives from a stem cell common with granulocytes and thrombocytes.
Monocytes vary considerably in size, i.e., from 20 to 40 μm in diameter, they have a large, usually kidney-shaped nucleus. The chromatin appears pale and has a lace-like or reticular appearance without compact chromatin blocks.
The cytoplasm is ample, greyish-blue and has fine azurophilic granulations. Cytoplasmic vacuoles are quite common Monocytes stay in the circulation between 15 and 30 h, after which they leave the blood randomly and regardless of age, by diapedesis, after they have become adherent.
In sites of inflammation, they accumulate very rapidly. The daily monocyte turnover is approximately 7 x 106 cells per hour per kg of body weight. The life span of macrophages is long and can attain 75 days and more.
The death of monocytes and histiocytes proceeds in an unknown manner, but it is known that the cells when damaged and particularly after intense phagocytosis can, in turn, be phagocytized by other macrophages.
Monocytes adhere very well to solid surfaces, have locomotion similar, though slower, to one of the polymorphonuclear cells, possess a quite marked sensitivity for chemotactic stimuli, and are very actively phagocytic.
They may ingest a variety of cells, including protozoa, bacilli, viruses as well as antigen antibody complexes, and a variety of inorganic substances (carbon, silica, asbestos, a.o.). They are endowed with receptors for immunoglobulins (Fc receptors) and complement components (C 3 b receptor).