The in vitro differentiation of homogeneous populations of monocyte-like cells from the unstimulated mouse peritoneal cavity is described. Under the conditions employed, a progressive increase in cell size occurs without significant cell division. This process is characterized morphologically by the accumulation of phase-dense and neutral red-positive granules, mitochondria, and lipid droplets. The phase-dense granules react strongly for acid phosphatase. Biochemical determinations indicate marked increases in the total content and specific activity of acid phosphatase, cathepsin, and ß-glucuronidase. The production of acid phosphatase is more rapid and extensive than that of the other two hydrolases. From these data it appears that the conversion of a monocyte-like cell to a mature macrophage is accompanied by the formation of increased numbers of lysosome-like cytoplasmic organelles.

Mouse peritoneal phagocytes stimulated in vivo with a bacterial lipopolysaccharide undergo a similar series of morphological and biochemical events.

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