The structure of unstimulated mouse peritoneal phagocytes has been examined by electron microscopy and compared to cells obtained from the inflamed peritoneum and from cultures maintained in vitro.

The unstimulated cell resembles the blood monocyte and contains a moderate amount of rough surfaced endoplasmic reticulum, a small but well defined Golgi apparatus and a few, small, electron-opaque granules in the cytoplasm.

During in vitro cultivation there are marked changes in cell ultrastructure. Most prominent is the formation of large electron-opaque granules, some of which have a complex matrix containing both electron-opaque and lucent vesicles. In addition, there is an increase in size of the Golgi apparatus with the appearance of new lamellae and tiny, smooth surfaced vesicles. With continued cultivation, large lipid droplets are found in apposition to the rough endoplasmic reticulum. The formation and size of electron-opaque granules as well as the enlargement of the Golgi region is stimulated by high concentrations of serum in the medium.

Cells obtained from the peritoneal cavity of lipopolysaccharide stimulated animals demonstrated changes in ultrastructure similar to those seen in cells cultured in vitro.

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