A semithin light microscopic study of peritoneal cells of the mouse embryo.

Using semithin plastic sections, sequential changes in the mouse peritoneal cells during their intrauterine life were observed by light microscopy. At 11-13 days of gestation, the peritoneal cavity contained a small number of free mononuclear cells, singly scattered. Most of them were 9 to 12 micron in cell diameter, and the N-C (nucleocytoplasmic) ratio was smaller than 0.7. The cytoplasm contained large phagocytic vacuoles, and long processes extended from the cell surface. These cells were considered mature macrophages. After 15 days, the peritoneal cavity contained small-sized mononuclear cells in addition to mature macrophages. The small mononuclear cells were 5-10 micron in cell diameter, and formed the major cell type at this time. The cytoplasm contained occasional small vesicles but no phagocytic vacuoles. The small mononuclear cells showed a larger N-C ratio, 0.8-1.7. In 18-day-old fetuses, the peritoneal cells consisted of mononuclear cells, 79.5%, and mast cells, 20.5%. During the initial 18 days of embryonic life, small lymphocytes, neutrophils and eosinophils were not contained in the mouse peritoneal space.