Distribution and activation of uterine mononuclear phagocytes in peripartum endometrium and myometrium of the mouse

The present study tested the hypothesis that macrophage distribution and ac tivation are enhanced in the uterus before term. Mid-uterine horn tissue st rips from mice on Days 15 and 18 of pregnancy, the day of birth (= Day 19), and one day postpartum were paraffin-embedded and then sectioned, stained with a monoclonal pan-macrophage marker (BM8), and processed for visualizat ion and quantification of resident macrophages per nuclear area. Macrophage s were dispersed throughout the endometrium and subluminal epithelium; cell numbers declined on the day before term, then increased postpartum. Within myometrium, macrophages congregated in stroma surrounding muscle bundles, and staining was enhanced near term. Macrophage numbers were similar in pre gnant and postpartum uteri, enhanced more than 2-fold over those in nonpreg nant controls. Uterine sections were also analyzed by laser-scanning cytome try to enumerate activated macrophages (i.e., those that express the interc ellular adhesion molecule marker CD54+) and to determine cell cycle (propid ium iodide fluorescence). Activated macrophages were directly proportional to cell numbers and, by cell cycle analysis, were not terminally differenti ated. Highest cell numbers occurred on Day 15: 4-fold greater than those in nonpregnant controls and 2-fold higher than those at Day 18 or in postpart um groups. These findings indicate a decline in endometrial macrophage numb ers at least one day before the onset of parturition and raise the possibil ity that trafficking of this immune cell may contribute to onset of labor.