The uterine lumen of the pregnant guinea-pig contains a large macrophage population

Cellular constituents of the uterine lumen were investigated. Fourteen pregnant sows of 40+ days' gestation were anaesthetized and naturally occurring peritoneal fluid was collected. A uterine horn was delivered and 0.25 ml Gey's solution injected into the uterine lumen to collect free cells. The fluid was aspirated into a syringe and the cells extracted, counted and prepared for phagocytosis experiments and microscopy. The cells were stained with alpha-naphthyl-acetate-esterase (ANAE) to determine the fraction that was non-specific esterase-positive, a feature of mononuclear phagocytes. Differential cells counts were also made. Both uterine and peritoneal compartments yielded large numbers of cells (greater than 10⁶/ml). Peritoneal fluid cells were 47 ± 6 per cent (SD) macrophages and 49 ± 6 per cent eosinophils (the remainder being 'other" cells); 47 ± 6 per cent also stained positively for ANAE. Uterine cells were 78 ± 12 per cent macrophages, the remainder being mostly lymphocytes (18 ± II per cent); 85 ± 13 per cent stained positively with ANAE. Electron microscopy of the uterine cells confirmed that most had morphology consistent with being mononuclear phagocytes. Uterine and peritoneal cells phagocytized carbon particles and yeast cells when incubated at 37°C. The origin and role of this macrophage population is unknown but uterine lumenal macrophages may be present to remove antigen-antibody complexes thus facilitating uptake of maternally derived IgG by the fetal yolk sac.