Abstract: Objective To investigate the effect of macrophages on embryo implantation by observing the distribution of
macrophages in mouse uterine tissues during the peri-implantation period. Methods Uterine tissues were collected from
pregnant (n=30) and pseudopregnant mice (n=30) during the peri-implantation period. The distributions of macrophages,
iNOS and leukemia inhibitory factor (LIF) were determined by immunohistochemistry and the correlations of macrophages
with iNOS and LIF were analyzed. Results Macrophages were located mainly in the endometrium before D4.5 in the pregnant
rats with D0.5 defined as the morning when a vaginal plug was observed. After D4.5, the macrophages was significantly
reduced in number (P<0.05) in the endometrium and gradually migrated to the perimetrium. In the psudopregnant mice,
macrophages were located mainly in the endometrium. Before D4.5, iNOS-positive cells were detected mainly in the
endometrium and the myometrium in the pregnant rats and became significantly reduced on D4.5 (P<0.05); in the
pseudopregnant mice, the positive cells were mostly detected in the endometrium. Significant differences were found in the
distribution of the macrophages and LIF between the implantation and non-implantation sites (P=0.013). LIF was mostly
located in the endometrium in the pregnant mice but scarcely detected in the pseudopregnant mice. Conclusion Macrophages
are located mainly in the endometrium and the implantation site where iNOS and LIF are expressed, suggesting the important
role of macrophages in the determination of implantation.