Macrophage trafficking in the uterus and cervix precedes parturition in the mouse

Immune activation is implicated in the etiology of preterm labor, but little is known about macrophage number or distribution in the uterus or cervix at term. This study tested the hypothesis that macrophages migrate into the reproductive tract before the onset of parturition. Paraffin-embedded sections from the mid-uterine horn and cervix of C3/HeN mice on Days 15 and 18 of pregnancy, the day of birth (Day 19), and 1 day postpartum were stained with a pan-macrophage marker to analyze cell numbers and distribution. During pregnancy, uterine macrophages were dispersed in endometrium, usually associated with vasculature and subluminal epithelium. In myometrium, macrophages were clustered in stromal connective tissue; near term and postpartum, cells appeared to surround the muscle bundles. Total macrophage numbers were increased on Day 15 relative to those in nonpregnant controls, declined before birth, and increased postpartum. In the cervix, macrophages congregated in subepithelium, often perivascular or near ganglia. Macrophage numbers in the cervix peaked on Day 18, then declined to nonpregnant levels by the day after birth. Thus, macrophage numbers in the uterus were inversely related to those in the cervix. These findings raise the possibility that macrophages and their products may be involved in uterine contractility and cervical remodeling during the processes of parturition.