Fetal macrophages are not present in the myometrium of women with labor at term.

Objective: The basic mechanisms responsible for human parturition remain to be elucidated. The influx of fetal leukocytes into the myometrium has been recently proposed to be a crucial event in the onset of murine parturition. Surfactant protein-A has been implicated in the initiation of labor. In mice, it is thought that surfactant protein-A induces migration and subsequent activation of amniotic fluid macrophages of fetal origin, which then invades the myometrium. The present study was conducted to determine whether fetal macrophages invade the myometrium of women in labor.

Study Design: Placental bed biopsy specimens were obtained from patients in labor who delivered male neonates at term (n = 7). Myometrial sections of postpartum hysterectomy specimens obtained from women who delivered of male neonates (n = 3) were also analyzed. Formalin-fixed, paraffin-embedded sections were immunostained for CD68 or CD14 (which are markers for macrophages); immunoreactive myometrial macrophages were specifically procured by laser capture microdissection. Sex typing was done by polymerase chain reaction for the amelogenin gene with genomic DNA that was isolated from the macrophages. Chromogenic in situ hybridization with a Y chromosome-specific probe was also performed on paraffin-embedded histologic sections.

Results: Amelogenin allelotypes of the macrophages were consistent with female alleles in all cases that were tested, indicating a maternal origin. Chromogenic in situ hybridization demonstrated the absence of Y chromosome-positive mononuclear cells in the myometrium in all cases.

Conclusion: These observations, from a limited number of cases, suggest that migration of fetal macrophages from the amniotic cavity or the chorioamniotic membranes into the myometrium does not occur during human labor. The trafficking of fetal macrophages in labor seems to be different in humans and mice.