Possible involvement of macrophages in embryo--maternal relationships during ovum implantation in the rat.

Distribution of phagocytic cells in the rat endometrium during the estrous cycle and early gestation was examined by histological, electron microscopic, and histochemical techniques. The results show that numerous macrophages emerge around the nidus shortly after the onset of ovum implantation. Such macrophages, however, were not present within the decidua, suggesting that this tissue may be a protective barrier against the migration of phagocytic cells towards the implants. Approximately 48 hours after the onset of implantation, the number of endometrial macrophages decreased dramatically.