Meeting Abstract
At the beginning of pregnancy, the females of most placental mammals undergo a specific transformation of the uterine lining, the endometrial decidualization, which accommodates the invasive implantation of the conceptus, and supports the fetus through the variably long pregnancy. This decidualization in most mammals requires the signal from conceptus- hence it occurs only in the case of fertilization. In primate females and a few other species (e.g., bats), decidualization has been assimilated into the spontaneous ovarian cycle, and occurs irrespective of the presence of conceptus. Thus primate maternal physiology spontaneously undergoes a transformation into a pregnancy-like state after every ovulation. This state ends with menstruation, the shedding of the decidualized endometrial lining, followed by the regeneration of uterus before new proliferation occurs. Interestingly, the processes involved in triggering menstrual shedding – the inflammatory cytokine profiles, prostaglandins, cramping- are highly similar to the processes triggering birth. We propose that the parturitive pathways have been coopted into menstrual cycle to shed decidualized endometrium. As birth of course is present in non-menstruating species, and thus predates spontaneous decidualization and menstruation, this serial homology is referred to as secondary. Such occurrence of pregnancy-like physiology in non-pregnant state offers a great asset for non-invasive research on human pregnancy and parturition.
