Placental neurohormones: secretion and physiological implications

Abstract

Though the placenta is a unique organ with a key role in feto-maternal communication, species specificity is a limiting factor in investigating the in vivo role of human placenta. Placentation in human pregnancy is defined as hemochorioendothelial, because maternal blood, villus trophoblast and fetal capillaries constitute its basic anatomical elements. Trophoblastic tissue is exposed to two distinct blood flows, though the maternal is a vascular lake rather than distinct vessels. In successful gestation the trophoblastmaternal interaction is a key event, and factors (hormones, cytokines, growth factors, prostanoids) influencing the process of implantation, gestation and delivery are produced by both trophoblast and maternal decidua. Such endocrine activity of human placenta thus seems to play a role in the physiology of pregnancy from early gestation till term. Both polypeptide and steroid hormones are produced by trophoblastic tissue; in particular, human chorionic gonadotropin (hCG) and human placental lactogen (hPL) are acknowledged as classical placental hormones, and progesterone, estradiol and estriol are also produced at high levels (Simpson and MacDonald, 1981). Considering their high levels in maternal and fetal circulations, maternal and/or fetal targets have been, at least in part, defined for each placental hormone.