Hormonal regulation and implication of cell signaling in calcium transfer by placenta.

During pregnancy, the human placenta transfers about 30 g of calcium (Ca2+) from the mother to the fetus. This transfer is mainly done during the third trimester, at a rate of 140 mg/(kg x d). It allows adequate growth and development of the fetus, since Ca2+ is vital for the mineralization of the fetus's skeleton and many cellular functions. Because Ca2+ flows through the placenta against an electrochemical gradient, calcemic hormones could also be involved to overcome this gradient. Hormones such as calcitonin, parathyroid hormone (PTH), and PTH-related peptide (PTHrP) have been found in maternal and fetal circulation, and they originate from both parties, as well as from the placenta in the case of PTHrP. As the placenta possesses most of the G-protein-coupled receptors to bind these hormones, it is likely that they play an important role in maternal-fetal Ca2+ homeostasis. More studies are needed to assess the importance of these hormones in the regulation of Ca2+ management during pregnancy, and to understand better the cell-signaling pathways involved. This article addresses the current knowledge in this field to guide future investigations on the roles, functions, and localizations of the components involved during Ca2+ transfer by syncytiotrophoblasts.