PLACENTAL AMINO-ACID-TRANSPORT

Normal fetal growth and development depend on a continuous supply of a mino acids from the mother to the fetus. The placenta is responsible f or the transfer of amino acids between the two circulations. The human placenta is hemomonochorial, meaning that the maternal and fetal circ ulations are separated by a single layer of polarized epithelium calle d the syncytiotrophoblast, which is in direct contact with maternal bl ood. Transport proteins located in the microvillous and basal membrane s of the syncytiotrophoblast are the principal mechanism for transfer from maternal blood to fetal blood. Knowledge of the function and regu lation of syncytiotrophoblast amino acid transporters is of great impo rtance in understanding the mechanism of placental transport and poten tially improving fetal and newborn outcomes. The development of method s for the isolation of microvillous and basal membrane vesicles from h uman placenta over the past two decades has contributed greatly to thi s understanding. Now a primary cultured trophoblast model is available to study amino acid transport and regulation as the cells differentia te. The types of amino acid transporters and their distribution betwee n the syncytiotrophoblast microvillous and basal membranes are somewha t unique compared with other polarized epithelia. These differences ma y reflect the unusual circumstance of this epithelium that is exposed to blood on both sides. The current state of knowledge as to the types of transport systems present in syncytiotrophoblast, their regulation , and the effects of maternal consumption of drugs on transport are di scussed.