Placental development, also known as placentation, is orchestrated by precise molecular and cellular interactions between extraembryonic cells and cells of the pregnant uterus. The major extraembryonic cell type of the placenta is the trophoblast cell, which arises from the trophectoderm cell lineage (1 ). Trophoblast cells make the physical connection between the embryo and the maternal environment and play important roles in the implantation process and placental function (2 ). In mice, trophoblast cells lead the embryonic invasion into the uterine stroma to achieve implantation, and contribute to the formation of a temporary yolk-sac placenta (choriovetelline placenta) during early postimplantation stages. Trophoblast cells eventually provide the major extraembryonic cell lineage that contribute to the formation of the mature chorioallantoic placenta (3 ). In the chorioallantoic placenta, the fetal and maternal circulatory systems are brought into close apposition to allow for a range of physiological interactions that are critical for fetal growth and development. Abnormal placentation is often associated with early embryonic mortality (4 -6 ) and can lead to serious pregnancy disorders such as preeclampsia (7 ).
