<unk> s are specialized cells of the placenta that play an important role in embryo implantation and interaction with the decidualized maternal uterus. The core of placental villi contain mesenchymal cells and placental blood vessels that are directly connected to the fetus’ circulation via the umbilical cord. This core is surrounded by two layers of<unk> s, the cytotrophoblast and the syncytiotrophoblast. The cytotrophoblast is a layer of mono-nucleated cells that resides underneath the syncytiotrophoblast. The syncytiotrophoblast is composed of fused cytotrophoblasts which then form a layer that covers the placental surface. The syncytiotrophoblast is in direct contact with the maternal blood that reaches the placental surface. It then facilitates the exchange of nutrients, wastes and gases between the maternal and fetal systems. In addition, cytotrophoblasts in the tips of villi can differentiate into another type of<unk>  called the extravillous<unk> . Extravillous<unk> s grow out from the placenta and penetrate into the decidualized uterus. This process is essential not only for physically attaching the placenta to the mother, but also for altering the vasculature in the uterus. This alteration allows an adequate blood supply to the growing fetus as pregnancy progresses. Some of these<unk> s even replace the endothelial cells in the uterine spiral arteries as they remodel these vessels into wide bore conduits that are independent of maternal vasoconstriction. This ensures that the fetus receives a steady supply of blood, and the placenta is not subjected to fluctuations in oxygen that could cause it damage.
Trophoblast
Hemangioblast
Nurse cell
Clue cell
Elliptocyte
Echinocyte
Lymphoblast
Neuroblast