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 trophoblast called the extravillous trophoblast. Extravillous trophoblasts grow out from the placenta