The syncytiotrophoblast forms the main component of the placental barrier because it is the outer multinucleated layer of the trophoblast that directly interfaces with maternal blood in the intervillous spaces. This layer facilitates nutrient and gas exchange while maintaining a selective barrier to protect the fetus from harmful substances. Unlike the cytotrophoblast, which is a cellular layer beneath, the syncytiotrophoblast has no cell boundaries, allowing efficient transport and secretion of hormones such as hCG. The amnion and decidua basalis are maternal or fetal membranes but do not contribute primarily to the placental barrier function. Understanding this is crucial in surgeries involving the placenta to avoid disrupting this key exchange layer.

Reference: Gray's Anatomy, Standring S., 42nd Edition.