The syncytiotrophoblast is the outer multinucleated layer of the trophoblast that directly interfaces with maternal blood in the intervillous spaces of the placenta. It forms a continuous, non-cellular layer that facilitates efficient maternal-fetal gas and nutrient exchange. This layer lacks a distinct cell membrane between nuclei, allowing for extensive surface area contact and prevention of maternal blood leakage. The cytotrophoblast lies beneath the syncytiotrophoblast and does not contact maternal blood directly. The amnion and chorion are fetal membranes located farther from maternal blood flow, involved in other supportive functions but not directly exposed to maternal circulation. Understanding the role of the syncytiotrophoblast is crucial for interpreting placental disorders and invasive trophoblastic diseases.

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