trophoblast

Trophoblast is the outer cell layer of the mammalian blastocyst that provides the initial contact with the maternal uterus and ultimately gives rise to the placenta. It forms a critical interface between the developing embryo and the mother, enabling implantation and nutrient exchange.

During early development, the trophoblast differentiates into two main lineages: cytotrophoblast and syncytiotrophoblast. The cytotrophoblast consists of proliferative mononucleated cells that reside beneath the surface layer. Through fusion, cytotrophoblasts form the syncytiotrophoblast, a multinucleated layer that covers the outer surface of the developing villi. The syncytiotrophoblast is invasive and remodels the maternal uterine tissue, while extravillous trophoblasts derive from cytotrophoblasts and invade the decidua and spiral arteries to establish and regulate placental blood flow.

The placental villi, lined by trophoblast, facilitate nutrient and gas exchange between mother and fetus. The placenta also secretes hormones, notably human chorionic gonadotropin (hCG), which supports pregnancy until placental function is fully established.

Clinical relevance: abnormal trophoblast proliferation or invasion can lead to trophoblastic diseases, such as hydatidiform mole and choriocarcinoma, or contribute to placental implantation abnormalities (e.g., placenta accreta spectrum). Trophoblasts interact with the maternal immune system, expressing immunomodulatory molecules like HLA-G to promote maternal-fetal tolerance.

Overall, the trophoblast is essential for implantation, placental formation, maternal blood flow adaptation, and hormonal support of pregnancy, while its dysfunction can underlie several obstetric conditions.