trofoblast

Trophoblast, sometimes spelled trofoblast, is the outer cell layer of the mammalian blastocyst that gives rise to the placenta and other extraembryonic tissues. After fertilization, the trophoblast differentiates from the surrounding trophectoderm of the blastocyst and interacts with the maternal endometrium to establish the fetal–maternal interface. The trophoblast forms the chorion and supports early placental development, including nutrient exchange and hormone production.

Two main lineages are present in the trophoblast: the cytotrophoblast, a proliferative, mononuclear layer; and the syncytiotrophoblast, a continuous, multi-nucleated network formed by fusion of cytotrophoblast cells. A third group, the extravillous trophoblast (EVT), invades the decidua and remodels maternal spiral arteries to improve uteroplacental blood flow. The syncytiotrophoblast is particularly important for hormone secretion, including human chorionic gonadotropin (hCG), while cytotrophoblast serves as a reservoir for renewing the trophoblast layers. EVTs contribute to maternal tissue remodeling and immune modulation at the implantation site.

Clinical relevance: Disruptions in trophoblast invasion or proliferation can impair placental development and function, contributing to conditions such as preeclampsia, intrauterine growth restriction, placenta accreta, and hydatidiform mole. Trophoblasts also participate in immune tolerance at the maternal–fetal interface and express proteases and receptors that facilitate invasion and placental remodeling.