Trophektoderm

Trophektoderm, also spelled trophectoderm, is the outer cell layer of the mammalian blastocyst that gives rise to the trophoblast lineage and forms the placenta and other extraembryonic structures such as the chorion. It emerges during the transition from the morula to the blastocyst, when outer cells acquire a distinct fate away from the inner cell mass, which forms the embryo proper. In most mammals, trophektoderm specification is regulated by a network of transcription factors including Cdx2, Eomes, and Tfap2c; Oct4 is downregulated in the trophektoderm. The trophektoderm proliferates and differentiates into cytotrophoblast, and in many species the cytotrophoblast further differentiates into syncytiotrophoblast and, for example, extravillous trophoblast in humans, which invade the maternal endometrium and remodel spiral arteries.

The trophoblast layer mediates implantation by attaching to and penetrating the uterine lining, establishing the fetal–maternal interface. The syncytiotrophoblast secretes pregnancy hormones such as human chorionic gonadotropin, while other trophoblast derivatives form the fetal portion of the placenta, including the chorion.

Clinical and research relevance: trophektoderm dysfunction or abnormal trophoblast differentiation is associated with implantation failure, pregnancy loss, and placental pathologies. Trophektoderm–derived trophoblast stem cells are used to study placental development and disease and to model early human development in vitro, subject to ethical considerations.