Trophoblast

The trophoblast forms the outer layer of the blastocyst, providing the embryo with nutrients and giving rise to the outer chorionic sac and the fetal component of the placenta. The trophoblast facilitates embryonic implantation via its inner layer of mononuclear cytotrophoblasts and its outer layer of multinuclear cytoplasm, the syncytiotrophoblasts, which form the early connections between the embryo and the maternal endometrium. The trophoblast is evident as early as day 6 in human embryos.