The trophoblast (from Greektrephein: to feed; and blastos: germinator) is the outer layer of cells of the blastocyst. Trophoblasts are present four days after fertilization in humans.[1] They provide nutrients to the embryo and develop into a large part of the placenta.[2][3] They form during the first stage of pregnancy and are the first cells to differentiate from the fertilizedegg to become extraembryonic structures that do not directly contribute to the embryo. After blastulation, the trophoblast is contiguous with the ectoderm of the embryo and is referred to as the trophectoderm. [4] After the first differentiation, the cells in the human embryo lose their totipotency because they can no longer form a trophoblast. They become pluripotent stem cells.
^Tang, Jiaqi; Liu, Bailin; Li, Na; Zhang, Mengshu; Li, Xiang; Gao, Qinqin; Zhou, Xiuwen; Sun, Miao; Xu, Zhice; Lu, Xiyuan (2020). "Development of Renin-Angiotensin-Aldosterone and Nitric Oxide System in the Fetus and Neonate". Maternal-Fetal and Neonatal Endocrinology. pp. 643–662. doi:10.1016/b978-0-12-814823-5.00038-6. ISBN978-0-12-814823-5. S2CID208378249.