trophoblast /trof″əblast′/ [Gk, trophe + blastos, germ] , the outermost layer of tissue that forms the wall of the blastocyst of placental mammals in the early stages of embryonic development. It functions in the implantation of the blastocyst in the uterine wall and in supplying nutrients to the embryo. At implantation the cells differentiate into two layers: the inner cytotrophoblast, which forms the chorion, and the syncytiotrophoblast, which develops into the outer layer of the placenta. Also called trophectoderm. −trophoblastic, adj.
