part_of part of part of extraembryonic cell trophoblast cell A cell lining the outside of the blastocyst. After binding to the endometrium, trophoblast cells develop into two distinct layers, an inner layer of mononuclear cytotrophoblast cells and an outer layer of continuous multinuclear cytoplasm, the syncytiotrophoblast cells, which form the early fetal-maternal interface. cell FMA:83028 trophoblastic cell blastocyst