Stem cell models of human embryo implantation and trophoblast invasion

Stem cell–based embryo models have taken the scientific community by storm as they enable investigation of previously inaccessible stages of human development. Here, we discuss how stem cell–based embryo and placenta models can shine a light on two elusive and intertwined aspects of human development that are critical for successful pregnancy: the implantation of the blastocyst into the endometrium and the subsequent invasion of placental villi deep inside the maternal tissues. Both of these processes are mediated by the trophoblast lineage, which is specified in the preimplantation embryo and can be modeled using naïve pluripotent stem cells. We review how embryo and placenta models built from naïve stem cells can be leveraged to obtain mechanistic insights into human implantation and trophoblast invasion.