The Universe before Embryo Implantation

A preimplanting embryo (pseudocolored and modified) superimposes on the luminal endometrial surface of an embryonic day 3.5 mouse uterus (scanning electron micrograph, pseudocolored). Implantation occurs during the ensuing day, and involves a new receptor-mediated pathway that utilizes lysophosphatidic acid (Ye et al, Nature, 2005).