Fig. P1.

Role of the two retrovirus-derived syncytin genes in mouse placentation. The placenta is the site where the fetal and maternal blood circulations come into close proximity. The interface between maternal and fetal blood is composed of two layers of multinucleated syncytial cells, ST-I and ST-II, which exchange nutrients and oxygen. In mice lacking the syncytin-A (SynA^−/−) or syncytin-B (SynB^−/−) gene, EM analyses revealed that the ST-I or ST-II layer, respectively, associated with those genes are unfused, unlike in normal placenta. Thus, two distinct retroviral genes, captured in evolutionary history by an ancestral rodent, are required for the formation of the two-layered placental syncytiotrophoblast. ST, syncytiotrophoblast layer; T, unfused trophoblast layer.