The creation of a synthetic stem cell-derived mouse embryo with natural brain, heart, and other organ development is reported in Nature this week. The embryo represents a new model that could be used to study mechanisms of development and disease without the need for experimental animals. Embryonic stem cells (ESCs) are capable of undergoing many aspects of mammalian embryogenesis in vitro, but they do not follow all of the stages of natural embryo development. In this week's study, a team led by scientists from the University of Cambridge used three types of mouse stem cells — ESCs, trophoblast stem cells, and inducible extraembryonic endoderm stem cells — to create a model that recapitulated whole natural mouse embryo development in utero to day 8.5 including the establishment of all brain regions, a neural tube, a beating heart, and a gut tube. Additional experimentation showed that knocking out the gene Pax6, which controls sensory organ development, in the model resulted in similar effects as those seen in natural Pax6 knockout embryos. "We anticipate the widespread application of this system to dissect molecular pathways and to screen for chemical entities that affect embryogenesis," the researchers write.