Molecular mechanism of symmetry breaking in a 3D model of a human epiblast

Breaking the anterior-posterior (AP) symmetry in mammals takes place at gastrulation. Much of the signaling network underlying this process has been elucidated in the mouse, however there is no direct molecular evidence of events driving axis formation in humans. Here, we use human embryonic stem cells to generate an in vitro 3D model of a human epiblast whose size, cell polarity, and gene expression are similar to a 10-day human epiblast. A defined dose of bone mor-phogenetic protein 4 (BMP4) spontaneously breaks axial symmetry, and induces markers of the primitive streak and epithelial to mesenchymal transition. By gene knockouts and live-cell imaging we show that, downstream of BMP4, WNT3 and its inhibitor DKK1 play key roles in this process. Our work demonstrates that a model human epiblast can break axial symmetry despite no asymmetry in the initial signal and in the absence of extraembryonic tissues or maternal cues. Our 3D model opens routes to capturing molecular events underlying axial symmetry breaking phenomena, which have largely been unexplored in model human systems.