Embryonic patterning is dependent on the establishment of the anteroposterior (AP) and dorsoventral axes early in development. In mammalian embryos, these axes are established by a breaking of symmetry in the epiblast, which involves signals from the extra-embryonic tissues. However, the molecular mechanisms that control this process are still not fully understood. On p. 3894, David Turner, Alfonso Martinez Arias and colleagues use gastruloids, three-dimensional aggregates of mouse embryonic stem cells, as a tool to unravel the signalling pathways that establish AP polarity in mammalian embryos. The authors demonstrate that these gastruloids can develop an AP axis in the absence of extra-embryonic tissue, instead depending on precisely timed interactions between Wnt and Nodal signalling. They also show that BMP signalling is dispensable for AP axis formation. This research demonstrates the powerful potential of gastruloids as a tool to understand the molecular mechanisms that underpin early embryonic development. Together, their results suggest that extra-embryonic tissues do not induce axis formation per se, but rather bias the critical symmetry-breaking event in embryo development, furthering our understanding of the molecular control of embryonic patterning.