Cell-cell communication controls the timing of gastruloid symmetry-breaking

In the embryo, morphogenetic signals instruct regional patterning thereby defining the body axes of the future animal. Remarkably, in the absence of such signals, collections of pluripotent stem cells can still self-organise and break symmetry in vitro . One such example is gastruloids, three-dimensional stem cell aggregates that form an anterior-posterior axis through the polarised expression of the gene Brachyury/T. How robust and reproducible cell proportions are achieved in these self-organised embryo-like structures is not understood. Here, through quantitative experiments and theoretical modelling, we dissect tissue rheology and cellular feedback in gastruloids. We show that the initial population of Brachyury-expressing cells critically influences the timing of symmetry-breaking. We propose a cell differentiation model, whereby pluripotent cells inhibit mesoderm differentiation, accounting for the observed cell fate dynamics. Our findings suggest that cell-cell communication dictates temporal cell proportions, while differential tissue mechanics governs spatial pole formation. Our work highlights the importance of initial cell heterogeneity in gastruloid development and offers a framework to identify feedback mechanisms in multicellular systems, advancing our understanding of how embryo-like structures self-organise.