Stochasticity in endodermal fate decision making

How are cells specified to different fates during embryogenesis to form appropriately sized organs, in the right place, at the right time? Specification of endoderm versus mesoderm has long been thought to be driven by the ligand Nodal, acting as a graded morphogen. However, we have revised this classical view and shown that in zebrafish embryos, Nodal signalling establishes a bipotential progenitor state from which cells can stochastically switch to endodermal fate; the likelihood of switching is reduced by Fgf/Erk signalling. We hypothesise that temporal and spatial heterogeneity in Erk activity governs the stochastic switching process. To address this, we will take an interdisciplinary and cross-species approach, implementing cutting- edge live imaging approaches, multi-omics, and proteomics in zebrafish embryos and human gastruloids. This stochastic mechanism raises the crucial question of how a functional gastrointestinal system is generated if the number of initially specified endodermal progenitors is random. We have evidence for a downstream buffering mechanism that corrects the number of endoderm progenitors, ensuring proper formation of endoderm derivatives. We will elucidate the mechanism by which this buffering is achieved. Our work will reveal how signalling heterogeneity regulates cell fate decisions and will provide new knowledge to guide future tissue engineering endeavours.