There are two clear phases to mammalian gastrulation each associated with a track of the primitive streak. The first one leads to the specification of anterior fates including the endoderm, craniofacial elements and the heart. A second phase, promotes axial elongation and fates associated with the trunk. Molecular work has shown that the anterior primitive streak is dependent on a GRN driven by Eomesodermin, whereas the posterior one depends on a different GRN driven by TbxT. Using gastruloids we have shown that these GRNs are independent. Furthermore, our work has revealed that each is driven by a different signal: Nodal for the Eomesodermin GRN and Wnt for the TbxT GRN. Using specific signalling regimes in gastruloids we can generate either the anterior or the posterior primitive streak suggesting that they are independent molecular modules. Our experimental system has allowed us to explore the relationship between the two modules and shown that gastrulation starts with high levels of Nodal and low levels of Wnt and that this relationship is inverted over time. allow. The inversion is driven by a cross inhibitory interactions of the two signalling molecules. The different fates and their sequence are dictated by the relative levels of Nodal and Wnt. Analysis of gene expression in mouse embryos shows that this relationship is associated with gastrulation. Furthermore, the transition from one module to another is associated with the suppression of Nodal signalling and we have shown that this is the case in gastruloids.
Analysis of gene expression in embryos of different species has shown that the two modules are conserved in all amniotes and that in all cases the transition coincides with the decay of Nodal signalling and the increase in Wnt signalling. This indicates that the two modules represent a conserved and universal feature of amniote gastrulation. Furthermore, we have shown the existence and function of both modules in human gastruloids, suggesting that they are likely to be functional in human embryos.
Additional outputs of the project are
• The standardization of the gastruloid culture that is now widely used to study early mammalian development with a significant reduction in the number of animals in experiments.
• A major contribution to the development of a replica model of primate gastrulation in a collaboration with Zhen Liu in Shanghai (China).
• Collaborations that have shown how gastruloids are a platform for disease modelling during somitogenesis and early blood formation.
• Significant contributions to the development of standards and an ethical framework for the field of stem cell based embryo models.
