A TgfβRI/Snai1-dependent developmental module at the core of vertebrate axial elongation

Formation of the vertebrate postcranial body axis follows two sequential but distinct phases. The first phase generates pre-sacral structures (the so-called primary body) through the activity of the primitive streak (PS) on axial progenitors within the epiblast. The embryo then switches to generate the secondary body (post-sacral structures), which depends on axial progenitors in the tail bud. Here we show that the mammalian tail bud is generated through an independent developmental module, concurrent but functionally different to that generating the primary body. This module is triggered by convergent TgfβRI and Snai1 activities that promote an incomplete epithelial to mesenchymal transition (EMT) on a subset of epiblast axial progenitors. This EMT is functionally different to that coordinated by the PS, as it does not lead to mesodermal differentiation but brings axial progenitors into a transitory state, keeping their progenitor activity to drive further axial body extension.