Michaut, A.; Chamolly, A.; ALEGRIA, O.; Phan, C.; Corson, F.; Gros, J.

Despite progress in probing tissue mechanics, direct long-term measurements in live embryonic epithelia are lacking. This limits our understanding of amniote embryonic morphogenesis, which takes place over hours or days. Here, we introduce a combined technological and analytical framework to directly measure active forces and material properties in developing quail embryos, in a minimally invasive way. We find that the embryonic epithelium behaves elastically on short timescales with a modulus {approx} 2 nN/m but flows over longer timescales with a viscosity {approx} 4 N.s/m, and that both properties are under active regulation. Notably, we demonstrate that cell division is an active epithelial fluidizer, with division rates correlating with tissue fluidity. This fluidization is crucial for the emergence of the primary embryonic axis, which we show is shaped by a force {approx} 2 N. Altogether, our approach delivers a complete and quantitative view of early embryonic mechanics, and opens new avenues for biomechanical studies in live embryos and tissues.