Biomechanics of anteroposterior axis elongation in the chicken embryo

In vertebrate embryos, anteroposterior axis elongation is a crucial developmental process resulting in the establishment of the basic body plan and the growth of the embryo from the tail region. In chicken, it has been proposed that a gradient of random cell motility along the presomitic mesoderm (PSM), rather than directed cell movements, drives axis elongation (Bénazéraf et al, Nature 2010). In order to access the physical mechanisms that could explain this process, we study the rheological properties of the PSM and the forces produced due to posterior PSM expansion. Both micropipette aspiration, as well as rounding experiments, show a liquid-like behavior for the PSM, and allow us to measure its viscoelastic properties. In addition, we have developed a novel cantilever-based system to measure the force of axis elongation and we relate it to the cell movements along the axis. We demonstrate that an isolated PSM explant elongates autonomously and contributes to the total elongation force of the embryo, highlighting its role in axis elongation. Taken together, our results provide a first quantitative description of the mechanics of the tail region in the chicken embryo, which will be essential for future modeling of axis elongation.