Cell density couples tissue mechanics to control the elongation speed of the body axis

SUMMARY The vertebrate body forms by addition of new tissues at the posterior end. This leads to body axis elongation which balances the anterior segmentation process to produce the stereotypic body plan. How elongation speed is constrained remains unknown. Here we utilised modeling and tissue force microscopy on chicken embryos to show that cell density of the posterior presomitic mesoderm (pPSM) dynamically modulates elongation speed in a negative feedback loop. Elongation alters the cell density in the pPSM, which in turn controls progenitor cell influx through the mechanical coupling of body axis tissues. This enables responsive cell dynamics in over- and under-elongated axes that consequently self-adjust speed to achieve long-term robustness in axial length. Our simulations and experiments further suggest that cell density and FGF activity act synergistically to drive elongation. Our work supports a simple mechanism of morphogenetic speed control where the cell density relates negatively to progress, and positively to force generation.