Activity-driven extracellular volume expansion drives vertebrate axis elongation

The vertebrate bauplan is primarily established via the formation of embryonic tissues in a head-to-tail progression. The biomechanics of this elongation, which requires the presomitic mesoderm (PSM), remains poorly understood. Here, we find that avian PSM explants can elongate autonomously when physically confined in vitro , producing a pushing force that can largely account for the posterior elongation of the embryo. Tissue elongation results from volumetric expansion that is driven by cellular activity and accompanied by inhomogeneous increase of the extracellular fraction along the AP axis. We show that FGF signaling promotes glycolysis-dependent production of Hyaluronic Acid (HA), which is required for expansion of the posterior PSM. Our findings link body axis elongation to tissue expansion through the metabolic control of extracellular matrix production downstream of FGF signaling. One-Sentence Summary Active tissue expansion propels body elongation independent of cell proliferation-driven growth ### Competing Interest Statement A.R.H. is listed as inventor on a patent describing nanopore analysis of HA. The other authors declare no competing interests.