Motility and adhesion gradient induced vertebrate body axis elongation and somite formation

The body of vertebrate embryos forms by posterior elongation from a terminal growth zone called the Tail Bud (TB). The TB produces highly motile cells that eventually constitute the presomitic mesoderm (PSM), a tissue playing an important role in elongation movements. PSM cells establish an anterior-posterior cell motility gradient which parallels a gradient associated with the degradation of a specific cellular signal (Fgf8) known to be implicated in cell motility. As Fgf8 degrades over time, anteriorly positioned cells move less, before eventually coming to a rest as they aggregate into epithelial somites. We show that simple microscopic and macroscopic mechano-chemical models for tissue extension that couple Fgf activity, cell motility, cell density and tissue rheology at both the cellular and continuum levels suffice to capture the speed and extent of elongation. These model qualitatively capture the condensation of cells into somites due to the effect of adhesion in the anterior region. These observations explain how the continuous addition of cells that exhibit an increase in cell density and a gradual reduction in motility combined with lateral confinement can be converted into somite formation in the anterior region and an oriented movement and drive body elongation.