| Elongation is crucial for the proper form and function of the adult. In vertebrates, much of the body elongation occurs after gastrulation. The tailbud, the site of posterior body elongation, gives rise to the skeletal musculature and neural elements of the vertebral column of the trunk and tail. Many perturbations disrupt body elongation of the developing zebrafish, yet little is known about the mechanics required for linear elongation. Using 4-D confocal imaging, cell tracking, and quantitative metrics we define the cell flow field of the tailbud. We find a posterior flow that is both effective and coherent, with a high polarization of cell motion. At the posterior tip of the tailbud, this flow divides bilaterally and turns. Here the flow transitions away from coherence as cells increase their directional variability allowing for extensive cell mixing. Reducing Wnt or Fgf signaling disrupts the flow mechanics. Reducing the level of Wnt signaling disrupts the flow coherence. Reducing Fgf signaling reduces coherence and flow rate in the tailbud. Paradoxically, the stronger flow alterations under reduced Fgf signaling maintain linear elongation while nonlinear elongation results from weaker flow alterations under reduced Wnt signaling. Loss of cadherin 2 function also disrupts the flow mechanics but is not the mediator of Wnt or Fgf signaling regulated cell flow. Mathematical modeling predicts that interplay between the coherence and cell flow rate determines whether jamming forms at the posterior and whether the elongation is linear. Here we show that a balance between flow rate and coherence is required for linear elongation. |
