Anterior expansion and posterior addition to the notochord mechanically coordinate embryo axis elongation

During development the embryo body progressively elongates from head-to-tail along the anterior-posterior (AP) axis. Multiple tissues contribute to this elongation through a combination of convergence and extension and/or volumetric growth. How force generated by the morphogenesis of one tissue impacts the morphogenesis of other axial tissues to achieve an elongated axis is not well understood. The notochord, a rod-shaped tissue possessed by all vertebrates, runs across the entire length of the somitic compartment and is flanked on either side by the developing somites in the segmented region of the axis and presomitic mesoderm in the posterior. Cells in the notochord undergo an expansion that is constrained by a stiff sheath of extracellular matrix, that increases the internal pressure in the notochord allowing it to straighten and elongate. Therefore, it is appropriately positioned to play a role in mechanically elongating the somitic compartment. Here, we use multi-photon mediated cell ablation to remove specific regions of the developing notochord and quantify the impact on axis elongation. We show that anterior notochord cell expansion generates a force that displaces notochord cells posteriorly relative to adjacent axial tissues and contributes to the elongation of segmented tissue during post-tailbud stages of development. Crucially, unexpanded cells derived from progenitors at the posterior end of the notochord provide resistance to anterior notochord cell expansion, allowing for force generation across the AP axis. Therefore, notochord cell expansion beginning in the anterior, and addition of cells to the posterior notochord, act as temporally coordinated morphogenetic events that shape the zebrafish embryo AP axis.