Dorsal forerunner cells transmit epiboly forces to extend the zebrafish notochord

Convergence and extension (C&E) cell movements promote anteroposterior axis extension and narrow both neuroectodermal and mesodermal tissues during gastrulation. Mediolateral cell intercalation is largely responsible for this morphogenetic process in vertebrates, but evidence suggests that additional cell-extrinsic forces generated by surrounding tissues contribute to the shaping of many structures. In zebrafish, for example, mechanical forces generated by the anteriorly migrating prechordal plate cooperate with tissue-autonomous cell intercalations to promote extension of the notochord. Here we propose a novel model for notochord morphogenesis by which mechanical epiboly forces within the enveloping layer are transmitted to the posterior end of the notochord via a cluster of dorsal forerunner cells (DFCs) that physically links the two. We found that scattered or absent DFCs caused by loss of crb2a or sox32, respectively, reduces notochord C&E and exacerbates axis extension defects in planar cell polarity signaling-deficient embryos. Using an automated image segmentation and cell shape analysis pipeline, we show that cells within the posterior notochord fail to properly elongate when DFCs are scattered or absent. Finally, we demonstrate that loss of crb2a and sox32 fails to disrupt C&E of zebrafish embryonic explants in which no epiboly occurs and all extension is driven by cell-intrinsic behaviors. Together, these findings support a model in which DFCs facilitate mechanical coupling of the enveloping layer to the posterior notochord during epiboly to ensure robust morphogenesis of the notochord during gastrulation.