Zebrafish embryos are robust to mechanical perturbations

The investigation of morphogenesis during organism development has massively benefited from the interaction between developmental biology and biophysics, which gave new quantitative insights into natures physical working principles. The process of zebrafish epiboly provides a beautiful model system as cells exhibit collective migration, meanwhile requiring symmetry breaking for gastrulation and the subsequent tissue differentiation. These elegantly robust processes are facilitated by both biochemical and mechanical cues. Here we test the hypothesis that the future body axis is robust against mechanical perturbations of tissue flow and tissue stress fields. For this, we trigger tissue flow by photoablation, which leads to an externally enforced symmetry breaking before natural symmetry breaking of gastrulation occurs. Using 3D light sheet microscopy, we obtained nuclei trajectories as a proxy for the migration of individual cells. Analyzing the derived tissue velocity field, we observed that epiboly is highly robust and mostly unaffected by ablation damage. However, the photoablation induced a convergent motion of cells towards an azimuthal angle resembling shield formation. We extensively characterized this convergent motion and checked if this tissue flow is able to reorient natural symmetry breaking during shield formation. While ablations can transiently disrupt or reorient the convergent motion, we cannot confirm any redirection of the location where the future body axis of the animal develops. The observations were also highly dependent on the developmental stage of the embryo, which cast a greater and more permanent impact, suggesting strong control exerted by factors other than mechanical signals. Significance statementMorphogenesis emerges from the interplay between biochemical patterning and mechanical forces, yet the extent to which mechanical cues influence embryo developmental remains unclear. Using zebrafish epiboly as model system, we imposed controlled mechanical perturbations through targeted photoablation and tracking resulting 3D cell movements. We show that although ablations generate transient convergent motions, epiboly is remarkably robust to externally induced tissue flows. Our findings indicate that the future body axis is strongly buffered against mechanical disturbances, highlighting the dominant role of developmental stage and non-mechanical cues in guiding development.