Tissue spreading couples gastrulation through extracellular matrix remodelling in early avian embryos

Tissue spreading (epiboly) couples gastrulation to shape the initial body plan of early vertebrate embryos. How these two large-scale collective cell movements cooperate remains unclear. Here, we examine the cell mechanics and tissue dynamics underlying epiboly of the chicken blastoderm. We found that cells at the blastoderm edge undergo a wetting-like process to spread on the vitelline membrane through stiffness sensing and cytoskeleton remodelling. This interaction is robust to edge cell loss and cooperates with cell-proliferation-based blastoderm growth to drive epiboly. Surprisingly, epiboly in turn remodels the extracellular matrix (ECM) to establish a basal lamina and maintain cell-cell connections. Impairing either edge cell wetting or the ECM causes tissue thickening and buckling across the blastoderm, disrupting gastrulation movements. We conclude that epiboly facilitates gastrulation by organizing an ECM that maintains a thin blastoderm. These findings suggest a general logic of mechanical coupling between distinctly controlled tissue movements during early development.