Ectoderm tensile stress controls mesoderm internalization during embryo gastrulation.

Gastrulation is the dynamic reorganization of embryonic cells into a multi-stratified epithelial system from which inner and outer organs originate and a mature animal emerges. Tissue folding is one of the key morphogenetic processes driving gastrulation. While the biochemical signals are well deciphered, the biomechanical bases of tissue folding are still poorly understood. Here we study mesoderm invagination during Drosophila gastrulation, a process that results from the folding of the ventral embryonic tissue. The prospective mesoderm forms a furrow and eventually internalizes. How mesoderm internalization is mechanically controlled is still unclear. By combining spatio-temporal stress mapping, two-photon optogenetic-based force patterning, and in toto embryo imaging, we show that the ectoderm tensile stress is developmentally fine-tuned on both lateral sides of the embryo to control the internalization of the mesoderm. Thus, the ectoderm-mesoderm mechanical interplay is key to ensuring folding in embryo gastrulation.