Response of epithelial cell and tissue shape to external forces in vivo

How actomyosin generates forces at epithelial adherens junctions has been extensively studied. However, less is known about how a balance between internal and external forces establishes epithelial cell, tissue and organ shape. We use the Drosophila egg chamber to investigate how contractility at adherens junction in the follicle epithelium is modulated to accommodate and resist forces arising from the growing germline. We find that between stages 6 and 9 adherens junction tension in the post-mitotic epithelium decreases, suggesting that the junctional network relaxes to accommodate germline growth. At that time, a prominent medial Myosin II network coupled to corrugating adherens junctions develops. Local enrichment of medial Myosin II in main body follicle cells resists germline-derived forces, thus constraining apical areas and consequently cuboidal cell shapes at stage 9. At the tissue and organ level, local reinforcement of medial-junctional architecture ensures the timely contact of main body cells with the expanding oocyte and imposes circumferential constraints on the germline guiding egg elongation. Our study provides insight into how adherens junction tension promotes cell and tissue shape transitions while integrating growth and shape of an internally enclosed structure in vivo.