Causes and consequences of contractile force integration. (A) Distribution of apical tensile forces that accompany ventral furrow formation. Black arrows indicate the movement of lateral cells toward the ventral midline. Red arrows indicate epithelial tension, which is predominantly directed along the length of the furrow (a-p axis). (B) Tissue-level forces influence individual cell shape changes. Ventral furrow cells attempt to constrict isotropically (black arrows), but a-p tension (red arrows) resists constriction in this direction, resulting in anisotropic constriction. (C) Model for how contractile forces are integrated to generate epithelial tension in the ventral furrow. In wild-type embryos, pulses of actin (red) and myosin II (green) contraction constrict cell apices, and actomyosin fibers that remain on the apical surface between pulses maintain cortical tension (black arrows above cells), thus stabilizing cell shape. Actomyosin fibers linked by spot AJs (blue) form a supracellular meshwork that allows forces to be stably transmitted between cells, generating global epithelial tension (red arrows). In the absence of Twist, cells lack actomyosin fibers between contractions and fail to assemble a supracellular actomyosin meshwork. Thus, contraction pulses in cells stretch their immediate neighbors, and tension fails to be propagated across the epithelium.