TADE unique and active mechanical properties. (A) Live cross-sections (XZ plane) of TADE, reconstructed from confocal Z stacks. Membranes are labeled with CMO. The cells unique T shape is seen, as well as membrane tubes on the apical surface. (B, Left) A snapshot from a movie that shows the animal’s dorsal and ventral epithelia moving independently (Movie S8). (B, Right) Optical plane separation analysis shows relative displacement between epithelia reaching 70 μm (∼10 cells) in 1 s. (C) A sketch comparing a single contraction in a thin, suspended tissue and in a cuboidal, adherent one (see model in SI Appendix, Supplementary Text 5). (D) A top view of live TADE stained with CMO shows membrane tubes. (Right) Zoom-in on a single cell, and stacking of the cell borders plane (magenta) and the excursing tubes plane (cyan). (E–G) TADE capability of extreme variation in cell size: (E) Applying compression in the Z direction on the animal results in 200–350% expansion in dorsal cell size before the first visible tear. (Inset) Whole animal view, FOV: 1.5 mm. (F) Treatment with ionomycin causes immediate contraction of all dorsal cells to ∼50% within a few seconds (Movie S9). (Inset) Whole animal view, FOV: 1.5 mm. (G) An animal left to die in the imaging chamber expanded its dorsal cell area by 400–700%. (H) Our hypothetical free body diagrams of a dorsal cell during contraction, expansion, and steady states. Red arrows mark regions of high tension and potential rupture (either membrane, cytoskeleton, or cell junctions). (I–N) Examples of variable TADE shapes in vivo: polygonal (I), wobbly (J), striated (K), elliptic/amorphic (L, Top Left). These shapes are commonly found in close proximity in space and change rapidly in time (L–N, Movie S10), implying local variability of stiffness. (O) A sketch of the animal from a top view, during locomotion. Color represents our hypothetical view of cells increasing (blue) or decreasing (red) their stiffness, and hence their shape, according to different patterns of external stress. (P) Schematics of possible scenarios in cellular sheets under tension: (i) Cell expansion due to softening may lead to cell rupture. (ii) Simultaneous contractions under the external constraint may lead to junctions’ detachment. (iii) The active cohesion hypothesis suggests active protection against the two rupture modes by asynchronous contractions and expansions, activated according to distinct, local mechanical cues.