Sheets of epithelial cells, which cover the surface of an organism’s tissues, can deform and fold into complex structures during development and other physiological processes. Xavier Trepat and colleagues study the mechanical properties of these sheets in three dimensions by producing arrays of epithelial ‘domes’ with controlled shape and size. They report that, at constant tension, individual cells within the sheet can reversibly stretch to different extents—from barely deformed cells to those reaching 1,000% areal strain. At a molecular level, the cellular instability is triggered by a stretch-induced dilution of the actin cortex and is rescued by the intermediate filament network. They conclude that epithelial sheets show a new type of mechanical behaviour, which they call active superelasticity. This is intriguing as superelasticity is a mechanical response that is generally attributed to microscopic material instabilities in metal alloys.
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