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Living tissues are active, multifunctional materials capable of generating, sensing, withstanding and responding to mechanical stress. These capabilities enable tissues to adopt complex shapes during development, to sustain those shapes during homeostasis and to restore them during healing and regeneration. Abnormal stress is associated with a broad range of pathological conditions, including developmental defects, inflammatory diseases, tumour growth and metastasis. A number of techniques are available to measure mechanical stress in living tissues at cellular and subcellular resolution. 2D techniques that map stress in cultured cell monolayers provide the highest resolution and accessibility, and include 2D traction force microscopy, micropillar arrays, monolayer stress microscopy and monolayer stretching between flexible cantilevers. Mapping stresses in tissues cultured in 3D can be achieved using 3D traction force microscopy and the microbulge test. Techniques for measuring stress in vivo include servo-null methods for measuring luminal pressure, deformable inclusions, Forster resonance energy transfer tension sensors, laser ablation and computational methods for force inference. Although these techniques are far from becoming everyday tools in biomedical laboratories, their rapid development is fostering key advances in our understanding of the role of mechanics in morphogenesis, homeostasis and disease. Methods for measuring stress in living cells, tissues and organs are advancing steadily and are increasingly being used for biomedical applications. In this Review, we discuss the concept of tissue stress and the techniques available to measure it in 2D and 3D cell and tissue cultures and in vivo.

Fundamental biological processes are carried out by curved epithelial sheets that enclose a pressurized lumen. How these sheets develop and withstand three-dimensional deformations has remained unclear. Here we combine measurements of epithelial tension and shape with theoretical modelling to show that epithelial sheets are active superelastic materials. We produce arrays of epithelial domes with controlled geometry. Quantification of luminal pressure and epithelial tension reveals a tensional plateau over several-fold areal strains. These extreme strains in the tissue are accommodated by highly heterogeneous strains at a cellular level, in seeming contradiction to the measured tensional uniformity. This phenomenon is reminiscent of superelasticity, a behaviour that is generally attributed to microscopic material instabilities in metal alloys. We show that in epithelial cells this instability is triggered by a stretch-induced dilution of the actin cortex, and is rescued by the intermediate filament network. Our study reveals a type of mechanical behaviour—which we term active superelasticity—that enables epithelial sheets to sustain extreme stretching under constant tension.