In the past decades numerous methods have been developed in order to achieve in situ stress measurements, using different and complementary techniques for reviews see Sugimura et al. Quantitative studies about the role of mechanical constraints in morphogenesis and development benefit from a precise and quantitative knowledge of the spatial distribution of mechanical stresses, from the subcellular scale to the tissue scale, and of its temporal evolution. Hence, the impact of mechanics on tissue fate and organization is considerable, either for healthy organisms during embryo development ( Krieg et al., 2008 Le Goff et al., 2013 Heisenberg and Bellaïche, 2013 Hiramatsu et al., 2013 Hamada, 2015 Herrera-Perez and Karen, 2018), or in pathological conditions ( Wells, 2013 Delarue et al., 2014b Angeli and Stylianopoulos, 2016). The complete process is thus regulated under the dual control of genetics and mechanics, which mutually feed back to each other, and drive the growth and shape of tissues ( Desprat et al., 2008). These rearrangements are possible because cells can generate and exert mechanical stresses on their surroundings, or conversely feel the stresses and transduce them into biological signals. The cohesion and morphogenesis of living tissues require coordinated processes at the cellular scale, based on changes in cell number, size, shape, position and packing ( Heisenberg and Bellaïche, 2013 Guirao et al., 2015). In vitro, within cell aggregates, and in vivo, in the prechordal plate of the zebrafish embryo during gastrulation, our pipeline of techniques demonstrates its efficiency to directly measure the three dimensional shear stress repartition within a tissue. The local shear stress tensor is retrieved from the sensor shape, accurately reconstructed through an active contour method. Its mechanical properties are carefully calibrated in situ, for a sensor embedded in a cell aggregate submitted to uniaxial compression. The elastomer rigidity after polymerization is adjusted to the tissue rigidity. Droplets of a polydimethylsiloxane mix, made fluorescent through specific covalent binding to a rhodamin dye, are produced by a microfluidics device. These techniques combine the advantages of incompressible liquid droplets, which have been used as precise in situ shear stress sensors, and of elastic compressible beads, which are easier to tune and to use. To investigate the role of mechanical constraints in morphogenesis and development, we have developed a pipeline of techniques based on incompressible elastic sensors.
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