In Vivo Quantification of Biomechanical Cell Properties within Deep Tissues

Biomechanical properties of cells play a critical role in sculpting organs and embryos duringdevelopment and regeneration. Yet, quantification of mechanical properties of internalorgans and tissues in vivo without causing severe physiological damage remains challenging.Here, we present a non-invasive approach for deep-tissue measurements of single cellviscoelasticity, including the loss and storage moduli, based on the thermal fluctuations ofoptically trapped nanoparticles. Quantifying cellular viscoelasticity as deep as 100-150 μmwithin living embryos, we demonstrate that liver and foregut morphogenesis in zebrafishentails progenitor populations with varying mechanical properties. Gut progenitors exhibit ahigher elasticity compared to the more viscous rearranging neighboring tissues, indicatingthat viscoelastic properties influence specific morphogenetic behaviours. Their higherelasticity correlates with an increased cellular concentration of microtubules. This approachopens new possibilities for quantitative in vivo investigation of cell mechanics in biologicalsystems with complex 3D organization, such as embryos, explants or organoids.