Mechanical characterization of regenerating Hydra tissue spheres Mechanics of regenerating Hydra.

Hydra vulgaris, long known for its remarkable regenerative capabilities, is also a longstanding source of inspiration for models of spontaneous patterning. Recently, it became clear that early patterning during Hydra regeneration is an integrated mechano-chemical process where morphogen dynamics is influenced by tissue mechanics. One roadblock to understand Hydra self-organization is our lack of knowledge about the mechanical properties of these organisms. In this paper, we combined microfluidic developments to perform parallelized microaspiration rheological experiments and numerical simulations to characterize these mechanical properties. We found three different behaviors depending on the applied stresses: an elastic response, a visco-elastic one and tissue rupture. Using models of deformable shells, we quantify their Young's modulus, shear viscosity as well as the critical stresses required to switch between behaviors. Based on these experimental results, we propose a description of the tissue mechanics during normal regeneration. Our results provide a first step towards the development of original mechano-chemical models of patterning grounded in quantitative, experimental data.