Our multi-cellular system of choice is the cnidarian Hydra vulgaris. However, this field is still in its very infancy and many questions remain to be investigated. as a solid or fluid, in order to explain our experimental findings. We try to close the gap and investigate physical phenomena at a mesoscopic level by combining a minimum of sub-cellular and molecular structures with a coarse-grained description, e.g. However, the findings about single cells have been integrated into the picture only rudimentarily so far. The question of what this complexity is required for may be partially answered by the living conditions of cells in a collective environment. The impressive recent results of cellular and molecular biophysics, however, have revealed a surprising complexity of the cytoskeletal dynamics. Regeneration and growth of tissues have been investigated mainly on two scales, the macroscopic one, where the tissue is considered as a piece of continuous material, and the molecular one, where tissue dynamics is reduced to biochemical signalling. This actin ring in the inner cell layer is assembled by myosin-driven length fluctuations of supra-cellular F-actin bundles (myonemes) in the outer cell layer. Its contraction can lead to the observed folding dynamics as we could confirm by finite element simulations. We found a supra-cellular actin ring assembled along the toroid's inner edge. In addition, we observed switching of cells from a tissue bound to a migrating state after folding failure as well as in tissue injury. Our observations are related to single-cell studies which explain the mechanical feasibility of the folding process. We found increasing mechanical fluctuations which break the toroidal symmetry, and discuss the evolution of their power spectra for various gel stiffnesses. The initial pattern selection dynamics was studied by embedding toroids into hydro-gels, allowing us to observe the deformation modes over longer periods of time. The time scale of folding is too fast for biochemical signalling or morphogenetic gradients, which forced us to assume purely mechanical self-organization. Tissue fragments undergo a specific toroid–spheroid folding process leading to complete regeneration towards a new organism. We studied regenerating bilayered tissue toroids dissected from Hydra vulgaris polyps and relate our macroscopic observations to the dynamics of force-generating mesoscopic cytoskeletal structures.
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