Fluctuations and differential contraction during regeneration of<i>Hydra vulgaris</i>tissue toroids

Abstract

We studied regenerating bilayered tissue toroids dissected from Hydra\nvulgaris polyps and relate our macroscopic observations to the dynamics of\nforce-generating mesoscopic cytoskeletal structures. Tissue fragments undergo a\nspecific toroid-spheroid folding process leading to complete regeneration\ntowards a new organism. The time scale of folding is too fast for biochemical\nsignalling or morphogenetic gradients which forced us to assume purely\nmechanical self-organization. The initial pattern selection dynamics was\nstudied by embedding toroids into hydro-gels allowing us to observe the\ndeformation modes over longer periods of time. We found increasing mechanical\nfluctuations which break the toroidal symmetry and discuss the evolution of\ntheir power spectra for various gel stiffnesses. Our observations are related\nto single cell studies which explain the mechanical feasibility of the folding\nprocess. In addition, we observed switching of cells from a tissue bound to a\nmigrating state after folding failure as well as in tissue injury.\n We found a supra-cellular actin ring assembled along the toroid's inner edge.\nIts contraction can lead to the observed folding dynamics as we could confirm\nby finite element simulations. This actin ring in the inner cell layer is\nassembled by myosin- driven length fluctuations of supra-cellular\n{\\alpha}-actin structures (myonemes) in the outer cell-layer.\n