Universal Calcium fluctuations in Hydra morphogenesis

Understanding how the collective physical processes drive robust morphological transitions in animal development requires the characterization of the relevant fields underlying morphogenesis. Calcium (Ca2+) is known to be such a field. Here we show that the Ca2+ spatial fluctuations, in whole-body Hydra regeneration, exhibit universal properties captured by a field-theoretic model describing fluctuations in a tilted double-well potential. We utilize an external electric field and Heptanol, a drug blocking gap junctions, as two separate controls affecting the Ca2+ activity and pausing the regeneration process in a reversible way. Subjecting the Hydra tissue to an electric field increases the calcium activity and its spatial correlations, while applying Heptanol inhibits the activity and weakens the spatial correlations. The statistical characteristics of the Ca2+ spatial fluctuations, i.e., the coefficient of variation and the skewness - exhibit universal shape distributions across tissue samples and conditions, demonstrating the existence of global constraints over this field. Our analysis shows that the Hydra's tissue resides near the onset of bistability; the local Ca2+ activity in different regions fluctuates between low and high excited states. The controls modulate the dynamics near that onset, preserving the universal characteristics of the Ca2+ fluctuations and, by that, maintaining the tissue's ability to regenerate.