  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.
