Hydra morphogenesis as phase transition Dynamics

A remarkable hallmark of animal morphogenesis is the robust convergence of this process into a stereotypic body form of a viable organism. Hydra regeneration provides a unique experimental setup, allowing us to develop a physics framework for this pattern-formation process. We demonstrated before that by tuning an external electric field, we can drive morphogenesis in whole-body Hydra regeneration, back- and-forth, around a critical point. Our control of morphogenesis allows us to identify and characterize the primary morphological transition in the regeneration process — from a spherical structure to an elongated cylindrical shape, the final body form of a mature Hydra. We found that spatial fluctuations of the calcium distribution in the Hydra’s tissue drive this transition, and constructed a field-theoretic model that explains the morphological transition as a first-order–like phase transition. The theory predicts and the experiments show the prominent role of noise in activating Hydra morphogenesis. Our experiments allow us to quantify the level of noise and to show that the tension between the apparent deterministic dynamics of morphogenesis and the inevitable influence of noise is resolved by extraordinary natural tuning of the intrinsic noise level, facilitating the self-organized developmental process. A periodically modulated electric field leads to a new induced phase of the regenerating tissue, showing stochastic morphological swings, back and forth, between a nearly spherical structure and an elongated cylindrical shape with dynamics characteristic of a stochastic resonance; the tissue’s response to the perturbation exhibits a resonance-like behavior as a function of the noise level. Finally, the geometrical size of the tissue limits the spectrum of fluctuations; large tissues exhibit enhanced morphological fluctuations, leading to a continuous, second-order-like, morphological transition. Our findings offer a novel physical framework for morphogenesis distinct from Turing-like mechanisms.