Hydra Molecular Network Reaches Criticality at the Symmetry-Breaking Axis-Defining Moment

We study biological, multicellular symmetry breaking on a hollow cell sphere as it occurs during hydra regeneration from a random cell aggregate. We show that even a weak temperature gradient directs the axis of the regenerating animal but only if it is applied during the symmetry-breaking moment. We observe that the spatial distribution of the early expressed, head-specific gene k s 1 has become scale-free and fractal at that point. We suggest the self-organized critical state to reflect long range signaling, which is required for axis definition and arises from cell next-neighbor communication