The dual nature of body-axis formation in Hydra regeneration: Polarity-morphology concurrency

The formation of a body axis is central to animal development and involves both polarity and morphology. While polarity is traditionally associated with biochemical patterning, the morphological aspect of axis formation remains elusive. In regenerating Hydra tissues, we find that morphological evolution in all tissue samples depends on inherited positional information from the donor's axis, and a foot precursor emerges early in the process. From the onset of regeneration, the Ca²⁺ excitations that drive actomyosin forces for tissue reshaping follow a gradient aligned with the inferred head-foot polarity direction. We conclude that polarity and morphological axis progression occur concurrently through interlinked processes. In this progression, the early and reproducible emergence of a foot precursor provides a robust morphological marker of axis formation, a role often discussed primarily in the context of the head organizer. A toy model accounts for the observed regeneration dynamics and illustrates the mechanochemical integration of polarity and morphogenesis. We expect the insights from Hydra to be relevant to broader developmental systems.