Mechano-Chemical Coupling in Hydra Regeneration and Patterning.

This article focuses on the mechanisms underlying axial patterning and symmetry breaking in the freshwater cnidarian Hydra, a model organism known for its remarkable regenerative abilities. It reviews the historical and current understanding of Hydra’s body axis formation, emphasizing the Gierer-Meinhardt reaction-diffusion model involving a short-range activator and a long-range inhibitor, with HyWnt3 identified as a key activator candidate. Despite extensive research, the molecular identity of the predicted inhibitor remains unknown, and the classical model cannot fully explain de novo axis formation in cellular aggregates lacking inherited polarity. The review highlights recent advances incorporating mechano-chemical feedback between tissue mechanics and morphogen dynamics, discusses limitations of existing models—especially regarding differences between tissue spheroids and cell aggregates—and proposes future experimental and theoretical directions, including identifying inhibitors, visualizing morphogen gradients in vivo, applying mechanical perturbations, and refining mathematical models to better capture Hydra’s complex patterning processes.