Body-Axis Reorientation in Regenerating Hydra under Geometric Confinement

Defining a body axis is a central aspect of animal morphogenesis. During regeneration from excised Hydra tissue pieces, the newly formed body axis typically preserves the orientation of the parent body axis and aligns with the inherited nematic organization of the supracellular actomyosin fibers. Here we show that this inherited orientation can be overridden by geometric confinement. Tissue spheroids confined in narrow cylindrical channels in a frustrating configuration, with the inherited axis initially perpendicular to the channel, regenerate with their body axis aligned along the channel. Using high-resolution live imaging we show that this reorientation is accompanied by remodeling of the nematic fiber organization. New fibers form parallel to the channel axis in the initially disordered closure regions, creating sharp domain boundaries with the inherited transverse fibers. These domain boundaries subsequently propagate, with perpendicular fibers dissolving and new fibers forming along the channel axis. The confined tissue behaves as a solid-like active nematic material, storing anisotropic strain over long timescales while allowing nematic reorganization relative to the material frame. Our results suggest that coupling between tissue strain and nematic alignment contributes to fiber reorientation and body-axis patterning, highlighting how external mechanical constraints can redirect the body axis during morphogenesis.