| Abstract: | Hydra, a small freshwater predator, exhibits remarkable whole-body regeneration from excised tissue pieces, making it an excellent model for studying animal morphogenesis. In previous work we showed that the nematic alignment of the supracellular muscle fibers in excised tissues confers a structural memory of the body axis in the parent animal, which persists and aligns with the body axis of the regenerated animal. The excised tissues first fold and seal into hollow spheroids, exhibiting partial loss of fibers in the closure region, while retaining the nematic fiber organization elsewhere. Subsequently, nematic alignment is induced in the disordered regions, and the spheroids develop into elongated mature animals, with muscle fibers aligned parallel to the body axis. To investigate the interactions underlying the nematic organization of the contractile muscle fibers and their relation to the establishment of the regenerated body axis, we confine regenerating spheroids within narrow cylindrical channels. Under frustrating configurations, where the inherited axis is oriented perpendicular to the channel, we find that the regenerated body axis typically reorients and develops along the channel. The formation of fibers in the initially disordered closure region occurs rapidly, with fibers forming parallel to the channel axis, aligned with the direction of mechanical strain in the confined tissue. This leads to the formation of an extended line defect between the domain of newly formed fibers aligned with the channel and the domain of inherited fibers oriented in a perpendicular orientation. The nematic rearranges from the domain boundary, with fibers initially oriented perpendicular to the boundary reorganizing and becoming parallel to the channel. This reorientation can occur without tissue rotation, so that the nematic field reorganizes within the elastic tissue and the regenerating body axis forms perpendicular to the original axis inherited from the parent animal. We further follow the calcium activity in confined tissues under these frustrating conditions. Regenerating Hydra spheroids were recently shown to have a polar calcium distribution with a bias towards the future head, which is present from the onset of regeneration and persists throughout the regeneration process. In samples confined in a frustrating configuration, we follow the redistribution of calcium activity as the body axis reorients along the channel axis. The ability to modify the body axis of regenerating Hydra using geometrical constraints highlights the role of mechanics in defining the body plan during morphogenesis, and provides new insights into the interactions underlying the nematic fiber alignment |