Morphogenesis in regenerating Hydra: actin dynamics and the influence of mechanical constraints

Morphogenesis is one of the most remarkable examples of biological self-organization. One of the great challenges in morphogenesis research is understanding the organizational principles responsible for the robust convergence of the process, across scales, to form viable organisms under variable conditions. In our research we utilize the fresh water animal Hydra as a model system to study the role of mechanics in morphogenesis, focusing on the dynamics of the actin cytoskeleton during whole-body regeneration. Previously we have found that actin fiber organization during Hydra morphogenesis is inherited from the parent Hydra and determines the body axis in regenerating tissue segments. Here, we present a framework for describing actin fiber organization in Hydra as an active nematic and examine the relation between defects in the nematic organization and the morphogenesis process. Tracking actin fiber organization and tissue geometry during regeneration, we find that defects in nematic organization are long-lived yet display rich dynamics, and can be related to morphological events in regeneration. We further examine the role of mechanics in morphogenesis by embedding regenerating tissues in confined geometries and applying various external mechanical constraints.