Topological defects in the nematic order of actin fibres as organization centres of Hydra morphogenesis

Animal morphogenesis arises from the complex interplay between multiple mechanical and biochemical processes with mutual feedback.  Developing an effective, coarse-grained description of morphogenesis is essential for understanding how these processes are coordinated across scales to form robust, functional outcomes. Here we show that the nematic order of the supra-cellular actin fibres in regenerating Hydra defines a slowly-varying field, whose dynamics provide an effective description of the morphogenesis process. We show that topological defects in this field, which are long-lived yet display rich dynamics, act as organization centres with morphological features developing at defect sites. These sites are characterized by a unique mechanical environment, undergoing repeated large, transient and localized deformations. These observations suggest that the nematic orientation field can be considered a “mechanical morphogen” whose dynamics, in conjugation with various biochemical and mechanical signalling processes, result in the robust emergence of functional patterns during morphogenesis.