Topological defects in the nematic order of actin fibers as organization centers of Hydra morphogenesis

Animal morphogenesis arises from the interaction of multiple biochemical and mechanical processes, spanning several orders of magnitude in space and time, from local dynamics at the molecular level to global, organism-scale morphology. How these numerous processes are coordinated and integrated across scales to form robust, functional outcomes remains an outstanding question. Developing an effective, coarse-grained description of morphogenesis can provide essential insight towards addressing this important challenge. Here we show that the nematic order of the supra-cellular actin fibers in regenerating Hydra defines a slowly-varying field, whose dynamics provide an effective description of the morphogenesis process. The nematic orientation field necessarily contains defects constrained by the topology of the regenerating tissue. These nematic topological defects are long-lived, yet display rich dynamics that can be related to the major morphological events during regeneration. In particular, we show that these defects act as organization centers, with the main functional morphological features developing at defect sites. To our knowledge, this provides the first demonstration of the significance of topological defects for establishing the body plan of a living animal. Importantly, the early identification of topological defect sites as precursors of morphological features, suggests that the nematic orientation field can be considered a 9mechanical morphogen9 whose dynamics, in conjugation with various biochemical signaling processes, result in the robust emergence of functional patterns during morphogenesis.