Topological Defects and their Role in Hydra Morphogenesis

Morphogenesis, the emergence of form and function in a developing organism, is one of the most beautiful examples of pattern formation in nature. We focus on the biophysical aspects of morphogenesis using Hydra, a small multicellular freshwater animal, as a model system. Hydra has a simple body plan and is famous for its ability to regenerate an entire animal from small tissue pieces, providing a flexible platform to explore how mechanical forces and feedback contribute to the establishment of the body plan. We use live imaging to follow the nematic organization of the supracellular muscle fibers and the tissue deformations in regenerating Hydra. We focus on topological defects in the nematic fiber organization, which emerge early in the regeneration process and identify the sites where the head and the foot develop in the regenerating animal. High-resolution imaging reveals that transient contractions of the supracellular fibers lead to mechanical strain focusing at defect sites. We suggest that these localized tissue deformations provide mechanical cues that are part of a mechanochemical feedback loop promoting the emergence of morphological features at defect sites.