Mechanics, nematics and chemical patterning in Hydra regeneration

Olivier Cochet-Escartin (ILM, Lyon)

Hydra vulgaris is a powerful model organism in the study of body axis definition and regeneration. Excised tissue pieces as well as cellular re-aggregates are able to regrow full, viable organisms in a matter of a few days. Recently, it became clear that early patterning during Hydra regeneration is an integrated mechano-chemical process involving morphogen dynamics, tissue mechanics and the nematic organization of long actin fibers. In this talk, I will start by presenting the characteristics of Hydra vulgaris and its regenerative process and introduce the known determinants of its axial patterning. Then, as the mechanical state of regenerating tissues was shown to be key in this process, I will describe our past work focusing on the mechanical characterization of Hydra tissue spheres. Finally, I will introduce more recent efforts to decipher the interplay between tissue mechanics, actin nematics and biochemical signalling during axial patterning. Our results suggest redundancy between two main mechanisms: 1- a mechano-chemical feedback loop involving mechanical stretch and biochemical (Wnt) signalling and 2- a nematic-based process by which topological defects induce the formation of a head organizer. These two mechanisms could be relevant for many developmental processes beyond the specific case of Hydra and could be key in explaining the remarkable robustness of regeneration in this organism