Hydra, named after the multi-headed monster in Greek mythology, is a radially symmetric freshwater polyp, a few mm in length. Hydra is famous for its regenerative capabilities, allowing it to regenerate from small tissue pieces and even from a "soup of cells" (cell aggregates). Because of its structural simplicity and strong regenerative potential, Hydra is a well-suited system for in vivo biophysical studies of regeneration and pattern formation.
The advent of modern molecular tools enable us to revisit long-standing questions about the interplay of mechanics and biochemistry on patterning during Hydra regeneration. My talk will summarize our recent work on two parts of the regeneration process: cell sorting and symmetry breaking. We have recently shown that tissue surface tensions drive cell sorting in Hydra cell aggregates and that discrepancies between experiments and theory were due to differences in aggregate geometry (Cochet-Escartin et al., Biophys. J. 2017). Once cell sorting is complete in aggregates - or excised tissue pieces have rounded up - the Hydra spheroid spits out excess cells and becomes a hollow bilayer sphere, which undergoes osmotically driven shape oscillations. The hollow sphere eventually breaks shape symmetry to form an ellipsoid, defining the future head-foot polarity of the adult polyp. How these mechanical oscillations are linked to shape (sphere à ellipsoid) and biochemical (head - foot) symmetry breaking has been an open question for nearly a decade. Different mechanisms have been proposed in an attempt to link mechanics and signaling, but a definitive answer is still missing. In my talk, I will present our recent work on this question, which provides the missing link between oscillation dynamics and biological events by deciphering the mechanism controlling the regeneration dynamics of Hydra tissue spheres.