Mouth Function Determines The Shape Oscillation Pattern In Regenerating Hydra Tissue Spheres

Hydra is a small freshwater polyp capable of regeneration from small tissue pieces and from aggregates of cells. During regeneration, a hollow bilayered sphere is formed that undergoes osmotically driven shape oscillations of inflation and rupture. These oscillations are necessary for successful regeneration. Eventually, the oscillating sphere breaks rotational symmetry along the future head-foot axis of the animal. Notably, the shape oscillations show an abrupt shift from large amplitude, long period oscillations to small amplitude, short period oscillations. It has been widely accepted that this shift in oscillation pattern is linked to symmetry breaking and axis formation. However, recent work showed that regenerating tissue pieces inherit the parent animals body axis and thus are asymmetric from the beginning. Thus, there is no mechanistic explanation for the observed shift in oscillation pattern and no clear understanding of its significance for Hydra regeneration. Using in vivo manipulation and imaging, we quantified the shape oscillation dynamics and dissected the timing and triggers of the pattern shift. Our experiments demonstrate that the shift in the shape oscillation pattern in regenerating Hydra tissue pieces is caused by the formation of a functional mouth, thereby linking morphological readouts to physiologically relevant events during regeneration. This study shows the power of using modern experimental techniques to revisit old questions in pattern formation and development.