Morphogenetic oscillations during symmetry breaking of regenerating Hydra vulgaris cells

During the process of regeneration, the fresh-water polyp Hydra vulgaris first builds a hollow sphere consisting of a cell bilayer. This cell ball undergoes subsequent shape transformations, at a later stage it creates tentacles and a foot to form an animal. We describe and analyze the transformation of the hollow sphere to the first non-spherical shape by means of contour analysis. We observe that the cell ball shows characteristic oscillations in size and shape which accompany symmetry breaking. Quantitative analysis of these oscillations provides information on the cell bilayer mechanics and hydrodynamic flows involved. In order to explain the origin of the observed oscillations, we propose three different physical scenarios of oscillation generation.