Hydra is a multicellular fresh-water polyp with uniaxial symmetry that
exhibits remarkable regeneration properties, making it an excellent model
system for studying animal morphogenesis. We follow actin dynamics during
the regeneration process using transgenic hydra strains expressing
lifeact-GFP. We find that the aligned supra-cellular actin organization in
freshly excised tissue fragments, which originates from the mother hydra,
persists throughout the regeneration process and defines the body axis in
the regenerating hydra. Moreover, the initial shape of the excised tissue
strongly influences the likelihood of developing deformed morphologies with
multiple body axes (i.e. more than one head and/or foot), and these defects
can be traced to the lack of global alignment of the actin cytoskeleton at
early stages of the regeneration process. The application of mechanical
constraints and external forces is also found to have a large influence on
the outcome of the regeneration process. In particular, we find that
imposing mechanical constraints by anchoring regenerating hydra on thin
metal wires promotes order during morphogenesis. Overall our results
demonstrate the importance of mechanical forces and feedbacks during the
early stages of axis formation in hydra, highlighting the intimate coupling
between mechanics and biochemistry in morphogenesis.