Topological defects in the nematic order of tissues drive morphogenesis of cellular tornadoes and bicephalous Hydra - Aurélien Roux (Université de Genève)

Cell assemblies can be describe as active nematics, which creates topological defects because of their activity. In the small animal Hydra, the regeneration of head and foot correlates with the position of integer topological defects in the nematic field of actin, suggesting that topological defects in muscle cells could control or drive morphogenesis. In vitro, flat assemblies of muscular cells forms +1/2 and -1/2 defects spontaneously in their nematic order. I will show that by confining these cells on micropatterns, we can trigger the formation of integer defects, initially spirals, and then, through proliferation, the cell layer transitions to an aster configuration, and further grow into a vortex, forming a protrusion of half a millimeter. I will further show that mechanical deformation of the nematic field of the actin in the Hydra causes bicephal regeneration. Altogether, our findings support that defects in the nematic order of muscles cells or actin controls the stress field in developping tissues, driving morphogenesis.