Hydrodynamics and cell motion during the rounding of two dimensional hydra cell aggregates

a two dimensional experimental geometry, we study hydrodynamics and cell motion during the rounding-up of three kinds of hydra aggregates (ectodermal, endodermal and mixed). The relaxation of initially elongated aggregates toward a circular shape is exponential and the relaxation time is proportional to the aggregate size as expected from hydrodynamics laws. As for viscous liquids, aggregate rounding is driven by tissue surface tensions sigma and resisted by tissue viscosity eta. The ratio eta/sigma is the same for the three kinds of aggregates. With a reasonable value sigma = 1 mN/m, we obtain an estimate larger than 104 Pas for the tissue viscosity. Cell motion during rounding is strongly cooperative and cell displacements exhibit some specific patterns in each aggregate. These results point out the role of adhesive bonds in the observed kinetics.