  We discuss a generic description of the dynamics of cell aggregates. We
describe cells as polar rotating objects which mechanically interact with each
other and with the surrounding medium. We use the framework of non-equilibrium
thermodynamics to derive generic constitutive equations for the interaction
forces, torques and polarity dynamics. We apply our framework to discuss
spontaneous motion of cell doublets. We find a rich phase diagram of possible
collective motion, including steady rotation arising from flow-polarity
coupling or coupling of polarity with cell position.
