The Active Regulation of Pressure and Volume in Cell Aggregates

We explore the relationship between the non-equilibrium generation of myosin-induced 'active' stress within the cell cytoskeleton and the pressure-volume relationship of cellular aggregates as models of simple tissues. We find that due to active stress, aggregate surface tension depends upon aggregate size. As a result, pressure and number density depend on size, although as they have the same dependence, the relationship between them resembles an equilibrium equation of state. This suggests that bulk and surface properties of tissues balance during growth to yield a constant average mechanical energy per cell. These results describe basic physical principles that govern the growth and form of simple tissues.