  Cell growth, division and death are defining features of biological tissues
that contribute to morphogenesis. In hydrodynamic descriptions of cohesive
tissues, their occurrence implies a non-zero rate of variation of cell density.
We show how linear nonequilibrium thermodynamics allows to express this rate as
a combination of relevant thermodynamic forces: chemical potential, velocity
divergence, and activity. We illustrate the resulting effects of the
non-conservation of cell density on simple examples inspired by recent
experiments on cell monolayers, considering first the velocity of a spreading
front, and second an instability leading to mechanical waves.
