  A theoretical model for stratified epithelium is presented. The viscoelastic
properties of the tissue is assumed to be dependent on the spatial distribution
of proliferative and differentiated cells. Based on this assumption, a
hydrodynamic description for tissue dynamics at long-wavelength, long-time
limit is developed, and the analysis reveals important insight for the dynamics
of an epithelium close to its steady state. When the proliferative cells occupy
a thin region close to the basal membrane, the relaxation rate towards the
steady state is enhanced by cell division and cell apoptosis. On the other
hand, when the region where proliferative cells reside becomes sufficiently
thick, a flow induced by cell apoptosis close to the apical surface could
enhance small perturbations. This destabilizing mechanism is general for
continuous self-renewal multi-layered tissues, it could be related to the
origin of certain tissue morphology and developing pattern.
