Substrate Viscosity Enhances Correlation in Epithelial Sheet Movement

The movement of the epithelium plays vital roles in the development and renewal of complex tissues, from the separation of tissues in the early embryo, to turnover in the homeostasis of the gastrointestinal mucosa. Yet, despite its importance, a clear interpretation of the mechanism for collective motion in epithelial sheets remains elusive. This interpretation is prohibited by the lack of understanding of the relationship between motion and cell-cell contact, and their mediation by the mechanical properties of the underlying substrate. To better mimic physiological substrates that have inherent viscosity, we probe this relationship using polydimethylsiloxane, a substrate whose mechanical properties can be tuned from predominantly elastic to viscous by altering its cross-linking content. We therefore characterize the comparative spatiotemporal correlations in cell velocity during the movement of an epithelial monolayer as a function of the viscoelasticity of the substrate. Our results show that high correlation in cell velocity is achieved when the substrate G"(omega) is similar to 0.4 x G'(omega). This correlation is driven by a balance between cell-cell contact and the adhesion and contraction of the extracellular matrix. For G"(omega) > G'(omega), this balance shifts, and contraction of the tissue drives the substrate to flow, further elevating the correlation in movement.