Epithelial polarity drives tissue tension in planar, free standing cell monolayers

Polar epithelial cells form thin but resilient sheets that resist mechanical in-plane stress by relying on strong conformal contacts with each mediated by dedicated cell-cell connections connected to the viscoelastic cortex. In this study, we investigate the mechanical response of free-standing cell monolayers to central indentation as a function of orientation using a colloidal probe. We determine tissue tension by treating the deformed tissue as a minimal surface area. Our findings reveal that the cortex tension of the basal side governs the purely elastic response to in-plane extension, while the apical side of the cells is soft and dissipative giving rise to a hysteresis at low indentation depth. At larger indentation depth, the apico-basal polarity is no longer relevant as the cells are apically compressed and the response is driven by the elastic in-plane response of the basal side of the tissue. These results are particularly significant for lumen-forming epithelial cells, which experience substantial compressive forces especially apically due to elevated Laplace pressure.