Competition Between Cell-Cell and Cell-Substrate Adhesion Determines Epithelial Monolayer Architecture in Culture

Organ surfaces are lined by epithelial monolayers - sheets of cells that are one-cell thick. This architecture underlies tissue function, and its loss is associated with disease, including cancer. Studies of in-plane epithelial cell behaviors show that a developing epithelium behaves as a fluid in respect to the tissue plane, and can therefore readily adapt to varying mechanical influences during morphogenesis. We asked the question of how monolayer architecture is achieved, and whether it demonstrates the same fluid behavior. To address this problem, we cultured MDCK (Madin-Darby Canine Kidney) cell layers at different densities and timepoints and analyzed their architectures using a novel tool, Automated Layer Analysis (ALAn), which we introduce here. Our experimental and theoretical results lead us to propose that epithelial monolayer architecture is governed by a balance of counteracting forces due to cell-cell and cell-substrate adhesion, and that this balance is influenced by cell density. MDCK cells do not undergo obvious rearrangement along the apical-basal axis; instead, cells that do not contact the substrate aggregate on top of the monolayer. Our findings therefore imply that monolayered architecture is under more rigid control than planar tissue shape in epithelia.