Elasticity and viscosity in an epithelial cell monolayer

Epithelial cells can tune their forces in response to environmental conditions. In development, the cells actively control their internal forces to produce changes in tissue shape. In response to external forces, the cells stretch or rearrange to accommodate the tissue deformation. Following tissue damage, the cells migrate to heal the wound. With these applications in mind, recent studies have focused on the question of whether the cell layer behaves a solid, with all cells jammed in place by their neighbors, or a fluid, with cells able to exchange places with their neighbors. Here, we ask, if the monolayer is a solid, how solid is it? And if it is a fluid, how fluid is it? To answer these questions, we have developed experimental methods and analyses to quantify the elastic modulus and fluid viscosity within a monolayer of epithelial cells. We quantify the kinematics of the cell layer by measuring the cell areas, velocities, and strain rates, and we quantify the stresses using monolayer stress microscopy. We present situations wherein the data demonstrate apparent elastic behavior, with a measurable elastic modulus, and other situations wherein the data demonstrate viscous behavior, with a measurable shear viscosity. Our results quantify how solid or how fluid is the cell layer, and more importantly, they hint at the underlying causes of elasticity and viscosity, which often result from active responses of the cells to their surroundings.