Biomechanics of stem cell fate decisions in multilayered tissues

Tissue homeostasis relies on a precise balance of fate choices between renewal and differentiation, which is dysregulated during tumor initiation. Although much progress has been done over recent years to characterize the dynamics of cellular fate choices at the single cell level, their underlying mechanistic basis often remains unclear. In particular, although physical forces are increasingly characterized as regulators of cell behaviors, a unifying description of how global tissue mechanics interplays with local cellular fate choices is missing. Concentrating on skin epidermis as a paradigm for multilayered tissues with complex fate choices, we develop a 3D vertex-based model with proliferation restrained in the basal layer, showing that mechanics and competition for space naturally gives rise to homeostasis and neutral drift dynamics that are seen experimentally. We then explore the effect of introducing mechanical inhomogeneities, whereby subpopulations have differential tensions. We uncover that relatively small mechanical disparities can be sufficient to heavily tilt cellular towards symmetric renewal and exponential growth. Importantly, the simulations predict that such mechanical inhomogeneities are reflected by distinct morphological changes in single-cell shapes. This led us to derive a master relationship between two very different experimentally measurable parameters, cell shape and long-term clonal dynamics, which we validated using a model of basal cell carcinoma (BCC) consisting in clonal Smoothened overexpression in mouse tail epidermis. Altogether, we propose a theoretical framework to link mechanical forces, quantitative cellular morphologies and cellular fate outcomes in complex tissues.