Density-driven alignment and defect annihilation in cell monolayers

A growing body of literature draws parallels between cell ordering and liquid crystals, where spindle-like cells co-align due to steric interactions. To understand their organization, we have innovated a continuous imaging platform for tracking thousands of cells, yielding a rich data set for unveiling cellular interactions. In 2D, we show that flat substrates with nematic order can induce global alignment on a millimeter scale. Remarkably, single cells are not sensitive to the substrate’s anisotropy. Rather, the emergence of global nematic order is a collective phenomenon that requires both steric effects and molecular-scale anisotropy of the substrate. We find that cells exhibit a density-dependent activation pattern of Yes-associated protein, a downstream mechanotransducer, corroborating its role in translating external mechanical and morphological patterns to decisions such as proliferation. Without an external alignment field, topological defects appear naturally and are postulated to serve key biological functions. We track defect dynamics in monolayers that include a fraction of overactive fibroblasts, also known as myofibroblasts, which are responsible for wound contraction but also known drivers for fibrotic diseases. Our findings reveal distinct collective dynamics indicative of variations in elastic constants. Ultimately, this difference could potentially be used as a quantitative diagnostic tool for monitoring fibrosis progression.