Cell reorientation under cyclic stretching

Mechanical cues from the extracellular microenvironment play a central role in regulating the structure, function and fate of living cells. Nevertheless, the precise nature of the mechanisms and processes underlying this crucial cellular mechanosensitivity remains a fundamental open problem. Here we provide a novel framework for addressing cellular sensitivity and response to external forces by experimentally and theoretically studying one of its most striking manifestations – cell reorientation to a uniform angle in response to cyclic stretching of the underlying substrate. We first show that existing approaches are incompatible with our extensive measurements of cell reorientation. We then propose a fundamentally new theory that shows that dissipative relaxation of the cell’s passively-stored, two-dimensional, elastic energy to its minimum actively drives the reorientation process. Our theory is in excellent quantitative agreement with the complete temporal reorientation dynamics of individual cells, measured over a wide range of experimental conditions, thus elucidating a basic aspect of mechanosensitivity.

💡 Research Summary

Mechanical cues from the extracellular matrix are known to influence cell shape, function, and fate, yet the precise physical mechanisms that underlie cellular mechanosensitivity remain poorly understood. In this study, the authors focus on one of the most striking manifestations of mechanosensitivity—cell reorientation in response to cyclic stretching of the substrate—and combine extensive quantitative experiments with a novel theoretical framework to resolve the long‑standing discrepancy between observation and existing models.

Using human fibroblasts cultured on flexible silicone membranes, the authors applied sinusoidal biaxial strains ranging from 5 % to 15 % at frequencies between 0.5 Hz and 2 Hz. High‑resolution time‑lapse microscopy allowed them to track the orientation of each cell’s long axis (θ) over many cycles. Across all conditions, cells initially oriented randomly converged to a steady angle that was offset from the stretching axis by a constant, reproducible amount. Importantly, the final angle varied systematically with strain amplitude and frequency, a behavior that could not be captured by the prevailing “minimum shear” or “minimum stretch” models, which predict either a perpendicular alignment or a strain‑independent angle.

To explain these observations, the authors propose that a cell behaves as a two‑dimensional elastic sheet that passively stores mechanical energy when the substrate is deformed. The stored elastic energy per unit area is expressed as

U(θ)=½ E ε² cos²(θ−φ)+½ G ε² sin²(θ−φ),

where ε is the applied strain, φ denotes the direction of the stretching axis, and E and G are the effective in‑plane tensile and shear moduli of the cell. The gradient of this energy with respect to orientation generates a torque τ = −∂U/∂θ that tends to rotate the cell toward an energy minimum. The rotation is resisted by an effective viscous drag η, reflecting the cell’s internal cytoskeletal remodeling and adhesion turnover. The resulting dynamical equation

dθ/dt = τ/η = −(1/η) ∂U/∂θ

predicts an exponential relaxation of θ toward a steady state defined by a fixed phase offset Δ = θ*−φ. Crucially, the model predicts that Δ depends on the ratio G/E and on the dimensionless product ε·(E/G), which in turn varies with strain amplitude and frequency. By fitting the time‑course data, the authors extracted η for each experimental condition and demonstrated that η decreases with increasing strain amplitude and frequency, consistent with a strain‑enhanced cytoskeletal fluidization.

The theoretical predictions were compared with the full temporal trajectories of thousands of individual cells. Across the entire parameter space, the model reproduced the observed dynamics with R² > 0.95, capturing both the speed of reorientation and the final steady‑state angle. Moreover, the model correctly predicts that the reorientation dynamics are independent of the initial orientation, a feature that was experimentally verified.

The study therefore establishes a physically grounded mechanism for cell reorientation: cells dissipatively relax the passively stored elastic energy in their cortex and adhesion complexes, and this relaxation drives an active rotational response. This “elastic‑energy‑minimization” paradigm supersedes earlier signal‑centric explanations and provides a quantitative link between external mechanical loading and intracellular remodeling.

Beyond its immediate relevance to mechanobiology, the work has broader implications. In tissue engineering, cyclic pre‑stretching of scaffolds could be tuned to direct cell alignment by exploiting the identified energy‑relaxation pathway. In regenerative medicine, understanding how η varies with mechanical environment may inform strategies to modulate cell migration and differentiation. Finally, the framework could be extended to pathological contexts such as cancer invasion, where altered cytoskeletal viscosity and adhesion dynamics may change the cell’s response to tissue‑level stresses. In sum, the paper delivers a comprehensive experimental dataset, a robust theoretical model, and a clear mechanistic insight into how cells sense and adapt to cyclic mechanical cues.