Alternative models of cell mechanics depict the living cell as a simple mechanical continuum, porous filament gel, tensed cortical membrane, or tensegrity network that maintains a stabilizing prestress through incorporation of discrete structural elements that bear compression. Real-time microscopic analysis of cells containing GFP-labeled microtubules and associated mitochondria revealed that living cells behave like discrete structures composed of an interconnected network of actin microfilaments and microtubules when mechanical stresses are applied to cell surface integrin receptors. Quantitation of cell tractional forces and cellular prestress by using traction force microscopy confirmed that microtubules bear compression and are responsible for a significant portion of the cytoskeletal prestress that determines cell shape stability under conditions in which myosin light chain phosphorylation and intracellular calcium remained unchanged. Quantitative measurements of both static and dynamic mechanical behaviors in cells also were consistent with specific a priori predictions of the tensegrity model. These findings suggest that tensegrity represents a unified model of cell mechanics that may help to explain how mechanical behaviors emerge through collective interactions among different cytoskeletal filaments and extracellular adhesions in living cells.cytoskeleton ͉ microtubules ͉ cell mechanics ͉ myosin light chain phosphorylation ͉ mechanotransduction M echanical stress-induced alterations in cell shape and structure are critical for control of many cell functions, including growth, motility, contraction, and mechanotransduction (1). These functional alterations are mediated through changes in the internal cytoskeleton (CSK), which is composed of an interconnected network of microfilaments, microtubules, and intermediate filaments that links the nucleus to surface adhesion receptors. Advances in cell biology have resulted in better understanding of the polymerization behavior and physical properties of individual CSK filaments as well as of gels composed of combinations of filaments. Yet, the material properties measured in vitro neither explain nor predict complex mechanical behaviors that are observed in living cells (2, 3). At the same time, engineers have approached the problem of how cells stabilize their shape by developing mechanical models, without considering molecular specificity. For example, the living cell is often modeled as a continuum that contains an elastic cortex that surrounds a viscous (4) or viscoelastic (5) fluid; a more complex variation includes an elastic nucleus within a viscous cytoplasm (6). These models provide reasonable empirical fits to measured elastic moduli and viscosity in cells under specific experimental conditions (4-6), but they cannot predict from mechanistic principles how these properties alter under different challenges to the cell. Continuum models also assume that the load-bearing elements are infinitesimally small relative to the size of the cell and thus, they...

Directed cell migration is critical for tissue morphogenesis and wound healing, but the mechanism of directional control is poorly understood. Here we show that the direction in which cells extend their leading edge can be controlled by constraining cell shape using micrometer-sized extracellular matrix (ECM) islands. When cultured on square ECM islands in the presence of motility factors, cells preferentially extended lamellipodia, filopodia, and microspikes from their corners. Square cells reoriented their stress fibers and focal adhesions so that tractional forces were concentrated in these corner regions. When cell tension was dissipated, lamellipodia extension ceased. Mechanical interactions between cells and ECM that modulate cytoskeletal tension may therefore play a key role in the control of directional cell motility.