-The tensegrity hypothesis holds that the cytoskeleton is a structure whose shape is stabilized predominantly by the tensile stresses borne by filamentous structures. Accordingly, cell stiffness must increase in proportion with the level of the tensile stress, which is called the prestress. Here we have tested that prediction in adherent human airway smooth muscle (HASM) cells. Traction microscopy was used to measure the distribution of contractile stresses arising at the interface between each cell and its substrate; this distribution is called the traction field. Because the traction field must be balanced by tensile stresses within the cell body, the prestress could be computed. Cell stiffness (G) was measured by oscillatory magnetic twisting cytometry. As the contractile state of the cell was modulated with graded concentrations of relaxing or contracting agonists (isoproterenol or histamine, respectively), the mean prestress (p t) ranged from 350 to 1,900 Pa. Over that range, cell stiffness increased linearly with the prestress: G (Pa) ϭ 0.18p t ϩ 92. While this association does not necessarily preclude other interpretations, it is the hallmark of systems that secure shape stability mainly through the prestress. Regardless of mechanism, these data establish a strong association between stiffness of HASM cells and the level of tensile stress within the cytoskeleton. tensegrity; mechanical stress; traction; cytoskeleton; actin microfilaments CONTROVERSY SURROUNDS the tensegrity hypothesis (23, 28). As described below, some part of this controversy is perhaps attributable to insufficient precision in the use of associated terminology and some part to insufficient emphasis on underlying mechanisms on which tensegrity rests. The major part of the controversy, however, is surely attributable to the fact that tensegrity is a hypothesis that has been rich in opinions but poor in quantitative data. Few, if any, data have been available that could be used to put the hypothesis to a rigorous test.The purpose of this series of companion papers is to amplify findings that have appeared recently in a brief report (55) and, in doing so, to bring to this controversy precision in the concepts, clarity about putative mechanisms, and new data that bear directly on the question itself. These data offer evidence that the tensegrity hypothesis, framed as it currently stands, captures certain central features of cell mechanical behavior but may be cast too narrowly.We begin by addressing a somewhat broader question: by what central mechanism does the cytoskeleton of adherent cells develop mechanical stresses that oppose distortion of cell shape? The answer to this question is important in understanding many cellular functions, including spreading, migration, contraction, growth, and mechanotransduction (9, 13, 29). To answer this question, several models of cell mechanics have been advanced, including tensegrity (1, 11, 12, 15, 16, 22, 24, 25, 27, 28, 30, 39, 40, 42, 44-46, 48, 51, 56-59, 60). This universe of cell models divid...

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...