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What Forces are at Work Here? Tom McKeag

papers on this subject and the process has comprised a rigorous but unpredictable combination of new discoveries and techniques, collaborations and insights. In 1993, Ingber submitted a commentary to the Journal of Cell Science in which he outlined his cellular tensegrity model. He offered that it would explain the range of behavior and activities of a cell being observed: shape, movement and even responses. Much was still unknown about the cytoskeleton, but he laid out the suggestive evidence bit by bit and made a compelling case for a mechanical theorem that seemed to address many scale behaviors seen to date. Most importantly, the paper changed the frame of reference for research in cell structure, which would now include, irrevocably, the principles of tensegrity (3). In 2000, many researchers had applied the tensegrity model to cell and tissue architecture and had demonstrated its plausibility in explaining complex mechanical forces in viruses, cells, tissues and organs of animals and plants. The idea was still being challenged strenuously enough that the editors of the Journal of Applied Physiology published Ingber’s summation of his views under the feature title “Controversies in Physiology� in a written debate with proponents of the viscous cytosol model,

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Heidemann et al from Michigan State University (5). The main difference in the two views of cell mechanics was this: the cellular tensegrity model held that the intricate network of cytoskeleton components within the cytoplasm of a cell, a cortical actin-ankyrinspectrin lattice, constituted an independent and prestressed structure which allowed the cell to absorb and adjust to outside mechanical forces. This adjustment, like that of all tensegrity stuctures, meant the whole lattice would shift as a system according to force and direction with asymmetric but stable results. The viscous cytosol model held that outside force placed upon the cell would result in a continuous absorption of this force equally and continuously along the cell membrane. Key to proving one theory over the other, according to Ingber, was demonstrating a transfer of force to a remote part of the cell (action at a distance), and he also offered proof of prestressing citing studies that removed the contractile capability of the cytoskeleton and thereby reduced the cellular shear modulus (resistance to shear force, a measure of cell stiffness). In summary he argued that the cellular tensegrity model was the only one extant that provided an explanation of the complexity, multimodality and hierarchical nature of cells, and from

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