What Forces are at Work Here? Tom McKeag
sub-membrane cortical network of filaments in a cell took the full load of externally applied stresses to the cell and did so equally along the entire periphery (4). The cytoplasm of a cell has been demonstrated, however, to be neither homogenous nor secondary in functional performance. Within it is an intricate structure network forming a cytoskeleton with important jobs to perform. Microtubules, and microfilaments serve as both internal structure for cell integrity and pathways for organelle and enzyme transfer for biochemical reactions. These are not static structures and respond to external stresses by initiating corrective chemical processes. Additionally, much of post-war molecular biology education had been dominated by the study of chemistry and Ingber thought it was curious that shapes and physical forces were not given more attention. The function of these three-dimensional molecular objects had been clearly demonstrated to be partially determined by shape. At the cellular and molecular level, he believed that mechanical forces played a much larger role in biological processes.
The New Theory Ingber adopted Fullerâ€™s complete definition of tensegrity in that it should comprise two structural concepts, that of stability through
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tension (prestressing) and geodesics, or minimal force pathways. He proposed that the entire cell is a prestressed tensegrity structure, with the cytoskeletal microfilaments and intermediate filaments providing the tensioning and the microtubules and adhesions from the extracellular matrix (ECM) providing the resistance to compression that counterbalances the tensioning. Ingber also noted that filaments can have dual functions and swing from tension to compression-bearing roles. The contractile actomyosin apparatus is believed to be the mechanism that activates the tensional prestressing that stabilizes the cell. The complexity of this â€œstructural homeostasisâ€? within the cell does not end with these components and forces, according to Ingber and others. Osmotic forces, the polymerization of filaments, and cell distension through adhesion to the ECM produce additional tensional stresses. Intermediate filaments that interconnect microtubules and microfilaments at many points serve to stiffen the matrix, and this matrix is in turn connected to a highly elastic cortical cytoskeletal network directly beneath the plasma membrane.
The Debate Nearly thirty years of debate within the scientific community has followed the first