Models of cytoskeletal mechanics based on tensegrity

Cell shape is an important determinant of cell function and it provides a regulatory mechanism to the cell. The idea that cell contractile stress may determine cell shape stability came with the model that depicts the cell as tensed membrane that surrounds viscous cytoplasm. Ingber has further advanced this idea of the stabilizing role of the contractile stress. However, he has argued that tensed intracellular cytoskeletal lattice, rather than the cortical membrane, confirms shape stability to adherent cells. Ingber introduced a special class of tensed reticulated structures, known as tensegrity architecture, as a model of the cytoskeleton. Tensegrity architecture belongs to a class of stress-supported structures, all of which require preexisting tensile stress (“prestress”) in their cable-like structural members, even before application of external loading, in order to maintain their structural integrity. Ordinary elastic materials such as rubber, polymers, or metals, by contrast, require no such prestress. A hallmark property that stems from this feature is that structural rigidity (stiffness) of the matrix is nearly proportional to the level of the prestress that it supports. As distinct from other stress-supported structures falling within the class, in tensegrity architecture the prestress in the cable network is balanced by compression of internal elements that are called struts. According to Ingber’s cellular tensegrity model, cytoskeletal prestress in generated by the cell contractile machinery and by mechanical distension of the cell. This prestress is carried mainly by the cytoskeletal actin network, and is balanced partly by compression of microtubules and partly by traction at the extracellular adhesions. The idea that the cytoskeleton maintains its structural stability through the agency of contractile stress rests on the premise that the cytoskeleton is a static network. In reality, the cytoskeleton is a dynamic network, which is exposed to dynamic loads and in which the dynamics of various biopolymers contribute to its rheological properties. Thus, the static model of the cytoskeleton provides only a limited insight into its mechanical properties (for example, near-steady-state conditions). However, our recent measurements have shown that cell rheological (dynamic) behavior may also be affected by the contractile prestress, suggesting thereby that the tensegrity idea may also account for some features of cell rheology. This chapter describes the basic idea of the cellular tensegrity hypothesis, how it applies to problems in cellular mechanics, and what its limitations are.