Tensegrity
Thirty years ago, when Donald Ingber was an undergraduate at Yale University, he was convinced that the view of the cell as a ‘rubber bag filled with jelly’ was somewhat oversimplified. Ingber was intrigued by the revolutionary architecture of Buckminster Fuller in the 1940s, who created a series of robust structures called geodomes (including his own house). Geodomes are constructed from a shell of multiple small rigid triangles without any major supporting structures such as beams or columns. Fuller himself had been infi uenced by the sculptures of Kenneth Snelson, where rigid stainless steel rods appeared to float in thin air, but are actually supported by a system of cables, rather like the rigging on a sailboat, where the mast is kept in place by a balance of tension and compression. The mast itself is rigid in order to resist the compression produced by the tension in the rigging. The structure is robust, and will only fail if the mast buckles or the rigging breaks. This is the principle of tensile integrity or tensegrity, which offers the maximum amount of strength for the minimum expenditure of energy and materials. Ingber reasoned that tensegrity exists in every cell, mediated by rigid microtubules which resist the compression produced by actin and intermediate filaments. Tensegrity thus generates strength within all cell shapes, be they fiattened hexagonal cells found in epithelia, or extended nerve axon cells which may be a metre in length. Tensegrity is at work even when a cell changes its shape, as happens in division. In culture, fiattened or elongated cells will round up at the start of division, then pinch off leaving two spherical daughter cells, which then fiatten and spread. As the edges fiatten, triangles formed by actin fibres are clearly visible around the edge, with six neighbouring triangles forming a hexagon, exactly like the edge of a Buckminster Fuller geodome. The cell then fiattens further, changing shape to a typical extended fibroblast form, forming attachments between the membrane and the base layer termed focal adhesions, before migrating as a single cell. In contrast, cells grown in culture from epithelial tissues will attach to their neighbours and move around as a sheet. As in living tissues, the epithelial cells attach to each other with structures called desmosomes, which are tough plaque-like structures formed by local reinforcement of the cell membrane, and anchored within the cell by intermediate filaments. In skin, a tissue constantly bent or stretched, epidermal cells have multiple desmosomes, and their intermediate filaments are strengthened by numerous keratin filaments ( Figure　6a, b ). When repeated through every cell, this arrangement creates an extremely tough tissue.