Does the concept of "tensegrity" help you understand formation or repair of shapes of organs and cells?

Prof. Donald Ingber believes that (Richard) Buckminster Fuller's idea of tensegrity can help us understand the forces that shape cells and organs. The inventor of tensegrity is Kenneth Snelson, who writes that Ingber slightly mis-uses the concept; but never mind. Ingber wrote a cover article about tensegrity of cell shapes in Scientific American (January issue, 1998), which was a shorter version of a review paper he had published in Annual Review of Physiology, vol. 59 (1997). His theories have gotten lots of attention. He has also published some very interesting research papers about changes in gene expression being caused by forces, and an elegant proof that "anchorage dependence" results from how much a cell is stretched, instead of the area of cell-substratum adhesions. Lots of people admire his ideas. Personally, I don't understand what is being hypothesized. Ingber has a very extensive set of interlinked web pages about himself and about tensegrity. Take a look at them and see whether you understand what biological theory he is proposing. [And Don, if you read this, my class and I would appreciate your explanation. Is it as simple as "Cells exert forces", or "Forces influence shapes and orientations of cells?"] Remember Karl Popper says a good theory is defined by what it says cannot happen. What does tensegrity predict doesn't happen, that we otherwise would have expected could happen?

What do people mean by tensegrity? Visualize a suspension bridge. Tension forces are concentrated in the cables; compression forces are concentrated in the two towers, to which the cables are attached. Both tension and compression are oriented along the axes of the structures that support them. The same is true of tents that have one pole: compression forces are exerted along the pole; tension forces are oriented along the cloth of the tent. Snelson's claim to be an artist depend on some nifty towers he built using combinations of steel cables and wooden struts. Fuller's claim to be an inventor was based mostly on geodesic domes.

Much of engineering depends on spatial arrangement of concrete, steel, etc. so that they resist forces most economically. In other words, the engineer figures out where and in what directions tensions will be largest. Then she (or he) gives instructions to the builders to put steel cables or other tension-resisting materials where the tensions are, and orient them parallel to the strongest tensions. These instructions are blue-prints.

Compression-resisting materials (stone, concrete, but also steel) are put wherever the strongest compression forces are located. These materials are aligned parallel to the direction of strongest compression. Stone arches resist compression around hemi-circles; domes resist compression around hemispheres. Truss bridges concentrate tension and compression along the axes of steel or wood beams. Sail boats focus compression into masts and tension into ropes and canvas. A good strategy for building anything is to figure out where (and in which directions) forces are concentrated, put the load-resisting materials at those locations, and orient them parallel to the loads.

The interior parts of bone ("spongy bone") have narrow rods (trabeculae) made of bone and oriented parallel to the directions of greatest compression and tension. If an abnormal geometric pattern of stress is imposed on a bone, trabeculae will form in the new directions. This proves (I think; most people think) either that the osteocytes or the bones themselves somehow "feel" the directions and amounts of tension and compression. More bone is produced where the loads are strongest. Does that mean we should call it a tensegrity structure? Actually, Ingber doesn't use this example. But it is important.

Bones get stronger in response to load. This is analogous to muscles becoming stronger in response to weight-lifting, or other loads. The serving arms of professional tennis players have much bigger muscles and also thicker and more dense bones. The mechanism is not known. It is not known how osteocytes "measure" the amounts and directions of forces imposed on them. But apparently they can. The best researcher on this subject is John D. Currey, who spent several years in the Duke Biology Department, and gave excellent seminars.

If the biological mechanisms were discovered they should be medically useful. For example, the mechanism could be used to stimulate increased bone formation in people with osteoporosis. Conversely, understanding the mechanism would probably guide you toward methods for inhibiting unwanted bone formation in people who suffer from that problem. Modern medicine is based on discovering normal mechanisms, and how to control them.

The most popular theory about how bones detect force article is piezoelectricity. This is not a different kind of electricity, but a method of inducing temporary changes in voltage. Quartz and many other crystals have the property that if you compress them along one axis, a small voltage is generated along that axis. If you stretch them in that axis, the opposite voltage is generated. C.A.L. Bassett and R. O. Becker (Science vol. 137, pages 1063-4, Sept 28, 1962) proposed the theory that bone has this property and that they use it to detect directions and strengths of compression and tension. Orthopedic Surgeons and Dentists have tried to use voltages to control bone formation. Some have told me voltages don't have much effect. Others are enthusiastic. If you search on line, you can find many research papers on the subject. A medical student, a PhD student and I collaborated on a paper about effects of DC electric voltages on tissue culture cells. We found evidence that supported completely different mechanisms that we expected. Also, we discovered that even very slowly oscillating voltages (once a second! once a minute!) produced no effect!

Many people wrongly believe that piezoelectric voltages continue over time. That would only be true if the crystal were insulated electrically. Otherwise, the voltage "short circuits" in a fraction of a second. Then you get a voltage in the opposite direction when you relax the mechanical load. In a water environment, these voltage cannot distinguish between exertion of compression and relaxation of tension. Another result of conductivity is that steady application of a force produces no long-term voltage - just little blips at the beginning and at the end.

Some have claimed that bone is piezoelectric (although calcium phosphate isn't!). The researchers making these claims did not consider the possibility that the voltages they detected were really caused by electro-osmosis. That is not the same thing as the Gibbs-Donnan effect, although practically all biologists think it is the same. Donnan's paper was specifically about effects involving semi-permeable membranes (That is in the title of his paper). But Donnan thought the same effects should occur with some gels. That turned out to be true, but by a different mechanism, in which voltages (instead of membranes) prevent free diffusion of ions. If you do a web search for "electro-osmosis" you will not get osmotic pressure produced by excess concentrations of ions held back by electric fields. What a search will produce is either the generation of voltage by water flowing across an ionized surface, or the exact reverse - production of water flow by a voltage along an ionized surface. Any surface that does one will also do the other. And if the surface is made of a porous or gelatinous material, then water will tend to flow into it, producing an osmotic pressure without the need for a membrane.

Biologists and physicians tend not to know enough physics and chemistry. Instead of knowledge, they substitute fancy words: Donnan Equilibrium, Tensegrity, Piezoelectricity, Galvanotaxis. This produces a simulation of understanding. Nothing holds back progress more effectively than plausible mistakes.

Please consider the following quote from a Wikipedia article about osseous tissues: osteocytes "release calcium, magnesium, and phosphate ions that ultimately combine chemically within the collagenous matrix into a crystalline mineral."

How do calcium and phosphate reach bone? They are released!

How are they chemically combined? Ultimately!

Etc. It sounds like information. In each case, we need to ask "As compared with what alternative". What if I claimed they are confined immediately? Which words make testable claims? Do any of them? Or is it what politicians often do: Combine words that seem to have a meaning.

People suffering from osteoporosis deserve a lot better than this.




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