1) Osteoporosis; 2) Electric Fields in tissues; 3) Transmembrane voltage functionsLiving bone is constantly dissolved and reformed. (this doesn't occur in Dentine of teeth)
The dissolving is done by a special cell type: "osteoclasts" which have many nuclei per cell, and develop by fusion of macrophages. Osteoclasts secrete protein-digesting enzymes and acids into closed-off spaces between themselves and surfaces of bone.
The remaking of bone is done by a different cell type: Osteocytes = Osteoblasts.
A very different possibility is that osteocytes might secrete some chemical that reduces the solubility product of calcium phosphate. Is this chemically possible? Does fluoride do this?
It is well known that chelators like EDTA increase ion solubilities?
Imposing tension and/or compression on bone (somehow) stimulates increased strength.
For example, tennis players develop stronger bones (and stronger muscles) in whichever arm
(the right arm in right-handed tennis players).
In all bones, trabeculi form preferentially along axes of maximum force; & if this axis is changed, that somehow causes new trabeculi to form parallel to the new axis of force.
Weakening of bone (osteoporosis) is a major medical problem for millions of Americans,
especially older women, and indirectly causes hundreds of thousands of deaths.
In principle, osteoporosis could either be caused by increased rates of destruction by osteoclasts or by decreased renewal of bone by osteocytes, or by a combination.
PLEASE TRY TO INVENT EXPERIMENTS OR OTHER KINDS OF TESTABLE PREDICTIONS BY WHICH YOU COULD FIND OUT WHICH POSSIBILITY IS TRUE.
How do bone cells detect amounts, locations and directions of forces exerted on/in bones? One popular theory (for many decades) has been that bone might be a piezoelectric crystal (including quartz and sucrose) that generate electrical voltages when distorted in shape.
The good news for this theory is that bone (and cartilage) emits voltages when stressed. The bad new is that calcium phosphate has the wrong symmetry to be piezoelectric. (Ions need to be arranged such that distorting a crystal causes unequal shifts of negative ions relative to positive ions) (The need to lack "inversion symmetry”.
I suspect that bone voltages are caused by electroosmosis, in which negative sulfate ions are covalently linked to collagen by chains of sugar molecules, and only positive ions are able to be squeezed out by pressure. This is another way to produce osmotic pressure, without semi-permeable membranes! Textbooks have mostly forgotten about this. What Wikipedia says on these subjects illustrates how confusing it can be. (Especially the "talk" section) Whenever distortion produces a voltage, then conversely voltages will produce shape change or movements of parts. In piezoelectric lighters, force generates voltage, and in atomic force microscopy voltage produces movements / forces.
In a salt water environment, small voltages will quickly drain away. Thus neither piezoelectric or electroosmotic effects can produce long-lasting electric currents.
Also, increasing pressure produces exactly the same effect as decreasing tension.
In effect, both these sources of voltage detect what amounts to the first derivative of shape changes.
Hint: Actual hospitals used direct current, but probably should have used alternating current.
Resting potentials occur in all cell types, not just nerves, muscles and oocytes.
Tissue culture cells crawl directionally in response to small voltages.
QUESTION FOR DISCUSSION: Whether this opposite electrical response of osteoclasts versus osteocytes is causally related to their bone-destruction versus bone construction?
Different possible reasons / mechanisms by which a voltage difference might alter cell movement or forces between one side of a cell and the other.
If we assume that plasma membranes have much lower electrical conductivity than the cytoplasm, therefore externally imposed voltage gradients will be much less steep inside cells as compared with in the surrounding medium (I think?). Try to figure this out.
Imagine if some positively charged membrane protein were pulled along cell surfaces accumulating near the side of each cell nearest the negative electrode. Would you consider that electrophoresis? What if accumulation of such a protein caused cells to crawl toward
higher concentrations? ..crawl away? … detach?
PS: Some danger of electrocution is hard to avoid in studies of voltage effects on cells.
back to syllabus