Electroosmosis causes cartilage to expand and elongate in developing embryos and during childhood. Negative charges on sulfated sugar chains in cartilage retain high concentrations of sodium, hydrogen and other cations nearby. These "counter-ions" produce a large osmotic pressure, without the need for semipermeable membranes, since the ions are prevented from dispersing by electric attraction toward the sulfates. A tempting mistake is to suppose that pressure is caused by repulsion between the negatively charged sulfates; but without water, the swelling pressure doesn't occur. |
The counter-ions produce other effects. One is that water flow can be produced by applying a voltage difference along the surface of a cartilage or through its interior. The word electroosmosis (or electro-osmosis) often gets defined as production of water flow along charged surfaces or through pores in materials having net electric charges (such as soil or glass). Many people (see the Wikipedia article on electro-osmosis) mistakenly believe that flow is caused by pores, per se, instead of by counter-ions that accumulate in the pores because of charged groups in the soil etc.
Yet another effect of the same cause is that forcing water to flow along the surface of a cartilage (or a piece of glass) will create a small voltage difference: positive at the end toward which the water is flowing (carrying counter-ions).
Rapid squeezing of a cartilage can produce brief voltage gradients of many hundreds of volts. This is because counter-ions get carried along by water being squeezed out. Equally big (and equally temporary) voltages are produced by relaxing the squeezing pressure. These voltages deserve much more attention than the comparatively tiny (and equally brief) voltages produced by bone piezoelectricity.
The ability to be inflated by osmotic pressure (from the inside out) is why nearly all of our skeleton is first made out of cartilage. Bone can only grow in size by deposition at its surface, which would be a problem where bones rub against each other. Most of our cartilage is progressively destroyed and bone is formed at the locations of cartilage "models". The shapes of bones therefore result mostly from the geometry of cartilage inflation. Osmotic pressure exerts physical resistance to penetration by blood vessels, regardless of whether there are chemical inhibitors of vascularization. Penetration by capillaries is equivalent to digging a hole through an inflated blimp, but does occur where cartilage is being replaced by bone.
Badly broken bones are healed by destruction of the bone at the break, formation of cartilage, and gradual replacement of the cartilage by bone.
Thin layers of "articular cartilage" are retained at many joints as lubrication. Other cartilages support the external ears, the tip of the nose, and a broad area where the distal ends of ribs converge. Flat blocks of especially fibrous cartilage lie between the vertebral centra of our backbones. The lateral surfaces of intervertebral disks are much more fibrous than their inner parts, which helps to spread pressure, but at the cost of increased susceptibility to surface herniation caused by combinations of pressure and twisting.
Revolutionary improvements in treatment of "slipped" disks will eventually be accomplished by stimulating active retraction of cartilage back into the space between the vertebral centra. The current treatment is to insert metal bolts to connect the vertebrae directly above and directly below the herniated disk. This reduces further herniation at the cost of serious surgery, which sometimes produces permanent paralysis. Because few MDs (including orthopedists) even know what electroosmosis means, who can believe surgical bolting is the best or only treatment for severely herniated discs? What you have learned in this course will put you in position to invent much easier and less incapacitating treatments.