Biology 446    Unsolved Problems Fall 2011



Cartilage and Electro-Osmosis

Cartilage : a very important building material of the body, that is often not understood.

Gristle is another word for cartilage.

Our joints are lined with thin layers of "articular cartilage", and many joints contain sheets of cartilage. Articular cartilage is very slippery, except where it is actually attached to bones. It feels like Teflon.

Damage to articular cartilage is the reason millions of people need to have knee replacements, hip replacements, etc. Sometimes cartilage gets physically damaged; sometimes it gets attacked by the immune system. Without it, bones will rub directly against each other, and there is terrible pain. If you could repair articular cartilages, for example by colonizing joints with enough new cartilage cells, that would save much suffering and expense. Progress has been made in this direction. Cartilage cells (chondrocytes) survive, grow and divide well in tissue culture. They are the only cell type I know that doesn't need vascularization; they tolerate low concentrations of oxygen.
Articular cartilages are made of a sub-type known as hyaline cartilage.

The flat blocks of cartilage between the central parts of your backbone are made of fibrous cartilage These are strongly attached to the bone on both sides: they are sort of the opposite of lubrication.

A so-called "slipped disk" is a herniation of the collagen layer that surrounds each intervertebral disk. Mechanically, it isn't at all analogous to a coin slipping out of a pile of coins (as suggested by "slipped").

Medically, it would be a huge step forward if someone invented a way to cause the surface tissues of these disks to contract and pull the herniation back in, and heal the "slip". No research is being done in that direction, as best I know. One reason is that the physics of cartilage is not correctly understood by most physicians and biologists. They don't understand electro-osmosis.

Our external ears and the tips of our noses are supported by a special version of cartilage, called elastic cartilage. This contains more elastin fibers, and fewer collagen fibers, than either hyaline or fibrous cartilage. Physically, elastic cartilage is much less stiff; it also springs back to its original shape quicker.

The shape, strength and elasticity of every cartilage is caused by a counter-balance of two forces:
One force is electro-osmotic swelling, caused by water being drawn inward by osmotic forces.
The other force is elastic stretching of collagen fibers (and sometimes also elastin fibers).
(Some of these collagen fibers run through the interior of cartilages; other collagen fibers are wrapped around the outside of cartilages. The interior ones are more important in articular cartilage.
The surface wrapping ones are most important in inter-vertebral disks (The "annulus fibrosis", which surrounds the "nucleus pulposis". What nifty old names; both from Latin.)

The kind of osmosis that you already know about depends on having a semi-permeable membrane that surrounds some water in which either molecules or ions are dissolved. The membrane needs to allow water to diffuse through it, but not allow the dissolved ions or molecules to diffuse through it. The third requirement is for a higher concentration of ions or molecules inside the surrounding membrane, as compared with their concentration outside the membrane. Water will then exert a surprisingly strong force diffusing toward wherever the concentration of dissolved ions or molecules are higher. This force obeys the ideal gas law (strange to say!), good old PV=nRT.

You remember that one from high school chemistry, right? If you put two grams of hydrogen in a one liter flask, it will push outward with a force of twenty two atmospheres. Eighteen grams of oxygen, ditto. A one molar solution of glucose dissolved in one liter of water, can produce 22 atmospheres of osmotic pressure. Please understand that osmosis is an entropic effect; it doesn't depend on attraction forces between water and dissolved materials, so long as they are soluble in water. Diffusing units per volume.

Suppose you had some other method able to confine diffusing ions to a certain limited volume? That could give you an osmotic pressure, proportional in strength to the concentration of ions whose diffusion is restricted. This pressure would push against whatever restricts the diffusion of the ions. Inside cartilage, there are many sulfate groups covalently bound to chains of sugars, which are bound to fibrous proteins.

Hydrogen ions, sodium ions and other cations are free to diffuse around amongst the sulfates, but are not free to leave their vicinity. Negative charges on the sulfates hold positive ions near them. Water tends to dilute the diffusing cations, producing an osmotic pressure equal to the cation's molar concentration. (times R, the gas constant, times T the Kelvin temperature, divided by volume).

The electric field serves the role of the semi-permeable membrane. So the force pulls on the sulfates.

You probably won't find this mentioned if you look up the meaning of electro-osmosis. And you probably won't find electro-osmosis mentioned in books about cartilage, or about back surgery or knee replacement.

If you squeeze a piece of cartilage, you can produce hundreds of volts (but a tiny amperage). Pressure squeezes out some of the water, which carries cations with it, producing a momentary voltage (positive outside, negative inside); then when you release the pressure, a brief but high voltage is generated in the opposite direction. These voltages are much bigger, and equally brief, as whatever piezo-electric voltages are produced by squeezing or twisting bones (a phenomenon which gets very much more attention.

Suppose you impose an electric voltage (using wires and electrodes, batteries, rectifiers, etc. a regular electric voltage) oriented somewhat parallel to the surface of a piece of cartilage,, what will happen? The same will happen if you create a voltage along the surface of a piece of glass, incidentally. Water flows toward the negative electrode. That is because of the slightly greater concentration of positive ions in the water next to the negatively charged immobile material. The same flow occurs if a voltage is imposed through pores of a piece of clay or glass, or cartilage.

This voltage-induced flow of water is what most people call electro-osmosis. (Those few people who have ever heard of it at all.) In your opinion, is one or the other definition of electro-osmosis correct, and the other meaning is incorrect? Or are these two sides of the same coin? You can't get one without the other?

Two results of the same set of causes?

How might knowledge of this subject help surgeons repair knees or slipped disks?

Some specific questions about cartilage that I don't know the answer to, and that might be important:

A) Does the cytoplasm of chondrocytes have a higher osmotic pressure than the cytoplasms of other kinds of differentiated cells?

B) If we dissected out a piece of cartilage and soaked it in water with a very high concentration of dissolved molecules or ions, would water flow out of the cartilage. If so, would the cartilage shrink in volume, change shape, shrivel up, or what?

C) Would an increased concentration of multivalent cations (Ca++, and Al+++) cause a weakening of electro-osmotic pressure by one half or two thirds, relative to what the pressure would be when most of the cations in the sulfated cartilage matrix are Na+, K+ and H+?

D) If we soaked a piece of cartilage in a non-poisonous dye that becomes a cation in solution, then would different amounts of this dye accumulate in different parts of the cartilage, in proportion to different amounts of electro-osmotic pressure at different locations?

E) Why does Alcian Blue stain cartilage with as much specificity as it does? Does this result from an especially large positive ionization by individual dye molecules?

F) If an embryologist injected microscopic plastic beads (fluorescent beads, perhaps?) into chick embryo limb buds, at locations where cells will later differentiate into chondrocytes, then will these beads become arranged into the same geometric rows as chondrocytes do?

G) Would the shape or size of a cartilage change if we imposed a voltage gradient on it?


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