All about bones....
First the words. Bone is a substance. Things can be made out of the material called bone. And, bones are skeletal parts which might not actually be made out of bone (the substance). Sharks have skeletons with lots of bones and those bones are not made out of bone. Baseball diamonds are not made out of diamond (at least not in our neck of the woods).
Bones - The Substance
Bone is a complex building material that can be made by a variety of tissues and not in a single way. It is easier to think of tissues placing requests or signaling for bone. That signal sets off processes that do the actual bone manufacture.
Some bone is built up in layers between or beneath
specialized tissue layers. That tissue is typically tough and not too unlike canvas in thickness and toughness. The plates of the skull are made wholly from bone layered between two such membranes.
Looking through the microscope at slivers of bone such as taken from a bone near the outer surface but in a spongy area, we can see a mix of types. The spongy bone is bone with lots of spaces. Those spaces are generally sprinkled with blood tissues. Bones do not only contain blood, they contain blood vessels and much of the machinery for making blood cells.
On the far right we see some lamellar bone, looking like strata of geologic layers of earth. Into such bone, cones of cutting blood vessels penetrate. They then organize bone around their path of penetration forming bone in long layered tubes. We see those in cross section on the left. They are called Haversian Tubules (take a wild guess who they are named after).
Cells are seen in layers around a central
canal (Haversian Canal). Radiating micro tubules reach out intercommunicating.
The layering and bundles of layered tubes are of cells and sheets of tissue made of very tough fiber called bone collagen. A bone with all of its calcium leached out can be tied in a knot. On and through that structure, a very special crystalline form of calcium is formed called hydroxyappetite (say "hy - drox - ee ahh - puh - tite"). That stiffens the bone to make it hard.
Bone gets strength from two things.
1) What it is made from (kind of bone), and
Hydroxyappetite figures in both aspects of strength. The first is kind of obvious. Some stuff is tougher than other stuff. But shape is very important. Tubes are stronger than rods. Tension and compression struts vastly support structures (look at power towers). As with certain crystals - such as those used in phonographs (remember those) - when pressure is placed on them, they polarize and exhibit an electric charge. A needle jiggling in a plastic track while pressing against a crystal will reflect the jiggling as a fluctuating charge which magnified gives us music - and we listen.
The compression of forces of daily activity on hydroxyappetite gives us zones of charge to which the osteoblasts listen. They respond by putting more bone substance where forces generate such charges. Where such charges fail to form, bone - always being dissolved - wheedles away. The form of bone follows function. In other words, as was spoken by Hypocrites a few years ago, "That which is not used, wastes away." The paraphrase is, "Use it or lose it."
This had to be rediscovered when perfected devices which held fractured bone pieces absolutely rigidly,
better than ever before ... produced poorer healing. Without SOME movement, bone formation is not very good.
Is Milk Really Important?
You betcha. I know, some kids get the snots. Others get cramps from lactose. Whatever. But the reality is that in most diets, minus milk - by far - there is very little vitamin D. To make bone substance you need vitamin D. In places where kids eat raw fish or consume certain beetles, there is plenty of vitamin D and so bone is happy.
Cows do NOT put vitamin D in milk. Most milk based products such as cheeses are NOT great sources of vitamin D. The vitamin D in milk is put there by a guy named Ernie - could be Roger, maybe even what's her face - Bernice. Anyway, whoever he or she is, it isn't the cow. Milk that is not vitamin D fortified is not advantageous. We have seen rickets in the children of health nuts who disdain "processed milk" and only allow the natural stuff - right from the cow - thus preserving all the natural tuberculosis that the animal can offer.
When milk cannot be used in the diet of children, you must consult a professional to get a substitute. Vitamin D can be overdosed and is dangerous when self administered without knowledge.
Seizure medications can and do interfere with vitamin D to produce very real and troublesome bone problems. "Normal" vitamin levels may be insufficient as there is an antagonist at work.
Diphosphonates are drugs which deliver dual phosphate molecules. Pairs of phosphates glom onto hydroxyappetite like egg dye on an egg. That tends to "stabilize" the otherwise very rapid turnover of the crystal. When there is a process that draws away calcium, the diphosphonates protect the crystals. But they can get in the way of build up when the process is in the direction of accumulation. Newer more clever chemistries are being used to see if the plusses can be made to outweigh the minuses. The very uneven results seen with these drugs as a class reflects the inconsistency of the application. There is tremendous variation and an experienced professional is needed to know if it is helping or hurting.
Calcium is an important substance beyond mere bone structure. In ionic form (the molecular water dissolved form) it is a charged atom as Ca++ which means it has charge that pairs it with negative charged things such as two OH- (hydroxyl) molecules or phosphate as PO4=. In this charged form it is used to regulate, control or initiate processes such as nerve conduction, muscle contraction, hormone release among other things. It is very tightly kept at optimum in the blood - even if that means taking calcium from bones. Calcium is the stuff that leaves water spots on dish ware or which makes bathtub rings when combined with certain substances. That is, it easily precipitates. X-rays of injured or inflamed tissue may show deposits of calcium. That does not mean calcium CAUSED the problem, but more likely that calcium is precipitating due to the problem.
Indeed, the combined levels of Ca++ and PO4= found normally in the blood exceed levels which can be achieved in water without precipitating. Blood is hyper saturated with calcium by means of other substances which stabilize calcium in solution. An interesting complication of this fact to babies - especially preemies - is that IV fluids used to maintain babies (who cannot eat) cannot contain EITHER enough Ca++ OR enough PO4= to sustain needs. Children can get a baby form of rickets which may look like bone loss with fractures. To get around this, if an IV is needed long term, solutions with high calcium have to be alternated with solutions of high phosphate. If the correct amounts of BOTH were placed in a single solution, the calcium phosphate would solidify in the bottle.
When calcium levels in the blood are off by substantial amounts then something else must be at the core of the matter: low protein, high protein, kidney disorder etc. The people who manage calcium and those other difficult blood salts and kidney function are the ones with tall foreheads, out of control hair and who horde all the back issues of the Journal of Clinical Investigation.