You may have considered the description of collagen in the previous blog post a digression, since collagen is tough and does not stretch, and we intuitively know that bone is not stretchy. Rather bone is rigid and resists getting mashed flat. Bone stands up to compression (scientific parlance for mashing) because it consists of calcium crystals deposited on a meshwork of, you guessed it, collagen—like plaster on lath.

To prove the complementary features of collagen and calcium for yourself, enjoy a roasted chicken dinner and save some of the long bones. Bake half of them at 250 degrees F for several hours. Soak the others in vinegar for a couple of weeks.  The baked ones becomes brittle and easily breakable, like sticks of chalk, because the heat has destroyed the collagen. See for yourself:

The soaked bones become rubbery because the vinegar dissolves the calcium, leaving the flexible collagen. See for yourself:

Chemistry books note that calcium crystals come in various compositions. These include calcium chloride (road de-icer), calcium citrate (water softener, diet supplement), calcium carbonate (Tums, chalk, coral, egg shell), calcium sulfate (gypsum, plaster of paris), and calcium hydroxide (slaked lime).

If you add a phosphorous compound to calcium hydroxide under the right conditions, you get hydroxyapatite, which is likely a new word for you. It has nothing to do with Hydrox cookies. Rather, it is the principal calcium crystal of bone. People might consider you a bit weird if you tossed the word out at a cocktail party, but it is what our bone crystals are called, and hydroxyapatite is holding you off the floor, so let’s learn a bit about it, starting with a little math.

Bone constitutes about 15% of our body weight, and bone is about one-third collagen and two-thirds calcium crystals. Accordingly, a 160-pound person possesses 24 pounds of bone (8 pounds of collagen and 16 pounds of hydroxyapatite). That’s enough to fill a carry-on roller bag, not that you would want to try to slip it past TSA, but at least you have a mental image now of one’s bone mass.

Here is what the dictionary says about the origin of the word hydroxyapatite. “German Apatit, from Greek apatē deceit. First known use: 1794.” Deceit? I guess that by that late date in the discovery and naming of calcium crystals all the easy Greek words were already taken, yet they had to name this one something. By the way, apatite has nothing to do with appetite, even for marshmallows and other collagen-derived delectables mentioned in Part I.

Imagine some osteoblasts floating around in a nutritious oxygen-water broth. The osteoblasts dutifully follow their genetically programmed urge to secrete collagen molecules and hydroxyapatite crystals. Voilà, the calcium crystals deposit themselves on the meshwork of collagen, and we have bone.

Gradually each osteoblast encases itself in a cocoon of bone, where it matures into an osteocyte, aka adult bone cell. Osteocytes maintain the structure of the bone but do not contribute much to further construction or to any destruction.

Various chemical messengers, mainly hormones from the pituitary, thyroid, and testes or ovaries, affect the vigor with which osteoblasts produce bone. Other chemical messengers, known as growth factors, are produced by different types of cells residing nearby. After a fracture, several of these growth factors can whip the osteoblasts into a bone-forming frenzy and can even convert other types of cells (stem cells) into osteoblasts.

Mature bone cells are tiny balloons of salt water rigidly encased in calcium. They are sensitive to changes in atmospheric pressure just like everything else in the world. For instance, a change in air pressure is why your ears pop when you rapidly gain or lose elevation and why champagne fizzes when you pop the cork and decompress the bubbly. The same thing happens to bone when a low-pressure storm front blows through.

The osteocytes in aged, arthritic bones are particularly sensitive to these subtle changes in air pressure and can cause Grandma’s knee to ache. The same is true for osteocytes in recently healed bone, such as after a fracture or joint replacement. These osteocytes will produce weather-related ache for over a year before they calm down to their usual life of solitude and anonymity.

This dynamic duo of collagen and calcium has been supporting vertebrate life for roughly 500 million years. Collagen provides the meshwork and a bit of resiliency. Yes, believe it or not, bone can bend a bit and spring back without breaking. Thank you, collagen. Calcium adheres to the meshwork and provides rigidity, which separates fish, frogs, snakes, birds, and us from jellyfish. Stand tall and be proud of your bone.

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