|MadSci Network: Anatomy|
Bone as a whole is an organ that is constantly remodeling itself in response to the needs of the organism for physical support, organ protection, and as a means to store minerals. In particular, the minerals calcium, phosphorous and magnesium are stored in bone. The bones have a matrix (a scaffolding) of protein in which crystalline mineral is deposited. Crystals of calcium-phosphate (phosphate is a chemical entity that contains phosphorous and oxygen) of different types (hydroxyapatite and osteocalcium phosphate, among others) are deposited by cells in the bone, and the means by which they determine how and where to "lay down" the crystal is a very complex and involved process, where the cells can sense stress and the need for stronger bones, or the need for bone breakdown to release minerals into the bloodstream. The matrix and the calcium-phosphate crystal structure are all outside the cells of bone, but the cells themselves are responsible for laying down the cyrstal structure. In typical solid bone, one can see rings of bone crystal around each of the cells when you see them under a microscope. The ring furthest from the cell is the oldest; that nearest the cell is the newest. Cells also sit in the structure of bone that are responsible for breaking the mineral structure down. When a person dies, the cells dissolve away and leave small spaces within the intact structure (the bone you're used to thinking makes up a skeleton in a grave or in your anatomy class) that is very strong and that takes much, much longer to degrade than the rest of the orgnaism.
The degree to which someone either builds bone or breaks it down depends on hormonal signals and supply of some particular chemical compounds, particularly vitamin D and the minerals (calcium and phosphorous, mainly) themselves. The parathyroid gland (located in the neck behind the thyroid gland), the intestine and the kidneys are typically recognized as the primary sources of our means to regulate calcium and phosphate. We gain some vitamin D in our diet, but our kidneys and skin together synthesize vitamin D as well. Ultraviolet light is required to convert native vitamin D to a form that works to build bone -- this is why rickets (caused by deficieny of vitamin D or of calcium, phosphorous, or both) occurs more often in people that are rarely exposed to sunlight. Other hormones including those from the thyroid, adrenals, gonads, pancreas and liver can affect bone production and breakdown. Many drugs affect bone metabolism as well.
Any basic physiology book in your local college bookstore (go before classes start the next semester) will have a clear section that describes bone mineralization. It's much too involved for me to spell out a detailed description. In fact, several scientists have entire careers figuring the details of small portions of the process. If it interests you sufficiently, you can dive in with them!
Tim Nicholls, MD
reference: Nelson's Textbook of Pediatrics, 17th ed.; Saunders, Philadelphia, PA, 2004.
Try the links in the MadSci Library for more information on Anatomy.