Bones

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represent a valuable mineral reserve that maintains normal concentrations of calcium and phosphate ions in body fluids. In addition, fat cells within marrow cavities store lipids that represent an important energy reserve. The bones of the skeleton function as levers that direct and modify the forces generated by skeletal muscles. The movements produced range from the delicate motion of a fingertip to powerful changes in the position of the entire body.
Osseous tissue contains specialized cells, cell products, and a fluid matrix. The distinctive solid, stony nature of bone results from the deposition of calcium salts within the matrix. Crystals of calcium phosphate account for almost two-thirds of the weight of the bone. The majority of
bone mineral occurs in the form of hydroxylated calcium phosphate crystals
(Ca 10 [PO 4 ] 6 [OH]2 ) referred to as hydroxyapatite. The remaining third is dominated by collagen fibers, with osteocytes and proteoglycans contributing only around 2 percent.
Calcium phosphate crystals are very strong, but inflexible. They can withstand compression, but the crystals are likely to shatter when exposed to bending, twisting, or sudden impacts. Collagen fibers are extremely tough, but quite flexible. They can easily tolerate stretching, twisting, and bending, but when compressed they simply bend out of the way. In bone, the collagen fibers provide an organic framework for the formation of mineral crystals. The combination has properties intermediate between those of collagen and those of pure mineral crystals.
When you examine sections of bone taken from different sites, you find two different architectural arrangements. Dense, or compact bone is relatively solid, whereas spongy, or cancellous bone resembles a network of bony shafts separated by marrow spaces. In both types of bone the individual bone cells, or osteocytes, reside within small pockets called lacunae. These lacunae are found between narrow s...