Emerging concepts of the structure and metabolic functions of bone

An attempt has been made to describe briefly the changing concepts of bone metabolism. Four new, important yet unproved postulates have been made and are summarized diagrammatically in Figure 2. The maintenance of normal bone physiology depends on: 1. 1. The absorption of calcium and phosphate from the gastrointestinal tract. Since serum is supersaturated, some active mechanism or ionpump must be postulated. Presumably, vitamin D is required for the normal operation of this mechanism. 2. 2. The mineralization of collagen. It is now fairly certain that the crystalline collagen initiates bone crystal formation. It is not yet settled whether the collagen molecule requires an “activation” mechanism. 3. 3. The continuous transfer of ions to and fro across the crystal-solution interface. Here, the crystals of the “available” bone reflect, by passive ion exchange, the electrolyte composition of the body fluids. It is necessary to postulate, however, that the osteocytes actively affect the composition of the fluid locally. Present indications are that local production of citric acid (possibly other acids) by the cells under the metabolic stimulus of parathyroid hormone (possibly also vitamin D) is responsible for maintenance of the serum in its supersaturated state. 4. 4. The continuous formation, maturation and resorption of osteones. While the natural course of events is toward complete mineralization, a state of dehydrated inertness, new and maturing osteones continuously provide an “available” skeletal reserve of mineral which can passively buffer against changes in electrolyte balance. 5. 5. The regulation of electrolyte composition by selective reabsorption in the kidney. This mechanism, with its intricate hormonal control is, of course, not a postulate [34,53].