Abstract
Mineralized bone tissue has a significant water component. Bone water is associated with the collagen fibers or mineral fraction or occurring as pore water of the Haversian and lacunocanalicular system. Among the multiple physiologic functions that include signaling and providing to bone its viscoelastic properties, bone water enables the transport of ions and nutrients to and waste products from the cells. In addition, it plays a key role during mineralization whereby collagen-bound water is gradually replaced by calcium apatite-like mineral. In this review it is shown how nuclear magnetic resonance (NMR) allows the study of various physiologically relevant properties of bone water nondestructively. Isotope exchange experiments are described from which the apparent water diffusion coefficient can be calculated. The method is based on monitoring the migration of H2O into the D2O after immersion of the specimen in heavy water. Data obtained from rabbit cortical bone in the normal and mineral-depleted skeleton provide evidence for the hypothesized reciprocal relationship between bone water and mineral. Further, from the diffusion coefficient (Da = (7.8+/-1.5) x 10(-7) cm2/s) measured at 40 degrees C it can be inferred that diffusive transport of small molecules from the bone's microvascular system to the osteocytes occurs within minutes. Finally, whereas isotope exchange is not feasible in vivo, it is shown that bone water can be imaged by proton MRI.
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