Hydroxyapatite bone mineral

Bone is a porous tissue composite material containing a fluid phase, a calcified bone mineral, hydroxyapatite (HA), and organic components (mainly, collagen type). The variety of cellular and noncellular components consist of approximately 69% organic and 22% inorganic material and 9% water. The principal constiments of bone tissue are calcium (Ca ), phosphate (PO ), and hydroxyl (OH ) ions and calcium carbonate. There are smaller quantities of sodium, magnesium, and fluoride. The major compound, HA, has the formula Caio(P04)g(OH)2 in its unit cell. The porosity of bone includes membrane-lined capillary blood vessels, which function to transport nutrients and ions in bone, canaliculi, and the lacunae occupied in vivo by bone cells (osteoblasts), and the micropores present in the matrix. 

The previously proposed uptake models were mathematical assumptions and had no physical or chemical basis. Millard and Hedges, on the other hand, considered the chemistry of bone-uranium interactions. With the D-A model, they proposed that U was diffusing into bone as uranyl complexes, and adsorbing to the large surface area presented by the bone mineral hydroxyapatite (Millard and Hedges 1996). Laboratory experiments showed a partition coefficient between uranyl and hydroxyapatite under oxic conditions of 10" -10, demonstrating U uptake in the U state without the need for reduction by protein decay products as proposed by Rae and Ivanovich (1986). 

The estimation of the crystallinity index (Cl) of bone is based on one of the four vibrational modes associated with the apatite phosphate group. In amorphous calcium phosphate, the absorption band at 550-600 cm-1 appears as a single broad peak, whilst in hydroxyapatite it is split into bands of unequal intensity by the apatite crystal field (Sillen and Parkington 1996). Based on the splitting factor introduced by Termine and Posner (1966), Weiner and Bar-Yosef (1990) proposed the use of a crystallinity index to measure the crystallinity of bone mineral. As illustrated in Fig. 4.7, the Cl is estimated by drawing a base line from 750 to 495 cm 1 and measuring the heights of the absorption peaks at 603 cm-1 (measurement a), 565 cm 1 (measurement b) and the distance from the base line to the lowest point between the two peaks (c). Cl is calculated from the formula ... 

In adults, bone is the repository for 98% of the body s calcium and 85% of the body s phosphate in infants, the skeleton contains about 96% of the body s calcium and 70% of its phosphate. The Ca/P ratio Of bone mineral (hydroxyapatite or SECa PO Jj-CafOH can be expressed in various units yielding different values ... 

Body fluids have a very high supersaturation with respect to hydroxyapatite, which cannot be explained by the small particle size of bone mineral. In fact, they behave as agueous solutions which are in metastable eguilibrium with DOHA. However, the minerals in bone, dentin, dental enamel and dental calculus contain considerable amounts of Na, Mg and CO3, in addition to calcium and phosphate which are the major components. Therefore, the phases mentioned above which all show a solubility comparable to that of DOHA, all come into consideration as components of these minerals. 

Bone dissolution is composed of two major processes mineral dissolution and protein degradation. Bone mineral is hydroxyapatite i.e. (Ca3(P04)2)3 x Ca(OH)2. It is dissolved into Ca, HP04, and H2O with the help of hydrochloric acid. For this, independently protons (H" ) and chloride (Cl ) are... 

Vitamin D hormone is derived from vitamin D (cholecalciferol). Vitamin D can also be produced in the body it is formed in the skin from dehydrocholesterol during irradiation with UV light. When there is lack of solar radiation, dietary intake becomes essential, cod liver oil being a rich source. Metaboli-cally active vitamin D hormone results from two successive hydroxylations in the liver at position 25 ( calcifediol) and in the kidney at position 1 ( calci-triol = vit. D hormone). 1-Hydroxylation depends on the level of calcium homeostasis and is stimulated by parathormone and a fall in plasma levels of Ca or phosphate. Vit D hormone promotes enteral absorption and renal reabsorption of Ca and phosphate. As a result of the increased Ca + and phosphate concentration in blood, there is an increased tendency for these ions to be deposited in bone in the form of hydroxyapatite crystals. In vit D deficiency, bone mineralization is inadequate (rickets, osteomalacia). Therapeutic Liillmann, Color Atlas of Pharmacology... 

The principal mineral species is an apatite which is best defined as an isomorphous mixture consisting of two endmembers hydroxyapatite and carbonate apatite, i.e. Ca10(P04)6(OH)2 and (Na, Ca)io(P04, C03)6(0H)2. Because of the isomorphous nature of the material the proportions of C03 that substitute for P04 most certainly vary from one unit cell of bone mineral to another and no uniform composition should actually exist. The extent to which carbonate groups proxy for phosphate groups in the crystal lattice is in the order of a few to a maximal of 10% by weight. 

How the osteoblast deposits mineral has long been an interesting mystery. Details of the process are still unclear, although some general principals can be derived from diseases of mineralization which point to the central mechanisms. The chemistry of bone mineral, hydroxyapatite, requires that any mechanism of bone formation include a supply of Ca2+ and H2PC>4 and some way to dispose of 1.4 H+ per each Ca2+ deposited. 

Collagen is a major structural element in connective tissues skin, tendons, muscle, and internal organs. It combines with inorganic compounds in bones and teeth. Cartilage is collagen mixed in an amorphous gel. Dentine, which makes up the bulk of a tooth, is a mixture of collagen and hydroxyapatite (a mineral), and water. 

Because of the importance of the surface chemistry of bone mineral in physiological systems, we have undertaken a series of gas adsorption studies on hydroxyapatite in the form of anorganic bone. In a recent publication from this laboratory (4) results of calorimetric studies of the adsorption of water and methanol vapors on bone mineral and on synthetic hydroxyapatite were reported. The adsorption potential for nitrogen on dehydrated hydroxyapatite, whether from bone or from synthetic sources, was rather profoundly altered by the addition to the surface of chemisorbed methanol or water prior to the adsorption of nitrogen at —195° C. This effect was reflected in the specific surface areas, in the BET C values, and in the resultant values of Ex — EL (net heats of adsorption) as shown in Table I of the above paper. 

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