Hydroxyapatite crystallization

Technetium-99m pyrophosphate is used for bone imaging. The compound appears to have an affinity for the hydroxyapatite crystals within bone, and is formed by addition of up to 7.4 GBq (200 mCi) pertechnetate. 

Fluorid ions stimulate bone formation by a direct mitogenic effect on osteoblasts mediated via protein kinase activation and other pathways. Further to these cellular effects, fluorides alter hydroxyapatite crystals in the bone matrix. In low doses, fluorides induce lamellar bone, while at higher doses abnormal woven bone with inferior quality is formed. The effect of fluorides on normal and abnormal (e.g. osteoporotic) bone therefore depends on the dose administered. 

The increased serum phosphorus binds to calcium in the serum, which leads to deposition of hydroxyapatite crystals throughout the body. The calcium-phosphorus (Ca-P) product reflects serum solubility. A Ca-P product greater than 75 mg2/dL2 promotes crystal deposition in the joints and eye, leading to arthritis and conjunctivitis, respectively. Soft tissue deposition primarily affects the coronary arteries of the heart, lungs, and vascular tissue and is associated with a Ca-P product greater than 55 mg2/dL2.36 The Ca-P product has been associated with increased mortality37 and is a risk factor for calcification of vascular and soft tissues.35... 

Hydroxyapatite Crystals / Prosthetic Coating. Wheeless Textbook of Orthopaedics, C. R. Wheeless, M.D., 1996. http //www.medmedia.com/o2/49.htm... 

Calcium phosphate precipitation may also be involved in the fixation of phosphate fertilizer in soils. Studies of the uptake of phosphate on calcium carbonate surfaces at low phosphate concentrations typical of those in soils, reveal that the threshold concentration for the precipitation of the calcium phosphate phases from solution is considerably increased in the pH range 8.5 -9.0 (3). It was concluded that the presence of carbonate ion from the calcite inhibits the nucleation of calcium phosphate phases under these conditions. A recent study of the seeded crystal growth of calcite from metastable supersaturated solutions of calcium carbonate, has shown that the presence of orthophosphate ion at a concentration as low as 10-6 mol L" and a pH of 8.5 has a remarkable inhibiting influence on the rate of crystallization (4). A seeded growth study of the influence of carbonate on hydroxyapatite crystallization has also shown an appreciable inhibiting influence of carbonate ion.(5). 

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... 

Figure 1 shows the schematic illustration of phosphate removal mechanism. Phosphate in wastewater contacts with seeds made of phosphate rock, after chemical conditioning such as supply of calcium and hydroxide ion. Then calcium phosphate, mainly hydroxyapatite, crystallizes, according to eq.(l), on the surface of phosphate rocks. 

Ability of a protein to bind in a bio-specific (selective) manner to a chosen immobilized ligand Ability of certain proteins to complex with zinc and copper Mechanism not fully understood. Involves ability of some proteins to bind to calcium and phosphate ions on the surface of hydroxyapatite crystals... 

The bisphosphonates are all analogues of pyrophosphate. They inhibit osteoclast resorption of bone and they are able to inhibit the formation and dissolution of hydroxyapatite crystals, however their exact mechanism is not well understood. Other effects which have relevance for bone homeostasis include inhibition of the activities of PTH, prostaglandins and 1,25-dihydroxy vitamin D. Bisphosphonates bind to bone with high affinity. They have therefore a duration of action that continues long after their use has been stopped. 

Arthropathies associated with crystals deposition are acute gouty arthritis, chronic gout and chronic tophaceous gout due to monosodium urate crystals. Then there is acute pseudogout and chronic pyrophosphate arthropathy caused by calcium pyrophosphate dehydrate crystals. Acute calcific periarthritis, acute hydroxylapatite arthritis and chronic hydroxyapatite arthritis including Milwaukee-shoulder-knee syndrome are due to basic calcium-phosphate-hydroxyapatite crystals. 

The bisphosphonates inhibit osteoclastic resorption of bone by binding to the hydroxyapatite crystals of bone. When osteoclasts first attach to bone in the active resorp-tive sites, the bisphosphonates are released from that bone. The release of these compounds locally prevents further osteoclastic attachment to those resorptive surfaces. The bisphosphonates also may inhibit resorption by inducing apoptosis of osteoclasts and by inhibiting release of interleukins and other compounds involved in bone resorption. The net result of actions of these compounds is inhibition of bone osteoclastic resorption. This action allows new bone formation to catch up in the remodeling process and can result in a net gain in bone density. 

In the presence of fluoride, calcium ions have been found to be more firmly anchored than in pure hydroxyapatite [67]. This enhances the overall resistance to dissolution. Thus, the presence of a thin stable film of fluorapatite on the surface of hydroxyapatite crystals has two effects, namely (i) resistance to diffusion and dissolution of the anion and (ii) firmer binding of calcium ions into the surface. Both of these make the resulting apatite structure more resistant to dissolution, regardless of the pH of the external medium, and they thereby increase the resistance of the mineral phase to the onset of caries. 

Hydroxyapatite (with some carbonate inclusions) is the most stable of the possible calcium phosphate salts that can be formed under physiological conditions. However, it is not the most rapid one to form. Instead, octacalcium phosphate (OCP) will precipitate more readily than hydroxyapatite. This led Brown in 1987 to propose that, as the kinetically favoured compound, OCP precipitates first, and then undergoes irreversible hydrolysis to a transition product OCP hydrolyzate [68]. This hypothesis is consistent with the observation that enamel comprises hydroxyapatite crystals that have the long, plate-like morphology that is generally considered characteristic of OCP crystals [69]. Overall, it seems that enamel crystals, with their elongated form, result from early precipitation of OCP, which forms a template on which hydroxyapatite units grow epitaxially [70,71]. This leads to enamel mineralisation with the observed thin, ribbon-like structure of crystals. 

More recently, it has been shown that topical fluoride preparations do not lead to fluoridation of the hydroxyapatite crystal [181]. Rather they form a calcium fluoride-like substance that is deposited onto the tooth surface and dissolves when the local pH is lowered [182]. The resulting dissolution adjacent to the tooth surface provides a source of soluble fluoride that can be incorporated into the mineral structure, and thus augment remineralisation. 

Christoffersen, J., Christoffersen, M. R., Larsen, R., Rostrup, E., Tingsgaard, P., Andersen, O. Grandjean, P. 1988. Interaction of cadmium ions with calcium hydroxyapatite crystals a possible mechanism contributing to the pathogenesis of cadmium-induced bone diseases. Calcified Tissue International, 42, 331 -339. 

Fig. 9.16. Single crystals of apatite compounds (a) V(z) measured with a line-focus-beam lens at 225 MHz (b) measured and calculated Rayleigh velocity in the surface of a hydroxyapatite crystal (c) measured and calculated Rayleigh velocity in the surface of... 

The bisphosphonates are analogs of pyrophosphate in which the P-O-P bond has been replaced with a nonhydrolyzable P-C-P bond (Figure 42-4). Etidronate, pamidronate, and alendronate have now been joined by risedronate, tiludronate, ibandronate, and zoledronate for clinical use. The bisphosphonates owe at least part of their clinical usefulness and toxicity to their ability to retard formation and dissolution of hydroxyapatite crystals within and outside the skeletal system. They localize to regions of bone resorption and so exert their greatest effects on osteoclasts. However, the exact mechanism by which they selectively inhibit bone resorption is not clear. 

Mechanism of Action Inhibits formation, growth, and dissolution of hydroxyapatite crystals and their amorphous precursors by chemisorption to calcium phosphate surfaces ... 

Selvig, K. A. Periodic lattice images of hydroxyapatite crystals in human bone and dental hard tissues. Calc. Tiss. Res. 6, 227 (1970)... 

Table 6. Diffracting planes in the hydroxyapatite crystal lattice demonstrated by electron microscopy362 ...

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