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What Do We Know about Biomineralization/Calcification?
Including humans, many multicellular organisms produce similar hard tissues, such as bones, teeth, shells, skeletal units, and spicules. These hard tissues are biocomposites and incorporate both structural macromolecules (lipids, proteins, and polysaccharides) and inorganic minerals.1We do not fully understand the control mechanism of biomineralization in primitive or in developed organisms. The mineral phase of hard tissue is sometimes called biologic apatite, that is, a nonstoichiometric hydroxylapatite. Pure hydroxyapatite has the formula Ca10(PO4)6(OH)2. In contrast, a biologic apatite (like in bone) is nonstoichiometric and contains several other ions, mainly carbonate and other elements in traces such as Mg2+, Na+, Fe2+, HPO4 2−, F−, and Cl−. Consequently, a more appropriate structural formula for the composition of bone is (Ca,X)10(PO4,CO3,Y)6(OH,Z)2, with X substituting cations and Y and Z substituting anions (with the indices 10, 6, and 2 changing according to stoichiometry).2
There is a paradox in medicine. Whereas some researchers have been discussing the cytotoxic effect of apatite in vitro,3,4others have been announcing the safety of in vivo apatite applications.5-8Although these disagreements have not been completely resolved, both biogenic and nonbiogenic apatite materials have been continuously used in drug delivery and transplantation.6,9We know that when apatite is found in soft tissue, it is considered to be pathologic calcification.10The causes of apatite-deposit formations in soft tissue have been discussed for decades but still remain speculative. For example, calcification in the coronary arteries has been widely regarded as an uncommon, end-stage, insignificant, passive, degenerative process of aging-a notion that has paralyzed research in this area for decades.11Interestingly, these same terms were once …
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