Biomineralization bones

Bone, shell, and coral are not, however, the only biominerals created by living organisms. The kidney and liver of animals, for example, often synthesize biominerals in the form of pathological stones (known as calculi) of varied composition (mostly of calcium oxalate, calcium phosphate, or... 

The bones and teeth of humans and other vertebrate animals, for example, consist mainly of a composite material made up of an organic substance, collagen, and a biomineral, calcium carbonate phosphate (see Textboxes 32 and 61). The latter, which makes up about two-thirds of the total dry weight of bone, is composed of calcium phosphate containing between 4-6% calcite (composed of calcium carbonate) as well as small amounts of sodium, magnesium, fluorine, and other trace elements. The formula Ca HPChXPChMCChXOH) approximately represents its composition its crystal structure is akin to that... 

As indicated above, the development of bio-nanohybrids by mimicking biomineralization represents an extraordinarily useful approach. This is, for instance, the case for those bio-nanocomposites based on bone biomimetic approaches, which show excellent structural properties and biocompatibility. They are prepared by... 

The collagen fibers leave small compartments where apatite nanocrystals are deposited during a controlled biomineralization process [20]. The collagen acts as a structural framework in which plate-like nanocrystals of carbonated hydroxyapatite (CHA) are embedded to strengthen the bone. The chemical formula of biological CHA can be represented as follows ... 

Ca2+ pumps are needed for the removal of calcium from cells, as in the maintenance of low Ca2+ levels in resting muscle, in bone deposition, and in biomineralization. The Na+/Ca2+ exchanger and Ca2+-ATPase (a Ca2+/H+-exchanger) act in parallel to pump Ca2+ across membranes, the latter being considerably more effective than the former. X-ray structure determinations have been carried out both for Ca2+-ATPase and for its Ca2+-bound form (443). The latter contains two Ca2+ ions... 

Calcium, a crucial second messenger signalling key changes in cellular metabolism, is also important in muscle activation, in the activation of many proteases, both intra- and extracellular, and as a major component of an range of biominerals, including bone. 

We now recognize that, while much of biology relies on inorganic structures, biominerals, to supply the tensile strength and the other material properties that we associate with, for example bone, the diversity of form and shape depends on the organic matrix in which the biomineral is allowed to form. It is a little like the construction of buildings with reinforced concrete—the mould determines in what shape and form the concrete will set. And it is... 

Bone and teeth in mammals and bony fishes all rely on calcium phosphates in the form of hydroxyapatite [Ca5(P04)30H]2, usually associated with around 5% carbonate (and referred to as carbonated apatite). The bones of the endoskeleton and the dentin and enamel of teeth have a high mineral content of carbonated apatite, and represent an extraordinary variety of structures with physical and mechanical properties exquisitely adapted to their particular function in the tissue where they are produced. We begin by discussing the formation of bone and then examine the biomineralization process leading to the hardest mineralized tissue known, the enamel of mammalian teeth. 

Biomineralization. In biomineralization, inorganic elements are extracted from the environment and selectively precipitated by organisms. Usually, templates consisting of suitable macro-molecules serve as a substrate for the heterogeneous nu-cleation of bulk mineralized structures such as bone, teeth and shells. Biological control mechanisms are reflected not only in the type of the mineral phase formed but also in its morphology and crystallographic orientation (Mann et al., 1989 Lowenstamm and Weiner, 1989). Two examples (perhaps oversimplified) may illustrate the principle (Ochial, 1991) ... 

The most important organic components of bone are collagens (mainly type 1 see p.344) and proteoglycans (see p. 346). These form the extracellular matrix into which the apatite crystals are deposited (biomineralization). Various proteins are involved in this not yet fully understood process of bone formation, including collagens and phosphatases. Alkaline phosphatase is found in osteoblasts and add phosphatase in osteoclasts. Both of these enzymes serve as marker enzymes for bone cells. 

The acoustic micrograph in Fig. 1.5(a) came from a 5-week-old preparation. It was fixed in alcohol, and stained for alkaline phosphatase and, with von Kossa stain, for biomineral material. The biomineral material of interest here is hydroxyapatite, the principal crystalline mineral constituent of bone. The ordered structure visible within the matrix is not seen with either the light or electron microscopes. But the acoustic microscope can also work perfectly well with unfixed, unstained specimens. Figure 1.5(b) is an acoustic micrograph of matrix and cells from a 17-year-old male. In addition to the standard ingredients of culture medium, these cells were specifically stimulated with beta-glycerolphosphate and a vitamin C preparation. Because the acoustic... 

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