Biomineralisation formation

Biological calcification processes are widely distributed in nature. They can be found in microorganisms, in plants, in the animal kingdom and in humans. Under physiological conditions, the results of mineral deposition in biological systems can be represented by the formation of bones, teeth and shell material as well as coccoliths, corals, pearls etc. The variety of biomineralisates can best be expressed by the fact that approximately 128,000 species of molluscs636 are known. The majority of them (Conchifera) form shells of different kinds of size and shape as well as of color. 

Biomineralisation is the study of processes that lead to the formation of hierarchically structured organic—inorganic materials generated by living organisms, such as shells, bone, and teeth. Over the last few decades, our ability to identify the often large number of macromolecules involved in the process of biomineralisation and the interactions between them has grown and expanded. 

While the initial stage of iron incorporation in mammalian ferritins requires the ferroxidase sites of the H-chains, thereafter the inner surface of the protein shell of the L chains provides nucleation sites which supply ligands that can partially coordinate iron but which leave some coordination spheres available for mineral phase anions. This enables the biomineralisation process to proceed, with formation of one or more small polynuclear ferrihydrite crystallites, which can then act as nucleation centres for mineral growth. Most probably, one of these clusters will become the dominant nucleation centre and growth of the mineral would then occur from this centre. 

Bone and teeth in mammals and bony fishes all rely on calcium phosphates in the form of hydroxyapatite [Caio(P04)6(OH)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 biomineralisation process leading to the hardest mineralised tissue known, the enamel of mammalian teeth. 

Finally, it is intriguing that in terms of biomineralisation, invertebrates have based their reliance on calcium carbonates, while vertebrates appear to have used almost exclusively calcium phosphate. We say almost, because, while the use of calcium phosphates for biomineralisation is an invention of some vertebrates, they still use calcium carbonate for the formation of otoliths" of the inner ear. It remains to be established if the equivalent of the gene starmaker required for otolith formation in zebrafish has homologues among invertebrates. 

Layer-silicates Recent studies have also demonstrated the potential microbial influence on clay mineral (layer silicates) formation at hydrothermal vents. Bacterial cells covered (or completely replaced) with a Fe-rich silicate mineral (putative nontronite), in some cases oriented in extracellular polymers (as revealed by TEM analysis), were found in deep-sea sediments of Iheya Basin, Okinawa Trough (Ueshima Tazaki, 2001), and in soft sediments, and on mineral surfaces in low-temperature (2-50°C) waters near vents at Southern Explorer Ridge in the northeast Pacific (Fortin etal., 1998 Fig. 8.6). The Fe-silicate is believed to form as a result of the binding and concentration of soluble Si and Fe species to reactive sites (e.g. carboxyl, phosphoryl) on EPS (Ueshima Tazaki, 2001). Formation of Fe-silicate may also involve complex binding mechanisms, whereas metal ions such as Fe possibly bridge reactive sites within cell walls to silicate anions to initiate silicate nucleation (Fortin etal., 1998). Alt (1988) also reported the presence of nontronite associated with Mn- and Fe-oxide-rich deposits from seamounts on the EPR. The presence of bacteria-like filaments within one nontronite sample was taken to indicate that bacterial activity may have been associated with nontronite formation. Although the formation of clay minerals at deep-sea hydrothermal vents has not received much attention, it seems probable that based on these studies, biomineralisation of clay minerals is ubiquitous in these environments. 

Biomineralisation is the study of the formation, structure and properties of inorganic solids deposited in biological systems (Mann, 2001). 

The processes of biomineralisation are grounded on the complex interaction of inorganic solids and organic templates. Following Mann (1996), biomineralisation, that is formation of hydroxyapatite-based bones and teeth of vertebrates, calcitic egg shells of birds and calcitic and aragonitic shells of mussels can be viewed as a sequence of four consecutive steps. 

Phospholipids often play a key role in biomineralisation processes. Examples are in the formation of coccolith conglomerations of CaCOg crystals and magnetosome arrangements of Fe304 crystals, found in nature [68]. It may be possible to utilise liposomes for the in situ formation of artificial biomaterials. 

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