Enamel mineralisation

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. 

T. Aoba, H. Komatsu, Y. Shimazu, H. Yagishita, Y. Taya, Enamel mineralisation and an initial crystalline phase. Connect. Tissue Res. 38 (1998) 129-137. 

Y. Miaka, S. Shimoda, M. Fukase, T. Aoba, Epitaxial overgrowth of apatite crystals on the thin-ribbon precursor at early stages of porcine enamel mineralisation, Calcif. Tissue Int. 53 (1993) 257-261. 

Fig. 2. Relationship between AZbase and subsequent change in mineralisation, AAZ, observed in human enamel during a recent in vitro pH-cycling study. A negative value for AAZ indicates net demineralisation, a positive value indicates net remineralisation.

Fig. 3. Mean mineral density profiles of two artificial caries lesions. Mineralisation, as a percentage of sound enamel, assumed to be 87% mineral by volume, is expressed as a function of depth into the lesion. The two mineral distributions are clearly different but the amount of mineral loss is almost identical in each case. In the text, the terms shallow and deep refer to lesion depth, whereas the terms small lesion and Targe lesion refer to amount of mineral loss, regardless of depth. The heavy line represents a lesion with a high R parameter and the lighter line, a lesion with a lower R parameter (see 4.6).

From a mechanistic viewpoint it is reasonable to anticipate an inverse clinical relationship between calculus and caries. Calculus formation is essentially a mineralisation process. The development of a caries lesion is the result of the net demineralisation of tooth enamel by plaque acid. These processes both involve crystalline calcium phosphate phases in contact with liquid, saliva and/or plaque fluid, containing their constituent ions. The oral environment also contains other salivary constituents and bacteria, which either inhibit or promote crystal growth or dissolution. 

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. 

The enamel of mammalian teeth is much more heavily mineralised than bone, which makes it much harder. In addition, it does not contain collagen, although in its final mature state it does contain small amounts of specialised matrix proteins. Early tooth development is a classical illustration of the interaction between two tissue types (epithelial cells and mesenchymal cells ), whereby a number of signaling molecules are involved in orchestrating reciprocal interactions between the two types of tissue. 

Mammalian teeth are Nature s well-designed functional gradient composites (FGC) consisting of a top layer of hard and inert enamel, underlain by dentin, a less mineralised but more resilient and vital hard connective tissue formed from... 

Figure 3.4 Mineralised structure of tooth, (a) Schematic drawing showing the enamel and dentin regions, (b) SEM micrograph of enamel and (c) SEM micrograph of dentin, showing the tubular morphology.

The composition of dentin is similar to that of bone, with crystal sizes of about 50 x 25 x 2 nm3, much smaller than those in enamel. Dentine connective tissue comprises a network formed by randomly intertwined mineralised collagen fibrils permeated by tubules that radiate from the pulp cavity towards the dentin-enamel junction (Figure 3.4c). 

The use of ion-selective electrodes in dental and mineralised tissue studies is quite extensive [162,190,192,197,288—360] and particular attention has been devoted to fluoride in saliva and enamel. 

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