Dentin apatite

The F content in recent bone or dentine apatite is normally less than 0.1 wt.%. For ancient specimen, F is known to diffuse during burial into bone material. Its enrichment is generally a part of many complex diagenetic changes of bone and tooth, which remains after their deposit. Fluorine can react with the bone and dentine mineral phase to form calcium fluoride compounds. It usually substitutes for hydroxyl ions in hydroxyapatite, leading to the less soluble fluorapatite compound (Ca10(PO4)6(F)2, FAP). 

The properties described above have important consequences for the way in which these skeletal tissues are subsequently preserved, and hence their usefulness or otherwise as recorders of dietary signals. Several points from the discussion above are relevant here. It is useful to ask what are the most important mechanisms or routes for change in buried bones and teeth One could divide these processes into those with simple addition of new non-apatitic material (various minerals such as pyrites, silicates and simple carbonates) in pores and spaces (Hassan and Ortner 1977), and those related to change within the apatite crystals, usually in the form of recrystallization and crystal growth. The first kind of process has severe implications for alteration of bone and dentine, partly because they are porous materials with high surface area initially and because the approximately 20-30% by volume occupied by collagen is subsequently lost by hydrolysis and/or consumption by bacteria and the void filled by new minerals. Enamel is much denser and contains no pores or Haversian canals and there is very, little organic material to lose and replace with extraneous material. Cracks are the only interstices available for deposition of material. 

One further difference between the tissues should be noted briefly—that of turnover—which holds implications for the nature of the isotopic signal recorded and its interpretation. Bone is constantly resorbed and reformed during life, i.e., it turns over , whereas enamel and dentine do not, although secondary dentine can be later accreted. Enamel and dentine form during a discrete period in the individual s life. This means that carbon isotope dietary signals in bone, for both collagen and apatite, reflect diet integrated over years, whereas those in enamel and dentine increments reflect diet at time of formation. 

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. 

Phosphate materials that may be analyzed are bone, dentine, enamel, fish scales and invertebrate shells. In contrast to bone and dentine, enamel is extremely dense, so it is least likely to be affected diageneticaUy and the prime candidate for paleoeviromnental reconstractions. Biogenic apatites contain besides the PO4 group CO3 that substitutes for PO " and OH as well as labile CO3 (Kohn and Cerling 2002), the latter is removed by pretreatment with a weak acid. The... 

Cement, dentin, and enamel are bone-like substances. The high proportion of inorganic matter they contain (about 97% in the dental enamel) gives them their characteristic hardness. The organic components of cement, dentin, and enamel mainly consist of collagens and proteoglycans their most important mineral component is apatite, as in bone (see above). 

Osteoclast cells generally require an apatitic substrate (dentine, enamel, bone slices or synthetic apatite coatings) to attach to and act on and the effect of fluoride ions in solution cannot be readily distinguished from the effect of fluoride on... 

The dimensions of bone crystallites can be as small as 96 10 A, with the order of magnitude even a factor of two or three smaller56. The mean size of the apatite crystals in dentine and cementum is of the same order as that found in bone. Enamel crystals, however, are at least an order of magnitude larger in all dimensions. There appears to be good agreement that the smallest dimension of the bone apatite crystal is about 50 A. On the other hand, there is a discrepancy in the reported size of the... 

Among the apatite substituents, carbonate has been the subject of many studies. The apatite present in dental enamel is not a pure HA, but rather a corbonate apatite with a carbonate content of 2—3 %. There is still a controversy about the location of the carbonate in enamel, dentine and bone. While most recent studies agree that carbonate appears to be substituting within the lattice rather than existing in an amorphous phase, there is still some disagreement as to the actual position of the carbonate. The presence and location of the carbonate in dental enamel may relate directly to the risk of carious attack. Carbonate has been shown to be leaked preferentially from early carious lesions81. ... 

Although about 80—90 percent of the total citric acid in humans are localized in hard tissues as enamel, dentine, cementum and bones, very little is known on the biological function of citric acid in biocalcification. HA crystals are reported to be dissolved by the action of citric acid. The acid dissolves the crystals in such a way that the destruction is a preferential attack along the c-axis. It is highly probable that the HA crystallites present in mineralized tissues also do have a dislocation in the centre of the material 165). Another assumption describes that citric acid is a constituent of the aqueous phase of enamel or that citrate is bound to the surface of apatite by adsorption166). 

The nature of mineral phases present in bone, dentin, enamel and other phosphatic tissues, and their mode of formation have been subjects of lively discussions among health scientists and crystallographers. Bioscientists most commonly accept the viewpoint that the inorganic phase of bones or teeth is principally hydroxyapatite, Caio(P04)6(OH)2, and deviation in Ca/P ratio from common hydroxyapatite (Ca/P = 1.667) observed in mineralized tissues is explained by the presence of amorphous phosphates. In contrast, many crystallographers favor the idea of carbonate apatite, i.e. dahllite, as the major crystalline phase in biophosphates and they doubt the existence of amorphous phases. The topic has been reviewed14,15,22,28, 37,44,47,348-358) no common consent has yet been reached. In the following an attempt is made to at least coordinate the controversial findings. 

In contrast to calcareous invertebrates with their species specific shell organic matrix213-221 apatite depositors follow a rather conservative pattern set by collagen for bone, scale, and dentin deposition, or by a keratin-type protein in the case of enamel formation412. Nevertheless, a certain spread in the concentration of amino acids can be recognized if one, for instance, compares dentin of species en-... 

Heteronuclear correlation (HETCOR) spectroscopy is a standard two-dimensional NMR technique. The resolution of 31P 1H CPMAS spectra is generally not sufficient to unequivocally detect the HP042 ions and the apatitic OH- ions in bone or dentin samples.15 In this regard, the breakthrough came from the first application of 31P 1H HETCOR to HAp and bone samples,24 where the correlation peak at 0.2 and 3 ppm of the 1H and 31P dimensions, respectively, has been established as the spectral marker of apatitic OH- ions. To enhance the spectral resolution in the XH dimension, 1H homonuclear decoupling was employed during the fi... 

Although the assumption that all apatite platelets are of equal thickness may not be well justified, it provides a useful approximation to the real situation. In fact, such assumption is inevitable for an ensemble technique like NMR. It is anticipated that HARDSHIP may be well suited to determine the relative thickness of apatite crystallites in different bone or dentin samples. We note in passing that C-REDOR, which is a hetero-nuclear recoupling technique with active suppression of homonuclear dipolar interaction,42 43 can be used to probe for the size of nanoparticles embedded in polymer matrix,44 provided that the nanoparticles do not... 

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