Microstructure and Chemistry of Human Dental Tissues
Many living organisms have a remarkable ability to form a diversity of biominerals and a variety of structures. Many of these biominerals are composite or agglomerated materials, where an organic matrix and crystalline or amorphous minerals are linked together . These exceptional combinations formed under particular conditions possess unique architectures and sophisticated compositions, which leads to highly improved materials characteristics compared to their inorganically formed mineralogical varieties [1, 2].
Human mineralized tissues, such as teeth and bones, consist of the inorganic Ca phosphate component hydroxylapatite (HA) and an organic component, mainly collagen. Biological HA is structurally and chemically very complex and it can be described as an impure Ca-deficient carbonate- containing apatite modified mainly with magnesium, sodium, potassium and zinc . Teeth consist of three unique dental hard tissues, enamel (E), dentine (D) and cementum (C). Their inorganic phase is based on HA, however they vary in Ca/P ratio and the concentration of minority elements. Numerous dentinal tubules (DT) penetrate the dentine, are surrounded by a wall of highly mineralized peritubular dentine (PD) and are separated by intertubular dentine (ID) .
In this study , permanent and primary human teeth were investigated. Enamel, dentine and DT from different locations in the teeth were studied in detail using energy-dispersive X-ray spectroscopy and electron energy-loss spectroscopy. Various energy-loss near-edge structures (ELNES) were acquired at high energy and high spatial resolution using the Zeiss SESAM and VG HB501UX microscopes.
Bright field scanning transmission electron microscopy (STEM) images of enamel, dentine and DT are shown above. An annular dark field-STEM image (above, right) of an enlarged area of DT clearly shows a well mineralized PD that appears denser than the ID. XEDS measurements were performed from several different areas in enamel, dentine and cementum. The values of the Ca, P and Mg atomic fractions were determined and the average Ca/P ratios compared to the average Mg/P ratios. The Ca/P ratios of enamel, cementum and dentine are rather constant; however there is a variation in the Mg/P ratio. Different areas of ID show quite constant Ca/P ratios and only minor variations for the Mg/P ratios. On the other hand, the Ca/P ratios of PD are significantly lower than those of ID, which, in turn, feature higher and variable Mg/P ratio values. Our results suggest that magnesium is most likely incorporated in the HA lattice by substituting for calcium.
C-K ELNES were acquired from enamel, dentine, PD and ID. The main C-K edge signal acquired from enamel (E) originates from the inorganic component. C-K ELNES from dentine, ID and PD exhibit several additional structural features compared to the spectrum from enamel, because of the much higher amount of organic component present. C-K ELNES recorded from ID appears identical to the spectrum acquired from dentine in the near vicinity of the dentine-enamel junction. These results are an important contribution to the understanding of differences in microstructure and chemical composition of mineralized human dental tissues.
 PUP Gilbert et al, Rev Mineral Geochem 59 (2005), 157-185.
 AP Jackson and JFV Vincent, J Mater Sci 25 (1990), 3173-3178.
 S Mann in "Biomineralization" (Oxford University Press, New York)
 JK Avery (Ed.) in "Oral development and histology" (Georg Thieme Verlag, Stuttgart)
 V Srot et al, Microscopy and Microanalysis 18 (2012), 509-523.