2, Environmental Protection College + Eurofins ERICo, Velenje, , Slovenia
3, Slovenian Forestry Institute, Ljubljana, , Slovenia
Mammalian teeth are a composite product of biomineralization consisting of an inorganic component, hydroxyapatite (HA), and an organic component, predominantly collagen. Biologically formed HA can be described as precisely adjusted Ca-deficient carbonate containing apatite with minor amounts of Mg, Na, K and Zn . Teeth are structured with three unique hard tissues – enamel, dentine and cementum. The most voluminous mineralized tissue forming the bulk of the tooth is dentine, constituted of 70 wt% of inorganic HA platelets, 20 wt% of organic material and 10 wt% of water . Dentinal tubules (DT) penetrate through the dentine, turning it into a highly permeable tissue [2,3]. Properties of biological functional composites are modulated starting at the nanoscale, making their characterization very demanding, especially as they are very beam sensitive.
The microstructure and chemistry of intertubular dentine (ID) and dentinal tubules (DT) of human teeth , continuously growing rodents (Myocastor coypus) incisors and molars  were inspected by advanced analytical and imaging transmission electron microscopy (TEM) techniques. Microstructural investigations have revealed relatively dense rim of peritubular dentine (PD) surrounding the DT in human teeth. Bulk of the tooth dentine between the DT is composed of ID. Average Ca/P at% ratios measured in ID are higher compared to PD and are combined with greatly higher Mg/P at% ratios (~1.4 times) obtained in PD. Considering the crystal chemistry perspective, there is a high probability for Mg incorporation into HA lattice by substituting Ca. In addition, smaller HA crystals in PD could be linked to higher Mg concentrations. Interestingly, ID of rodents molars is chemically identical to ID in human teeth. DT of rodents incisors appear to be partially filled with flake-like amorphous material. Surprisingly, the amount of Mg in ID of incisors is notably higher, and the Mg/P at% ratio is 2-4 times higher compared to ID in rodents molars or human teeth. The flake-like material within the incisors DT had an unprecedentedly high amount of Mg, that is around 5 times higher compared to the values in ID, suggesting the presence of an amorphous (Mg,Ca)-phosphate phase. The presence of such high Mg concentrations only in continuously growing incisors but in not molars could be closely associated with the permanent growth of incisors.
Living organisms have ability to precisely adjust organic and inorganic components into masterpiece compounds. Detailed knowledge of such natural biocomposites is important for better understanding of the functionality of human dental tissues and in preventive and restorative dentistry.
 S Mann, Biomineralization; Oxford University Press: Oxford, UK, 2001
 JK Avery, Essentials of Oral Histology and Embryology; Mosby Elsevier, USA, 2006
 IA Mjör and I Nordahl, Arch Oral Biol 41 (1996), 401
 V Srot et al., Microsc Microanal 18 (2012), 509
 V Srot et al., ACS Nano 11 (2017), 239