These results suggested that the change in fine structure, filling in inter-rod spaces of tooth enamel, was related to progression of calcification in the inter-rod spaces with advancing age and one of the factors of yellowness of human tooth.

This study was intended to clarify the changes in the color of teeth and enamel rod sheath with age. A total of 14 first premolars taken from patients between the ages of 10-70 years old were used in this study. Photographs of the buccal surface of teeth were taken and the tone was converted into color system marked as L*, a* and b* using image processing software. Thick sections of 1.5 mm traversing the tooth axis were prepared and the enamel translucency and color of enamel and dentin were determined in the same manner. Correlation analysis was carried out between the clinical color of the buccal surface and age, enamel color and age, dentin color and age and translucency and age. The sections were embedded in resin and the polished cross section was observed a backscattered electron image with SEM. The observation sites were the following: outer of the enamel, middle layer and deep layer (near the dentin-enamel junction). The structure of the enamel rod sheath was the focus of observation. The area and width of enamel rod sheath were measured from SEM images. Correlation analysis between color, area of enamel rod sheath and age were carried out with these data. Results showed that color of enamel and dentin became darker with age; the color of the entire buccal surface became dark reddish and yellowish but it was not due to increased permeability of enamel. The enamel rod sheath was clear in teeth of young people but it showed a rapid decrease with age. From the results, the crystal gap and enamel rod sheath suppress the reflection of the color of underlying dentin with the tendency of scattering the light from the outer layer to the deeper layer in teeth of young patients. However with increasing age, the crystal gap and enamel rod sheath decrease thereby strongly reflecting the color of dentin resulting to a darker color. Thus, microstructural changes in enamel affect the color of the tooth.

β‐Glycyrrhetinic acid (BGA) is a natural antibacterial agent. Previous studies reported that BGA has antibacterial effects against several bacteria. This study evaluated the effects of BGA on the regulation of supragingival plaque bacteria. First, the minimum inhibitory concentrations (MICs) of BGA against oral bacteria were measured. Next, the minimum concentrations for inhibition of biofilm formation were evaluated against Streptococcus mutans and Streptococcus sobrinus, possessing insoluble glucan synthesis abilities. The MICs of biofilm formation by these bacteria ranged from 1/8 to 2× MIC. Furthermore, the inhibition effects of BGA against the coaggregation of Porphyromonas gingivalis and Streptococcus gordonii were evaluated. BGA at 32 or 64 μg/mL inhibited the coaggregation of these bacteria after a 30 min incubation. Lastly, the inhibition effects of BGA against human supragingival plaque bacteria were evaluated. Human supragingival plaque samples were obtained from 12 healthy donors. The inhibition effects of BGA against biofilm formation by these plaque bacteria were evaluated. Of 12 samples, the biofilm formation by 11 was significantly attenuated by 128–256 μg/mL of BGA. The number of colony forming units in these biofilms was also significantly attenuated. In conclusion, it was revealed that BGA inhibits the growth and biofilm formation of bacteria, furthermore, the same effect was confirmed with supragingival plaque bacteria. BGA is a good candidate for a natural agent that prevents the outbreak and progression of periodontal disease because it suppresses not only the growth and biofilm formation of bacteria, but also the coaggregation of P. gingivalis with plaque bacteria.

The aim of this study was to clarify the fine structure of accumulated deposits on the surface of teeth that are considered to affect the gloss of teeth. The study was carried out using, as specimens, human incisor teeth having gloss, which were extracted from teenage donors and those incapable of showing gloss even by brushing which were extracted from donors in their 50s. Thin longitudinal sections of tooth enamel with accumulated deposits on the surface were prepared by focused ion beam (FIB) milling, and the fine structure was analyzed using a scanning transmission electron microscope (STEM) and a high resolution transmission electron microscope (HRTEM). By FIB, thin longitudinal sections could be prepared from tooth enamel together with organic and inorganic substances accumulated on the surface without artifacts. The accumulated deposits on the surface of teeth having gloss were composed of organic substances. However, it was first revealed by STEM observation that the accumulated solid deposits on the surface of teeth having no gloss had a complicated structure wherein inorganic and organic substances coexisted. It is suggested that the organic substances contain proteins derived from saliva. The inorganic substances were spherical and needle-like hydroxyapatites (HAs). It is considered that amino acids constituting the proteins affected the nucleus formation and the crystal formation of HA. It is considered that the unevenness of the accumulated deposits existing on the surface of tooth enamel having no gloss causes the decrease in gloss of teeth due to diffuse reflection of light.

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