Figure 5. The amelogenin assemblies manipulate the arrangement of prenucleation clusters into organized mesostructures.[193] Copyright 2020, National Academy of Science.
Physiological processes and disease diagnosis
Enamel mineralization
Teeth are biomineralized body tissues exposed to the external environment, which differs from bones. Enamel is more susceptible to exogenous damage from bacteria, food debris, etc., causing plaque, dental caries, periodontitis, and other diseases in the teeth.[198,210] Meanwhile, the enamel is produced under the regulation of ameloblasts, which undergo programmed apoptosis after enamel maturation. If there is faulty signaling by ameloblasts in the regulation of enamel maturation, it can lead to irreversible amelogenesis imperfect.[2,152] Therefore, studying exogenous and endogenous factors that affect the physicochemical properties of enamel mineralization is an essential basis for maintaining enamel function (Figure 6 ).
The full-scale Raman imaging (~7 mm in diameter) covering the cross-sectional area of the laser beam can be realized to illustrate biomineralization by using a high-power laser.[211] The laser can be guided to the sample without passing through the objective lens, which greatly saves the time of Raman scanning. The full-scale Raman image based on mineral intensity distribution can be used to recognize and distinguish enamel and dentin, as well as the normally healthy and carious enamel.[211] Polarization resolution hyperspectral stimulated Raman imaging technology can analyze the direction change of biomolecules in dental caries on the submicron scale. The depolarization rate of the microscope objective used is ~ 0.005, which is much smaller than the difference between the depolarization rates of normal enamel and caries, allowing the marking of caries.[56] Polarized Raman spectroscopy revealed that healthy enamel exhibited strong Raman polarization anisotropy, while all early caries showed low Raman polarization anisotropy.[212] The Raman spectroscopy generated by ν1PO43- vibrations was strongly polarized, while the polarization dependence was weaker in the caries region. This difference in the degree of polarization anisotropy of Raman spectra becomes a distinct marker to distinguish early caries from sound enamel. Meanwhile, Raman spectroscopy has also been used in combination with other techniques for the synergistic diagnosis of dental caries, such as optical coherence tomography.[57]
Pathological enamel damage causes a change in the crystal structure of hydroxyapatite, and the result is a change in the characteristic peaks of Raman spectroscopy.[58,199,213,214] Raman spectroscopy was especially used to analyze the relationship between Msx2 gene expression in ameloblasts and enamel.[105] There were differences in enamel composition after knocking out Msx2 in ameloblasts. Figure 7ashows the Raman images reflecting the phosphate vibration modes ν1 PO43- and ν4 PO43- in enamel. The position of ν4PO43- in enamel after Msx2 knockout is uneven, which may be due to the disorder of spatial distribution during mineralization.[105] And it will affect the content of hydroxyapatite in enamel (Figure 7b ). After periodontitis treatment, the main structural changes in tooth tissue occur in the cementum. The main reasons for the structural changes of cementum are microbial infection, susceptibility, and periodontal pocket susceptibility. The alteration of the characteristic peaks of Raman spectra caused by this phenomenon was used by the Timchenko group to identify the effect of the treatment of periodontitis.[198] Marco Antonio diagnosed dental fluorosis by improving the classification algorithm.[215] It was found that the b-type carbonate content increased with fluorosis severity. And the specificity of the PCA-LDA model, i.e., the combination of principal component analysis with linear discriminant analysis, for different fluorosis severity groups was higher than 93%, which could be utilized to effectively distinguish different degrees of fluorosis.[215]