# High Resolution STEM Images of the Human Tooth Enamel Crystals
## 4 Discussion  
### TEM and STEM Contrasts  
The high-resolution STEM-HAADF images of human tooth enamel crystals presented in this work indicated that the CDL thickness was between 3 and 8 A. The CDL plane was formed by the [1-210] and [0001] directions, and it divided the enamel crystal into two parts, which were related by a mirror plane \(m\). Table 1 summarizes the contrasts presented by the CDL when it was observed by TEM, HRTEM, and STEM in BF and dark-field (DF) modes.  
In TEM, the CDL behaved as a Fresnel contrast, because it changed from dark in overfocus conditions to bright in underfocus conditions, and disappears at the Gaussian focus. This Fresnel contrast suggests a change in the internal mean potential produced by the local deviation in stoichiometry in the central region of the enamel crystal [6] similar to the one presented in the TEM by the oriented-in-edge-on-direction grain boundary dislocations and voids [19]. Thus, the origin of the CDL would be a local deviation in stoichiometry in the HAP unit cell at the center of the enamel crystal.  
The STEM-ADF contrasts suggested that enamel crystals had a \"core-shell\" type structure (Figure 3d). [PERSON] et al. [2] indicated that the dark contrast around the CDL was a strained zone and the light contrast in the periphery was free of strain. [PERSON] et al. [2] and [PERSON] et al. [3] have also indicated that the dark zone is rich in Mg and Na. The presence of Mg\({}^{2+}\) in human tooth enamel crystals has been well established [3; 20], together with high carbon concentrations due to the presence of carbonate (CO\({}_{3}\)\({}^{2-}\)) ions [2; 3]. Therefore, the size relationship between Ca\({}^{2+}\) and Mg\({}^{2+}\) results in the lattice contraction deformation observed [2]. In this way, residual stresses due to chemical gradients agree quite well with the preferential dissolution from the CDL of the enamel crystal in acidic media.  
[PERSON] et al. [3] commented that an antiphase boundary (APB) can also be produced when enamel crystals grow during amelogenesis, as it happens in the monoclinic crystals [3]. The APB has higher energy than the rest of the crystal provided surely by ions such as Mg\({}^{2+}\) and Na\({}^{+}\), and it would also explain why CDL is susceptible to demineralization. Although the APB shows the same structure on both sides, the arrangement should be staggered by half an atomic plane. However, this displacement was not observed in the high-resolution STEM-HAADF images.  
Therefore, it is reasonable to believe that the contrast behavior of the CDL is an indication that in the CDL the mean internal potential is lower than in the perfect crystal and produced by the local deviation of the stoichiometry of the HAP unit cell.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Image** & **TEM** & **HTEM** & \multicolumn{2}{c}{**STEM**} \\ \hline Contrast & \begin{tabular}{c} Amplitude \\ (1-beam) \\ \end{tabular} & \begin{tabular}{c} Phase \\ (N-beams) \\ \end{tabular} & \begin{tabular}{c} Amplitude \\ (N-beams) \\ \end{tabular} & \begin{tabular}{c} \\ (N-beams) \\ \end{tabular} & \begin{tabular}{c} \\ (N-beams) \\ \end{tabular} & \\ \begin{tabular}{c} BF \\ (Detural) \\ \end{tabular} & \begin{tabular}{c} DF \\ (Detural) \\ \end{tabular} & \begin{tabular}{c} BF \\ (Detural) \\ \end{tabular} & \begin{tabular}{c} BF \\ (Detural) \\ \end{tabular} & \begin{tabular}{c} ADF \\ (Detural) \\ \end{tabular} & \begin{tabular}{c} HAADF \\ (Detural) \\ \end{tabular} \\ \
\begin{tabular}{c} Dark or white \\ (detocusing function) \\ \end{tabular} & Dark & Dark & White &
\begin{tabular}{c} Dark \\ (but weak) \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table}
Table 1: CDL contrast observed when it is viewed by TEM, HRTEM, and STEM in the BF and dark field (DF) modes.