The hidden structure of mouse and human enamel

Enamel is the hardest and most resilient tissue in the human body. The morphology of human and mouse enamel is well established: it consists of space-filling¹, aligned, parallel, ~100 nm wide, microns-long nanocrystals, bundled into 5-micron-wide rods. The orientation and arrangement of enamel crystals, however, are poorly understood yet they confer outstanding materials properties. We show with polarization-dependent imaging contrast (PIC) mapping^2,3 that in mouse enamel, within a rod, crystals are co-oriented with one another but not with the long axis of the rod⁴, in human enamel they are not co-oriented with either: the c-axes of adjacent crystals are most frequently mis-oriented by 1°-30°, and their orientation gradually changes up to 30°-90° within a rod^5,6. Molecular dynamics simulations demonstrate that the mis-orientations of adjacent crystals observed in human enamel induce crack deflection6. This toughening mechanism, therefore, contributes to make our enamel last a lifetime.
1 L Yang 2011 DOI: 10.1039/C0NR00697A
2 PUPA Gilbert 2011 DOI: 10.1073/pnas.1107917108
3 CE Killian 2011 DOI: 10.1002/adfm.201001546
4 CA Stifler 2018 DOI: 10.1021/jacs.8b05547
5 RT DeVol 2015 DOI: 10.1021/jacs.5b07931
6 E Beniash 2018