Chemical gradients revealed in human tooth enamel

Biting, chewing, slicing—human teeth withstand wear and tear over the course of a lifetime. Their resilience is partially due to the hardness of tooth enamel, a biomaterial that new research shows is more chemically complex than previously expected. These new chemical details could help scientists better understand how teeth grow and decay.

Our tooth enamel is composed of closely packed crystallites of hydroxylapatite (Ca₅(PO₄)₃(OH)). There are long-standing mysteries about the inner construction of these crystallites. For example, the cores of the crystallites dissolve in acid, but the outer surfaces do not. That suggests that human enamel crystallites do not contain a uniform chemical structure as seen in other mammalian teeth.

“We thought that with modern techniques now at our disposal, we might be able to solve an old question of what’s in the core of human crystallites,” says Derk Joester, a material scientist at Northwestern University.

Joester and his team used a combination of scanning transmission electron microscopy (STEM) and atom probe tomography (APT) to visualize the internal structure of enamel crystallites in human teeth (Nature 2020, DOI: 10.1038/s41586-020-2433-3). The STEM images showed that crystallites in human enamel have distinct shell and core regions, an architecture never before seen in enamel. Moreover, APT revealed that the core region was further segregated into a sandwich of two magnesium layers that flank a central zone rich in carbonate, sodium, and fluoride.

“I would never have expected that there would be two layers of magnesium parallel to the core,” says Joester, who hopes to investigate this surprise moving forward.

The team went on to create a computational model that showed how incorporation of magnesium into the core could distort the hydroxylapatite lattice and add stress to the material. In this case, that stress strengthens the crystallites. Therefore, Mg gradients may contribute to the hardness of human enamel and the tendency for crystallite cores to dissolve preferentially with acid.

Janet Moradian-Oldak, a structural biologist who studies enamel at the University of Southern California and was not involved in the research, says she was impressed by the “elegant” use of both microscopy and spectroscopy to reveal the elusive inner structure of enamel crystallites. She also notes that this is the first time a mechanical function has been proposed for Mg in human enamel.

Joester says that this new insight into the complex structure of human enamel crystallites could help scientists understand how tooth decay develops and progresses. It may also help dentists study enamel diseases. “Now we can look at anomalies in really unprecedented detail and look at composition as well as structure,” Joester says. That means that “some of the pathologies that affect enamel formation might leave clues to their mechanisms.”