Rodents inspire dentistry

Researchers discover an iron-containing material in the outer enamel of rodent teeth that could also make human teeth more resistant

Image of the lower incisors of nutria (coypu) and beaver.  adapted from ACS Nan
Image of the lower incisors of nutria (coypu) and beaver. adapted from ACS Nano

Rodents such as beavers, nutrias (coypu) , squirrels and rats have particularly strong, elongated front teeth that grow continuously over the course of their lives. Using state-of-the-art imaging techniques, researchers at the Max Planck Institute for Solid State Research in Stuttgart have now elucidated the tooth structure of various rodent species at the nanometre scale. In the process, they discovered an iron-containing material in the outer enamel that is crucial for the extreme resilience of rodent teeth. The discovery could be the starting point for the development of completely new biomaterials for human dentistry.

Teeth are an excellent example of a natural composite material consisting of optimally arranged, simple inorganic and organic components. Dental enamel is the most mineralised and hardest tissue in our body. It consists mainly of elongated, calcium-containing hydroxyapatite crystals that are mixed with organic material and water. Both human and animal teeth are coated with this crystalline and extremely resistant structural component.

The continuously growing, rootless incisors of rodents are perfectly adapted to gnawing activity through structural and chemical optimisation and are therefore especially robust. Their labial side is covered with particularly hard enamel, making them a self-sharpening device. Rodent incisors are striking due to their characteristic orange-brown colour.

Vesna Srot and her colleagues at the Max Planck Institute for Solid State Research in Stuttgart have now discovered what makes rodent teeth so resistant. In the tooth enamel of seven different rodent species, they found an iron-containing, ferrihydrite-like material in the nanometre-sized spaces between the elongated hydroxyapatite crystals. "These filled pockets make up less than two percent of the iron-rich enamel by volume, yet they are decisive for the mechanical properties and resistance to acid attack," says Vesna Srot.

The newly discovered iron-rich layer is similar in colour to normal tooth enamel. So, it is not the cause of the orange-brown colour of rodent teeth. Instead, the two overlying layers - the surface layer and the transition zone - are decisive for the colour. The former consists of an organic matrix and an inorganic component that also contains iron. It varies in thickness, even in different teeth of an individual. The thicker the surface layer, the darker the tooth.

"The results represent a paradigm shift, as it was previously thought that the colour was caused by iron-rich enamel." The researchers therefore recommend re-evaluating the conventional terminology that has been used for seven decades: the enamel previously referred to as "pigmented enamel" should now be defined as "iron-rich enamel".

As part of the study, the researchers analysed the incisors of seven rodent species from different habitats: Beaver, nutria, marmot, squirrel, vole, mouse and rat. To visualise the architecture of the teeth at the macroand nanoscale, the researchers used nanoscale-imaging techniques in combination with 3D tomography, optical microscopy and scanning electron microscopy.

Although the seven species studied had different living conditions, their teeth generally adapted the same microstructure. Tooth development in rodents therefore appears to be a general adaptation in which individual environmental factors do not play a role. "Rodent teeth are masterpieces of architecture," says Vesna Srot. "They have remarkable physical properties that make them very different from human teeth." These differences are of great interest for human dentistry.

Inspiration for dentistry

As the newly discovered iron-rich material improves the properties of tooth enamel without changing its colour, it could be the starting point for the development of an entirely new class of dental biomaterials. Various applications are also conceivable in restorative dentistry. "The addition of small amounts of amorphous or nanocrystalline ferrihydrite-like material or other biocompatible iron oxyhydroxides to dental care products could protect human tooth enamel exceptionally well," says Vesna Srot. In addition, tiny amounts of iron oxyhydroxides could be incorporated into synthetic enamel to make dental repairs more durable.

V. Srot, S. Houari, G. Kapun, B. Bussmann, F. Predel, B. Pokorny, E. Bužan, U. Salzberger, B. Fenk, M. Kelsch, P.A. van Aken