MicroCT images of the molars of mice, showing the bridge region that separates the roots. Image credit: Jing et al. (CC BY 4.0)
Different teeth have different numbers of roots. Incisors and canines each have one, and molars have two or three. Roots anchor the teeth to the jawbone, and provide a route for blood and nerves to reach the tooth. Getting the shape and number of the roots right during development is important to make sure that each tooth has proper support and function.
A protein called Ezh2 helps the bones of the face to develop, but it was not known how it affects how the roots of teeth grow. Teeth form from two layers of tissue; epithelium on the outside and mesenchyme on the inside. Jing et al. have now looked at what happens when Ezh2 is not present in these tissues in the molar teeth of developing mice.
The teeth of the mice were affected in different ways depending on which tissue Ezh2 was missing from. When the mesenchyme lacked Ezh2, the roots of the molar teeth did not form properly: the teeth formed too few roots, and the ‘bridge’ region between the roots did not develop correctly. When the epithelium lacked Ezh2, molar teeth formed the correct number of roots but the bridges between the roots developed later than normal. This suggests that signals from the mesenchyme determine how many roots each tooth grows.
Further investigation revealed that Ezh2 opposes the activity of another protein called Arid1a, and together they regulate the production of a protein that influences when cells divide. A balanced interaction between Ezh2 and Arid1a is important for tooth root development. This controls how bridges form between tooth roots, and ultimately determines how many roots a tooth grows.
Neanderthal teeth show evidence of forming bridges between roots later than modern human teeth, suggesting that similar regulation mechanisms have been important throughout human evolution. In the future, understanding how the roots of teeth form could help researchers to develop ways to regenerate teeth.
