Patterning of enamel knots in tooth development

The primary enamel knot (EK), which is located in the center of bud and cap stage tooth germs, is a transitory cluster of non-dividing epithelial cells. The primary EK acts as a signaling center that provides positional information for crown morphogenesis and regulates the growth of tooth cusps by inducing seven or eight secondary EKs. The formation of these cusps is composed of three major processes such as the appearance of the primary EK, the relationship between the primary EK and secondary EKs, and the patterning of secondary EKs. However, details in these processes are not known clearly. Therefore, we chased the change and movement of the EKs during tooth development to clarify the patterning of the EKs in these processes. As results, the patterning of the EK in tooth germs always started from a primary EK by the interaction between the epithelium and mesenchyme. Even after being removed from tooth germs, the primary EK was regenerated in the center of these tooth germs. 　Differently from previous reports, the primary EK was incorporated into the first buccal secondary EK rather disappeared from bell stage. This result supports previous evolutional studies suggesting that the first cusps in tooth development are the paracone and the protoconid, which are the first buccal cusps in the upper in the lower jaws. In addition, the distance of secondary EKs was determined by the epithelium, while the number of secondary EKs, the number of cusps, was co-regulated by the epithelium and mesenchyme. The reaction diffusion mechanism can be applied to explain this patterning of secondary EKs.

In conclusion, our results suggest that the interaction between the epithelium and mesenchyme is involved in the patterning of the primary and secondary EKs, which are important structures determining the cusp patterning and crown morphogenesis.