β-catenin is required in odontoblasts for tooth root formation.

The tooth root is an important part of the tooth that works together with the surrounding periodontium to maintain the tooth in the alveolar socket. The root develops after crown morphogenesis. While the molecular and cellular mechanisms of early tooth development and crown morphogenesis have been extensively studied, little is known about the molecular mechanisms controlling tooth root formation. Here, we show that β-catenin is strongly expressed in odontoblast-lineage cells and is required for root formation. Tissue-specific inactivation of β-catenin in developing odontoblasts produced molars lacking roots and aberrantly thin incisors. At the beginning of root formation in the mutant molars, the cervical loop epithelium extended apically to form Hertwig's epithelial root sheath (HERS), but root odontoblast differentiation was disrupted and followed by the loss of some HERS inner layer cells. However, the outer layer of the HERS extended without the root, and the mutant molars finally erupted. The periodontal tissues extensively invaded the dental pulp. These results indicate that there is a cell-autonomous requirement for Wnt/β-catenin signaling in the dental mesenchyme for root formation.