Impaired breakdown of Herwig's epithelial root sheath disturbs tooth root development

Background: Wnt/β-catenin signaling plays a variety of roles in both the dental epithelium and mesenchyme at most stages of tooth development. In this study, we verified the roles of Hertwig's epithelial root sheath (HERS) breakdown in tooth root development. This breakdown results in formation of epithelial cell rests of Malassez (ERM). Results: Following induction of β-catenin stabilization in the epithelium of developing tooth at the moment of HERS breakdown, HERS failed to break down for ERM formation. HERS with stabilized β-catenin was altered into a multicellular layer enveloping elongated root dentin with higher expression of junctional proteins such as Zo-1 and E-cadherin. Importantly, this impairment of HERS breakdown led to arrest of further root elongation. In addition, the portion of root dentin enveloped by the undissociated HERS remained in a hypomineralized state. The odontoblasts showed ectopically higher expression of pyrophosphate regulators including Ank and Npp1, whereas Tnap expression was unchanged. Conclusions: Our data suggest that Wnt/β-catenin signaling is decreased in HERS for ERM formation during root development. Furthermore, ERM formation is important for further elongation and dentin mineralization of the tooth roots. These findings may provide new insight to understand the contribution of ERM to root formation.