Hidemitsu Harada

Background and Objectives Hertwig's epithelial root sheath (HERS) plays a role in root dentin formation. It produces the epithelial rests of Malassez (ERM) for the induction of periodontal tissue development during root formation. Although ERM is thought to be caused by epithelial–mesenchymal transition (EMT), the mechanism by which HERS is maintained as epithelium is unknown. Here, we aimed to elucidate the molecular mechanisms regulating the relationship between HERS maintenance and ERM development. Methods To understand the relationship between HERS and ERM development during root formation, we observed the developing molar root using cytokeratin14 (CK14) Cre/tdTomato mice via stereomicroscopy. The relationship between semaphorin and transforming growth factor (TGF) signaling in the maintenance of HERS and ERM development was examined using CK14cre/R26‐tdTomato mice and a HERS cell line. Results tdTomato‐positive cells were observed on HERS and the migrating cells from HERS. The migrating cells showed reduced E‐cadherin expression. In contrast, HERS cells expressed semaphorin receptors and active RhoA. Semaphorin signaling was associated with RhoA activation and cell–cell adhesion, while TGF‐β induced decreased E‐cadherin and active RhoA expression, and consequently enhanced cell migration. Conclusion HERS induces root formation by controlling epithelial maintenance and EMT through the opposing effects of semaphorin and TGF‐β signaling.