Orthodontic loading modulates epithelial progenitor dynamics and enamel formation in the mouse mandibular incisor

BACKGROUND: Adult stem cells (SCs) maintain tissue homeostasis through a balance of self-renewal and differentiation regulated by biochemical and mechanical cues. While biochemical pathways are well studied, the in vivo mechanisms by which external forces influence SCs and their progeny remain poorly understood. The continuously growing mouse incisor offers a tractable model for investigating these mechanisms because dental epithelial stem cells (DESCs) reside in a spatially restricted apical niche, and their progeny undergo stereotyped proliferation and differentiation along the apical-incisal axis. We investigated how intrusive mechanical loading affects dental epithelial cell kinetics and enamel regeneration during injury repair. METHODS: A modified orthodontic tooth movement device was adapted to apply controlled intrusive forces (0.05-0.5 N) to the mouse mandibular incisor. To stimulate regeneration, the incisal tip was trimmed, and mice were subjected to force application for three days, followed by recovery after device removal. Dental epithelial responses were evaluated using histology, dual EdU/BrdU labeling, in situ hybridization, and micro-computed tomography. Expression of proliferation (Ccnb1) and differentiation (Igfbpl1) markers, as well as nuclear localization of the mechano-responsive transcription co-factor YAP, were analyzed. RESULTS: Forces ≥ 0.2 N caused adverse health effects, whereas 0.05 N induced consistent epithelial changes without systemic consequences. At 0.05 N loading, incisors showed delayed regeneration and hypomineralized enamel. The apical epithelium exhibited folds, tears, reduced proliferation, and premature amelogenin expression. These effects reversed after load removal, coinciding with restored proliferation and enamel deposition. Gene expression analysis revealed apical-incisal shifts in Ccnb1 and Igfbpl1 domains, consistent with altered cell kinetics. Mechanistically, loading decreased nuclear YAP localization in the central body of the labial cervical loop, suggesting a role for this mechanosensitive pathway in mediating the cellular response to mechanical force. CONCLUSIONS: This study presents an in vivo model for examining mechanical influences on dental epithelial progenitor cells. Intrusive loading suppresses proliferation, promotes precocious differentiation, and alters epithelial architecture. These findings support a role for mechanical cues in modulating tissue regeneration and provide a framework for future mechanistic studies in tooth biology.