Mechanical loading is critical for enamel and dentin formation

Objectives: Mechanical loading is essential for maintenance of healthy bone. Yet, the effects of loading on dentin and enamel are unclear. The goals of this study were 1) to address mechanical unloading effects on enamel and dentin formation; and 2) to identify a molecule, which is potentially responsive to loading in dentin and enamel.

Methods: Left lower incisors in 1-month-old mice were unloaded by trimming 2-3 mms from occlusal surface two times a week for 4 weeks. Each mouse was sacrificed at 2-months of age. Incisors were also trimmed in periostin null mice, as this adhesive molecule is a likely candidate gene for response to mechanical loading. Real-time RT-PCR, histology, calcein/Alizarin red double labeling, backscattered electronic microscopy, image analysis, and radiography were used to characterize incisor dentin and enamel.

Results: (1) Thickness and mineral contents in both enamel and dentin were dramatically reduced by removing mechanical force. Interestingly, mineral loss was more severe in enamel while thickness reduction was greater in dentin. At the EM level, enamel rods were very small with low P content. The enamel was fragile and displayed macro- and micro-fracture/cracks following tissue preparation for microscopy, and the enamel-dentin junction was not tightly connected. (2) Periostin was identified in the pre-odontoblasts and mature ameloblasts. The same unloading on periostin null mice has much less effect on enamel and dentine thickness and mineral content suggesting that periostin is responsive to changes in mechanical loading. (3) Mechanical tension in cells of odontoblast lineage inhibited periostin expression in vitro. These results suggest that mechanical loading is critical for enamel and dentin formation and has a significant effect on mineral content and tissue integrity.

Conclusion: Periostin is likely a potent negative regulator of tissue growth in response to mechanical loading for both enamel and dentin. (This work is supported by AR51587).