Abstract:The purpose of this study was to evaluate the responses of immature young pulp cells under conditions of hypoxia with or without reoxygenation. Proliferative activity and bone related protein mRNAs were evaluated. Proliferation ratios in the hypoxia only and hypoxia with reoxygenation groups were significantly lower than those in the control group on days 2 and 3. On day 4, there was no significant difference between the control and the reoxygenation group. There was no significant difference in alkaline phosphatase (ALP) mRNA among the control, hypoxia and reoxygenation groups at any of the time periods. Expression of dentinsialophosphoprotein (DSPP) mRNA was significantly lower in the hypoxia group than in the control and the reoxygenation group on days 2 and 3. Hypoxic condition suppressed the expression of a gene specific to odontoblasts in immature young pulp cells, suggesting that it affects the differentiation ability of odontoblasts.

Background: Molecular mechanisms of bone resorption during odontogenesis are entirely unknown. In this study, we investigated the expressions of receptor activator nuclear factor κB ligand (RANKL), osteoprotegerin (OPG) and tumor necrosis factor-α (TNF-α) during early-stage tooth germ development. Methods: Developing tooth germs in rats, including Hertwig's epithelial root sheaths (HERS) at prenatal day 20 and postnatal days 1, 3, 5, 7 and 9, were examined. Expressions for RANKL, OPG, and TNF-α in mRNA, as well as protein levels, were evaluated by RT-PCR and immunohistochemistry. Results: mRNA expression levels for RANKL were significantly increased at postnatal days 3 to 9. Those for OPG increased from 1 to 5 days. However, those for TNF-α did not change between prenatal day 20 and postnatal day 5. On each postnatal day, RANKL was immunohistochemically localized in dental follicle cells attached to HERS, as well as in the crown region. HERS cells were also immunopositive for RANKL at postnatal day 7 and later. OPG was weakly immunolocalized in dental follicle cells from postnatal days 1 to 5, whereas after postnatal day 7 it disappeared. Conclusions: These findings revealed that the RANKL-RANK signaling, but not TNF-α signaling, plays a major role during bone resorption in the dental follicle surface to the HERS during early-stage tooth development.