Occlusion Influences Tooth Root Development In Mice Molars

Objective: To establish a non-occlusal model,and to investigate the effects of the absence of occlusion in root development.And we hope to explore the reason how root develops and grows in molecular level and give more information for root regeneration in the future,and reduce the risk of oral treatment while providing more extensive indications for orthodontic treatment.Method: The molars of C57 BL / 6J inbred mice at postnatal day(PN)7.5,14.5,15.5,21.5,28.5,35.5,42.5,49.5,56.5,63.5,70.5,77.5,84.5 are selected,to observe the occlusal relationship and the change of the root length of the mandibular first molar.Then,PN15.5 C57 BL / 6J mice were used to perform a bite force change animal by extracting the unerupted counterpart upper first molars.Animals were sacrificed at 6 days,10 days,25 days,and 75 days later(namely PN21.5,25.5,40.5,90.5).As control,wide-type(Wt)at PN21.5,PN25.5,PN40.5,PN90.5 were selected.All samples were processed into paraffin-emdedded serial sections using standard procedures for Hematoxylin-Eosin(HE)staining,PCNA and Shh immunohistochemical staining,and TRAP staining.The histopathological change,PCNA expression level,the number of TRAP staining positive cells of the bone tissue in each groups were investigated and the results were analyzed by the Computer Image Analyzing System and treated by statistics test.Results: 1.In this study,after the continuous observation of the occlusal relationship and root length of the upper and lower mandibular first molar in C57 BL / 6J wild type mice,it was found that the tooth was exposed to the oral cavity at PN15.5.So,we choose the PN15.5 mice to establish the non-occlusal animal model to investigate the effects of the absence of occlusion in root development.2.Root lengths in the hypofunctional condition increased more than in hyperfunctional condition.Histological findings of hypofunctional condition showed the molars with a long root,narrow the periodontal ligament(PDL)and increased number of HERS cells.While,in hyperfunctional condition,molars were with a short root,wide PDL and decreased number of HERS cell.3.Proliferating cells localized in Hertwig’s epithelial root sheath(HERS),the apical pulp,and the PDL of hypofunctional condition groups.Furthermore,cell-proliferative activity in hypofunctional condition and control groups exceeded that in hyperfunctional condition,as well as TRAP-positive cells.Shh expression was changed in different groups.Conclusion: We conclude that cell proliferation and bone resorption at the apical base is decreased by occlusion during root formation.Thus,occlusion is one factors that regulate root elongation.At the same time,the expression of Shh in the apical part was affected by occlusion,which may affect the length of the root.