| Tooth development is characterized by reciprocal interaction between dental epithelium and mesenchyme. Compared with many studies of early tooth development, little was known about the root development. Some researches have indicated that at E12d, the dental mesenchyme which regulate the tooth shape and can induce the non-dental epithelium to form tooth structure, has acquired the odontogenetic potential to instruct tooth development. So we proposed that the apical papilla may also have this potential, which can regulate tooth root formation. However the coronal and apical papilla may have different function in inducing the dental epithelium, which causes the differente developmental patterning between crown and root. Methods and the main results: By using histological and cell biological technologies, such as cell culture, immunohistology, organ culture, renal capsule implantation, and RT-PCR, we attempted to investigate the interaction of enamel organ with coronal or apical papilla, and discuss the possibility whether apical papilla can induce enamel organ epthilium to form HERS.This study obtained the following results. Part 1: Observation of rat first and second molar germs developmentThe sections were prepared by the mandibe of rat from PN1-7days.We observed the HERS has formed in the PN 6-7days in rat mandibular first molar. And it could be distinctly dividied into the crown and root parts. So the undifferentiating apical papilla in this stage could be used in the recombination. We also observed that the second molar in PN1-2days was still in early bell stage and the inner enamel epithelial and papilla cells has not differentiated, which was suitable for the recombination.Part 2: Study on implanting rat tooth enamel organ in with apical or coronal papilla in renal capsuleBased on parts 1, the PN2d mandibular second molar enamel organ with coronal papilla or PN7d mandibular first molar apical papilla were recombined respectively. Reconstructed tooth germs elements were implanted under renal capsule for 2 weeks. We observed that coronal papilla can induce the enamel organ epithelium to differentiate into ameloblast, which form enamel. However the apical papilla did not have the ability to make enamel organ epithelium differentiation. Reconstructied tooth germ developed bone like tissue where some epithelial cells were embided into. And the implanted enamel organ also developed into cornified epithelium. It indicated that apical papilla can not induce the enamel epithelium differentiation, and the dental epthilium without the suitable environment do not form dental structure their own.Part 3: Study on implanting rat tooth enamel organ epithelial cells with apical or coronal papilla cells in renal capsuleEnamel organ epithelial cells and apical papilla cells prepared from PN2 second molar and PN7 first molar were recombined. The implantation in renal capsule for 2 weeks regenerated calcified structures, which was more disordered. The enamel organ epithelial cells could be found in the fibrous tissue and embedded into the calcified structure. And we observed that the fibrous tissues were embedded into the bone like tissue, which was similar with the periodontal fiber. It indicated that induced enamel organ eithilium did not maintain the ability to form enamel but transform to root pattern.Part 4: The inductive affect of apical papilla cells to enamel organ epithelial cells in the expression of EMPWe use RT-PCR to evaluate the expression of enamel matrix protein mRNA in the induced enamel oragan epithial cells by coronal or apical papilla cells. We found that the apical papilla cells could not induce epithelial cell to express amlogenin mRNA, which is similar with the HERS cells cultured in vitro.Conclusion: from the above study, we inferred that the apical papilla can not induce enamel organ epithelia cells to differentiate into ameloblast and express amelogenin mRNA . It may inferred that the different developmental patterning of tooth crown and root.
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