| Tooth morphogenesis is a series of process including the initiation of organogenesis and the regulation of structures. Morphogenesis in cellular level mainly means the proliferation and differentiation of cells. The migration of ectomesenchymal cells derived from neurocrest plays a vital role in the initiation of dental development. Apoptosis has a great value on the maintenance of histo-differatiation, organogenesis and homestasis of multicellular organ. During dental development, the regulatory molecules include signaling molecules and recepotors, transcription factors, which involve in the signaling networks. The signaling networks mediate cellular proliferation, differentiation, adhesion, apoptosis.Recently the homeobox genes family is found during the course of embryogenesis. Homeobox genes are the highly conservative DNA sequences throughout evolution, which encode the homeodomain proteins binding with DNA. The homeobox genes are regulators of place-dependent morphogenesis during embryogenesis. Muscle segment homeobox genes (Msx) are sort of master regulator gene during embryogenesis, includingMsxl, Msx2, and Msx3, which participate in embryogenesis. The functional deficiency of human Msx genes can result in prenatal dysgenesis.So in our research, we observed the dynamic expression of Msx 1/2, BMP-4, and the distribution of apoptosis in the growth phase of mouse teeth by molecular biological experimental technology, then exploring the regulatory roles of Msxl/2, BMP-4, the significance of apoptosis and their reciprocal relationship. The study consisted of two parts described as below:l.The study on the expression of Msx gene during mouse odontogenesisIn this research, we mapped out the dynamic localization of Msxl/2 during every phase of mouse tooth germ (bud stage, cap stage, bell stage) by in situ hybridization. The results were :Localization of Msx 1Localization of Msx2Bud stagedental mesenchymeepithelium of dental budCap stagedental sac, dental papilla near enamel epitheliuminner and outer enamel epithelium, enamel knotBell stagepreodontoblastinner and outer enamel epithelium, preodontoblastIt suggested that there were apparent difference in regulatory roles of tooth development between Msxl and Msx2. At bud stage, Msxl participateed in the adhesion of ectomesenchymal cells and migration of signaling BMP-4 from epithelium to mesenchyme; Msx2 could induce Ca +-dependent adhesion molecules to mediate the proliferation anddifferentiation of mesenchyme cells. At cap stage, Msxl could induce BMP-4 to play a role in the differentiation of odontoblast, and Msx2 could induce differentiation of dental papilla cells, dental sac cells and enamel epithelium cells, and it also initiated the apoptosis of enamel knots. At bell stage, Msxl could induce the dentinogenesis; Msx2 could participate in the differientiation of odontoblast and ameloblasts.2.The study on localization of BMP-4 and apoptosis2.1 In this research, we mapped out the localization of BMP-4 during every phase of rat tooth germ (bud stage, cap stage, bell stage) by in situ hybridization. The results indicated that BMP-4 was confined to dental epithelium and adhered dental mesenchyme at bud stage, and BMP-4 positive signals distribute in inner and outer enamel epithelium, enamel knot, dental sac and preodontoblast. It suggested that BMP-4 was not only growth factor but also the important signaling molecule to regulate the interaction between epithelium and mesenchyme.2.2 In this research, we observed the localization of apoptosis during the growth phase of rat tooth germ by TUNEL. The results indicated that apoptosis distribute in head of bud protrusion at bud stage; primary enamel knot and cervical round at cap stage; secondary enamel knot and cervical round at bell stage. This suggested that apoptosis could be regulated by interaction of epithelium-mesenchyme, and could control the quantities of proliferating cells, remove the aging cells and temporary structures, and play a role in pat...
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