| Enamel matrix proteins are secreted by the epithelial cells during the tooth germ development. They constitute the shape and structure of enamel, and play an important role during dentin formation and mineralization. Ameloblastin (AMBN, also known as amelin or sheathlin), which is distributed in enamel rod sheath, is one of the most important non-amelogenins and can be also regarded as tooth-specific protein. In recent years, much work on the characteristics of gene, protein structure and expression has been done. Up to now, people have gained considerable knowledge about the AMBN gene series of mouse, rat, pig, human and caiman, and the expression pattern of AMBN protein and mRNA. However, many problems about this protein remain unsolved yet. In order to understand the function of AMBN in tooth development, we underwent the following study with the methods of molecular biology, organ culture and electron microscope.This study was divided into two parts: 1 Expression of AMBN during mouse tooth germ developmentExpression of AMBN during mouse molar germ development was examinedby immunohistochemistry. The results showed that AMBN was strongly expressed in ameloblasts and enamel matrix, weakly in odontoblasts at the bell stage of postnatal Id. At postnatal 3 d, it was mainly detected in odontoblasts and weakly in ameloblasts. At postnatal 7 d, AMBN only expressed in Tomes processes. AMBN was not found at the other stages and positions. Those results tended to suggest that AMBN might get involved in enamel matrix formation and might play a role in the signal transduction during dentin matrix formation. 2 Effects of AMBN antisense oligodeoxynucleotide (AS-ODN) on mouse molar germ development in vitroIn order to understand the role of AMBN during tooth development, two experiments were carried out. Firstly, we set up a model of cultivation of mouse maxillary first molar germs in vitro. The germs from embryonic 17 th days mice were cultured in agarose semi-solid medium without serum and antibiotics under chemically defined conditions. We observed the epithelial root sheath formation and the structure of epithelial diaphragm in the cultivation after 12 d. Secondly, we designed and prefabricated AMBN AS-ODN. In the control experiments, maxillary first molars obtained from E17 d mice were cultured in agarose semi-solid medium for 12 d. Some explants were cultured in the presence of 18bp antisense or sense oligodeoxynucleotide targeted to AMBN mRNA. On the termination date, the cultured explants were all examined by Western blot, HE and transmission electron microscope. Our results showed that after 12-days in culture, the cultivation treated with AS-ODN reduced the synthesis of AMBN and had a deformed dental cusp with thinner enamel matrix. Ultrastructure analyses showed that there was hardly any cisternae of the rough endoplasmic reticulum (RER) in the ameloblasts at the tip of the cusp of AS-ODN treatedexplants. However, on average the enamel matrix was thinner compared with that in the control group. Furthermore, the collagen fibers in extracellular matrix were found disorganized. These findings seemed to provide a direct experimental evidence that tended to indicate that the arrested AMBN translation in cultured tooth germs might result in the delay of the tooth development. At the same time, these findings seemed also to suggest that AMBN might play an important role in keeping the normal tooth development by maintaining secreting ability of ameloblasts and in controlling the structure and direction of the fiber.
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