Molecular biological study of the function of matrix proteins in the early stage of tooth development, with special reference to cytodifferentiation and initial calcification

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Sheath protein (ameloblastin), one of the main constituent of the enamel matrix protein, is expressed not only in the secretory ameloblast but also in other cell types such as immature odontoblasts and epithelial cell rest of Malassez. In order to understand the role of Sheath protein in the epithelial-mesenchymal interaction, we have attempted to demonstrate the expression of sheathe protein and the relation of cement blast differentiation of the earliest stage of the cementgenesis in the rat moral tooth germs and incisors. The results are summarized as follows.1. A part of cells in the Hertwig's epithelial root sheath and the epithelial cell rest of Malassez expressed sheath protein whose molecular form is identical to that expressed in the secretory ameloblast. Electron microscopic immunocytochemitry revealed that sheath protein is contained in the Golgi apparatus located periodontal side of the epithelial cell rest of Malassez. Cement matrix itself did not contain sheath protein.2. Mesenchymal cells located near the surface of the root dentin expressed osteopontin, one of the marker of initial cementum, and have processes extending to the surface of the root dentin. Thses cells are usually attached to the epithelial cell rest of Malassez.3. After the administration of Monesin, one of the inhibitor of the cell secretory activity, osteopontin are clearly localized in the cytoplasmic processes extending from these mesenchynal cells.4. Expression of sheath protein was always prior to that of osteopontin.These results suggested that sheath protein secreted from the cells of Hertwig's epithelial root sheath and the epithelial cell rest of Malassez induced differentiation of mesenchymal cells to odontoblasts which extend cytoplasmmic processes to the surface of calcified dentin and form initial cement layer.