Identification of the cell type origin of odontoma-like cell masses in microphthalmic (mi/mi) mice by in situ hybridization.

Tooth abnormalities occur in microphthalmic (mi/mi) mice. The elongated odontogenic epithelium is interrupted by unresorbed bone at the basal end of the mi/mi incisor, with the epithelium gathered into cell clusters. These clusters develop to odontoma-like masses. To identify the origin of the cell types of these odontoma-like masses, the localization of osteonectin (Osn), osteocalcin (Osc), osteopontin (Osp), matrix Gla protein (MGP) and amelogenin (Am) mRNA in the process of tooth development in mi/mi and +/+ mice was investigated by means of in situ hybridization. Decalcified mandibles of neonatal, 5-, 10-, 14-day-old mice were examined. Osn and Osc mRNA, which localized in osteoblasts and odontoblasts, were also detected in the cells of odontoma-like masses in mi/mi mice. The cells expressing these mRNA were short, columnar and odontoblast-like. Am mRNA was detected in ameloblasts. In mi/mi mice, Am mRNA was also detected in ameloblastic cell clusters, which were formed by the tall columnar cells in the odontoma-like masses. No apparent Osp mRNA expression was detected in the masses. These results indicated that even in odontogenic abnormal cells resulting from physical obstruction in mi/mi mice, the genes that are involved in normal tooth development were still expressed.