EXPRESSION OF MATRIX PROTEINS DURING THE DEVELOPMENT OF MINERALIZED TISSUES

The specific properties of mineralized tissues are defined by the comp osition of the fraction of the noncollagenous matrix proteins, Because these proteins play a pivotal role in the processes of cell different iation and activation and of mineralization, their temporal and spatia l expression is tightly regulated, Within this study, the expression o f the enamel protein amelogenin and of the bone matrix proteins osteop ontin, bone sialoprotein, osteocalcin, and osteonectin was investigate d by in situ hybridization. Two models that allow observation of the f ormation of mineralized tissues were chosen, The development of bone a nd cartilage was observed on murine metatarsals from W-day-old embryos up to I-day-old mice, This time covers the periods of initial bone fo rmation as well as onset of resorption of mineralized cartilage and bo ne, To study gene expression in the mineralized tissues of the dental organ, enamel, dentin, and cementum, developing molars ranging in age from 16-day-old embryos to 14 days after delivery were chosen, Within this time frame, the molars develop from an immature state to the diff erentiated organ which erupts through the mandibular bone. In the deve loping metatarsals, osteopontin and bone sialoprotein mRNAs were detec ted in osteoblasts and hypertrophic chondrocytes at the onset of miner alization, In the tooth organ, only cementoblasts expressed transcript s encoding the two proteins; odontoblasts and ameloblasts did not expr ess these genes, Osteonectin was expressed by osteoblasts and hypertro phic chondrocytes as well, whereas in the molars it was produced exclu sively by odontoblasts, Osteocalcin was expressed specifically by oste oblasts in the developing metatarsals. In tooth, osteocalcin transcrip ts were detected in odontoblasts. Finally, amelogenin was a specific p roduct of ameloblasts, Thus, a sequential and cell type-restricted exp ression of matrix proteins takes place during the development of the m ineralized tissues, The expression patterns of the transcripts encodin g the bone matrix proteins suggest different biological roles dependin g on the time and site of expression.