Expression of matrix proteins during the development of mineralized tissues

The specific properties of mineralized tissues are defined by the composition of the fraction of the noncollagenous matrix proteins. Because these proteins play a pivotal role in the processes of cell differentiation and activation and of mineralization, their temporal and spatial expression is tightly regulated. Within this study, the expression of the enamel protein amelogenin and of the bone matrix proteins osteopontin, bone sialoprotein, osteocalcin, and osteonectin was investigated by in situ hybridization. Two models that allow observation of the formation of mineralized tissues were chosen. The development of bone and cartilage was observed on murine metatarsals from 15-day-old embryos up to 1-day-old mice. This time covers the periods of initial bone formation as well as onset of resorption of mineralized cartilage and bone. 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 differentiated organ which erupts through the mandibular bone. In the developing metatarsals, osteopontin and bone sialoprotein mRNAs were detected in osteoblasts and hypertrophic chondrocytes at the onset of mineralization. In the tooth organ, only cementoblasts expressed transcripts encoding the two proteins; odontoblasts and ameloblasts did not express these genes. Osteonectin was expressed by osteoblasts and hypertrophic chondrocytes as well, whereas in the molars it was produced exclusively by odontoblasts. Osteocalcin was expressed specifically by osteoblasts in the developing metatarsals. In tooth, osteocalcin transcripts were detected in odontoblasts. Finally, amelogenin was a specific product of ameloblasts. Thus, a sequential and cell type-restricted expression of matrix proteins takes place during the development of the mineralized tissues. The expression patterns of the transcripts encoding the bone matrix proteins suggest different biological roles depending on the time and site of expression.