Dissimilar expression patterns for the extracellular matrix proteins osteopontin (OPN) and collagen type I in dental tissues and alveolar bone of the neonatal rat.
Osteopontin (OPN) is a phosphorylated, sialic acid containing glycoprotein that can be extracted from the mineralized extracellular matrix of bone. In the present study we determined the expression patterns of OPN in dental tissues and alveolar bone of 1-3 day old (neonatal) rats by means of 1) immunohistochemistry, 2) Northern blotting and 3) in situ hybridization. We compared these patterns with those of type I collagen. We localized collagen type I expression in osteoblasts adjacent to alveolar bone and in odontoblasts lining predentin/dentin, but not in the epithelial ameloblasts. For OPN, we observed a weak antigenicity in predentin. Although generally no cellular immunostaining was found, very occasionally a minor immunoreactivity was detected in a small number of pre-mineralizing incisor odontoblasts. On the mRNA level, however, no OPN transcripts could be detected in odontoblasts, either by in situ or by Northern hybridization analyses. Also the odontoblasts of the bone-like dentin (osteodentin) region in the tip of incisors were negative for OPN. In contrast, however, osteoblasts of alveolar bone showed strong positive signals with all three techniques, confirming the sensitivity and specificity of the detection methods. From the data obtained in this study, it can be concluded that during early stages of dentinogenesis OPN presumably is not expressed in developing rat tooth germs. The weak immunostaining observed sporadically in some young odontoblasts is probably due to resorption of OPN of non-dental origin entrapped in the predentin.