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 glycopro tein that can be extracted from the mineralized extracellular matrix o f 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 s itu hybridization. We compared these patterns with those of type I col lagen. 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 antigenici ty in predentin. Although generally no cellular immunostaining was fou nd, very occasionally a minor immunoreactivity was detected in a small number of premineralizing incisor odontoblasts. On the mRNA level, ho wever, 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 we re negative for OPN. In contrast, however, osteoblasts of alveolar bon e showed strong positive signals with all three techniques, confirming the sensitivity and specificity of the detection methods. From the da ta obtained in this study, it can be concluded that during early stage s of dentinogenesis OPN presumably is not expressed in developing rat tooth germs. The weak immunostaining observed sporadically in some you ng odontoblasts is probably due to resorption of OPN of non-dental ori gin entrapped in the predentin.