Development 101, 289-2% (1987) Printed in Great Britain © The Company of Biologists Limited 1987 289 Changes in the distribution of tenascin during tooth development IRMA THESLEFF 1 , ELEANOR MACKIE 2 , SEPPO VAINIO 1 and RUTH CHIQUET-EHRISMANN 2 1 Department of Pedodontics and Orthodontics, Institute of Dentistry, University of Helsinki, SF-00280 Helsinki, Finland 2 Friedrich Miescher Institute, Basel, Switzerland Summary Tenascin is an extracellular matrix molecule that was earlier shown to be enriched in embryonic mesen- chyme surrounding the budding epithelium in various organs including the tooth. In the present study tenascin was localized by immunohistology through- out the course of tooth development in the mouse and rat using polyclonal antibodies against chick tenascin. The results indicate that tenascin is expressed by the lineage of dental mesenchymal cells throughout tooth ontogeny. The intensity of staining with tenascin antibodies in the dental papilla mesenchyme was temporarily reduced at cap stage when the tooth grows rapidly and undergoes extensive morphogenetic changes. During the bell stage of morphogenesis, the staining intensity increased and tenascin was accumu- lated in the dental pulp even after completion of crown development and eruption. Tenascin was present in the dental basement membrane at the time of odonto- blast differentiation. The dental papilla cells ceased to express tenascin upon differentiation into odontoblasts and tenascin was completely absent from dentin. It can be speculated that the remarkable expression of tenascin in the dental mesenchymal cells as compared to other connective tissues is associated with their capacity to differentiate into hard-tissue-forming cells. Key words: tenascin, extracellular matrix, morphogenesis, odontogenesis, tissue interactions. Introduction The extracellular matrix plays an important role in embryonic morphogenesis and cell differentiation. For example, branching morphogenesis in the sali- vary gland is accompanied by remarkable changes in the tissue distribution of collagens and proteoglycans (Grobstein & Cohen, 1965; Bernfield & Banerjee, 1982). Interactions between many defined matrix molecules and the cell surface have been associated with cell differentiation (Saxen, Ekblom & Thesleff, 1982; Hay, 1982; Orkin, Knudson & Toole, 1985; Thiery, 1985; Zanetti & Solursh, 1986). The regulat- ive role of the extracellular matrix in tooth develop- ment has been emphasized in numerous descriptive as well as experimental studies (Kollar, 1978; Thesleff, 1980; Thesleff & Hurmerinta, 1981; Ruch, 1985). We have shown earlier that the distribution of collagens and fibronectin change during tooth morphogenesis (Thesleff, Stenman, Vaheri & Timpl, 1979) and we have suggested that the differentiation of odonto- blasts and ameloblasts in the developing tooth is mediated by interactions between the cells and the extracellular matrix (Thesleff & Hurmerinta, 1981). Tenascin is a matrix protein that was previously described as chick myotendinous antigen (Chiquet & Fambrough, 1984a,b). It is identical to the protein described as hexabrachion (Erickson & Inglesias, 1984; Vaughan, Huber, Chiquet & Winterhalter, 1987) and probably also to the proteins described as cytotactin (Grumet, Hoffman, Crossin & Edelman, 1985), Jl (Kruse etal. 1985) and GMEM (Bourdon, Matthews, Pizzo & Bigner, 1985). In embryonic teeth, as well as mammary glands and hair follicles, tenascin is present in the dense, organ-specific mesen- chyme surrounding the invaginating epithelial bud, but not in the more distant mesenchyme (Chiquet- Ehrismann, Mackie, Pearson & Sakakura, 1986). It has been proposed that tenascin plays a role in the tissue interactions that govern the early development of these embryonic organs. The aim of the present study was to analyse whether changes in the distri- bution of tenascin can be correlated with certain stages of tooth morphogenesis and cell differen- tiation. For this purpose, tenascin was localized in mouse and rat teeth from the stage of epithelial bud formation until tooth eruption.