Initial aspects of mineralization at the dentino-enamel junction in embryonic mouse incisor in vivo and in vitro: A TEM comparative study

The frontier between the enamel organ and the dental papilla, the future de ntino-enamel junction, undergoes coordinated modifications. The mineralizat ion of the extracellular matrix starts within the predentine, which is a pr erequisite for the formation of the first enamel crystallites in vivo. We i nvestigated the dentino-enamel junction using the embryonic mouse incisor a s a model. Our data showed that the notion of the dentino-enamel junction s hould not be restricted to the thin interface classically described, a temp oro-spatial survey from the epithelio-mesenchymal junction to the dentino-e namel junction delineated a clear sequence of events characterized by the e arly deposition of electron-dense granules, followed by the appearance of p atches of stippled material at the dentino-enamel junction. The first tiny enamel crystallites appeared in the vicinity of this material which present ed a well-ordered alignment. The comparison of data obtained in vivo on 17- , 18-, 19-d-old embryonic incisors with those obtained in vitro using 15-d- old embryonic incisors cultured for 7 d emphasizes the relevance of this se quence. Helicoidal growing crystals were observed in cultured tooth germs b ut never in vivo.