New Insights About Early Tongue Teeth Development in the Salmon Trout (Oncorhynchus mykiss)

The salmon trout or rainbow trout (Oncorhynchus mykiss) is a teleost fish are characterized by the presence of lingual teeth. The objective of this study is to investigate their development, particularly in relation to the expression of genes associated with mammalian odontogenesis. Sixty fry were reared in a freshwater aquarium at 7°C. Specimens at developmental stages ranging from D0 (hatching) to D35 were collected and subjected to histological (Masson's trichrome) and immunohistochemical analysis, including the expression of proteins associated with the Pitx2, Shh, MSX1, Pax9, Bmp4 and β-catenin genes. The developmental patterns of the lingual teeth are similar to those of the maxillary and mandibular teeth. These are not giant papillae or pseudoteeth. The initiator gene Pitx2 is expressed in the epithelium during early stages but subsequently diminishes. Bmp4, Shh and β-Cat exhibit a pattern similar to that of mammals, except for the absence of the enamel node. Surprisingly, however, we observe expression of Msx1 and Pax9 in the dental mesenchyme as well as in the epithelium, a phenomenon unknown in mammals. The second week marks an increase in the expression of these two proteins and of Shh, while Bmp4 does not appear in large quantities until 10 days. Incidentally, the mucous glands (cement glands) express Shh, Pitx2, Msx1, Pax9 and β-catenin. In conclusion, tongue teeth are 'classical' teeth; their gene expression patterns are similar to those in mammals, except that Msx1 and Pax9 are expressed in both epithelial and mesenchymal tissues in this species.