Autoradiographic study of the formation of enameloid and dentine matrices in teleost fishes using tritiated amino acids

The patterns of synthesis and secretion of the matrix proteins of dentine and enameloid were studied in developing teleost teeth by light microscope autoradiography after injection of 3H-proline in the ballan wrasse and after injection of 3H-proline or 3H-tyrosine in the common eel. In both species, the collagenous matrix of dentine was laid down incrementally, as in the teeth of tetrapods. In the wrasse, it was observed that both the odontoblasts and the inner dental epithelium secreted protein into the matrix of the enameloid forming the tooth tip (cap enameloid). It is concluded that the protein secreted by the odontoblasts is collagen and that the matrix of cap enameloid increases in bulk by deposition of collagen at the surface of the papilla. The protein secreted by the inner dental epithelium diffused into the preformed enameloid matrix. In the eel, the epithelial and odontoblastic components of enameloid matrix were not clearly distinguishable in the experiment with 3H-proline but, when 3H-tyrosine was employed, the labelling of collagen was reduced to a low level and the protein secreted by the inner dental epithelium was demonstrable. In both the wrasse and the eel, the inner dental epithelium continued to secrete protein into the enameloid for some time after mineralization of the tissue had commenced. Because the epithelial com­ponent of enameloid diffused into the matrix, unlike the collagenous odontoblastic component, and because the former protein was labelled more intensely with 3H-tyrosine than the latter, it is concluded that the enameloid protein originating from the inner dental epithelium resembles the matrix proteins (amelogenins) of mammalian enamel in organization and composition. Enameloid matrix is, therefore, a composite tissue containing collagen and an amelogenin-like protein. It is suggested that the epithelial protein plays an important part in enameloid mineralization and may be responsible for initiating the removal of collagen from the matrix by a non-enzymatic mechanism. Enameloid as found in fishes could have evolved into true enamel either by a prolongation of the secretory activity of the inner dental epithelium (Poole 1971) or by a delay in the onset of this activity. It was found that collar enameloid is homologous with cap enameloid, not with enamel, and that the cuticle overlying the cap enameloid in teleosts is produced entirely by the inner dental epithelium.