Enameloid

Abstract: A systematic SEM survey of tooth micro- structure in (primarily) fossil taxa spanning chon- drichthyan phylogeny demonstrates the presence of a su- perficial cap of single crystallite enameloid (SCE) on the teeth of several basal elasmobranchs, as well as on the tooth plates of Helodus (a basal holocephalan). This sug- gests that the epithelial-mesenchymal interactions required for the development of enameloid during odonto- genesis are plesiomorphic in chondrichthyans, and most likely in toothed gnathostomes, and provides phylogenetic support for the homology of chondrichthyan and actino- pterygian enameloid. Along the neoselachian stem, we see a crownward progression, possibly modulated by hetero- chrony, from a monolayer of SCE lacking microstructural differentiation to the complex triple-layered tooth ename- loid fabric of neoselachians. Finally, the occurrence of fully-differentiated neoselachian enameloid microstruc- ture (including compression-resistant tangle fibered enameloid and bending-resistant parallel fibered ename- loid) in Chlamydoselachus anguineus, a basal Squalean with teeth that are functionally ''cladodont,'' is evidence that triple-layered enameloid microstructure was a prea- daption to the cutting and gouging function of many neo- selachian teeth, and thus may have played an integral role in the Mesozoic radiation of the neoselachian crown group. J. Morphol. 268:33-49, 2007. ! 2006 Wiley-Liss, Inc.

Abstract: 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 $^{3}$H-proline in the ballan wrasse and after injection of $^{3}$H-proline or $^{3}$H-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 $^{3}$H-proline but, when $^{3}$H-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 $^{3}$H-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.

Abstract: Diverse hard tissues constituted a tooth-like skeletal element in extinct jawless vertebrates. Today, similar tissues are found in our teeth. These tissues mineralize in the extracellular matrix and involve various macromolecules. Among these molecules are secretory calcium-binding phosphoproteins (SCPPs) coded by genes that arose by duplication. Although the repertoire of SCPPs may vary in different lineages, some SCPPs are unusually acidic and are thought to participate in the mineralization of a collagenous matrix, principally either bone or dentin. Other SCPPs are rich in Pro and Gln (P/Q) and are employed to form the tooth surface. In tetrapods, the tooth surface is usually covered with enamel which develops in a matrix comprised of P/Q-rich SCPPs. By contrast, the tooth surface tissue in teleosts is called enameloid and it forms in a dentin-like collagenous matrix. Despite the difference in their matrix, both enamel and enameloid mature into hypermineralized inorganic tissues. Notably, some P/Q-rich SCPP genes are primarily expressed at this stage and their proteins localize between the tooth surface and overlying dental epithelium. Moreover, an orthologous gene is used for maturation of these 2 different tissues. These findings suggest distinct roles of acidic and P/Q-rich SCPPs during the evolution of hard tissues. Acidic SCPPs initially regulated the mineralization of bone, dentin, or a similar ancient collagenous tissue through interaction with calcium ions. P/Q-rich SCPPs arose next and originally assembled a structure or a space that facilitated the hypermineralization of dentin or a dentin-like tissue. Subsequently, some P/Q-rich SCPPs were coopted for the mineralizing enamel matrix. More recently, however, many SCPP genes were lost in toothless birds and mammals. Thus, it appears that, in vertebrates, the phenotypic complexity of hard tissues correlates with gain and loss of SCPP genes.