Enameloid/enamel transition through successive tooth replacements in Pleurodeles waltl (Lissamphibia, Caudata)

Abstract

Study of the evolutionary enameloid/enamel transition suffers from discontinuous data in the fossil record, although a developmental enameloid/enamel transition exists in living caudates, salamanders and newts. The timing and manner in which the enameloid/enamel transition is achieved during caudate ontogeny is of great interest, because the caudate situation could reflect events that have occurred during evolution. Using light and transmission electron microscopy, we have monitored the formation of the upper tooth region in six successive teeth of a tooth family (position I) in Pleurodeles waltl from late embryos to young adult. Enameloid has only been identified in embryonic tooth I(1) and in larval teeth I(2) and I(3). A thin layer of enamel is deposited later by ameloblasts on the enameloid surface of these teeth. From post-metamorphic juvenile onwards, teeth are covered with enamel only. The collagen-rich enameloid matrix is deposited by odontoblasts, which subsequently form dentin. Enameloid, like enamel, mineralizes and then matures but ameloblast participation in enameloid matrix deposition has not been established. From tooth I(1) to tooth I(3), the enameloid matrix becomes ever more dense and increasingly comes to resemble the dentin matrix, although it is still subjected to maturation. Our data suggest the absence of an enameloid/enamel transition and, instead, the occurrence of an enameloid/dentin transition, which seems to result from a progressive slowing down of odontoblast activity. As a consequence, the ameloblasts in post-metamorphic teeth appear to synthesize the enamel matrix earlier than in larval teeth.

The Origin and Evolution of Enamel Mineralization Genes

“…Odontoblasts progressively reduced their production of loose collagenous matrix, which characterizes forming enameloid, while ameloblast activity increased with the secretion of large amounts of enamel-specific products at the dentin surface. This evolutionary 'tran sition' between enameloid and enamel was, in fact, probably an enameloid-dentin transition, as recently demo nstrated in the ontogeny of caudate amphibians [Davit-Béal et al, 2007]. However, enamel did not replace enameloid in all vertebrate lineages.…”

Chemical and mechanical properties of snake fangs

“…[13] Although these tissues are similar when mineralized, they differ in structure and organization of their organic matrix. [14] Dental tissues are complex hydrated biological composites consisting of inorganic material, organic components and water. The composition of these materials, and their interaction, will determine the mechanical properties of the individual dental tissues.…”

Materials Engineering by Ameloblasts

“…Enameloid develops from dental epithelial and odontoblast activity, and it is composed, at least initially, of matrices containing predominantly collagen (Sasagawa et al 2009). This suggests that enameloid may be associated with dentin formation and is thus different from enamel (Davit-Beal et al 2007;Assaraf-Weill et al 2014). While it is not fully understood if enameloid and enamel are related evolutionarily, it has been shown that several species (newts, salamanders, some fish) that produce enameloid structures also have a mineral coating that appears to be enamel, since it is synthesized by ameloblasts Davit-Beal et al 2007;Diekwisch et al 2009).…”

“…This suggests that enameloid may be associated with dentin formation and is thus different from enamel (Davit-Beal et al 2007;Assaraf-Weill et al 2014). While it is not fully understood if enameloid and enamel are related evolutionarily, it has been shown that several species (newts, salamanders, some fish) that produce enameloid structures also have a mineral coating that appears to be enamel, since it is synthesized by ameloblasts Davit-Beal et al 2007;Diekwisch et al 2009). These layers reveal structural and compositional characteristics of enamel, since their matrices are composed of tuftelins, enamelins, amelogenins, and enamel-specific enzymes (Diekwisch et al 2002;Satchell et al 2002).…”

“…Enameloid is the term for the mineralized layer that covers teeth in chondrichthyans (sharks and rays), actinopterygians (bony fish), and early larval stages of caudate amphibians, e.g., newts and salamanders (Davit-Beal et al 2007). Enameloid develops from dental epithelial and odontoblast activity, and it is composed, at least initially, of matrices containing predominantly collagen (Sasagawa et al 2009).…”