In order to understand the process of ganoine formation on the ganoid scales, scale regeneration has been studied to overcome the lack of a growth series of scale ontogeny. Seven stages of ganoid scale regeneration have been defined over a period of five months in the polypterid fish Calamoichthys calabaricus. The study has been carried out using transmission electron microscopic techniques. After wound healing and differentiation of the osseous basal plate, a layer of vascular dentin is deposited at the upper surface of the basal plate owing to the presence there of odontoblasts closely applied to the dentin. When these cells move away, a close contact is then established between the stratified epidermis and the regenerating scale. Numerous alterations of the epidermal-dermal boundary occur until its disappearance and a thick layer of pre-ganoine is formed. This layer is progressively mineralized; and finally an organic intermediate layer differentiates between the ganoine, which is a hyper-mineralized tissue, and the overlying epidermis. This ultrastructural study demonstrates rather unequivocally the involvement of the inner epidermal layer (IEL) in the appearance and growth of the ganoine. It is suggested that these epidermal cells can be compared functionally to the inner dental epithelium (IDE) described during mammal tooth morphogenesis. Consequently, our results allow us to propose that ganoine can be identified as true enamel, although additional data are necessary to analyze the proteinaceous component or its organic matrix.
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.
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