First-generation Teeth Research Articles

Continuous tooth replacement: what can teleost fish teach us?

Most tooth-bearing non-mammalian vertebrates have the capacity to replace their teeth throughout life. This capacity was lost in mammals, which replace their teeth only once at most. Not surprisingly, continuous tooth replacement has attracted much attention. Classical morphological studies (e.g. to analyse patterns of replacement) are now being complemented by molecular studies that investigate the expression of genes involved in tooth formation. This review focuses on ray-finned fish (actinopterygians), which have teeth often distributed throughout the mouth and pharynx, and more specifically on teleost fish, the largest group of extant vertebrates. First we highlight the diversity in tooth distribution and in tooth replacement patterns. Replacement tooth formation can start from a distinct (usually discontinuous and transient) dental lamina, but also in the absence of a successional lamina, e.g. from the surface epithelium of the oropharynx or from the outer dental epithelium of a predecessor tooth. The relationship of a replacement tooth to its predecessor is closely related to whether replacement is the result of a prepattern or occurs on demand. As replacement teeth do not necessarily have the same molecular signature as first-generation teeth, the question of the actual trigger for tooth replacement is discussed. Much emphasis has been laid in the past on the potential role of epithelial stem cells in initiating tooth replacement. The outcome of such studies has been equivocal, possibly related to the taxa investigated, and the permanent or transient nature of the dental lamina. Alternatively, replacement may result from local proliferation of undifferentiated progenitors, stimulated by hitherto unknown, perhaps mesenchymal, factors. So far, the role of the neurovascular link in continuous tooth replacement has been poorly investigated, despite the presence of a rich vascularisation surrounding actinopterygian (as well as chondrichthyan) teeth and despite a complete arrest of tooth replacement after nerve resection. Lastly, tooth replacement is possibly co-opted as a process to expand the number of teeth in a dentition ontogenetically whilst conserving features of the primary dentition. That neither a dental lamina, nor stem cells appear to be required for tooth replacement places teleosts in an advantageous position as models for tooth regeneration in humans, where the dental lamina regresses and epithelial stem cells are considered lost.

Effect of Embryonic Alcohol Exposure on Craniofacial and Skin Melanocyte Development: Insights from Zebrafish (Danio rerio).

Alcohol is a common addictive substance and prenatal alcohol exposure could cause fetal alcohol spectrum disorder (FASD) and can lead to various birth defects. The small teleost zebrafish (Danio rerio) has been identified as a fine animal model in developmental biology and toxicological research. Zebrafish models are widely used to study the harmful effects of alcohol and limited studies are available on the craniofacial and skin malformations associated with FASD. The present study attempts to investigate the effect of alcohol on early zebrafish embryonic development. The effects of prenatal alcohol exposure on neural crest cell-derived organ formation, including pharyngeal dentition, palatal bones and skin melanocytes were analysed. Whole-mount cartilage and bone staining and imaging techniques were applied to determine the effects of alcohol on the above-mentioned structures. The tooth size and shape were affected by alcohol exposure, but the number of teeth in the pharyngeal dentition was not affected. Only first-generation teeth showed size differences. The alcohol-exposed ethmoid bone, which is homologous to the human hard palate, was smaller and less dense in cell arrangement compared with the control medial ethmoid bone. The skin pigmentation defects included reduced melanocyte density, melanin contraction, smaller melanocyte surface area and aberrations in melanosome dispersion, revealing that alcohol significantly influenced and downregulated each and every step of the melanocyte developmental process. This descriptive study summarises the effects of alcohol on the development of neural crest cell-derived structures and highlights the importance of zebrafish in studying the phenotypic characteristics of fetal alcohol spectrum disorder.

Evolution and development of parrot pseudoteeth.

Parrot embryos carry peculiar appendages at their developing beak that have been described as pseudoteeth. To better characterize the pattern of development responsible for the emergence of these dental appendages, we examined parrot embryos combining conventional histology and microtomography approaches. Using immunohistochemistry, we observed the epithelial and mesenchymal expression of several proteins involved in tooth development in mammals. Parrot pseudoteeth arose by epithelial and mesenchymal evagination, and their early development was similar to the ontogeny of scales and feathers. There was no enamel tissue, and the evaginations were surrounded by the rhamphotheca. In adults, the rhamphotheca covers entirely the appendages, now represented by bone evaginations, which were more numerous in the lower than in the upper beak, being similar to the osseous teeth of the fossil Pelagornithidae. These embryonic pseudoteeth resembled reptile's first-generation teeth and dental appendages of chicken talpid2 mutants. Proteins involved in mammalian odontogenesis, such as SHH, BMP4, PITX2, and PAX9, were found to be generally expressed in beak epithelium and mesenchyme during parrot pseudoteeth development, with clusters of high-level expression in the pseudoteeth rudiments. This suggests that a similar, highly conserved gene expression program gives rise to the appearance of odontode derivatives in numerous species, despite their divergent developmental paths. These results provide new insights into the development and evolution of odontode-derived structures in vertebrates.

Miniaturization: How many cells are needed to build a tooth?

Organs that develop early in life, and are replaced by a larger version as the animal grows, often represent a miniature version of the adult organ. Teeth constituting the first functional dentition in small-sized teleost fish, such as medaka (Oryzias latipes), are examples of such miniature organs. With a dentin cone as small as the size of one human cell, or even smaller, these teeth raise the question how many dentin-producing cells (odontoblasts) are required to build such a tooth, and whether this number can be as little as one. Based on detailed observations with transmission electron microscopy (TEM) and TEM-based 3D-reconstructions, we show that only one mesenchymal cell qualifies as a true odontoblast. A second mesenchymal cell potentially participates in dentin formation, but only at a late stage of tooth development. Moreover, the fate of these cells appears to be specified very early during tooth development. Our observations indicate that in this system, one single odontoblast fulfills roles normally exerted by a large and communicating cell population. First-generation teeth in medaka thus provide an exciting model to study integration of multiple functions into a single cell.

Spatially restricted dental regeneration drives pufferfish beak development

Vertebrate dentitions are extraordinarily diverse in both morphology and regenerative capacity. The teleost order Tetraodontiformes exhibits an exceptional array of novel dental morphologies, epitomized by constrained beak-like dentitions in several families, i.e., porcupinefishes, three-toothed pufferfishes, ocean sunfishes, and pufferfishes. Modification of tooth replacement within these groups leads to the progressive accumulation of tooth generations, underlying the structure of their beaks. We focus on the dentition of the pufferfish (Tetraodontidae) because of its distinct dental morphology. This complex dentition develops as a result of (i) a reduction in the number of tooth positions from seven to one per quadrant during the transition from first to second tooth generations and (ii) a dramatic shift in tooth morphogenesis following the development of the first-generation teeth, leading to the elongation of dental units along the jaw. Gene expression and 1,1'-Dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) lineage tracing reveal a putative dental epithelial progenitor niche, suggesting a highly conserved mechanism for tooth regeneration despite the development of a unique dentition. MicroCT analysis reveals restricted labial openings in the beak, through which the dental epithelium (lamina) invades the cavity of the highly mineralized beak. Reduction in the number of replacement tooth positions coincides with the development of only four labial openings in the pufferfish beak, restricting connection of the oral epithelium to the dental cavity. Our data suggest the spatial restriction of dental regeneration, coupled with the unique extension of the replacement dental units throughout the jaw, are primary contributors to the evolution and development of this unique beak-like dentition.

Development of cyclic shedding teeth from semi-shedding teeth: the inner dental arcade of the stem osteichthyan Lophosteus.

The numerous cushion-shaped tooth-bearing plates attributed to the stem group osteichthyan Lophosteus superbus, which are argued here to represent an early form of the osteichthyan inner dental arcade, display a previously unknown and presumably primitive mode of tooth shedding by basal hard tissue resorption. They carry regularly spaced, recumbent, gently recurved teeth arranged in transverse tooth files that diverge towards the lingual margin of the cushion. Three-dimensional reconstruction from propagation phase-contrast synchrotron microtomography (PPC-SRµCT) reveals remnants of the first-generation teeth embedded in the basal plate, a feature never previously observed in any taxon. These teeth were shed by semi-basal resorption with the periphery of their bases retained as dentine rings. The rings are highly overlapped, which evidences tooth shedding prior to adding the next first-generation tooth at the growing edge of the plate. The first generation of teeth is thus diachronous. Successor teeth at the same sites underwent cyclical replacing and shedding through basal resorption, producing stacks of buried resorption surfaces separated by bone of attachment. The number and spatial arrangement of resorption surfaces elucidates that basal resorption of replacement teeth had taken place at the older tooth sites before the addition of the youngest first-generation teeth at the lingual margin. Thus, the replacement tooth buds cannot have been generated by a single permanent dental lamina at the lingual edge of the tooth cushion, but must have arisen either from successional dental laminae associated with the individual predecessor teeth, or directly from the dental epithelium of these teeth. The virtual histological dissection of these Late Silurian microfossils broadens our understanding of the development of the gnathostome dental systems and the acquisition of the osteichthyan-type of tooth replacement.

Blocking VEGF Signaling Delays Development of Replacement Teeth in Zebrafish

The dentition in zebrafish is extremely and richly vascularized, but the function of the vasculature, in view of the continuous replacement of the teeth, remains elusive. Through application of SU5416, a vascular endothelial growth factor receptor inhibitor, we studied the role of the blood vessels in the dentition of the zebrafish. We were unable to show an effect on the development of first-generation teeth as well as first tooth replacement. However, in juvenile fish, a delay was observed in the developmental state of the replacement tooth compared with what was expected based on the maturation state of the functional tooth. Furthermore, we observed a difference between treated and nontreated fish in the distance between blood vessels and developing replacement teeth. In conclusion, our results provide support for a nutritive, rather than an inductive, function of the vasculature in the process of tooth development and replacement.

Wnt signaling during tooth replacement in zebrafish (Danio rerio): pitfalls and perspectives.

The canonical (β-catenin dependent) Wnt signaling pathway has emerged as a likely candidate for regulating tooth replacement in continuously renewing dentitions. So far, the involvement of canonical Wnt signaling has been experimentally demonstrated predominantly in amniotes. These studies tend to show stimulation of tooth formation by activation of the Wnt pathway, and inhibition of tooth formation when blocking the pathway. Here, we report a strong and dynamic expression of the soluble Wnt inhibitor dickkopf1 (dkk1) in developing zebrafish (Danio rerio) tooth germs, suggesting an active repression of Wnt signaling during morphogenesis and cytodifferentiation of a tooth, and derepression of Wnt signaling during start of replacement tooth formation. To further analyse the role of Wnt signaling, we used different gain-of-function approaches. These yielded disjunct results, yet none of them indicating enhanced tooth replacement. Thus, masterblind (mbl) mutants, defective in axin1, mimic overexpression of Wnt, but display a normally patterned dentition in which teeth are replaced at the appropriate times and positions. Activating the pathway with LiCl had variable outcomes, either resulting in the absence, or the delayed formation, of first-generation teeth, or yielding a regular dentition with normal replacement, but no supernumerary teeth or accelerated tooth replacement. The failure so far to influence tooth replacement in the zebrafish by perturbing Wnt signaling is discussed in the light of (i) potential technical pitfalls related to dose- or time-dependency, (ii) the complexity of the canonical Wnt pathway, and (iii) species-specific differences in the nature and activity of pathway components. Finally, we emphasize the importance of in-depth knowledge of the wild-type pattern for reliable interpretations. It is hoped that our analysis can be inspiring to critically assess and elucidate the role of Wnt signaling in tooth development in polyphyodonts.

A comparison of the larval and juvenile dentition in Polypterus senegalus

Summary Polypterus senegalus is a freshwater fish belonging to the earliest diverged group of the actinopterygian lineage with currently living representatives. Its dentition has been well characterized in terms of distribution and shape of teeth in different life stages. Additionally, structural features of the first-generation teeth have been briefly described. However, at present it is not known how the primary dentition is patterned and if this pattern is similar to that found in juveniles. In this preliminary study, to answer this question we investigate the tooth pattern of a larval P. senegalus based on serial sections and 3D reconstructions, and compare dental features in larvae and juveniles. Polypterus senegalus passive apterolarvae possess a well developed primary dentition. Dentigerous bones develop either prior to the teeth (dentary, retroarticular), or as an attachment site concomitant with the expanding dentition (coronoid 1). Most teeth are attached and erupted, and thus considered functional, and their pulp houses a blood vessel, two features that contrast with earlier findings. The primary teeth do not display a level of maturation that could reflect a particular order of development. New teeth are added anteriorly, and likely also posteriorly, to allow the tooth row to grow to keep pace with the growing jaw. Replacement teeth are absent at this stage. Thus, the pattern observed in juveniles, with alternate (odd and even) positions displaying a different level of tooth maturation, is not present in the larva. Most strikingly, in the larva, considerable symmetry exists between apparently random patterns on both dentaries, eliciting intriguing questions on left-right control to be addressed in future studies.

Ameloblasts Express Type I Collagen during Amelogenesis

Enamel and enameloid, the highly mineralized tooth-covering tissues in living vertebrates, are different in their matrix composition. Enamel, a unique product of ameloblasts, principally contains enamel matrix proteins (EMPs), while enameloid possesses collagen fibrils and probably receives contributions from both odontoblasts and ameloblasts. Here we focused on type I collagen (COL1A1) and amelogenin (AMEL) gene expression during enameloid and enamel formation throughout ontogeny in the caudate amphibian, Pleurodeles waltl. In this model, pre-metamorphic teeth possess enameloid and enamel, while post-metamorphic teeth possess enamel only. In first-generation teeth, qPCR and in situ hybridization (ISH) on sections revealed that ameloblasts weakly expressed AMEL during late-stage enameloid formation, while expression strongly increased during enamel deposition. Using ISH, we identified COL1A1 transcripts in ameloblasts and odontoblasts during enameloid formation. COL1A1 expression in ameloblasts gradually decreased and was no longer detected after metamorphosis. The transition from enameloid-rich to enamel-rich teeth could be related to a switch in ameloblast activity from COL1A1 to AMEL synthesis. P. waltl therefore appears to be an appropriate animal model for the study of the processes involved during enameloid-to-enamel transition, especially because similar events probably occurred in various lineages during vertebrate evolution.

N-cadherin is Required for Cytodifferentiation during Zebrafish Odontogenesis

N-cadherin is a well-studied classic cadherin involved in multiple developmental processes and is also known to have a signaling function. Using the zebrafish (Danio rerio) as a model, we tested the hypothesis that tooth morphogenesis is accompanied by dynamic changes in N-cadherin distribution and that absence of N-cadherin disturbs tooth development. N-cadherin, encoded by the gene cdh2, is absent during the initiation and morphogenesis stages of both primary (first-generation) and replacement teeth, as demonstrated by immunohistochemistry. However, N-cadherin is up-regulated at the onset of differentiation of cells of the inner dental epithelium and the dental papilla, i.e., the ameloblasts and odontoblasts, respectively. In the inner dental epithelium, N-cadherin is co-expressed with E-cadherin, excluding the occurrence of cadherin switching such as observed during human tooth development. While early lethality of N-cadherin knockout mice prevents any functional study of N-cadherin in mouse odontogenesis, zebrafish parachute (pac) mutants, deficient for N-cadherin, survive beyond the age when primary teeth normally start to form. In these mutants, the first tooth forms, but its development stops at the early cytodifferentiation stage. N-cadherin deficiency also completely inhibits the development of the other first-generation teeth, possibly due to the absence of N-cadherin signaling once the first tooth has differentiated.

Replacing the first-generation dentition in pufferfish with a unique beak.

Teleost fishes comprise approximately half of all living vertebrates. The extreme range of diversity in teleosts is remarkable, especially, extensive morphological variation in their jaws and dentition. Some of the most unusual dentitions are found among members of the highly derived teleost order Tetraodontiformes, which includes triggerfishes, boxfishes, ocean sunfishes, and pufferfishes. Adult pufferfishes (Tetraodontidae) exhibit a distinctive parrot-like beaked jaw, forming a cutting edge, unlike in any other group of teleosts. Here we show that despite novelty in the structure and development of this "beak," it is initiated by formation of separate first-generation teeth that line the embryonic pufferfish jaw, with timing of development and gene expression patterns conserved from the last common ancestor of osteichthyans. Most of these first-generation larval teeth are lost in development. Continuous tooth replacement proceeds in only four parasymphyseal teeth, as sequentially stacked, multigenerational, jaw-length dentine bands, before development of the functional beak. These data suggest that dental novelties, such as the pufferfish beak, can develop later in ontogeny through modified continuous tooth addition and replacement. We conclude that even highly derived morphological structures like the pufferfish beak form via a conserved developmental bauplan capable of modification during ontogeny by subtle respecification of the developmental module.

Early Iron Deposition in Teeth of the Streamside Salamander, Ambystoma barbouri

Abstract In some salamander species, as well as various other vertebrates, iron-rich molecules are sequestered in the enamel and enameloid layers of the crowns of teeth. In adults, the presence of iron can be detected easily because the cusps are often tinted orange-brown. However, visual confirmation is not possible in embryos or larvae. Consequently, the ontogenetic stage during which iron is first deposited in teeth is not known. We used scanning electron microscopy and energy dispersive X-ray spectroscopy to examine the teeth of embryonic and larval Streamside Salamanders, Ambystoma barbouri, to determine the earliest developmental stage during which iron can be detected in teeth and to quantify the relative iron concentration along the length of the enamel cap. Iron was present in first-generation teeth of embryos when individuals were still acquiring nutrients from yolk, suggesting that yolk, rather than diet, was the source of iron deposited in early generation teeth of salamanders. Furthermore, as...

Zebrafish teeth as a model for repetitive epithelial morphogenesis: dynamics of E-cadherin expression

BackgroundThe development of teeth is the result of interactions between competent mesenchyme and epithelium, both of which undergo extensive morphogenesis. The importance of cell adhesion molecules in morphogenesis has long been acknowledged but remarkably few studies have focused on the distribution and function of these molecules in tooth development.ResultsWe analyzed the expression pattern of an important epithelial cadherin, E-cadherin, during the formation of first-generation teeth as well as replacement teeth in the zebrafish, using in situ hybridization and whole mount immunostaining to reveal mRNA expression and protein distribution. E-cadherin was detected in every layer of the enamel organ during the different stages of tooth development, but there were slight differences between first-generation and replacement teeth in the strength and distribution of the signal. The dental papilla, which is derived from the mesenchyme, did not show any expression. Remarkably, the crypts surrounding the functional teeth showed an uneven distribution of E-cadherin throughout the pharyngeal region.ConclusionsThe slight differences between E-cadherin expression in zebrafish teeth and developing mouse and human teeth are discussed in the light of fundamental differences in structural and developmental features of the dentition between zebrafish and mammals. Importantly, the uninterrupted expression of E-cadherin indicates that down-regulation of E-cadherin is not required for formation of an epithelial tooth bud. Further research is needed to understand the role of other cell adhesion systems during the development of teeth and the formation of replacement teeth.

Autocrine and paracrine Shh signaling are necessary for tooth morphogenesis, but not tooth replacement in snakes and lizards (Squamata)

Here we study the role of Shh signaling in tooth morphogenesis and successional tooth initiation in snakes and lizards (Squamata). By characterizing the expression of Shh pathway receptor Ptc1 in the developing dentitions of three species (Eublepharis macularius, Python regius, and Pogona vitticeps) and by performing gain- and loss-of-function experiments, we demonstrate that Shh signaling is active in the squamate tooth bud and is required for its normal morphogenesis. Shh apparently mediates tooth morphogenesis by separate paracrine- and autocrine-mediated functions. According to this model, paracrine Shh signaling induces cell proliferation in the cervical loop, outer enamel epithelium, and dental papilla. Autocrine signaling within the stellate reticulum instead appears to regulate cell survival. By treating squamate dental explants with Hh antagonist cyclopamine, we induced tooth phenotypes that closely resemble the morphological and differentiation defects of vestigial, first-generation teeth in the bearded dragon P. vitticeps. Our finding that these vestigial teeth are deficient in epithelial Shh signaling further corroborates that Shh is needed for the normal development of teeth in snakes and lizards. Finally, in this study, we definitively refute a role for Shh signaling in successional dental lamina formation and conclude that other pathways regulate tooth replacement in squamates.

An evolutionary view on tooth development and replacement in wild Atlantic salmon (Salmo salar L.)

To gain an insight into the evolution of tooth replacement mechanisms, we studied the development of first-generation and replacement teeth on the dentary of wild Atlantic salmon (Salmo salar L.), a protacanthopterygian teleost, using serially sectioned heads of early posthatching stages as well as adults. First-generation teeth develop within the oral epithelium. The anlage of the replacement tooth is first seen as a placode-like thickening of the outer dental epithelium of the predecessor, at its lingual and caudal side. Ongoing development of the replacement tooth germ is characterized by the elaboration of a population of epithelial cells, termed here the middle dental epithelium, apposed to the inner dental epithelium on the lingual side of the tooth germ. Before the formation of the new successor, a single-layered outer dental epithelium segregates from the middle dental epithelium. The dental organs of the predecessor and the successor remain broadly interconnected. The absence of a discrete successional dental lamina in salmon stands in sharp contrast to what is observed in other teleosts, even those that share with salmon the extraosseous formation of replacement teeth. The mode of tooth replacement in Atlantic salmon displays several characters similar to those observed in the shark Squalus acanthias. To interpret similarities in tooth replacement between Atlantic salmon and chondrichthyans as a case of convergence, or to see them as a result of a heterochronic shift, requires knowledge on the replacement process in more basal actinopterygian lineages. The possibility that the middle dental epithelium functionally substitutes for a successional lamina, and could be a source of stem cells, whose descendants subsequently contribute to the placode of the new replacement tooth, needs to be explored.

Establishment, maintenance and modifications of the lower jaw dentition of wild Atlantic salmon (Salmo salar L.) throughout its life cycle

In this paper we elucidate the pattern of initiation of the first teeth and the pattern of tooth replacement on the dentary of wild Atlantic salmon (Salmo salar L.), throughout nearly all stages of its life cycle, using serially sectioned heads and jaws, cleared and stained animals, and X-rays. The dentary teeth are set in one row. Tooth germs appear around hatching, first in odd positions, followed by even positions. From position 8 further backwards, teeth are added in adjacent positions. The first replacement teeth appear in animals of about 30 mm fork length. On the dentary of early life stages (alevins and fry), every position in the tooth row holds a functional (i.e. attached and erupted) tooth and a replacement tooth. The alternating pattern set up anteriorly in the dentary by the first-generation teeth changes in juveniles (parr) whereby teeth are in a similar functional (for the erupted teeth) or developmental stage (for the replacement teeth) every three positions. This pattern is also observed in marine animals during their marine life phase and in both sexes of adult animals prior to spawning (grilse and salmon), but every position now holds either a functional tooth or a mineralised replacement tooth. This is likely due to the fact that replacement tooth germs have to grow to a larger size before mineralisation starts. In the following spring, the dentary tooth pattern of animals that have survived spawning (kelts) is highly variable. The abundance of functional teeth in post-spawning animals nevertheless indicates that teeth are not lost over winter. We confirm the earlier reported lack of evidence for the existence of an edentulous life phase, preceding the appearance of so-called breeding teeth during upstream migration to the spawning grounds, and consider breeding teeth to be just another tooth generation in a regularly replacing dentition. This study shows how Atlantic salmon maintains a functional adaptive dentition throughout its complex life cycle.

The Development of Archosaurian First-Generation Teeth in a Chicken Mutant

Modern birds do not have teeth. Rather, they develop a specialized keratinized structure, called the rhamphotheca, that covers the mandible, maxillae, and premaxillae. Although recombination studies have shown that the avian epidermis can respond to tooth-inductive cues from mouse or lizard oral mesenchyme and participate in tooth formation, attempts to initiate tooth development de novo in birds have failed. Here, we describe the formation of teeth in the talpid2 chicken mutant, including the developmental processes and early molecular changes associated with the formation of teeth. Additionally, we show recapitulation of the early events seen in talpid2 after in vivo activation of beta-catenin in wild-type embryos. We compare the formation of teeth in the talpid2 mutant with that in the alligator and show the formation of decidedly archosaurian (crocodilian) first-generation teeth in an avian embryo. The formation of teeth in the mutant is coupled with alterations in the specification of the oral/aboral boundary of the jaw. We propose an epigenetic model of the developmental modification of dentition in avian evolution; in this model, changes in the relative position of a lateral signaling center over competent odontogenic mesenchyme led to loss of teeth in avians while maintaining tooth developmental potential.

Developmental mechanisms underlying tooth patterning in continuously replacing osteichthyan dentitions

The dentition of osteichthyans presents an astonishing diversity with regard to the distribution of teeth in the oral cavity, tooth numbers, arrangements, shapes, and sizes. Taking examples from three unrelated teleosts--the most speciose group of osteichthyans--and from the literature, this study explores how the initial tooth pattern is set up, and how this relates to the establishment and maintenance (or modification) of the tooth replacement pattern. In teleosts, first-generation teeth (the very first teeth in ontogeny to develop at a particular locus) are commonly initiated in adjacent or in alternate (odd and even) positions. The mechanisms responsible for these divergent developmental patterns remain to be elucidated, in particular, whether they reflect a field or local type of control. However, patterns of adjacent or alternate tooth initiation, set up by the first-generation teeth, can easily turn into replacement patterns where new teeth are initiated simultaneously every second, or even every third position, by synchronizing the formation of new first-generation teeth to the formation of replacement teeth at older loci. Our observations suggest that, once established, the replacement pattern appears to be maintained, as a kind of "default" state. Variations and modifications in this pattern are nevertheless common and suggest that tooth replacement is under local control, exerted at the level of the initiation of replacement teeth. Further studies are needed to test the hypothesis that regular replacement patterns are more frequent in association with the plesiomorphic condition of extramedullary replacement (replacement on the surface of the dentigerous bone) and more rare in the derived condition of intramedullary replacement (replacement within the medullary cavity of the dentigerous bone).

Tooth development in a scincid lizard, Chalcides viridanus (Squamata), with particular attention to enamel formation

Comparative analysis of tooth development in the main vertebrate lineages is needed to determine the various evolutionary routes leading to current dentition in living vertebrates. We have used light, scanning and transmission electron microscopy to study tooth morphology and the main stages of tooth development in the scincid lizard, Chalcides viridanus, viz., from late embryos to 6-year-old specimens of a laboratory-bred colony, and from early initiation stages to complete differentiation and attachment, including resorption and enamel formation. In C. viridanus, all teeth of a jaw have a similar morphology but tooth shape, size and orientation change during ontogeny, with a constant number of tooth positions. Tooth morphology changes from a simple smooth cone in the late embryo to the typical adult aspect of two cusps and several ridges via successive tooth replacement at every position. First-generation teeth are initiated by interaction between the oral epithelium and subjacent mesenchyme. The dental lamina of these teeth directly branches from the basal layer of the oral epithelium. On replacement-tooth initiation, the dental lamina spreads from the enamel organ of the previous tooth. The epithelial cell population, at the dental lamina extremity and near the bone support surface, proliferates and differentiates into the enamel organ, the inner (IDE) and outer dental epithelium being separated by stellate reticulum. IDE differentiates into ameloblasts, which produce enamel matrix components. In the region facing differentiating IDE, mesenchymal cells differentiate into dental papilla and give rise to odontoblasts, which first deposit a layer of predentin matrix. The first elements of the enamel matrix are then synthesised by ameloblasts. Matrix mineralisation starts in the upper region of the tooth (dentin then enamel). Enamel maturation begins once the enamel matrix layer is complete. Concomitantly, dental matrices are deposited towards the base of the dentin cone. Maturation of the enamel matrix progresses from top to base; dentin mineralisation proceeds centripetally from the dentin-enamel junction towards the pulp cavity. Tooth attachment is pleurodont and tooth replacement occurs from the lingual side from which the dentin cone of the functional teeth is resorbed. Resorption starts from a deeper region in adults than in juveniles. Our results lead us to conclude that tooth morphogenesis and differentiation in this lizard are similar to those described for mammalian teeth. However, Tomes' processes and enamel prisms are absent.