Formation of a successional dental lamina in the zebrafish (Danio rerio): support for a local control of replacement tooth initiation

In order to test whether the formation of a replacement tooth bud in a continuously replacing dentition is linked to the functional state of the tooth predecessor, I examined the timing of development of replacement teeth with respect to their functional predecessors in the pharyngeal dentition of the zebrafish. Observations based on serial semithin sections of ten specimens, ranging in age from four week old juveniles to adults, indicate that (i) a replacement tooth germ develops at the distal end of an epithelial structure, called the successional dental lamina, budding off from the crypt epithelium surrounding the erupted part of a functional tooth; (ii) there appears to be a developmental link between the eruption of a tooth and the formation of a successional dental lamina and (iii) there can be a time difference between successional lamina formation and initiation of the new tooth germ, i.e., the successional dental lamina can remain quiescent for some time. The data suggest that the formation of a successional lamina and the differentiation of a replacement tooth germ from this lamina, are two distinct phases of a process and possibly under a different control. The strong spatio-temporal coincidence of eruption of a tooth and development of a successional dental lamina is seen as evidence for a local control over tooth replacement.     

Origin and evolution of gnathostome dentitions: a question of teeth and pharyngeal denticles in placoderms

The fossil group Placodermi is the most phylogenetically basal of the clade of jawed vertebrates but lacks a marginal dentition comparable to that of the dentate Chondrichthyes, Acanthodii and Osteichthyes (crown-group Gnathostomata). The teeth of crown-group gnathostomes are part of an ordered dentition replaced from, and patterned by, a dental lamina, exemplified by the elasmobranch model. A dentition recognised by these criteria has been previously judged absent in placoderms, based on structural evidence such as absence of tooth whorls and typical vertebrate dentine. However, evidence for regulated tooth addition in a precise spatiotemporal order can be observed in placoderms, but significantly, only within the group Arthrodira. In these fossils, as in other jawed vertebrates with statodont, non-replacing dentitions, new teeth are added at the ends of rows below the bite, but in line with biting edges of the dentition. The pattern is different on each gnathal bone and probably arises from single odontogenic primordia on each, but tooth rows are arranged in a distinctive placoderm pattern. New teeth are made of regular dentine comparable to that of crown-gnathostomes, formed from a pulp cavity. This differs from semidentine previously described for placoderm gnathalia, a type present in the external dermal tubercles. The Arthrodira is a derived taxon within the Placodermi, hence origin of teeth in placoderms occurs late in the phylogeny and teeth are convergently derived, relative to those of other jawed vertebrates. More basal placoderm taxa adopted other strategies for providing biting surfaces and these vary substantially, but include addition of denticles to the growing gnathal plates, at the margins of pre-existing denticle patches. These alternative strategies and apparent absence of regular dentine have led to previous interpretations that teeth were entirely absent from the placoderm dentition. A consensus view emerged that a dentition, as developed within a dental lamina, is a synapomorphy characterising the clade of crown-group gnathostomes. Recent comparisons between sets of denticle whorls in the pharyngeal region of the jawless fish Loganellia scotica (Thelodonti) and those in sharks suggest homology of these denticle sets on gill arches. Although the placoderm pharyngeal region appears to lack denticles (placoderm gill arches are poorly known), the posterior wall of the pharyngeal cavity, formed by a bony flange termed the postbranchial lamina, is covered in rows of patterned denticle arrays. These arrays differ significantly, both in morphology and arrangement, from those of the denticles located externally on the head and trunkshield plates. Denticles in these arrays are homologous to denticles associated with the gill arches in other crown-gnathostomes, with pattern similarities for order and position of pharyngeal denticles. From their location in the pharynx these are inferred to be under the influence of a cell lineage from endoderm, rather than ectoderm. Tooth sets and tooth whorls in crown-group gnathostomes are suggested to derive from the pharyngeal denticle whorls, at least in sharks, with the patterning mechanisms co-opted to the oral cavity. A comparable co-option is suggested for the Placodermi.     

Developmental and evolutionary origins of the vertebrate dentition: molecular controls for spatio-temporal organisation of tooth sites in osteichthyans

The rainbow trout (Oncorhynchus mykiss) as a developmental model surpasses both zebrafish and mouse for a more widespread distribution of teeth in the oro-pharynx as the basis for general vertebrate odontogenesis, one in which replacement is an essential requirement. Studies on the rainbow trout have led to the identification of the initial sequential appearance of teeth, through differential gene expression as a changing spatio-temporal pattern, to set in place the primary teeth of the first generation, and also to regulate the continuous production of replacement tooth families. Here we reveal gene expression data that address both the field and clone theories for patterning a polyphyodont osteichthyan dentition. These data inform how the initial pattern may be established through up-regulation at tooth loci from a broad odontogenic band. It appears that control and regulation of replacement pattern resides in the already primed dental epithelium at the sides of the predecessor tooth. A case is presented for the developmental changes that might have occurred during vertebrate evolution, for the origin of a separate successional dental lamina, by comparison with an osteichthyan tetrapod dentition (Ambystoma mexicanum). The evolutionary origins of such a permanent dental lamina are proposed to have occurred from the transient one demonstrated here in the trout. This has implications for phylogenies based on the homology of teeth as only those developed from a dental lamina. Utilising the data generated from the rainbow trout model, we propose this as a standard for comparative development and evolutionary theories of the vertebrate dentition.     

Early development of rostrum saw-teeth in a fossil ray tests classical theories of the evolution of vertebrate dentitions

In classical theory, teeth of vertebrate dentitions evolved from co-option of external skin denticles into the oral cavity. This hypothesis predicts that ordered tooth arrangement and regulated replacement in the oral dentition were also derived from skin denticles. The fossil batoid ray Schizorhiza stromeri (Chondrichthyes; Cretaceous) provides a test of this theory. Schizorhiza preserves an extended cartilaginous rostrum with closely spaced, alternating saw-teeth, different from sawfish and sawsharks today. Multiple replacement teeth reveal unique new data from micro-CT scanning, showing how the ‘cone-in-cone’ series of ordered saw-teeth sets arrange themselves developmentally, to become enclosed by the roots of pre-existing saw-teeth. At the rostrum tip, newly developing saw-teeth are present, as mineralized crown tips within a vascular, cartilaginous furrow; these reorient via two 90° rotations then relocate laterally between previously formed roots. Saw-tooth replacement slows mid-rostrum where fewer saw-teeth are regenerated. These exceptional developmental data reveal regulated order for serial self-renewal, maintaining the saw edge with ever-increasing saw-tooth size. This mimics tooth replacement in chondrichthyans, but differs in the crown reorientation and their enclosure directly between roots of predecessor saw-teeth. Schizorhiza saw-tooth development is decoupled from the jaw teeth and their replacement, dependent on a dental lamina. This highly specialized rostral saw, derived from diversification of skin denticles, is distinct from the dentition and demonstrates the potential developmental plasticity of skin denticles.     

Vertebrate dentitions at the origin of jaws: when and how pattern evolved

New evidence shows that teeth evolved with a greater degree of independence from jaws than previously considered. Pharyngeal denticles occur in jawless fish and also in early gnathostomes and precede jaw teeth in phylogeny. Many of these denticles form joined polarized sets on each branchial arch; these resemble whorl-shaped tooth sets on the jaws of stem and crown gnathostomes and are proposed as homologous units. Therefore, the source of patterning of these pharyngeal denticle and tooth sets is conserved from jawless conditions. It is proposed that developmental regulatory systems, responsible for all such tooth patterns on the jaws, are co-opted from the pharyngeal region and not from the skin as classically understood. This strongly implicates embryonic endoderm as opposed to ectoderm in the genetic control of dentition patterning. New interpretations of ontogenetic data on patterning dentitions of extant sharks are proposed, together with those of osteichthyan fish. Two entirely fossil groups, placoderms and acanthodians, at the base of gnathostome phylogeny are reassessed on the basis of a new model. It is concluded that within stem group and crown group gnathostomes several different strategies, unique to each taxon, were adopted to produce different developmental models of dentition patterning from pharyngeal denticles. One shared developmental pattern is that of initiation from primordial tooth sites, independently in each dentate zone of the jaws. The new model is proposed as a framework for data on evolutionary developmental genetics.     

A revised hypothesis on the evolutionary origin of the vertebrate dentition

Despite claims to the contrary, the evolutionary origin of teeth has not been definitely established. The classical ‘outside in’ theory stating that teeth derive from odontodes that invaded the oral cavity in conjunction with the origin of jaws has been challenged by an alternative, ‘inside out’, hypothesis suggesting that teeth evolved from pharyngeal denticles, as endodermal derivatives, prior to the origin of jaws. We propose a third scenario, a revised ‘outside in’ hypothesis ( Huysseune et al., 2009 ). Our hypothesis is consistent with the current data and avoids speculations about convergent tooth evolution. We suggest that teeth may indeed have arisen before the origin of jaws, a pillar of the ‘inside out’ hypothesis, but not from the endodermally lined posterior pharynx. Rather, teeth would have been the result of competent, odontode‐forming ectoderm invading the oropharyngeal cavity through the mouth as well as through the gill slits, interacting with neural‐crest derived mesenchyme. Arguments in support of this hypothesis are: (i) the observation that pharyngeal teeth are present only in species known to possess gill slits, and disappear from the pharyngeal region in early tetrapods concomitant with the closure of gill slits; (ii) the assumption that endoderm alone, together with neural crest, cannot form teeth; (iii) observations on pharyngeal tooth and gill slit formation in extant species; (iv) the observation that the dental lamina (sensu Reif, 1982 ) is not a prerequisite for tooth formation; (v) evidence that patterning does not distinguish pharyngeal from skin denticles, and (vi) the observation on zebrafish mutants affected in the dermal skeleton. This ‘modified outside in’ hypothesis can be tested both on paleontological data (it predicts a correlation of the presence of pharyngeal teeth and of gill slits), and on developmental data in extant species (it predicts the necessity of an ectodermal signal to make [pharyngeal] teeth).     

Plate-like permanent dental laminae of upper jaw dentition in adult gobiid fish, Sicyopterus japonicus

Sicyopterus japonicus (Teleostei, Gobiidae) possesses a unique upper jaw dentition different from that known for any other teleosts. In the adults, many (up to 30) replacement teeth, from initiation to attachment, are arranged orderly in a semicircular-like strand within a capsule of connective tissue on the labial side of each premaxillary bone. We have applied histological, ultrastructural, and three-dimensional imaging from serial sections to obtain insights into the distribution and morphological features of the dental lamina in the upper jaw dentition of adult S. japonicus. The adult fish has numerous permanent dental laminae, each of which is an infolding of the oral epithelium at the labial side of the functional tooth and forms a thin plate-like structure with a wavy contour. All replacement teeth of a semicircular-like strand are connected to the plate-like dental lamina by the outer dental epithelium and form a tooth family; neighboring tooth families are completely separated from each other. The new tooth germ directly buds off from the ventro-labial margin of the dental lamina, whereas no distinct free end of the dental lamina is present, even adjacent to this region. Cell proliferation concentrated at the ventro-labial margin of the dental lamina suggests that this region is the site for repeated tooth initiation. During tooth development, the replacement tooth migrates along a semicircular-like strand and eventually erupts through the dental lamina into the oral epithelium at the labial side of the functional tooth. This unique thin plate-like permanent dental lamina and the semicircular-like strand of replacement teeth in the upper jaw dentition of adult S. japonicus probably evolved as a dental adaptation related to the rapid replacement of teeth dictated by the specialized feeding habit of this algae-scraping fish.     

From Embryo to Adult: The Complete Development and Unusual Replacement of the Dentition of the Tammar Wallaby (Macropus eugenii)

Unlike their reptile-like ancestors with continuous tooth replacement, mammals have evolved to replace each tooth either only once, or not at all. In previous large-scale comparative studies, it has been suggested that this tooth replacement only occurs from a successional dental lamina produced lingually to the primary tooth. This study aims to document the complete tooth development and replacement pattern of the tammar wallaby (Macropus eugenii). The tammar wallaby is a diprotodont marsupial, a group defined by their two procumbent lower incisors. To provide a comprehensive documentation of the spatio-temporal pattern of tooth development, we used Lugol’s Iodine staining and microCT scanning (diceCT) of embryos and pouch young into adulthood, resulting in high resolution 3D models for both soft and mineralised stages of development for all tooth positions. Our results reveal that the eponymous lower incisors are the successional generation at the third incisor locus, where the primary dentition initiates but never erupts. Furthermore, we track the development of the only replacement tooth, the permanent third premolar (P3), from initiation to eruption, and found it develops from the primary dental lamina, mesial to the dP3. This is contrary to the conventional view of lingual replacement from successional lamina in mammals. Our findings indicate that no functional tooth replacement occurs in the tammar wallaby, and expands the diversity of tooth replacement patterns found in mammals. We also conclude that since almost all marsupial and placental mammals produce replacement teeth from the distalmost deciduous premolar, this tooth should be considered homologous in these two groups.

     

Placoderm fishes,pharyngeal denticles,and the vertebrate dentition

The correlation of the origin of teeth with jaws in vertebrate history has recently been challenged with an alternative to the canonical view of teeth deriving from separate skin denticles. This alternative proposes that organized denticle whorls on the pharyngeal (gill) arches in the fossil jawless fish Loganellia are precursors to tooth families developing from a dental lamina along the jaw, such as those occurring in sharks, acanthodians, and bony fishes. This not only indicates that homologs of tooth families were present, but also illustrates that they possessed the relevant developmental controls, prior to the evolution of jaws. However, in the Placodermi, a phylogenetically basal group of jawed fishes, the state of pharyngeal denticles is poorly known, tooth whorls are absent, and the presence of teeth homologous to those in extant jawed fishes (Chondrichthyes + Osteichthyes) is controversial. Thus, placoderms would seem to provide little evidence for the early evolution of dentitions, or of denticle whorls, or tooth families, at the base of the clade of jawed fishes. However, organized denticles do occur at the rear of the placoderm gill chamber, but are associated with the postbranchial lamina of the anterior trunkshield, assumed to be part of the dermal cover. Significantly, these denticles have a different organization and morphology relative to the external dermal trunkshield tubercles. We propose that they represent a denticulate part of the visceral skeleton, under the influence of pharyngeal patterning controls comparable to those for pharyngeal denticles in other jawed vertebrates and Loganellia.     

Teeth outside the mouth: The evolution and development of shark denticles

Vertebrate skin appendages are incredibly diverse. This diversity, which includes structures such as scales, feathers, and hair, likely evolved from a shared anatomical placode, suggesting broad conservation of the early development of these organs. Some of the earliest known skin appendages are dentine and enamel-rich tooth-like structures, collectively known as odontodes. These appendages evolved over 450 million years ago. Elasmobranchs (sharks, skates, and rays) have retained these ancient skin appendages in the form of both dermal denticles (scales) and oral teeth. Despite our knowledge of denticle function in adult sharks, our understanding of their development and morphogenesis is less advanced. Even though denticles in sharks appear structurally similar to oral teeth, there has been limited data directly comparing the molecular development of these distinct elements. Here, we chart the development of denticles in the embryonic small-spotted catshark (Scyliorhinus canicula) and characterize the expression of conserved genes known to mediate dental development. We find that shark denticle development shares a vast gene expression signature with developing teeth. However, denticles have restricted regenerative potential, as they lack a sox2+ stem cell niche associated with the maintenance of a dental lamina, an essential requirement for continuous tooth replacement. We compare developing denticles to other skin appendages, including both sensory skin appendages and avian feathers. This reveals that denticles are not only tooth-like in structure, but that they also share an ancient developmental gene set that is likely common to all epidermal appendages.     

Development of the dentition in Alligator mississippiensis: upper jaw dental and craniofacial development in embryos, hatchlings, and young juveniles, with a comparison to lower jaw development

Development of the upper dentition in Alligator mississippiensis was investigated using a close series of accurately staged and aged embryos, hatchlings, and young juveniles up to 11 days posthatching, as well as some young and old adult specimens. Studies from scanning electron microscopy, light microscopy, acetate and computer reconstructions, radiography and macroscopy were combined to elucidate the details of embryonic dental development, tooth initiation pattern, dentitional growth, and erupted functional dentition. The results were compared with those from the lower jaw and related to the development of other craniofacial structures. Approximately 17 early teeth in each jaw half develop as surface teeth, of which 13 project for 1 to 12 days before sinking into the mesenchyme. The first three teeth initiate directly from the oral epithelium at Ferguson stages 14-15 (days 15-19 after egg laying), before there is any local trace of dental lamina formation. All other teeth develop from a dental prolamina or lamina; and with progressive lamina development, submerged teeth initiate from the aboral end leading to the formation of replacement teeth. All teeth form dentin matrix, but 12 early teeth do not form enamel. Approximately 20 embryonic teeth are resorbed, 6 are transitional, and 42 function for longer periods after hatching. The embryonic tooth initiation pattern (illustrated by defining a tooth position formula) does not support the previous models of Odontostichi, Zahnreihen, and Tooth Families, each of which postulates perfect regularity. Up to three interstitial tooth positions develop between sites of primary tooth initiation, and families with up to five generations at hatching are at first arbitrarily defined.     

An Ancient Gene Network Is Co-opted for Teeth on Old and New Jaws

Vertebrate dentitions originated in the posterior pharynx of jawless fishes more than half a billion years ago. As gnathostomes (jawed vertebrates) evolved, teeth developed on oral jaws and helped to establish the dominance of this lineage on land and in the sea. The advent of oral jaws was facilitated, in part, by absence of hox gene expression in the first, most anterior, pharyngeal arch. Much later in evolutionary time, teleost fishes evolved a novel toothed jaw in the pharynx, the location of the first vertebrate teeth. To examine the evolutionary modularity of dentitions, we asked whether oral and pharyngeal teeth develop using common or independent gene regulatory pathways. First, we showed that tooth number is correlated on oral and pharyngeal jaws across species of cichlid fishes from Lake Malawi (East Africa), suggestive of common regulatory mechanisms for tooth initiation. Surprisingly, we found that cichlid pharyngeal dentitions develop in a region of dense hox gene expression. Thus, regulation of tooth number is conserved, despite distinct developmental environments of oral and pharyngeal jaws; pharyngeal jaws occupy hox-positive, endodermal sites, and oral jaws develop in hox-negative regions with ectodermal cell contributions. Next, we studied the expression of a dental gene network for tooth initiation, most genes of which are similarly deployed across the two disparate jaw sites. This collection of genes includes members of the ectodysplasin pathway, eda and edar, expressed identically during the patterning of oral and pharyngeal teeth. Taken together, these data suggest that pharyngeal teeth of jawless vertebrates utilized an ancient gene network before the origin of oral jaws, oral teeth, and ectodermal appendages. The first vertebrate dentition likely appeared in a hox-positive, endodermal environment and expressed a genetic program including ectodysplasin pathway genes. This ancient regulatory circuit was co-opted and modified for teeth in oral jaws of the first jawed vertebrate, and subsequently deployed as jaws enveloped teeth on novel pharyngeal jaws. Our data highlight an amazing modularity of jaws and teeth as they coevolved during the history of vertebrates. We exploit this diversity to infer a core dental gene network, common to the first tooth and all of its descendants.     

Fate of the Molar Dental Lamina in the Monophyodont Mouse

The successional dental lamina (SDL) plays an essential role in the development of replacement teeth in diphyodont and polyphyodont animals. A morphologically similar structure, the rudimental successional dental lamina (RSDL), has been described in monophyodont (only one tooth generation) lizards on the lingual side of the developing functional tooth. This rudimentary lamina regresses, which has been proposed to play a role in preventing the formation of future generations of teeth. A similar rudimentary lingual structure has been reported associated with the first molar in the monophyodont mouse, and we show that this structure is common to all murine molars. Intriguingly, a lingual lamina is also observed on the non-replacing molars of other diphyodont mammals (pig and hedgehog), initially appearing very similar to the successional dental lamina on the replacing teeth. We have analyzed the morphological as well as ultrastructural changes that occur during the development and loss of this molar lamina in the mouse, from its initiation at late embryonic stages to its disappearance at postnatal stages. We show that loss appears to be driven by a reduction in cell proliferation, down-regulation of the progenitor marker Sox2, with only a small number of cells undergoing programmed cell death. The lingual lamina was associated with the dental stalk, a short epithelial connection between the tooth germ and the oral epithelium. The dental stalk remained in contact with the oral epithelium throughout tooth development up to eruption when connective tissue and numerous capillaries progressively invaded the dental stalk. The buccal side of the dental stalk underwent keratinisation and became part of the gingival epithelium, while most of the lingual cells underwent programmed cell death and the tissue directly above the erupting tooth was shed into the oral cavity.     

Early development and replacement of the stickleback dentition

Teeth have long served as a model system to study basic questions about vertebrate organogenesis, morphogenesis, and evolution. In nonmammalian vertebrates, teeth typically regenerate throughout adult life. Fish have evolved a tremendous diversity in dental patterning in both their oral and pharyngeal dentitions, offering numerous opportunities to study how morphology develops, regenerates, and evolves in different lineages. Threespine stickleback fish (Gasterosteus aculeatus) have emerged as a new system to study how morphology evolves, and provide a particularly powerful system to study the development and evolution of dental morphology. Here, we describe the oral and pharyngeal dentitions of stickleback fish, providing additional morphological, histological, and molecular evidence for homology of oral and pharyngeal teeth. Focusing on the ventral pharyngeal dentition in a dense developmental time course of lab‐reared fish, we describe the temporal and spatial consensus sequence of early tooth formation. Early in development, this sequence is highly stereotypical and consists of seventeen primary teeth forming the early tooth field, followed by the first tooth replacement event. Comparing this detailed morphological and ontogenetic sequence to that described in other fish reveals that major changes to how dental morphology arises and regenerates have evolved across different fish lineages. J. Morphol. 277:1072–1083, 2016. © 2016 Wiley Periodicals, Inc.     

Making teeth to order: conserved genes reveal an ancient molecular pattern in paddlefish (Actinopterygii)

Ray-finned fishes (Actinopterygii) are the dominant vertebrate group today (+30 000 species, predominantly teleosts), with great morphological diversity, including their dentitions. How dental morphological variation evolved is best addressed by considering a range of taxa across actinopterygian phylogeny; here we examine the dentition of Polyodon spathula (American paddlefish), assigned to the basal group Acipenseriformes. Although teeth are present and functional in young individuals of Polyodon, they are completely absent in adults. Our current understanding of developmental genes operating in the dentition is primarily restricted to teleosts; we show that shh and bmp4, as highly conserved epithelial and mesenchymal genes for gnathostome tooth development, are similarly expressed at Polyodon tooth loci, thus extending this conserved developmental pattern within the Actinopterygii. These genes map spatio-temporal tooth initiation in Polyodon larvae and provide new data in both oral and pharyngeal tooth sites. Variation in cellular intensity of shh maps timing of tooth morphogenesis, revealing a second odontogenic wave as alternate sites within tooth rows, a dental pattern also present in more derived actinopterygians. Developmental timing for each tooth field in Polyodon follows a gradient, from rostral to caudal and ventral to dorsal, repeated during subsequent loss of teeth. The transitory Polyodon dentition is modified by cessation of tooth addition and loss. As such, Polyodon represents a basal actinopterygian model for the evolution of developmental novelty: initial conservation, followed by tooth loss, accommodating the adult trophic modification to filter-feeding.     

Gene deployment for tooth replacement in the rainbow trout (Oncorhynchus mykiss): a developmental model for evolution of the osteichthyan dentition

Repeated tooth initiation occurs often in nonmammalian vertebrates (polyphyodontism), recurrently linked with tooth shedding and in a definite order of succession. Regulation of this process has not been genetically defined and it is unclear if the mechanisms for constant generation of replacement teeth (secondary dentition) are similar to those used to generate the primary dentition. We have therefore examined the expression pattern of a sub-set of genes, implicated in tooth initiation in mouse, in relation to replacement tooth production in an osteichthyan fish (Oncorhynchus mykiss). Two epithelial genes pitx2, shh and one mesenchymal bmp4 were analyzed at selected stages of development for O. mykiss. pitx2 expression is upregulated in the basal outer dental epithelium (ODE) of the predecessor tooth and before cell enlargement, on the postero-lingual side only. This coincides with the site for replacement tooth production identifying a region responsible for further tooth generation. This corresponds with the expression of pitx2 at focal spots in the basal oral epithelium during initial (first generation) tooth formation but is now sub-epithelial in position and associated with the dental epithelium of each predecessor tooth. Co-incidental expression of bmp4 and aggregation of the mesenchymal cells identifies the epithelial-mesenchymal interactions and marks initiation of the dental papilla. These together suggest a role in tooth site regulation by pitx2 together with bmp4. Conversely, the expression of shh is confined to the inner dental epithelium during the initiation of the first teeth and is lacking from the ODE in the predecessor teeth, at sites identified as those for replacement tooth initiation. Importantly, these genes expressed during replacement tooth initiation can be used as markers for the sites of "set-aside cells," the committed odontogenic cells both epithelial and mesenchymal, which together can give rise to further generations of teeth. This information may show how initial pattern formation is translated into secondary tooth replacement patterns, as a general mechanism for patterning the vertebrate dentition. Replacement of the marginal sets of teeth serves as a basis for discussion of the evolutionary significance, as these dentate bones (dentary, premaxilla, maxilla) form the restricted arcades of oral teeth in many crown-group gnathostomes, including members of the tetrapod stem group.     

Reptilian tooth development

Dental patterns in vertebrates range from absence of teeth to multiple sets of teeth that are replaced throughout life. Despite this great variation, most of our understanding of tooth development is derived from studies on just a few model organisms. Here we introduce the reptile as an excellent model in which to study the molecular basis for early dental specification and, most importantly, for tooth replacement. We review recent snake studies that highlight the conserved role of Shh in marking the position of the odontogenic band. The distinctive molecular patterning of the dental lamina in the labial-lingual and oral-aboral axes is reviewed. We explain how these early signals help to specify the tooth-forming and non-tooth forming sides of the dental lamina as well as the presumptive successional lamina. Next, the simple architecture of the reptilian enamel organ is contrasted with the more complex, mammalian tooth bud and we discuss whether or not there is an enamel knot in reptilian teeth. The role of the successional lamina during tooth replacement in squamate reptiles is reviewed and we speculate on the possible formation of a vestigial, post-permanent dentition in mammals. In support of these ideas, we present data on agamid teeth in which development of a third generation is arrested. We suggest that in diphyodont mammals, similar mechanisms may be involved in reducing tooth replacement capacity. Finally, we review the location of label-retaining cells and suggest ways in which these putative dental epithelial stem cells contribute to continuous tooth replacement.