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

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.

Evolutionary developmental genetics of teeth and odontodes in jawed vertebrates: a perspective from the study of elasmobranchs

Most extant vertebrates display a high variety of tooth and tooth-like organs (odontodes) that vary in shape, position over the body and nature of composing tissues. The development of these structures is known to involve similar genetic cascades and teeth and odontodes are believed to share a common evolutionary history. Gene expression patterns have previously been compared between mammalian and teleost tooth development but we highlight how the comparative framework was not always properly defined to deal with different tooth types or tooth developmental stages. Larger-scale comparative analyses also included cartilaginous fishes: sharks display oral teeth and dermal scales for which the gene expression during development started to be investigated in the small-spotted catshark Scyliorhinus canicula during the past decade. We report several descriptive approaches to analyse the embryonic tooth and caudal scale gene expressions in S. canicula. We compare these expressions wih the ones reported in mouse molars and teleost oral and pharyngeal teeth and highlight contributions and biases that arise from these interspecific comparisons. We finally discuss the evolutionary processes that can explain the observed intra and interspecific similarities and divergences in the genetic cascades involved in tooth and odontode development in jawed vertebrates.

Distinct tooth regeneration systems deploy a conserved battery of genes

BACKGROUND

Vertebrate teeth exhibit a wide range of regenerative systems. Many species, including most mammals, reptiles, and amphibians, form replacement teeth at a histologically distinct location called the successional dental lamina, while other species do not employ such a system. Notably, a 'lamina-less' tooth replacement condition is found in a paraphyletic array of ray-finned fishes, such as stickleback, trout, cod, medaka, and bichir. Furthermore, the position, renewal potential, and latency times appear to vary drastically across different vertebrate tooth regeneration systems. The progenitor cells underlying tooth regeneration thus present highly divergent arrangements and potentials. Given the spectrum of regeneration systems present in vertebrates, it is unclear if morphologically divergent tooth regeneration systems deploy an overlapping battery of genes in their naïve dental tissues.

RESULTS

In the present work, we aimed to determine whether or not tooth progenitor epithelia could be composed of a conserved cell type between vertebrate dentitions with divergent regeneration systems. To address this question, we compared the pharyngeal tooth regeneration processes in two ray-finned fishes: zebrafish (Danio rerio) and threespine stickleback (Gasterosteus aculeatus). These two teleost species diverged approximately 250 million years ago and demonstrate some stark differences in dental morphology and regeneration. Here, we find that the naïve successional dental lamina in zebrafish expresses a battery of nine genes (bmpr1aa, bmp6, cd34, gli1, igfbp5a, lgr4, lgr6, nfatc1, and pitx2), while active Wnt signaling and Lef1 expression occur during early morphogenesis stages of tooth development. We also find that, despite the absence of a histologically distinct successional dental lamina in stickleback tooth fields, the same battery of nine genes (Bmpr1a, Bmp6, CD34, Gli1, Igfbp5a, Lgr4, Lgr6, Nfatc1, and Pitx2) are expressed in the basalmost endodermal cell layer, which is the region most closely associated with replacement tooth germs. Like zebrafish, stickleback replacement tooth germs additionally express Lef1 and exhibit active Wnt signaling. Thus, two fish systems that either have an organized successional dental lamina (zebrafish) or lack a morphologically distinct successional dental lamina (sticklebacks) deploy similar genetic programs during tooth regeneration.

CONCLUSIONS

We propose that the expression domains described here delineate a highly conserved "successional dental epithelium" (SDE). Furthermore, a set of orthologous genes is known to mark hair follicle epithelial stem cells in mice, suggesting that regenerative systems in other epithelial appendages may utilize a related epithelial progenitor cell type, despite the highly derived nature of the resulting functional organs.

The Effects of Premature Tooth Extraction and Damage on Replacement Timing in the Green Iguana

Reptiles with continuous tooth replacement, or polyphyodonty, replace their teeth in predictable, well-timed waves in alternating tooth positions around the mouth. This process is thought to occur irrespective of tooth wear or breakage. In this study, we aimed to determine if damage to teeth and premature tooth extraction affects tooth replacement timing long-term in juvenile green iguanas (Iguana iguana). First, we examined normal tooth development histologically using a BrdU pulse-chase analysis to detect label-retaining cells in replacement teeth and dental tissues. Next, we performed tooth extraction experiments for characterization of dental tissues after functional tooth (FT) extraction, including proliferation and β-Catenin expression, for up to 12 weeks. We then compared these results to a newly analyzed historical dataset of X-rays collected up to 7 months after FT damage and extraction in the green iguana. Results show that proliferation in the dental and successional lamina (SL) does not change after extraction of the FT, and proliferation occurs in the SL only when a tooth differentiates. Damage to an FT crown does not affect the timing of the tooth replacement cycle, however, complete extraction shifts the replacement cycle ahead by 4 weeks by removing the need for resorption of the FT. These results suggest that traumatic FT loss affects the timing of the replacement cycle at that one position, which may have implications for tooth replacement patterning around the entire mouth.

Development and regeneration of the crushing dentition in skates (Rajidae)

Sharks and rays (elasmobranchs) have the remarkable capacity to continuously regenerate their teeth. The polyphyodont system is considered the ancestral condition of the gnathostome dentition. Despite this shared regenerative ability, sharks and rays exhibit dramatic interspecific variation in their tooth morphology. Ray (batoidea) teeth typically constitute crushing pads of flattened teeth, whereas shark teeth are pointed, multi-cuspid units. Although recent research has addressed the molecular development of the shark dentition, little is known about that of the ray. Furthermore, how dental diversity within the elasmobranch lineage is achieved remains unknown. Here, we examine dental development and regeneration in two Batoid species: the thornback skate (Raja clavata) and the little skate (Leucoraja erinacea). Using in situ hybridization and immunohistochemistry, we examine the expression of a core gnathostome dental gene set during early development of the skate dentition and compare it to development in the shark. Elasmobranch tooth development is highly conserved, with sox2 likely playing an important role in the initiation and regeneration of teeth. Alterations to conserved genes expressed in an enamel knot-like signalling centre may explain the morphological diversity of elasmobranch teeth, thereby enabling sharks and rays to occupy diverse dietary and ecological niches.

The alternative regenerative strategy of bearded dragon unveils the key processes underlying vertebrate tooth renewal

Deep understanding of tooth regeneration is hampered by the lack of lifelong replacing oral dentition in most conventional models. Here, we show that the bearded dragon, one of the rare vertebrate species with both polyphyodont and monophyodont teeth, constitutes a key model for filling this gap, allowing direct comparison of extreme dentition types. Our developmental and high-throughput transcriptomic data of microdissected dental cells unveils the critical importance of successional dental lamina patterning, in addition to maintenance, for vertebrate tooth renewal. This patterning process happens at various levels, including directional growth but also gene expression levels, dynamics, and regionalization, and involves a large number of yet uncharacterized dental genes. Furthermore, the alternative renewal mechanism of bearded dragon dentition, with dual location of slow-cycling cells, demonstrates the importance of cell migration and functional specialization of putative epithelial stem/progenitor niches in tissue regeneration, while expanding the diversity of dental replacement strategies in vertebrates.

All multicellular organisms, from lizards to humans, must be able to repair and regrow damaged tissue. This includes not only healing after an injury, but also replacing parts of the body that suffer wear and tear. For example, many animals shed and replace worn out teeth throughout their life, but the number of times this occurs varies greatly between species.

Much of the understanding about how teeth grow and develop has come from researching mice. However, mice only develop one set of teeth, making them a poor ‘model’ for studying how species such as fish and reptiles can re-grow and replace their teeth. Recent studies of these species has shown that regenerating teeth relies on a specialised structure known as the dental lamina. In mice, the dental lamina forms but then quickly disappears, preventing new sets of teeth from developing. In most animals that regrow their teeth, however, the dental lamina keeps growing beyond the most recently produced tooth to create an area where its replacement will emerge. Now, Salomies et al. have identified other strategies involved in tooth replacement from studying the bearded dragon lizard, a rare example of an animal that continuously regenerates some, but not all, of its teeth.

Analysing the cells in different parts of the re-growing teeth from bearded dragon lizards revealed new features of the dental lamina. Specifically, Salomies et al. found that a previously uncharacterized set of genes within the dental lamina could determine whether or not a tooth will be replaced. Further experiments using microscope imaging revealed that bearded dragon lizards use two distinct groups of stem cells – specialised cells that have the potential to develop into various cell types in the body – to re-grow their teeth. These experiments demonstrate how the bearded dragon lizard uses a previously unknown mechanism to regenerate its teeth, combining elements used by gecko lizards and sharks.

These findings are an important step towards understanding the different strategies animals can use to maintain and regenerate their teeth. The knowledge gained could one day help design better therapies for patients suffering from inherited dental disorders or tooth loss.

Revitalising the rudimentary replacement dentition in the mouse

Most mammals have two sets of teeth (diphyodont) - a deciduous dentition replaced by a permanent dentition; however, the mouse possesses only one tooth generation (monophyodont). In diphyodonts, the replacement tooth forms on the lingual side of the first tooth from the successional dental lamina. This lamina expresses the stem/progenitor marker Sox2 and has activated Wnt/β-catenin signalling at its tip. Although the mouse does not replace its teeth, a transient rudimentary successional dental lamina (RSDL) still forms during development. The mouse RSDL houses Sox2-positive cells, but no Wnt/β-catenin signalling. Here, we show that stabilising Wnt/β-catenin signalling in the RSDL in the mouse leads to proliferation of the RSDL and formation of lingually positioned teeth. Although Sox2 has been shown to repress Wnt activity, overexpression of Wnts leads to a downregulation of Sox2, suggesting a negative-feedback loop in the tooth. In the mouse, the first tooth represses the formation of the replacement, and isolation of the RSDL is sufficient to induce formation of a new tooth germ. Our data highlight key mechanisms that may have influenced the evolution of replacement teeth.This article has an associated 'The people behind the papers' interview.

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.

Complex patterns of tooth replacement revealed in the fruit bat (Eidolon helvum)

How teeth are replaced during normal growth and development has long been an important question for comparative and developmental anatomy. Non-standard model animals have become increasingly popular in this field due to the fact that the canonical model laboratory mammal, the mouse, develops only one generation of teeth (monophyodonty), whereas the majority of mammals possess two generations of teeth (diphyodonty). Here we used the straw-coloured fruit bat (Eidolon helvum), an Old World megabat, which has two generations of teeth, in order to observe the development and replacement of tooth germs from initiation up to mineralization stages. Our morphological study uses 3D reconstruction of histological sections to uncover differing arrangements of the first and second-generation tooth germs during the process of tooth replacement. We show that both tooth germ generations develop as part of the dental lamina, with the first generation detaching from the lamina, leaving the free edge to give rise to a second generation. This separation was particularly marked at the third premolar locus, where the primary and replacement teeth become positioned side by side, unconnected by a lamina. The position of the replacement tooth, with respect to the primary tooth, varied within the mouth, with replacements forming posterior to or directly lingual to the primary tooth. Development of replacement teeth was arrested at some tooth positions and this appeared to be linked to the timing of tooth initiation and the subsequent rate of development. This study adds an additional species to the growing body of non-model species used in the study of tooth replacement, and offers a new insight into the development of the diphyodont condition.

Epithelial Label-Retaining Cells Are Absent during Tooth Cycling in Salmo salar and Polypterus senegalus

The Atlantic salmon (Salmo salar) and African bichir (Polypterus senegalus) are both actinopterygian fish species that continuously replace their teeth without the involvement of a successional dental lamina. Instead, they share the presence of a middle dental epithelium: an epithelial tier enclosed by inner and outer dental epithelium. It has been hypothesized that this tier could functionally substitute for a successional dental lamina and might be a potential niche to house epithelial stem cells involved in tooth cycling. Therefore, in this study we performed a BrdU pulse chase experiment on both species to (1) determine the localization and extent of proliferating cells in the dental epithelial layers, (2) describe cell dynamics and (3) investigate if label-retaining cells are present, suggestive for the putative presence of stem cells. Cells proliferate in the middle dental epithelium, outer dental epithelium and cervical loop at the lingual side of the dental organ to form a new tooth germ. Using long chase times, both in S. salar (eight weeks) and P. senegalus (eight weeks and twelve weeks), we could not reveal the presence of label-retaining cells in the dental organ. Immunostaining of P. senegalus dental organs for the transcription factor Sox2, often used as a stem cell marker, labelled cells in the zone of outer dental epithelium which grades into the oral epithelium (ODE transition zone) and the inner dental epithelium of a successor only. The location of Sox2 distribution does not provide evidence for epithelial stem cells in the dental organ and, more specifically, in the middle dental epithelium. Comparison of S. salar and P. senegalus reveals shared traits in tooth cycling and thus advances our understanding of the developmental mechanism that ensures lifelong replacement.

An ancient dental gene set governs development and continuous regeneration of teeth in sharks

The evolution of oral teeth is considered a major contributor to the overall success of jawed vertebrates. This is especially apparent in cartilaginous fishes including sharks and rays, which develop elaborate arrays of highly specialized teeth, organized in rows and retain the capacity for life-long regeneration. Perpetual regeneration of oral teeth has been either lost or highly reduced in many other lineages including important developmental model species, so cartilaginous fishes are uniquely suited for deep comparative analyses of tooth development and regeneration. Additionally, sharks and rays can offer crucial insights into the characters of the dentition in the ancestor of all jawed vertebrates. Despite this, tooth development and regeneration in chondrichthyans is poorly understood and remains virtually uncharacterized from a developmental genetic standpoint. Using the emerging chondrichthyan model, the catshark (Scyliorhinus spp.), we characterized the expression of genes homologous to those known to be expressed during stages of early dental competence, tooth initiation, morphogenesis, and regeneration in bony vertebrates. We have found that expression patterns of several genes from Hh, Wnt/β-catenin, Bmp and Fgf signalling pathways indicate deep conservation over ~450 million years of tooth development and regeneration. We describe how these genes participate in the initial emergence of the shark dentition and how they are redeployed during regeneration of successive tooth generations. We suggest that at the dawn of the vertebrate lineage, teeth (i) were most likely continuously regenerative structures, and (ii) utilised a core set of genes from members of key developmental signalling pathways that were instrumental in creating a dental legacy redeployed throughout vertebrate evolution. These data lay the foundation for further experimental investigations utilizing the unique regenerative capacity of chondrichthyan models to answer evolutionary, developmental, and regenerative biological questions that are impossible to explore in classical models.

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.

Evolutionary origins and development of saw-teeth on the sawfish and sawshark rostrum (Elasmobranchii; Chondrichthyes)

A well-known characteristic of chondrichthyans (e.g. sharks, rays) is their covering of external skin denticles (placoid scales), but less well understood is the wide morphological diversity that these skin denticles can show. Some of the more unusual of these are the tooth-like structures associated with the elongate cartilaginous rostrum 'saw' in three chondrichthyan groups: Pristiophoridae (sawsharks; Selachii), Pristidae (sawfish; Batoidea) and the fossil Sclerorhynchoidea (Batoidea). Comparative topographic and developmental studies of the 'saw-teeth' were undertaken in adults and embryos of these groups, by means of three-dimensional-rendered volumes from X-ray computed tomography. This provided data on development and relative arrangement in embryos, with regenerative replacement in adults. Saw-teeth are morphologically similar on the rostra of the Pristiophoridae and the Sclerorhynchoidea, with the same replacement modes, despite the lack of a close phylogenetic relationship. In both, tooth-like structures develop under the skin of the embryos, aligned with the rostrum surface, before rotating into lateral position and then attaching through a pedicel to the rostrum cartilage. As well, saw-teeth are replaced and added to as space becomes available. By contrast, saw-teeth in Pristidae insert into sockets in the rostrum cartilage, growing continuously and are not replaced. Despite superficial similarity to oral tooth developmental organization, saw-tooth spatial initiation arrangement is associated with rostrum growth. Replacement is space-dependent and more comparable to that of dermal skin denticles. We suggest these saw-teeth represent modified dermal denticles and lack the 'many-for-one' replacement characteristic of elasmobranch oral dentitions.

Indeterminate Growth: Could It Represent the Ancestral Condition?

Although we are used to the idea that many organisms stop growing when they reach a predictable size, in many taxa, growth occurs throughout the life of an organism, a phenomenon referred to as indeterminate growth. Our comparative analysis suggests that indeterminate growth may indeed represent the ancestral condition, whereas the permanent arrest of growth may be a more derived state. Consistent with this idea, in diverse taxa, the basal branches show indeterminate growth, whereas more derived branches arrest their growth. Importantly, in some closely related taxa, the termination of growth has evolved in mechanistically distinct ways. Also, even within a single organism, different organs can differ with respect to whether they terminate their growth or not. Finally, the study of tooth development indicates that, even at the level of a single tissue, multiple determinate patterns of growth can evolve from an ancestral one that is indeterminate.

Distinct developmental genetic mechanisms underlie convergently evolved tooth gain in sticklebacks

Teeth are a classic model system of organogenesis, as repeated and reciprocal epithelial and mesenchymal interactions pattern placode formation and outgrowth. Less is known about the developmental and genetic bases of tooth formation and replacement in polyphyodonts, which are vertebrates with continual tooth replacement. Here, we leverage natural variation in the threespine stickleback fish Gasterosteus aculeatus to investigate the genetic basis of tooth development and replacement. We find that two derived freshwater stickleback populations have both convergently evolved more ventral pharyngeal teeth through heritable genetic changes. In both populations, evolved tooth gain manifests late in development. Using pulse-chase vital dye labeling to mark newly forming teeth in adult fish, we find that both high-toothed freshwater populations have accelerated tooth replacement rates relative to low-toothed ancestral marine fish. Despite the similar evolved phenotype of more teeth and an accelerated adult replacement rate, the timing of tooth number divergence and the spatial patterns of newly formed adult teeth are different in the two populations, suggesting distinct developmental mechanisms. Using genome-wide linkage mapping in marine-freshwater F2 genetic crosses, we find that the genetic basis of evolved tooth gain in the two freshwater populations is largely distinct. Together, our results support a model whereby increased tooth number and an accelerated tooth replacement rate have evolved convergently in two independently derived freshwater stickleback populations using largely distinct developmental and genetic mechanisms.

Evolved tooth gain in sticklebacks is associated with a cis-regulatory allele of Bmp6

Developmental genetic studies of evolved differences in morphology have led to the hypothesis that cis-regulatory changes often underlie morphological evolution. However, because most of these studies focus on evolved loss of traits, the genetic architecture and possible association with cis-regulatory changes of gain traits are less understood. Here we show that a derived benthic freshwater stickleback population has evolved an approximate twofold gain in ventral pharyngeal tooth number compared with their ancestral marine counterparts. Comparing laboratory-reared developmental time courses of a low-toothed marine population and this high-toothed benthic population reveals that increases in tooth number and tooth plate area and decreases in tooth spacing arise at late juvenile stages. Genome-wide linkage mapping identifies largely separate sets of quantitative trait loci affecting different aspects of dental patterning. One large-effect quantitative trait locus controlling tooth number fine-maps to a genomic region containing an excellent candidate gene, Bone morphogenetic protein 6 (Bmp6). Stickleback Bmp6 is expressed in developing teeth, and no coding changes are found between the high- and low-toothed populations. However, quantitative allele-specific expression assays of Bmp6 in developing teeth in F1 hybrids show that cis-regulatory changes have elevated the relative expression level of the freshwater benthic Bmp6 allele at late, but not early, stages of stickleback development. Collectively, our data support a model where a late-acting cis-regulatory up-regulation of Bmp6 expression underlies a significant increase in tooth number in derived benthic sticklebacks.

Evolution and developmental diversity of tooth regeneration

This review considers the diversity observed during both the development and evolution of tooth replacement throughout the vertebrates in a phylogenetic framework from basal extant chondrichthyan fish and more derived teleost fish to mammals. We illustrate the conservation of the tooth regeneration process among vertebrate clades, where tooth regeneration refers to multiple tooth successors formed de novo for each tooth position in the jaws from a common set of retained dental progenitor cells. We discuss the conserved genetic mechanisms that might be modified to promote morphological diversity in replacement dentitions. We review current research and recent progress in this field during the last decade that have promoted our understanding of tooth diversity in an evolutionary developmental context, and show how tooth replacement and dental regeneration have impacted the evolution of the tooth-jaw module in vertebrates.

Aetiology of supernumerary teeth: a literature review

BACKGROUND

Supernumerary teeth are teeth, or tooth-like structures that have either erupted or remain unerupted in addition to the 20 primary and 32 permanent teeth.

AIMS

This paper attempts to (a) provide an overview of the proposed hypotheses and the current understanding of the aetiology of supernumerary teeth, and (b) review the published cases of supernumerary teeth occurring in families.

REVIEW

No studies have been able to distinguish between different aetiologies for the different locations of supernumerary teeth, while, from a developmental or molecular perspective, the proposed hypotheses may be plausible and explains the origin of different types of supernumerary teeth.

CONCLUSION

The only clearly evident feature, based on the existing published reports, is that it is logical to state that supernumerary teeth have a genetic component in their aetiology.

Common developmental pathways link tooth shape to regeneration

In many non-mammalian vertebrates, adult dentitions result from cyclical rounds of tooth regeneration wherein simple unicuspid teeth are replaced by more complex forms. Therefore and by contrast to mammalian models, the numerical majority of vertebrate teeth develop shape during the process of replacement. Here, we exploit the dental diversity of Lake Malawi cichlid fishes to ask how vertebrates generally replace their dentition and in turn how this process acts to influence resulting tooth morphologies. First, we used immunohistochemistry to chart organogenesis of continually replacing cichlid teeth and discovered an epithelial down-growth that initiates the replacement cycle via a labial proliferation bias. Next, we identified sets of co-expressed genes from common pathways active during de novo, lifelong tooth replacement and tooth morphogenesis. Of note, we found two distinct epithelial cell populations, expressing markers of dental competence and cell potency, which may be responsible for tooth regeneration. Related gene sets were simultaneously active in putative signaling centers associated with the differentiation of replacement teeth with complex shapes. Finally, we manipulated targeted pathways (BMP, FGF, Hh, Notch, Wnt/β-catenin) in vivo with small molecules and demonstrated dose-dependent effects on both tooth replacement and tooth shape. Our data suggest that the processes of tooth regeneration and tooth shape morphogenesis are integrated via a common set of molecular signals. This linkage has subsequently been lost or decoupled in mammalian dentitions where complex tooth shapes develop in first generation dentitions that lack the capacity for lifelong replacement. Our dissection of the molecular mechanics of vertebrate tooth replacement coupled to complex shape pinpoints aspects of odontogenesis that might be re-evolved in the lab to solve problems in regenerative dentistry.

Reprogramming of single-cell-derived mesenchymal stem cells into hair cell-like cells

HYPOTHESIS

Adult mesenchymal stem cells (MSCs) can be converted into hair cell-like cells by transdetermination.

BACKGROUND

Given the fundamental role sensory hair cells play in sound detection and the irreversibility of their loss in mammals, much research has focused on developing methods to generate new hair cells as a means of treating permanent hearing loss. Although MSCs can differentiate into multiple cell lineages, no efficient means of reprogramming them into sensory hair cells exists. Earlier work has shown that the transcription factor Atoh1 is necessary for early development of hair cells, but it is not clear whether Atoh1 can be used to convert MSCs into hair cells.

METHODS

Clonal MSC cell lines were established and reprogrammed into hair cell-like cells by a combination of protein transfer, adenoviral based gene transfer, and co-culture with neurons. During transdetermination, inner ear molecular markers were analyzed using reverse transcriptase-polymerase chain reaction, and cell structures were examined using immunocytochemistry.

RESULTS

Atoh1 overexpression in MSCs failed to convert MSCs into hair cell-like cells, suggesting that the ability of Atoh1 to induce hair cell differentiation is context dependent. Because Atoh1 overexpression successfully transforms VOT-E36 cells into hair cell-like cells, we modified the cell context of MSCs by performing a total protein transfer from VOT-E36 cells before overexpressing Atoh1. The modified MSCs were transformed into hair cell-like cells and attracted contacts from spiral ganglion neurons in a co-culture model.

CONCLUSION

We established a new procedure, consisting of VOT-E36 protein transfer, Atoh1 overexpression, and co-culture with spiral ganglion neurons, which can transform MSCs into hair cell-like cells.

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.

Molecular genetics of supernumerary tooth formation

Despite advances in the knowledge of tooth morphogenesis and differentiation, relatively little is known about the aetiology and molecular mechanisms underlying supernumerary tooth formation. A small number of supernumerary teeth may be a common developmental dental anomaly, while multiple supernumerary teeth usually have a genetic component and they are sometimes thought to represent a partial third dentition in humans. Mice, which are commonly used for studying tooth development, only exhibit one dentition, with very few mouse models exhibiting supernumerary teeth similar to those in humans. Inactivation of Apc or forced activation of Wnt/β(catenin signalling results in multiple supernumerary tooth formation in both humans and in mice, but the key genes in these pathways are not very clear. Analysis of other model systems with continuous tooth replacement or secondary tooth formation, such as fish, snake, lizard, and ferret, is providing insights into the molecular and cellular mechanisms underlying succesional tooth development, and will assist in the studies on supernumerary tooth formation in humans. This information, together with the advances in stem cell biology and tissue engineering, will pave ways for the tooth regeneration and tooth bioengineering.

The odontode explosion: the origin of tooth-like structures in vertebrates

Essentially we show recent data to shed new light on the thorny controversy of how teeth arose in evolution. Essentially we show (a) how teeth can form equally from any epithelium, be it endoderm, ectoderm or a combination of the two and (b) that the gene expression programs of oral versus pharyngeal teeth are remarkably similar. Classic theories suggest that (i) skin denticles evolved first and odontode-inductive surface ectoderm merged inside the oral cavity to form teeth (the 'outside-in' hypothesis) or that (ii) patterned odontodes evolved first from endoderm deep inside the pharyngeal cavity (the 'inside-out' hypothesis). We propose a new perspective that views odontodes as structures sharing a deep molecular homology, united by sets of co-expressed genes defining a competent thickened epithelium and a collaborative neural crest-derived ectomesenchyme. Simply put, odontodes develop 'inside and out', wherever and whenever these co-expressed gene sets signal to one another. Our perspective complements the classic theories and highlights an agenda for specific experimental manipulations in model and non-model organisms.

Evolution of developmental pattern for vertebrate dentitions: an oro-pharyngeal specific mechanism

Classically the oral dentition with teeth regulated into a successional iterative order was thought to have evolved from the superficial skin denticles migrating into the mouth at the stage when jaws evolved. The canonical view is that the initiation of a pattern order for teeth at the mouth margin required development of a sub-epithelial, permanent dental lamina. This provided regulated tooth production in advance of functional need, as exemplified by the Chondrichthyes. It had been assumed that teeth in the Osteichthyes form in this way as in tetrapods. However, this has been shown not to be true for many osteichthyan fish where a dental lamina of this kind does not form, but teeth are regularly patterned and replaced. We question the evolutionary origin of pattern information for the dentition driven by new morphological data on spatial initiation of skin denticles in the catshark. We review recent gene expression data for spatio-temporal order of tooth initiation for Scyliorhinus canicula, selected teleosts in both oral and pharyngeal dentitions, and Neoceratodus forsteri. Although denticles in the chondrichthyan skin appear not to follow a strict pattern order in space and time, tooth replacement in a functional system occurs with precise timing and spatial order. We suggest that the patterning mechanism observed for the oral and pharyngeal dentition is unique to the vertebrate oro-pharynx and independent of the skin system. Therefore, co-option of a successional iterative pattern occurred in evolution not from the skin but from mechanisms existing in the oro-pharynx of now extinct agnathans.

Ectoderm, endoderm, and the evolution of heterodont dentitions

Mammalian dentitions consist of different shapes/types of teeth that are positioned in different regions of the jaw (heterodont) whereas in many fish and reptiles all teeth are of similar type (homodont). The process by which heterodont dentitions have evolved in mammals is not understood. In many teleosts teeth develop in the pharynx from endoderm (endodermal teeth), whereas mammalian teeth develop from the oral ectoderm indicating that teeth can develop (and thus possibly evolve) via different mechanisms. In this article, we compare the molecular characteristics of pharyngeal/foregut endoderm with the molecular characteristics of oral ectoderm during mouse development. The expression domains of Claudin6, Hnf3beta, alpha-fetoprotein, Rbm35a, and Sox2 in the embryonic endoderm have boundaries overlapping the molar tooth-forming region, but not the incisor region in the oral ectoderm. These results suggest that molar teeth (but not incisors) develop from epithelium that shares molecular characteristics with pharyngeal endoderm. This opens the possibility that the two different theories proposed for the evolution of teeth may both be correct. Multicuspid (eg. molars) having evolved from the externalization of endodermal teeth into the oral cavity and monocuspid (eg. incisors) having evolved from internalization of ectodermal armour odontodes of ancient fishes. The two different mechanisms of tooth development may have provided the developmental and genetic diversity on which evolution has acted to produce heterodont dentitions in mammals.

Current knowledge of tooth development: patterning and mineralization of the murine dentition

The integument forms a number of different types of mineralized element, including dermal denticles, scutes, ganoid scales, elasmoid scales, fin rays and osteoderms found in certain fish, reptiles, amphibians and xenarthran mammals. To this list can be added teeth, which are far more widely represented and studied than any of the other mineralized elements mentioned above, and as such can be thought of as a model mineralized system. In recent years the focus for studies on tooth development has been the mouse, with a wealth of genetic information accrued and the availability of cutting edge techniques. It is the mouse dentition that this review will concentrate on. The development of the tooth will be followed, looking at what controls the shape of the tooth and how signals from the mesenchyme and epithelium interact to lead to formation of a molar or incisor. The number of teeth generated will then be investigated, looking at how tooth germ number can be reduced or increased by apoptosis, fusion of tooth germs, creation of new tooth germs, and the generation of additional teeth from existing tooth germs. The development of mineralized tissue will then be detailed, looking at how the asymmetrical deposition of enamel is controlled in the mouse incisor. The continued importance of epithelial-mesenchymal interactions at these later stages of tooth development will also be discussed. Tooth anomalies and human disorders have been well covered by recent reviews, therefore in this paper we wish to present a classical review of current knowledge of tooth development, fitting together data from a large number of recent research papers to draw general conclusions about tooth development.

Evolutionary and developmental origins of the vertebrate dentition

According to the classical theory, teeth derive from odontodes that invaded the oral cavity in conjunction with the origin of jaws (the 'outside in' theory). A recent alternative hypothesis suggests that teeth evolved prior to the origin of jaws as endodermal derivatives (the 'inside out' hypothesis). We compare the two theories in the light of current data and propose a third scenario, a revised 'outside in' hypothesis. We suggest that teeth may have arisen before the origin of jaws, as a 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. This hypothesis revives the homology between skin denticles (odontodes) and teeth. Our hypothesis is based on (1) the assumption that endoderm alone, together with neural crest, cannot form teeth; (2) 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, and (3) the observation that the dental lamina (sensu Reif, 1982) is not a prerequisite for teeth to form. We next discuss the progress that has been made to understand the spatially restricted loss of teeth from certain arches, and the many questions that remain regarding the ontogenetic loss of teeth in specific taxa. The recent advances that have been made in our knowledge on the molecular control of tooth formation in non-mammalians (mostly in some teleost model species) will undoubtedly contribute to answering these questions in the coming years.

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.

Molecular genetics of tooth development

Organogenesis depends upon a well-ordered series of inductive events involving coordination of molecular pathways that regulate the generation and patterning of specific cell types. Key questions in organogenesis involve the identification of the molecular mechanisms by which proteins interact to organize distinct pattern formation and cell fate determination. Tooth development is an excellent context for investigating this complex problem because of the wealth of information emerging from studies of model organisms and human mutations. Since there are no obvious sources of stem cells in adult human teeth, any attempt to create teeth de novo will probably require the reprogramming of other cell types. Thus, the fundamental understanding of the control mechanisms responsible for normal tooth patterning in the embryo will help us understand cell fate specificity and may provide valuable information towards tooth organ regeneration.

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.

The importance of signal pathway modulation in all aspects of tooth development

Most characteristics of tooth shape and pattern can be altered by modulating the signal pathways mediating epithelial-mesenchymal interactions in developing teeth. These regulatory signals function in complex networks, characterized by an abundance of activators or inhibitors. In addition, multiple specific inhibitors of all conserved signal pathways have been identified as modulators in tooth development. The number of teeth as well as molar cusp patterns can be modified by tinkering with several different signal pathways. The inhibition of any of the major conserved signal pathways in knockout mice leads to arrested tooth formation. On the other hand, the stimulation of the Wnt pathway in the oral epithelium in transgenic mice leads to abundant de novo tooth formation. The modulation of some of the signal pathways can rescue the development of vestigial tooth rudiments in the incisor and molar regions resulting in extra premolar-like teeth. The size and the degree of asymmetry of the continuously growing mouse incisor can be modulated by modifying the complex network of FGF, bone morphogenetic protein, and Activin signals, which regulate the proliferation and differentiation of epithelial stem cells. Follistatin, Sprouty, and Sostdc1 are important endogenous inhibitors antagonizing these pathways and they are also involved in regulation of enamel formation, and patterning of teeth in crown and root domains. All these findings support the hypothesis that the diversity of tooth types and dental patterns may have resulted from tinkering with the conserved signal pathways, organized into complex networks, during evolution.

Dental lamina as source of odontogenic stem cells: evolutionary origins and developmental control of tooth generation in gnathostomes

This study considers stem cells for odontogenic capability in biological tooth renewal in the broad context of gnathostome dentitions and the derivation of them from oral epithelium. The location of the developmental site and cell dynamics of the dental lamina are parameters of a possible source for odontogenic epithelial stem cells, but the phylogenetic history is not known. Understanding the phylogenetic basis for stem cell origins throughout continuous tooth renewal in basal jawed vertebrates is the ultimate objective of this study. The key to understanding the origin and location of stem cells in the development of the dentition is sequestration of stem cells locally for programmed tooth renewal. We suggest not only the initial pattern differences in each dentate field but local control subsequently for tooth renewal within each family. The role of the specialized odontogenic epithelium (odontogenic band) is considered as that in which the stem cells reside and become partitioned. These regulate time, position and shape in sequential tooth production. New histological data for chondrichthyan fish show first a thickening of the oral epithelium (odontogenic band). After this, all primary and successive teeth are only generated deep to the oral epithelium from a dental lamina. In contrast, in osteichthyan fish the first teeth develop directly within the odontogenic band. In addition, successors are initiated at each tooth site in the predecessor tooth germ (without a dental lamina). We suggest that stem cells specified for each tooth family are set up and located in intermediate cells between the outer and inner dental epithelia.

Genetic basis of tooth agenesis

Tooth agenesis or hypodontia, failure to develop all normally developing teeth, is one of the most common developmental anomalies in man. Common forms, including third molar agenesis and hypodontia of one or more of the incisors and premolars, constitute the great majority of cases. They typically affect those teeth that develop latest in each tooth class and these teeth are also most commonly affected in more severe and rare types of tooth agenesis. Specific vulnerability of the last developing teeth suggests that agenesis reflects quantitative defects during dental development. So far molecular genetics has revealed the genetic background of only rare forms of tooth agenesis. Mutations in MSX1, PAX9, AXIN2 and EDA have been identified in familial severe agenesis (oligodontia) and mutations in many other genes have been identified in syndromes in which tooth agenesis is a regular feature. Heterozygous loss of function mutations in many genes reduce the gene dose, whereas e.g. in hypohidrotic ectodermal dysplasia (EDA) the complete inactivation of the partially redundant signaling pathway reduces the signaling centers. Although these mechanisms involve quantitative disturbances, the phenotypes associated with mutations in different genes indicate that in addition to an overall reduction of odontogenic potential, tooth class-specific and more complex mechanisms are also involved. Although several of the genes so far identified in rare forms of tooth agenesis are being studied as candidate genes of common third molar agenesis and incisor and premolar hypodontia, it is plausible that novel genes that contribute to these phenotypes will also become identified.

The role of the dental lamina in mammalian tooth replacement

We have applied the ferret, Mustela putorius furo, as a model for tooth replacement. Ferret has a heterodont dentition, which includes all tooth families, and all antemolar teeth are replaced. Compared with mouse, the ferret therefore has a less derived mammalian dentition resembling that of humans. We have studied tooth replacement in serial histological sections in embryonic and young postnatal ferrets. Our observations indicate that the replacement teeth form from the dental lamina that is intimately connected to the lingual aspect of the deciduous tooth enamel organ. It grows as an offshoot from the enamel organ, elongates in cervical direction and later buds to give rise to the replacement tooth. The extent of the dental lamina growth, preceding replacement tooth budding, varied between different teeth. The dynamic gene expression patterns of Sostdc1, Shh and Axin2 brought new insight into the signal networks regulating the tooth replacement process. The distinct expression of Sostdc1 at the interface between the dental lamina and the deciduous tooth is the first indication of a specific tissue identity of the dental lamina. We suggest that the reactivation of a competent dental lamina is pivotal for the replacement tooth formation.

Functional analysis of Pitx during asexual regeneration in a basal chordate

Embryogenesis in ascidians is the classic example of mosaic development, yet within this phyla a number of colonial species exist which as adults can reproduce entire bodies asexually. The colonial ascidian Botryllus schlosseri is an excellent model to study this process: on a weekly basis it regenerates all somatic and germline tissues, and while these processes have been characterized morphologically at high-resolution over the last 70 years, almost nothing is known regarding the genetic basis of asexual development and its relationship to embryogenesis. In this study, we functionally characterized the role of the paired-related homeobox transcription factor, Pitx, during this regenerative process. During ascidian embryogenesis Pitx seems to be multifunctional and involved in the formation of multiple tissues, including the stomodeum, pituitary gland, and determination of left-right asymmetry, similar to other deuterostomes. Previous spatial-temporal expression studies during asexual regeneration in Botryllus adults suggest the same roles in this developmental program. Here, we analyzed Pitx function using RNA interference at distinct stages of asexual development. Pitx phenotypes were described focusing on each developmental stage both in vivo, and via histological analysis, and were found to correspond to expression patterns; with the exception that normal asymmetries in the gut were not affected by knockdown. As mRNA destruction is not instantaneous, we found that by tailoring our short interfering double-stranded RNA delivery different developmental processes could be studied independently. This allows a reverse genetic approach to dissect asexual developmental pathways, even in cases involving multifunctional, ubiquitously expressed genes like Pitx.

Spatial and temporal pattern for the dentition in the Australian lungfish revealed with sonic hedgehog expression profile

We report a temporal order of tooth addition in the Australian lungfish where timing of tooth induction is sequential in the same pattern as osteichthyans along the lower jaw. The order of tooth initiation in Neoceratodus starts from the midline tooth, together with left and right ones at jaw position 2, followed by 3 and then 1. This is the pattern order for dentary teeth of several teleosts and what we propose represents a stereotypic initiation pattern shared with all osteichthyans, including the living sister group to all tetrapods, the Australian lungfish. This is contrary to previous opinions that the lungfish dentition is otherwise derived and uniquely different. Sonic hedgehog (shh) expression is intensely focused on tooth positions at different times corresponding with their initiation order. This deployment of shh is required for lungfish tooth induction, as cyclopamine treatment results in complete loss of these teeth when applied before they develop. The temporal sequence of tooth initiation is possibly regulated by shh and is know to be required for dentition pattern in other osteichthyans, including cichlid fish and snakes. This reflects a shared developmental process with jawed vertebrates at the level of the tooth module but differs with the lack of replacement teeth.

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.

Reiterative pattern of sonic hedgehog expression in the catshark dentition reveals a phylogenetic template for jawed vertebrates

For a dentition representing the most basal extant gnathostomes, that of the shark can provide us with key insights into the evolution of vertebrate dentitions. To detail the pattern of odontogenesis, we have profiled the expression of sonic hedgehog, a key regulator of tooth induction. We find in the catshark (Scyliorhinus canicula) that intense shh expression first occurs in a bilaterally symmetrical pattern restricted to broad regions in each half of the dentition in the embryo jaw. As in the mouse, there follows a changing temporal pattern of shh spatial restriction corresponding to epithelial bands of left and right dental fields, but also a subfield for symphyseal teeth. Then, intense shh expression is restricted to loci coincident with a temporal series of teeth in iterative jaw positions. The developmental expression of shh reveals previously undetected timing within epithelial stages of tooth formation. Each locus at alternate, even then odd, jaw positions establishes precise sequential timing for successive replacement within each tooth family. Shh appears first in the central cusp, iteratively along the jaw, then reiteratively within each tooth for secondary cusps. This progressive, sequential restriction of shh is shared by toothed gnathostomes and conserved through 500 million years of evolution.

Unique and shared gene expression patterns in Atlantic salmon (Salmo salar) tooth development

To validate the use of Atlantic salmon (Salmo salar L.) as a model species in research on the mechanism of continuous tooth replacement, we have started to collect data on the molecular control underlying tooth formation in this species. This study reports expression patterns in the lower jaw dentition of a number of key regulatory genes such as bmp2, bmp4, and sox9 and structural genes such as col1alpha 1 and osteocalcin (= bgp, Bone Gla Protein) by means of in situ hybridization using salmon-specific, digoxygenin-labeled antisense riboprobes. We compare expression of these genes to that in other skeletogenic cells in the lower jaw (osteoblasts, chondroblasts, and chondrocytes). Our studies reveal both expression patterns that are in accordance to studies on mammalian tooth development and patterns that are specific to salmon, or teleosts. The epithelial expression of sox9 and a shift of the expression of bmp2 from epithelium to mesenchyme have also been observed during mammalian tooth development. Different from previous reports are the expressions of col1alpha 1 and osteocalcin. In contrast to what has been reported for zebrafish, osteocalcin is not expressed in odontoblasts, nor in the osteoblasts involved in the attachment of the teeth. At the lower jaw, osteocalcin is expressed in mature and/or resting osteoblasts only. As expected, col1alpha 1 is expressed in odontoblasts. Surprisingly, it is also strongly expressed in the inner dental epithelium, representing the first report of ameloblast involvement in collagen type I transcription. Whether the collagen is translated and secreted into the enameloid remains to be demonstrated.

A periodic pattern generator for dental diversity

Background

Periodic patterning of iterative structures is a fundamental process during embryonic organization and development. Studies have shown how gene networks are employed to pattern butterfly eyespots, fly bristles and vertebrate epithelial appendages such as teeth, feathers, hair and mammary glands. Despite knowledge of how these features are organized, little is known about how diversity in periodic patterning is generated in nature. We address this problem through the molecular analysis of oral jaw dental diversity in Lake Malawi cichlids, where closely related species exhibit from 1 to 20 rows of teeth, with total teeth counts ranging from around 10 to 700.

Results

We investigate the expression of conserved gene networks (involving bmp2, bmp4, eda, edar, fgf8, pax9, pitx2, runx2, shh and wnt7b) known to pattern iterative structures and teeth in other vertebrates. We show that spatiotemporal variation in expression pattern reflects adult morphological diversity among three closely related Malawi cichlid species. Combinatorial epithelial expression of pitx2 and shh appears to govern the competence both of initial tooth sites and future tooth rows. Epithelial wnt7b and mesenchymal eda are expressed in the inter-germ and inter-row regions, and likely regulate the spacing of these shh-positive units. Finally, we used chemical knockdown to demonstrate the fundamental role of hedgehog signalling and initial placode formation in the organization of the periodically patterned cichlid dental programme.

Conclusion

Coordinated patterns of gene expression differ among Malawi species and prefigure the future-ordered distribution of functional teeth of specific size and spacing. This variation in gene expression among species occurs early in the developmental programme for dental patterning. These data show how a complex multi-rowed vertebrate dentition is organized and how developmental tinkering of conserved gene networks during iterative pattern formation can impact upon the evolution of trophic novelty.

Zebrafish dentition in comparative context

Studies of the zebrafish (Danio rerio) promise to contribute much to an understanding of the developmental genetic mechanisms underlying diversification of the vertebrate dentition. Tooth development, structure, and replacement in the zebrafish largely reflect the primitive condition of jawed vertebrates, providing a basis for comparison with features of the more extensively studied mammalian dentition. A distinctive derived feature of the zebrafish dentition is restriction of teeth to a single pair of pharyngeal bones. Such reduction of the dentition, characteristic of the order Cypriniformes, has never been reversed, despite subsequent and extensive diversification of the group in numbers of species and variety of feeding modes. Studies of the developmental genetic mechanism of dentition reduction in the zebrafish suggest a potential explanation for irreversibility in that tooth loss seems to be associated with loss of developmental activators rather than gain of repressors. The zebrafish and other members of the family Cyprinidae exhibit species-specific numbers and arrangements of pharyngeal teeth, and extensive variation in tooth shape also occurs within the family. Mutant screens and experimental alteration of gene expression in the zebrafish are likely to yield variant tooth number and shape phenotypes that can be compared with those occurring naturally within the Cyprinidae. Such studies may reveal the relative contribution to trends in dental evolution of biases in the generation of variation and sorting of this variation by selection or drift.