The ins and outs of the evolutionary origin of teeth

The origin of vertebrate teeth and evolution of sensory exoskeletons

The earliest record of tooth antecedents and the tissue dentine1,2, an early-vertebrate novelty, has been controversially represented by fragmentary Cambrian fossils identified as Anatolepis heintzi3-5. Anatolepis exoskeletons have the characteristic tubules of dentine that prompted their interpretation as the first precursors of teeth3, known as odontodes. Debates over whether Anatolepis is a legitimate vertebrate6-8 have arisen because of limitations in imaging and the lack of comparative exoskeletal tissues. Here, to resolve this controversy and understand the origin of dental tissues, we synchrotron-scanned diverse extinct and extant vertebrate and invertebrate exoskeletons. We find that the tubules of Anatolepis have been misidentified as dentine tubules and instead represent aglaspidid arthropod sensory sensilla structures9,10. Synchrotron scanning reveals that deep ultrastructural similarities between odontodes and sensory structures also extend to definitive vertebrate tissues. External odontodes of the Ordovician vertebrate Eriptychius11-13 feature large dentine tubules1 that are morphologically convergent with invertebrate sensilla. Immunofluorescence analysis shows that the external odontodes of extant chondrichthyans and teleosts retain extensive innervation suggestive of a sensory function akin to teeth14-16. These patterns of convergence and innervation reveal that dentine evolved as a sensory tissue in the exoskeleton of early vertebrates, a function retained in modern vertebrate teeth16. Middle-Ordovician fossils now represent the oldest known evidence for vertebrate dental tissues.

Evolution, development, and regeneration of tooth-like epithelial appendages in sharks

Sharks and their relatives are typically covered in highly specialized epithelial appendages embedded in the skin called dermal denticles; ancient tooth-like units (odontodes) composed of dentine and enamel-like tissues. These 'skin teeth' are remarkably similar to oral teeth of vertebrates and share comparable morphological and genetic signatures. Here we review the histological and morphological data from embryonic sharks to uncover characters that unite all tooth-like elements (odontodes), including teeth and skin denticles in sharks. In addition, we review the differences between the skin and oral odontodes that reflect their varied capacity for renewal. Our observations have begun to decipher the developmental and genetic shifts that separate these seemingly similar dental units, including elements of the regenerative nature in both oral teeth and the emerging skin denticles from the small-spotted catshark (Scyliorhinus canicula) and other chondrichthyan models. Ultimately, we ask what defines a tooth at both the molecular and morphological level. These insights aim to help us understand how nature makes, replaces and evolves a vast array of odontodes.

Morphogenesis of pteraspid heterostracan oral plates and the evolutionary origin of teeth

Teeth are a key vertebrate innovation; their evolution is generally associated with the origin of jawed vertebrates. However, tooth-like structures already occur in jawless stem-gnathostomes; heterostracans bear denticles and morphologically distinct tubercles on their oral plates. We analysed the histology of the heterostracan denticles and plates to elucidate their morphogenesis and test their homology to the gnathostome oral skeleton. We identified a general model of growth for heterostracan oral plates that exhibit proximal episodic addition of tubercle rows. The distal hook exhibits truncated lamellae compatible with resorption, but we observe growth layers to be continuous between denticles. The denticles show no evidence of patterns of apposition or replacement indicating tooth homology. The oral plates and dermal skeleton share the same histological layers. The denticles grew in a manner comparable to the oral plate tubercles and the rest of the dermal skeleton. Our test of phylogenetic congruence reveals that the distribution of internal odontodes is discontinuous, indicating that the capacity to form internal odontodes evolved independently several times among stem-gnathostomes. Our results support the 'outside-in' hypothesis and the origin of teeth through the spread of odontogenic competence from extra-oral to oral epithelia and the subsequent co-option to a tooth function in gnathostomes.

Periderm fate and independence of tooth formation are conserved across osteichthyans

BACKGROUND

Previous studies have reported that periderm (the outer ectodermal layer) in zebrafish partially expands into the mouth and pharyngeal pouches, but does not reach the medial endoderm, where the pharyngeal teeth develop. Instead, periderm-like cells, arising independently from the outer periderm, cover prospective tooth-forming epithelia and are crucial for tooth germ initiation. Here we test the hypothesis that restricted expansion of periderm is a teleost-specific character possibly related to the derived way of early embryonic development. To this end, we performed lineage tracing of the periderm in a non-teleost actinopterygian species possessing pharyngeal teeth, the sterlet sturgeon (Acipenser ruthenus), and a sarcopterygian species lacking pharyngeal teeth, the axolotl (Ambystoma mexicanum).

RESULTS

In sturgeon, a stratified ectoderm is firmly established at the end of gastrulation, with minimally a basal ectodermal layer and a surface layer that can be homologized to a periderm. Periderm expands to a limited extent into the mouth and remains restricted to the distal parts of the pouches. It does not reach the medial pharyngeal endoderm, where pharyngeal teeth are located. Thus, periderm in sturgeon covers prospective odontogenic epithelium in the jaw region (oral teeth) but not in the pharyngeal region. In axolotl, like in sturgeon, periderm expansion in the oropharynx is restricted to the distal parts of the opening pouches. Oral teeth in axolotl develop long before mouth opening and possible expansion of the periderm into the mouth cavity.

CONCLUSIONS

Restricted periderm expansion into the oropharynx appears to be an ancestral feature for osteichthyans, as it is found in sturgeon, zebrafish and axolotl. Periderm behavior does not correlate with presence or absence of oral or pharyngeal teeth, whose induction may depend on 'ectodermalized' endoderm. It is proposed that periderm assists in lumenization of the pouches to create an open gill slit. Comparison of basal and advanced actinopterygians with sarcopterygians (axolotl) shows that different trajectories of embryonic development converge on similar dynamics of the periderm: a restricted expansion into the mouth and prospective gill slits.

Fibroblast growth factor signalling regulates the development of tooth root

Fibroblast growth factor (FGF) signalling plays a crucial role in the morphogenesis of multiple tissues including teeth. While the role of the signal has been studied in tooth crown development, little is known about root development. Of several FGF ligands involved in hard tissue formation, we suggest that FGF18 regulates the development of murine tooth roots. We implanted FGF18-soaked heparin beads into the lower first molar tooth buds at postnatal day 6 (P6), followed by transplantation under the kidney capsule. After 3 weeks, FGF18 significantly facilitated root elongation and periodontal tissue formation compared to the control. In situ hybridisation showed that Fgf18 transcripts were initially localised in the dental pulp along Hertwig's epithelial root sheath at P6 and P10 and subsequently in the dental follicle cells at P14. Fgf receptors were expressed in various dental tissues during these stages. In vitro analysis using the dental pulp stem cells revealed that FGF18 inhibited cell proliferation and decreased expression levels of osteogenic markers, Runx2, Alpl and Sp7. Consistently, after 1 week of kidney capsule transplantation, FGF18 application did not induce the expression of Sp7 and Bsp, but upregulated Periostin in the apical region of dental mesenchyme in the grafted molar. These findings suggest that FGF18 facilitates molar root development by regulating the calcification of periodontal tissues.

Tooth pattern, development, and replacement in the yellow catfish, Pelteobagrus fulvidraco

Studies of teleost teeth are important for understanding the evolution and mechanisms of tooth development, replacement, and regeneration. Here, we used gross specimens, microcomputed tomography, and histological analysis to characterize tooth structure, development, and resorption patterns in adult Pelteobagrus fulvidraco. The oral and pharyngeal teeth are villiform and conical. Multiple rows of dentition are densely distributed and the tooth germ is derived from the epithelium. P. fulvidraco exhibits a discontinuous and non-permanent dental lamina. Epithelial cells surround the teeth and are separated into distinct tooth units by mesenchymal tissue. Tooth development is completed in the form of independent tooth units. P. fulvidraco does not undergo simultaneous tooth replacement. Based on tooth development and resorption status, five forms of teeth are present in adult P. fulvidraco: developing tooth germs, accompanied by relatively immature tooth germs; mature and well-mineralized tooth accompanied by one tooth germ; teeth that have begun resorption, but not completely fractured; fractured teeth with only residual attachment to the underlying bone; and teeth that are completely resorbed and detached. Seven biological stages of a tooth in P. fulvidraco were also described.

Evo Devo of the Vertebrates Integument

All living jawed vertebrates possess teeth or did so ancestrally. Integumental surface also includes the cornea. Conversely, no other anatomical feature differentiates the clades so readily as skin appendages do, multicellular glands in amphibians, hair follicle/gland complexes in mammals, feathers in birds, and the different types of scales. Tooth-like scales are characteristic of chondrichthyans, while mineralized dermal scales are characteristic of bony fishes. Corneous epidermal scales might have appeared twice, in squamates, and on feet in avian lineages, but posteriorly to feathers. In contrast to the other skin appendages, the origin of multicellular glands of amphibians has never been addressed. In the seventies, pioneering dermal-epidermal recombination between chick, mouse and lizard embryos showed that: (1) the clade type of the appendage is determined by the epidermis; (2) their morphogenesis requires two groups of dermal messages, first for primordia formation, second for appendage final architecture; (3) the early messages were conserved during amniotes evolution. Molecular biology studies that have identified the involved pathways, extending those data to teeth and dermal scales, suggest that the different vertebrate skin appendages evolved in parallel from a shared placode/dermal cells unit, present in a common toothed ancestor, c.a. 420 mya.

A shark-inspired general model of tooth morphogenesis unveils developmental asymmetries in phenotype transitions

Developmental complexity stemming from the dynamic interplay between genetic and biomechanic factors canalizes the ways genotypes and phenotypes can change in evolution. As a paradigmatic system, we explore how changes in developmental factors generate typical tooth shape transitions. Since tooth development has mainly been researched in mammals, we contribute to a more general understanding by studying the development of tooth diversity in sharks. To this end, we build a general, but realistic, mathematical model of odontogenesis. We show that it reproduces key shark-specific features of tooth development as well as real tooth shape variation in small-spotted catsharks Scyliorhinus canicula. We validate our model by comparison with experiments in vivo. Strikingly, we observe that developmental transitions between tooth shapes tend to be highly degenerate, even for complex phenotypes. We also discover that the sets of developmental parameters involved in tooth shape transitions tend to depend asymmetrically on the direction of that transition. Together, our findings provide a valuable base for furthering our understanding of how developmental changes can lead to both adaptive phenotypic change and trait convergence in complex, phenotypically highly diverse, structures.

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.

Complex enameloid microstructure of †Ischyrhiza mira rostral denticles

Serving in a foraging or self-defense capacity, pristiophorids, pristids, and the extinct sclerorhynchoids independently evolved an elongated rostrum lined with modified dermal denticles called rostral denticles. Isolated rostral denticles of the sclerorhynchoid Ischyrhiza mira are commonly recovered from Late Cretaceous North American marine deposits. Although the external morphology has been thoroughly presented in the literature, very little is known about the histological composition and organization of these curious structures. Using acid-etching techniques and scanning electron microscopy, we show that the microstructure of I. mira rostral denticles are considerably more complex than that of previously described dermal denticles situated elsewhere on the body. The apical cap consists of outer single crystallite enameloid (SCE) and inner bundled crystallite enameloid (BCE) overlying a region of orthodentine. The BCE has distinct parallel bundled enameloid (PBE), tangled bundled enameloid (TBE), and radial bundled enameloid (RBE) components. Additionally, the cutting edge of the rostral denticle is produced by a superficial layer of SCE and a deeper ridges/cutting edge layer (RCEL) of the BCE. The highly organized enameloid observed in the rostral denticles of this batomorph resembles that of the multifaceted tissue architecture observed in the oral teeth of selachimorphs and demonstrates that dermal scales have the capacity to evolve histologically similar complex tooth-like structures both inside and outside the oropharyngeal cavity.

The oldest gnathostome teeth

Mandibular teeth and dentitions are features of jawed vertebrates that were first acquired by the Palaeozoic ancestors^1-3 of living chondrichthyans and osteichthyans. The fossil record currently points to the latter part of the Silurian period^4-7 (around 425 million years ago) as a minimum date for the appearance of gnathostome teeth and to the evolution of growth and replacement mechanisms of mandibular dentitions in the subsequent Devonian period^2,8-10. Here we provide, to our knowledge, the earliest direct evidence for jawed vertebrates by describing Qianodus duplicis, a new genus and species of an early Silurian gnathostome based on isolated tooth whorls from Guizhou province, China. The whorls possess non-shedding teeth arranged in a pair of rows that demonstrate a number of features found in modern gnathostome groups. These include lingual addition of teeth in offset rows and maintenance of this patterning throughout whorl development. Our data extend the record of toothed gnathostomes by 14 million years from the late Silurian into the early Silurian (around 439 million years ago) and are important for documenting the initial diversification of vertebrates. Our analyses add to mounting fossil evidence that supports an earlier emergence of jawed vertebrates as part of the Great Ordovician Biodiversification Event (approximately 485-445 million years ago).

An epithelial signalling centre in sharks supports homology of tooth morphogenesis in vertebrates

Development of tooth shape is regulated by the enamel knot signalling centre, at least in mammals. Fgf signalling regulates differential proliferation between the enamel knot and adjacent dental epithelia during tooth development, leading to formation of the dental cusp. The presence of an enamel knot in non-mammalian vertebrates is debated given differences in signalling. Here, we show the conservation and restriction of fgf3, fgf10, and shh to the sites of future dental cusps in the shark (Scyliorhinus canicula), whilst also highlighting striking differences between the shark and mouse. We reveal shifts in tooth size, shape, and cusp number following small molecule perturbations of canonical Wnt signalling. Resulting tooth phenotypes mirror observed effects in mammals, where canonical Wnt has been implicated as an upstream regulator of enamel knot signalling. In silico modelling of shark dental morphogenesis demonstrates how subtle changes in activatory and inhibitory signals can alter tooth shape, resembling developmental phenotypes and cusp shapes observed following experimental Wnt perturbation. Our results support the functional conservation of an enamel knot-like signalling centre throughout vertebrates and suggest that varied tooth types from sharks to mammals follow a similar developmental bauplan. Lineage-specific differences in signalling are not sufficient in refuting homology of this signalling centre, which is likely older than teeth themselves.

Osteodentin in the Atlantic wolffish (Anarhichas lupus): Dentin or bone?

The teeth of actinopterygian fish, like those of mammals, consist of a thin outer hyper-mineralized layer (enamel or enameloid) that surrounds a core of dentin. While all mammalian species have a single type of dentin (called orthodentin), various dentin types have been reported in the teeth of actinopterygian fish. The most common type of actinopterygian fish dentin is orthodentin. However, the second most common type of actinopterygian fish dentin, called osteodentin, found in several teleost species and in many Selachians, is structurally radically different from orthodentin. Osteodentin, comprising denteons and inter-denteonal matrix, is characterized by an appearance that is similar to mammalian osteonal bone, however, it lacks cells and a lacuno-canalicular system. The current consensus is that although osteodentin is morphologically different from orthodentin, it is a true dentinal material, the product of odontoblast cells. We present the results of a study of osteodentin found in the teeth of the Atlantic wolffish, Anarhichas lupus. Using a variety of microscopy techniques, high-resolution microCT scans, and micro-indentation we describe the three-dimensional structure of both its components (denteons and inter-denteonal matrix), as well as their mineral density distribution and mechanical properties, at several length-scales. We show that wolffish osteodentin is remarkably similar to the anosteocytic bone of the swords of several swordfish species. We also describe the three-dimensional network of canals found in mature osteodentin. The high density of these canals in a metabolically inactive, acellular tissue casts doubt upon the accepted paradigm, that the canals house a vascular network.

Tooth structure, mechanical properties, and diet specialization of Piranha and Pacu (Serrasalmidae): A comparative study

The relationship between diet, bite performance, and tooth structure is a topic of common interest for ecologists, biologists, materials scientists, and engineers. The highly specialized group of biters found in Serrasalmidae offers a unique opportunity to explore their functional diversity. Surprisingly, the piranha, whose teeth have a predominantly cutting function and whose main diet is soft flesh, is capable of exerting a greater bite force than a similarly sized pacu, who feeds on a hard durophagous diet. Herein, we expand our understanding of diet specialization in the Serrasalmidae family by investigating the influence of elemental composition and hierarchical structure on the local mechanical properties, stress distribution, and deformation mechanics of teeth from piranha (Pygocentrus nattereri) and pacu (Colossoma macropomum). Microscopic and spectroscopic analyses combined with nanoindentation and finite element simulations are used to probe the hierarchical features to uncover the structure-property relationships in piranha and pacu teeth. We show that the pacu teeth support a durophagous diet through its broad cusped-shaped teeth, thicker-irregular enameloid, interlocking interface of the dentin-enameloid junction, and increased hardness of the cuticle layer due to the larger concentrations of iron present. Comparatively, the piranha teeth are well suited for piercing due to their conical-shape which we report as having the greatest stiffness at the tip and evenly distributed enameloid. STATEMENT OF SIGNIFICANCE: The hierarchical structure and local mechanical properties of the piranha and pacu teeth are characterized and related to their feeding habits. Finite element models of the anterior teeth are generated to map local stress distribution under compressive loading. Bioinspired designs from the DEJ interface are developed and 3D printed. The pacu teeth are hierarchically structured and have local mechanical properties more suitable to a durophagous diet than the piranha. The findings here can provide insight into the design and fabrication of layered materials with suture interfaces for applications that require compressive loading conditions.

The conundrum of pharyngeal teeth origin: the role of germ layers, pouches, and gill slits

There are several competing hypotheses on tooth origins, with discussions eventually settling in favour of an 'outside-in' scenario, in which internal odontodes (teeth) derived from external odontodes (skin denticles) in jawless vertebrates. The evolution of oral teeth from skin denticles can be intuitively understood from their location at the mouth entrance. However, the basal condition for jawed vertebrates is arguably to possess teeth distributed throughout the oropharynx (i.e. oral and pharyngeal teeth). As skin denticle development requires the presence of ectoderm-derived epithelium and of mesenchyme, it remains to be answered how odontode-forming skin epithelium, or its competence, were 'transferred' deep into the endoderm-covered oropharynx. The 'modified outside-in' hypothesis for tooth origins proposed that this transfer was accomplished through displacement of odontogenic epithelium, that is ectoderm, not only through the mouth, but also via any opening (e.g. gill slits) that connects the ectoderm to the epithelial lining of the pharynx (endoderm). This review explores from an evolutionary and from a developmental perspective whether ectoderm plays a role in (pharyngeal) tooth and denticle formation. Historic and recent studies on tooth development show that the odontogenic epithelium (enamel organ) of oral or pharyngeal teeth can be of ectodermal, endodermal, or of mixed ecto-endodermal origin. Comprehensive data are, however, only available for a few taxa. Interestingly, in these taxa, the enamel organ always develops from the basal layer of a stratified epithelium that is at least bilayered. In zebrafish, a miniaturised teleost that only retains pharyngeal teeth, an epithelial surface layer with ectoderm-like characters is required to initiate the formation of an enamel organ from the basal, endodermal epithelium. In urodele amphibians, the bilayered epithelium is endodermal, but the surface layer acquires ectodermal characters, here termed 'epidermalised endoderm'. Furthermore, ectoderm-endoderm contacts at pouch-cleft boundaries (i.e. the prospective gill slits) are important for pharyngeal tooth initiation, even if the influx of ectoderm via these routes is limited. A balance between sonic hedgehog and retinoic acid signalling could operate to assign tooth-initiating competence to the endoderm at the level of any particular pouch. In summary, three characters are identified as being required for pharyngeal tooth formation: (i) pouch-cleft contact, (ii) a stratified epithelium, of which (iii) the apical layer adopts ectodermal features. These characters delimit the area in which teeth can form, yet cannot alone explain the distribution of teeth over the different pharyngeal arches. The review concludes with a hypothetical evolutionary scenario regarding the persisting influence of ectoderm on pharyngeal tooth formation. Studies on basal osteichthyans with less-specialised types of early embryonic development will provide a crucial test for the potential role of ectoderm in pharyngeal tooth formation and for the 'modified outside-in' hypothesis of tooth origins.

The Dental Lamina: An Essential Structure for Perpetual Tooth Regeneration in Sharks

In recent years, nonclassical models have emerged as mainstays for studies of evolutionary, developmental, and regenerative biology. Genomic advances have promoted the use of alternative taxa for the study of developmental biology, and the shark is one such emerging model vertebrate. Our research utilizes the embryonic shark (Scyliorhinus canicula) to characterize key developmental and regenerative processes that have been overlooked or not possible to study with more classic developmental models. Tooth development is a major event in the construction of the vertebrate body plan, linked in part with the emergence of jaws. Early development of the teeth and morphogenesis is well known from the murine model, but the process of tooth redevelopment and regeneration is less well known. Here we explore the role of the dental lamina in the development of a highly regenerative dentition in sharks. The shark represents a polyphyodont vertebrate with continuously repeated whole tooth regeneration. This is presented as a major developmental shift from the more derived renewal process that the murine model offers, where incisors exhibit continuous renewal and growth of the same tooth. Not only does the shark offer a study system for whole unit dental regeneration, it also represents an important model for understanding the evolutionary context of vertebrate tooth regeneration.

Non-trophic Functional Ecology of Vertebrate Teeth: A Review

Teeth are critical to the functional ecology of vertebrate trophic abilities, but are also used for a diversity of other non-trophic tasks. Teeth can play a substantial role in how animals move, manipulate their environment, positively interact with conspecifics, antagonistically interact with other organisms, and sense the environment. We review these non-trophic functions in an attempt to place the utility of human and all other vertebrate dentitions in a more diverse framework that emphasizes an expanded view of the functional importance and ecological diversity of teeth. In light of the extensive understanding of the developmental genetics, trophic functions, and evolutionary history of teeth, comparative studies of vertebrate dentitions will continue to provide unique insights into multi-functionality, many-to-one mapping, and the evolution of novel abilities.

Hard Dental Tissues Regeneration-Approaches and Challenges

With the development of the modern concept of tissue engineering approach and the discovery of the potential of stem cells in dentistry, the regeneration of hard dental tissues has become a reality and a priority of modern dentistry. The present review reports the recent advances on stem-cell based regeneration strategies for hard dental tissues and analyze the feasibility of stem cells and of growth factors in scaffolds-based or scaffold-free approaches in inducing the regeneration of either the whole tooth or only of its component structures.

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.

A large Middle Devonian eubrachythoracid 'placoderm' (Arthrodira) jaw from northern Gondwana

For the understanding of the evolution of jawed vertebrates and jaws and teeth, 'placoderms' are crucial as they exhibit an impressive morphological disparity associated with the early stages of this process. The Devonian of Morocco is famous for its rich occurrences of arthrodire 'placoderms'. While Late Devonian strata are rich in arthrodire remains, they are less common in older strata. Here, we describe a large tooth-bearing jaw element of Leptodontichthys ziregensis gen. et sp. nov., an eubrachythoracid arthrodire from the Middle Devonian of Morocco. This species is based on a large posterior superognathal with a strong dentition. The jawbone displays features considered synapomorphies of Late Devonian eubrachythoracid arthrodires, with one posterior and one lateral row of conical teeth oriented postero-lingually. μCT-images reveal internal structures including pulp cavities and dentinous tissues. The posterior orientation of the teeth and the traces of a putative occlusal contact on the lingual side of the bone imply that these teeth were hardly used for feeding. Similar to Compagopiscis and Plourdosteus, functional teeth were possibly present during an earlier developmental stage and have been worn entirely. The morphological features of the jaw element suggest a close relationship with plourdosteids. Its size implies that the animal was rather large.

The developmental relationship between teeth and dermal odontodes in the most primitive bony fish Lophosteus

The ontogenetic trajectory of a marginal jawbone of Lophosteus superbus (Late Silurian, 422 Million years old), the phylogenetically most basal stem osteichthyan, visualized by synchrotron microtomography, reveals a developmental relationship between teeth and dermal odontodes that is not evident from the adult morphology. The earliest odontodes are two longitudinal founder ridges formed at the ossification center. Subsequent odontodes that are added lingually to the ridges turn into conical teeth and undergo cyclic replacement, while those added labially achieve a stellate appearance. Stellate odontodes deposited directly on the bony plate are aligned with the alternate files of teeth, whereas new tooth positions are inserted into the files of sequential addition when a gap appears. Successive teeth and overgrowing odontodes show hybrid morphologies around the oral-dermal boundary, suggesting signal cross-communication. We propose that teeth and dermal odontodes are modifications of a single system, regulated and differentiated by the oral and dermal epithelia.

Tusks, the extra-oral teeth

OBJECTIVE

The present review aims to: a) describe the features that support tusks in extra-oral position, and b) represent distinctive features of tusks, which provide insights into tusks adaptation to ambient conditions.

DESIGN

A comprehensive review of scientific literature relevant to tusks and comparable dental tissues was conducted.

RESULTS

The oral cavity provides a desirable condition which is conducive to tooth health. Therefore, it remains questionable how the bare (exposed) tusks resist the extra-oral conditions. The common features among tusked mammals indicate that the structural (e.g. the peculiar dentinal alignment), cellular (e.g. low or lack of cell populations in the tusk), hormonal (e.g. androgens), and behavioral traits have impact on a tusk's preservation and occurrence.

CONCLUSIONS

Understanding of bare mineralized structures, such as tusks and antlers, and their compatibility with different environments, can provide important insight into oral biology.

Multiple epithelia are required to develop teeth deep inside the pharynx

To explain the evolutionary origin of vertebrate teeth from odontodes, it has been proposed that competent epithelium spread into the oropharyngeal cavity via the mouth and other possible channels such as the gill slits [Huysseune et al., 2009, J. Anat. 214, 465-476]. Whether tooth formation deep inside the pharynx in extant vertebrates continues to require external epithelia has not been addressed so far. Using zebrafish we have previously demonstrated that cells derived from the periderm penetrate the oropharyngeal cavity via the mouth and via the endodermal pouches and connect to periderm-like cells that subsequently cover the entire endoderm-derived pharyngeal epithelium [Rosa et al., 2019, Sci. Rep. 9, 10082]. We now provide conclusive evidence that the epithelial component of pharyngeal teeth in zebrafish (the enamel organ) is derived from medial endoderm, as hitherto assumed based on position deep in the pharynx. Yet, dental morphogenesis starts only after the corresponding endodermal pouch (pouch 6) has made contact with the skin ectoderm, and only after periderm-like cells have covered the prospective tooth-forming endodermal epithelium. Manipulation of signaling pathways shown to adversely affect tooth development indicates they act downstream of these events. We demonstrate that pouch-ectoderm contact and the presence of a periderm-like layer are both required, but not sufficient, for tooth initiation in the pharynx. We conclude that the earliest interactions to generate pharyngeal teeth encompass those between different epithelial populations (skin ectoderm, endoderm, and periderm-like cells in zebrafish), in addition to the epithelial-mesenchymal interactions that govern the formation of all vertebrate teeth.

Tooth replacement in early sarcopterygians

Teeth were an important innovation in vertebrate evolution but basic aspects of early dental evolution remain poorly understood. Teeth differ from other odontode organs, like scales, in their organized, sequential pattern of replacement. However, tooth replacement patterns also vary between the major groups of jawed vertebrates. Although tooth replacement in stem-osteichthyans and extant species has been intensively studied it has been difficult to resolve scenarios for the evolution of osteichthyan tooth replacement because of a dearth of evidence from living and fossil sarcopterygian fishes. Here we provide new anatomical data informing patterns of tooth replacement in the Devonian sarcopterygian fishes Onychodus, Eusthenopteron and Tiktaalik and the living coelacanth Latimeria based on microfocus- and synchrotron radiation-based X-ray microtomography. Early sarcopterygians generated replacement teeth on the jaw surface in a pattern similar to stem-osteichthyans, with damaged teeth resorbed and replacement teeth developed on the surface of the bone. However, resorption grades and development of replacement teeth vary spatially and temporally within the jaw. Particularly in Onychodus, where teeth were also shed through anterior rotation and resorption of bone at the base of the parasymphyseal tooth whorl, with new teeth added posteriorly. As tooth whorls are also present in more stem-osteichthyans, and statodont tooth whorls are present among acanthodians (putative stem-chondrichthyans), rotational replacement of the anterior dentition may be a stem-osteichthyan character. Our results suggest a more complex evolutionary history of tooth replacement.

Retention of fish-like odontode overgrowth in Permian tetrapod dentition supports outside-in theory of tooth origins

Teeth are often thought of as structures that line the margins of the mouth; however, tooth-like structures called odontodes are commonly found on the dermal bones of many Palaeozoic vertebrates including early jawless fishes. 'Odontode' is a generalized term for all tooth-like dentine structures that have homologous tissues and development. This definition includes true teeth and the odontodes of early 'fishes', which have been recently examined to gain new insights into the still unresolved origin of teeth. Two leading hypotheses are frequently referenced in this debate: the 'outside-in' hypothesis, which posits that dermal odontodes evolutionarily migrate into the oral cavity, and the 'inside-out' hypothesis, which posits that teeth originated in the oropharyngeal cavity and then moved outwards into the oral cavity. Here, we show that, unlike the well-known one-to-one replacement patterns of marginal dentition, the palatal dentition of the early Permian tetrapods, including the dissorophoid amphibian Cacops and the early reptile Captorhinus, is overgrown by a new layer of bone to which the newest teeth are then attached. This same overgrowth pattern has been well documented in dermal and oral odontodes (i.e. teeth) of early fishes. We propose that this pattern represents the primitive condition for vertebrates and may even predate the origin of jaws. Therefore, this pattern crosses the fish-tetrapod transition, and the retention of this ancestral pattern in the palatal dentition of early terrestrial tetrapods provides strong support for the 'outside-in' hypothesis of tooth origins.

Regrowing a tooth: in vitro and in vivo approaches

Biologically oriented regenerative dentistry in an attempt to regrow a functional tooth by harnessing the natural healing capabilities of dental tissues has become a recent trend challenging the current dental practice on repairing the damaged or missing tooth. In this review, we outline the conceptual development on the in situ revitalization of the tooth replacement capability lost during evolution, the updated progress in stem-cell-based in vivo repair of the damaged tooth, and the recent endeavors for in vitro generation of an implantable bioengineered tooth germ. Thereafter, we summarize the major challenges that need to be overcome in order to provide the rationale and directions for the success of fully functional tooth regeneration in the near future.

Expression of new genes in vertebrate tooth development and p63 signaling

BACKGROUND

p63 is an evolutionarily ancient transcription factor essential to vertebrate tooth development. Our recent gene expression screen comparing wild-type and "toothless" p63^-/- mouse embryos implicated in tooth development several new genes that we hypothesized act downstream of p63 in dental epithelium, where p63 is also expressed.

RESULTS

Via in situ hybridization and immunohistochemistry, we probed mouse embryos (embryonic days 10.5-14.5) and spotted gar fish embryos (14 days postfertilization) for these newly linked genes, Cbln1, Cldn23, Fermt1, Krt15, Pltp and Prss8, which were expressed in mouse and gar dental epithelium. Loss of p63 altered expression levels but not domains. Expression was comparable between murine upper and lower tooth organs, implying conserved gene functions in maxillary and mandibular dentitions. Our meta-analysis of gene expression databases supported that these genes act within a p63-driven gene regulatory network important to tooth development in mammals and more evolutionary ancient vertebrates (fish, amphibians).

CONCLUSIONS

Cbln1, Cldn23, Fermt1, Krt15, Pltp, and Prss8 were expressed in mouse and fish dental epithelium at placode, bud, and/or cap stages. We theorize that these genes participate in cell-cell adhesion, cell polarity, and extracellular matrix signaling to support dental epithelium integrity, folding, and epithelial-mesenchymal cross talk during tooth development.

Tissue Mechanical Forces and Evolutionary Developmental Changes Act Through Space and Time to Shape Tooth Morphology and Function

Efforts from diverse disciplines, including evolutionary studies and biomechanical experiments, have yielded new insights into the genetic, signaling, and mechanical control of tooth formation and functions. Evidence from fossils and non-model organisms has revealed that a common set of genes underlie tooth-forming potential of epithelia, and changes in signaling environments subsequently result in specialized dentitions, maintenance of dental stem cells, and other phenotypic adaptations. In addition to chemical signaling, tissue forces generated through epithelial contraction, differential growth, and skeletal constraints act in parallel to shape the tooth throughout development. Here recent advances in understanding dental development from these studies are reviewed and important gaps that can be filled through continued application of evolutionary and biomechanical approaches are discussed.

An ancient Turing-like patterning mechanism regulates skin denticle development in sharks

Vertebrates have a vast array of epithelial appendages, including scales, feathers, and hair. The developmental patterning of these diverse structures can be theoretically explained by Alan Turing's reaction-diffusion system. However, the role of this system in epithelial appendage patterning of early diverging lineages (compared to tetrapods), such as the cartilaginous fishes, is poorly understood. We investigate patterning of the unique tooth-like skin denticles of sharks, which closely relates to their hydrodynamic and protective functions. We demonstrate through simulation models that a Turing-like mechanism can explain shark denticle patterning and verify this system using gene expression analysis and gene pathway inhibition experiments. This mechanism bears remarkable similarity to avian feather patterning, suggesting deep homology of the system. We propose that a diverse range of vertebrate appendages, from shark denticles to avian feathers and mammalian hair, use this ancient and conserved system, with slight genetic modulation accounting for broad variations in patterning.

Evolution of dental tissue mineralization: an analysis of the jawed vertebrate SPARC and SPARC-L families

BACKGROUND

The molecular bases explaining the diversity of dental tissue mineralization across gnathostomes are still poorly understood. Odontodes, such as teeth and body denticles, are serial structures that develop through deployment of a gene regulatory network shared between all gnathostomes. Dentin, the inner odontode mineralized tissue, is produced by odontoblasts and appears well-conserved through evolution. In contrast, the odontode hypermineralized external layer (enamel or enameloid) produced by ameloblasts of epithelial origin, shows extensive structural variations. As EMP (Enamel Matrix Protein) genes are as yet only found in osteichthyans where they play a major role in the mineralization of teeth and others skeletal organs, our understanding of the molecular mechanisms leading to the mineralized odontode matrices in chondrichthyans remains virtually unknown.

RESULTS

We undertook a phylogenetic analysis of the SPARC/SPARC-L gene family, from which the EMPs are supposed to have arisen, and examined the expression patterns of its members and of major fibrillar collagens in the spotted catshark Scyliorhinus canicula, the thornback ray Raja clavata, and the clawed frog Xenopus tropicalis. Our phylogenetic analyses reveal that the single chondrichthyan SPARC-L gene is co-orthologous to the osteichthyan SPARC-L1 and SPARC-L2 paralogues. In all three species, odontoblasts co-express SPARC and collagens. In contrast, ameloblasts do not strongly express collagen genes but exhibit strikingly similar SPARC-L and EMP expression patterns at their maturation stage, in the examined chondrichthyan and osteichthyan species, respectively.

CONCLUSIONS

A well-conserved odontoblastic collagen/SPARC module across gnathostomes further confirms dentin homology. Members of the SPARC-L clade evolved faster than their SPARC paralogues, both in terms of protein sequence and gene duplication. We uncover an osteichthyan-specific duplication that produced SPARC-L1 (subsequently lost in pipidae frogs) and SPARC-L2 (independently lost in teleosts and tetrapods).Our results suggest the ameloblastic expression of the single chondrichthyan SPARC-L gene at the maturation stage reflects the ancestral gnathostome situation, and provide new evidence in favor of the homology of enamel and enameloids in all gnathostomes.

Upper jaw development in the absence of teeth: New insights for craniodental evo-devo integration

In p63-null mice (p63^-/- ), teeth fail to form but the mandible forms normally; conversely, the upper jaw skeleton is malformed. Here we explored whether lack of dental tissues contributed to midfacial dysmorphologies in p63^-/- mice by testing if facial prominence defects appeared before odontogenesis failed. We also investigated gene dose effects by testing if one wild type (WT) p63 allele (p63^+/- ) was sufficient for normal upper jaw skeleton formation. We micro-CT scanned PFA-fixed p63^-/- , p63^+/- , and WT (p63^+/+ ) adult and embryonic mice aged E10-E14. Next, we landmarked mandibular (MdP), maxillary (MxP) and nasal prominences (NPs), and facial bones. 3D landmark data were assessed using Principal Component, Canonical Variate, Partial Least Squares, and other statistical analyses. The p63^-/- embryos showed MxP and NP malformations by E12, despite the presence of dental tissues. MdP shape was comparable among p63^-/- , p63^+/- , and p63^+/+ embryos. Upper jaw shape was comparable between p63^+/+ and p63^+/- adults. The upper jaw and its dentition both require p63 signaling, but not each other's presence, to form properly. One WT p63 allele enables normal midfacial morphogenesis; gene dose may be a target for jaw macroevolution. Jaw-specific genetic mechanisms likely integrate the evo-devo of dentitions with upper versus lower jaws.

Trunk dental tissue evolved independently from underlying dermal bony plates but is associated with surface bones in living odontode-bearing catfish

Although oral dental tissue is a vertebrate attribute, trunk dental tissue evolved in several extinct vertebrate lineages but is rare among living species. The question of which processes trigger dental-tissue formation in the trunk remains open, and would shed light on odontogenesis evolution. Extra-oral dental structures (odontodes) in the trunk are associated with underlying dermal bony plates, leading us to ask whether the formation of trunk bony plates is necessary for trunk odontodes to emerge. To address this question, we focus on Loricarioidei: an extant, highly diverse group of catfish whose species all have odontodes. We examined the location and cover of odontodes and trunk dermal bony plates for all six loricarioid families and 17 non-loricarioid catfish families for comparison. We inferred the phylogeny of Loricarioidei using a new 10-gene dataset, eight time-calibration points, and noise-reduction techniques. Based on this phylogeny, we reconstructed the ancestral states of odontode and bony plate cover, and find that trunk odontodes emerged before dermal bony plates in Loricarioidei. Yet we discovered that when bony plates are absent, other surface bones are always associated with odontodes, suggesting a link between osteogenic and odontogenic developmental pathways, and indicating a remarkable trunk odontogenic potential in Loricarioidei.

Sox2+ progenitors in sharks link taste development with the evolution of regenerative teeth from denticles

Teeth and denticles belong to a specialized class of mineralizing epithelial appendages called odontodes. Although homology of oral teeth in jawed vertebrates is well supported, the evolutionary origin of teeth and their relationship with other odontode types is less clear. We compared the cellular and molecular mechanisms directing development of teeth and skin denticles in sharks, where both odontode types are retained. We show that teeth and denticles are deeply homologous developmental modules with equivalent underlying odontode gene regulatory networks (GRNs). Notably, the expression of the epithelial progenitor and stem cell marker sex-determining region Y-related box 2 (sox2) was tooth-specific and this correlates with notable differences in odontode regenerative ability. Whereas shark teeth retain the ancestral gnathostome character of continuous successional regeneration, new denticles arise only asynchronously with growth or after wounding. Sox2+ putative stem cells associated with the shark dental lamina (DL) emerge from a field of epithelial progenitors shared with anteriormost taste buds, before establishing within slow-cycling cell niches at the (i) superficial taste/tooth junction (T/TJ), and (ii) deep successional lamina (SL) where tooth regeneration initiates. Furthermore, during regeneration, cells from the superficial T/TJ migrate into the SL and contribute to new teeth, demonstrating persistent contribution of taste-associated progenitors to tooth regeneration in vivo. This data suggests a trajectory for tooth evolution involving cooption of the odontode GRN from nonregenerating denticles by sox2+ progenitors native to the oral taste epithelium, facilitating the evolution of a novel regenerative module of odontodes in the mouth of early jawed vertebrates: the teeth.

Romundina and the evolutionary origin of teeth

Theories on the origin of vertebrate teeth have long focused on chondrichthyans as reflecting a primitive condition—but this is better informed by the extinct placoderms, which constitute a sister clade or grade to the living gnathostomes. Here, we show that ‘supragnathal’ toothplates from the acanthothoracid placoderm Romundina stellina comprise multi-cuspid teeth, each composed of an enameloid cap and core of dentine. These were added sequentially, approximately circumferentially, about a pioneer tooth. Teeth are bound to a bony plate that grew with the addition of marginal teeth. Homologous toothplates in arthrodire placoderms exhibit a more ordered arrangement of teeth that lack enameloid, but their organization into a gnathal, bound by layers of cellular bone associated with the addition of each successional tooth, is the same. The presence of enameloid in the teeth of Romundina suggests that it has been lost in other placoderms. Its covariation in the teeth and dermal skeleton of placoderms suggests a lack of independence early in the evolution of jawed vertebrates. It also appears that the dentition—manifest as discrete gnathal ossifications—was developmentally discrete from the jaws during this formative episode of vertebrate evolution.

Tooth and scale morphogenesis in shark: an alternative process to the mammalian enamel knot system

BACKGROUND

The gene regulatory network involved in tooth morphogenesis has been extremely well described in mammals and its modeling has allowed predictions of variations in regulatory pathway that may have led to evolution of tooth shapes. However, very little is known outside of mammals to understand how this regulatory framework may also account for tooth shape evolution at the level of gnathostomes. In this work, we describe expression patterns and proliferation/apoptosis assays to uncover homologous regulatory pathways in the catshark Scyliorhinus canicula.

RESULTS

Because of their similar structural and developmental features, gene expression patterns were described over the four developmental stages of both tooth and scale buds in the catshark. These gene expression patterns differ from mouse tooth development, and discrepancies are also observed between tooth and scale development within the catshark. However, a similar nested expression of Shh and Fgf suggests similar signaling involved in morphogenesis of all structures, although apoptosis assays do not support a strictly equivalent enamel knot system in sharks. Similarities in the topology of gene expression pattern, including Bmp signaling pathway, suggest that mouse molar development is more similar to scale bud development in the catshark.

CONCLUSIONS

These results support the fact that no enamel knot, as described in mammalian teeth, can be described in the morphogenesis of shark teeth or scales. However, homologous signaling pathways are involved in growth and morphogenesis with variations in their respective expression patterns. We speculate that variations in this topology of expression are also a substrate for tooth shape evolution, notably in regulating the growth axis and symmetry of the developing structure.

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.