The homology and phylogeny of chondrichthyan tooth enameloid

The Notch-mediated circuitry in the evolution and generation of new cell lineages: the tooth model

The Notch pathway is an ancient, evolutionary conserved intercellular signaling mechanism that is involved in cell fate specification and proper embryonic development. The Jagged2 gene, which encodes a ligand for the Notch family of receptors, is expressed from the earliest stages of odontogenesis in epithelial cells that will later generate the enamel-producing ameloblasts. Homozygous Jagged2 mutant mice exhibit abnormal tooth morphology and impaired enamel deposition. Enamel composition and structure in mammals are tightly linked to the enamel organ that represents an evolutionary unit formed by distinct dental epithelial cell types. The physical cooperativity between Notch ligands and receptors suggests that Jagged2 deletion could alter the expression profile of Notch receptors, thus modifying the whole Notch signaling cascade in cells within the enamel organ. Indeed, both Notch1 and Notch2 expression are severely disturbed in the enamel organ of Jagged2 mutant teeth. It appears that the deregulation of the Notch signaling cascade reverts the evolutionary path generating dental structures more reminiscent of the enameloid of fishes rather than of mammalian enamel. Loss of interactions between Notch and Jagged proteins may initiate the suppression of complementary dental epithelial cell fates acquired during evolution. We propose that the increased number of Notch homologues in metazoa enabled incipient sister cell types to form and maintain distinctive cell fates within organs and tissues along evolution.

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.

Bricks, trusses and superstructures: Strategies for skeletal reinforcement in batoid fishes (rays and skates)

Crushing and eating hard prey (durophagy) is mechanically demanding. The cartilage jaws of durophagous stingrays are known to be reinforced relative to non-durophagous relatives, with a thickened external cortex of mineralized blocks (tesserae), reinforcing struts inside the jaw (trabeculae), and pavement-like dentition. These strategies for skeletal strengthening against durophagy, however, are largely understood only from myliobatiform stingrays, although a hard prey diet has evolved multiple times in batoid fishes (rays, skates, guitarfishes). We perform a quantitative analysis of micro-CT data, describing jaw strengthening mechanisms in Rhina ancylostoma (Bowmouth Guitarfish) and Rhynchobatus australiae (White-spotted Wedgefish), durophagous members of the Rhinopristiformes, the sister taxon to Myliobatiformes. Both species possess trabeculae, more numerous and densely packed in Rhina, albeit simpler structurally than those in stingrays like Aetobatus and Rhinoptera. Rhina and Rhynchobatus exhibit impressively thickened jaw cortices, often involving >10 tesseral layers, most pronounced in regions where dentition is thickest, particularly in Rhynchobatus. Age series of both species illustrate that tesserae increase in size during growth, with enlarged and irregular tesserae associated with the jaws’ oral surface in larger (older) individuals of both species, perhaps a feature of ageing. Unlike the flattened teeth of durophagous myliobatiform stingrays, both rhinopristiform species have oddly undulating dentitions, comprised of pebble-like teeth interlocked to form compound “meta-teeth” (large spheroidal structures involving multiple teeth). This is particularly striking in Rhina, where the upper/lower occlusal surfaces are mirrored undulations, fitting together like rounded woodworking finger-joints. Trabeculae were previously thought to have arisen twice independently in Batoidea; our results show they are more widespread among batoid groups than previously appreciated, albeit apparently absent in the phylogenetically basal Rajiformes. Comparisons with several other durophagous and non-durophagous species illustrate that batoid skeletal reinforcement architectures are modular: trabeculae can be variously oriented and are dominant in some species (e.g. Rhina, Aetobatus), whereas cortical thickening is more significant in others (e.g. Rhynchobatus), or both reinforcing features can be lacking (e.g. Raja, Urobatis). We discuss interactions and implications of character states, framing a classification scheme for exploring cartilage structure evolution in the cartilaginous fishes.

Parallel evolution of ameloblastic scpp genes in bony and cartilaginous vertebrates

In bony vertebrates, skeletal mineralization relies on the secretory calcium-binding phosphoproteins (Scpp) family whose members are acidic extracellular proteins posttranslationally regulated by the Fam20°C kinase. As scpp genes are absent from the elephant shark genome, they are currently thought to be specific to bony fishes (osteichthyans). Here, we report a scpp gene present in elasmobranchs (sharks and rays) that evolved from local tandem duplication of sparc-L 5′ exons and show that both genes experienced recent gene conversion in sharks. The elasmobranch scpp is remarkably similar to the osteichthyan scpp members as they share syntenic and gene structure features, code for a conserved signal peptide, tyrosine-rich and aspartate/glutamate-rich regions, and harbor putative Fam20°C phosphorylation sites. In addition, the catshark scpp is coexpressed with sparc-L and fam20°C in tooth and scale ameloblasts, similarly to some osteichthyan scpp genes. Despite these strong similarities, molecular clock and phylogenetic data demonstrate that the elasmobranch scpp gene originated independently from the osteichthyan scpp gene family. Our study reveals convergent events at the sparc-L locus in the two sister clades of jawed vertebrates, leading to parallel diversification of the skeletal biomineralization toolkit. The molecular evolution of sparc-L and its coexpression with fam20°C in catshark ameloblasts provides a unifying genetic basis that suggests that all convergent scpp duplicates inherited similar features from their sparc-L precursor. This conclusion supports a single origin for the hypermineralized outer odontode layer as produced by an ancestral developmental process performed by Sparc-L, implying the homology of the enamel and enameloid tissues in all vertebrates.

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.

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.

Mineralized Cartilage and Bone-Like Tissues in Chondrichthyans Offer Potential Insights Into the Evolution and Development of Mineralized Tissues in the Vertebrate Endoskeleton

The impregnation of biominerals into the extracellular matrix of living organisms, a process termed biomineralization, gives rise to diverse mineralized (or calcified) tissues in vertebrates. Preservation of mineralized tissues in the fossil record has provided insights into the evolutionary history of vertebrates and their skeletons. However, current understanding of the vertebrate skeleton and of the processes underlying its formation is biased towards biomedical models such as the tetrapods mouse and chick. Chondrichthyans (sharks, skates, rays, and chimaeras) and osteichthyans are the only vertebrate groups with extant (living) representatives that have a mineralized skeleton, but the basal phylogenetic position of chondrichthyans could potentially offer unique insights into skeletal evolution. For example, bone is a vertebrate novelty, but the internal supporting skeleton (endoskeleton) of extant chondrichthyans is commonly described as lacking bone. The molecular and developmental basis for this assertion is yet to be tested. Subperichondral tissues in the endoskeleton of some chondrichthyans display mineralization patterns and histological and molecular features of bone, thereby challenging the notion that extant chondrichthyans lack endoskeletal bone. Additionally, the chondrichthyan endoskeleton demonstrates some unique features and others that are potentially homologous with other vertebrates, including a polygonal mineralization pattern, a trabecular mineralization pattern, and an unconstricted perichordal sheath. Because of the basal phylogenetic position of chondrichthyans among all other extant vertebrates with a mineralized skeleton, developmental and molecular studies of chondrichthyans are critical to flesh out the evolution of vertebrate skeletal tissues, but only a handful of such studies have been carried out to date. This review discusses morphological and molecular features of chondrichthyan endoskeletal tissues and cell types, ultimately emphasizing how comparative embryology and transcriptomics can reveal homology of mineralized skeletal tissues (and their cell types) between chondrichthyans and other vertebrates.

Role of Cell Death in Cellular Processes During Odontogenesis

The development of a tooth germ in a precise size, shape, and position in the jaw, involves meticulous regulation of cell proliferation and cell death. Apoptosis, as the most common type of programmed cell death during embryonic development, plays a number of key roles during odontogenesis, ranging from the budding of the oral epithelium during tooth initiation, to later tooth germ morphogenesis and removal of enamel knot signaling center. Here, we summarize recent knowledge about the distribution and function of apoptotic cells during odontogenesis in several vertebrate lineages, with a special focus on amniotes (mammals and reptiles). We discuss the regulatory roles that apoptosis plays on various cellular processes during odontogenesis. We also review apoptosis-associated molecular signaling during tooth development, including its relationship with the autophagic pathway. Lastly, we cover apoptotic pathway disruption, and alterations in apoptotic cell distribution in transgenic mouse models. These studies foster a deeper understanding how apoptotic cells affect cellular processes during normal odontogenesis, and how they contribute to dental disorders, which could lead to new avenues of treatment in the future.

Hierarchical Microstructure of Tooth Enameloid in Two Lamniform Shark Species, Carcharias taurus and Isurus oxyrinchus

Shark tooth enameloid is a hard tissue made up of nanoscale fluorapatite crystallites arranged in a unique hierarchical pattern. This microstructural design results in a macroscopic material that is stiff, strong, and tough, despite consisting almost completely of brittle mineral. In this contribution, we characterize and compare the enameloid microstructure of two modern lamniform sharks, Isurus oxyrinchus (shortfin mako shark) and Carcharias taurus (spotted ragged-tooth shark), based on scanning electron microscopy images. The hierarchical microstructure of shark enameloid is discussed in comparison with amniote enamel. Striking similarities in the microstructures of the two hard tissues are found. Identical structural motifs have developed on different levels of the hierarchy in response to similar biomechanical requirements in enameloid and enamel. Analyzing these structural patterns allows the identification of general microstructural design principles and their biomechanical function, thus paving the way for the design of bioinspired composite materials with superior properties such as high strength combined with high fracture resistance.

Shape-preserving erosion controlled by the graded microarchitecture of shark tooth enameloid

The teeth of all vertebrates predominantly comprise the same materials, but their lifespans vary widely: in stark contrast to mammals, shark teeth are functional only for weeks, rather than decades, making lifelong durability largely irrelevant. However, their diets are diverse and often mechanically demanding, and as such, their teeth should maintain a functional morphology, even in the face of extremely high and potentially damaging contact stresses. Here, we reconcile the dilemma between the need for an operative tooth geometry and the unavoidable damage inherent to feeding on hard foods, demonstrating that the tooth cusps of Port Jackson sharks, hard-shelled prey specialists, possess unusual microarchitecture that controls tooth erosion in a way that maintains functional cusp shape. The graded architecture in the enameloid provokes a location-specific damage response, combining chipping of outer enameloid and smooth wear of inner enameloid to preserve an efficient shape for grasping hard prey. Our discovery provides experimental support for the dominant theory that multi-layered tooth enameloid facilitated evolutionary diversification of shark ecologies.

Shark teeth have short lifespans yet can be subject to significant mechanical damage. Here, the authors report on a site-specific damage mechanism in shark teeth enameloid, which maintains tooth functional shape, providing experimental evidence that tooth architecture may have influenced the diversification of shark ecologies over evolution.

3D microstructural study of selachimorph enameloid evolution

Enameloid, the hyper-mineralized tissue covering shark teeth is a complex structure resulting from both ameloblast and odontoblast activity. The way these two types of cells interact to set up this tissue is not fully understood and results in the formation of subunits in the enameloid: the Single Crystallite Enameloid (SCE) and the Bundled Crystallite Enameloid (BCE). Using the Focused Ion Beam Nanotomography (FIB-nt), 3D images were produced to assess the relationship between the SCE and BCE of one fossil and one recent neoselachian shark teeth. 3D analysis of crystallite bundles reveals a strong connection between the crystallites forming the SCE and those forming the bundles of the Radial Bundle Enameloid (RBE), a component of the BCE, although it has been suggested that SCE and BCE have a different origin: epithelial for the SCE and mesenchymal for the BCE. Another significant result of the use of FIB-nt is the visualization of frequent branching among the radial bundles forming the RBE, including horizontal link between adjacent bundles. FIB-nt demonstrates therefore a strong potential to decipher the complex evolution of hyper-mineralised tissue in shark teeth, and, therefore, to better understand the evolution of tooth structure among basal Gnathostomes.

Dental morphology and microstructure of the Prickly Dogfish Oxynotus bruniensis (Squaliformes: Oxynotidae)

This study describes and illustrates the jaws, teeth, and tooth microstructure of the Prickly Dogfish Oxynotus bruniensis. Detailed accounts of the dental morphology of O. bruniensis are rare and have not addressed the tissue arrangement or microstructure of the teeth. These features are documented and discussed in the contexts of interspecific comparisons with other elasmobranchs and the dietary specialization of O. bruniensis. The overall tooth morphology of O. bruniensis is similar to those of other closely related members in the order Squaliformes, as is the tissue arrangement, or histotype. Oxynotus bruniensis exhibits a simplified enameloid microstructure, which we compare with previously documented enameloid microstructures of other elasmobranchs. Though subtle interspecific differences in dental characters are documented, neither overall tooth morphology nor histotype and microstructure are unique to O. bruniensis. We conclude that in the case of O. bruniensis, dietary specialization is facilitated by behavioral rather than morphological specialization.

The hidden structure of human enamel

Enamel is the hardest and most resilient tissue in the human body. Enamel includes morphologically aligned, parallel, ∼50 nm wide, microns-long nanocrystals, bundled either into 5-μm-wide rods or their space-filling interrod. The orientation of enamel crystals, however, is poorly understood. Here we show that the crystalline c-axes are homogenously oriented in interrod crystals across most of the enamel layer thickness. Within each rod crystals are not co-oriented with one another or with the long axis of the rod, as previously assumed: the c-axes of adjacent nanocrystals are most frequently mis-oriented by 1°-30°, and this orientation within each rod gradually changes, with an overall angle spread that is never zero, but varies between 30°-90° within one rod. Molecular dynamics simulations demonstrate that the observed mis-orientations of adjacent crystals induce crack deflection. This toughening mechanism contributes to the unique resilience of enamel, which lasts a lifetime under extreme physical and chemical challenges.

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.

Vascularization and odontode structure of a dorsal ridge spine of Romundina stellina Ørvig 1975

There are two types of dermal skeletons in jawed vertebrates: placoderms and osteichthyans carry large bony plates (macromery), whereas chondrichthyans and acanthodians are covered by small scales (micromery). Fin spines are one of the last large dermal structures found on micromeric taxa and offer a potential source of histology and morphology that can be compared to those found on macromeric groups. Dermal fin spines offer a variety of morphology but aspects of their growth modes and homology are unclear. Here, we provide detailed descriptions of the microstructure and growth of a dorsal ridge spine from the acanthothoracid placoderm, Romundina stellina, using virtual three-dimensional paleohistological datasets. From these data we identify several layers of dentine ornamentation covering the lateral surfaces of the spine and reconstructed their growth pattern. We show that this spine likely grew posteriorly and proximally from a narrow portion of bone located along the leading edge of the spine. The spine is similarly constructed to the scales with a few exceptions, including the absence of polarized fibers distributed throughout the bone and the presence of a thin layer of perichondral bone. The composition of the spine (semidentine odontodes, dermal bone, perichondral bone) is identical to that of the Romundina dermal plates. These results illustrate the similarities and differences between the dermal tissues in Romundina and indicate that the spine grew differently from the dentinous fin spines from extant and fossil chondrichthyans. The morphology and histology of Romundina is most similar to the fin spine of the probable stem osteichthyan Lophosteus, with a well-developed inner cellular bony base and star-shaped odontodes on the surface. Results from these studies will undoubtedly have impact on our understanding of fossil fin spine histology and evolution, contributing to the on-going revision of early gnathostome phylogeny.

Parrotfish Teeth: Stiff Biominerals Whose Microstructure Makes Them Tough and Abrasion-Resistant To Bite Stony Corals

Parrotfish (Scaridae) feed by biting stony corals. To investigate how their teeth endure the associated contact stresses, we examine the chemical composition, nano- and microscale structure, and the mechanical properties of the steephead parrotfish Chlorurus microrhinos tooth. Its enameloid is a fluorapatite (Ca₅(PO₄)₃F) biomineral with outstanding mechanical characteristics: the mean elastic modulus is 124 GPa, and the mean hardness near the biting surface is 7.3 GPa, making this one of the stiffest and hardest biominerals measured; the mean indentation yield strength is above 6 GPa, and the mean fracture toughness is ∼2.5 MPa·m^1/2, relatively high for a highly mineralized material. This combination of properties results in high abrasion resistance. Fluorapatite X-ray absorption spectroscopy exhibits linear dichroism at the Ca L-edge, an effect that makes peak intensities vary with crystal orientation, under linearly polarized X-ray illumination. This observation enables polarization-dependent imaging contrast mapping of apatite, a method to quantitatively measure and display nanocrystal orientations in large, pristine arrays of nano- and microcrystalline structures. Parrotfish enameloid consists of 100 nm-wide, microns long crystals co-oriented and assembled into bundles interwoven as the warp and the weave in fabric and therefore termed fibers here. These fibers gradually decrease in average diameter from 5 μm at the back to 2 μm at the tip of the tooth. Intriguingly, this size decrease is spatially correlated with an increase in hardness.

Trunk neural crest origin of dermal denticles in a cartilaginous fish

Cartilaginous fishes (e.g., sharks and skates) possess a postcranial dermal skeleton consisting of tooth-like "denticles" embedded within their skin. As with teeth, the principal skeletal tissue of dermal denticles is dentine. In the head, cranial neural crest cells give rise to the dentine-producing cells (odontoblasts) of teeth. However, trunk neural crest cells are generally regarded as nonskeletogenic, and so the embryonic origin of trunk denticle odontoblasts remains unresolved. Here, we use expression of FoxD3 to pinpoint the specification and emigration of trunk neural crest cells in embryos of a cartilaginous fish, the little skate (Leucoraja erinacea). Using cell lineage tracing, we further demonstrate that trunk neural crest cells do, in fact, give rise to odontoblasts of trunk dermal denticles. These findings expand the repertoire of vertebrate trunk neural crest cell fates during normal development, highlight the likely primitive skeletogenic potential of this cell population, and point to a neural crest origin of dentine throughout the ancestral vertebrate dermal skeleton.

SCPP Genes and Their Relatives in Gar: Rapid Expansion of Mineralization Genes in Osteichthyans

Gar is an actinopterygian that has bone, dentin, enameloid, and ganoin (enamel) in teeth and/or scales. Mineralization of these tissues involves genes encoding various secretory calcium-binding phosphoproteins (SCPPs) in osteichthyans, but no SCPP genes have been identified in chondrichthyans to date. In the gar genome, we identified 38 SCPP genes, seven of which encode "acidic-residue-rich" proteins and 31 encode "Pro/Gln (P/Q) rich" proteins. These gar SCPP genes constitute the largest known repertoire, including many newly identified P/Q-rich genes expressed in teeth and/or scales. Among gar SCPP genes, six acidic and three P/Q-rich genes were identified as orthologs of sarcopterygian genes. The sarcopterygian orthologs of most of these acidic genes are involved in bone and/or dentin formation, and sarcopterygian orthologs of all three P/Q-rich genes participate in enamel formation. The finding of these genes in gar suggests that an elaborate SCPP gene-based genetic system for tissue mineralization was already present in stem osteichthyans. While SCPP genes have been thought to originate from ancient SPARCL1, SPARCL1L1 appears to be more closely related to these genes, because it established a structure similar to acidic SCPP genes probably in stem gnathostomes, perhaps at about the same time with the origin of tissue mineralization. Assuming enamel evolved in stem osteichthyans, all P/Q-rich SCPP genes likely arose within the osteichthyan lineage. Furthermore, the absence of acidic SCPP genes in chondrichthyans might be explained by the secondary loss of earliest acidic genes. It appears that many SCPP genes expanded rapidly in stem osteichthyans and in basal actinopterygians.

High-precision (34) S/(32) S measurements in vertebrate bioapatites using purge-and-trap elemental analyser/isotope ratio mass spectrometry technology

RATIONALE

In ecological studies, the sulfur isotope compositions (δ(34) S values) of soft tissues (e.g. hair, skin, nail, muscle, collagen) allow the determination of both past and present-day living environments of organisms. However, technical limitations have so far prevented reliable sulfur isotope analyses of minerals having low sulfur content, such as bioapatite, which is the crystalline component of skeletal tissues. The development of 'purge-and-trap' technology in elemental analysers recently demonstrated new possibilities to solve some of those technical difficulties.

METHODS

We have used a VarioPYROcube elemental analyser (EA) equipped with 'purge-and-trap' technology, interfaced in continuous flow mode to an Isoprime 100 isotope ratio mass spectrometer, to measure the sulfur isotope compositions of bioapatite samples. We first characterised a working calibrated material of chemical composition close to those of our samples, the low-S bearing phosphorite BCR32, against the two δ(34) SV-CDT reference-calibrated materials, NBS-127 and IAEA-SO-5. We have confirmed a δ(34) SV-CDT value of +18.4‰ (1σ = 0.5; n = 18) in agreement with the previously published value. Using BCR32 as a compositional and isotopic reference material, we have then measured the δ(34) SV-CDT values of various bioapatite tissues (bone, dentine and enamel) from both modern and fossil vertebrates living in different environments (marine, freshwater and terrestrial).

RESULTS

Our results demonstrate the capacity of this analytical setup to measure the δ(34) SV-CDT values of low-S bioapatite samples (0.14 to 1.19 wt%) with a good analytical precision (1σ = 0.5; n = 14). Our results also show that the δ(34) SV-CDT values of modern and fossil vertebrate bioapatites allow discrimination between marine environments and freshwater or terrestrial ones.

CONCLUSIONS

Sulfur isotope analysis of bioapatite has great potential to track the living environment of extinct vertebrates for which only fossilised bones or teeth have been preserved. Copyright © 2016 John Wiley & Sons, Ltd.

Tooth development and histology patterns in lamniform sharks (Elasmobranchii, Lamniformes) revisited

The dentition of lamniforme sharks exhibits several characters that have been used extensively to resolve the phylogenetic relationships of extant taxa, yet some uncertainties remain. Also, the development of different teeth of a tooth file within the jaws of most extant lamniforms has not been documented to date. High-resolution micro-computed tomography is used here to re-evaluate the importance of two dental characters within the order Lamniformes, which were considered not to be phylogenetically informative, the histotype and the number of teeth per tooth file. Additionally, the development and mineralization patterns of the teeth of the two osteodont lamniforms Lamna nasus and Alopias superciliosus were compared. We discuss the importance of these dental characters for phylogenetic interpretations to assess the quality of these characters in resolving lamniform relationships. The dental characters suggest that (1) Lamniformes are the only modern-level sharks exhibiting the osteodont histotype, (2) the osteodont histotype in lamniform sharks is a derived state in modern-level sharks (Elasmobranchii), (3) the osteodont type, conversely is convergently achieved when the clade Chondrichthyes is considered and thus might comprise a functional rather than a phylogenetic signal, and (4) there is an increase in the number of teeth per file throughout lamniform phylogeny. Structural development of the teeth of L. nasus and A. superciliosus is congruent with a previous investigation of the lamniform shark Carcharodon carcharias. J. Morphol. 277:1584-1598, 2016. © 2016 Wiley Periodicals, Inc.

Histology and affinity of anaspids, and the early evolution of the vertebrate dermal skeleton

The assembly of the gnathostome bodyplan constitutes a formative episode in vertebrate evolutionary history, an interval in which the mineralized skeleton and its canonical suite of cell and tissue types originated. Fossil jawless fishes, assigned to the gnathostome stem-lineage, provide an unparalleled insight into the origin and evolution of the skeleton, hindered only by uncertainty over the phylogenetic position and evolutionary significance of key clades. Chief among these are the jawless anaspids, whose skeletal composition, a rich source of phylogenetic information, is poorly characterized. Here we survey the histology of representatives spanning anaspid diversity and infer their generalized skeletal architecture. The anaspid dermal skeleton is composed of odontodes comprising spheritic dentine and enameloid, overlying a basal layer of acellular parallel fibre bone containing an extensive shallow canal network. A recoded and revised phylogenetic analysis using equal and implied weights parsimony resolves anaspids as monophyletic, nested among stem-gnathostomes. Our results suggest the anaspid dermal skeleton is a degenerate derivative of a histologically more complex ancestral vertebrate skeleton, rather than reflecting primitive simplicity. Hypotheses that anaspids are ancestral skeletonizing lampreys, or a derived lineage of jawless vertebrates with paired fins, are rejected.

Comments on “Dental lessons from past to present: ultrastructure and composition of teeth from plesiosaurs, dinosaurs, extinct and recent sharks” by A. Lübke, J. Enax, K. Loza, O. Prymak, P. Gaengler, H.-O. Fabritius, D. Raabe and M. Epple, RSC Adv., 2015, 5, 61612

The noted paper suggested that high fluoride levels in teeth of extinct animals proves the use of fluoroapatite when alive, but it is well-know that F⁻ increases during fossilization due to diagenetic passage of hydroxy to more stable fluoroapatite.

SCPP genes in the coelacanth: tissue mineralization genes shared by sarcopterygians

The coelacanth is the basal-most extant sarcopterygian that has teeth and tooth-like structures, comprising bone, dentin, and enamel or enameloid. Formation of these tissues involves many members of the secretory calcium-binding protein (SCPP) family. In tetrapods, acidic-residue-rich SCPPs are used in mineralization of bone and dentin, whereas Pro/Gln-rich SCPPs participate in enamel formation. Teleosts also employ many SCPPs for tissue mineralization. Nevertheless, the repertoire of SCPPs is largely different in teleosts and tetrapods; hence, filling this gap would be critical to elucidate early evolution of mineralized tissues in osteichthyans. In the present study, we searched for SCPP genes in the coelacanth genome and identified 11, of which two have clear orthologs in both tetrapods and teleosts, seven only in tetrapods, and two in neither of them. Given the divergence times of these vertebrate lineages, our discovery of this many SCPP genes shared between the coelacanth and tetrapods, but not with teleosts, suggests a complicated evolutionary scheme of SCPP genes in early osteichthyans. Our investigation also revealed both conserved and derived characteristics of SCPPs in the coelacanth and other vertebrates. Notably, acidic SCPPs independently evolved various acidic repeats in different lineages, while maintaining high acidity, presumably important for interactions with calcium. Furthermore, the three Pro/Gln-rich SCPP genes, required for mineralizing enamel matrix and confirmed only in tetrapods, were all identified in the coelacanth, strongly suggesting that enamel is equivalent in the coelacanth and tetrapods. This finding corroborates the previous proposition that true enamel evolved much earlier than the origin of tetrapods.

Development and microstructure of tooth histotypes in the blue shark, Prionace glauca (Carcharhiniformes: Carcharhinidae) and the great white shark, Carcharodon carcharias (Lamniformes: Lamnidae)

Elasmobranchs exhibit two distinct arrangements of mineralized tissues in the teeth that are known as orthodont and osteodont histotypes. Traditionally, it has been said that orthodont teeth maintain a pulp cavity throughout tooth development whereas osteodont teeth are filled with osteodentine and lack a pulp cavity when fully developed. We used light microscopy, scanning electron microscopy, and high-resolution micro-computed tomography to compare the structure and development of elasmobranch teeth representing the two histotypes. As an example of the orthodont histotype, we studied teeth of the blue shark, Prionace glauca (Carcharhiniformes: Carcharhinidae). For the osteodont histotype, we studied teeth of the great white shark, Carcharodon carcharias (Lamniformes: Lamnidae). We document similarities and differences in tooth development and the microstructure of tissues in these two species and review the history of definitions and interpretations of elasmobranch tooth histotypes. We discuss a possible correlation between tooth histotype and tooth replacement and review the history of histotype differentiation in sharks. We find that contrary to a long held misconception, there is no orthodentine in the osteodont teeth of C. carcharias.

Histology of the heterostracan dermal skeleton: Insight into the origin of the vertebrate mineralised skeleton

Living vertebrates are divided into those that possess a fully formed and fully mineralised skeleton (gnathostomes) versus those that possess only unmineralised cartilaginous rudiments (cyclostomes). As such, extinct phylogenetic intermediates of these living lineages afford unique insights into the evolutionary assembly of the vertebrate mineralised skeleton and its canonical tissue types. Extinct jawless and jawed fishes assigned to the gnathostome stem evidence the piecemeal assembly of skeletal systems, revealing that the dermal skeleton is the earliest manifestation of a homologous mineralised skeleton. Yet the nature of the primitive dermal skeleton, itself, is poorly understood. This is principally because previous histological studies of early vertebrates lacked a phylogenetic framework required to derive evolutionary hypotheses. Nowhere is this more apparent than within Heterostraci, a diverse clade of primitive jawless vertebrates. To this end, we surveyed the dermal skeletal histology of heterostracans, inferred the plesiomorphic heterostracan skeleton and, through histological comparison to other skeletonising vertebrate clades, deduced the ancestral nature of the vertebrate dermal skeleton. Heterostracans primitively possess a four‐layered skeleton, comprising a superficial layer of odontodes composed of dentine and enameloid; a compact layer of acellular parallel‐fibred bone containing a network of vascular canals that supply the pulp canals (L1); a trabecular layer consisting of intersecting radial walls composed of acellular parallel‐fibred bone, showing osteon‐like development (L2); and a basal layer of isopedin (L3). A three layered skeleton, equivalent to the superficial layer L2 and L3 and composed of enameloid, dentine and acellular bone, is possessed by the ancestor of heterostracans + jawed vertebrates. We conclude that an osteogenic component is plesiomorphic with respect to the vertebrate dermal skeleton. Consequently, we interpret the dermal skeleton of denticles in chondrichthyans and jawless thelodonts as independently and secondarily simplified. J. Morphol. 276:657–680, 2015. © 2015 The Authors Journal of Morphology Published by Wiley Periodicals, Inc.

Materials engineering by ameloblasts

Enamel is unique. It is the only epithelial-derived mineralized tissue in mammals and has a distinct micro- and nanostructure with nanofibrous apatite crystals as building blocks. It is synthesized by a highly specialized cell, the ameloblast, which secretes matrix proteins with little homology to any other known amino acid sequence, but which is composed of a primary structure that makes it competent to self-assemble and control apatite crystal growth at the nanometer scale. The end-product of ameloblast activity is a marvel of structural engineering: a material optimized to provide the tooth with maximum biting force, withstanding millions of cycles of loads without catastrophic failure, while also protecting the dental pulp from bacterial attack. This review attempts to bring into context the mechanical behavior of enamel with the developmental process of amelogenesis and structural development, since they are linked to tissue function, and the importance of controlling calcium phosphate mineralization at the nanometer scale. The origins of apatite nanofibers, the development of a stiffness gradient, and the biological processes responsible for the synthesis of a hard and fracture-resistant dental tissue are discussed with reference to the evolution of enamel from a fibrous composite to a complex, tough, and damage-tolerant coating on dentin.

Ultrastructural organization and micromechanical properties of shark tooth enameloid

The outer part of shark teeth is formed by the hard and mineral-rich enameloid that has excellent mechanical properties, which makes it a very interesting model system for the development of new bio-inspired dental materials. We characterized the microstructure, chemical composition and resulting local mechanical properties of the enameloid from teeth of Isurus oxyrinchus (shortfin mako shark) by performing an in-depth analysis using various high-resolution analytical techniques, including scanning electron microscopy, qualitative energy-dispersive X-ray spectroscopy and nanoindentation. Shark tooth enameloid reveals an intricate hierarchical arrangement of thin (50-80nm) and long (>1μm) crystallites of fluoroapatite with a high degree of structural anisotropy, which leads to exceptional mechanical properties. Both stiffness and hardness are surprisingly homogeneous in the shiny layer as well as in the enameloid: although both tooth phases differ in structure and composition, they show almost no orientation dependence with respect to the loading direction of the enameloid crystallites. The results were used to determine the structural hierarchy of shark teeth, which can be used as a base for establishing design criteria for synthetic bio-inspired and biomimetic dental composites.

Histology of "placoderm" dermal skeletons: Implications for the nature of the ancestral gnathostome

The vertebrate dermal skeleton has long been interpreted to have evolved from a primitive condition exemplified by chondrichthyans. However, chondrichthyans and osteichthyans evolved from an ancestral gnathostome stem‐lineage in which the dermal skeleton was more extensively developed. To elucidate the histology and skeletal structure of the gnathostome crown‐ancestor we conducted a histological survey of the diversity of the dermal skeleton among the placoderms, a diverse clade or grade of early jawed vertebrates. The dermal skeleton of all placoderms is composed largely of a cancellar architecture of cellular dermal bone, surmounted by dermal tubercles in the most ancestral clades, including antiarchs. Acanthothoracids retain an ancestral condition for the dermal skeleton, and we record its secondary reduction in antiarchs. We also find that mechanisms for remodeling bone and facilitating different growth rates between adjoining plates are widespread throughout the placoderms. J. Morphol., 2013. © 2013 Wiley Periodicals, Inc.

Structure, composition, and mechanical properties of shark teeth

The teeth of two different shark species (Isurus oxyrinchus and Galeocerdo cuvier) and a geological fluoroapatite single crystal were structurally and chemically characterized. In contrast to dentin, enameloid showed sharp diffraction peaks which indicated a high crystallinity of the enameloid. The lattice parameters of enameloid were close to those of the geological fluoroapatite single crystal. The inorganic part of shark teeth consisted of fluoroapatite with a fluoride content in the enameloid of 3.1 wt.%, i.e., close to the fluoride content of the geological fluoroapatite single crystal (3.64 wt.%). Scanning electron micrographs showed that the crystals in enameloid were highly ordered with a special topological orientation (perpendicular towards the outside surface and parallel towards the center). By thermogravimetry, water, organic matrix, and biomineral in dentin and enameloid of both shark species were determined. Dentin had a higher content of water, organic matrix, and carbonate than enameloid but contained less fluoride. Nanoindentation and Vicker's microhardness tests showed that the enameloid of the shark teeth was approximately six times harder than the dentin. The hardness of shark teeth and human teeth was comparable, both for dentin and enamel/enameloid. In contrast, the geological fluoroapatite single crystal was much harder than both kinds of teeth due to the absence of an organic matrix. In summary, the different biological functions of the shark teeth ("tearing" for Isurus and "cutting" for Galeocerdo) are controlled by the different geometry and not by the chemical or crystallographic composition.

The homology of odontodes in gnathostomes: insights from Dlx gene expression in the dogfish, Scyliorhinus canicula

BACKGROUND

Teeth and tooth-like structures, together named odontodes, are repeated organs thought to share a common evolutionary origin. These structures can be found in gnathostomes at different locations along the body: oral teeth in the jaws, teeth and denticles in the oral-pharyngeal cavity, and dermal denticles on elasmobranch skin. We, and other colleagues, had previously shown that teeth in any location were serially homologous because: i) pharyngeal and oral teeth develop through a common developmental module; and ii) the expression patterns of the Dlx genes during odontogenesis were highly divergent between species but almost identical between oral and pharyngeal dentitions within the same species. Here we examine Dlx gene expression in oral teeth and dermal denticles in order to test the hypothesis of serial homology between these odontodes.

RESULTS

We present a detailed comparison of the first developing teeth and dermal denticles (caudal primary scales) of the dogfish (Scyliorhinus canicula) and show that both odontodes develop through identical stages that correspond to the common stages of oral and pharyngeal odontogenesis. We identified six Dlx paralogs in the dogfish and found that three showed strong transcription in teeth and dermal denticles (Dlx3, Dlx4 and Dlx5) whereas a weak expression was detected for Dlx1 in dermal denticles and teeth, and for Dlx2 in dermal denticles. Very few differences in Dlx expression patterns could be detected between tooth and dermal denticle development, except for the absence of Dlx2 expression in teeth.

CONCLUSIONS

Taken together, our histological and expression data strongly suggest that teeth and dermal denticles develop from the same developmental module and under the control of the same set of Dlx genes. Teeth and dermal denticles should therefore be considered as serial homologs developing through the initiation of a common gene regulatory network (GRN) at several body locations. This mechanism of heterotopy supports the 'inside and out' model that has been recently proposed for odontode evolution.

Origin and evolution of the integumentary skeleton in non-tetrapod vertebrates

Most non-tetrapod vertebrates develop mineralized extra-oral elements within the integument. Known collectively as the integumentary skeleton, these elements represent the structurally diverse skin-bound contribution to the dermal skeleton. In this review we begin by summarizing what is known about the histological diversity of the four main groups of integumentary skeletal tissues: hypermineralized (capping) tissues; dentine; plywood-like tissues; and bone. For most modern taxa, the integumentary skeleton has undergone widespread reduction and modification often rendering the homology and relationships of these elements confused and uncertain. Fundamentally, however, all integumentary skeletal elements are derived (alone or in combination) from only two types of cell condensations: odontogenic and osteogenic condensations. We review the origin and diversification of the integumentary skeleton in aquatic non-tetrapods (including stem gnathostomes), focusing on tissues derived from odontogenic (hypermineralized tissues, dentines and elasmodine) and osteogenic (bone tissues) cell condensations. The novelty of our new scenario of integumentary skeletal evolution resides in the demonstration that elasmodine, the main component of elasmoid scales, is odontogenic in origin. Based on available data we propose that elasmodine is a form of lamellar dentine. Given its widespread distribution in non-tetrapod lineages we further propose that elasmodine is a very ancient tissue in vertebrates and predict that it will be found in ancestral rhombic scales and cosmoid scales.

Enameloid microstructure of the serrated cutting edges in certain fossil carcharhiniform and lamniform sharks

The triple-layered enameloid organization of neoselachian teeth has proven to be a reliable systematic character of the group. This study uses scanning electron microscopy to investigate the orientation of the parallel enameloid bundles in the area of the serrated cutting edges in certain fossil elasmobranchs. The examined teeth come from two Upper Cretaceous Squalicorax species and the Upper Miocene carcharhiniforms Galeocerdo sp., Carcharhinus sp., and Hemipristis serra. The parallel bundles are revealed by surface etching, which removes the superficial shiny-layered enameloid. In the teeth of Squalicorax, the bundles around the cutting edge bend once, before they reach the serrations. The studied carcharhiniform species show a more complicated pattern with a change of parallel bundle course inside the serrations. H. serra teeth do not display the first bending of the bundles, whereas it was present in the other two carcharhiniforms. The course of the crystalline bundles in both Squalicorax species is not affected by the presence of the serrations, regardless of the twofold difference in tooth size between them. In the carcharhiniform species, the bended bundles occur within the primary and secondary serrations and are always associated with them. This feature might have functional significance by strengthening the cutting edge or could simply develop as a consequence of the enameloid mineralization around the individual serrae.

A new elasmobranch fauna from the Middle Jurassic of southern Thailand

We describe a new elasmobranch fauna from the lower part of the Khlong Min Formation in Thailand. The fauna includesHybodussp.,Asteracanthussp.,Lonchidion reesunderwoodisp. nov.,Belemnobatis aominensissp. nov., and possibly a second species ofBelemnobatis.This fauna supports a Bathonian–Callovian age for the Khlong Min Formation, and suggests a close taxonomic relationship between the Middle Jurassic elasmobranch faunas of Europe and Thailand. The presence of a monolayered enameloid inBelemnobatis aominensissp. nov. and other primitive batoids is interpreted as the retention of a primitive character for neoselachians, which would suggest a divergence time between the batoids and the rest of the neoselachian sharks as early as the Carboniferous–Permian boundary.

SCPP gene evolution and the dental mineralization continuum

Many genes critical to vertebrate skeletal mineralization are members of the secretory calcium-binding phosphoprotein (SCPP) gene family, which has evolved by gene duplication from a single ancestral gene. In humans, mutations in some of these SCPP genes have been associated with various diseases related to dentin or enamel hypoplasia. Recently, systematic searches for SCPP genes of various species have allowed us to investigate the history of phylogenetically variable dental tissues as a whole. One important conclusion is that not all disease-associated SCPP genes are present in tetrapods, and teleost fish probably have none, even in toothed species, having acquired their complement of SCPP genes through an independent duplication history. Here, we review comparative analyses of mineralized dental tissues, with particular emphasis on the use of SCPPs, within and between tetrapods and teleosts. Current knowledge suggests a close relationship among bone, dentin, teleost fish enameloid (enamel-like hard tissue), and tetrapod enamel. These tissues thus form a mineralized-tissue continuum. Contemporary dental tissues have evolved from an ancestral continuum through lineage-specific modifications.

Conserved features and evolutionary shifts of the EDA signaling pathway involved in vertebrate skin appendage development

It is widely accepted that evolutionary changes in conserved developmental signaling pathways play an important role in morphological evolution. However, few in silico studies were interested in tracking such changes in a signaling pathway. The Ectodysplasin (EDA) pathway provides an opportunity to fill this gap because it is involved in vertebrate skin appendage development such as scales, teeth, hair, and feathers that take an obvious part in the adaptation of species to their environment. We benefited from the large amount of genomic data now available to explore the evolution of the upstream genes of the EDA pathway. In mammals, these genes are eda (encoding 2 ligands, EDA-A1 and EDA-A2), edar (EDA-A1 receptor), edaradd (EDA receptor [EDAR] adapter), xedar (EDA-A2 receptor), and troy (a XEDAR-related receptor). We show that the evolution of EDA pathway genes combines both strongly conserved features and evolutionary shifts. These shifts are found at different signaling levels (from the ligand to intracellular signaling) and at different taxonomic levels (class, suborder, and genera). Although conserved features likely participate to the similarities found in the early development of vertebrate skin appendages, these shifts might account for innovations and specializations. Moreover, our study demonstrates that we can now benefit from the large number of sequenced vertebrate genomes to explore the evolution of specific signaling pathways and thereby to open new perspectives for developmental biology and evolutionary developmental biology.