Morphology and development of teeth and epidermal brushes in loricariid catfishes

Loricariidae or suckermouth armored catfishes are one of several aquatic taxa feeding on epilithic and epiphytic algae. Their upper and lower jaws bear exquisitely curved teeth, which usually are asymmetrically bicuspid. The enlarged lower lip carries papillae with keratinous unicellular epidermal brushes or unculi. Teeth, and probably unculi too, assist in scraping food off substrates. Their morphology, growth, and replacement is examined and compared among several loricariid species, using cleared and stained specimens, serial sections, and SEM. Apart from the general tooth form and crown shape, the anterior layer of soft tissue on the lower shaft region, present in several species, appears to be a specialization for enhancing the mobility of individual teeth when scraping on uneven surfaces. During early ontogeny, a transition from simple conical to mature tooth occurs. The first unculi appear together with the first teeth carrying a bicuspid crown, 2 days after the first exogenous feeding, but synchronous with the complete resorption of the yolk sac.     

Fine structure of the secretory and non-secretory ameloblasts in the frog. II. Fine structure of the non-secretory ameloblast.

The non-secretory ameloblasts present at the enamel-free surfaces of maxillary teeth in the frog Rana pipiens were examined by electron microscopy at different stages of tooth development. Their main fine structural features seem to reflect a transport function. During early tooth development, the non-secretory ameloblasts adjacent to odontoblasts and predentin exhibit extensive lateral surface specializations and numerous cytoplasmic vesicles. During late tooth development, the non-secretory ameloblasts adjacent to mineralizing dentin show numerous cellular junctions, well-developed intercellular channels with numerous interdigitating processes and labyrinthine configurations at their distal surfaces. An intact basal lamina is present between the non-secretory ameloblasts and the dentin surface until the dentin becomes fully mineralized. At this stage the adjacent cells no longer exhibit surface specializations. It is suggested that the non-secretory ameloblasts may participate in the mineralization of adjacent dentin at the enamel-free surfaces. This surface dentin becomes fully mineralized at a later stage of development than the underlying dentin.     

Osteocalcin and matrix GLA protein in developing teleost teeth: identification of sites of mRNA and protein accumulation at single cell resolution

In this study, the tissue distribution and accumulation of osteocalcin or bone Gla protein (BGP) and matrix Gla protein (MGP) were determined during tooth development in a teleost fish, Argyrosomus regius. In this species, the presence of BGP and MGP mRNA in teeth was revealed by in situ hybridization. mRNA for BGP was detected in the odontoblasts as well as in its cytoplasmic processes emerging through dentinal tubules, while mRNA for MGP was expressed in the enamel portion within the apical portion of the elongated cell bodies of enameloblasts, adjacent to the root of the teeth as well as in cells within the pulpal space. Immunolocalization of BGP and MGP demonstrated that these proteins accumulate mainly in the mineralized dentin or in enameloblastic processes, confirming in situ hybridization results. In this study, we examined for the first time the localization of both BGP and MGP gene expression and protein accumulation within the different regions of the vertebrate tooth. We clearly demonstrated that although the overall pattern of BGP and MGP gene expression and protein accumulation in A. regius teeth was in general agreement to what is known for other vertebrates such as rats or rodents, our study provided novel information and highlighted some species-differences between fish and higher vertebrates.     

Dentition in the African catfishes Andersonia (Amphiliidae) and Siluranodon (Schilbeidae) previously considered toothless

Based on light and scanning electron microscopic examination of their morphology, the dentition on both the premaxilla and dentary of Andersonia (Amphiliidae) and Siluranodon (Schilbeidae) catfishes is described from samples taken from tributaries of the White Nile in south‐western Ethiopia. These monotypic African genera were previously believed to lack teeth on the lower jaw in Andersonia and on both jaws in Siluranodon . Siluranodon exhibits an ontogenetic reduction: teeth were less frequently found in larger individuals than in smaller ones. In contrast to the adults of all other schilbeids, whose oral teeth are arranged in multiserial (or at least, biserial) bands, Siluranodon has uniserial teeth on both the premaxilla and the dentary. The adaptive, ontogenetic and phylogenetic aspects of jaw‐tooth reduction in catfishes are discussed.     

Evolutionary genetics of vertebrate tissue mineralization: the origin and evolution of the secretory calcium-binding phosphoprotein family

Three principal mineralized tissues are present in teeth; a highly mineralized surface layer (enamel or enameloid), body dentin, and basal bone. Similar tissues have been identified in the dermal skeleton of Paleozoic jawless vertebrates, suggesting their ancient origin. These dental tissues form on protein matrix and their mineralization is controlled by distinctive proteins. We have shown that many secretory calcium-binding phosphoproteins (SCPPs) are involved in tetrapod tissue mineralization. These SCPPs all originated from the common ancestral gene SPARCL1 (secreted protein, acidic, cysteine-rich like 1) that initially arose from SPARC. The SCPP family also includes a bird eggshell matrix protein, mammalian milk casein, and salivary proteins. The eggshell SCPP plays crucial roles in rigid eggshell production, milk SCPPs in efficient lactation and in the evolution of complex dentition, and salivary SCPPs in maintaining tooth integrity. A comparative analysis of the mammalian, avian, and amphibian genomes revealed a tandem duplication history of the SCPP genes in tetrapods. Although these tetrapod SCPP genes are fewer in teleost genomes, independent parallel duplication has created distinct SCPP genes in this lineage. These teleost SCPPs are also used for enameloid and dentin mineralization, implying essential roles of SCPPs for dental tissue mineralization in osteichthyans. However, the SCPPs used for tetrapod enamel and teleost enameloid, as well as tetrapod dentin and teleost dentin, are all different. Thus, the evolution of vertebrate mineralized tissues seems to be explained by phenogenetic drift: while mineralized tissues are retained during vertebrate evolution, the underlying genetic basis has extensively drifted.     

Structure of the radially asymmetrical uncalcified region of the teeth of the red-backed salamander,Plethodon cinereus (Amphibia,Plethodontidae)

The teeth of the adult plethodontid salamander, Plethodon cinereus, were examined by light and electron microscopy with emphasis on the ringlike zone of uncalcified dentin that divides the calcified portion of each tooth into a proximal pedestal and a distal apex. The uncalcified region displays radial asymmetry, forming an integral part of the posterior wall of the tooth but bulging into the pulp cavity anteriorly, thus forming a hingelike structure. All portions of the dentin, including the uncalcified region, are composed predominantly of collagenous fibers but lack elastin. In scanning electron micrographs of teeth from which the oral mucosa has been removed, the location of the anterior uncalcified hinge is marked externally by a notch-like articulation of the apex and pedestal. Sites of transition between calcified and uncalcified areas of the dentin show no special modifications in transmission electron micrographs, but collagenous fibers in calcified portions are associated with more electron-dense amorphous material than are those in the uncalcified region. Odontoblasts associated with the uncalcified region possess ultrastructural features closely resembling those of odontoblasts found in calcified areas. The uncalcified region seems to afford the teeth a certain degree of flexibility, and the asymmetry of the region appears to allow the teeth to flex only in a posterior direction, thus facilitating the entry of living prey but hindering its escape. The uncalcified region also seems to permit the apex of a tooth to break away from its pedestal without damage to underlying bone.     

Enamel dictates whole tooth deformation: A finite element model study validated by a metrology method

In order to understand whole tooth behavior under load the biomechanical role of enamel and dentin has to be determined. We approach this question by comparing the deformation pattern and stiffness of intact teeth under load with the deformation pattern and stiffness of the same teeth after the enamel has been mechanically compromised by introducing a defect. FE models of intact human premolars, based on high resolution micro-CT scans, were generated and validated by in vitro electronic speckle pattern interferometry (ESPI) experiments. Once a valid FE model was established, we exploit the flexibility of the FE model to gain more insight into whole tooth function. Results show that the enamel cap is an intrinsically stiff biological structure and its morphology dictates the way a whole tooth will mechanically behave under load. The mechanical properties of the enamel cap were sufficient to mechanically maintain almost its entire stiffness function under load even when a small defect (cavity simulating caries) was introduced into its structure and breached the crown integrity. We conclude that for the most part, that enamel and not dentin dictates the mechanical behavior of the whole tooth.     

Morphological variations in a tooth family through ontogeny in Pleurodeles waltl (Lissamphibia, Caudata)

Most nonmammalian species replace their teeth continuously (so-called polyphyodonty), which allows morphological and structural modifications to occur during ontogeny. We have chosen Pleurodeles waltl, a salamander easy to rear in the laboratory, as a model species to establish the morphological foundations necessary for future molecular approaches aiming to understand not only molecular processes involved in tooth development and replacement, but also their changes, notably during metamorphosis, that might usefully inform studies of modifications of tooth morphology during evolution. In order to determine when (in which developmental stage) and how (progressively or suddenly) tooth modifications take place during ontogeny, we concentrated our observations on a single tooth family, located at position I, closest to the symphysis on the left lower jaw. We monitored the development and replacement of the six first teeth in a large growth series ranging from 10-day-old embryos (tooth I1) to adult specimens (tooth I6), using light (LM), scanning (SEM), and transmission electron (TEM) microscopy. A timetable of the developmental and functional period is provided for the six teeth, and tooth development is compared in larvae and young adults. In P. waltl the first functional tooth is not replaced when the second generation tooth forms, in contrast to what occurs for the later generation teeth, leading to the presence of two functional teeth in a single position during the first 2 months of life. Larval tooth I1 shows dramatically different features when compared to adult tooth I6: a dividing zone has appeared between the dentin cone and the pedicel; the pulp cavity has enlarged, allowing accommodation of large blood vessels; the odontoblasts are well organized along the dentin surface; tubules have appeared in the dentin; and teeth have become bicuspidate. Most of these modifications take place progressively from one tooth generation to the next, but the change from monocuspid to bicuspid tooth occurs during the tooth I3 to tooth I4 transition at metamorphosis.     

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

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

Dentin regeneration based on tooth tissue engineering: A review

Missing or damaged teeth due to caries, genetic disorders, oral cancer, or infection may contribute to physical and mental impairment that reduces the quality of life. Despite major progress in dental tissue repair and those replacing missing teeth with prostheses, clinical treatments are not yet entirely satisfactory, as they do not regenerate tissues with natural teeth features. Therefore, much of the focus has centered on tissue engineering (TE) based on dental stem/progenitor cells to create bioengineered dental tissues. Many in vitro and in vivo studies have shown the use of cells in regenerating sections of a tooth or a whole tooth. Tooth tissue engineering (TTE), as a promising method for dental tissue regeneration, can form durable biological substitutes for soft and mineralized dental tissues. The cell-based TE approach, which directly seeds cells and bioactive components onto the biodegradable scaffolds, is currently the most potential method. Three essential components of this strategy are cells, scaffolds, and growth factors (GFs). This study investigates dentin regeneration after an injury such as caries using TE and stem/progenitor cell-based strategies. We begin by discussing about the biological structure of a dentin and dentinogenesis. The engineering of teeth requires knowledge of the processes that underlie the growth of an organ or tissue. Then, the three fundamental requirements for dentin regeneration, namely cell sources, GFs, and scaffolds are covered in the current study, which may ultimately lead to new insights in this field.     

Do all geckos hatch in the same way? Histological and 3D studies of egg tooth morphogenesis in the geckos Eublepharis macularius Blyth 1854 and Lepidodactylus lugubris Duméril & Bibron 1836

The egg tooth of squamates evolved to facilitate hatching from mineralized eggshells. Squamate reptiles can assist their hatching with a single unpaired egg tooth (unidentates) or double egg teeth (geckos and dibamids). Egg tooth ontogeny in two gekkotan species, the leopard gecko Eublepharis macularius and the mourning gecko Lepidodactylus lugubris, was compared using microtomography, scanning electron microscopy, and light microscopy. Investigated species are characterized by different hardnesses of their eggshells. Leopard geckos eggs have a relatively soft and flexible parchment (leathery) shell, while eggshells of mourning geckos are hard and rigid. Embryos of both species, like other Gekkota, have double egg teeth, but the morphology of these structures differs between the investigated species. These differences in shape, localization, and spatial orientation were present from the earliest stages of embryonic development. In mourning gecko, anlagen of differentiating egg teeth change their position on the palate during embryonic development. Initially they are separated by condensed mesenchyme, but later in development, their enamel organs are connected. In leopard geckos, the localization of egg tooth germs does not change, but their spatial orientation does. Egg teeth of this species shift from inward to outward orientation. This is likely related to differences in structure and mechanical properties of eggshells in the studied species. In investigated species, two hatching mechanisms are possible during emergence of young individuals. We speculate that mourning geckos break the eggshell through puncturing action with egg teeth, similar to the pipping phase of chick and turtles embryos. Egg teeth of leopard geckos cut egg membranes similarly to most squamates. Our results also revealed differences in egg tooth implantation between Gekkota and Unidentata: gekkotan egg teeth are subthecodont (in shallow sockets), while those in unidentates are acrodont (attached to the top of the alveolar ridge). © 2020 Wiley Periodicals LLC     

Mechanical wear of the gastropod radula: a scanning electron microscope study

The wear sustained by the teeth of the highly mineralized radula of Patella vulgata and the unmineralized radula of Agriolomax reticulatus has been studied with the scanning electron microscope. The unworn teeth of P. vulgata have tall pointed cusps and wear is first seen as a chipping of the tips then as abrasion. There is a well defined abraded surface giving the worn teeth a chisel shape. In cleaved teeth fibres ˜ 800 Å in diameter are observed parallel to the axis of the tooth. Evidence is presented for chemical etching and for the existence of a surface coat on the tooth. A. reticulatus teeth show wear over their whole surface. With the aid of the grooves in the jaw caused by the teeth the role of the jaw in flattening the radula and protracting and retracting the odontophore is confirmed. From the arrangement of the abraded surfaces on the teeth of P. vulgata it was deduced that the teeth rows are abraded one at a time while on a convex surface at or near the bending plane.     

Tissue-specific expression of dentin sialophosphoprotein (DSPP) and its polymorphisms in mouse tissues

Dentin sialophosphoprotein (DSPP) consists of dentin sialoprotein (DSP) and dentin phosphoprotein (DPP). DSPP is highly expressed in mineralized tissues. However, recent studies have shown that DSPP is also expressed in several active metabolic ductal epithelial tissues and exists in a variety of sequences. We have investigated DSPP expression in various mouse tissues using RT-PCR, in situ hybridization and immunohistochemical analyses. To identify DSPP gene polymorphisms, we screened a mouse tooth cDNA library as well as isolated and characterized DSPP variations. Our results show that DSPP is predominantly expressed in teeth and moderately in bone tissues. We also have characterized a full-length DSPP cDNA clone with an open-reading frame of 940 codons and this polyadenylation signal. Compared to previously reported mouse DSPP cDNAs, 13 sequence variations were identified, including 8 non-synonymous single nucleotide polymorphisms and an in-frame indel (8 amino acids) at DPP domain of the mouse DSPP. These 8 amino acids are rich in aspartic acid and serine residues. Northern blot assay showed a prominent band at 4.4 kb. RT-PCR demonstrated that this mouse DSPP gene was dominantly expressed in teeth. The predicted secondary structure of DPP domain of this DSPP showed differences from the previously published mouse DPPs, implying that they play different roles during tooth development and formation.     

The NH2-terminal and COOH-terminal fragments of dentin matrix protein 1 (DMP1) localize differently in the compartments of dentin and growth plate of bone

Multiple studies have shown that dentin matrix protein 1 (DMP1) is essential for bone and dentin mineralization. After post-translational proteolytic cleavage, DMP1 exists within the extracellular matrix of bone and dentin as an NH2-terminal fragment, a COOH-terminal fragment, and the proteoglycan form of the NH2-terminal fragment (DMP1-PG). To begin to assess the biological function of each fragment, we evaluated the distribution of both fragments in the rat tooth and bone using antibodies specific to the NH2-terminal and COOH-terminal regions of DMP1 and confocal microscopy. In rat first molar organs, the NH2-terminal fragment localized to predentin, whereas the COOH-terminal fragment was mainly restricted to mineralized dentin. In the growth plate of bone, the NH2-terminal fragment appeared in the proliferation and hypertrophic zones, whereas the COOH-terminal fragment occupied the ossification zone. Forster resonance energy transfer analysis showed colocalization of both fragments of DMP1 in odontoblasts and predentin, as well as hypertrophic chondrocytes within the growth plates of bone. The biochemical analysis of bovine teeth showed that predentin is rich in DMP1-PG, whereas mineralized dentin primarily contains the COOH-terminal fragment. We conclude that the differential patterns of expression of NH2-terminal and COOH-terminal fragments of DMP1 reflect their potentially distinct roles in the biomineralization of dentin and bone matrices.     

Aluminum concentrations in human deciduous enamel and dentin related to dental caries

The aluminum (Al) concentrations in the enamel and dentin of 314 human deciduous teeth were determined in order to examine the relationship between Al and dental caries. The sample teeth were divided into three groups: the sound tooth group, carious tooth group and filled tooth group. The teeth of the carious tooth group were further classified into three groups depending on the stage of caries. The Al content was determined using graphite furnace atomic absorption spectrometry. In both the enamel and dentin, the Al concentrations were unaffected by sex, but did depend on tooth type. In enamel, the Al concentration was significantly higher in the sound tooth group (42.8 +/- 37.3 microg/g) than in the three carious groups (20.7 +/- 17.1-24.9 +/- 22.0 microg/g) and the filled tooth group (27.3 +/- 25.5 microg/g). As for dentin, the Al concentration was also significantly higher in the sound tooth group (36.2 +/- 35.1 microg/g) than in the three carious groups (15.1 +/- 13.3-24.5 +/- 23.4 microg/g) and the filled tooth group (17.2 +/- 20.6 microg/g). Even when analyzing incisors alone, the Al concentrations were significantly higher in the sound tooth group than in the other groups, for both enamel and dentin. Furthermore, the Al levels in carious enamel and dentin did not decrease with the advance of caries. These findings indicated that the deciduous teeth containing higher Al concentrations on average had less caries than the teeth with lower Al concentrations, and suggest that Al acts as a possible cariostatic agent by itself.     

A neontological interpretation of conodont elements based on agnathan cyclostome tooth structire function, and development

Krejsa, R. J., Bringas, P. Jr. & Slavkin, H. C. 1990 10 15: A neontological interpretation of conodont elements based on agnathan cyclostome tooth structure, function, and development. Lethaia , Vol. 23, pp. 359–378. Oslo. ISSN 0024–1164.
Speculation about a conodont-cyclostome connection has led us to search for and establish a biological basis for various characteristic structures in conodont elements. Measurements of juvenile hagfish palatal and lingual teeth overlap those of representative conodont elements, demonstrating a size correspondence of conodonts with teeth of living vertebrates. When hagfish tooth histology is compared with internal and surface topography (SEM) of hagfish, keratinous teeth and mineralized conodont elements, microspaces and tubules similar to those found in hagfish functional tooth coverings and replacement elements are also found within the white matter' of conodont elements. It is provisionally suggested that the primary organic matrix of conodont elements could be keratin and/or keratin-related molecules, and that individual conodont elements could represent shed tooth coverings. The basal bodies' found in certain conodont elements could be replacement elements. These interpretations are contrary to several paradigms of orthodox conodontology. ▭ Agnatha, conodonts, cyclostomes, hagfish, keratin, paleo-biology, shedding, teeth .     

Apoptosis in developmental and repair-related human tooth remodeling: a view from the inside

Apoptosis is a key phenomenon in the regulation of the life span of odontoblasts, which are responsible for dentin matrix production of the teeth. The mechanism controlling odontoblasts loss in developing, normal, and injured human teeth is largely unknown. A possible correlation between apoptosis and dental pulp volume reduction was examined. Histomorphometric analysis was performed on intact 10 to 14 year-old premolars to follow dentin deposition and evaluate the total number of odontoblasts. Apoptosis in growing healthy teeth as well as in mature irritated human teeth was determined using a modified TUNEL technique and an anti-caspase-3 antibody. In intact growing teeth, the sequential rearrangement of odontoblasts into a multi-layer structure during tooth crown formation was correlated with an apoptotic wave that leads to the massive elimination of odontoblasts. These data suggest that apoptosis, coincident with dentin deposition changes, plays a role in tooth maturation and homeostasis. Massive apoptotic events were observed after dentin irritation. In carious and injured teeth, apoptosis was detected in cells surrounding the lesion sites, as well as in mono-nucleated cells nearby the injury. These results indicate that apoptosis is a part of the mechanism that regulate human dental pulp chamber remodeling during tooth development and pathology.     

MEPE-Derived ASARM Peptide Inhibits Odontogenic Differentiation of Dental Pulp Stem Cells and Impairs Mineralization in Tooth Models of X-Linked Hypophosphatemia

Mutations in PHEX (phosphate-regulating gene with homologies to endopeptidases on the X-chromosome) cause X-linked familial hypophosphatemic rickets (XLH), a disorder having severe bone and tooth dentin mineralization defects. The absence of functional PHEX leads to abnormal accumulation of ASARM (acidic serine- and aspartate-rich motif) peptide − a substrate for PHEX and a strong inhibitor of mineralization − derived from MEPE (matrix extracellular phosphoglycoprotein) and other matrix proteins. MEPE-derived ASARM peptide accumulates in tooth dentin of XLH patients where it may impair dentinogenesis. Here, we investigated the effects of ASARM peptides in vitro and in vivo on odontoblast differentiation and matrix mineralization. Dental pulp stem cells from human exfoliated deciduous teeth (SHEDs) were seeded into a 3D collagen scaffold, and induced towards odontogenic differentiation. Cultures were treated with synthetic ASARM peptides (phosphorylated and nonphosphorylated) derived from the human MEPE sequence. Phosphorylated ASARM peptide inhibited SHED differentiation in vitro, with no mineralized nodule formation, decreased odontoblast marker expression, and upregulated MEPE expression. Phosphorylated ASARM peptide implanted in a rat molar pulp injury model impaired reparative dentin formation and mineralization, with increased MEPE immunohistochemical staining. In conclusion, using complementary models to study tooth dentin defects observed in XLH, we demonstrate that the MEPE-derived ASARM peptide inhibits both odontogenic differentiation and matrix mineralization, while increasing MEPE expression. These results contribute to a partial mechanistic explanation of XLH pathogenesis: direct inhibition of mineralization by ASARM peptide leads to the mineralization defects in XLH teeth. This process appears to be positively reinforced by the increased MEPE expression induced by ASARM. The MEPE-ASARM system can therefore be considered as a potential therapeutic target.     

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. J. Morphol. 276:797–817, 2015. © 2015 Wiley Periodicals, Inc.     

Enameloid/enamel transition through successive tooth replacements in <Emphasis Type="Italic">Pleurodeles waltl</Emphasis> (Lissamphibia,Caudata)

Study of the evolutionary enameloid/enamel transition suffers from discontinuous data in the fossil record, although a developmental enameloid/enamel transition exists in living caudates, salamanders and newts. The timing and manner in which the enameloid/enamel transition is achieved during caudate ontogeny is of great interest, because the caudate situation could reflect events that have occurred during evolution. Using light and transmission electron microscopy, we have monitored the formation of the upper tooth region in six successive teeth of a tooth family (position I) in Pleurodeles waltl from late embryos to young adult. Enameloid has only been identified in embryonic tooth I₁ and in larval teeth I₂ and I₃. A thin layer of enamel is deposited later by ameloblasts on the enameloid surface of these teeth. From post-metamorphic juvenile onwards, teeth are covered with enamel only. The collagen-rich enameloid matrix is deposited by odontoblasts, which subsequently form dentin. Enameloid, like enamel, mineralizes and then matures but ameloblast participation in enameloid matrix deposition has not been established. From tooth I₁ to tooth I₃, the enameloid matrix becomes ever more dense and increasingly comes to resemble the dentin matrix, although it is still subjected to maturation. Our data suggest the absence of an enameloid/enamel transition and, instead, the occurrence of an enameloid/dentin transition, which seems to result from a progressive slowing down of odontoblast activity. As a consequence, the ameloblasts in post-metamorphic teeth appear to synthesize the enamel matrix earlier than in larval teeth.