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Tooth crest formation and the epithelial bulge. Frontal sections through reptilian teeth as they develop from cap to bell stage and final differentiation. (A–E) Unicuspid snake, Python molurus. No bulge is found in the inner enamel epithelium. (F–J) Bicuspid gecko Paroedura picta. A bulge is found in the central part of the inner enamel epithelium. See arrow in (G). (K–O) Lingual-labial crest formation in the central cusp of the chameleon, Chamaeleo calyptratus. (P–T) Lingual-labial crest formation in the central cusp of the anole. Anolis allisoni. A bulge is found in the inner enamel epithelium in both species. See arrow in (L,S). Scale bar = 50 μm.
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Reptiles have a diverse array of tooth shapes, from simple unicuspid to complex multicuspid teeth, reflecting functional adaptation to a variety of diets and eating styles. In addition to cusps, often complex longitudinal labial and lingual enamel crests are widespread and contribute to the final shape of reptile teeth. The simplest shaped unicuspi...
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The dentition and tooth crown microstructure of gekkonids and eublepharids are examined. Scanning electron microscopy shows that the lingual surface of teeth in these lizards has one, two, or, occasionally, several cusps separated by grooves. The teeth of geckoes usually have two (lingual and labial) cusps in the apical region. With respect to the...
Citations
... By contrast, in reptiles with unicuspid teeth, dental epithelial structure like mammalian primary EK has recently been identified (Landova Sulcova et al., 2020; but see Handrigan & Richman, 2010a) and similar signaling molecules as in the EK are expressed in the dental epithelium (Handrigan & Richman, 2010b). Some reptiles have more complex multicuspid teeth, although their morphologies are much less diverse than those of mammals (Zahradnicek et al., 2014). However, it remains unclear whether multiple cusp formation in reptiles is controlled by the arrangement of secondary EKs, as in mammals, or by a different mechanism (Handrigan & Richman, 2011). ...
Phylogenetically, the tribosphenic molars—prototypes of multi-cusped cheek teeth in marsupial and placental mammals—are derived from the single-cusped conical teeth of reptiles through the addition of cusps. Ontogenetically, mammalian molars are formed through the interface between the dental epithelium and mesenchyme (future enamel–dentin junction), becoming geometrically complex by adding epithelial signaling centers, called enamel knots, which determine future cusp positions. To re-evaluate cusp homologies in Mesozoic mammals from an ontogenetic perspective, this study tracked molar development in a living placental mammal species, the house shrew (Suncus murinus), whose molars are morphologically the least derived from tribosphenic prototypes. The development of shrew molars proceeded as if it replayed the evolutionary process of tribosphenic molars. The first formed enamel knots gave rise to the evolutionarily oldest cusps—upper paracone and lower protoconid. The order of formation of other enamel knots and their location in development seemed to trace the order of cusp appearance in evolution. The parallel relationship between ontogeny and phylogeny of mammalian molars, if any, suggests that a change in the timing between developmental events rather than a change in the morphogenetic mechanism itself, should have been a major causal factor for the evolutionary transformation of tooth morphology.
... The single teeth can have nevertheless highly complex morphologies, and their positioning and orientation within the jaw is thought to confer a certain level of functional specialization [88]. Reptiles and amphibians possess relatively simple teeth, which are often continuously replaced [72,89,90]. Mammals display more complex dental structures and generally exhibit a reduced tooth turnover [68,89]. ...
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.
... The asymmetrical distribution of enamel in snakes is similar to what has been observed in mammals, fish, and other reptiles (e.g., Buchtova et al. 2008;Seidel et al. 2010;Zahradnicek et al. 2012;Zahradnicek et al. 2014). This particular enamel distribution arises from differences between lingual and labial cervical loops, where only the latter contain the stem cells responsible for the deposition of the enamel (e.g., Buchtova et al. 2008;Seidel et al. 2010;Handrigan and Richman 2011). ...
... Biomechanically, the asymmetrical distribution of enamel in rodents and primates (aye-aye) results in asymmetrical wear of the teeth and continual sharpening of the occlusal surface (e.g., Tatersall and Schwartz 1974;Druzinsky et al. 2012;Müller et al. 2014). This particular arrangement, in addition to the creation of an epithelial bulge during tooth formation, has been suggested to be a mechanism responsible for the formation of more complex tooth shapes (e.g., multicuspid teeth) or dental ornaments (like enamel ridges and crests) via an increase of the enamel thickness (Buchtova et al. 2008;Zahradnicek et al. 2014). However, previous research showed that snakes lack this epithelial bulge (Buchtova et al. 2008;Handrigan and Richman 2011;Zahradnicek et al. 2014), possibly explaining the simple unicuspid morphology and lack of ornamentation. ...
... This particular arrangement, in addition to the creation of an epithelial bulge during tooth formation, has been suggested to be a mechanism responsible for the formation of more complex tooth shapes (e.g., multicuspid teeth) or dental ornaments (like enamel ridges and crests) via an increase of the enamel thickness (Buchtova et al. 2008;Zahradnicek et al. 2014). However, previous research showed that snakes lack this epithelial bulge (Buchtova et al. 2008;Handrigan and Richman 2011;Zahradnicek et al. 2014), possibly explaining the simple unicuspid morphology and lack of ornamentation. Most research has been done on Colubridae (corn snake E. guttata) and Pythonidae (Python regius, Python sebae, and Python molurus). ...
Synopsis
Teeth are composed of the hardest tissues in the vertebrate body and have been studied extensively to infer diet in vertebrates. The morphology and structure of enamel is thought to reflect feeding ecology. Snakes have a diversified diet, some species feed on armored lizards, others on soft invertebrates. Yet, little is known about how tooth enamel, and specifically its thickness, is impacted by diet. In this study, we first describe the different patterns of enamel distribution and thickness in snakes. Then, we investigate the link between prey hardness and enamel thickness and morphology by comparing the dentary teeth of 63 species of snakes. We observed that the enamel is deposited asymmetrically at the antero-labial side of the tooth. Both enamel coverage and thickness vary a lot in snakes, from species with thin enamel, only at the tip of the tooth to a full facet covered with enamel. There variations are related with prey hardness: snakes feeding on hard prey have a thicker enamel and a lager enamel coverage while species. Snakes feeding on softer prey have a thin enamel layer confined to the tip of the tooth.
... Teeth are crucial for food acquisition and processing. They have different shapes and sizes, which is related to the method of feeding, the type of food and behavior [1]. It is assumed that heterodonty (incisors, canines, premolars and molars) is a feature typical of mammals [2]. ...
... This classic division is currently being questioned, as some researchers point to the occurrence of heterodonts also in other taxa [1,[4][5][6]. ...
Simple Summary
The goal of this study was a detailed analysis of the mandibular teeth structure of the Komodo dragon (Varanus komodoensis), based on macroscopic, histological and computed microtomography examinations. The samples were collected post mortem from two adult female Komodo dragons from Wroclaw Zoo. Macroscopically, each mandibular tooth is laterally flattened and its dental crown has an arcuate shape with the typical denticles of a tooth’s caudal margin. The base of the tooth is filled with abundant plicidentine with secondary lamellae. The cavity in the central part of the tooth is well visible, while collagen fibers fill the spaces between the inner lamellae. In the epiphyseal part, the tooth wall is thin and contains numerous dentin trabeculae that grow into the tooth cavity. The dentine is most developed in the mid-tooth, where numerous tubules are observed. The presence of plicidentine, a small number of odontoblasts and a relatively large amount of adipose tissue cells are typical of the mandibular teeth of V. komodoensis.
Abstract
The present study aimed to characterize the macrostructure and microstructure of the mandibular teeth of the Komodo dragon (Varanus komodoensis) and the methods it uses to obtain food. Examinations were performed using a stereoscopic microscope, autofluorescence method, histological method and computed microtomography. A detailed macro- and micro-structural description of V. komodoensis mandibular teeth were made. The mandibular teeth are laterally flattened along their entire length and the dental crown is hooked caudally. The part of the nasal margin of the tooth crown is irregular, while the caudal margin of the tooth is characteristically serrated, except for the tooth base area. There are longitudinal grooves on the lingual and vestibular surfaces up to the lower third of the tooth height. The mandibular tooth is surrounded by a cuff made of the oral mucosa, containing the opening of the venom gland. In the histological structure of the tooth, the enamel covering the tooth crown and the dentin under the enamel are distinguished. The inside of the tooth, except its basal part, is filled with the tooth chamber, while the inside of the lower part of the tooth is filled with plicidentine, which corresponds to external furrows on the enamel. The plicidentine arrangement resembles a honeycomb. A small amount of dentine folds reach up to the tooth apex. Characteristic features of the structure of the mandibular teeth in V. komodoensis may indicate their significant role, in addition to the venom glands, in obtaining food in the natural environment of this species.
... Teeth are crucial for food acquisition and processing. They have different shapes and sizes, which is related to the method of feeding, the type of food and behavior [1]. It is assumed that heterodonty (incisors, canines, premolars and molars) is a feature typical of mammals [2]. ...
... This classic division is currently being questioned, as some researchers point to the occurrence of heterodonts also in other taxa [1,[4][5][6]. ...
... Out of all these animals we have selected the leopard gecko, Eublepharis macularius for our research because the animals replace their teeth frequently in a highly patterned manner (Brink et al. 2022). Molecular studies have been carried out on the gecko, partially characterizing the main processes of tooth initiation, and differentiation (Brink et al. 2021;Handrigan et al. 2010;Handrigan and Richman 2011;Kim et al. 2020;Zahradnicek et al. 2014). ...
... The leopard gecko has fusion of dentin (contains tubules) to the alveolar bone (woven bone with embedded osteocytes). There is no evidence from our studies (Brink et al. 2021;Brink et al. 2020;Buchtová et al. 2007;Handrigan et al. 2010;Handrigan and Richman 2010; and those of others (Buchtova et al. 2013;Vonk et al. 2008;Zahradnicek et al. 2014) on prehatching snakes and lizards of a follicle in surrounding the developing teeth. In contrast, the mammals and crocodilians form a thecodont tooth attachment which consists of tooth inside a socket that is connected to the bone via a non-calcified periodontal ligament. ...
The aim of this study is to profile the transcriptomes of teeth and the surrounding tissues of an adult lizard dentition (Eublepharis macularius) that is actively replacing teeth throughout life. Bulk RNAseq was used to compare teeth that are in function versus unerupted, developing teeth and single cell RNA-seq was carried out on jaw segments containing the dental forming tissues. In bulk RNAseq data, we found that functional teeth expressed genes involved in bone and tooth resorption. Indeed, multinucleated odontoclasts were abundant in tissue sections of functional teeth undergoing resorption. These are the first steps towards characterizing odontoclasts that participate in physiological resorption. Unexpectedly, chemotaxis gene SEMA3A was expressed within odontoblasts and in adjacent mesenchyme, confirmed using RNAscope. Semaphorins may be involved in regulating odontoclasts during tooth resorption. The scRNA-seq experiment successfully isolated dental mesenchyme and epithelial cells. We confirmed that some of these genes are expressed in the earliest tooth buds within the tooth forming field. In addition, we found evidence of convergent evolution in the tooth eruption trait. Geckos evolved a means for second generation teeth to communicate with the functional teeth. Instead of a dental follicle inducing an eruption pathway as in the mammal, the gecko and other squamate reptiles use the enamel organ of the successional teeth to trigger tooth resorption of the functional teeth, thus creating an eruption pathway. New molecules such as SEMA3A may also participate in this process. Future studies on the gecko will uncover the molecular basis of convergent evolution in the dentition.
... The overall stages of tooth development and composition have remained largely unchanged between mammals and reptiles (Sulcova et al., 2020;Zahradnicek et al., 2014). In both mammals and reptiles, Sonic hedgehog (SHH) is expressed in the early dental epithelium and acts to promote initiation of a tooth (Buchtova et al., 2008;Fons et al., 2020;Li et al., 2016). ...
In the echidna, after development in utero, the egg is laid in the pouch and incubated for 10 days. During this time, the fetuses develop an egg tooth and caruncle to help them hatch. Using rare and unprecedented access to limited echidna pre- and post-hatching tissues, the egg tooth and caruncle development were assessed by micro-CT, histology and immunofluorescence. Unlike mammalian tooth germs that develop by placode invagination, the echidna egg tooth developed by evagination, similar to the first teeth in some reptiles and fish. The egg tooth ankylosed to the premaxilla, rather than forming a tooth root with ligamentous attachment found in all other mammals, with loss of the egg tooth associated with high levels of odontoclasts and by apoptosis. The caruncle formed as a separate mineralisation from the adjacent nasal capsule, and as observed in birds and turtles, the nasal region epithelium on top of the nose expressed markers of cornification. Together, this highlights that the monotreme egg tooth shares many similarities with typical reptilian teeth, suggesting that this tooth has been conserved from a common ancestor of mammals and reptiles.
... In the subset of reptiles studied to date, there appears to be no clear histologically definable EK (Buchtová et al., 2008;Weeks et al., 2013), although in some species there is a thickening of the inner dental epithelium which leads to the asymmetric deposition of enamel and the formation of cusps (Zahradnicek et al., 2014). Furthermore, whilst in reptiles, cells of the inner dental epithelium are non-proliferative, this region is not as highly restricted to the cusp as in mammalian molars (Buchtová et al., 2008;Handrigan and Richman, 2011). ...
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.
... Squamates can have simple teeth in homodont dentitions or complex, multicuspid teeth in heterodont dentitions 17 , and squamate ecology spans a broad range of past and present niches. Squamates express dental marker genes broadly conserved across vertebrates 15 , with varying patterns of localisation and expression compared to mammals, and structures at least partially homologous to mammalian enamel knots (non-proliferative signalling centres of ectodermal cells) determine tooth shape in some squamate clades 16,18,19 . In mammalsthe most commonly studied dental system-novel morphologies arise from developmental changes in tooth morphogenesis 20 . ...
Teeth act as tools for acquiring and processing food, thus holding a prominent role in vertebrate evolution. In mammals, dental-dietary adaptations rely on tooth complexity variations controlled by cusp number and pattern. Complexity increase through cusp addition has dominated the diversification of mammals. However, studies of Mammalia alone cannot reveal patterns of tooth complexity conserved throughout vertebrate evolution. Here, we use morphometric and phylogenetic comparative methods across fossil and extant squamates to show they also repeatedly evolved increasingly complex teeth, but with more flexibility than mammals. Since the Late Jurassic, multiple-cusped teeth evolved over 20 times independently from a single-cusped common ancestor. Squamates frequently lost cusps and evolved varied multiple-cusped morphologies at heterogeneous rates. Tooth complexity evolved in correlation with changes in plant consumption, resulting in several major increases in speciation. Complex teeth played a critical role in vertebrate evolution outside Mammalia, with squamates exemplifying a more labile system of dental-dietary evolution. Tooth morphology has provided many insights into the tempo and mode of dietary evolution in mammals. A study of fossil and extant squamates shows that this group also repeatedly evolved increasingly complex teeth with more flexibility than mammals, and that higher tooth complexity and herbivory likely led to higher speciation rates.
... In all enamel-bearing teeth, the first hard tissue to form is the dentine, which develops centripetally towards the inner core of the tooth (Berkovitz and Shellis 2018;Nanci 2003). Once the contour of the tooth crown has been established within the dentine, the enamel either mirrors the underlying structure and forms parallel to the DEJ, or the enamel-producing ameloblasts form more complex structures independent of the underlying dentine and DEJ via differential enamel deposition (Sander 1999;Zahradnicek et al. 2014). The contours of the DEJ and OES in thin section therefore provide enough information to distinguish between crown ornamentations formed solely by amelogenesis and those that are formed prior to the formation of the first layers of dentine. ...
Mosasaur researchers have used varieties of tooth crown ornamentation as diagnostic and phylogenetic characters for decades. Such tooth crown features include facets, flutes, striations, serrated carinae, and coarse anastomosing texture. is study investigates the relative contributions of dentine and enamel to the development of these dental characters and assesses homology statements between these structures. Histological analysis of isolated mosasaur teeth reveals that flutes and facets develop initially from the dentine, and the external enamel morphology we observe macroscopically mirrors the shape of the underlying dentine. Striations combine underlying contributions from the dentine with additional and irregular enamel deposition resulting strictly from amelogenesis. In both serrated carinae and anastomosing texture the Dentine-Enamel Junction is smooth, and these external ornamentations form exclusively through variations in enamel development. Based on these observations, we infer that flutes and facets form a morphological spectrum and should not be treated as separate phylogenetic characters. Conversely, striations develop differently than flutes and facets, and should therefore be treated as a distinct character. We recommend referring to serrations on mosasaur carinae as false denticulations to differentiate these enamel-only structures from true denticles possessing a dentine core. Anastomosing texture can also coincide with significant apical enamel thickening, both of which could be adaptations for processing harder prey, as they are in modern reptiles. Care must be taken when using tooth crown features as diagnostic or phylogenetic characters because seemingly different morphologies can have similar developmental origins, and tooth morphology can be more closely tied to diet than common ancestry.
