Allometric and Metameric Shape Variation in Pan Mandibular Molars: A Digital Morphometric Analysis

The predominance of molar teeth in fossil hominin assemblages makes the patterning of molar shape variation a topic of bioanthropological interest. Extant models are the principal basis for understanding dental variation in the fossil record. As the sister taxon to the hominin clade, Pan is one such model and the only widely accepted extant hominid model for both interspecific and intraspecific variation. To explore the contributions of allometric scaling and meristic variation to molar variation in Pan, we applied geometric shape analysis to 3D landmarks collected from virtual replicas of chimpanzee and bonobo mandibular molars. Multivariate statistical analysis and 3D visualization of metameric and allometric shape vectors were used to characterize shape differences and test the hypothesis that species of Pan share patterns of metameric variation and molar shape allometry. Procrustes‐based shape variables were found to effectively characterize crown shape, sorting molars into species and tooth‐row positions with ≥95% accuracy. Chimpanzees and bonobos share a common pattern of M₁–M₂ metameric variation, which is defined by differences in the relative position of the metaconid, size of the hypoconulid, curvature of the buccal wall, and proportions of the basins and foveae. Allometric scaling of molar shape is homogeneous for M₁ and M₂ within species, but bonobo and chimpanzee allometric vectors are significantly different. Nevertheless, the common allometric shape trend explains most molar‐shape differences between P. paniscus and P. troglodytes. When allometric effects are factored out, chimpanzee and bonobo molars are not morphometrically distinguishable. Implications for hominid taxonomy and dietary reconstruction are discussed. Anat Rec, 2011. © 2010 Wiley‐Liss, Inc.     

Exploring metameric variation in human molars: a morphological study using morphometric mapping

Human molars exhibit a type of metameric variation, which is the difference in serially repeated morphology within an organism. Various theories have been proposed to explain how this variation is brought about in the molars. Actualistic data that support the theories, however, are still relatively scarce because of methodological limitations. Here we propose new methods to analyse detailed tooth crown morphologies. We applied morphometric mapping to the enamel–dentine junction of human maxillary molars and examined whether odontogenetic models were adaptable to human maxillary molars. Our results showed that the upper first molar is phenotypically distinct among the maxillary molars. The average shape of the upper first molar is characterized by four well‐defined cusps and precipitous surface relief of the occlusal table. On the other hand, upper third molar is characterized by smooth surface relief of the occlusal table and shows greater shape variation and distinct distribution patterns in morphospace. The upper second molar represents an intermediate state between first and third molar. Size‐related shape variation was investigated by the allometric vector analysis, and it appeared that human maxillary molars tend to converge toward the shape of the upper first molar as the size increases. Differences between the upper first molar and the upper second and third molar can thus be largely explained as an effect of allometry. Collectively, these results indicate that the observed pattern of metameric variation in human molars is consistent with odontogenetic models of molar row structure (inhibitory cascade model) and molar crown morphology (patterning cascade model). This study shows that morphometric mapping is a useful tool to visualize and quantify the morphological features of teeth, which can provide the basis for a better understanding of tooth evolution linking morphology and development.     

可注射性磷酸钙骨水泥修复下颌阻生第三磨牙微创拔除后牙槽骨缺损

目的:研究一种可注射性磷酸钙骨水泥(ICPCS)修复下颌阻生第三磨牙微创拔除后牙槽骨缺损的临床效果。方法:将下颌阻生第三磨牙需拔除,同时X线片显示其近中第二磨牙远中牙槽骨已丧失1/3以上的33例患者分为2组。试验组17例,17颗下颌阻生第三磨牙微创拔除后应用ICPCS注入第二磨牙远中牙槽骨缺损。对照组16例,16颗下颌阻生第三磨牙微创拔除后采取血凝块机化愈合。分别于拔牙术后1周及术后1、3个月复查,比较两组患者术后伤口愈合情况、牙周临床指标及X线片。结果:全部33例患者伤口均顺利愈合,牙周评估指标显示术后1、3个月的ICPCS组下颌第二磨牙牙周状况明显改善。X线片示术后1、3个月下颌第二磨牙远中骨缺损区的骨密度明显增高,远中牙槽嵴高度有明显增高,均位于远中牙根1/3以上。结论:ICPCS具有良好的可注射性、操作方便、组织相容性好、降解时间适当和骨引导骨诱导能力,能修复下颌阻生第三磨牙微创拔除后下颌第二磨牙远中缺损骨壁,明显改善其牙周状况及牙松动。     

Activin is an essential early mesenchymal signal in tooth development that is required for patterning of the murine dentition

Development of the mammalian tooth has been intensively studied as a model system for epithelial/mesenchymal interactions during organogenesis, and progress has been made in identifying key molecules involved in this signaling. We show that activin βA is expressed in presumptive tooth-germ mesenchyme and is thus a candidate for a signaling molecule in tooth development. Analysis of tooth development in activin βA mutant embryos shows that incisor and mandibular molar teeth fail to develop beyond the bud stage. Activin βA is thus an essential component of tooth development. Development of maxillary molars, however, is unaffected in the mutants. Using tissue recombination experiments we show that activin is required in the mesenchyme prior to bud formation and that although activin signaling from mesenchyme to epithelium takes place, mutant epithelium retains its ability to support tooth development. Implantation of beads soaked in activin A, into developing mandibles, is able to completely rescue tooth development from E11.5, but not E12.5 or E13.5, confirming that activin is an early, essential mesenchyme signal required before tooth bud formation. Normal development of maxillary molars in the absence of activin shows a position specific role for this pathway in development of dentition. Functional redundancy with activin B or other TGFβ family members that bind to activin receptors cannot explain development of maxillary molars in the mutants since the activin-signaling pathway appears not to be active in these tooth germs. The early requirement for activin signaling in the mesenchyme in incisor and mandibular molar tooth germs must be carried-out in maxillary molar mesenchyme by other independent signaling pathways.     

Modeling the dental development of fossil hominins through the inhibitory cascade

The inhibitory cascade is a mathematical model for interpreting the relative size of the occlusal surfaces of mammalian molars in terms of developmental mechanisms. The cascade is derived from experimental studies of mouse molars developed in culture, and has been tested and applied to the dentitions of rodents, ungulates, carnivores, and platyrrhines. Results from such applications have provided new information regarding the origins of plesiomorphic traits in mammalian clade and how derived morphologies may arise. In this study we apply the inhibitory cascade model to the postcanine dentition of a sample of Old World primates that includes fossil hominins. The results of this study suggest that the inhibitory cascade (i.e. M₁ < M₂ < M₃) describes the relative sizes of the molar occlusal areas of Old World primates and is likely the plesiomorphic condition for this clade. Within that clade, whereas most Old World monkeys have a M₁ < M₂ < M₃ pattern, most apes have a M₁ < M₂ ≈ M₃ pattern. This modified cascade suggests that greater levels of inhibition (or less activation) are acting on the posterior molars of apes, thus facilitating the reduction of M₃s within the apes. With the exception of the baboon genus Papio, extant congeners typically share the same molar inhibitory cascade. The differences in the relative size relationships observed in the molar and premolar-molar cascades of the species included in the fossil hominin genus Paranthropus suggest that although large postcanine teeth are a shared derived trait within this genus, the developmental basis for postcanine megadontia may not be the same in these two Paranthropus taxa. Our results show that phenotypic characters such as postcanine megadontia may not reflect common development.     

Identification of a nonsense mutation in the PAX9 gene in molar oligodontia.

Development of dentition is controlled by numerous genes, as has been shown by experimental animal studies and mutations that have been identified by genetic studies in man. Here we report a nonsense mutation in the PAX9 gene that is associated with molar tooth agenesis in a Finnish family. The A340T transversion creates a stop codon at lysine 114, and truncates the coded PAX9 protein at the end of the DNA-binding paired-box. All the affected members of the family were heterozygous for the mutation. The tooth agenesis phenotype involves all permanent second and third molars and most of the first molars and resembles the earlier reported phenotype that was also associated with a PAX9 mutation. The phenotype is presumably a consequence of haploinsufficiency of PAX9. In another Finnish family with molar tooth agenesis, we could not find similar sequence changes in PAX9.     

Static genetic and phenotypic allometry of dental traits in inbred and hybrid house mice

Static allometric coefficients were estimated by regression and reduced major axis methods for second and third mandibular molar widths scaled on body weight in 42-day-old inbred and hybrid house mice. Phenotypic slopes were calculated by combining inbred and hybrid strains, and genetic slopes were obtained from differences among the strains. It was hypothesized that the genetic scaling, which reflects the expected evolutionary allometry between populations if they have been produced from directional selection on body weight alone, would exceed the phenotypic scaling. The genetic coefficients were generally higher (average regression estimates in inbreds = 0.65) than the phenotypic ones (comparable average = 0.47), but the differences were non-significant. It was concluded that the higher scaling of tooth dimensions in interspecies versus intraspecies comparisons typically found in previous studies therefore could not necessarily be explained as a secondary response to selection for body size, although more information is needed about trends in the scaling of mammalian teeth at different ages.     

Root Canal Morphology of Chalcolithic and Early Bronze Age Human Populations of El Mirador Cave (Sierra de Atapuerca,Spain)

This study provides a morphological characterization of the inner anatomy of the root canals of permanent first and second molars in Chalcolithic and early Bronze Age human fossils using cone‐beam computed tomography. The general evolutionary trend in present‐day human dentition is related to morphological simplification. As little is known about when this trend appeared in Homo sapiens populations, the aim of this work is to test the presence of modern radicular morphology 4,400 years ago. Fifty‐four permanent first and second maxillary and mandibular molars of 17 individuals were included in the study. All maxillary first and second molars showed three separate roots. Almost all the lower molars analyzed (100% of first molars and 75% of second molars) had two separate roots. More differences in the canal system configuration were documented in the maxillary mesiobuccal roots than in the palatal or distobuccal roots. The most variable tooth in root and canal configuration is the maxillary second molar. It should be pointed out that 12.5% of the teeth analyzed showed a C‐shaped root configuration. Anat Rec, 297:2342–2348, 2014. © 2014 Wiley Periodicals, Inc.     

Development and size of the teeth of Macaca mulatta

A cross-sectional sample of 151 skulls from Macaca mulatta of known age and similar rearing in U.S. Primate Centers was analyzed to determine age-related “norms” of stages of development and size of teeth. The stages of development from the follicle of a deciduous incisor in the fetus to completion of the root with apex closed of the permanent third molar were related to age. The age range observed for eruption of each tooth was noted and related to its stage of development. The crown of each erupted tooth was found to be completely developed, but growth of its root continued for a longer, indeterminate period. When a deciduous tooth was exfoliated, the crown of the permanent successor was found to be completed and root growth had begun. Measurements of both mesiodistal and faciolingual diameters and of crown length of the teeth in situ and of total length and root length on roentgenograms were examined for sexual dimorphism. The faciolingual diameter of the deciduous mandibular second incisor and of both second molars showed the greatest sexual dimorphism among both diameters of all deciduous teeth. The mesiodistal and faciolingual diameters of the mandibular premolars were found to be the best dimensions in discriminant functions for identifying sex in the absence of permanent canines.     

Evolution and development of the mammalian dentition: Insights from the marsupial Monodelphis domestica

To understand developmental mechanisms of evolutionary change, we must first know how different morphologies form. The vast majority of our knowledge on the developmental genetics of tooth formation derives from studies in mice, which have relatively derived mammalian dentitions. The marsupial Monodelphis domestica has a more plesiomorphic heterodont dentition with incisors, canines, premolars, and molars on both the upper and the lower jaws, and a deciduous premolar. The complexity of the M. domestica dentition ranges from simple, unicusped incisors to conical, sharp canines to multicusped molars. We examine the development of the teeth in M. domestica, with a specific focus on the enamel knot, a signaling center in the embryonic tooth that controls shape. We show that the tooth germs of M. domestica express fibroblast growth factor (FGF) genes and Sprouty genes in a manner similar to wild‐type mouse molar germs, but with a few key differences. Developmental Dynamics, 2011. © 2010 Wiley‐Liss, Inc.     

Comparative cranial ontogeny of Tapirus (Mammalia: Perissodactyla: Tapiridae)

Skull morphology in tapirs is particularly interesting due to the presence of a proboscis with important trophic, sensory and behavioral functions. Several studies have dealt with tapir skull osteology but chiefly in a comparative framework between fossil and recent species of tapirs. Only one study examined an aspect of cranial ontogeny, development of the sagittal crest (Holbrook. J Zool Soc Lond 2002; 256; 215). Our goal is to describe in detail the morphological changes that occur during the postnatal ontogeny of the skull in two representative tapir species, Tapirus terrestris and Tapirus indicus, and to explore possible functional consequences of their developmental trajectories. We compared qualitative features of the skull on a growth series of 46 specimens of T. terrestris ordered on the basis of the sequence of eruption and tooth wear, dividing the sample into three age classes: class Y (very young juvenile), class J (from young juvenile to young adult) and class A (full and old adult). The qualitative morphological analysis consisted of describing changes in the series in each skull bone and major skull structure, including the type and degree of transformation (e.g. appearance, fusion) of cranial features (e.g. processes, foramina) and articulations (sutures, synchondroses, and synovial joints). We then measured 23 cranial variables in 46 specimens of T. terrestris that included the entire ontogenetic series from newborn to old adults. We applied statistical multivariate techniques to describe allometric growth, and compared the results with the allometric trends calculated for a sample of 25 specimens of T. indicus. Results show that the skull structure was largely conserved throughout the postnatal ontogeny in T. terrestris, so class Y was remarkably similar to class A in overall shape, with the most significant changes localized in the masticatory apparatus, specifically the maxillary tuber as a support of the large‐sized permanent postcanine dentition, and correlated changes in diastemata, mandibular body, and sagittal and nuchal crests. In the nasal region, ontogenetic remodeling affected the space for the meatal diverticulum and the surfaces for the origin of the proboscis musculature. Overall, ontogenetic trajectories exhibited more negative allometric components in T. indicus than in T. terrestris, and they shared 47.83% of allometric trends. Tapirus indicus differed most significantly from T. terrestris in the allometry of postcanine toothrows, diastemata and mandibular body. Thus, some allometric trends seem to be highly conserved among the species studied, and the changes observed showed a strong functional and likely adaptive basis in this lineage of ungulates.     

Morphodigital study of bone quality in the mandibular angle in patients with third molar impacted

The aim of this study was to analyze if the presence of impacted third molars, and their positions in the mandibular angle, can change the bone quality in this area, considering the measure of the cortical thickness in this region as representative or not for mandible fracture risk. Software was used to analyze 50 digital images from panoramic radiographs of patients who had one or two impacted third molars in the mandible, and 30 digital images of patients with agenesis of the mandibular third molar. The thickness of the cortical region of the mandible was measured; it was possible to draw a parallel line to the posterior portion of the mandible and a parallel line to the body of this bone on each side of the image. At the intersection of these lines near the distal portion of the second molar, another line was set up to serve as reference in the cortical thickness measurement. It could be concluded that the cortical thickness of the mandibular angle in male patients without impacted third molars was greater than the thickness in patients with these teeth, and no difference in thickness was found for the female group.     

Short-term mineralization of dentin and enamel in the mouse embryonic molars cultured in serum-free, chemically-defined medium.

The present study was designed to demonstrate the short term mineralization of dentin and enamel, and to investigate the effects of sodium beta-glycerophosphate (Na-beta-GPO4) on calcification in a serum-free, chemically-defined medium. The first mandibular molars at the bell stage dissected from 18-day-old mouse embryos were used as explants, which were cultured by an improved flotation method. Calcification of enamel in the 18-day-old embryonic molars occurred within the 6th day of culture. In another experiment, the molar germs were cultured in a serum-free, chemically-defined medium supplemented with 1, 5 and 10 mM Na-beta-GPO4. Promotion of tooth mineralization was recognizable at very low concentrations, such as 1 mM Na-beta-GPO4, in 18-day-old embryonic tooth germs. The culturing system reported here shortens the time required for dentin and enamel calcification to one half or one third of that reported previously and therefore should prove useful for examining regulations for cytodifferentiation and morphogenesis in tooth germs and the mineralization of dentin and enamel.     

Evolutionary aspects of the development of teeth and baleen in the bowhead whale

In utero, baleen whales initiate the development of several dozens of teeth in upper and lower jaws. These tooth germs reach the bell stage and are sometimes mineralized, but toward the end of prenatal life they are resorbed and no trace remains after birth. Around the time that the germs disappear, the keratinous baleen plates start to form in the upper jaw, and these form the food‐collecting mechanism. Baleen whale ancestors had two generations of teeth and never developed baleen, and the prenatal teeth of modern fetuses are usually interpreted as an evolutionary leftover. We investigated the development of teeth and baleen in bowhead whale fetuses using histological and immunohistochemical evidence. We found that upper and lower dentition initially follow similar developmental pathways. As development proceeds, upper and lower tooth germs diverge developmentally. Lower tooth germs differ along the length of the jaw, reminiscent of a heterodont dentition of cetacean ancestors, and lingual processes of the dental lamina represent initiation of tooth bud formation of replacement teeth. Upper tooth germs remain homodont and there is no evidence of a secondary dentition. After these germs disappear, the oral epithelium thickens to form the baleen plates, and the protein FGF‐4 displays a signaling pattern reminiscent of baleen plates. In laboratory mammals, FGF‐4 is not involved in the formation of hair or palatal rugae, but it is involved in tooth development. This leads us to propose that the signaling cascade that forms teeth in most mammals has been exapted to be involved in baleen plate ontogeny in mysticetes.     

Mandibular lingual canals distribute to the dental crypts in prenatal stage

Purpose

To describe configurations of human prenatal mandibular, lingual canals using a limited-field cone beam computed tomography (CBCT) to examine their origin and anatomical significance.

Materials and methods

Nine fetal mandibles were examined using a CBCT. Mandibular maturity was assessed according to the mandibular size measured directly and image findings on development of dental crypts. Mandibular, lingual canals and the related foramina (mandibular, mental, and lingual foramina) were observed on axial, sagittal, and cross-sectional images. The horizontal position of mental and lingual foramina was assessed by direct observation using a loupe.

Results

In all nine mandibles, CBCT images depicted three separate mandibular canals, which individually occurred at the ramus area. One was a short canal directly connecting to the permanent molar crypt. The other two showed a parallel course, following the mandibular corpus toward the frontal area; the upper one connected to the mental foramen, and the lower one distributed anterior area of canine and incisor crypts. Lingual foramina were observed bilaterally in eight of nine mandibles, whose horizontal position was lingual against the crypt of deciduous canine. The lingual canals occurred from lingual foramina, and connected to the close deciduous tooth crypt or the mandibular canal distributing in the frontal area.

Conclusions

It was suggested human dentition could be developed by plural mandibular and lingual canals.     

Effects of dexamethasone on tooth eruption in rats: differences in incisor and molar eruption

A requirement for tooth eruption is the resorption of alveolar bone. Because bone resorption is stimulated by dexamethasone both in vivo and in vitro, dexamethasone 21-phosphate, a soluble form of dexamethasone, was injected into rats to determine its effect on tooth eruption. Such dexamethasone injections accelerate the time of intra-osseous eruption in rat incisors but do not accelerate the eruption time of rat molars when injected into rats. The injections of dexamethasone 21-phosphate also accelerate the time of eyelid opening in the postnatal rats, as well as retarding growth, as measured by body weight. These effects of dexamethasone 21-phosphate parallel the effects of epidermal growth factor injections, including the absence of an effect on molar eruption. This suggests that the molecular signals for the initiation of tooth eruption (i.e., onset of bone resorption) differ between rat incisors and molars. Given that rat incisors are teeth of continuous eruption whereas rat molars are teeth of limited eruption, as are human teeth, care must be taken in extrapolating results derived from rat incisors to human dentition. In vitro, dexamethasone has no effect on the gene expression of either osteoprotegerin or epidermal growth factor in dental follicle cells derived from molars. Because osteoprotegerin expression during normal tooth eruption is transitorily inhibited early postnatally in the molar dental follicle to allow osteoclast formation, the absence of inhibition of its expression by dexamethasone could explain why dexamethasone does not accelerate eruption in molars.     

Mouse rudimentary diastema tooth primordia are devoid of peripheral nerve fibers

The tooth is a well-defined peripheral target organ for trigeminal nerve fibers. However, only limited information is available regarding pioneer axon guidance to the developing tooth target field. In rodents there is a toothless diastema region between incisors and molars that in the mouse maxilla contains three rudimentary tooth anlagen. Their development stop at the early bud stage when the primary nerve axons grow towards the developing first molar tooth germs. In order to provide background information for studies of regulatory mechanisms of pioneer axon guidance to the developing tooth germs, we investigated the distribution of nerve fibers in the mouse diastema tooth buds, and compared it to the axon growth to the maxillary and mandibular first molar tooth germs by immunohistochemical localization of peripherin and PGP9.5. Analysis of serial sections showed that trigeminal nerve fibers emerging from the trigeminal maxillary and mandibular nerve trunks started to grow towards the developing molar tooth germ at the early bud stage, and subsequently they diverged into buccal and lingual branches next to the condensed dental mesenchyme. During the cap stage, nerve fibers were observed around the tooth germ in the dental follicle region. In contrast, no nerve fibers were located in the vicinity of the diastema tooth primordia at any stage studied, nor did any nerve fibers appear to grow towards this region. Our results show that the development and subsequent disappearance of the diastema tooth primordia takes place without peripheral trigeminal innervation. The diastema tooth primordia may therefore be a useful model system for future studies on molecular regulatory mechanisms of pioneer axon guidance to the tooth germs, and possibly also for evolutionary studies of peripheral axon guidance mechanisms.     

Course and composition of the nerves that supply the mandibular teeth of the rat

The rodent dentition has become an important model for investigations of interactions between dental tissues and peripheral neurons. Although experimental nerve injury has been widely used for such studies, there is uncertainty about the courses of nerve fibers supplying the mandibular teeth. In order to clarify this, we used a mixture of monoclonal antibodies against neurofilament proteins to enhance demonstration of nerve fibers so that small nerves could be readily traced in serial frozen sections of mandibles of Sprague Dawley rats ranging in age from embryonic day (E) 18 to postnatal day (P) 90. The 1st molar and anterior portion of the 2nd molar were innervated by small nerves that emerged as distinct branches of the IAN trunk at or near the mandibular foramen. In contrast, the nerve supply to the 3rd molar and posterior part of the 2nd molar was a branch of the lingual nerve that bypassed the mandibular canal altogether. The IAN trunk split into the mental nerve and a large branch to the incisor about 2 mm anterior to the mandibular foramen. Thick branches of the incisor nerve descended into the incisor socket to form a dense plexus of nerve fiber bundles extending along the length of the incisor periodontium. The sparse pulpal innervation of the incisor was provided by a few thin fascicles that emerged from the caudal portion of the periodontal plexus to enter the incisor apex. The dental branches of the IAN and lingual nerve seen in the adult were well established and readily identifiable at age E18 even though their targets were limited to the follicles of the developing teeth. These studies show that the trigeminal branches that supply the mandibular teeth can be identified at a wide range of ages as distinct nerves at a considerable distance proximal to their targets. This detailed information on the courses taken by the dental nerves can provide an anatomical basis for increased precision in characterization and perturbation of neural pathways from the molars and incisor.