Enamel Secretion Research Articles

Analysis of enamel defects in a cave bear maxillary molar, with remarks on incremental markings in bear enamel

AbstractThe paper discusses the formation of an enamel defect in the crown of a cave bear (Ursus spelaeus sensu lato) left maxillary second molar (M2), based on macroscopic and microscopic analysis. The tooth belongs to a cranium recovered from the Cerovac caves in Croatia that exhibits a partially healed, depressed lesion in the left squama frontalis and a further lesion in the left maxilla associated with loss of the M1. Microscopic inspection demonstrated an accentuated incremental line in both enamel and dentin of the left M2. It is suggested that in the defect area the outer enamel had been posteruptively lost along the accentuated line in the enamel that constituted a zone of reduced mechanical resistance. Presence of enamel hypoplasia in both M2 indicated that these developmental lesions reflect a systemic stress event during crown formation of the teeth. The underlying cause of this stress is assumed to have been a trauma to the skull that caused the lesion in the left squama frontalis. It is further suggested that a later trauma to the left maxilla had led to the loss of the left M1 and the flaking‐off of enamel along the accentuated incremental line in the left M2. The defect in the left M2 is thus diagnosed as the result of a developmental lesion during crown formation, related to systemic stress due to a skull trauma, followed by posteruptive damage from a second traumatic impact. In addition to reconstructing the formation of the defect in the crown of the left M2, the paper, for the first time, describes daily and subdaily incremental markings in ursid enamel and provides preliminary information on enamel secretion rate in a cave bear molar.

Human-like enamel growth in Homo naledi.

A modern pattern (rate and duration) of dental development occurs relatively recently during human evolution. Given the temporal overlap of Homo naledi with the first appearance of fossil Homo sapiens in Africa, this small-bodied and small-brained hominin presents an opportunity to elucidate the evolution of enamel growth in the hominin clade. Here we conduct the first histological study of two permanent mandibular canines and one permanent maxillary first molar, representing three individuals attributed to H. naledi. We reconstruct the rate and duration of enamel growth and compare these findings to those reported for other fossil hominins and recent humans. Thin sections of each tooth were produced using standard histological methods. Daily and longer period incremental markings were measured to reconstruct enamel secretion and extension rates, Retzius periodicity, canine crown and molar cusp formation time. Daily enamel secretion rates overlapped with those from recent hominins. Canine crown formation time is similar to that observed in recent Europeans but is longer than canine formation times reported for most other hominins including Australopithecus and H. neanderthalensis. The extended period of canine formation appears to be due to a relatively tall enamel crown and a sustained slow rate of enamel extension in the cervical portion of the crown. A Retzius periodicity of 11 days for the canines, and nine days for the molar, in H. naledi parallel results found in recent humans. An 11-day periodicity has not been reported for Late Pleistocene Homo (H. erectus, H. neanderthalensis) and is rarely found in Australopithecus and Paranthropus species. Enamel growth of H. naledi is most similar to recent humans though comparative data are limited for most fossil hominin species. The high Retzius periodicity values do not follow expectations for a small-brained hominin.

Relating metric crown dimensions to underlying internal daily secretion rates in antimeric premolars

ObjectivesThe underlying cause for metric differences in antimeric tooth pairs is an important question for understanding dental variation. We hypothesize that localized variation in crown dimensions will be reflected in localized variation in daily enamel secretion rate. DesignCasts of pairs of human premolars from a tissue bank were 3D scanned using an optical scanning system (n = 32). Histological slides were created, and daily secretion rates (DSRs) were recorded in two areas of enamel that corresponded to scanner measurements. Antimeres were compared for both metric measurements and DSRs. Outliers for scanner measures were compared to significant differences in DSRs measurements in right and left teeth in corresponding areas. ResultsThirteen of the 16 individuals differed significantly in antimeric metric measurements, but only ten of those also differed in the underlying DSR. Fifteen of the 16 total individuals differed significantly in at least one DSR area. DiscussionWhile some individuals were outliers for metric measurements and had multiple areas of DSR differences, the majority of individuals had antimeric DSR differences regardless of metric differences. While there was no conclusive correlation between 3D metric analysis and underlying DSR differences, the most important result of this study is that DSR differences between antimeres are common.

Immunohistochemical Localization of Endothelin- 1 and Endothelin A Receptor in Human Primary Tooth Enamel Organ.

Enamel organ (EO) is an ectodermal derived structure, which is involved in the different aspects of tooth development. Tooth development shares the same regulatory molecules and genes expressed in other developing organs. Endothelin- 1 (ET-1) and Endothelin A receptor (ETAR), (ET-1/ETAR) axis, are involved in differentiation of embryonic stem cells and organ development. The present study aimed to investigate the ET-1 and ETAR expression profiles during the development of human primary tooth EO with the relatively large sample size. In this experimental study, 33 human fetuses aged from 13 to 23 weeks (3 samples from each fetal age) were collected. The sampleswere divided into three age groups (<16 weeks, <19 weeks, ≥19 weeks) and cut for hematoxylin and eosin (H&E) and immunohistochemistry (IHC) staining. A two-way ANOVA test was conducted to examine the expression levels of ET-1 and ETAR in different layers of human primary tooth EO. The statistical significance was assumed at p ≤ 0.05. There were statistically significant differences between the expression levels of ET-1/ETAR axis in the four-layered human primary tooth EO in different fetal ages (13-23 weeks). Besides, there were significant differences between the expression levels of ET-1/ETAR axis in all layers of human primary enamel organ and types of teeth. Due to the profile of expression of ET-1/ETAR axis, it can be concluded that this axis contributes to the differentiation of all human primary EO layers and secretion of enamel. ET-1/ETAR axis is one of the signaling molecules, which may have crucial roles in tooth development.

Epigenetics in developmental defects of enamel:A scoping review.

The significant role of epigenetics has been revealed in normal enamel formation process and occurrence of developmental defects. This presented literature is aiming at summarizing the regulatory function of epigenetics in physiological amelogenesis process and reviewing the epigenetic mechanisms in occurrence of developmental defects of enamel (DDE), so as to provide biological foundation evidence to support early predication and clinical management of DDE. An extensive literature review was conducted using electronic databases MEDLINE (through PubMed), Web of Science and EMBASE up to November 30,2022. Studies about epigenetic effects on enamel tissue or cells associated with amelogenesis, including in vivo studies using humans or animal models, and in vitro studies, are selected. 22 studies were included. Epigenetic factors or effects specifically activate or silence certain genes, which may regulate related biological activites including cell proliferation, cell differentiation, enamel secretion and mineralization during the process of amelogenesis. Once the status of epigenetic modification is altered, the quantity and quality of enamel may both be disturbed, which can finally result in DDE. Epigenetics plays a noteworthy role of regulating the amelogenesis process and DDE potentially by altering the expression level of genes related to enamel formation, providing a new perspective of early predication and clinical management of DDE.

Stress exposure histories revealed by biochemical changes along accentuated lines in teeth

The regular incremental secretion of enamel and dentine can be interrupted during periods of stress resulting in accentuated growth lines. These accentuated lines, visible under light microscopy, provide a chronology of an individual's stress exposure. Previously, we showed that small biochemical changes along accentuated growth lines detected by Raman spectroscopy, coincided with the timing of medical history events and disruptions of weight trajectory in teeth from captive macaques. Here, we translate those techniques to study biochemical changes related to illness and prolonged medical treatment during early infancy in humans. Chemometric analysis revealed biochemical changes related to known stress-induced changes in circulating phenylalanine as well as other biomolecules. Changes in phenylalanine are also known to affect biomineralization which is reflected in changes in the wavenumbers of hydroxyapatite phosphate bands associated with stress in the crystal lattice. Raman spectroscopy mapping of teeth is an objective, minimally-destructive technique that can aid in the reconstruction of an individual's stress response history and provide important information on the mixture of circulating biochemicals associated with medical conditions, as applied in epidemiological and clinical samples.

Wild boar versus domestic pig-Deciphering of crown growth in porcine second molars.

Based on the previously established periodicity of enamel growth marks, we reconstructed crown growth parameters of mandibular second molars from two wild boar and two domestic pigs of the Linderöd breed. Body weight gain and progression of dental development were markedly faster in the domestic pigs than the wild boar. While the final crown dimensions of the M2 did not differ between domestic pigs and wild boar, mean crown formation time (CFT) of this tooth was considerably shorter in the domestic pigs (162 days) than in the wild boar (205 days). The difference in CFT was mainly attributable to a higher enamel extension rate (EER) in the domestic pig. Generally, EER was highest in the cuspalmost deciles of the length of the enamel-dentine-junction and markedly dropped in cervical direction, with lowest values occurring in the cervicalmost decile. In consequence, the cuspal half of the M2 crown was formed about three times faster than the cervical half. In contrast to the EER, no marked difference in daily enamel secretion rate (DSR) was recorded between domestic pigs and wild boar. The duration of enamel matrix apposition as well as linear enamel thickness in corresponding crown portions was only slightly lower in the domestic pigs than the wild boar. Thus, the earlier completion of M2 crown growth in the domestic pig was mainly achieved by a higher EER and not by an increased DSR. The more rapid recruitment of secretory ameloblasts in the course of molar crown formation of domestic pigs compared to wild boar is considered a side-effect of the selection for rapid body growth during pig domestication.

Time of occurrence and width of accentuated lines in the enamel of primary incisors from mediaeval skeletal remains from north-central Poland: A further contribution to the explanation of early childhood mortality in past populations

ObjectivesPhysiological disturbances experienced during the early stages of biological development may have an impact on later periods of ontogenesis. The purpose of this study was to assess whether there is a correlation between early childhood mortality in archaeological populations and the age of onset of such disruptions in the course of prenatal and postnatal development. Material and methodsPrimary incisors were taken from 55 individuals who died at the age of 2–6 years and were buried in a mediaeval cemetery located in Brześć Kujawski in the region of Kujawy. The study material was divided into children who died at the age of 2–3 years (n = 29) and those who died at the age of 4 years and later (n = 26). Each tooth had at least one visible postnatal accentuated line. The distance between the neonatal line (NNL) in the lateral enamel and any visible accentuated line (AL) was measured and divided by the local daily enamel secretion rate in order to estimate the time of onset of the AL. The width of ALs was also measured. ResultsThe average age of onset of the first AL in children who died before 4 years of age and in those who died older was 56 and 59 days after birth, respectively, with the difference not being statistically significant (Z = 0.56; p = 0.576). However, children who died younger revealed wider ALs (Z = 2.06; p = 0.039). ConclusionThese results indicate that in past populations childhood mortality was not influenced by the exact time of occurrence of unfavourable factors in the early postnatal period, but rather by environmental conditions in the perinatal period and the first month of life shaping the individual predispositions of the response to them.

Virtual histology of archaeological human deciduous prenatal enamel through synchrotron X-ray computed microtomography images

Virtual histology is increasingly utilized to reconstruct the cell mechanisms underlying dental morphology for fragile fossils when physical thin sections are not permitted. Yet, the comparability of data derived from virtual and physical thin sections is rarely tested. Here, the results from archaeological human deciduous incisor physical sections are compared with virtual ones obtained by phase-contrast synchrotron radiation computed microtomography (SRµCT) of intact specimens using a multi-scale approach. Moreover, virtual prenatal daily enamel secretion rates are compared with those calculated from physical thin sections of the same tooth class from the same archaeological skeletal series. Results showed overall good visibility of the enamel microstructures in the virtual sections which are comparable to that of physical ones. The highest spatial resolution SRµCT setting (effective pixel size = 0.9 µm) produced daily secretion rates that matched those calculated from physical sections. Rates obtained using the lowest spatial resolution setup (effective pixel size = 2.0 µm) were higher than those obtained from physical sections. The results demonstrate that virtual histology can be applied to the investigated samples to obtain reliable and quantitative measurements of prenatal daily enamel secretion rates.

Calcium Sets the Clock in Ameloblasts

BackgroundStromal interaction molecule 1 (STIM1) is one of the main components of the store operated Ca2+ entry (SOCE) signaling pathway. Individuals with mutated STIM1 present severely hypomineralized enamel characterized as amelogenesis imperfecta (AI) but the downstream molecular mechanisms involved remain unclear. Circadian clock signaling plays a key role in regulating the enamel thickness and mineralization, but the effects of STIM1-mediated AI on circadian clock are unknown.ObjectivesThe aim of this study is to examine the potential links between SOCE and the circadian clock during amelogenesis.MethodsWe have generated mice with ameloblast-specific deletion of Stim1 (Stim1fl/fl/Amelx-iCre+/+, Stim1 cKO) and analyzed circadian gene expression profile in Stim1 cKO compared to control (Stim1fl/fl/Amelx-iCre–/–) using ameloblast micro-dissection and RNA micro-array of 84 circadian genes. Expression level changes were validated by qRT-PCR and immunohistochemistry.ResultsStim1 deletion has resulted in significant upregulation of the core circadian activator gene Brain and Muscle Aryl Hydrocarbon Receptor Nuclear Translocation 1 (Bmal1) and downregulation of the circadian inhibitor Period 2 (Per2). Our analyses also revealed that SOCE disruption results in dysregulation of two additional circadian regulators; p38α mitogen-activated protein kinase (MAPK14) and transforming growth factor-beta1 (TGF-β1). Both MAPK14 and TGF-β1 pathways are known to play major roles in enamel secretion and their dysregulation has been previously implicated in the development of AI phenotype.ConclusionThese data indicate that disruption of SOCE significantly affects the ameloblasts molecular circadian clock, suggesting that alteration of the circadian clock may be partly involved in the development of STIM1-mediated AI.

Disturbances in primary dental enamel in Polish autistic children

Dental enamel is a structure that is formed as a result of the regular functioning of ameloblasts. The knowledge of the patterns of enamel secretion allows an analysis of their disruptions manifested in pronounced additional accentuated lines. These lines represent a physiological response to stress experienced during enamel development. The aim of this study was to assess the occurrence of accentuated lines in the tooth enamel of autistic boys. The width of the neonatal line and the periodicity of the striae of Retzius were also assessed. The study material consisted of longitudinal ground sections of 56 primary teeth (incisors and molars): 22 teeth from autistic children and 34 teeth from the control group. The Mann–Whitney U test indicates that the accentuated lines were found significantly more often in autistic children (Z = 3.03; p = 0.002). No differentiation in the rate of enamel formation and in the rate of regaining homeostasis after childbirth were found. The obtained results may indicate a higher sensitivity of autistic children to stress factors, manifested in more frequent disturbances in the functioning of ameloblasts or may be a reflection of differences in the occurrence of stress factors in the first years of life in both analyzed groups.

Variation in the timing of enamel formation in modern human deciduous canines

ObjectivesDeciduous canines are now used increasingly in archaeological and forensic studies to establish the time of birth and as a retrospective source of trace elements incorporated into enamel before and after birth. However, data on the variability of deciduous enamel formation times are scarce. Our objectives were to use daily incremental markings to estimate daily secretion rates, the timing of prenatal, postnatal and total enamel formation and any changes in enamel coverage or prism and stria orientation that occur during enamel formation. Materials and MethodsLongitudinal ground sections of 81 deciduous canines were studied with transmitted light microscopy. High-resolution digital images were imported from an Olympus VS-120 virtual slide scanning system into a geographic information system (ArcGIS, ESRI USA) for quantitative and statistical analyses of linear, angular and area measurements of buccal enamel. ResultsDaily rates of enamel secretion close to the EDJ were faster than in permanent enamel (3.23 μm/day, SD = 0.54). Prism and stria angles subtended to the EDJ both increased through crown formation. Enamel coverage was low in the cusp and cervix but maximal ∼150 days after birth. The mean prenatal enamel formation time was 118 days (range 60−150, SD, 29.2, n = 24). The overall mean postnatal enamel formation time was 319 days (range 210–420, SD 50.6, n = 67). ConclusionsDaily enamel secretion rates compared well with previous studies of deciduous enamel, however, enamel extension rates in deciduous cuspal enamel were notably lower. The variability of both prenatal and postnatal deciduous enamel formation times was greater than previously reported.

Adaptive amelogenesis during unimpeded eruption of rat mandibular incisor.

The aim of the study was to see whether the length of the enamel secretion zone in unimpeded rat incisors, measured precisely, is in agreement with the observed decrease in enamel thickness. Unimpeded eruption of mandibular incisors of five experimental and two control rats was induced by cutting off the erupted part of the incisors three times per week for 5weeks. The length of the zone of enamel secretion in unimpeded and impeded control incisors was measured on longitudinal and serial transverse histological sections of fixed, demineralised and embedded hemimandibles. Impeded contralateral incisors were also included in the study. The length of the zone of enamel secretion in unimpeded incisors showed an increase to 8,398±558µm, that is 161% of the length in control incisors (5,213±95µm). The contralateral incisor showed a reduction in eruption rate, in length of the secretion zone, and the whole tooth was shifted somewhat apically. The measured length of the secretion zone is in agreement with the observed thickness of enamel (98µm) in unimpeded incisors. The reduced eruption rate and the apical shift of the contralateral incisor are probably due to an increased occlusal load.

Enamel thickness and growth rates in modern human permanent first molars over a 2000 year period in Britain.

This study explores variation and trends in first molar enamel thickness and daily enamel secretion rates over a 2000 year period in Britain. Permanent first molars (n = 89) from the Roman, Anglo-Saxon, and Medieval periods, as well as modern-day Britain, were analyzed using standard histological methods. Relative enamel thickness (RET) and linear measurements of cuspal and lateral thickness were calculated for mesial cusps. Daily secretion rates (DSRs) were calculated for inner, mid, and outer enamel regions in both cuspal and lateral enamel. Significant differences and trends were identified between samples using nonparametric statistical tests. Enamel thickness differed between some populations, but no temporal trends were identified. Early Anglo-Saxon molars had significantly thinner RET than both Late Anglo-Saxon (p < .00) and Medieval (p < .00) molars. Lateral enamel from the Roman molars was significantly thinner than the modern-day sample (p = .04). In contrast, a significant slowing trend in DSRs was observed across the more ancient to modern-day samples in every measured region except the mid-lateral enamel region. This study presents the first evidence for a gradual slowing in the daily rate that enamel is secreted in molars over the past 2000 years in Britain. However, this trend was not matched by consistent or significant positive or negative shifts in enamel thickness. These findings suggest that modern human molars of similar enamel thickness, from different modern and ancient populations, formed at different rates.

Enamel mineralization and compositional time-resolution in human teeth evaluated via histologically-defined LA-ICPMS profiles

Abstract Mammalian dental enamel is a key archive for the reconstruction of past environments. Sequentially mineralizing enamel provides continuous, several year-long records, which spatially-resolved sampling can ‘read’ at seasonal or higher time resolution. Yet it remains underexplored how much an initially incorporated compositional signal is overprinted by the two-stage enamel mineralization process, which affects the finally achievable time-resolution. We report results of a systematic investigation into histologically-defined compositional profiles from human enamel obtained by laser-ablation inductively-coupled-plasma mass spectrometry (LA-ICPMS). Sr/Ca, Zn/Ca, Ba/Ca and Pb/Ca are investigated as commonly-utilized proxies of (palaeo)diet, mineralization and/or pollution in modern and archaeological samples. Our modern human teeth are from unequivocally breast-fed and formula-fed individuals, respectively. By focusing on two profiles that are time-equivalent yet topographically different, namely the enamel-dentine junction (EDJ) vs. enamel prisms (P), we evaluate the compositional effect of enamel secretion vs. maturation on elemental proxies throughout enamel thickness. These two broadly orthogonal orientations are compared with the connecting Neonatal/Retzius (NNL/R) profiles that represent nominal isogrowth events during enamel secretion, which – if later maturation had no effect – should show invariant compositional signals across enamel. However, we find that all NNL/R profiles record systematically varying compositions across enamel, with Sr/Ca always decreasing by 30–80% and Zn/Ca increasing near-exponentially 20–35 times towards outer enamel. As such they can be used to benchmark the extent of enamel maturation. Since all prism profiles reveal signals similar to NNL/R for Sr, Zn and in part Ba, this unequivocally demonstrates that P-orientations are equally overprinted by maturation, in all cases with increasing intensity towards outer enamel. EDJ and P profiles do not match one-another despite representing coeval secretion time, and we infer that the highest degree of initial signal variability can be retrieved along the EDJ (approx. ≤ 100 μm) where maturation had the least effect. Using a simple two-component mixing model we show that during maturation only a moderate increase (1.3–1.9×) in discrimination against Sr is required to explain the Sr/Ca patterns. The Zn/Ca increase on the other hand is interpreted to reflect the Zn-binding motifs of the enzymes MMP-20 (matrix metalloproteinase-20) and KLK-4 (kallikrein-4) active during enamel secretion and maturation, which may preferentially imprint a Zn compositional signal in maturing enamel. In the case of Ba/Ca we find no systematic patterns analogous to that of Sr and Zn in modern samples. Moreover, cryptic diagenesis affecting Ba in otherwise well-preserved fossil teeth argues for caution when using Ba/Ca to infer nutritional signals without additional elemental/isotopic corroboration. Unlike all other investigated elements, Pb/Ca shows broadly similar EDJ-P profiles, and nearly invariant R-signals across enamel, which may reflect the affinity of Pb to both organic molecules and inorganic apatite during enamel mineralization. Overall our results reveal the element-specific behaviour of Sr, Zn, Ba and Pb during enamel mineralization.

Pig enamel revisited – Incremental markings in enamel of wild boars and domestic pigs

The nature and periodicity of incremental markings in pig enamel is currently debated. To broaden the basis for a correct interpretation of growth marks in pig enamel, we analyzed their periodicity in teeth of wild boars and domestic pigs. For that, the numbers of enamel incremental markings were recorded in ground sections and compared with crown formation times for the respective teeth derived from literature data on tooth development and eruption in Sus scrofa. Our study revealed that laminations with a daily periodicity are the dominant incremental feature of pig enamel. In wild boar M3s, daily enamel secretion (apposition) rates ranged between a minimum of 6.1 µm in the inner and a maximum of 30.6 µm in the outer enamel.Long-period (supra-daily) incremental markings were present as perikymata at the outer enamel surface (OES). Contrary to the situation in primate enamel, in pig enamel the long-period incremental lines terminating in perikyma grooves were mostly structurally indistinguishable from the daily laminations. Typically, five sub-daily increments were present between successive laminations. The incremental pattern in pig enamel can be misinterpreted if the laminations are mistaken for long-period markings (striae of Retzius) and the sub-daily growth marks for daily prism cross-striations. The findings of the present study demonstrate the critical importance of correctly characterizing the incremental markings and their periodicity in enamel, and caution against an uncritical transfer of the interpretation of the nature of incremental markings in primate enamel to other mammalian taxa.

Quantitative reconstruction of seasonality from stable isotopes in teeth

Seasonal variation in rainfall and temperature are major determinants of ecosystem structure and productivity and influence physical processes such as erosion rates and glaciation. By extension, seasonality can drive evolutionary change, and is hypothesized to have influenced early human subsistence and technological development. Rainfall patterns are reflected by oxygen isotope ratios (δ18O values) in mammalian tooth enamel, which records environmental chemistry as reflected by blood chemistry during mineralization. Fossilized herbivore molars are commonly used to study paleoseasonality, since temporal variation in precipitation δ18O results in spatial variation in tooth mineral δ18O. However, this approach has been hampered by incomplete knowledge of isotope incorporation during tooth mineralization. Here we test a new synchrotron-based mineralization model and demonstrate its potential to quantitatively reconstruct original seasonal input histories under a Bayesian computational framework. To accomplish this, we first integrate the mineralization model with blood water oxygen isotope turnover to produce high-resolution spatial δ18O predictions. We test these predictions using fine-scaled tooth enamel phosphate δ18O measurements (n = 109 locations sampled in a 2D grid format) in a sheep (Ovis aries) subjected to a controlled water switch (switch δ18O magnitude of 12‰). Tooth mineralization, blood oxygen turnover and isotopic measurements demonstrate that enamel secretion and maturation waves advance at nonlinear rates with distinct geometries. Final enamel isotopic composition may be influenced by isotopic shifts during enamel maturation from amorphous precursors. We combine these observations to produce a Bayesian inverse model for reconstructing water δ18O inputs from tooth isotopic measurements. The system accurately reproduces the controlled switch of the experimental animal and reveals transient documented meteorological events that were unplanned during the experiment. A simulation approach shows that given appropriate priors and search methodologies, this system also reconstructs precipitation histories at different latitudes with striking fidelity. We show that in some contexts even simple sampling approaches (i.e. 1D transects) can yield accurate reconstructions of seasonality. Finally, we refine our mineralization model further by exploring the possible extent of isotopic resetting during mineral phase transitions. This work demonstrates that tooth isotopic measurements can be employed to reconstruct past seasonal isotopic variation in rainfall quantitatively, allowing exploration of the relationships among climate, earth history and evolution.

Pleiotropic function of DLX3 in amelogenesis: from regulating pH and keratin expression to controlling enamel rod decussation

ABSTRACTDLX3 is essential for tooth enamel development and is so far the only transcription factor known to be mutated in a syndromic form of amelogenesis imperfecta. Through conditional deletion of Dlx3 in the dental epithelium in mouse, we have previously established the involvement of DLX3 in enamel pH regulation, as well as in controlling the expression of sets of keratins that contribute to enamel rod sheath formation. Here, we show that the decussation pattern of enamel rods was lost in conditional knockout animals, suggesting that DLX3 controls the coordinated migration of ameloblasts during enamel secretion. We further demonstrate that DLX3 regulates the expression of some components of myosin II complexes potentially involved in driving the movement of ameloblasts that leads to enamel rod decussation.

Aspects of the final phase of enamel formation as evidenced by observations of superficial enamel of human third molars using scanning electron microscopy

ObjectiveEnamel structure reflects ameloblast function. By studying the structure of the superficial enamel, information about ameloblast function toward the end of the secretory stage may be obtained. DesignThe superficial enamel in midcoronal areas of acid-etched facio-lingual sections from human third molars was studied in the scanning electron microscope (SEM). ResultsA great variation was observed in occurrence of prism-free enamel. Prism-free enamel dominated in 40% (mandibular) and 47% (maxillary) of observed areas and had a mean thickness of about 30μm. Striations in the prism-free enamel had an interstriae distance of about 3.3–3.8μm. The angle between prisms and enamel surface was about 60°, between prisms and Retzius lines about 45° and between Retzius lines and enamel surface about 15°. The distances between regularly occurring Retzius lines and between striations in the prism-free enamel both tended to decrease toward the enamel surface. Prisms could change direction as they approached the enamel surface, mostly in cervical direction. Where Retzius lines curved and converged occlusally, prisms tended to deviate in an occlusal direction. ConclusionsJudged from the incremental lines and occurrence of prism-free enamel, ameloblasts slow down and tend to lose their Tomes’ process as they approach the end of secretion. The crystals of prism-free enamel belong to the same system as the interprism crystals of prismatic enamel. A method, based on the disposition of fine incremental lines, is suggested for evaluation of ameloblast dynamics in the last stage of enamel secretion.

Synchrotron imaging and Markov Chain Monte Carlo reveal tooth mineralization patterns

The progressive character of tooth formation records aspects of mammalian life history, diet, seasonal behavior and climate. Tooth mineralization occurs in two stages: secretion and maturation, which overlap to some degree. Despite decades of study, the spatial and temporal pattern of elemental incorporation during enamel mineralization remains poorly characterized. Here we use synchrotron X-ray microtomography and Markov Chain Monte Carlo sampling to estimate mineralization patterns from an ontogenetic series of sheep molars (n = 45 M1s, 18 M2s). We adopt a Bayesian approach that posits a general pattern of maturation estimated from individual- and population-level mineral density variation over time. This approach converts static images of mineral density into a dynamic model of mineralization, and demonstrates that enamel secretion and maturation waves advance at nonlinear rates with distinct geometries. While enamel secretion is ordered, maturation geometry varies within a population and appears to be driven by diffusive processes. Our model yields concrete expectations for the integration of physiological and environmental signals, which is of particular significance for paleoseasonality research. This study also provides an avenue for characterizing mineralization patterns in other taxa. Our synchrotron imaging data and model are available for application to multiple disciplines, including health, material science, and paleontological research.