Epithelial-mesenchymal signaling during tooth development

A Narrative Review on Advancing Pediatric Oral Health: Comprehensive Strategies for the Prevention and Management of Dental Challenges in Children

Oral health is fundamental to overall well-being, especially in childhood, when dental structures and lifelong habits are established. This review provides a comprehensive analysis of children's dental development, common oral health challenges, and evidence-based preventive strategies. Key topics include the mechanisms of tooth development, the eruption processes of primary and permanent dentition, and the management of developmental abnormalities. The prevalence, risk factors, and health impacts of dental caries in children are examined, underscoring the need for early intervention and targeted prevention. This review evaluates the effectiveness of preventive measures such as dental sealants, fluoride varnishes, and fluoride mouth rinses while highlighting the influence of dietary habits, toothbrush selection, and parental involvement on oral health outcomes. Additionally, it explores the synergistic benefits of combining preventive approaches, such as the concurrent use of dental sealants and fluoride applications, which have demonstrated superior caries prevention compared to either method alone. The effectiveness of these strategies is analyzed across different age groups, from early childhood to adolescence, with tailored recommendations for each developmental stage. Furthermore, the role of education, policy interventions, and community-based programs in addressing oral health disparities is discussed. By integrating developmental insights with epidemiological data and clinical evidence, this review provides a comprehensive framework for advancing pediatric dentistry, informing best practices, and enhancing preventive strategies to reduce the burden of oral diseases in diverse pediatric populations.

Unraveling the hidden complexity: Exploring dental tissues through single-cell transcriptional profiling

Understanding the composition and function of cells constituting tissues and organs is vital for unraveling biological processes. Single-cell analysis has allowed us to move beyond traditional methods of categorizing cell types. This innovative technology allows the transcriptional and epigenetic profiling of numerous individual cells, leading to significant insights into the development, homeostasis, and pathology of various organs and tissues in both animal models and human samples. In this review, we delve into the outcomes of major investigations using single-cell transcriptomics to decipher the cellular composition of mammalian teeth and periodontal tissues. The recent single-cell transcriptome-based studies have traced in detail the dental epithelium-ameloblast lineage and dental mesenchyme lineages in the mouse incisors and the tooth germ of both mice and humans; unraveled the microenvironment, the identity of niche cells, and cellular intricacies in the dental pulp; shed light on the molecular mechanisms orchestrating root formation; and characterized cellular dynamics of the periodontal ligament. Additionally, cellular components in dental pulps were compared between healthy and carious teeth at a single-cell level. Each section of this review contributes to a comprehensive understanding of tooth biology, offering valuable insights into developmental processes, niche cell identification, and the molecular secrets of the dental environment.

Spatiotemporal expression of fibroblast growth factor 4 and 10 during the morphogenesis of deciduous molars in miniature pigs

OBJECTIVE

Fibroblast growth factors (FGFs) play pivotal roles in mediating interactions between dental epithelium and mesenchyme throughout tooth initiation and morphogenesis. This study aimed to elucidate the roles of FGF4 and FGF10 in the regulation of tooth development.

DESIGN

In this study, we investigated spatiotemporal expression patterns of FGF4 and FGF10 in the third deciduous molars (DM3) of miniature pigs at the cap, early bell, and late bell stages. Pregnant miniature pigs were obtained, and the samples were processed for histological staining. Non-radioactive in situ hybridization, immunohistochemistry, and real-time PCR were used to detect mRNA and protein expression levels of FGF4 and FGF10.

RESULTS

FGF4 was expressed in the dental epithelium and mesenchyme at the cap stage. At the early bell stage, epithelial expression of FGF4 was reduced while mesenchymal expression got stronger. At the late bell stage, the FGF4 expression was restricted to the inner enamel epithelium (IEE) and differentiating odontoblasts. FGF10 was expressed intensely in both epithelium and mesenchyme at the cap stage. The expression of FGF10 was concentrated in the secondary enamel knots and surrounding mesenchyme at the early bell stage. FGF10 was weakly detected in the IEE by the late bell stage.

CONCLUSIONS

Our results indicated that FGF4 and FGF10 might have partially redundant functions in regulating epithelium morphogenesis. FGF4 may be involved in regulatory signaling cascades mediating interactions between the epithelium and mesenchyme. In addition, the downregulation of FGF10 expression may be associated with the cessation of mesenchymal cell proliferation and initiation of preodontoblast polarization.

Morphological characteristics of beak-like tooth in spotted knifejaw (Oplegnathus punctatus) and mechanisms of dental development regulation by the Wnt, BMP, FGF and SHH signalling pathways

Oplegnathus punctatus is a fish species with beak-like tooth that feeds on algae, oysters, sea urchins, and other organisms attached to rocks. Currently, there are no research reports on the development and regulatory mechanisms of O. punctatus beak-like tooth. This present study firstly elucidated the nesting structure pattern of the beak-like tooth with dental formula (4, 15-16, 10-1) for O. punctatus. Four critical periods during early beak-like tooth development (28dph, 40dph, 50dph, 60dph) were also identified. In addition, 11 key genes (bmp2, bmpr2, smad1, wnt5a, msx, axin2, fgfr1a, fgfr2, pitx2, ptch1, cyp27a1) closely related to the development of beak-like tooth were discovered, with the highest expression levels in the initial stages of functional teeth and replacement teeth development, and expression in the mesenchymal and epithelial tissues of the teeth. Further research found that the cyp27a1 gene, related to vitamin D metabolism and calcium accumulation, was expressed in the maxilla and base of the tooth in O. punctatus. This study provides support for the biological theory of tooth development and healing and provides a reference for the adaptive evolution of tooth healing in special habitats.

Mesenchymal condensation in tooth development and regeneration: a focus on translational aspects of organogenesis

The teeth are vertebrate-specific, highly specialized organs performing fundamental functions of mastication and speech, the maintenance of which is crucial for orofacial homeostasis and is further linked to systemic health and human psychosocial well-being. However, with limited ability for self-repair, the teeth can often be impaired by traumatic, inflammatory, and progressive insults, leading to high prevalence of tooth loss and defects worldwide. Regenerative medicine holds the promise to achieve physiological restoration of lost or damaged organs, and in particular an evolving framework of developmental engineering has pioneered functional tooth regeneration by harnessing the odontogenic program. As a key event of tooth morphogenesis, mesenchymal condensation dictates dental tissue formation and patterning through cellular self-organization and signaling interaction with the epithelium, which provides a representative to decipher organogenetic mechanisms and can be leveraged for regenerative purposes. In this review, we summarize how mesenchymal condensation spatiotemporally assembles from dental stem cells (DSCs) and sequentially mediates tooth development. We highlight condensation-mimetic engineering efforts and mechanisms based on ex vivo aggregation of DSCs, which have achieved functionally robust and physiologically relevant tooth regeneration after implantation in animals and in humans. The discussion of this aspect will add to the knowledge of development-inspired tissue engineering strategies and will offer benefits to propel clinical organ regeneration.

Enamel Phenotypes: Genetic and Environmental Determinants

Dental enamel is a specialized tissue that has adapted over millions of years of evolution to enhance the survival of a variety of species. In humans, enamel evolved to form the exterior protective layer for the crown of the exposed tooth crown. Its unique composition, structure, physical properties and attachment to the underlying dentin tissue allow it to be a resilient, although not self-repairing, tissue. The process of enamel formation, known as amelogenesis, involves epithelial-derived cells called ameloblasts that secrete a unique extracellular matrix that influences the structure of the mineralizing enamel crystallites. There are over 115 known genetic conditions affecting amelogenesis that are associated with enamel phenotypes characterized by either a reduction of enamel amount and or mineralization. Amelogenesis involves many processes that are sensitive to perturbation and can be altered by numerous environmental stressors. Genetics, epigenetics, and environment factors can influence enamel formation and play a role in resistance/risk for developmental defects and the complex disease, dental caries. Understanding why and how enamel is affected and the enamel phenotypes seen clinically support diagnostics, prognosis prediction, and the selection of treatment approaches that are appropriate for the specific tissue defects (e.g., deficient amount, decreased mineral, reduced insulation and hypersensitivity). The current level of knowledge regarding the heritable enamel defects is sufficient to develop a new classification system and consensus nosology that effectively communicate the mode of inheritance, molecular defect/pathway, and the functional aberration and resulting enamel phenotype.

Runx2 and Nell-1 in dental follicle progenitor cells regulate bone remodeling and tooth eruption

Dental follicles are necessary for tooth eruption, surround the enamel organ and dental papilla, and regulate both the formation and resorption of alveolar bone. Dental follicle progenitor cells (DFPCs), which are stem cells found in dental follicles, differentiate into different kinds of cells that are necessary for tooth formation and eruption. Runt‐related transcription factor 2 (Runx2) is a transcription factor that is essential for osteoblasts and osteoclasts differentiation, as well as bone remodeling. Mutation of Runx2 causing cleidocranial dysplasia negatively affects osteogenesis and the osteoclastic ability of dental follicles, resulting in tooth eruption difficulties. Among a variety of cells and molecules, Nel-like molecule type 1 (Nell-1) plays an important role in neural crest-derived tissues and is strongly expressed in dental follicles. Nell-1 was originally identified in pathologically fused and fusing sutures of patients with unilateral coronal synostosis, and it plays indispensable roles in bone remodeling, including roles in osteoblast differentiation, bone formation and regeneration, craniofacial skeleton development, and the differentiation of many kinds of stem cells. Runx2 was proven to directly target the Nell-1 gene and regulate its expression. These studies suggested that Runx2/Nell-1 axis may play an important role in the process of tooth eruption by affecting DFPCs. Studies on short and long regulatory noncoding RNAs have revealed the complexity of RNA-mediated regulation of gene expression at the posttranscriptional level. This ceRNA network participates in the regulation of Runx2 and Nell-1 gene expression in a complex way. However, non-study indicated the potential connection between Runx2 and Nell-1, and further researches are still needed.

Single cell atlas of developing mouse dental germs reveals populations of CD24+ and Plac8+ odontogenic cells

The spatiotemporal relationships in high-resolution during odontogenesis remain poorly understood. We report a cell lineage and atlas of developing mouse teeth. We performed a large-scale (92,688 cells) single cell RNA sequencing, tracing the cell trajectories during odontogenesis from embryonic days 10.5 to 16.5. Combined with an assay for transposase-accessible chromatin with high-throughput sequencing, our results suggest that mesenchymal cells show the specific transcriptome profiles to distinguish the tooth types. Subsequently, we identified key gene regulatory networks in teeth and bone formation and uncovered spatiotemporal patterns of odontogenic mesenchymal cells. CD24+ and Plac8+ cells from the mesenchyme at the bell stage were distributed in the upper half and preodontoblast layer of the dental papilla, respectively, which could individually induce nonodontogenic epithelia to form tooth-like structures. Specifically, the Plac8+ tissue we discovered is the smallest piece with the most homogenous cells that could induce tooth regeneration to date. Our work reveals previously unknown heterogeneity and spatiotemporal patterns of tooth germs that may lead to tooth regeneration for regenerative dentistry.

The implication of holocytochrome c synthase mutation in Korean familial hypoplastic amelogenesis imperfecta

Objectives

This study aimed to comprehensively characterise genetic variants of amelogenesis imperfecta in a single Korean family through whole-exome sequencing and bioinformatics analysis.

Material and methods

Thirty-one individuals of a Korean family, 9 of whom were affected and 22 unaffected by amelogenesis imperfecta, were enrolled. Whole-exome sequencing was performed on 12 saliva samples, including samples from 8 affected and 4 unaffected individuals. The possible candidate genes associated with the disease were screened by segregation analysis and variant filtering. In silico mutation impact analysis was then performed on the filtered variants based on sequence conservation and protein structure.

Results

Whole-exome sequencing data revealed an X-linked dominant, heterozygous genomic missense mutation in the mitochondrial gene holocytochrome c synthase (HCCS). We also found that HCCS is potentially related to the role of mitochondria in amelogenesis. The HCCS variant was expected to be deleterious in both evolution-based and large population-based analyses. Further, the variant was predicted to have a negative effect on catalytic function of HCCS by in silico analysis of protein structure. In addition, HCCS had significant association with amelogenesis in literature mining analysis.

Conclusions

These findings suggest new evidence for the relationship between amelogenesis and mitochondria function, which could be implicated in the pathogenesis of amelogenesis imperfecta.

Clinical relevance

The discovery of HCCS mutations and a deeper understanding of the pathogenesis of amelogenesis imperfecta could lead to finding solutions for the fundamental treatment of this disease. Furthermore, it enables dental practitioners to establish predictable prosthetic treatment plans at an early stage by early detection of amelogenesis imperfecta through personalised medicine.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00784-022-04413-0.

Testing the inhibitory cascade model in a recent human sample

The Inhibitory Cascade Model was proposed by Kavanagh and colleagues (Nature, 449, 427-433 [2007]) after their experimental studies on the dental development of murine rodent species. These authors described an activator-inhibitor mechanism that has been employed to predict evolutionary size patterns of mammalian teeth, including hominins. In the present study, we measured the crown area of the three lower permanent molars (M1, M2, and M3) of a large recent modern human sample of male and female individuals from a collection preserved at the Institute of Anthropology of the University of Coimbra (Portugal). The main aim of the present study is to test if the size molar patterns observed in this human sample fits the Inhibitory Cascade Model. For this purpose, we first measured the crown area in those individuals preserving the complete molar series. Measurements were taken in photographs, using a planimeter and following well-tested techniques used in previous works. We then plot the M3 /M1 and M2 /M1  size ratios. Our results show that the premise of the Inhibitory Cascade Model, according to which the average of the crown area of M2 is approximately one-third of the sum of the crown area of the three molars, is fulfilled. However, our results also show that the individual values of a significant number of males and females are out of the 95% confidence interval predicted by the Inhibitory Cascade Model in rodents. As a result, the present analyses suggest that neither the sample of males, nor that of females, nor the pooled sample fits the Inhibitory Cascade Model. It is important to notice that, although this model has been successfully tested in a large number of current human populations, to the best of our knowledge this is the first study in which individual data have been obtained in a recent human population rather than using the average of the sample. Our results evince that, at the individual level, some factors not yet known could interfere with this model masking the modulation of the size on the molar series in modern humans. We suggest that the considerable delay in the onset of M3 formation in modern humans could be related to a weakening of the possible activation/inhibition process for this tooth. Finally, and in support of our conclusions, we have checked that the absolute and relative size of M1 and M2 is not related to the M3 agenesis in our sample. In line with other studies in primates, our results do not support the Inhibitory Cascade Model in a recent human sample. Further research is needed to better understand the genetic basis of this mechanism and its relationship to the phenotype. In this way, we may be able to find out which evolutionary changes may be responsible for the deviations observed in many species, including Homo sapiens.

Formation and Developmental Specification of the Odontogenic and Osteogenic Mesenchymes

Within the mandible, the odontogenic and osteogenic mesenchymes develop in a close proximity and form at about the same time. They both originate from the cranial neural crest. These two condensing ecto-mesenchymes are soon separated from each other by a very loose interstitial mesenchyme, whose cells do not express markers suggesting a neural crest origin. The two condensations give rise to mineralized tissues while the loose interstitial mesenchyme, remains as a soft tissue. This is crucial for proper anchorage of mammalian teeth. The situation in all three regions of the mesenchyme was compared with regard to cell heterogeneity. As the development progresses, the early phenotypic differences and the complexity in cell heterogeneity increases. The differences reported here and their evolution during development progressively specifies each of the three compartments. The aim of this review was to discuss the mechanisms underlying condensation in both the odontogenic and osteogenic compartments as well as the progressive differentiation of all three mesenchymes during development. Very early, they show physical and structural differences including cell density, shape and organization as well as the secretion of three distinct matrices, two of which will mineralize. Based on these data, this review highlights the consecutive differences in cell-cell and cell-matrix interactions, which support the cohesion as well as mechanosensing and mechanotransduction. These are involved in the conversion of mechanical energy into biochemical signals, cytoskeletal rearrangements cell differentiation, or collective cell behavior.

Differentiation and Establishment of Dental Epithelial-Like Stem Cells Derived from Human ESCs and iPSCs

Tooth development and regeneration occur through reciprocal interactions between epithelial and ectodermal mesenchymal stem cells. However, the current studies on tooth development are limited, since epithelial stem cells are relatively difficult to obtain and maintain. Human embryonic stem cells (hESCs) and induced pluripotent stem cells (hiPSCs) may be alternative options for epithelial cell sources. To differentiate hESCs/hiPSCs into dental epithelial-like stem cells, this study investigated the hypothesis that direct interactions between pluripotent stem cells, such as hESCs or hiPSCs, and Hertwig's epithelial root sheath/epithelial rests of Malassez (HERS/ERM) cell line may induce epithelial differentiation. Epithelial-like stem cells derived from hES (EPI-ES) and hiPSC (EPI-iPSC) had morphological and immunophenotypic characteristics of HERS/ERM cells, as well as similar gene expression. To overcome a rare population and insufficient expansion of primary cells, EPI-iPSC was immortalized with the SV40 large T antigen. The immortalized EPI-iPSC cell line had a normal karyotype, and a short tandem repeat (STR) analysis verified that it was derived from hiPSCs. The EPI-iPSC cell line co-cultured with dental pulp stem cells displayed increased amelogenic and odontogenic gene expression, exhibited higher dentin sialoprotein (DSPP) protein expression, and promoted mineralized nodule formation. These results indicated that the direct co-culture of hESCs/hiPSCs with HERS/ERM successfully established dental epithelial-like stem cells. Moreover, this differentiation protocol could help with understanding the functional roles of cell-to-cell communication and tissue engineering of teeth.

Sonic hedgehog signaling in epithelial tissue development

The Sonic hedgehog (SHH) signaling pathway is essential for embryonic development and tissue regeneration. The dysfunction of SHH pathway is involved in a variety of diseases, including cancer, birth defects, and other diseases. Here we reviewed recent studies on main molecules involved in the SHH signaling pathway, specifically focused on their function in epithelial tissue and appendages development, including epidermis, touch dome, hair, sebaceous gland, mammary gland, tooth, nail, gastric epithelium, and intestinal epithelium. The advance in understanding the SHH signaling pathway will give us more clues to the mechanisms of tissue repair and regeneration, as well as the development of new treatment for diseases related to dysregulation of SHH signaling pathway.

Phylogenetic and Developmental Constraints Dictate the Number of Cusps on Molars in Rodents

Mammal tooth morphology and function correlate strongly with dietary ecology, and convergence is a major feature of mammalian tooth evolution. Yet, function and ecology are insufficient to explain morphological diversification and convergence within mammalian molar evolution; suggesting that development and phylogeny also limit possible structural solutions to selective pressures. Here, I use in silico models and empirical studies of extant and fossil rodent teeth to identify morphogenetic rules that influence molar morphology. Because rodents are the most diverse group of mammals with corresponding dental disparity they represent an excellent system for investigating how genetic interactions limit morphology. I find that lower first molars are limited to a minimum of four cusps and a maximum of nine cusps. Multiple developmental pathways produce the same numbers of cusps, despite highly variable cusp morphologies, indicating the existence of limits on how morphological evolution can fill a morphospace defined by cusp numbers. These constraints are both developmental and phylogenetic in nature and the identification of their influence on rodent molar shape provides a framework for investigation of how tooth batteries evolved an array of functions despite fundamental structural limits. The data presented here increase predictability of cusp number and evolutionary outcomes of rodent cheek dentition.

Perception of attractiveness of missing maxillary lateral incisors replaced by canines

Objective:

The aim of this study was to evaluate the degree of perception of attractiveness of the smile among dentists, dental students, and lay persons in cases of agenesis of the maxillary lateral incisors replaced by canines for space closure.

Methods:

A smiling front view extraoral photograph of a 20-year-old woman was digitally altered simulating agenesis and its treatment, by means of: repositioning, reshaping or bleaching the canine, and gingival contour. A questionnaire was distributed to individuals of the three groups (n= 150), with a view to evaluating their degree of esthetic perception. An attractiveness scale was also used, with ‘0’ representing unattractive and ‘10’, very attractive.

Results:

In the comparative evaluation among all the photographs, the original image obtained the highest level of acceptance. Photograph ‘i’ (agenesis of both lateral incisors treated with reposition and reshaping of the canines) was ranked as the least attractive by the dentists, whereas the student and lay persons ranked photograph ‘f’ (agenesis of both lateral incisors treated with reposition of the canines, gingival contour, bleaching and reshaping) as the worst.

Conclusion:

The methods of treatment most accepted among the dentists and students were those that involved changes in the gingival contour, whereas among lay persons, they were those that involved only reshaping.

The versatile hippo pathway in oral-maxillofacial development and bone remodeling

The Hippo signaling pathway is implicated in key aspects of cell proliferation, control of organ size, stem cell functions and tumor suppression. Its functions are primarily mediated either through direct effects on transcription factors to influence target gene expression or through crosstalk with other signaling pathways that regulate multiple physiological activities. Studies are revealing Hippo pathway involvement in diverse functions including renewal of intestinal epithelium, promotion of myocardial cell proliferation, cancer suppression, etc. In this review we discuss Hippo pathway signaling in oral-maxillofacial development and bone remodeling under normal and pathological conditions and highlight promising future research directions.

Type 1 diabetes mellitus effects on dental enamel formation revealed by microscopy and microanalysis

BACKGROUND

Type 1 diabetes mellitus (T1DM) largely affects children, occurring therefore at the same period of deciduous and permanent teeth development. The aim of this work was to investigate birefringence and morphology of the secretory stage enamel organic extracellular matrix (EOECM), and structural and mechanical features of mature enamel from T1DM rats.

METHODS

Adult Wistar rats were maintained alive for a period of 56 days after the induction of experimental T1DM with a single dose of streptozotocin (60 mg/kg). After proper euthanasia of the animals, fixed upper incisors were accurately processed, and secretory stage EOECM and mature enamel were analyzed by transmitted polarizing and bright field light microscopies (TPLM and BFLM), energy-dispersive x-ray (EDX) analysis, scanning electron microscopy (SEM), and microhardness testing.

RESULTS

Bright field light microscopies and transmitted polarizing light microscopies showed slight morphological changes in the secretory stage EOECM from diabetic rats, which also did not exhibit statistically significant alterations in birefringence brightness when compared to control animals (P > .05). EDX analysis showed that T1DM induced statistically significant little increases in the amount of calcium and phosphorus in outer mature enamel (P < .01) with preservation of calcium/phosphorus ratio in that structure (P > .05). T1DM also caused important ultrastructural alterations in mature enamel as revealed by SEM and induced a statistically significant reduction of about 13.67% in its microhardness at 80 μm from dentin-enamel junction (P < .01).

CONCLUSIONS

This study shows that T1DM may disturb enamel development, leading to alterations in mature enamel ultrastructure and in its mechanical features.

Homeobox protein MSX-1 inhibits expression of bone morphogenetic protein 2, bone morphogenetic protein 4, and lymphoid enhancer-binding factor 1 via Wnt/β-catenin signaling to prevent differentiation of dental mesenchymal cells during the late bell stage

Homeobox protein MSX-1 (hereafter referred to as MSX-1) is essential for early tooth-germ development. Tooth-germ development is arrested at bud stage in Msx1 knockout mice, which prompted us to study the functions of MSX-1 beyond this stage. Here, we investigated the roles of MSX-1 during late bell stage. Mesenchymal cells of the mandibular first molar were isolated from mice at embryonic day (E)17.5 and cultured in vitro. We determined the expression levels of β-catenin, bone morphogenetic protein 2 (Bmp2), Bmp4, and lymphoid enhancer-binding factor 1 (Lef1) after knockdown or overexpression of Msx1. Our findings suggest that knockdown of Msx1 promoted expression of Bmp2, Bmp4, and Lef1, resulting in elevated differentiation of odontoblasts, which was rescued by blocking the expression of these genes. In contrast, overexpression of Msx1 decreased the expression of Bmp2, Bmp4, and Lef1, leading to a reduction in odontoblast differentiation. The regulation of Bmp2, Bmp4, and Lef1 by Msx1 was mediated by the Wnt/β-catenin signaling pathway. Additionally, knockdown of Msx1 impaired cell proliferation and slowed S-phase progression, while overexpression of Msx1 also impaired cell proliferation and prolonged G1-phase progression. We therefore conclude that MSX-1 maintains cell proliferation by regulating transition of cells from G1-phase to S-phase and prevents odontoblast differentiation by inhibiting expression of Bmp2, Bmp4, and Lef1 at the late bell stage via the Wnt/β-catenin signaling pathway.

Activin and Bmp4 Signaling Converge on Wnt Activation during Odontogenesis

Previous studies show that both activin and Bmp4 act as crucial mesenchymal odontogenic signals during early tooth development. Remarkably, mice lacking activin-βA ( Inhba^-/-) and mice with neural crest-specific inactivation of Bmp4 ( Bmp4^ncko/ncko) both exhibit bud-stage developmental arrest of the mandibular molar tooth germs while their maxillary molar tooth germs completed morphogenesis. In this study, we found that, whereas expression of Inhba and Bmp4 in the developing tooth mesenchyme is independent of each other, Bmp4^ncko/nckoInhba^-/- compound mutant mice exhibit early developmental arrest of all tooth germs. Moreover, genetic inactivation of Osr2, a negative regulator of the odontogenic function of the Bmp4-Msx1 signaling pathway, rescues mandibular molar morphogenesis in Inhba^-/- embryos. We recently reported that Osr2 and the Bmp4-Msx1 pathway control the bud-to-cap transition of tooth morphogenesis through antagonistic regulation of expression of secreted Wnt antagonists, including Dkk2 and Sfrp2, in the developing tooth mesenchyme. We show here that expression of Dkk2 messenger RNAs was significantly upregulated and expanded into the tooth bud mesenchyme in Inhba^-/- embryos in comparison with wild-type littermates. Furthermore, in utero treatment with either lithium chloride, an agonist of canonical Wnt signaling, or the DKK inhibitor IIIC3a rescued mandibular molar tooth morphogenesis in Inhba^-/- embryos. Together with our finding that the developing mandibular molar tooth bud mesenchyme expresses significantly higher levels of Dkk2 than the developing maxillary molar tooth mesenchyme, these data indicate that Bmp4 and activin signaling pathways converge on activation of the Wnt signaling pathway to promote tooth morphogenesis through the bud-to-cap transition and that the differential effects of loss of activin or Bmp4 signaling on maxillary and mandibular molar tooth morphogenesis are mainly due to the differential expression of Wnt antagonists, particularly Dkk2, in the maxillary and mandibular tooth mesenchyme.

Angled Growth of the Dental Lamina Is Accompanied by Asymmetrical Expression of the WNT Pathway Receptor Frizzled 6

Frizzled 6 (FZD6) belongs to a family of proteins that serve as receptors in the WNT signaling pathway. FZD6 plays an important role in the establishment of planar cell polarity in many embryonic processes such as convergent extension during gastrulation, neural tube closure, or hair patterning. Based on its role during hair development, we hypothesized that FZD6 may have similar expression pattern and function in the dental lamina, which is also a distinct epithelial protrusion growing characteristically angled into the mesenchyme. Diphyodont minipig was selected as a model species because its dentition closely resemble human ones with successional generation of teeth initiated from the dental lamina. We revealed asymmetrical expression of FZD6 in the dental lamina of early as well as late stages during its regression with stronger expression located on the labial side of the dental lamina. During lamina regression, FZD6-positive cells were found in its superficial part and the signal coincided with the upregulation of molecules involved in epithelial-mesenchymal transition and increased migratory potential of epithelial cells. FZD6-expression was also turned on during differentiation of cells producing hard tissues, in which mature odontoblasts, ameloblasts, or surrounding osteoblasts were FZD6-positive. On the other hand, the tip of successional lamina and its lingual part, in which progenitor cells are located, exhibited FZD6-negativity. In conclusion, asymmetrical expression of FZD6 correlates with the growth directionality and side-specific morphological differences in the dental lamina of diphyodont species. Based on observed expression pattern, we propose that the dental lamina is other epithelial tissue, where planar cell polarity signaling is involved during its asymmetrical growth.

Jawbone microenvironment promotes periodontium regeneration by regulating the function of periodontal ligament stem cells

During tooth development, the jawbone interacts with dental germ and provides the development microenvironment. Jawbone-derived mesenchymal stem cells (JBMSCs) maintain this microenvironment for root and periodontium development. However, the effect of the jawbone microenvironment on periodontium tissue regeneration is largely elusive. Our previous study showed that cell aggregates (CAs) of bone marrow mesenchymal stem cells promoted periodontium regeneration on the treated dentin scaffold. Here, we found that JBMSCs enhanced not only the osteogenic differentiation of periodontal ligament stem cells (PDLSCs) but also their adhesion to titanium (Ti) material surface. Importantly, the compound CAs of PDLSCs and JBMSCs regenerated periodontal ligament-like fibers and mineralized matrix on the Ti scaffold surface, both in nude mice ectopic and minipig orthotopic transplantations. Our data revealed that an effective regenerative microenvironment, reconstructed by JBMSCs, promoted periodontium regeneration by regulating PDLSCs function on the Ti material.

The Neurovascular Properties of Dental Stem Cells and Their Importance in Dental Tissue Engineering

Within the field of tissue engineering, natural tissues are reconstructed by combining growth factors, stem cells, and different biomaterials to serve as a scaffold for novel tissue growth. As adequate vascularization and innervation are essential components for the viability of regenerated tissues, there is a high need for easily accessible stem cells that are capable of supporting these functions. Within the human tooth and its surrounding tissues, different stem cell populations can be distinguished, such as dental pulp stem cells, stem cells from human deciduous teeth, stem cells from the apical papilla, dental follicle stem cells, and periodontal ligament stem cells. Given their straightforward and relatively easy isolation from extracted third molars, dental stem cells (DSCs) have become an attractive source of mesenchymal-like stem cells. Over the past decade, there have been numerous studies supporting the angiogenic, neuroprotective, and neurotrophic effects of the DSC secretome. Together with their ability to differentiate into endothelial cells and neural cell types, this makes DSCs suitable candidates for dental tissue engineering and nerve injury repair.

αE-catenin inhibits YAP/TAZ activity to regulate signalling centre formation during tooth development

Embryonic signalling centres are specialized clusters of non-proliferating cells that direct the development of many organs. However, the mechanisms that establish these essential structures in mammals are not well understood. Here we report, using the murine incisor as a model, that αE-catenin is essential for inhibiting nuclear YAP localization and cell proliferation. This function of αE-catenin is required for formation of the tooth signalling centre, the enamel knot (EK), which maintains dental mesenchymal condensation and epithelial invagination. EK formation depends primarily on the signalling function of αE-catenin through YAP and its homologue TAZ, as opposed to its adhesive function, and combined deletion of Yap and Taz rescues the EK defects caused by loss of αE-catenin. These findings point to a developmental mechanism by which αE-catenin restricts YAP/TAZ activity to establish a group of non-dividing and specialized cells that constitute a signalling centre.

It is unclear which signals regulate the establishment of the tooth signalling centre, the enamel knot (EK). Here, the authors show that EK formation depends on αE-catenin, which acts to restrict the Hippo pathway transcription factors Yes-associated protein (YAP) and TAZ.

Cellular Reprogramming Using Defined Factors and MicroRNAs

Development of human bodies, organs, and tissues contains numerous steps of cellular differentiation including an initial zygote, embryonic stem (ES) cells, three germ layers, and multiple expertized lineages of cells. Induced pluripotent stem (iPS) cells have been recently developed using defined reprogramming factors such as Nanog, Klf5, Oct3/4 (Pou5f1), Sox2, and Myc. This outstanding innovation is largely changing life science and medicine. Methods of direct reprogramming of cells into myocytes, neurons, chondrocytes, and osteoblasts have been further developed using modified combination of factors such as N-myc, L-myc, Sox9, and microRNAs in defined cell/tissue culture conditions. Mesenchymal stem cells (MSCs) and dental pulp stem cells (DPSCs) are also emerging multipotent stem cells with particular microRNA expression signatures. It was shown that miRNA-720 had a role in cellular reprogramming through targeting the pluripotency factor Nanog and induction of DNA methyltransferases (DNMTs). This review reports histories, topics, and idea of cellular reprogramming.

Effects of Pamidronate on Dental Enamel Formation Assessed by Light Microscopy, Energy-Dispersive X-Ray Analysis, Scanning Electron Microscopy, and Microhardness Testing

The aim of the present work was to investigate birefringence and morphology of the secretory-stage enamel organic extracellular matrix (EOECM), and structural and mechanical properties of mature enamel of upper incisors from adult rats that had been treated with pamidronate disodium (0.5 mg/kg/week for 56 days), using transmitted polarizing and bright-field light microscopies (TPLM and BFLM), energy-dispersive X-ray (EDX) analysis, scanning electron microscopy (SEM) and microhardness testing. BFLM showed no morphological changes of the EOECM in pamidronate and control groups, but TPLM revealed a statistically significant reduction in optical retardation values of birefringence brightness of pamidronate-treated rats when compared with control animals (p0.05). The present study indicates that pamidronate can affect birefringence of the secretory-stage EOECM, which does not seem to be associated with significant changes in morphological and/or mechanical properties of mature enamel.

Odontoblastic inductive potential of epithelial cells derived from human deciduous dental pulp

For the dentin regeneration, dental epithelial cells are indispensible and must possess odontoblastic induction capability. Epithelial cell-like stem cells were recently identified in human deciduous dental pulp (DPESCs). However, their cellular characteristics remain poorly defined. The purpose of this study was to characterize DPESCs compared to HAT-7 ameloblastic cells. Expression levels of ameloblast-specific markers [odontogenic ameloblast-associated protein (Odam), matrix metalloproteinase (Mmp)-20, amelogenin, and ameloblastin] were detected in DPESCs. Co-culturing odontoblastic MDPC-23 cells with DPESCs increased expression of odontoblast differentiation markers (Dmp1 and Dspp) from days 4 to 10, while the expression of bone sialoprotein rapidly decreased. MDPC-23 cells cultured in DPESC-conditioned medium (CM) showed increased Dspp promoter activity compared with control MDPC-23 cultures. Mineralization was first observed in the CM groups from day 4 and proceeded rapidly until day 14, whereas mineralized nodules were found from day 7 in control media-cultured cells. In conclusion, DPESCs in human deciduous pulp possess ameloblast-like characteristics and differentiation properties, and substances derived from DPESCs promote odontoblastic differentiation. Thus, our results indicate that DPESCs can be a realistic epithelial source for use in odontoblastic induction and dentin formation of dental mesenchymal cells.

Bisphosphonates: Pharmacokinetics, bioavailability, mechanisms of action, clinical applications in children, and effects on tooth development

Bisphosphonates (BPs) avidly bind to calcium crystals and inhibit osteoclastic bone resorption, making them useful for treatment of skeletal disorders such as osteoporosis, Paget's disease, osteogenesis imperfecta and metastatic bone diseases. BPs therapeutically act by causing toxic effects on osteoclasts or interfering with specific intracellular pathways in those cells. BPs that possess nitrogen in their composition are called nitrogen-containing BPs (NBPs) and include alendronate, pamidronate, risedronate, ibandronate, and zoledronate. Simple BPs or non-NBPs do not have nitrogen in their composition, include etiodronate and clodronate, and were the first to be tested in animals and clinically used. Because BPs may be administered to pregnant women or children during deciduous and permanent teeth development, it is expected that they might disturb tooth eruption and development. A review of current literature on pharmacokinetics, bioavailability, mechanisms of action, and clinical applications of BPs in children, and their effects on tooth eruption and development is presented.

Matrix metalloproteinase-20 mediates dental enamel biomineralization by preventing protein occlusion inside apatite crystals

Reconstruction of enamel-like materials is a central topic of research in dentistry and material sciences. The importance of precise proteolytic mechanisms in amelogenesis to form a hard tissue with more than 95% mineral content has already been reported. A mutation in the Matrix Metalloproteinase-20 (MMP-20) gene results in hypomineralized enamel that is thin, disorganized and breaks from the underlying dentin. We hypothesized that the absence of MMP-20 during amelogenesis results in the occlusion of amelogenin in the enamel hydroxyapatite crystals. We used spectroscopy and electron microscopy techniques to qualitatively and quantitatively analyze occluded proteins within the isolated enamel crystals from MMP-20 null and Wild type (WT) mice. Our results showed that the isolated enamel crystals of MMP-20 null mice had more organic macromolecules occluded inside them than enamel crystals from the WT. The crystal lattice arrangements of MMP-20 null enamel crystals analyzed by High Resolution Transmission Electron Microscopy (HRTEM) were found to be significantly different from those of the WT. Raman studies indicated that the crystallinity of the MMP-20 null enamel crystals was lower than that of the WT. In conclusion, we present a novel functional mechanism of MMP-20, specifically prevention of unwanted organic material entrapped in the forming enamel crystals, which occurs as the result of precise amelogenin cleavage. MMP-20 action guides the growth morphology of the forming hydroxyapatite crystals and enhances their crystallinity. Elucidating such molecular mechanisms can be applied in the design of novel biomaterials for future clinical applications in dental restoration or repair.

Cell-to-cell communication--periodontal regeneration

BACKGROUND

Although regenerative treatment options are available, periodontal regeneration is still regarded as insufficient and unpredictable.

AIM

This review article provides scientific background information on the animated 3D film Cell-to-Cell Communication - Periodontal Regeneration.

RESULTS

Periodontal regeneration is understood as a recapitulation of embryonic mechanisms. Therefore, a thorough understanding of cellular and molecular mechanisms regulating normal tooth root development is imperative to improve existing and develop new periodontal regenerative therapies. However, compared to tooth crown and earlier stages of tooth development, much less is known about the development of the tooth root. The formation of root cementum is considered the critical element in periodontal regeneration. Therefore, much research in recent years has focused on the origin and differentiation of cementoblasts. Evidence is accumulating that the Hertwig's epithelial root sheath (HERS) has a pivotal role in root formation and cementogenesis. Traditionally, ectomesenchymal cells in the dental follicle were thought to differentiate into cementoblasts. According to an alternative theory, however, cementoblasts originate from the HERS. What happens when the periodontal attachment system is traumatically compromised? Minor mechanical insults to the periodontium may spontaneously heal, and the tissues can structurally and functionally be restored. But what happens to the periodontium in case of periodontitis, an infectious disease, after periodontal treatment? A non-regenerative treatment of periodontitis normally results in periodontal repair (i.e., the formation of a long junctional epithelium) rather than regeneration. Thus, a regenerative treatment is indicated to restore the original architecture and function of the periodontium. Guided tissue regeneration or enamel matrix proteins are such regenerative therapies, but further improvement is required. As remnants of HERS persist as epithelial cell rests of Malassez in the periodontal ligament, these epithelial cells are regarded as a stem cell niche that can give rise to new cementoblasts. Enamel matrix proteins and members of the transforming growth factor beta (TGF-ß) superfamily have been implicated in cementoblast differentiation.

CONCLUSION

A better knowledge of cell-to-cell communication leading to cementoblast differentiation may be used to develop improved regenerative therapies to reconstitute periodontal tissues that were lost due to periodontitis.

Cementum and Periodontal Ligament Regeneration

The unique anatomy and composition of the periodontium make periodontal tissue healing and regeneration a complex process. Periodontal regeneration aims to recapitulate the crucial stages of wound healing associated with periodontal development in order to restore lost tissues to their original form and function and for regeneration to occur, healing events must progress in an ordered and programmed sequence both temporally and spatially, replicating key developmental events. A number of procedures have been employed to promote true and predictable regeneration of the periodontium. Principally, the approaches are based on the use of graft materials to compensate for the bone loss incurred as a result of periodontal disease, use of barrier membranes for guided tissue regeneration and use of bioactive molecules. More recently, the concept of tissue engineering has been integrated into research and applications of regenerative dentistry, including periodontics, to aim to manage damaged and lost oral tissues, through reconstruction and regeneration of the periodontium and alleviate the shortcomings of more conventional therapeutic options. The essential components for generating effective cellular based therapeutic strategies include a population of multi-potential progenitor cells, presence of signalling molecules/inductive morphogenic signals and a conductive extracellular matrix scaffold or appropriate delivery system. Mesenchymal stem cells are considered suitable candidates for cell-based tissue engineering strategies owing to their extensive expansion rate and potential to differentiate into cells of multiple organs and systems. Mesenchymal stem cells derived from multiple tissue sources have been investigated in pre-clinical animal studies and clinical settings for the treatment and regeneration of the periodontium.

From regenerative dentistry to regenerative medicine: progress, challenges, and potential applications of oral stem cells

Adult mesenchymal stem cells (MSCs) and epithelial stem cells play essential roles in tissue repair and self-healing. Oral MSCs and epithelial stem cells can be isolated from adult human oral tissues, for example, teeth, periodontal ligament, and gingiva. Cocultivated adult oral epithelial stem cells and MSCs could represent some developmental events, such as epithelial invagination and tubular structure formation, signifying their potentials for tissue regeneration. Oral epithelial stem cells have been used in regenerative medicine over 1 decade. They are able to form a stratified cell sheet under three-dimensional culture conditions. Both experimental and clinical data indicate that the cell sheets can not only safely and effectively reconstruct the damaged cornea in humans, but also repair esophageal ulcer in animal models. Oral MSCs include dental pulp stem cells (DPSCs), stem cells from exfoliated deciduous teeth (SHED), stem cells from apical papilla (SCAP), periodontal ligament stem cells (PDLSCs), and mesenchymal stem cells from gingiva (GMSCs). They are widely applied in both regenerative dentistry and medicine. DPSCs, SHED, and SCAP are able to form dentin–pulp complex when being transplanted into immunodeficient animals. They have been experimentally used for the regeneration of dental pulp, neuron, bone muscle and blood vessels in animal models and have shown promising results. PDLSCs and GMSCs are demonstrated to be ideal cell sources for repairing the damaged tissues of periodontal, muscle, and tendon. Despite the abovementioned applications of oral stem cells, only a few human clinical trials are now underway to use them for the treatment of certain diseases. Since clinical use is the end goal, their true regenerative power and safety need to be further examined.

Bioengineered post-natal recombinant tooth bud models

The long-term goal of this study is to devise reliable methods to regenerate full-sized and fully functional biological teeth in humans. In this study, three-dimensional (3D) tissue engineering methods were used to characterize intact postnatal dental tissue recombinant constructs, and dental cell suspension recombinant constructs, as models for bioengineered tooth development. In contrast to studies using mouse embryonic dental tissues and cells, here the odontogenic potential of intact dental tissues and dental cell suspensions harvested from post natal porcine teeth and human third molar wisdom tooth dental pulp were examined. The recombinant 3D tooth constructs were cultured in osteogenic media in vitro for 1 week before subcutaneous transplantation in athymic nude rat hosts for 1 month or 3 months. Subsequent analyses using X-ray, histological and immunohistochemical methods showed that the majority of the recombinant tooth structures formed calcified tissues, including osteodentin, dentin cementum, enamel and morphologically typical tooth crowns composed of dentin and enamel. The demonstrated formation of mineralized dental tissues and tooth crown structures from easily obtained post-natal dental tissues is an important step toward reaching the long-term goal of establishing robust and reliable models for human tooth regeneration. Copyright © 2014 John Wiley & Sons, Ltd.

Primary cilia integrate hedgehog and Wnt signaling during tooth development

Many ciliopathies have clinical features that include tooth malformations but how these defects come about is not clear. Here we show that genetic deletion of the motor protein Kif3a in dental mesenchyme results in an arrest in odontogenesis. Incisors are completely missing, and molars are enlarged in Wnt1(Cre+)Kif3a(fl/fl) embryos. Although amelogenesis and dentinogenesis initiate in the molar tooth bud, both processes terminate prematurely. We demonstrate that loss of Kif3a in dental mesenchyme results in loss of Hedgehog signaling and gain of Wnt signaling in this same tissue. The defective dental mesenchyme then aberrantly signals to the dental epithelia, which prompts an up-regulation in the Hedgehog and Wnt responses in the epithelia and leads to multiple attempts at invagination and an expanded enamel organ. Thus, the primary cilium integrates Hedgehog and Wnt signaling between dental epithelia and mesenchyme, and this cilia-dependent integration is required for proper tooth development.

YAP Overexpression Affects Tooth Morphogenesis and Enamel Knot Patterning

Teeth develop through distinct morphological stages. At the cap stage, a compactly clustered and concentrically arranged cell mass, the enamel knot, appears at the tip of the enamel organ. Cells in this knot express sets of key molecules, and as such have been proposed to act as a signaling center directing tooth morphogenesis and tooth cusp formation. YAP is a transcriptional co-activator of the Hippo signaling pathway that is essential for the proper regulation of organ growth. In this study, we analyzed the tooth phenotype in transgenic mice that overexpressed a constitutively active form of YAP in the dental epithelium. We found that overexpression of YAP resulted in deformed tooth morphogenesis with widened dental lamina. In addition, the enamel knot was mislocated to the upper portion of the enamel organ, where it remained devoid of proliferating cells and contained apoptotic cells with intense Edar transcripts and reduced E-cadherin expression. Interestingly, some signaling molecules, such as Shh, Fgf4, and Wnt10a, were not expressed in this mislocated enamel knot, but remained at the tip of the enamel organ. Analysis of these data suggests that the signaling center is induced by reciprocal epithelial-mesenchymal interactions, and its induction may be independent of the enamel knot.

Novel PAX9 Mutations Cause Non-syndromic Tooth Agenesis

PAX9 is a transcription factor expressed in the tooth mesenchyme during tooth morphogenesis. In Pax9-null mice, tooth development is arrested at the bud stage. In humans, heterozygous mutations in PAX9 have been associated with non-syndromic tooth agenesis, predominantly in the molars. Here, we report 2 novel mutations in the paired domain of PAX9, a three-nucleotide deletion (73-75 delATC) and a missense mutation (C146T), in two unrelated Japanese patients with non-syndromic tooth agenesis. The individual with the 73-75del ATC mutation was missing all maxillary molars and mandibular second and third molars. The individual with the C146T mutation was missing the mandibular central incisors, maxillary second premolars, and first molars, along with all second and third molars. Both mutations affected amino acids that are highly conserved among different species and are critical for DNA binding. When both mutants were transfected to COS7 cells, nuclear localization of PAX9 proteins was not affected. However, reduced expression of the mutant proteins and almost no transcriptional activity of the target BMP4 gene were observed, suggesting haploinsufficiency of PAX9 as the cause of non-syndromic tooth agenesis.

Stem cells, tissue engineering and periodontal regeneration

The aim of this review is to discuss the clinical utility of stem cells in periodontal regeneration by reviewing relevant literature that assesses the periodontal-regenerative potential of stem cells. We consider and describe the main stem cell populations that have been utilized with regard to periodontal regeneration, including bone marrow-derived mesenchymal stem cells and the main dental-derived mesenchymal stem cell populations: periodontal ligament stem cells, dental pulp stem cells, stem cells from human exfoliated deciduous teeth, stem cells from apical papilla and dental follicle precursor cells. Research into the use of stem cells for tissue regeneration has the potential to significantly influence periodontal treatment strategies in the future.

Dental enamel development: proteinases and their enamel matrix substrates

This review focuses on recent discoveries and delves in detail about what is known about each of the proteins (amelogenin, ameloblastin, and enamelin) and proteinases (matrix metalloproteinase-20 and kallikrein-related peptidase-4) that are secreted into the enamel matrix. After an overview of enamel development, this review focuses on these enamel proteins by describing their nomenclature, tissue expression, functions, proteinase activation, and proteinase substrate specificity. These proteins and their respective null mice and human mutations are also evaluated to shed light on the mechanisms that cause nonsyndromic enamel malformations termed amelogenesis imperfecta. Pertinent controversies are addressed. For example, do any of these proteins have a critical function in addition to their role in enamel development? Does amelogenin initiate crystallite growth, does it inhibit crystallite growth in width and thickness, or does it do neither? Detailed examination of the null mouse literature provides unmistakable clues and/or answers to these questions, and this data is thoroughly analyzed. Striking conclusions from this analysis reveal that widely held paradigms of enamel formation are inadequate. The final section of this review weaves the recent data into a plausible new mechanism by which these enamel matrix proteins support and promote enamel development.

C-Jun N-terminal kinase (JNK) mediates Wnt5a-induced cell motility dependent or independent of RhoA pathway in human dental papilla cells

Wnt5a plays an essential role in tissue development by regulating cell migration, though the molecular mechanisms are still not fully understood. Our study investigated the pathways involved in Wnt5a-dependent cell motility during the formation of dentin and pulp. Over-expression of Wnt5a promoted cell adhesion and formation of focal adhesion complexes (FACs) in human dental papilla cells (hDPCs), while inhibiting cell migration. Instead of activating the canonical Wnt signal pathway in hDPCs, Wnt5a stimulation induced activation of the JNK signal in a RhoA-dependent or independent manner. Inhibiting JNK abrogated Wnt5a-induced FACs formation but not cytoskeletal rearrangement. Both dominant negative RhoA (RhoA T19N) and constitutively active RhoA mutants (RhoA Q63L) blocked the Wnt5a-dependent changes in hDPCs adhesion, migration and cytoskeletal rearrangement here too, with the exception of the formation of FACs. Taken together, our study suggested that RhoA and JNK signaling have roles in mediating Wnt5a-dependent adhesion and migration in hDPCs, and the Wnt5a/JNK pathway acts both dependently and independently of the RhoA pathway.

Self-renewal and multilineage differentiation of mouse dental epithelial stem cells

Understanding the cellular and molecular mechanisms underlying the self-renewal and differentiation of dental epithelial stem cells (DESCs) that support the unlimited growth potential of mouse incisors is critical for developing novel tooth regenerative therapies and unraveling the pathogenesis of odontogenic tumors. However, analysis of DESC properties and regulation has been limited by the lack of an in vitro assay system and well-documented DESC markers. Here, we describe an in vitro sphere culture system to isolate the DESCs from postnatal mouse incisor cervical loops (CLs) where the DESCs are thought to reside. The dissociated cells from CLs were able to expand and form spheres for multiple generations in the culture system. Lineage tracing indicated that DESC within the spheres were epithelial in origin as evident by lineage tracing. Upon stimulation, the sphere cells differentiated into cytokeratin 14- and amelogenin-expressing and mineral material-producing cells. Compared to the CL tissue, sphere cells expressed high levels of expression of Sca-1, CD49f (also designated as integrin α6), and CD44. Fluorescence-activated cell sorting (FACS) analyses of mouse incisor CL cells further showed that the CD49f(Bright) population was enriched in sphere-forming cells. In addition, the CD49f(Bright) population includes both slow-cycling and Lgr5(+) DESCs. The in vitro sphere culture system and identification of CD49f(Bright) as a DESC marker provide a novel platform for enriching DESCs, interrogating how maintenance, cell fate determination, and differentiation of DESCs are regulated, and developing tooth regenerative therapies.

Current concepts and occurrence of epithelial odontogenic tumors: I. Ameloblastoma and adenomatoid odontogenic tumor

Ameloblastomas and adenomatoid odontogenic tumors (AOTs) are common epithelial tumors of odontogenic origin. Ameloblastomas are clinico-pathologically classified into solid/multicystic, unicystic, desmoplastic, and peripheral types, and also divided into follicular, plexiform, acanthomatous, granular types, etc., based on their histological features. Craniopharyngiomas, derived from the remnants of Rathke's pouch or a misplaced enamel organ, are also comparable to the odontogenic tumors. The malignant transformation of ameloblastomas results in the formation of ameloblastic carcinomas and malignant ameloblastomas depending on cytological dysplasia and metastasis, respectively. AOTs are classified into follicular, extrafollicular, and peripheral types. Ameloblastomas are common, have an aggressive behavior and recurrent course, and are rarely metastatic, while AOTs are hamartomatous benign lesions derived from the complex system of the dental lamina or its remnants. With advances in the elucidation of molecular signaling mechanisms in cells, the cytodifferentiation of epithelial tumor cells in ameloblastomas and AOTs can be identified using different biomarkers. Therefore, it is suggested that comprehensive pathological observation including molecular genetic information can provide a more reliable differential diagnosis for the propagation and prognosis of ameloblastomas and AOTs. This study aimed to review the current concepts of ameloblastomas and AOTs and to discuss their clinico-pathological features relevant to tumorigenesis and prognosis.

Three-dimensional analysis of tooth dimensions in the MSX1-missense mutation

OBJECTIVES

A novel, 3D technique to measure the differences in tooth crown morphology between the MSX1 cases and non-affected controls was designed to get a better understanding of dental phenotype-genotype associations.

MATERIALS AND METHODS

Eight Dutch subjects from a single family with tooth agenesis, all with an established nonsense mutation c.332 C > A, p. Ser 111 Stop in exon 1 of MSX1, were compared with unaffected controls regarding several aspects of tooth crown morphology of incisor and molar teeth. A novel method of quantitative three-dimensional analysis was used to detect differences.

RESULTS

Statistically significant shape differences were observed for the maxillary incisor in the MSX1 family compared with the controls on the following parameters: surface area, buccolingual dimension, squareness, and crown volume (P ≤ 0.002). Molar crown shape was unaffected.

CONCLUSIONS

A better understanding of dental phenotype-genotype associations may contribute to earlier diagnosis of some multiple-anomaly congenital syndromes involving dental anomalies.

CLINICAL RELEVANCE

A "shape database" that includes associated gene mutations resulting from developmental syndromes may facilitate the genetic identification of hypodontia cases.

Tertiary dentinogenesis with calcium hydroxide: a review of proposed mechanisms

Calcium hydroxide has been used extensively in dentistry for a century. Despite its widespread use as a pulp-capping agent, its mechanisms of action still remain ambiguous. Understanding its modes of action will lead to a broader understanding of the mechanisms associated with induced dentinogenesis and help in optimizing the currently available agents to target specific regenerative processes to obtain the best possible clinical outcomes. A literature search relating to mechanisms of dentinogenesis of calcium hydroxide up to December 2011 was carried out using pubmed and MEDLINE database searches as well as manual searching of cross-references from identified studies. Resulting suggestions regarding dentinogenic mechanisms of calcium hydroxide range from direct irritating action of the material to induction of release of biologically active molecules. The purpose of this article is to discuss various mechanisms through which calcium hydroxide may induce tertiary dentinogenesis in the light of observations made in included studies.