Early epithelial signaling center governs tooth budding morphogenesis

Notum regulates the cusp and root patterns in mouse molar

Notum is a direct target of Wnt/β-catenin signaling and plays a crucial role as a Wnt inhibitor within a negative feedback loop. In the tooth, Notum is known to be expressed in odontoblasts, and severe dentin defects and irregular tooth roots have been reported in Notum-deficient mice. However, the precise expression pattern of Notum in early tooth development, and the role of Notum in crown and root patterns remain elusive. In the present study, we identified a novel Notum expression in primary enamel knot (EK), secondary EKs, and dental papilla during tooth development. Notum-deficient mice exhibited enlarged secondary EKs, resulting in broader cusp tips, altered cusp patterns, and reduced concavity in crown outline. These alterations in crown outline led to a reduction in cervical tongue length, thereby inducing root fusion in Notum-deficient mice. Overall, these results suggest that the secondary EK size, regulated by the Wnt/Notum negative feedback loop, has a significant impact on the patterns of crown and root during tooth morphogenesis.

Proliferation-driven mechanical compression induces signalling centre formation during mammalian organ development

Localized sources of morphogens, called signalling centres, play a fundamental role in coordinating tissue growth and cell fate specification during organogenesis. However, how these signalling centres are established in tissues during embryonic development is still unclear. Here we show that the main signalling centre orchestrating development of rodent incisors, the enamel knot (EK), is specified by a cell proliferation-driven buildup in compressive stresses (mechanical pressure) in the tissue. Direct mechanical measurements indicate that the stresses generated by cell proliferation are resisted by the surrounding tissue, creating a circular pattern of mechanical anisotropy with a region of high compressive stress at its centre that becomes the EK. Pharmacological inhibition of proliferation reduces stresses and suppresses EK formation, and application of external pressure in proliferation-inhibited conditions rescues the formation of the EK. Mechanical information is relayed intracellularly through YAP protein localization, which is cytoplasmic in the region of compressive stress that establishes the EK and nuclear in the stretched anisotropic cells that resist the pressure buildup around the EK. Together, our data identify a new role for proliferation-driven mechanical compression in the specification of a model signalling centre during mammalian organ development.

Mobility gene expression differences among wild-type, Mmp20 null and Mmp20 over-expresser mice plus visualization of 3D mouse ameloblast directional movement

Enamel forming ameloblasts move away from the dentino-enamel junction and also move relative to each other to establish enamel shape during the secretory stage of enamel development. Matrix metalloproteinase-20 (MMP20) is a tooth specific proteinase essential for proper enamel formation. We previously reported that MMP20 cleaves cadherins and may regulate ameloblast movement. Here, we used an Amelx promoter driven tdTomato reporter to label mouse ameloblasts. With these transgenic mice, we assessed ameloblast mobility group dynamics and gene expression. Three-dimensional imaging of mouse ameloblasts were observed in hemi-mandibles by using a tissue clearing technique. The three-dimensional ameloblast layer in Tg(Amelx-Mmp20) mice that overexpress MMP20 was uneven and the ameloblasts migrated away from this layer. Mouse ameloblast movement toward incisal tips was monitored by ex vivo time-lapse imaging. Gene expression related to cell migration and adhesion was analyzed in ameloblasts from wild-type mice, Mmp20-/- mice with no functional MMP20 and from Tg(Amelx-Mmp20) overexpressing mice. Gene expression was altered in Mmp20-/- and Tg(Amelx-Mmp20) mice compared to wild type. Among the genes assessed, those encoding laminins and a gap junction protein were upregulated in Mmp20-/- mice. New techniques and findings described in this study may lead to an improved understanding of ameloblast movement during enamel formation.

Using Ex Vivo Live Imaging to Investigate Cell Divisions and Movements During Mouse Dental Renewal

The continuously growing mouse incisor is emerging as a highly tractable model system to investigate the regulation of adult epithelial and mesenchymal stem cells and tooth regeneration. These progenitor populations actively divide, move, and differentiate to maintain tissue homeostasis and regenerate lost cells in a responsive manner. However, traditional analyses using fixed tissue sections could not capture the dynamic processes of cellular movements and interactions, limiting our ability to study their regulations. This paper describes a protocol to maintain whole mouse incisors in an explant culture system and live-track dental epithelial cells using multiphoton timelapse microscopy. This technique adds to our existing toolbox for dental research and allows investigators to acquire spatiotemporal information on cell behaviors and organizations in a living tissue. We anticipate that this methodology will help researchers further explore mechanisms that control the dynamic cellular processes taking place during both dental renewal and regeneration.

Single-cell census of human tooth development enables generation of human enamel

Tooth enamel secreted by ameloblasts (AMs) is the hardest material in the human body, acting as a shield to protect the teeth. However, the enamel is gradually damaged or partially lost in over 90% of adults and cannot be regenerated due to a lack of ameloblasts in erupted teeth. Here, we use single-cell combinatorial indexing RNA sequencing (sci-RNA-seq) to establish a spatiotemporal single-cell census for the developing human tooth and identify regulatory mechanisms controlling the differentiation process of human ameloblasts. We identify key signaling pathways involved between the support cells and ameloblasts during fetal development and recapitulate those findings in human ameloblast in vitro differentiation from induced pluripotent stem cells (iPSCs). We furthermore develop a disease model of amelogenesis imperfecta in a three-dimensional (3D) organoid system and show AM maturation to mineralized structure in vivo. These studies pave the way for future regenerative dentistry.

Count Me in, Count Me out: Regulation of the Tooth Number via Three Directional Developmental Patterns

Tooth number anomalies, including hyperdontia and hypodontia, are common congenital dental problems in the dental clinic. The precise number of teeth in a dentition is essential for proper speech, mastication, and aesthetics. Teeth are ectodermal organs that develop from the interaction of a thickened epithelium (dental placode) with the neural-crest-derived ectomesenchyme. There is extensive histological, molecular, and genetic evidence regarding how the tooth number is regulated in this serial process, but there is currently no universal classification for tooth number abnormalities. In this review, we propose a novel regulatory network for the tooth number based on the inherent dentition formation process. This network includes three intuitive directions: the development of a single tooth, the formation of a single dentition with elongation of the continual lamina, and tooth replacement with the development of the successional lamina. This article summarizes recent reports on early tooth development and provides an analytical framework to classify future relevant experiments.

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.

Eda controls the size of the enamel knot during incisor development

Ectodysplasin (Eda) plays important roles in both shaping the developing tooth and establishing the number of teeth within the tooth row. Sonic hedgehog (Shh) has been shown to act downstream of Eda and is involved in the initiation of tooth development. Eda-/- mice possess hypoplastic and hypomineralized incisors and show changes in tooth number in the molar region. In the present study we used 3D reconstruction combined with expression analysis, cell lineage tracing experiments, and western blot analysis in order to investigate the formation of the incisor germs in Eda-/- mice. We show that a lack of functional Eda protein during early stages of incisor tooth germ development had minimal impact on development of the early expression of Shh in the incisor, a region proposed to mark formation of a rudimental incisor placode and act as an initiating signalling centre. In contrast, deficiency of Eda protein had a later impact on expression of Shh in the primary enamel knot of the functional tooth. Eda-/- mice had a smaller region where Shh was expressed, and a reduced contribution from Shh descendant cells. The reduction in the enamel knot led to the formation of an abnormal enamel organ creating a hypoplastic functional incisor. Eda therefore appears to influence the spatial formation of the successional signalling centres during odontogenesis.

Tooth number abnormality: from bench to bedside

Tooth number abnormality is one of the most common dental developmental diseases, which includes both tooth agenesis and supernumerary teeth. Tooth development is regulated by numerous developmental signals, such as the well-known Wnt, BMP, FGF, Shh and Eda pathways, which mediate the ongoing complex interactions between epithelium and mesenchyme. Abnormal expression of these crutial signalling during this process may eventually lead to the development of anomalies in tooth number; however, the underlying mechanisms remain elusive. In this review, we summarized the major process of tooth development, the latest progress of mechanism studies and newly reported clinical investigations of tooth number abnormality. In addition, potential treatment approaches for tooth number abnormality based on developmental biology are also discussed. This review not only provides a reference for the diagnosis and treatment of tooth number abnormality in clinical practice but also facilitates the translation of basic research to the clinical application.

Dental niche cells directly contribute to tooth reconstitution and morphogenesis

Mammalian teeth develop from the inductive epithelial-mesenchymal interaction, an important mechanism shared by many organs. The cellular basis for such interaction remains elusive. Here, we generate a dual-fluorescence model to track and analyze dental cells from embryonic to postnatal stages, in which Pitx2⁺ epithelium and Msx1⁺ mesenchyme are sufficient for tooth reconstitution. Single-cell RNA sequencing and spatial mapping further revealed critical cellular dynamics during molar development, where tooth germs are organized by Msx1⁺Sdc1⁺ dental papilla and surrounding dental niche. Surprisingly, niche cells are more efficient in tooth reconstitution and can directly regenerate papilla cells through interaction with dental epithelium. Finally, from the dental niche, we identify a group of previously unappreciated migratory Msx1⁺ Sox9⁺ cells as the potential cell origin for dental papilla. Our results indicate that the dental niche cells directly contribute to tooth organogenesis and provide critical insights into the essential cell composition for tooth engineering.

Cellular mechanisms of reverse epithelial curvature in tissue morphogenesis

Epithelial bending plays an essential role during the multiple stages of organogenesis and can be classified into two types: invagination and evagination. The early stages of invaginating and evaginating organs are often depicted as simple concave and convex curves respectively, but in fact majority of the epithelial organs develop through a more complex pattern of curvature: concave flanked by convex and vice versa respectively. At the cellular level, this is far from a geometrical truism: locally cells must passively adapt to, or actively create such an epithelial structure that is typically composed of opposite and connected folds that form at least one s-shaped curve that we here, based on its appearance, term as "reverse curves." In recent years, invagination and evagination have been studied in increasing cellular detail. A diversity of mechanisms, including apical/basal constriction, vertical telescoping and extrinsic factors, all orchestrate epithelial bending to give different organs their final shape. However, how cells behave collectively to generate reverse curves remains less well-known. Here we review experimental models that characteristically form reverse curves during organogenesis. These include the circumvallate papillae in the tongue, crypt-villus structure in the intestine, and early tooth germ and describe how, in each case, reverse curves form to connect an invaginated or evaginated placode or opposite epithelial folds. Furthermore, by referring to the multicellular system that occur in the invagination and evagination, we attempt to provide a summary of mechanisms thought to be involved in reverse curvature consisting of apical/basal constriction, and extrinsic factors. Finally, we describe the emerging techniques in the current investigations, such as organoid culture, computational modelling and live imaging technologies that have been utilized to improve our understanding of the cellular mechanisms in early tissue morphogenesis.

Understanding the development of oral epithelial organs through single cell transcriptomic analysis

During craniofacial development, the oral epithelium begins as a morphologically homogeneous tissue that gives rise to locally complex structures, including the teeth, salivary glands and taste buds. How the epithelium is initially patterned and specified to generate diverse cell types remains largely unknown. To elucidate the genetic programs that direct the formation of distinct oral epithelial populations, we mapped the transcriptional landscape of embryonic day 12 mouse mandibular epithelia at single cell resolution. Our analysis identified key transcription factors and gene regulatory networks that define different epithelial cell types. By examining the spatiotemporal patterning process along the oral-aboral axis, our results propose a model in which the dental field is progressively confined to its position by the formation of the aboral epithelium anteriorly and the non-dental oral epithelium posteriorly. Using our data, we also identified Ntrk2 as a proliferation driver in the forming incisor, contributing to its invagination. Together, our results provide a detailed transcriptional atlas of the embryonic mandibular epithelium, and unveil new genetic markers and regulators that are present during the specification of various oral epithelial structures.

What sharks and mammals share

The tooth shape of sharks and mice are regulated by a similar signaling center despite their teeth having very different geometries.

An epithelial signalling centre in sharks supports homology of tooth morphogenesis in vertebrates

Development of tooth shape is regulated by the enamel knot signalling centre, at least in mammals. Fgf signalling regulates differential proliferation between the enamel knot and adjacent dental epithelia during tooth development, leading to formation of the dental cusp. The presence of an enamel knot in non-mammalian vertebrates is debated given differences in signalling. Here, we show the conservation and restriction of fgf3, fgf10, and shh to the sites of future dental cusps in the shark (Scyliorhinus canicula), whilst also highlighting striking differences between the shark and mouse. We reveal shifts in tooth size, shape, and cusp number following small molecule perturbations of canonical Wnt signalling. Resulting tooth phenotypes mirror observed effects in mammals, where canonical Wnt has been implicated as an upstream regulator of enamel knot signalling. In silico modelling of shark dental morphogenesis demonstrates how subtle changes in activatory and inhibitory signals can alter tooth shape, resembling developmental phenotypes and cusp shapes observed following experimental Wnt perturbation. Our results support the functional conservation of an enamel knot-like signalling centre throughout vertebrates and suggest that varied tooth types from sharks to mammals follow a similar developmental bauplan. Lineage-specific differences in signalling are not sufficient in refuting homology of this signalling centre, which is likely older than teeth themselves.

Parallels in signaling between development and regeneration in ectodermal organs

Ectodermal organs originate from the outermost germ layer of the developing embryo and include the skin, hair, tooth, nails, and exocrine glands. These organs develop through tightly regulated, sequential and reciprocal epithelial-mesenchymal crosstalk, and they eventually assume various morphologies and functions while retaining the ability to regenerate. As with many other tissues in the body, the development and morphogenesis of these organs are regulated by a set of common signaling pathways, such as Shh, Wnt, Bmp, Notch, Tgf-β, and Eda. However, subtle differences in the temporal activation, the multiple possible combinations of ligand-receptor activation, the various cofactors, as well as the underlying epigenetic modulation determine how each organ develops into its adult form. Although each organ has been studied separately in considerable detail, the mechanisms underlying the parallels and differences in signaling that regulate their development have rarely been investigated. First, we will use the tooth, the hair follicle, and the mammary gland as representative ectodermal organs to explore how the development of signaling centers and establishment of stem cell populations influence overall growth and morphogenesis. Then we will compare how some of the major signaling pathways (Shh, Wnt, Notch and Yap/Taz) differentially regulate developmental events. Finally, we will discuss how signaling regulates regenerative processes in all three.

Intertwined Signaling Pathways Governing Tooth Development: A Give-and-Take Between Canonical Wnt and Shh

Teeth play essential roles in life. Their development relies on reciprocal interactions between the ectoderm-derived dental epithelium and the underlying neural crest-originated mesenchyme. This odontogenic process serves as a prototype model for the development of ectodermal appendages. In the mouse, developing teeth go through distinct morphological phases that are tightly controlled by epithelial signaling centers. Crucial molecular regulators of odontogenesis include the evolutionarily conserved Wnt, BMP, FGF and sonic hedgehog (Shh) pathways. These signaling modules do not act on their own, but are closely intertwined during tooth development, thereby outlining the path to be taken by specific cell populations including the resident dental stem cells. Recently, pivotal Wnt-Shh interaction and feedback loops have been uncovered during odontogenesis, showing conservation in other developing ectodermal appendages. This review provides an integrated overview of the interplay between canonical Wnt and Shh throughout mouse tooth formation stages, extending from the initiation of dental placode to the fully formed adult tooth.

Programmed Cell Death Not as Sledgehammer but as Chisel: Apoptosis in Normal and Abnormal Craniofacial Patterning and Development

Coordination of craniofacial development involves an complex, intricate, genetically controlled and tightly regulated spatiotemporal series of reciprocal inductive and responsive interactions among the embryonic cephalic epithelia (both endodermal and ectodermal) and the cephalic mesenchyme — particularly the cranial neural crest (CNC). The coordinated regulation of these interactions is critical both ontogenetically and evolutionarily, and the clinical importance and mechanistic sensitivity to perturbation of this developmental system is reflected by the fact that one-third of all human congenital malformations affect the head and face. Here, we focus on one element of this elaborate process, apoptotic cell death, and its role in normal and abnormal craniofacial development. We highlight four themes in the temporospatial elaboration of craniofacial apoptosis during development, namely its occurrence at (1) positions of epithelial-epithelial apposition, (2) within intra-epithelial morphogenesis, (3) during epithelial compartmentalization, and (4) with CNC metameric organization. Using the genetic perturbation of Satb2, Pbx1/2, Fgf8, and Foxg1 as exemplars, we examine the role of apoptosis in the elaboration of jaw modules, the evolution and elaboration of the lambdoidal junction, the developmental integration at the mandibular arch hinge, and the control of upper jaw identity, patterning and development. Lastly, we posit that apoptosis uniquely acts during craniofacial development to control patterning cues emanating from core organizing centres.

Unexpected variation of human molar size patterns

A tenet of mammalian, including primate dental evolution, is the Inhibitory Cascade Model, where first molar (M1) size predicts in a linear cline the size and onset time of the second (M2) and third (M3) molars: a larger M1 portends a progressively smaller and later-developing M2 and M3. In contemporary modern Homo sapiens, later-developing M3s are less likely to erupt properly. The Inhibitory Cascade Model is also used to predict molar sizes of extinct taxa, including fossil Homo. The extent to which Inhibitory Cascade Model predictions hold in contemporary H. sapiens molars is unclear, including whether this tenet informs about molar initiation, development, and eruption. We tested these questions here. In our radiographic sample of 323 oral quadrants and molar rows from contemporary humans based on mesiodistal crown lengths, we observed the distribution of molar proportions with a central tendency around parity (M1 = M2 = M3) that parsed into 13 distinct molar size ratio patterns. These patterns presented at different frequencies (e.g., M1 > M2 > M3 in about one-third of cases) that reflected whether the molar row was located in the maxilla or mandible and included both linear (e.g., M1 < M2 < M3) and nonlinear molar size ratio progressions (e.g., M1 > M2 < M3). Up to four patterns were found in the same subject's mouth. Lastly, M1 size alone does not predict M3 size, developmental timing, or eruption; rather, M2 size is integral to predicting M3 size. Our study indicates that human molar size is genetically 'softwired' and sensitive to factors local to the human upper jaw vs. lower jaw. The lack of a single stereotypical molar size ratio for contemporary H. sapiens suggests that predictions of fossil H. sapiens molar sizes using the Inhibitory Cascade Model must be made with caution.

Comparative transcriptional profiling of canine acanthomatous ameloblastoma and homology with human ameloblastoma

Ameloblastomas are odontogenic tumors that are rare in people but have a relatively high prevalence in dogs. Because canine acanthomatous ameloblastomas (CAA) have clinicopathologic and molecular features in common with human ameloblastomas (AM), spontaneous CAA can serve as a useful translational model of disease. However, the molecular basis of CAA and how it compares to AM are incompletely understood. In this study, we compared the global genomic expression profile of CAA with AM and evaluated its dental origin by using a bulk RNA-seq approach. For these studies, healthy gingiva and canine oral squamous cell carcinoma served as controls. We found that aberrant RAS signaling, and activation of the epithelial-to-mesenchymal transition cellular program are involved in the pathogenesis of CAA, and that CAA is enriched with genes known to be upregulated in AM including those expressed during the early stages of tooth development, suggesting a high level of molecular homology. These results support the model that domestic dogs with spontaneous CAA have potential for pre-clinical assessment of targeted therapeutic modalities against AM.

Wnt Signalling in Regenerative Dentistry

Teeth are complex structures where a soft dental pulp tissue is enriched with nerves, vasculature and connective tissue and encased by the cushioning effect of dentin and the protection of a hard enamel in the crown and cementum in the root. Injuries such as trauma or caries can jeopardise these layers of protection and result in pulp exposure, inflammation and infection. Provision of most suitable materials for tooth repair upon injury has been the motivation of dentistry for many decades. Wnt signalling, an evolutionarily conserved pathway, plays key roles during pre- and post-natal development of many organs including the tooth. Mutations in the components of this pathway gives rise to various types of developmental tooth anomalies. Wnt signalling is also fundamental in the response of odontoblasts to injury and repair processes. The complexity of tooth structure has resulted in diverse studies looking at specific compartments or cell types of this organ. This review looks at the current advances in the field of tooth development and regeneration. The objective of the present review is to provide an updated vision on dental biomaterials research, focusing on their biological properties and interactions to act as evidence for their potential use in vital pulp treatment procedures. We discuss the outstanding questions and future directions to make this knowledge more translatable to the clinics.

R-Spondin 3 Regulates Mammalian Dental and Craniofacial Development

Development of the teeth requires complex signaling interactions between the mesenchyme and the epithelium mediated by multiple pathways. For example, canonical WNT signaling is essential to many aspects of odontogenesis, and inhibiting this pathway blocks tooth development at an early stage. R-spondins (RSPOs) are secreted proteins, and they mostly augment WNT signaling. Although RSPOs have been shown to play important roles in the development of many organs, their role in tooth development is unclear. A previous study reported that mutating Rspo2 in mice led to supernumerary lower molars, while teeth forming at the normal positions showed no significant anomalies. Because multiple Rspo genes are expressed in the orofacial region, it is possible that the relatively mild phenotype of Rspo2 mutants is due to functional compensation by other RSPO proteins. We found that inactivating Rspo3 in the craniofacial mesenchyme caused the loss of lower incisors, which did not progress beyond the bud stage. A simultaneous deletion of Rspo2 and Rspo3 caused severe disruption of craniofacial development from early stages, which was accompanied with impaired development of all teeth. Together, these results indicate that Rspo3 is an important regulator of mammalian dental and craniofacial development.

Cell influx and contractile actomyosin force drive mammary bud growth and invagination

The mammary gland develops from the surface ectoderm during embryogenesis and proceeds through morphological phases defined as placode, hillock, bud, and bulb stages followed by branching morphogenesis. During this early morphogenesis, the mammary bud undergoes an invagination process where the thickened bud initially protrudes above the surface epithelium and then transforms to a bulb and sinks into the underlying mesenchyme. The signaling pathways regulating the early morphogenetic steps have been identified to some extent, but the underlying cellular mechanisms remain ill defined. Here, we use 3D and 4D confocal microscopy to show that the early growth of the mammary rudiment is accomplished by migration-driven cell influx, with minor contributions of cell hypertrophy and proliferation. We delineate a hitherto undescribed invagination mechanism driven by thin, elongated keratinocytes-ring cells-that form a contractile rim around the mammary bud and likely exert force via the actomyosin network. Furthermore, we show that conditional deletion of nonmuscle myosin IIA (NMIIA) impairs invagination, resulting in abnormal mammary bud shape.

The initiation knot is a signaling center required for molar tooth development

Signaling centers, or organizers, regulate many aspects of embryonic morphogenesis. In the mammalian molar tooth, reiterative signaling in specialized centers called enamel knots (EKs) determines tooth patterning. Preceding the primary EK, transient epithelial thickening appears, the significance of which remains debated. Using tissue confocal fluorescence imaging with laser ablation experiments, we show that this transient thickening is an earlier signaling center, the molar initiation knot (IK), that is required for the progression of tooth development. IK cell dynamics demonstrate the hallmarks of a signaling center: cell cycle exit, condensation and eventual silencing through apoptosis. IK initiation and maturation are defined by the juxtaposition of cells with high Wnt activity to Shh-expressing non-proliferating cells, the combination of which drives the growth of the tooth bud, leading to the formation of the primary EK as an independent cell cluster. Overall, the whole development of the tooth, from initiation to patterning, is driven by the iterative use of signaling centers.

Summary: During tooth morphogenesis, transient thickening of the epithelium in the diastema anterior to the first developing molar is an early signaling center, the molar initiation knot (IK), which is required for the progression of mammalian molar tooth development.

Tooth agenesis: What do we know and is there a connection to cancer?

Like all developmental processes, odontogenesis is highly complex and dynamically regulated, with hundreds of genes co-expressed in reciprocal networks. Tooth agenesis (missing one or more/all teeth) is a common human craniofacial anomaly and may be caused by genetic variations and/or environmental factors. Variants in PAX9, MSX1, AXIN2, EDA, EDAR, and WNT10A genes are associated with tooth agenesis. Currently, variants in ATF1, DUSP10, CASC8, IRF6, KDF1, GREM2, LTBP3, and components and regulators of WNT signaling WNT10B, LRP6, DKK, and KREMEN1 are at the forefront of interest. Due to the interconnectedness of the signaling pathways of carcinogenesis and odontogenesis, tooth agenesis could be a suitable marker for early detection of cancer predisposition. Variants in genes associated with tooth agenesis could serve as prognostic or therapeutic targets in cancer. This review aims to summarize existing knowledge of development and clinical genetics of teeth. Concurrently, the review proposes possible approaches for future research in this area, with particular attention to roles in monitoring, early diagnosis and therapy of tumors associated with defective tooth development.

Exploiting teeth as a model to study basic features of signaling pathways

Teeth constitute a classical model for the study of signaling pathways and their roles in mediating interactions between cells and tissues in organ development, homeostasis and regeneration. Rodent teeth are mostly used as experimental models. Rodent molars have proved fundamental in the study of epithelial–mesenchymal interactions and embryonic organ morphogenesis, as well as to faithfully model human diseases affecting dental tissues. The continuously growing rodent incisor is an excellent tool for the investigation of the mechanisms regulating stem cells dynamics in homeostasis and regeneration. In this review, we discuss the use of teeth as a model to investigate signaling pathways, providing an overview of the many unique experimental approaches offered by this organ. We discuss how complex networks of signaling pathways modulate the various aspects of tooth biology, and the models used to obtain this knowledge. Finally, we introduce new experimental approaches that allow the study of more complex interactions, such as the crosstalk between dental tissues, innervation and vascularization.

m6A RNA methylation impacts fate choices during skin morphogenesis

N6-methyladenosine is the most prominent RNA modification in mammals. Here, we study mouse skin embryogenesis to tackle m6A's functions and physiological importance. We first landscape the m6A modifications on skin epithelial progenitor mRNAs. Contrasting with in vivo ribosomal profiling, we unearth a correlation between m6A modification in coding sequences and enhanced translation, particularly of key morphogenetic signaling pathways. Tapping physiological relevance, we show that m6A loss profoundly alters these cues and perturbs cellular fate choices and tissue architecture in all skin lineages. By single-cell transcriptomics and bioinformatics, both signaling and canonical translation pathways show significant downregulation after m6A loss. Interestingly, however, many highly m6A-modified mRNAs are markedly upregulated upon m6A loss, and they encode RNA-methylation, RNA-processing and RNA-metabolism factors. Together, our findings suggest that m6A functions to enhance translation of key morphogenetic regulators, while also destabilizing sentinel mRNAs that are primed to activate rescue pathways when m6A levels drop.

Live Tissue Imaging Sheds Light on Cell Level Events During Ectodermal Organ Development

Embryonic development of ectodermal organs involves a very dynamic range of cellular events and, therefore, requires advanced techniques to visualize them. Ectodermal organogenesis proceeds in well-defined sequential stages mediated by tissue interactions. Different ectodermal organs feature shared morphological characteristics, which are regulated by conserved and reiterative signaling pathways. A wealth of genetic information on the expression patterns and interactions of specific signaling pathways has accumulated over the years. However, the conventional developmental biology methods have mainly relied on two-dimensional tissue histological analyses at fixed time points limiting the possibilities to follow the processes in real time on a single cell resolution. This has complicated the interpretation of cause and effect relationships and mechanisms of the successive events. Whole-mount tissue live imaging approaches are now revealing how reshaping of the epithelial sheet for the initial placodal thickening, budding morphogenesis and beyond, involve coordinated four dimensional changes in cell shapes, well-orchestrated cell movements and specific cell proliferation and apoptosis patterns. It is becoming evident that the interpretation of the reiterative morphogenic signals takes place dynamically at the cellular level. Depending on the context, location, and timing they drive different cell fate choices and cellular interactions regulating a pattern of behaviors that ultimately defines organ shapes and sizes. Here we review how new tissue models, advances in 3D and live tissue imaging techniques have brought new understanding on the cell level behaviors that contribute to the highly dynamic stages of morphogenesis in teeth, hair and related ectodermal organs during development, and in dysplasia contexts.

Pitx2-Sox2-Lef1 interactions specify progenitor oral/dental epithelial cell signaling centers

Epithelial signaling centers control epithelial invagination and organ development, but how these centers are specified remains unclear. We report that Pitx2 (the first transcriptional marker for tooth development) controls the embryonic formation and patterning of epithelial signaling centers during incisor development. We demonstrate using Krt14Cre /Pitx2flox/flox (Pitx2cKO ) and Rosa26CreERT/Pitx2flox/flox mice that loss of Pitx2 delays epithelial invagination, and decreases progenitor cell proliferation and dental epithelium cell differentiation. Developmentally, Pitx2 regulates formation of the Sox2+ labial cervical loop (LaCL) stem cell niche in concert with two signaling centers: the initiation knot and enamel knot. The loss of Pitx2 disrupted the patterning of these two signaling centers, resulting in tooth arrest at E14.5. Mechanistically, Pitx2 transcriptional activity and DNA binding is inhibited by Sox2, and this interaction controls gene expression in specific Sox2 and Pitx2 co-expression progenitor cell domains. We demonstrate new transcriptional mechanisms regulating signaling centers by Pitx2, Sox2, Lef1 and Irx1.

Six1 is required for signaling center formation and labial-lingual asymmetry in developing lower incisors

BACKGROUND

The structure of the mouse incisor is characterized by its asymmetric accumulation of enamel matrix proteins on the labial side. The asymmetric structure originates from the patterning of the epithelial incisor placode through the interaction with dental mesenchymal cells. However, the molecular basis for the asymmetric patterning of the incisor germ is largely unknown.

RESULTS

A homeobox transcription factor SIX1 was shown to be produced in the mandibular mesenchyme, and its localization patterns changed dynamically during lower incisor development. Six1^-/- mice exhibited smaller lower incisor primordia than wild-type mice. Furthermore, Six1^-/- mice showed enamel matrix production on both the lingual and labial sides and disturbed odontoblast maturation. In the earlier stages of development, the formation of signaling centers, the initiation knot and the enamel knot, which are essential for the morphogenesis of tooth germs, were impaired in Six1^-/- embryos. Notably, Wnt signaling activity, which shows an anterior-posterior gradient, and the expression patterns of genes involved in incisor formation were altered in the mesenchyme in Six1^-/- embryos.

CONCLUSION

Our results indicate that Six1 is required for signaling center formation in lower incisor germs and the labial-lingual asymmetry of the lower incisors by regulating the anterior-posterior patterning of the mandibular mesenchyme.

Molecular and cellular mechanisms of tooth development, homeostasis and repair

The tooth provides an excellent system for deciphering the molecular mechanisms of organogenesis, and has thus been of longstanding interest to developmental and stem cell biologists studying embryonic morphogenesis and adult tissue renewal. In recent years, analyses of molecular signaling networks, together with new insights into cellular heterogeneity, have greatly improved our knowledge of the dynamic epithelial-mesenchymal interactions that take place during tooth development and homeostasis. Here, we review recent progress in the field of mammalian tooth morphogenesis and also discuss the mechanisms regulating stem cell-based dental tissue homeostasis, regeneration and repair. These exciting findings help to lay a foundation that will ultimately enable the application of fundamental research discoveries toward therapies to improve oral health.

Modeling Edar expression reveals the hidden dynamics of tooth signaling center patterning

When patterns are set during embryogenesis, it is expected that they are straightly established rather than subsequently modified. The patterning of the three mouse molars is, however, far from straight, likely as a result of mouse evolutionary history. The first-formed tooth signaling centers, called MS and R2, disappear before driving tooth formation and are thought to be vestiges of the premolars found in mouse ancestors. Moreover, the mature signaling center of the first molar (M1) is formed from the fusion of two signaling centers (R2 and early M1). Here, we report that broad activation of Edar expression precedes its spatial restriction to tooth signaling centers. This reveals a hidden two-step patterning process for tooth signaling centers, which was modeled with a single activator–inhibitor pair subject to reaction–diffusion (RD). The study of Edar expression also unveiled successive phases of signaling center formation, erasing, recovering, and fusion. Our model, in which R2 signaling center is not intrinsically defective but erased by the broad activation preceding M1 signaling center formation, predicted the surprising rescue of R2 in Edar mutant mice, where activation is reduced. The importance of this R2–M1 interaction was confirmed by ex vivo cultures showing that R2 is capable of forming a tooth. Finally, by introducing chemotaxis as a secondary process to RD, we recapitulated in silico different conditions in which R2 and M1 centers fuse or not. In conclusion, pattern formation in the mouse molar field relies on basic mechanisms whose dynamics produce embryonic patterns that are plastic objects rather than fixed end points.

The generation of patterns during development is generally viewed as a direct process, but this study of mouse tooth development shows how embryonic patterns can be constructed and erased in a very dynamic manner before the correct pattern is achieved.

Organs, such as teeth, that form regular patterns are of particular interest to developmental biologists. These patterns are established early in the embryo, and it has generally been thought the organs appear in what is their final position. Recent studies that focus on the dynamics of patterning events challenge this view, suggesting that pattern formation can be more complex than previously thought. For example, mouse molars form from “organizing centers,” which appear, disappear, or fuse in a complex sequence of events, until the final pattern is stabilized. Based on the dynamics of expression of the Edar gene, we built a mathematical model of how tooth “organizing centers” form. We reveal that a newly formed organizing center can actively impair or erase a previously formed one. We call this phenomenon a developmental “palimpsest,” from the terminology of old manuscripts that were scraped to be reused again. This indirect developmental process likely reflects the evolutionary history of mice, which lost premolars while maintaining their embryonic organizing centers. More broadly, we believe that overwriting or correcting previously established patterns during development might be more common than anticipated, simply due to the fact that developmental programs are modified by incrementation during evolution.

Temporal Control of WNT Activity Regulates Tooth Number in Fish

Wnts determine cell polarity, cell proliferation, and cell differentiation during embryogenesis and play an essential role during tooth development initiation and morphogenesis. Wnt/β-catenin signaling has a time-dependent role in development because various signaling molecules that mutually interact are involved in the pathway, and tight regulation of the pathway is essential for normal development. Studies investigating how the Wnt/β-catenin signaling pathway controls the different stages of tooth development are rare. Specifically, the effects of Wnt/β-catenin signaling loss of function on different stages of tooth development are currently unknown. Here, we report the stage-dependent role of Wnt/β-catenin signaling in tooth development. In vivo loss and gain of function of Wnt/β-catenin signaling were implemented through the genetic overexpression of DKK1 with heat shock-inducible transgenic models and the pharmacologic inhibition of β-catenin destruction complex formation in zebrafish, respectively. We demonstrated that transient inhibition of Wnt/β-catenin signaling interrupted tooth development in a stage-dependent manner and conditional activation of Wnt/β-catenin signaling during 4V morphogenesis inhibited the development of 3V. These findings suggest that Wnt/β-catenin signaling plays an important role in the morphogenesis of teeth and the initiation of sequential tooth development in a stage-dependent manner.

Tissue Mechanical Forces and Evolutionary Developmental Changes Act Through Space and Time to Shape Tooth Morphology and Function

Efforts from diverse disciplines, including evolutionary studies and biomechanical experiments, have yielded new insights into the genetic, signaling, and mechanical control of tooth formation and functions. Evidence from fossils and non-model organisms has revealed that a common set of genes underlie tooth-forming potential of epithelia, and changes in signaling environments subsequently result in specialized dentitions, maintenance of dental stem cells, and other phenotypic adaptations. In addition to chemical signaling, tissue forces generated through epithelial contraction, differential growth, and skeletal constraints act in parallel to shape the tooth throughout development. Here recent advances in understanding dental development from these studies are reviewed and important gaps that can be filled through continued application of evolutionary and biomechanical approaches are discussed.

Genetic Evidence Supporting the Role of the Calcium Channel, CACNA1S, in Tooth Cusp and Root Patterning

In this study, we report a unique dominantly inherited disorganized supernumerary cusp and single root phenotype presented by 11 affected individuals belonging to 5 north-eastern Thai families. Using whole exome sequencing (WES) we identified a common single missense mutation that segregates with the phenotype in exon 6 of CACNA1S (Ca_v1.1) (NM_000069.2: c.[865A > G];[=] p.[Ile289Val];[=]), the Calcium Channel, Voltage-Dependent, L Type, Alpha-1s Subunit, OMIM ^∗ 114208), affecting a highly conserved amino-acid isoleucine residue within the pore forming subdomain of CACNA1S protein. This is a strong genetic evidence that a voltage-dependent calcium ion channel is likely to play a role in influencing tooth morphogenesis and patterning.

Bioengineered Tooth Buds Exhibit Features of Natural Tooth Buds

Tooth loss is a significant health issue currently affecting millions of people worldwide. Artificial dental implants, the current gold standard tooth replacement therapy, do not exhibit many properties of natural teeth and can be associated with complications leading to implant failure. Here we propose bioengineered tooth buds as a superior alternative tooth replacement therapy. We describe improved methods to create highly cellularized bioengineered tooth bud constructs that formed hallmark features that resemble natural tooth buds such as the dental epithelial stem cell niche, enamel knot signaling centers, transient amplifying cells, and mineralized dental tissue formation. These constructs were composed of postnatal dental cells encapsulated within a hydrogel material that were implanted subcutaneously into immunocompromised rats. To our knowledge, this is the first report describing the use of postnatal dental cells to create bioengineered tooth buds that exhibit evidence of these features of natural tooth development. We propose future bioengineered tooth buds as a promising, clinically relevant tooth replacement therapy.

The Ectodysplasin receptor EDAR acts as a tumor suppressor in melanoma by conditionally inducing cell death

Ectodysplasin receptor EDAR is seen as a typical Tumor Necrosis Factor receptor (TNFR) family member known to interact with its ligand Eda-A1, and signaling mainly through the nuclear factor-kappaB (NF-κB) and c-jun N-terminal kinases pathways. Mutations in genes that encode proteins involved in EDAR transduction cascade cause anhidrotic ectodermal dysplasia. Here, we report an unexpected pro-apoptotic activity of EDAR when unbound to its ligand Eda-A1, which is independent of NF-κB pathway. Contrarily to other death receptors, EDAR does recruit caspase-8 to trigger apoptosis but solely upon ligand withdrawal, thereby behaving as the so-called dependence receptors. We propose that pro-apoptotic activity of unbound EDAR confers it a tumor suppressive activity. Along this line, we identified loss-of-pro-apoptotic function mutations in EDAR gene in human melanoma. Moreover, we show that the invalidation of EDAR in mice promotes melanoma progression in a B-Raf mutant background. Together, these data support the view that EDAR constrains melanoma progression by acting as a dependence receptor.

Cell cycle of the enamel knot during tooth morphogenesis

Enamel knot (EK) is known to be a central organ in tooth development, especially for cusp patterning. To trace the exact position and movement among the inner dental epithelium (IDE) and EK cells, and to monitor the relationship between the EK and cusp patterning, it is essential that we understand the cell cycle status of the EK in early stages of tooth development. In this study, thymidine analogous (IdU, BrdU) staining was used to evaluate the cell cycle phase of the primary EK at the early casp stage (E13.0) and the gerbil embryo (E19) in a developing mouse embryo. The centerpiece of this study was to describe the cell cycle phasing and sequencing during proliferation in the IDE according to the expression of IdU and BrdU following their injection at calculated time points. The interval time between IdU injection and BrdU injection was set at 4 h. As a result, the cell cycle in the IDE of the mouse and gerbil was found to be synchronous. Conversely, the cell cycle in primary EKs of mice was much longer than that of the IDE. Therefore, the difference of cell cycle of the IDE and the EK is related to the diversity of cusp patterning and would provide a new insight into tooth morphogenesis.

Basement Membranes in Development and Disease

The basement membrane is a thin but dense, sheet-like specialized type of extracellular matrix that has remarkably diverse functions tailored to individual tissues and organs. Tightly controlled spatial and temporal changes in its composition and structure contribute to the diversity of basement membrane functions. These different basement membranes undergo dynamic transformations throughout animal life, most notably during development. Numerous developmental mechanisms are regulated or mediated by basement membranes, often by a combination of molecular and mechanical processes. A particularly important process involves cell transmigration through a basement membrane because of its link to cell invasion in disease. While developmental and disease processes share some similarities, what clearly distinguishes the two is dysregulation of cells and extracellular matrices in disease. With its relevance to many developmental and disease processes, the basement membrane is a vitally important area of research that may provide novel insights into biological mechanisms and development of innovative therapeutic approaches. Here we present a review of developmental and disease dynamics of basement membranes in Caenorhabditis elegans, Drosophila, and vertebrates.

Mesenchymal Wnt/β-catenin signaling limits tooth number

Tooth agenesis is one of the predominant developmental anomalies in humans, usually affecting the permanent dentition generated by sequential tooth formation and, in most cases, caused by mutations perturbing epithelial Wnt/β-catenin signaling. In addition, loss-of-function mutations in the Wnt feedback inhibitor AXIN2 lead to human tooth agenesis. We have investigated the functions of Wnt/β-catenin signaling during sequential formation of molar teeth using mouse models. Continuous initiation of new teeth, which is observed after genetic activation of Wnt/β-catenin signaling in the oral epithelium, was accompanied by enhanced expression of Wnt antagonists and a downregulation of Wnt/β-catenin signaling in the dental mesenchyme. Genetic and pharmacological activation of mesenchymal Wnt/β-catenin signaling negatively regulated sequential tooth formation, an effect partly mediated by Bmp4. Runx2, a gene whose loss-of-function mutations result in sequential formation of supernumerary teeth in the human cleidocranial dysplasia syndrome, suppressed the expression of Wnt inhibitors Axin2 and Drapc1 in dental mesenchyme. Our data indicate that increased mesenchymal Wnt signaling inhibits the sequential formation of teeth, and suggest that Axin2/Runx2 antagonistic interactions modulate the level of mesenchymal Wnt/β-catenin signaling, underlying the contrasting dental phenotypes caused by human AXIN2 and RUNX2 mutations.

Cell volume changes contribute to epithelial morphogenesis in zebrafish Kupffer's vesicle

How epithelial cell behaviors are coordinately regulated to sculpt tissue architecture is a fundamental question in biology. Kupffer’s vesicle (KV), a transient organ with a fluid-filled lumen, provides a simple system to investigate the interplay between intrinsic cellular mechanisms and external forces during epithelial morphogenesis. Using 3-dimensional (3D) analyses of single cells we identify asymmetric cell volume changes along the anteroposterior axis of KV that coincide with asymmetric cell shape changes. Blocking ion flux prevents these cell volume changes and cell shape changes. Vertex simulations suggest cell shape changes do not depend on lumen expansion. Consistent with this prediction, asymmetric changes in KV cell volume and shape occur normally when KV lumen growth fails due to leaky cell adhesions. These results indicate ion flux mediates cell volume changes that contribute to asymmetric cell shape changes in KV, and that these changes in epithelial morphology are separable from lumen-generated forces.

Watching a deep dive: Live imaging provides lessons about tooth invagination

Invagination of epithelium into the surrounding mesenchyme is a critical step that marks the developmental onset of many ectodermal organs. In this issue, Ahtiainen et al. (2016. J. Cell. Biol. http://dx.doi.org/10.1083/jcb.201512074) use the mouse incisor as a model to advance our understanding of the cellular mechanisms underlying ectodermal organ morphogenesis.

Epithelial cell behaviours during neurosensory organ formation

Perception of the environment in vertebrates relies on a variety of neurosensory mini-organs. These organs develop via a multi-step process that includes placode induction, cell differentiation, patterning and innervation. Ultimately, cells derived from one or more different tissues assemble to form a specific mini-organ that exhibits a particular structure and function. The initial building blocks of these organs are epithelial cells that undergo rearrangements and interact with neighbouring tissues, such as neural crest-derived mesenchymal cells and sensory neurons, to construct a functional sensory organ. In recent years, advances in in vivo imaging methods have allowed direct observation of these epithelial cells, showing that they can be displaced within the epithelium itself via several modes. This Review focuses on the diversity of epithelial cell behaviours that are involved in the formation of small neurosensory organs, using the examples of dental placodes, hair follicles, taste buds, lung neuroendocrine cells and zebrafish lateral line neuromasts to highlight both well-established and newly described modes of epithelial cell motility.

The Biology Underlying Abnormalities of Tooth Number in Humans

In past decades, morphologic, molecular, and cellular mechanisms that govern tooth development have been extensively studied. These studies demonstrated that the same signaling pathways regulate development of the primary and successional teeth. Mutations of these pathways lead to abnormalities in tooth development and number, including aberrant tooth shape, tooth agenesis, and formation of extra teeth. Here, we summarize the current knowledge on the development of the primary and successional teeth in animal models and describe some of the common tooth abnormalities in humans.

Lineage tracing of epithelial cells in developing teeth reveals two strategies for building signaling centers

An important event in organogenesis is the formation of signaling centers, which are clusters of growth factor-secreting cells. In the case of tooth development, sequentially formed signaling centers known as the initiation knot (IK) and the enamel knot (EK) regulate morphogenesis. However, despite the importance of signaling centers, their origin, as well as the fate of the cells composing them, remain open questions. Here, using lineage tracing of distinct epithelial populations, we found that the EK of the mouse incisor is derived de novo from a group of SRY-box 2 (Sox2)-expressing cells in the posterior half of the tooth germ. Specifically, EK progenitors are located in the posterior ventral basal layer, as demonstrated by DiI labeling of cells. Lineage tracing the formed EK with Shh^CreER , which encodes an inducible Cre recombinase under the control of the Sonic hedgehog promoter, at subsequent developmental stages showed that, once formed, some EK cells in the incisor give rise to differentiated cells, whereas in the molar, EK cells give rise to the buccal secondary EK. This work thus establishes the developmental origin as well as the fate of the EK and reveals two strategies for the emergence of serially formed signaling centers: one through de novo establishment and the other by incorporation of progeny from previously formed signaling centers.

From snapshots to movies: Understanding early tooth development in four dimensions

The developing tooth offers a model for the study of ectodermal appendage organogenesis. The signaling networks that regulate tooth development have been intensively investigated, but how cell biological responses to signaling pathways regulate dental morphogenesis remains an open question. The increasing use of ex vivo imaging techniques has enabled live tracking of cell behaviors over time in high resolution. While recent studies using these techniques have improved our understanding of tooth morphogenesis, important gaps remain that require additional investigation. In addition, some discrepancies have arisen between recent studies, and resolving these will advance our knowledge of tooth development. Developmental Dynamics 246:442-450, 2016. © 2017 Wiley Periodicals, Inc.