Resolving stem and progenitor cells in the adult mouse incisor through gene co-expression analysis

Spatiotemporal cell landscape of human embryonic tooth development

Understanding the cellular composition and trajectory of human tooth development is valuable for dentistry and stem cell engineering research. Previous single-cell studies have focused on mature human teeth and developing mouse teeth, but the cell landscape of human embryonic dental development is still unknown. In this study, tooth germ tissues were collected from aborted foetus (17-24 weeks) for single-cell RNA sequence and spatial transcriptome analysis. The cells were classified into seven subclusters of epithelium, and seven clusters of mesenchyme, as well as other cell types such as Schwann cell precursor and pericyte. For epithelium, the stratum intermedium branch and the ameloblast branch diverged from the same set of outer enamel-inner enamel-ALCAM+ epithelial cell lineage, but their spatial distribution of two branches was not clearly distinct. This trajectory received spatially adjacent regulation signals from mesenchyme and pericyte, including JAG1 and APP. The differentiation of pulp cell and pre-odontoblast showed four waves of temporally distinct gene expression, which involved regulation networks of LHX9, DLX5 and SP7, and these genes were regulated by upstream ligands such as the BMP family. This provides a reference landscape for the research on early human tooth development, covering different spatial structures and developmental periods.

Computational identification of key transcription factors for embryonic and postnatal Sox2+ dental epithelial stem cell

While many reptiles can replace their tooth throughout life, human loss the tooth replacement capability after formation of the permanent teeth. It was thought that the difference in tooth regeneration capability depends on the persistence of a specialized dental epithelial structure, the dental lamina that contains dental epithelial stem cells (DESC). Currently, we know very little about DESC such as what genes are expressed or its chromatin accessibility profile. Multiple markers of DESC have been proposed such as Sox2 and Lgr5 . Few single cell RNA-seq experiments have been performed previously, but no obvious DESC cluster was identified in these scRNA-seq datasets, possible due to that the expression level of DESC markers such as Sox2 and Lgr5 is too low or the percentage of DESC is too low in whole tooth. We utilize a mouse line Sox2-GFP to enrich Sox2+ DESC and use Smart-Seq2 protocol and ATAC-seq protocol to generate transcriptome profile and chromatin accessibility profile of P2 Sox2+ DESC. Additionally, we generate transcriptome profile and chromatin accessibility profile of E11.5 Sox2+ dental lamina cells. With transcriptome profile and chromatin accessibility profile, we systematically identify potential key transcription factors for E11.5 Sox2+ cells and P2 Sox2+ cells. We identified transcription factors including Pitx2, Id3, Pitx1, Tbx1, Trp63, Nkx2-3, Grhl3, Dlx2, Runx1, Nfix, Zfp536 , etc potentially formed the core transcriptional regulatory networks of Sox2+ DESC in both embryonic and postnatal stages.

Adult dental epithelial stem cell-derived organoids deposit hydroxylapatite biomineral

Ameloblasts are specialized cells derived from the dental epithelium that produce enamel, a hierarchically structured tissue comprised of highly elongated hydroxylapatite (OHAp) crystallites. The unique function of the epithelial cells synthesizing crystallites and assembling them in a mechanically robust structure is not fully elucidated yet, partly due to limitations with in vitro experimental models. Herein, we demonstrate the ability to generate mineralizing dental epithelial organoids (DEOs) from adult dental epithelial stem cells (aDESCs) isolated from mouse incisor tissues. DEOs expressed ameloblast markers, could be maintained for more than five months (11 passages) in vitro in media containing modulators of Wnt, Egf, Bmp, Fgf and Notch signaling pathways, and were amenable to cryostorage. When transplanted underneath murine kidney capsules, organoids produced OHAp crystallites similar in composition, size, and shape to mineralized dental tissues, including some enamel-like elongated crystals. DEOs are thus a powerful in vitro model to study mineralization process by dental epithelium, which can pave the way to understanding amelogenesis and developing regenerative therapy of enamel.

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.

Craniofacial developmental biology in the single-cell era

The evolution of a unique craniofacial complex in vertebrates made possible new ways of breathing, eating, communicating and sensing the environment. The head and face develop through interactions of all three germ layers, the endoderm, ectoderm and mesoderm, as well as the so-called fourth germ layer, the cranial neural crest. Over a century of experimental embryology and genetics have revealed an incredible diversity of cell types derived from each germ layer, signaling pathways and genes that coordinate craniofacial development, and how changes to these underlie human disease and vertebrate evolution. Yet for many diseases and congenital anomalies, we have an incomplete picture of the causative genomic changes, in particular how alterations to the non-coding genome might affect craniofacial gene expression. Emerging genomics and single-cell technologies provide an opportunity to obtain a more holistic view of the genes and gene regulatory elements orchestrating craniofacial development across vertebrates. These single-cell studies generate novel hypotheses that can be experimentally validated in vivo. In this Review, we highlight recent advances in single-cell studies of diverse craniofacial structures, as well as potential pitfalls and the need for extensive in vivo validation. We discuss how these studies inform the developmental sources and regulation of head structures, bringing new insights into the etiology of structural birth anomalies that affect the vertebrate head.

Dental Pulp Stem Cells and Current in vivo Approaches to Study Dental Pulp Stem Cells in Pulp Injury and Regeneration

Dental pulp stem cells (DPSCs) have garnered significant interest in dental research for their unique characteristics and potential in tooth development and regeneration. While there were many studies to define their stem cell-like characteristics and osteogenic differentiation functions that are considered ideal candidates for regenerating damaged dental pulp tissue, how endogenous DPSCs respond to dental pulp injury and supply new dentin-forming cells has not been extensively investigated in vivo. Here, we review the recent progress in identity, function, and regulation of endogenous DPSCs and their clinical potential for pulp injury and regeneration. In addition, we discuss current advances in new mouse models, imaging techniques, and its practical uses and limitations in the analysis of DPSCs in pulp injury and regeneration in vivo.

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.

Emerging Roles of YAP/TAZ in Tooth and Surrounding: from Development to Regeneration

Yes associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ) are ubiquitous transcriptional co-activators that control organ development, homeostasis, and tissue regeneration. Current in vivo evidence suggests that YAP/TAZ regulates enamel knot formation during murine tooth development, and is indispensable for dental progenitor cell renewal to support constant incisor growth. Being a critical sensor for cellular mechano-transduction, YAP/TAZ lays at the center of the complex molecular network that integrates mechanical cues from the dental pulp chamber and surrounding periodontal tissue into biochemical signals, dictating in vitro cell proliferation, differentiation, stemness maintenance, and migration of dental stem cells. Moreover, YAP/TAZ-mediated cell-microenvironment interactions also display essential regulatory roles during biomaterial-guided dental tissue repair and engineering in some animal models. Here, we review recent advances in YAP/TAZ functions in tooth development, dental pulp, and periodontal physiology, as well as dental tissue regeneration. We also highlight several promising strategies that harness YAP/TAZ activation for promoting dental tissue regeneration.

Plasticity of Dental Cell Types in Development, Regeneration, and Evolution

Recent years have improved our understanding of the plasticity of cell types behind inducing, building, and maintaining different types of teeth. The latest efforts were aided by progress in single-cell transcriptomics, which helped to define not only cell states with mathematical precision but also transitions between them. This includes new aspects of dental epithelial and mesenchymal stem cell niches and beyond. These recent efforts revealed continuous and fluid trajectories connecting cell states during dental development and exposed the natural plasticity of tooth-building progenitors. Such "developmental" plasticity seems to be employed for organizing stem cell niches in adult continuously growing teeth. Furthermore, transitions between mature cell types elicited by trauma might represent a replay of embryonic continuous cell states. Alternatively, they could constitute transitions that evolved de novo, not known from the developmental paradigm. In this review, we discuss and exemplify how dental cell types exhibit plasticity during dynamic processes such as development, self-renewal, repair, and dental replacement. Hypothetically, minor plasticity of cell phenotypes and greater plasticity of transitions between cell subtypes might provide a better response to lifetime challenges, such as damage or dental loss. This plasticity might be additionally harnessed by the evolutionary process during the elaboration of dental cell subtypes in different animal lineages. In turn, the diversification of cell subtypes building teeth brings a diversity of their shape, structural properties, and functions.

HYPOTHESIS: Do LRIG Proteins Regulate Stem Cell Quiescence by Promoting BMP Signaling?

Leucine-rich repeats and immunoglobulin-like domains (LRIG) proteins are evolutionarily conserved integral membrane proteins. Mammalian LRIG1 regulates stem cell quiescence in various tissue compartments, including compartments in the epidermis, oral mucosa, intestines, neural system, and incisors. The planarian LRIG1 homolog regulates the quiescence of multipotent neoblasts. The mechanism through which LRIG proteins regulate stem cell quiescence has not been well documented, although it is generally assumed that LRIG1 regulates the epidermal growth factor receptor (EGFR) or other receptor tyrosine kinases. However, Lrig-null (Lrig1-/-;Lrig2-/-; and Lrig3-/-) mouse embryonic fibroblasts (MEFs) have been recently found to exhibit apparently normal receptor tyrosine kinase functions. Moreover, bone morphogenetic protein (BMP) signaling has been shown to depend on LRIG1 and LRIG3 expression. BMPs are well-known regulators of stem cell quiescence. Here, we hypothesize that LRIG1 might regulate stem cell quiescence by promoting BMP signaling. HYPOTHESIS: Based on recent findings, it is hypothesized that LRIG1 regulates stem cell quiescence in mammalian tissues as well as in planarian neoblasts by promoting BMP signaling.

Rodent incisor as a model to study mesenchymal stem cells in tissue homeostasis and repair

The homeostasis of adult tissues, such as skin, hair, blood, and bone, requires continuous generation of differentiated progeny of stem cells. The rodent incisor undergoes constant renewal and can provide an extraordinary model for studying stem cells and their progeny in adult tissue homeostasis, cell differentiation and injury-induced regeneration. Meanwhile, cellular heterogeneity in the mouse incisor also provides an opportunity to study cell-cell communication between different cell types, including interactions between stem cells and their niche environment. More importantly, the molecular and cellular regulatory mechanisms revealed by the mouse incisor have broad implications for other organs. Here we review recent findings and advances using the mouse incisor as a model, including perspectives on the heterogeneity of cells in the mesenchyme, the niche environment, and signaling networks that regulate stem cell behavior. The progress from this field will not only expand the knowledge of stem cells and organogenesis, but also bridge a gap between animal models and tissue regeneration.

Establishment of inclusive single-cell transcriptome atlases from mouse and human tooth as powerful resource for dental research

Single-cell (sc) omics has become a powerful tool to unravel a tissue’s cell landscape across health and disease. In recent years, sc transcriptomic interrogation has been applied to a variety of tooth tissues of both human and mouse, which has considerably advanced our fundamental understanding of tooth biology. Now, an overarching and integrated bird’s-view of the human and mouse tooth sc transcriptomic landscape would be a powerful multi-faceted tool for dental research, enabling further decipherment of tooth biology and development through constantly progressing state-of-the-art bioinformatic methods as well as the exploration of novel hypothesis-driven research. To this aim, we re-assessed and integrated recently published scRNA-sequencing datasets of different dental tissue types (healthy and diseased) from human and mouse to establish inclusive tooth sc atlases, and applied the consolidated data map to explore its power. For mouse tooth, we identified novel candidate transcriptional regulators of the ameloblast lineage. Regarding human tooth, we provide support for a developmental connection, not advanced before, between specific epithelial compartments. Taken together, we established inclusive mouse and human tooth sc atlases as powerful tools to potentiate innovative research into tooth biology, development and disease. The maps are provided online in an accessible format for interactive exploration.

CGRP and Shh Mediate the Dental Pulp Cell Response to Neuron Stimulation

Dental pain is a persistent, detrimental public health issue that requires a better understanding of the mechanisms of tooth pain and inflammation in order to develop more effective treatments. Calcitonin gene-related peptide (CGRP) and dental pulp cells are promising candidates for mediating tooth pain and generating reparative dental tissues, respectively, but their behavior in the context of pulpitis remains elusive. The mouse incisor requires Sonic hedgehog (Shh) secreted from sensory nerves to continuously regenerate. However, it is unknown whether sensory nerves also regulate the comparatively nonregenerative mouse molar through CGRP and Shh. This is an important knowledge gap to fill since mouse incisors differ biologically from human teeth, while mouse and human molars are similar. In this work, we identified that molar pulp cells express CGRP receptor and Gli1, a Hedgehog (Hh) signaling protein found to label a dental stem cell population in the mouse incisor. We also observed in a mouse molar injury model that Hh signaling was activated and Shh expression was upregulated in vivo. We then determined in vitro that Shh and CGRP regulate differentiation of primary mouse molar and incisor pulp cells and a human dental pulp stem cell line. Furthermore, conditioned media from stimulated sensory neurons induced Hh signaling activation and inflammatory gene expression in primary molar pulp cells, which was abolished by inhibition of either Shh or CGRP. Our results suggest that CGRP and Shh signaling may promote an inflammatory response after injury in the molar and that activated sensory nerves secrete CGRP and Shh to regulate molar pulp cell expansion and differentiation into odontoblast-like cells for dentin repair. Thus, CGRP/Shh signaling should be considered for new strategies that seek to manage pain or dentin regeneration in the molar.

Oral stem cells, decoding and mapping the resident cells populations

The teeth and their supporting tissues provide an easily accessible source of oral stem cells. These different stem cell populations have been extensively studied for their properties, such as high plasticity and clonogenicity, expressing stem cell markers and potency for multilineage differentiation in vitro. Such cells with stem cell properties have been derived and characterised from the dental pulp tissue, the apical papilla region of roots in development, as well as the supporting tissue of periodontal ligament that anchors the tooth within the alveolar socket and the soft gingival tissue. Studying the dental pulp stem cell populations in a continuously growing mouse incisor model, as a traceable in vivo model, enables the researchers to study the properties, origin and behaviour of mesenchymal stem cells. On the other side, the oral mucosa with its remarkable scarless wound healing phenotype, offers a model to study a well-coordinated system of healing because of coordinated actions between epithelial, mesenchymal and immune cells populations. Although described as homogeneous cell populations following their in vitro expansion, the increasing application of approaches that allow tracing of individual cells over time, along with single-cell RNA-sequencing, reveal that different oral stem cells are indeed diverse populations and there is a highly organised map of cell populations according to their location in resident tissues, elucidating diverse stem cell niches within the oral tissues. This review covers the current knowledge of diverse oral stem cells, focusing on the new approaches in studying these cells. These approaches "decode" and "map" the resident cells populations of diverse oral tissues and contribute to a better understanding of the "stem cells niche architecture and interactions. Considering the high accessibility and simplicity in obtaining these diverse stem cells, the new findings offer potential in development of translational tissue engineering approaches and innovative therapeutic solutions.

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.

SRSF1 governs progenitor-specific alternative splicing to maintain adult epithelial tissue homeostasis and renewal

Alternative splicing generates distinct mRNA variants and is essential for development, homeostasis, and renewal. Proteins of the serine/arginine (SR)-rich splicing factor family are major splicing regulators that are broadly required for organ development as well as cell and organism viability. However, how these proteins support adult organ function remains largely unknown. Here, we used the continuously growing mouse incisor as a model to dissect the functions of the prototypical SR family protein SRSF1 during tissue homeostasis and renewal. We identified an SRSF1-governed alternative splicing network that is specifically required for dental proliferation and survival of progenitors but dispensable for the viability of differentiated cells. We also observed a similar progenitor-specific role of SRSF1 in the small intestinal epithelium, indicating a conserved function of SRSF1 across adult epithelial tissues. Thus, our findings define a regulatory mechanism by which SRSF1 specifically controls progenitor-specific alternative splicing events to support adult tissue homeostasis and renewal.

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.

Spatiotemporal Changes in Transcriptome of Odontogenic and Non-odontogenic Regions in the Dental Arch of Mus musculus

Over the past 40 years, studies on tooth regeneration have been conducted. These studies comprised two main flows: some focused on epithelial-mesenchymal interaction in the odontogenic region, whereas others focused on creating a supernumerary tooth in the non-odontogenic region. Recently, the scope of the research has moved from conventional gene modification and molecular therapy to genome and transcriptome sequencing analyses. However, these sequencing data have been produced only in the odontogenic region. We provide RNA-Seq data of not only the odontogenic region but also the non-odontogenic region, which loses tooth-forming capacity during development and remains a rudiment. Sequencing data were collected from mouse embryos at three different stages of tooth development. These data will expand our understanding of tooth development and will help in designing developmental and regenerative studies from a new perspective.

The Dental Lamina: An Essential Structure for Perpetual Tooth Regeneration in Sharks

In recent years, nonclassical models have emerged as mainstays for studies of evolutionary, developmental, and regenerative biology. Genomic advances have promoted the use of alternative taxa for the study of developmental biology, and the shark is one such emerging model vertebrate. Our research utilizes the embryonic shark (Scyliorhinus canicula) to characterize key developmental and regenerative processes that have been overlooked or not possible to study with more classic developmental models. Tooth development is a major event in the construction of the vertebrate body plan, linked in part with the emergence of jaws. Early development of the teeth and morphogenesis is well known from the murine model, but the process of tooth redevelopment and regeneration is less well known. Here we explore the role of the dental lamina in the development of a highly regenerative dentition in sharks. The shark represents a polyphyodont vertebrate with continuously repeated whole tooth regeneration. This is presented as a major developmental shift from the more derived renewal process that the murine model offers, where incisors exhibit continuous renewal and growth of the same tooth. Not only does the shark offer a study system for whole unit dental regeneration, it also represents an important model for understanding the evolutionary context of vertebrate tooth regeneration.

Distinct tooth regeneration systems deploy a conserved battery of genes

Background

Vertebrate teeth exhibit a wide range of regenerative systems. Many species, including most mammals, reptiles, and amphibians, form replacement teeth at a histologically distinct location called the successional dental lamina, while other species do not employ such a system. Notably, a ‘lamina-less’ tooth replacement condition is found in a paraphyletic array of ray-finned fishes, such as stickleback, trout, cod, medaka, and bichir. Furthermore, the position, renewal potential, and latency times appear to vary drastically across different vertebrate tooth regeneration systems. The progenitor cells underlying tooth regeneration thus present highly divergent arrangements and potentials. Given the spectrum of regeneration systems present in vertebrates, it is unclear if morphologically divergent tooth regeneration systems deploy an overlapping battery of genes in their naïve dental tissues.

Results

In the present work, we aimed to determine whether or not tooth progenitor epithelia could be composed of a conserved cell type between vertebrate dentitions with divergent regeneration systems. To address this question, we compared the pharyngeal tooth regeneration processes in two ray-finned fishes: zebrafish (Danio rerio) and threespine stickleback (Gasterosteus aculeatus). These two teleost species diverged approximately 250 million years ago and demonstrate some stark differences in dental morphology and regeneration. Here, we find that the naïve successional dental lamina in zebrafish expresses a battery of nine genes (bmpr1aa, bmp6, cd34, gli1, igfbp5a, lgr4, lgr6, nfatc1, and pitx2), while active Wnt signaling and Lef1 expression occur during early morphogenesis stages of tooth development. We also find that, despite the absence of a histologically distinct successional dental lamina in stickleback tooth fields, the same battery of nine genes (Bmpr1a, Bmp6, CD34, Gli1, Igfbp5a, Lgr4, Lgr6, Nfatc1, and Pitx2) are expressed in the basalmost endodermal cell layer, which is the region most closely associated with replacement tooth germs. Like zebrafish, stickleback replacement tooth germs additionally express Lef1 and exhibit active Wnt signaling. Thus, two fish systems that either have an organized successional dental lamina (zebrafish) or lack a morphologically distinct successional dental lamina (sticklebacks) deploy similar genetic programs during tooth regeneration.

Conclusions

We propose that the expression domains described here delineate a highly conserved “successional dental epithelium” (SDE). Furthermore, a set of orthologous genes is known to mark hair follicle epithelial stem cells in mice, suggesting that regenerative systems in other epithelial appendages may utilize a related epithelial progenitor cell type, despite the highly derived nature of the resulting functional organs.

From Bite to Byte: Dental Structures Resolved at a Single-Cell Resolution

The systematic classification of the cells that compose a tissue or an organ is key to understanding how these cells cooperate and interact as a functional unit. Our capacity to detect features that define cell identity has evolved from morphological and chemical analyses, through the use of predefined genetic markers, to unbiased transcriptomic and epigenetic profiling. The innovative technology of single-cell RNA sequencing (scRNA-seq) enables transcriptional profiling of thousands of individual cells. Since its development, scRNA-seq has been extensively applied to numerous organs and tissues in a wide range of animal models and human samples, thereby providing a plethora of fundamental biological insights into their development, homeostasis, and pathology. In this review, we present the findings of 3 recent studies that employed scRNA-seq to unravel the complexity of cellular composition in mammalian teeth. These findings offer an unprecedented catalogue of cell types in the mouse incisor, which is a convenient model system for studying continuous tooth growth. These studies identified novel cell types in the tooth epithelium and mesenchyme, as well as new markers for known cell types. Computational analyses of the data also uncovered the lineage and dynamics of cell states during ameloblast and odontoblast differentiation during both normal homeostasis and injury repair. The transcriptional differences between the mouse incisor and mouse and human molars uncover species-specific as well as shared features in tooth cell composition. Here, we highlight these findings and discuss important similarities and differences between these studies. We also discuss potential future applications of scRNA-seq in dental research and dentistry. Together, these studies demonstrate how the rapidly evolving technology of scRNA-seq can advance the study of tooth development and function and provide putative targets for regenerative approaches.

Bisphenol A Exposure Disrupts Enamel Formation via EZH2-Mediated H3K27me3

Enamel formation is a serial and complex biological process, during which related genes are expressed progressively in a spatiotemporal manner. This process is vulnerable to environmental cues, resulting in developmental defects of enamel (DDE). However, how environmental factors are biologically integrated during enamel formation is still poorly understood. Here, we investigated the mechanism of DDE elicited by a model endocrine-disrupting chemical, bisphenol A (BPA), in mouse incisors. We show that BPA exposure leads to DDE in mouse incisors, as well as excessive proliferation in dental epithelial stem/progenitor cells. Western blotting, chromatin immunoprecipitation sequencing, and immunofluorescence staining revealed that this effect was accompanied by upregulation of a repressive mark, H3K27me3, in the labial cervical loop of mouse incisors. Perturbation of H3K27me3 methyltransferase EZH2 repressed the level of H3K27me3 and partially attenuated the excessive proliferation in dental epithelial stem/progenitor cells and DDE induced by BPA exposure. Overall, our results demonstrate the essential role of repressive histone modification H3K27me3 in DDE elicited by exposure to an endocrine-disrupting chemical.

Oxygen regulates epithelial stem cell proliferation via RhoA-actomyosin-YAP/TAZ signal in mouse incisor

Stem cells are maintained in specific niches that strictly regulate their proliferation and differentiation for proper tissue regeneration and renewal. Molecular oxygen (O₂) is an important component of the niche microenvironment, but little is known about how O₂ governs epithelial stem cell (ESC) behavior. Here, we demonstrate that O₂ plays a crucial role in regulating the proliferation of ESCs using the continuously growing mouse incisors. We have revealed that slow-cycling cells in the niche are maintained under relatively hypoxic conditions compared with actively proliferating cells, based on the blood vessel distribution and metabolic status. Mechanistically, we have demonstrated that, during hypoxia, HIF1α upregulation activates the RhoA signal, thereby promoting cortical actomyosin and stabilizing the adherens junction complex, including merlin. This leads to the cytoplasmic retention of YAP/TAZ to attenuate cell proliferation. These results shed light on the biological significance of blood-vessel geometry and the signaling mechanism through microenvironmental O₂ to orchestrate ESC behavior, providing a novel molecular basis for the microenvironmental O₂-mediated stem cell regulation during tissue development and renewal.

The vital role of Gli1+ mesenchymal stem cells in tissue development and homeostasis

The hedgehog (Hh) signaling pathway plays an essential role in both tissue development and homeostasis. Glioma-associated oncogene homolog 1 (Gli1) is one of the vital transcriptional factors as well as the direct target gene in the Hh signaling pathway. The cells expressing the Gli1 gene (Gli1⁺ cells) have been identified as mesenchymal stem cells (MSCs) that are responsible for various tissue developments, homeostasis, and injury repair. This review outlines some recent discoveries on the crucial roles of Gli1⁺ MSCs in the development and homeostasis of varieties of hard and soft tissues.

LRIG1, a regulator of stem cell quiescence and a pleiotropic feedback tumor suppressor

LRIG1, leucine-rich repeats and immunoglobulin-like domains protein 1, was discovered more than 20 years ago and has been shown to be downregulated or lost, and to function as a tumor suppressor in several cancers. Another well-reported biological function of LRIG1 is to regulate and help enforce the quiescence of adult stem cells (SCs). In both contexts, LRIG1 regulates SC quiescence and represses tumor growth via, primarily, antagonizing the expression and activities of ERBB and other receptor tyrosine kinases (RTKs). We have recently reported that in treatment-naïve human prostate cancer (PCa), LRIG1 is primarily regulated by androgen receptor (AR) and is prominently overexpressed. In castration-resistant PCa (CRPC), both LRIG1 and AR expression becomes heterogeneous and, frequently, discordant. Importantly, in both androgen-dependent PCa and CRPC models, LRIG1 exhibits tumor-suppressive functions. Moreover, LRIG1 induction inhibits the growth of pre-established AR⁺ and AR⁻ PCa. Here, upon a brief introduction of the LRIG1 and the LRIG family, we provide an updated overview on LRIG1 functions in regulating SC quiescence and repressing tumor development. We further highlight the expression, regulation and functions of LRIG1 in treatment-naïve PCa and CRPC. We conclude by offering the perspectives of identifying novel cancer-specific LRIG1-interacting signaling partners and developing LRIG1-based anti-cancer therapeutics and diagnostic/prognostic biomarkers.

New insight into dental epithelial stem cells: Identification, regulation, and function in tooth homeostasis and repair

Tooth enamel, a highly mineralized tissue covering the outermost area of teeth, is always damaged by dental caries or trauma. Tooth enamel rarely repairs or renews itself, due to the loss of ameloblasts and dental epithelial stem cells (DESCs) once the tooth erupts. Unlike human teeth, mouse incisors grow continuously due to the presence of DESCs that generate enamel-producing ameloblasts and other supporting dental epithelial lineages. The ready accessibility of mouse DESCs and wide availability of related transgenic mouse lines make mouse incisors an excellent model to examine the identity and heterogeneity of dental epithelial stem/progenitor cells; explore the regulatory mechanisms underlying enamel formation; and help answer the open question regarding the therapeutic development of enamel engineering. In the present review, we update the current understanding about the identification of DESCs in mouse incisors and summarize the regulatory mechanisms of enamel formation driven by DESCs. The roles of DESCs during homeostasis and repair are also discussed, which should improve our knowledge regarding enamel tissue engineering.

Expression Patterns of Claudin Family Members During Tooth Development and the Role of Claudin-10 (Cldn10) in Cytodifferentiation of Stratum Intermedium

There is growing evidence showing that tight junctions play an important role in developing enamel. Claudins are one of the main components of tight junctions and may have pivotal functions in modulating various cellular events, such as regulating cell differentiation and proliferation. Mutations in CLDN10 of humans are associated with HELIX syndrome and cause enamel defects. However, current knowledge regarding the expression patterns of claudins and the function of Cldn10 during tooth development remains fragmented. In this study, we aimed to analyze the expression patterns of claudin family members during tooth development and to investigate the role of Cldn10 in amelogenesis. Using cap analysis gene expression of developing mouse tooth germs compared with that of the whole body, we found that Cldn1 and Cldn10 were highly expressed in the tooth. Furthermore, single-cell RNA-sequence analysis using 7-day postnatal Krt14-RFP mouse incisors revealed Cldn1 and Cldn10 exhibited distinct expression patterns. Cldn10 has two isoforms, Cldn10a and Cldn10b, but only Cldn10b was expressed in the tooth. Immunostaining of developing tooth germs revealed claudin-10 was highly expressed in the inner enamel epithelium and stratum intermedium. We also found that overexpression of Cldn10 in the dental epithelial cell line, SF2, induced alkaline phosphatase (Alpl) expression, a marker of maturated stratum intermedium. Our findings suggest that Cldn10 may be a novel stratum intermedium marker and might play a role in cytodifferentiation of stratum intermedium.

Microbiota-Derived Short-Chain Fatty Acids Promote BMP Signaling by Inhibiting Histone Deacetylation and Contribute to Dentinogenic Differentiation in Murine Incisor Regeneration

Microbiota and their metabolites short-chain fatty acids (SCFAs) have important roles in regulating tissue regeneration and mesenchymal stem cell (MSC) differentiation. In this study, we explored the potential effects of SCFAs on murine incisor regeneration and dental MSCs. We observed that SCFA deficiency induced by depletion of microbiota through antibiotic treatment led to lower renewal rate and delayed dentinogenesis in mice incisors. Supplementation with SCFAs in drinking water during antibiotic treatment can rescue the renewal rate and dentinogenesis effectively. In vitro, stimulation with SCFAs could promote differentiation of dental MSCs to odontoblasts. We further found that SCFAs could contribute to dentinogenic differentiation of dental MSCs by increasing bone morphogenetic protein (BMP) signal activation. SCFAs could inhibit deacetylation and increase BMP7 transcription of dental MSCs, which promoted BMP signaling. Our results suggested that SCFAs were required for incisor regeneration as well as differentiation of dental MSCs. Microbiota and their metabolites should be concerned as important factors in the tissue renewal and regeneration.

Heterogeneity within Stratified Epithelial Stem Cell Populations Maintains the Oral Mucosa in Response to Physiological Stress

Stem cells in stratified epithelia are generally believed to adhere to a non-hierarchical single-progenitor model. Using lineage tracing and genetic label-retention assays, we show that the hard palatal epithelium of the oral cavity is unique in displaying marked proliferative heterogeneity. We identify a previously uncharacterized, infrequently-dividing stem cell population that resides within a candidate niche, the junctional zone (JZ). JZ stem cells tend to self-renew by planar symmetric divisions, respond to masticatory stresses, and promote wound healing, whereas frequently-dividing cells reside outside the JZ, preferentially renew through perpendicular asymmetric divisions, and are less responsive to injury. LRIG1 is enriched in the infrequently-dividing population in homeostasis, dynamically changes expression in response to tissue stresses, and promotes quiescence, whereas Igfbp5 preferentially labels a rapidly-growing, differentiation-prone population. These studies establish the oral mucosa as an important model system to study epithelial stem cell populations and how they respond to tissue stresses.

Grand Challenges in Comparative Tooth Biology

Teeth are a model system for integrating developmental genomics, functional morphology, and evolution. We are at the cusp of being able to address many open issues in comparative tooth biology and we outline several of these newly tractable and exciting research directions. Like never before, technological advances and methodological approaches are allowing us to investigate the developmental machinery of vertebrates and discover both conserved and excitingly novel mechanisms of diversification. Additionally, studies of the great diversity of soft tissues, replacement teeth, and non-trophic functions of teeth are providing new insights into dental diversity. Finally, we highlight several emerging model groups of organisms that are at the forefront of increasing our appreciation of the mechanisms underlying tooth diversification.

LGRs in Skeletal Tissues: An Emerging Role for Wnt-Associated Adult Stem Cell Markers in Bone

Leucine‐rich repeat‐containing G protein‐coupled receptors (LGRs) are adult stem cell markers that have been described across various stem cell niches, and expression of LGRs and their corresponding ligands (R‐spondins) has now been reported in multiple bone‐specific cell types. The skeleton harbors elusive somatic stem cell populations that are exceedingly compartment‐specific and under tight regulation from various signaling pathways. Skeletal progenitors give rise to multiple tissues during development and during regenerative processes of bone, requiring postnatal endochondral and intramembranous ossification. The relevance of LGRs and the LGR/R‐spondin ligand interaction in bone and tooth biology is becoming increasingly appreciated. LGRs may define specific stem cell and progenitor populations and their behavior during both development and regeneration, and their role as Wnt‐associated receptors with specific ligands poses these proteins as unique therapeutic targets via potential R‐spondin agonism. This review seeks to outline the current literature on LGRs in the context of bone and its associated tissues, and points to key future directions for studying the functional role of LGRs and ligands in skeletal biology. © 2020 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

Tissue Engineering Approaches for Enamel, Dentin, and Pulp Regeneration: An Update

Stem/progenitor cells are undifferentiated cells characterized by their exclusive ability for self-renewal and multilineage differentiation potential. In recent years, researchers and investigations explored the prospect of employing stem/progenitor cell therapy in regenerative medicine, especially stem/progenitor cells originating from the oral tissues. In this context, the regeneration of the lost dental tissues including enamel, dentin, and the dental pulp are pivotal targets for stem/progenitor cell therapy. The present review elaborates on the different sources of stem/progenitor cells and their potential clinical applications to regenerate enamel, dentin, and the dental pulpal tissues.

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.

A large pool of actively cycling progenitors orchestrates self-renewal and injury repair of an ectodermal appendage

The classical model of tissue renewal posits that small numbers of quiescent stem cells (SCs) give rise to proliferating transit-amplifying cells before terminal differentiation. However, many organs house pools of SCs with proliferative and differentiation potentials that diverge from this template. Resolving SC identity and organization is therefore central to understanding tissue renewal. Here, using a combination of single-cell RNA sequencing (scRNA-seq), mouse genetics and tissue injury approaches, we uncover cellular hierarchies and mechanisms that underlie the maintenance and repair of the continuously growing mouse incisor. Our results reveal that, during homeostasis, a group of actively cycling epithelial progenitors generates enamel-producing ameloblasts and adjacent layers of non-ameloblast cells. After injury, tissue repair was achieved through transient increases in progenitor-cell proliferation and through direct conversion of Notch1-expressing cells to ameloblasts. We elucidate epithelial SC identity, position and function, providing a mechanistic basis for the homeostasis and repair of a fast-turnover ectodermal appendage.

Quantitative analysis of the core 2D arrangement and distribution of enamel rods in cross-sections of mandibular mouse incisors

Considerable descriptive information about the overall organization of mouse mandibular incisor enamel is available but almost nothing is known about the quantitative characteristics of enamel rod arrangement and distribution in these teeth. This has important implications concerning cell movement during the secretory stage because each ameloblast makes one enamel rod. Knowing how many enamel rods are cut open in a cross-section of the enamel layer could provide insights into understanding the dynamics of how groups of ameloblasts form the enamel layer. In this study, cross-sections of fully mineralized enamel were cut on 24 mandibular mouse incisors, polished and etched, and imaged by scanning electron microscopy in backscatter mode. Montaged maps of the entire enamel layer were made at high magnification and the enamel rod profiles in each map were color-coded based upon rod category. Quantitative analyses of each color layer in the maps were then performed using standard routines available in imagej. The data indicated that that there were on average 7233 ± 575 enamel rod profiles per cross-section in mandibular incisors of 7-week-old mice, with 70% located in the inner enamel layer, 27% located in the outer enamel layer, and 3% positioned near the mesial and lateral cementoenamel junctions. All enamel rod profiles showed progressive increases in tilt angles, some very large in magnitude, from the lateral to mesial sides of the enamel layer, whereas only minor variations in tilt angle were found relative to enamel thickness at given locations across the enamel layer. The decussation angle between alternating rows of rod profiles within the inner enamel layer was fairly constant from the lateral to central labial sides of the enamel layer, but it increased dramatically in the mesial region of the enamel layer. The packing density of all rod profiles decreased from lateral to central labial regions of the enamel layer and then in progressing mesially, decreased slightly (inner enamel, mesial tilt), increased slightly (outer enamel layer) or almost doubled in magnitude (inner enamel, lateral tilt). It was concluded that these variations in rod tilt angle and packing densities are adaptations that allow the tooth to maintain a sharp incisal edge and shovel-shape as renewing segments formed by around 7200 ameloblasts are brought onto the occluding surface of the tooth by continuous renewal.

A proteomic network approach across the ALS-FTD disease spectrum resolves clinical phenotypes and genetic vulnerability in human brain

Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are neurodegenerative diseases with overlap in clinical presentation, neuropathology, and genetic underpinnings. The molecular basis for the overlap of these disorders is not well established. We performed a comparative unbiased mass spectrometry‐based proteomic analysis of frontal cortical tissues from postmortem cases clinically defined as ALS, FTD, ALS and FTD (ALS/FTD), and controls. We also included a subset of patients with the C9orf72 expansion mutation, the most common genetic cause of both ALS and FTD. Our systems‐level analysis of the brain proteome integrated both differential expression and co‐expression approaches to assess the relationship of these differences to clinical and pathological phenotypes. Weighted co‐expression network analysis revealed 15 modules of co‐expressed proteins, eight of which were significantly different across the ALS‐FTD disease spectrum. These included modules associated with RNA binding proteins, synaptic transmission, and inflammation with cell‐type specificity that showed correlation with TDP‐43 pathology and cognitive dysfunction. Modules were also examined for their overlap with TDP‐43 protein–protein interactions, revealing one module enriched with RNA‐binding proteins and other causal ALS genes that increased in FTD/ALS and FTD cases. A module enriched with astrocyte and microglia proteins was significantly increased in ALS cases carrying the C9orf72 mutation compared to sporadic ALS cases, suggesting that the genetic expansion is associated with inflammation in the brain even without clinical evidence of dementia. Together, these findings highlight the utility of integrative systems‐level proteomic approaches to resolve clinical phenotypes and genetic mechanisms underlying the ALS‐FTD disease spectrum in human brain.

Prominin-1 controls stem cell activation by orchestrating ciliary dynamics

Proper temporal and spatial activation of stem cells relies on highly coordinated cell signaling. The primary cilium is the sensory organelle that is responsible for transmitting extracellular signals into a cell. Primary cilium size, architecture, and assembly-disassembly dynamics are under rigid cell cycle-dependent control. Using mouse incisor tooth epithelia as a model, we show that ciliary dynamics in stem cells require the proper functions of a cholesterol-binding membrane glycoprotein, Prominin-1 (Prom1/CD133), which controls sequential recruitment of ciliary membrane components, histone deacetylase, and transcription factors. Nuclear translocation of Prom1 and these molecules is particularly evident in transit amplifying cells, the immediate derivatives of stem cells. The absence of Prom1 impairs ciliary dynamics and abolishes the growth stimulation effects of sonic hedgehog (SHH) treatment, resulting in the disruption of stem cell quiescence maintenance and activation. We propose that Prom1 is a key regulator ensuring appropriate response of stem cells to extracellular signals, with important implications for development, regeneration, and diseases.

Biology Explaining Tooth Repair and Regeneration: A Mini-Review

The tooth is an intricate composition of precisely patterned, mineralized matrices and soft tissues. Mineralized tissues include enamel (produced by the epithelial cells called ameloblasts), dentin and cementum (produced by mesenchymal cells called odontoblasts and cementoblasts, respectively), and soft tissues, which include the dental pulp and the periodontal ligament along with the invading nerves and blood vessels. It was perceived for a very long time that teeth primarily serve an esthetical function. In recent years, however, the role of healthy teeth, as well as the impact of oral health on general well-being, became more evident. Tooth loss, caused by tooth decay, congenital malformations (tooth agenesis), trauma, periodontal diseases, or age-related changes, is usually replaced by artificial materials which lack many of the important biological characteristics of the natural tooth. Human teeth have very low to almost absent regeneration potential, due to early loss of cell populations with regenerative capacity, namely stem cells. Significant effort has been made in recent decades to identify and characterize tooth stem cells, and to unravel the developmental programs which these cells follow in order to generate a tooth.

Role of the Inferior Alveolar Nerve in Rodent Lower Incisor Stem Cells

In developing teeth, the sequential and reciprocal interactions between epithelial and mesenchymal tissues promote stem/progenitor cell differentiation. However, the origin of the stem/progenitor cells has been the subject of considerable debate. According to recent studies, mesenchymal stem cells originate from periarterial cells and are regulated by neurons in various organs. The present study examined the role of innervation in tooth development and rodent incisor stem/progenitor cell homeostasis. Rodent incisors continuously grow throughout their lives, and the lower incisors are innervated by the inferior alveolar nerve (IAN). In this study, we resected the IAN in adult rats, and the intact contralateral side served as a nonsurgical control. Sham control rats received the same treatment as the resected rats, except for the resection process. The extent of incisor eruption was measured, and both mesenchymal and epithelial stem/progenitor cells were visualized and compared between the IAN-resected and sham-operated groups. One week after surgery, the IAN-resected incisors exhibited a chalky consistency, and the eruption rate was decreased. Micro–computed tomography and histological analyses performed 4 wk after surgery revealed osteodentin formation, disorganized ameloblast layers, and reduced enamel thickness in the IAN-resected incisors. Immunohistochemical analysis revealed a reduction in the CD90- and LRIG1-positive mesenchymal cell ratio in the IAN-resected incisors. However, the p40-positive epithelial stem/progenitor cell ratio was comparable between the 2 groups. Thus, mesenchymal stem/progenitor cell homeostasis is more related to IAN innervation than to epithelial stem/progenitor cells. Furthermore, sensory nerve innervation influences subsequent incisor growth and formation.

Salivary glands regenerate after radiation injury through SOX2-mediated secretory cell replacement

Salivary gland acinar cells are routinely destroyed during radiation treatment for head and neck cancer that results in a lifetime of hyposalivation and co-morbidities. A potential regenerative strategy for replacing injured tissue is the reactivation of endogenous stem cells by targeted therapeutics. However, the identity of these cells, whether they are capable of regenerating the tissue, and the mechanisms by which they are regulated are unknown. Using in vivo and ex vivo models, in combination with genetic lineage tracing and human tissue, we discover a SOX2+ stem cell population essential to acinar cell maintenance that is capable of replenishing acini after radiation. Furthermore, we show that acinar cell replacement is nerve dependent and that addition of a muscarinic mimetic is sufficient to drive regeneration. Moreover, we show that SOX2 is diminished in irradiated human salivary gland, along with parasympathetic nerves, suggesting that tissue degeneration is due to loss of progenitors and their regulators. Thus, we establish a new paradigm that salivary glands can regenerate after genotoxic shock and do so through a SOX2 nerve-dependent mechanism.

Plasticity within the niche ensures the maintenance of a Sox2+ stem cell population in the mouse incisor

In mice, the incisors grow throughout the animal's life, and this continuous renewal is driven by dental epithelial and mesenchymal stem cells. Sox2 is a principal marker of the epithelial stem cells that reside in the mouse incisor stem cell niche, called the labial cervical loop, but relatively little is known about the role of the Sox2⁺ stem cell population. In this study, we show that conditional deletion of Sox2 in the embryonic incisor epithelium leads to growth defects and impairment of ameloblast lineage commitment. Deletion of Sox2 specifically in Sox2⁺ cells during incisor renewal revealed cellular plasticity that leads to the relatively rapid restoration of a Sox2-expressing cell population. Furthermore, we show that Lgr5-expressing cells are a subpopulation of dental Sox2⁺ cells that also arise from Sox2⁺ cells during tooth formation. Finally, we show that the embryonic and adult Sox2⁺ populations are regulated by distinct signalling pathways, which is reflected in their distinct transcriptomic signatures. Together, our findings demonstrate that a Sox2⁺ stem cell population can be regenerated from Sox2^- cells, reinforcing its importance for incisor homeostasis.

Meeting report: a hard look at the state of enamel research

The Encouraging Novel Amelogenesis Models and Ex vivo cell Lines (ENAMEL) Development workshop was held on 23 June 2017 at the Bethesda headquarters of the National Institute of Dental and Craniofacial Research (NIDCR). Discussion topics included model organisms, stem cells/cell lines, and tissues/3D cell culture/organoids. Scientists from a number of disciplines, representing institutions from across the United States, gathered to discuss advances in our understanding of enamel, as well as future directions for the field.

Isl1 Controls Patterning and Mineralization of Enamel in the Continuously Renewing Mouse Incisor

Rodents are characterized by continuously renewing incisors whose growth is fueled by epithelial and mesenchymal stem cells housed in the proximal compartments of the tooth. The epithelial stem cells reside in structures known as the labial (toward the lip) and lingual (toward the tongue) cervical loops (laCL and liCL, respectively). An important feature of the rodent incisor is that enamel, the outer, highly mineralized layer, is asymmetrically distributed, because it is normally generated by the laCL but not the liCL. Here, we show that epithelial-specific deletion of the transcription factor Islet1 (Isl1) is sufficient to drive formation of ectopic enamel by the liCL stem cells, and also that it leads to production of altered enamel on the labial surface. Molecular analyses of developing and adult incisors revealed that epithelial deletion of Isl1 affected multiple, major pathways: Bmp (bone morphogenetic protein), Hh (hedgehog), Fgf (fibroblast growth factor), and Notch signaling were upregulated and associated with liCL-generated ectopic enamel; on the labial side, upregulation of Bmp and Fgf signaling, and downregulation of Shh were associated with premature enamel formation. Transcriptome profiling studies identified a suite of differentially regulated genes in developing Isl1 mutant incisors. Our studies demonstrate that ISL1 plays a central role in proper patterning of stem cell-derived enamel in the incisor and indicate that this factor is an important upstream regulator of signaling pathways during tooth development and renewal. © 2017 American Society for Bone and Mineral Research.