Wnt/beta-catenin signaling plays an essential role in activation of odontogenic mesenchyme during early tooth development

Pax9 drives development of the upper jaw but not teeth in zebrafish

Loss of dentition has occurred repeatedly throughout vertebrate evolution. Cyprinid fish, including zebrafish, form teeth only deep within the pharynx, not on their oral jaws. However, zebrafish still robustly express transcription factors associated with mammalian tooth development in the neural crest-derived mesenchyme surrounding the mouth. We investigated whether this expression is vestigial or whether these factors contribute to the formation of non-tooth mesenchymal structures in the oral region, using Pax9 as a test case. Zebrafish homozygous for two different pax9 mutant alleles develop the normal complement of pharyngeal teeth but fail to form the premaxilla bone, most of the maxilla, and nasal and maxillary barbels. Lack of most of the upper jaw complex does not preclude effective feeding in the laboratory environment. We observe a significant deficit of sp7:EGFP + osteoblasts and adjacent alx4a:DsRed+ condensing mesenchyme around the maxilla, and no accumulation of either in the premaxillary domain. Loss of pax9 may prevent osteoprogenitors from maintaining the state of condensation required for full osteogenic differentiation. We conclude that Pax9 is not unequivocally required for all vertebrate tooth development but instead may be involved in the development of a variety of organs forming through mesenchymal condensation around the mouth.

Crucial roles of mesenchymal Gata2 in murine epididymal development

Androgens drive the morphogenesis and differentiation of the Wolffian duct (WD) into the epididymis, an essential organ for male reproduction, by binding to the androgen receptor (AR). However, it remains unclear whether other transcriptional programs operate beyond the central androgen/AR signaling in promoting WD development. We discovered that mesenchyme-specific deletion of the transcription factor Gata2 resulted in defective epididymal coiling in the corpus and caudal regions. The defective coiling in the absence of mesenchymal Gata2 did not result from androgen signaling deficiency, as there were no abnormalities in testicular morphology, androgen production, or AR/Ar expression, and dihydrotestosterone supplementation did not restore epididymal coiling in cultured WDs. Instead, Gata2 deletion reduced the expression of the mesenchyme-derived factor Inhba and epithelial proliferation, both of which play critical roles in epididymal coiling. The epididymal defect persisted into adulthood, with the uncoiled corpus and caudal epididymis exhibiting abnormal epithelial morphology and lumen environments, resulting in an unfavorable environment for sperm storage. Our results demonstrate the androgen-independent role of mesenchymal GATA2 in promoting epididymal development through Inhba induction and highlight the importance of proper fetal development in male reproduction.

Mutanobactin-D, a Streptococcus mutans Non-Ribosomal Cyclic Lipopeptide, Induces Osteogenic/Odontogenic Differentiation of Human Dental Pulp Stem Cells and Human Bone Marrow Stem Cells

Bacterium-triggered carious lesions implicate dental hard tissue destruction and the simultaneous initiation of regenerative events comprising dental stem cell activation. Streptococcus mutans (S. mutans) is a prominent pathogen of the oral cavity and the principal cause of caries. S. mutans generates complex products involved in interbacterial interactions, including Mutanobactin-D (Mub-D), which belongs to a group of non-ribosomal cyclic lipopeptides. In the present study, we aimed to analyse the potential role of the synthetic Mub-D peptide in cell populations involved in tissue regenerative processes. To this end, we assessed the in vitro effects of Mub-D in human dental pulp stem cells (hDPSCs) and human bone marrow stem cells (hBMSCs). Our data demonstrated a concentration-dependent effect of Mub-D on their viability and a significant increase in their proliferation and osteogenic/odontogenic differentiation. These events were associated with specific changes in gene expression, where CCDN-1, RUNX-2, OSX, OCN, DMP-1, DSPP, and BMP-2 genes were upregulated. The ability of Mub-D to modulate the osteogenic/odontogenic differentiation of both hDPSCs and hBMSCs and considerably enhance mineralisation in a controlled and concentration-dependent manner opens new perspectives for stem cell-based regenerative approaches in the clinics.

MLL4 regulates postnatal palate growth and midpalatal suture development

MLL4, also known as KMT2D, is a histone methyltransferase that acts as an important epigenetic regulator in various organogenesis programs. Mutations in the MLL4 gene are the major cause of Kabuki syndrome, a human developmental disorder that involves craniofacial birth defects, including anomalies in the palate. This study aimed to investigate the role of MLL4 and the underlying mechanisms in the development and growth of the palate. We generated a novel conditional knockout (cKO) mouse model with tissue-specific deletion of Mll4 in the palatal mesenchyme. Using micro-computed tomography (CT), histological analysis, cell mechanism assays, and gene expression profiling, we examined palate development and growth in the Mll4-cKO mice. Gross craniofacial examination at adult stages revealed mild midfacial hypoplasia and midline defects of the palate in Mll4-cKO mice, including a widened midpalatal suture and disrupted midline rugae pattern. Micro-CT-based time-course skeletal analysis during postnatal palatogenesis through adulthood demonstrated a transverse growth deficit in overall palate width in Mll4-cKO mice. Whole-mount and histological staining at perinatal stages identified that the midline defects in the Mll4-cKO mice emerged as early as 1 day prior to birth, presenting as a widened midpalatal suture, accompanied by increased cell apoptosis in the suture mesenchyme. Genome-wide mRNA expression analysis of the midpalatal suture tissue revealed that MLL4 is essential for the timely expression of major cartilage development genes, such as Col2a1 and Acan, at birth. Immunofluorescence staining for osteochondral differentiation markers demonstrated a marked decrease in the chondrogenic marker COL2A1, while the expression of the osteogenic marker RUNX2 remained unchanged, in the Mll4-cKO midpalatal suture. Additionally, SOX9, a master regulator of chondrogenesis, exhibited a significant decrease in protein expression. Indeed, time-course histological analysis during postnatal palate growth revealed retardation in the development of the suture cartilage in Mll4-cKO mice. Taken together, our results demonstrate that MLL4 is essential for orchestrating key cellular and molecular events that ensure proper midpalatal suture development and palate growth.

A Chemically Defined Culture for Tooth Reconstitution

It is known for decades that dental epithelium and mesenchyme can reconstitute and regenerate a functional tooth. However, the mechanism of tooth reconstitution remains largely unknown due to the lack of an efficient in vitro model. Here, a chemically defined culture system is established that supports tooth reconstitution, further development with normal anatomy, and prompt response to chemical interference in key developmental signaling pathways, termed as toothoids. By using such a system, it is discovered that, during reconstitution, instead of resetting the developmental clock, dental cells reorganized and restarted from the respective developmental stage where they are originally isolated. Moreover, co-stimulation of Activin A and Hedgehog/Smoothened agonist (SAG) sustained the initial induction of tooth fate from the first branchial arch, which would be otherwise quickly lost in culture. Furthermore, activation of Bone Morphogenetic Protein (BMP) signaling triggered efficient enamel formation in the late-stage toothoids, without affecting the normal development of ameloblasts. Together, these data highlight the toothoid culture as a powerful tool to dissect the molecular mechanisms of tooth reconstitution and regeneration.

MLL4 regulates postnatal palate growth and midpalatal suture development

MLL4, also known as KMT2D, is a histone methyltransferase that acts as an important epigenetic regulator in various organogenesis programs. Mutations in the MLL4 gene are the major cause of Kabuki syndrome, a human developmental disorder that involves craniofacial birth defects, including anomalies in the palate. This study aimed to investigate the role of MLL4 and the underlying mechanisms in the development and growth of the palate. We generated a novel conditional knockout (cKO) mouse model with tissue-specific deletion of Mll4 in the palatal mesenchyme. Using micro-computed tomography (CT), histological analysis, cell mechanism assays, and gene expression profiling, we examined palate development and growth in the Mll4-cKO mice. Gross craniofacial examination at adult stages revealed mild midfacial hypoplasia and midline defects of the palate in Mll4-cKO mice, including a widened midpalatal suture and disrupted midline rugae pattern. Micro-CT-based time-course skeletal analysis during postnatal palatogenesis through adulthood demonstrated a transverse growth deficit in overall palate width in Mll4-cKO mice. Whole-mount and histological staining at perinatal stages identified that the midline defects in the Mll4-cKO mice emerged as early as one day prior to birth, presenting as a widened midpalatal suture, accompanied by increased cell apoptosis in the suture mesenchyme. Genome-wide mRNA expression analysis of the midpalatal suture tissue revealed that MLL4 is essential for the timely expression of major cartilage development genes, such as Col2a1 and Acan, at birth.Immunofluorescence staining for osteochondral differentiation markers demonstrated a marked decrease in the chondrogenic marker COL2A1, while the expression of the osteogenic marker RUNX2 remained unchanged, in the Mll4-cKO midpalatal suture. Additionally, SOX9, a master regulator of chondrogenesis, exhibited a significant decrease in protein expression. Indeed, time-course histological analysis during postnatal palate growth revealed retardation in the development of the suture cartilage in Mll4-cKO mice. Taken together, our results demonstrate that MLL4 is essential for orchestrating key cellular and molecular events that ensure proper midpalatal suture development and palate growth.

Derivation of dental epithelial-like cells from murine embryonic stem cells for tooth regeneration

Teeth are comprised of epithelial and mesenchymal cells, and regenerative teeth rely on the regeneration of both cell types. Transcription factors play a pivotal role in cell fate determination. In this study, we establish fluorescence models based on transcription factors to monitor and analyze dental epithelial cells. Using Pitx2-P2A-copGFP mice, we observe that Pitx2+ epithelial cells, when combined with E14.5 dental mesenchymal cells, are sufficient for the reconstitution of teeth. Induced-Pitx2+ cells, directly isolated from the embryoid body that employs the Pitx2-GFP embryonic stem cell line, exhibit the capacity to differentiate into ameloblasts and develop into teeth when combined with dental mesenchymal cells. The regenerated teeth exhibit a complete structure, including dental pulp, dentin, enamel, and periodontal ligaments. Subsequent exploration via RNA-seq reveals that induced-Pitx2+ cells exhibit enrichment in genes associated with FGF receptors and WNT ligands compared with induced-Pitx2- cells. Our results indicate that both primary Pitx2+ and induced Pitx2+ cells possess the capability to differentiate into enamel-secreting ameloblasts and grow into teeth when combined with dental mesenchymal cells.

Vascular smooth muscle cell-derived exosomes promote osteoblast-to-osteocyte transition via β-catenin signaling

Blood vessel growth and osteogenesis in the skeletal system are coupled; however, fundamental aspects of vascular function in osteoblast-to-osteocyte transition remain unclear. Our study demonstrates that vascular smooth muscle cells (VSMCs), but not endothelial cells, are sufficient to drive bone marrow mesenchymal stromal cell-derived osteoblast-to-osteocyte transition via β-catenin signaling and exosome-mediated communication. We found that VSMC-derived exosomes are loaded with transcripts encoding proteins associated with the osteocyte phenotype and members of the WNT/β-catenin signaling pathway. In contrast, endothelial cell-derived exosomes facilitated mature osteoblast differentiation by reprogramming the TGFB1 gene family and osteogenic transcription factors osterix (SP7) and RUNX2. Notably, VSMCs express significant levels of tetraspanins (CD9, CD63, and CD81) and drive the intracellular trafficking of exosomes with a lower membrane zeta potential than those from other cells. Additionally, the high ATP content within these exosomes supports mineralization mechanisms, as ATP is a substrate for alkaline phosphatase. Osteocyte function was further validated by RNA sequencing, revealing activity in genes related to intermittent mineralization and sonic hedgehog signaling, alongside a significant increase in TNFSF11 levels. Our findings unveil a novel role of VSMCs in promoting osteoblast-to-osteocyte transition, thus offering new insights into bone biology and homeostasis, as well as in bone-related diseases. Clinically, these insights could pave the way for innovative therapeutic strategies targeting VSMC-derived exosome pathways to treat bone-related disorders such as osteoporosis. By manipulating these signaling pathways, it may be possible to enhance bone regeneration and improve skeletal health in patients with compromised bone structure and function.

Transcriptional programs of Pitx2 and Tfap2a/Tfap2b controlling lineage specification of mandibular epithelium during tooth initiation

How the dorsal-ventral axis of the vertebrate jaw, particularly the position of tooth initiation site, is established remains a critical and unresolved question. Tooth development starts with the formation of the dental lamina, a localized thickened strip within the maxillary and mandibular epithelium. To identify transcriptional regulatory networks (TRN) controlling the specification of dental lamina from the naïve mandibular epithelium, we utilized Laser Microdissection coupled low-input RNA-seq (LMD-RNA-seq) to profile gene expression of different domains of the mandibular epithelium along the dorsal-ventral axis. We comprehensively identified transcription factors (TFs) and signaling pathways that are differentially expressed along mandibular epithelial domains (including the dental lamina). Specifically, we found that the TFs Sox2 and Tfap2 (Tfap2a/Tfap2b) formed complimentary expression domains along the dorsal-ventral axis of the mandibular epithelium. Interestingly, both classic and novel dental lamina specific TFs-such as Pitx2, Ascl5 and Zfp536-were found to localize near the Sox2:Tfap2a/Tfap2b interface. To explore the functional significance of these domain specific TFs, we next examined loss-of-function mouse models of these domain specific TFs, including the dental lamina specific TF, Pitx2, and the ventral surface ectoderm specific TFs Tfap2a and Tfap2b. We found that disruption of domain specific TFs leads to an upregulation and expansion of the alternative domain's TRN. The importance of this cross-repression is evident by the ectopic expansion of Pitx2 and Sox2 positive dental lamina structure in Tfap2a/Tfap2b ectodermal double knockouts and the emergence of an ectopic tooth in the ventral surface ectoderm. Finally, we uncovered an unappreciated interface of mesenchymal SHH and WNT signaling pathways, at the site of tooth initiation, that were established by the epithelial domain specific TFs including Pitx2 and Tfap2a/Tfap2b. These results uncover a previously unknown molecular mechanism involving cross-repression of domain specific TFs including Pitx2 and Tfap2a/Tfap2b in patterning the dorsal-ventral axis of the mouse mandible, specifically the regulation of tooth initiation site.

wnt10a is required for zebrafish median fin fold maintenance and adult unpaired fin metamorphosis

BACKGROUND

Mutations of human WNT10A are associated with odonto-ectodermal dysplasia syndromes. Here, we present analyses of wnt10a loss-of-function mutants in the zebrafish.

RESULTS

wnt10a mutant zebrafish embryos display impaired tooth development and a collapsing median fin fold (MFF). Rescue experiments show that wnt10a is essential for MFF maintenance both during embryogenesis and later metamorphosis. The MFF collapse could not be attributed to increased cell death or altered proliferation rates of MFF cell types. Rather, wnt10a mutants show reduced expression levels of dlx2a in distal-most MFF cells, followed by compromised expression of col1a1a and other extracellular matrix proteins encoding genes. Transmission electron microscopy analysis shows that although dermal MFF compartments of wnt10a mutants initially are of normal morphology, with regular collagenous actinotrichia, positioning of actinotrichia within the cleft of distal MFF cells becomes compromised, coinciding with actinotrichia shrinkage and MFF collapse.

CONCLUSIONS

MFF collapse of wnt10a mutant zebrafish is likely caused by the loss of distal properties in the developing MFF, strikingly similar to the proposed molecular pathomechanisms underlying the teeth defects caused by the loss of Wnt10 in fish and mammals. In addition, it points to thus fur unknown mechanisms controlling the linear growth and stability of actinotrichia and their collagen fibrils.

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.

Impaired breakdown of Herwig's epithelial root sheath disturbs tooth root development

BACKGROUND

Wnt/β-catenin signaling plays a variety of roles in both the dental epithelium and mesenchyme at most stages of tooth development. In this study, we verified the roles of Hertwig's epithelial root sheath (HERS) breakdown in tooth root development. This breakdown results in formation of epithelial cell rests of Malassez (ERM).

RESULTS

Following induction of β-catenin stabilization in the epithelium of developing tooth at the moment of HERS breakdown, HERS failed to break down for ERM formation. HERS with stabilized β-catenin was altered into a multicellular layer enveloping elongated root dentin with higher expression of junctional proteins such as Zo-1 and E-cadherin. Importantly, this impairment of HERS breakdown led to arrest of further root elongation. In addition, the portion of root dentin enveloped by the undissociated HERS remained in a hypomineralized state. The odontoblasts showed ectopically higher expression of pyrophosphate regulators including Ank and Npp1, whereas Tnap expression was unchanged.

CONCLUSIONS

Our data suggest that Wnt/β-catenin signaling is decreased in HERS for ERM formation during root development. Furthermore, ERM formation is important for further elongation and dentin mineralization of the tooth roots. These findings may provide new insight to understand the contribution of ERM to root formation.

Crucial Roles of the Mesenchymal Androgen Receptor in Wolffian Duct Development

Wolffian duct (WD) maintenance and differentiation is predominantly driven by the androgen action, which is mediated by the androgen receptor (AR). It is well established that the mesenchyme indicates the fate and differentiation of epithelial cells. However, in vivo developmental requirement of mesenchymal AR in WD development is still undefined. By designing a mesenchyme-specific Ar knockout (ARcKO), we discovered that the loss of mesenchymal Ar led to the bilateral or unilateral degeneration of caudal WDs and cystic formation at the cranial WDs. Ex vivo culture of ARcKO WDs invariably resulted in bilateral defects, suggesting that some factor(s) originating from surrounding tissues in vivo might promote WD survival and growth even in the absence of mesenchymal Ar. Mechanistically, we found cell proliferation was significantly reduced in both epithelial and mesenchymal compartments; but cell apoptosis was not affected. Transcriptomic analysis by RNA sequencing of E14.5 mesonephroi revealed 131 differentially expressed genes. Multiple downregulated genes (Top2a, Wnt9b, Lama2, and Lamc2) were associated with morphological and cellular changes in ARcKO male embryos (ie, reduced cell proliferation and decreased number of epithelial cells). Mesenchymal differentiation into smooth muscle cells that are critical for morphogenesis was also impaired in ARcKO male embryos. Taken together, our results demonstrate the crucial roles of the mesenchymal AR in WD maintenance and morphogenesis in mice.

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.

Identification of copy neutral loss of heterozygosity on chromosomes 1p, 1q, and 6p among nonsyndromic cleft lip and/or without cleft palate with hypodontia

BACKGROUND

Nonsyndromic cleft lip and/or without cleft palate (NSCL/P) with or without hypodontia is a common developmental aberration in humans and animals. This study aimed to identify the loss of heterozygosity (LOH) involved in hypodontia and NSCL/P pathogenesis.

METHODS

This is a cross-sectional study that conducted genome-wide copy number analysis using CytoScan 750K array on salivary samples from Malay subjects with NSCL/P with or without hypodontia aged 7-13 years. To confirm the significant results, simple logistic regression was employed to conduct statistical data analysis using SPSS software.

RESULTS

The results indicated the most common recurrent copy neutral LOH (cnLOH) observed at 1p33-1p32.3, 1q32.2-1q42.13 and 6p12.1-6p11.1 loci in 8 (13%), 4 (7%), and 3 (5%) of the NSCL/P subjects, respectively. The cnLOHs at 1p33-1p32.3 (D1S197), 1q32.2-1q42.13 (D1S160), and 6p12.1-6p11.1 (D1S1661) were identified observed in NSCL/P and noncleft children using microsatellite analysis markers as a validation analysis. The regions affected by the cnLOHs at 1p33-1p32.3, 1q32.2-1q42.13, and 6p12.1-6p11.1 loci contained selected genes, namely FAF1, WNT3A and BMP5, respectively. There was a significant association between the D1S197 (1p33-32.3) markers containing the FAF1 gene among NSCL/P subjects with or without hypodontia compared with the noncleft subjects (p-value = 0.023).

CONCLUSION

The results supported the finding that the genetic aberration on 1p33-32.3 significantly contributed to the development of NSCL/P with or without hypodontia. These results have an exciting prospect in the promising field of individualized preventive oral health care.

Mesenchymal β-catenin signaling affects palatogenesis by regulating α-actinin-4 and F-actin

OBJECTIVE

Our previous research have found that mesenchymal β-catenin may be involved in palatal shelf (PS) elevation by regulating F-actin. Here, we further investigated the exact mechanism of β-catenin/F-actin in the PS mesenchyme to regulate palatal reorientation.

MATERIALS AND METHODS

(1) Firstly, Ctnnb1ex3f (β-catenin) mice were conditionally overexpressed in the palatal mesenchyme by crossing with the Sox9-creERT2 mice (induced by Tamoxifen injections); (2) Subsequently, histology and immunohistochemistry were used to characterize the variations of PS morphology and expression of key molecules associated with developmental process; (3) Finally, experiments in vivo and ex vivo were employed to identify the critical mechanisms in β-catenin silenced and overexpressed models.

RESULTS

We found that the Sox9CreER; Ctnnb1ex3f mice exhibited failed palatal elevation and visible cleft palate, and overexpression of β-catenin disturbed the F-actin responsible for cytoskeletal remodeling in palatal mesenchymal cells. qRT-PCR results showed mRNA levels of α-actinin4, a gene involved in F-actin cross-linking, were associated with knockdown or overexpression of β-catenin in ex vivo, respectively. Experiments in vivo revealed that mesenchymal specific inactivation or overexpression of β-catenin exhibited decreased or increased α-actinin-4 expression.

CONCLUSIONS

Mesenchymal β-catenin/F-actin plays an essential role in PS reorientation, which mediate α-actinin-4 to regulate F-actin cytoskeleton reorganization.

The odontoblastic differentiation of dental mesenchymal stem cells: molecular regulation mechanism and related genetic syndromes

Dental mesenchymal stem cells (DMSCs) are multipotent progenitor cells that can differentiate into multiple lineages including odontoblasts, osteoblasts, chondrocytes, neural cells, myocytes, cardiomyocytes, adipocytes, endothelial cells, melanocytes, and hepatocytes. Odontoblastic differentiation of DMSCs is pivotal in dentinogenesis, a delicate and dynamic process regulated at the molecular level by signaling pathways, transcription factors, and posttranscriptional and epigenetic regulation. Mutations or dysregulation of related genes may contribute to genetic diseases with dentin defects caused by impaired odontoblastic differentiation, including tricho-dento-osseous (TDO) syndrome, X-linked hypophosphatemic rickets (XLH), Raine syndrome (RS), hypophosphatasia (HPP), Schimke immuno-osseous dysplasia (SIOD), and Elsahy-Waters syndrome (EWS). Herein, recent progress in the molecular regulation of the odontoblastic differentiation of DMSCs is summarized. In addition, genetic syndromes associated with disorders of odontoblastic differentiation of DMSCs are discussed. An improved understanding of the molecular regulation and related genetic syndromes may help clinicians better understand the etiology and pathogenesis of dentin lesions in systematic diseases and identify novel treatment targets.

Single-cell RNA-Seq reveals transcriptional regulatory networks directing the development of mouse maxillary prominence

During vertebrate embryonic development, neural crest-derived ectomesenchyme within the maxillary prominences undergoes precisely coordinated proliferation and differentiation to give rise to diverse craniofacial structures, such as tooth and palate. However, the transcriptional regulatory networks underpinning such an intricate process have not been fully elucidated. Here, we perform single-cell RNA-Seq to comprehensively characterize the transcriptional dynamics during mouse maxillary development from embryonic day (E) 10.5-E14.5. Our single-cell transcriptome atlas of ∼28,000 cells uncovers mesenchymal cell populations representing distinct differentiating states and reveals their developmental trajectory, suggesting that the segregation of dental from the palatal mesenchyme occurs at E11.5. Moreover, we identify a series of key transcription factors (TFs) associated with mesenchymal fate transitions and deduce the gene regulatory networks directed by these TFs. Collectively, our study provides important resources and insights for achieving a systems-level understanding of craniofacial morphogenesis and abnormality.

[Characteristics and microRNA expression profile of exosomes derived from odontogenic dental pulp stem cells]

OBJECTIVE

To investigate the characteristics of exosomes derived from dental pulp stem cells (DPSCs) in the direction of odontogenic differentiation, to analyze the differences in microRNA expression profile between exosomes derived from undifferentiated and odontogenic DPSCs, and to analyze their possible signal transduction pathways.

METHODS

(1) DPSCs were cultured in α minimum Eagle' s medium (α-MEM), and odontogenic DPSCs were cultured in odontogenic differentiation medium for 21 days, using alizarin red staining and alkaline phosphatase staining to identify the odontogenic differentiation. Exosomes from the cell supernatant were isolated respectively, named as dental pulp stem cells-exosomes (DPSCs-Exo) and dental pulp stem cells-odontogenic-exosomes (DPSCs-OD-Exo). The exosomes were identified by transmission electron microscopy, nanoparticle tracking analysis and Western blot. (2) The microRNA expression profiles of DPSCs-Exo and DPSCs-OD-Exo were investigated by microRNA microarray. To validate the result of the microRNA microarray, real-time quantitative polymerase chain reaction (real-time PCR) assay was applied on 3 most significantly differential expressed microRNA. Pathway analysis was taken to detect enriched pathways associated with the predicted target genes of microRNA.

RESULTS

(1) The DPSCs were isolated and cultured in vitro showed typical fibroblast-like morphology. The odontogenic differentiated DPSCs were spindle-shaped, polygonal, and uniform in size. Odontogenic differentiation group showed a large number of dark deposits in alizarin red staining and the cells were darkly stained in alkaline phosphatase staining, while the cells in normal culture medium group did not show obvious dyeing. The DPSCs-Exo and DPSCs-OD-Exo had the same morphology, both showed bilayer membrane and cup-shape. The peak sizes of DPSCs-Exo and DPSCs-OD-Exo were (114.67±9.07) nm and (134.00±8.54) nm, respectively. The difference between the two was statistically significant. DPSCs-Exo and DPSCs-OD-Exo both expressed the markers of exosomes, tumor susceptibility gene (TSG)101 and CD63. (2) microRNA microarray results showed that the expression profiles of DPSCs-Exo and DPSCs-OD-Exo were different. Nineteen increased by more than two times, and one decreased by 64%. Real-time PCR results showed that the expression levels of microRNA-1246, microRNA-1246-100-5p and microRNA-1246-494-3p in DPSCs-OD-Exo were significantly up-regulated. The difference was statistically significant. microRNA target prediction database and gene signaling pathway database were used to analyze differentially expressed microRNA, and it was predicted that differentially expressed microRNA could target axis inhibition protein 2(AXIN2) gene and Wnt/β-catenin signaling pathway.

CONCLUSION

DPSCs-OD-Exo and DPSCs-Exo had differences in their microRNA expression profile. Those differentially expressed microRNA may be involved in the regulation of DPSCs odontogenic differentiation.

Disruption of fos causes craniofacial anomalies in developing zebrafish

Craniofacial development is a complex and tightly regulated process and disruptions can lead to structural birth defects, the most common being nonsyndromic cleft lip and palate (NSCLP). Previously, we identified FOS as a candidate regulator of NSCLP through family-based association studies, yet its specific contributions to oral and palatal formation are poorly understood. This study investigated the role of fos during zebrafish craniofacial development through genetic disruption and knockdown approaches. Fos was expressed in the periderm, olfactory epithelium and other cell populations in the head. Genetic perturbation of fos produced an abnormal craniofacial phenotype with a hypoplastic oral cavity that showed significant changes in midface dimensions by quantitative facial morphometric analysis. Loss and knockdown of fos caused increased cell apoptosis in the head, followed by a significant reduction in cranial neural crest cells (CNCCs) populating the upper and lower jaws. These changes resulted in abnormalities of cartilage, bone and pharyngeal teeth formation. Periderm cells surrounding the oral cavity showed altered morphology and a subset of cells in the upper and lower lip showed disrupted Wnt/β-catenin activation, consistent with modified inductive interactions between mesenchymal and epithelial cells. Taken together, these findings demonstrate that perturbation of fos has detrimental effects on oral epithelial and CNCC-derived tissues suggesting that it plays a critical role in zebrafish craniofacial development and a potential role in NSCLP.

Tideglusib-incorporated nanofibrous scaffolds potently induce odontogenic differentiation

Pulp-Dentin regeneration is a key aspect of maintain tooth vitality and enabling good oral-systemic health. This study aimed to investigate a nanofibrous scaffold loaded with a small molecule i.e. Tideglusib to promote odontogenic differentiation. Tideglusib (GSK-3β inhibitor) interaction with GSK-3β was determined using molecular docking and stabilization of β-catenin was examined by confocal microscopy. 3D nanofibrous scaffolds were fabricated through electrospinning and their physicochemical characterizations were performed. Scaffolds were seeded with mesenchymal stem cells or pre-odontoblast cells to determine the cells proliferation and odontogenic differentiation. Our results showed that Tideglusib (TG) binds with GSK-3β at Cys199 residue. Stabilization and nuclear translocation of β-catenin was increased in the odontoblast cells treated with TG. SEM analysis revealed that nanofibers exhibited controlled architectural features that effectively mimicked the natural ECM. UV-Vis spectroscopy demonstrated that TG was incorporated successfully and released in a controlled manner. Both kinds of biomimetic nanofibrous matrices (PCLF-TG100, PCLF-TG1000) significantly stimulated cells proliferation. Furthermore, these scaffolds significantly induced dentinogenic markers (ALP, and DSPP) expression and biomineralization. In contrast to current pulp capping material driving dentin repair, the sophisticated, polymeric scaffold systems with soluble and insoluble spatiotemporal cues described here can direct stem cell differentiation and dentin regeneration. Hence, bioactive small molecule-incorporated nanofibrous scaffold suggests an innovative clinical tool for dentin tissue engineering.

PAX9 mutations and genetic synergism in familial tooth agenesis

Familial tooth agenesis (FTA) is one of the most common craniofacial anomalies in humans. Loss-of-function mutations in PAX9 and WNT10A have been known to cause FTA with various expressivity. In this study, we identified five FTA kindreds with novel PAX9 disease-causing mutations: p.(Glu7Lys), p.(Val83Leu), p.(Pro118Ser), p.(Ser197Argfs*23), and c.771+4A>G. Concomitant PAX9 and WNT10A pathogenic variants found in two probands with severe phenotypes suggested an effect of mutational synergism. All overexpressed PAX9s showed proper nuclear localization, excepting the p.(Pro118Ser) mutant. Various missense mutations caused differential loss of PAX9 transcriptional ability. PAX9 overexpression in dental pulp cells upregulated LEF1 and AXIN2 expression, indicating a positive regulatory role for PAX9 in canonical Wnt signaling. Analyzing 176 cases with 63 different mutations, we observed a distinct pattern of tooth agenesis for PAX9-associated FTA: Maxillary teeth are in general more frequently affected than mandibular ones. Along with all second molars, maxillary bicuspids and first molars are mostly involved, while maxillary lateral incisors and mandibular bicuspids are relatively less affected. Genotypically, missense mutations are associated with fewer missing teeth than frameshift and nonsense variants. This study significantly expands the phenotypic and genotypic spectrums of PAX9-associated disorders and reveals a molecular mechanism of genetic synergism underlying FTA variable expressivity.

A Genome-wide association study of premolar agenesis in a chinese population

OBJECTIVE

Premolar agenesis is a common subtype of tooth agenesis. Although a genome-wide study (GWAS) has identified some variants involved in tooth agenesis in Europeans, the genetic mutation related to premolar agenesis in the Chinese population remains unclear.

MATERIALS AND METHODS

We present a GWAS in 218 premolar agenesis cases and 1,222 controls using the Illumina Infinium^® Global Screening Array. 5,585,618 single nucleotide polymorphisms (SNPs) were used for tests of associations with premolar agenesis.

RESULTS

Four independent SNPs on chromosome 2 were identified as susceptibility loci, including rs147680216, rs79743039, rs60540881, and rs6738629. The genome-wide significant SNP rs147680216 (p = 6.09 × 10^-9 ) was predicted to change the structure of the WNT10A protein and interact with hedgehog signaling pathway components. Meta-analysis showed that the rs147680216 A allele significantly increased the risk of tooth agenesis (p = 0.000). The other three SNPs with nominal significance are novel susceptibility loci. Of them, rs6738629 (p = 5.40 × 10^-6 ) acts as a potential transcriptional regulator of GCC2, a gene playing a putative role in dental and craniofacial development.

CONCLUSION

Our GWAS indicates that rs147680216 and additional three novel susceptibility loci on chromosome 2 are associated with the risk of premolar agenesis in the Chinese population.

Canonical Wnt signaling regulates soft palate development by mediating ciliary homeostasis

Craniofacial morphogenesis requires complex interactions involving different tissues, signaling pathways, secreted factors and organelles. The details of these interactions remain elusive. In this study, we have analyzed the molecular mechanisms and homeostatic cellular activities governing soft palate development to improve regenerative strategies for individuals with cleft palate. We have identified canonical Wnt signaling as a key signaling pathway primarily active in cranial neural crest (CNC)-derived mesenchymal cells surrounding soft palatal myogenic cells. Using Osr2-Cre;β-cateninfl/fl mice, we show that Wnt signaling is indispensable for mesenchymal cell proliferation and subsequently for myogenesis through mediating ciliogenesis. Specifically, we have identified that Wnt signaling directly regulates expression of the ciliary gene Ttll3. Impaired ciliary disassembly leads to differentiation defects in mesenchymal cells and indirectly disrupts myogenesis through decreased expression of Dlk1, a mesenchymal cell-derived pro-myogenesis factor. Moreover, we show that siRNA-mediated reduction of Ttll3 expression partly rescues mesenchymal cell proliferation and myogenesis in the palatal explant cultures from Osr2-Cre;β-cateninfl/fl embryos. This study highlights the role of Wnt signaling in palatogenesis through the control of ciliary homeostasis, which establishes a new mechanism for Wnt-regulated craniofacial morphogenesis.

Transcriptome heterogeneity of congenital cleft palate model in congener New Zealand rabbits induced by dexamethasone

OBJECTIVES

This work aimed to investigate the transcriptome heterogeneity of dexamethasone-induced congenital cleft palate in homozygous New Zealand rabbits and determine the molecular mechanism underlying the occurrence of congenital cleft palate.

METHODS

Dexamethasone (1.0 mg per day) was administered intramuscularly to 20 New Zealand pregnant rabbits from day 14 to day 17 of gestation, and the palatal phenotype of all offspring of each pregnant rabbit was observed. Eight embryos with a 4∶4 ratio of cleft palate to non-cleft palate were selected and divided into the cleft palate group (CP) and non-cleft palate group (NCP). Their palatal tissues were collected for RNA sequencing.

RESULTS

A total of 225 differentially expressed genes (Q<0.05) were found in the CP group compared with the NCP group, of which 120 genes were upregulated and 105 genes were downregulated. The GO and KEGG enrichment analyses of these differentially expressed genes were carried out. The results showed significant enrichment in GO classification, which included heterotrimeric G protein complex, extracellular matrix, transcription factor complex, and basement membrane. Meanwhile, GABA ergic synapse, morphine addiction, retrograde endocannabinoid signaling, glutamate synapse, serotonergic synapse, regulation of actin cytoskeleton, and the Apelin signaling pathway were significantly enriched in the KEGG pathway. Compared with the NCP group, the gene expression levels of ARHGEF6 (P<0.05) and ABI2 (P<0.001) decreased in the CP group, and APC increased (P<0.001); these results were confirmed by real-time polymerase chain reaction.

CONCLUSIONS

Abnormal expression levels of the ARHGEF6, APC, and ABI2 genes involved in the regulation of the actin cytoskeleton in the palatal synapse may be associated with the dexamethasone-induced congenital cleft palate in New Zealand rabbits.

Genome wide analysis of dexamethasone stimulated mineralization in human dental pulp cells by RNA sequencing

Human dental pulp cells (hDPCs) contain mesenchymal stem cells and are therefore indispensible for reparative dentin formation. Here, we present a pilot study of transcriptomic profiles of mineralized hDPCs isolated from sound human maxillary third molars. We observed altered gene expression of hDPCs between control (dexamethasone free) and experimental (dexamethasone 1 nm) groups. Differential expression analysis revealed up-regulation of several inflammation and mineralization-related genes in the experimental group. After a Gene Ontology analysis for predicting genes involved in biological process, cellular component and molecular function, we found enrichment of genes related to protein binding. Based on the results of Kyoto Encylopedia of Genes and Genomes pathway analysis, it is suggested up-regulated genes in mineralized hDPCs were mostly enriched in the mitogen-activated protein kinase (MAPK) signaling pathway, fluid shear stress and the atherosclerosis signaling pathway, etc. Importantly, Gene Set Enrichment Analysis revealed dexamethasone was positively related to the Janus kinase/signal transducer and activator of transcription, MAPK and Notch signaling pathway. Moreover, it was suggested that dexamethasone regulates signaling pathway in pluripotency of stem cells. Collectively, our work highlights transcriptome level gene regulation and intercellular interactions in mineralized hDPCs. The database produced in the present study paves the way for further investigations looking to explore genes that are involved in dental pulp cells mineralization.

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.

Teeth outside the mouth: The evolution and development of shark denticles

Vertebrate skin appendages are incredibly diverse. This diversity, which includes structures such as scales, feathers, and hair, likely evolved from a shared anatomical placode, suggesting broad conservation of the early development of these organs. Some of the earliest known skin appendages are dentine and enamel-rich tooth-like structures, collectively known as odontodes. These appendages evolved over 450 million years ago. Elasmobranchs (sharks, skates, and rays) have retained these ancient skin appendages in the form of both dermal denticles (scales) and oral teeth. Despite our knowledge of denticle function in adult sharks, our understanding of their development and morphogenesis is less advanced. Even though denticles in sharks appear structurally similar to oral teeth, there has been limited data directly comparing the molecular development of these distinct elements. Here, we chart the development of denticles in the embryonic small-spotted catshark (Scyliorhinus canicula) and characterize the expression of conserved genes known to mediate dental development. We find that shark denticle development shares a vast gene expression signature with developing teeth. However, denticles have restricted regenerative potential, as they lack a sox2+ stem cell niche associated with the maintenance of a dental lamina, an essential requirement for continuous tooth replacement. We compare developing denticles to other skin appendages, including both sensory skin appendages and avian feathers. This reveals that denticles are not only tooth-like in structure, but that they also share an ancient developmental gene set that is likely common to all epidermal appendages.

TGF-β signaling and Creb5 cooperatively regulate Fgf18 to control pharyngeal muscle development

The communication between myogenic cells and their surrounding connective tissues is indispensable for muscle morphogenesis. During late embryonic development in mice, myogenic progenitors migrate to discrete sites to form individual muscles. The detailed mechanism of this process remains unclear. Using mouse levator veli palatini (LVP) development as a model, we systematically investigated how a distinct connective tissue subpopulation, perimysial fibroblasts, communicates with myogenic cells to regulate mouse pharyngeal myogenesis. Using single-cell RNAseq data analysis, we identified that TGF-β signaling is a key regulator for the perimysial fibroblasts. Loss of TGF-β signaling in the neural crest-derived palatal mesenchyme leads to defects in perimysial fibroblasts and muscle malformation in the soft palate in Osr2Cre;Tgfbr1fl/fl mice. In particular, Creb5, a transcription factor expressed in the perimysial fibroblasts, cooperates with TGF-β signaling to activate expression of Fgf18. Moreover, Fgf18 supports pharyngeal muscle development in vivo and exogenous Fgf18 can partially rescue myogenic cell numbers in Osr2Cre;Tgfbr1fl/fl samples, illustrating that TGF-β-regulated Fgf18 signaling is required for LVP development. Collectively, our findings reveal the mechanism by which TGF-β signaling achieves its functional specificity in defining the perimysial-to-myogenic signals for pharyngeal myogenesis.

Transcriptome sequencing analysis of the role of β-catenin in F-actin reorganization in embryonic palatal mesenchymal cells

Background

Palatogenesis is a highly regulated and coordinated developmental process that is coordinated by multiple transcription factors and signaling pathways. Our previous studies identified that defective palatal shelf reorientation due to aberrant mesenchymal β-catenin signaling is associated with Filamentous actin (F-actin) dysregulation. Herein, the underlying mechanism of mesenchymal β-catenin in regulating F-actin cytoskeleton reorganization is further investigated.

Methods

Firstly, β-catenin silenced and overexpressed mouse embryonic palatal mesenchymal (MEPM) cells were established by adenovirus-mediated transduction. Subsequently, we compared transcriptomes of negative control (NC) group, β-catenin knockdown (KD) group, or β-catenin overexpression group respectively using RNA-sequencing (RNA-seq), and differentially expressed genes (DEGs) screened were further identified by quantitative real-time polymerase chain reaction (qRT-PCR). Finally, in vivo experiments further verified the expression change of critical molecules.

Results

We discovered 184 and 522 DEGs in the knockdown and overexpression groups compared to the NC group, respectively (adjusted P<0.05; |fold change| >2.0). Among these, 106 DEGs were altered in both groups. Gene Ontology (GO) enrichment analysis relating to biological functions identified cytokine-cytokine receptor interaction, and positive modulation of cellular migration. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment assessment indicated that these DEGs were chiefly linked by the regulation of signaling receptor activity and chemokine signaling pathways. Quantitative real-time polymerase chain reaction (qRT-PCR) results showed that the similar expression trend of serum amyloid A3 (Saa3) and CXC-chemokine ligand 5 (Cxcl5) possibly involved in cytoskeletal rearrangement with RNA-seq data. Experiments in vivo displayed that no significant expression change of Saa3 and Cxcl5 was observed in β-catenin knockout and overexpressed mouse models.

Conclusions

Our study provides an expression landscape of DEGs in β-catenin silenced and overexpressed MEPM cells, which emphasizes the important role of processes such as chemotactic factor and cell migration. Our data gain deeper insight into genes associated with F-actin reorganization that is regulated by β-catenin either directly or by another route, which will contribute to further investigation of the exact mechanism of mesenchymal β-catenin/F-actin in palatal shelf reorientation.

Disrupted tenogenesis in masseter as a potential cause of micrognathia

Micrognathia is a severe craniofacial deformity affecting appearance and survival. Previous studies revealed that multiple factors involved in the osteogenesis of mandibular bone have contributed to micrognathia, but concerned little on factors other than osteogenesis. In the current study, we found that ectopic activation of Fgf8 by Osr2-cre in the presumptive mesenchyme for masseter tendon in mice led to micrognathia, masseter regression, and the disrupted patterning and differentiation of masseter tendon. Since Myf5-cre;Rosa26R-Fgf8 mice exhibited the normal masseter and mandibular bone, the possibility that the micrognathia and masseter regression resulted directly from the over-expressed Fgf8 was excluded. Further investigation disclosed that a series of chondrogenic markers were ectopically activated in the developing Osr2-cre;Rosa26R-Fgf8 masseter tendon, while the mechanical sensing in the masseter and mandibular bone was obviously reduced. Thus, it suggested that the micrognathia in Osr2-cre;Rosa26R-Fgf8 mice resulted secondarily from the reduced mechanical force transmitted to mandibular bone. Consistently, when tenogenic or myogenic components were deleted from the developing mandibles, both the micrognathia and masseter degeneration took place with the decreased mechanical sensing in mandibular bone, which verified that the loss of mechanical force transmitted by masseter tendon could result in micrognathia. Furthermore, it appeared that the micrognathia resulting from the disrupted tenogenesis was attributed to the impaired osteogenic specification, instead of the differentiation in the periosteal progenitors. Our findings disclose a novel mechanism for mandibular morphogenesis, and shed light on the prevention and treatment for micrognathia.

MSX1 Drives Tooth Morphogenesis Through Controlling Wnt Signaling Activity

Tooth agenesis is a common structural birth defect in humans that results from failure of morphogenesis during early tooth development. The homeobox transcription factor Msx1 and the canonical Wnt signaling pathway are essential for "bud to cap" morphogenesis and are causal factors for tooth agenesis. Our recent study suggested that Msx1 regulates Wnt signaling during early tooth development by suppressing the expression of Dkk2 and Sfrp2 in the tooth bud mesenchyme, and it demonstrated partial rescue of Msx1-deficient molar teeth by a combination of DKK inhibition and genetic inactivation of SFRPs. In this study, we found that Sostdc1/Wise, another secreted Wnt antagonist, is involved in regulating the odontogenic pathway downstream of Msx1. Whereas Sostdc1 expression in the developing tooth germ was not increased in Msx1-/- embryos, genetic inactivation of Sostdc1 rescued maxillary molar, but not mandibular molar, morphogenesis in Msx1-/- mice with full penetrance. Since the Msx1-/-;Sostdc1-/- embryos exhibited ectopic Dkk2 expression in the developing dental mesenchyme, similar to Msx1-/- embryos, we generated and analyzed tooth development in Msx1-/-;Dkk2-/- double and Msx1-/-;Dkk2-/-;Sostdc1-/- triple mutant mice. The Msx1-/-;Dkk2-/- double mutants showed rescued maxillary molar morphogenesis at high penetrance, with a small percentage also exhibiting mandibular molars that transitioned to the cap stage. Furthermore, tooth development was rescued in the maxillary and mandibular molars, with full penetrance, in the Msx1-/-;Dkk2-/-;Sostdc1-/- mice. Together, these data reveal 1) that a key role of Msx1 in driving tooth development through the bud-to-cap transition is to control the expression of Dkk2 and 2) that modulation of Wnt signaling activity by Dkk2 and Sostdc1 plays a crucial role in the Msx1-dependent odontogenic pathway during early tooth morphogenesis.

A Neural Crest-specific Overexpression Mouse Model Reveals the Transcriptional Regulatory Effects of Dlx2 During Maxillary Process Development

Craniofacial morphogenesis is a complex process that requires precise regulation of cell proliferation, migration, and differentiation. Perturbations of this process cause a series of craniofacial deformities. Dlx2 is a critical transcription factor that regulates the development of the first branchial arch. However, the transcriptional regulatory functions of Dlx2 during craniofacial development have been poorly understood due to the lack of animal models in which the Dlx2 level can be precisely modulated. In this study, we constructed a Rosa26 site-directed Dlx2 gene knock-in mouse model Rosa26 ^{CAG-LSL-Dlx2-3xFlag} for conditionally overexpressing Dlx2. By breeding with wnt1 ^cre mice, we obtained wnt1 ^cre ; Rosa26 ^Dlx2/- mice, in which Dlx2 is overexpressed in neural crest lineage at approximately three times the endogenous level. The wnt1 ^cre ; Rosa26 ^Dlx2/- mice exhibited consistent phenotypes that include cleft palate across generations and individual animals. Using this model, we demonstrated that Dlx2 caused cleft palate by affecting maxillary growth and uplift in the early-stage development of maxillary prominences. By performing bulk RNA-sequencing, we demonstrated that Dlx2 overexpression induced significant changes in many genes associated with critical developmental pathways. In summary, our novel mouse model provides a reliable and consistent system for investigating Dlx2 functions during development and for elucidating the gene regulatory networks underlying craniofacial development.

The protoconid: a key cusp in lower molars. Evidence from a recent modern human population

BACKGROUND

The molar (M) size sequence in the genus Homo is decreasing and the general pattern in Homo sapiens is M1 > M2 > M3.

AIM

To gain a better understanding of the reduction patterns of molar components (cusps), we aim to assess the area of the protoconid, the phylogenetically oldest cusp of the lower molars.

SUBJECT AND METHODS

We measured the protoconid and the total crown area in the scaled photographs of a recent modern human sample of lower molars (76 males and 39 females). The values were statistically analysed.

RESULTS

The absolute size of the protoconid increases significantly between M1 and M2/M3, whereas the relative size of this cusp increases significantly from M1 to M3. In the latter, reduction or disappearance of the cusps of the talonid is common.

CONCLUSIONS

The results can be explained in the framework of the patterning cascade model. As the first cusp to appear developmentally, the protoconid forms in response to signals from the primary enamel knot, likely contributing to its stability. Inhibitory signals emitted during the protoconid formation may lead to the reduction or disappearance of the talonid cusps, if these do not have enough time to form before the end of the molar morphogenetic process.

Redeployment of odontode gene regulatory network underlies dermal denticle formation and evolution in suckermouth armored catfish

Odontodes, i.e., teeth and tooth-like structures, consist of a pulp cavity and dentin covered by a mineralized cap. These structures first appeared on the outer surface of vertebrate ancestors and were repeatedly lost and gained across vertebrate clades; yet, the underlying genetic mechanisms and trajectories of this recurrent evolution remain long-standing mysteries. Here, we established suckermouth armored catfish (Ancistrus sp.; Loricariidae), which have reacquired dermal odontodes (dermal denticles) all over most of their body surface, as an experimental model animal amenable to genetic manipulation for studying odontode development. Our histological analysis showed that suckermouth armored catfish develop dermal denticles through the previously defined odontode developmental stages. De novo transcriptomic profiling identified the conserved odontode genetic regulatory network (oGRN) as well as expression of paired like homeodomain 2 (pitx2), previously known as an early regulator of oGRN in teeth but not in other dermal odontodes, in developing dermal denticles. The early onset of pitx2 expression in cranial dermal denticle placodes implies its function as one of the inducing factors of the cranial dermal denticles. By comprehensively identifying the genetic program for dermal odontode development in suckermouth armored catfish, this work illuminates how dermal odontodes might have evolved and diverged in distinct teleost lineages via redeployment of oGRN.

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.

Hypoxia-inducible factor 1α induces osteo/odontoblast differentiation of human dental pulp stem cells via Wnt/β-catenin transcriptional cofactor BCL9

Accelerated dental pulp mineralization is a common complication in avulsed/luxated teeth, although the mechanisms underlying this remain unclear. We hypothesized that hypoxia due to vascular severance may induce osteo/odontoblast differentiation of dental pulp stem cells (DPSCs). This study examined the role of B-cell CLL/lymphoma 9 (BCL9), which is downstream of hypoxia-inducible factor 1α (HIF1α) and a Wnt/β-catenin transcriptional cofactor, in the osteo/odontoblastic differentiation of human DPSCs (hDPSCs) under hypoxic conditions. hDPSCs were isolated from extracted healthy wisdom teeth. Hypoxic conditions and HIF1α overexpression induced significant upregulation of mRNAs for osteo/odontoblast markers (RUNX2, ALP, OC), BCL9, and Wnt/β-catenin signaling target genes (AXIN2, TCF1) in hDPSCs. Overexpression and suppression of BCL9 in hDPSCs up- and downregulated, respectively, the mRNAs for AXIN2, TCF1, and the osteo/odontoblast markers. Hypoxic-cultured mouse pulp tissue explants showed the promotion of HIF1α, BCL9, and β-catenin expression and BCL9-β-catenin co-localization. In addition, BCL9 formed a complex with β-catenin in hDPSCs in vitro. This study demonstrated that hypoxia/HIF1α-induced osteo/odontoblast differentiation of hDPSCs was partially dependent on Wnt/β-catenin signaling, where BCL9 acted as a key mediator between HIF1α and Wnt/β-catenin signaling. These findings may reveal part of the mechanisms of dental pulp mineralization after traumatic dental injury.

KMT2D deficiency disturbs the proliferation and cell cycle activity of dental epithelial cell line (LS8) partially via Wnt signaling

Lysine methyltransferase 2D (KMT2D), as one of the key histone methyltransferases responsible for histone 3 lysine 4 methylation (H3K4me), has been proved to be the main pathogenic gene of Kabuki syndrome disease. Kabuki patients with KMT2D mutation frequently present various dental abnormalities, including abnormal tooth number and crown morphology. However, the exact function of KMT2D in tooth development remains unclear. In this report, we systematically elucidate the expression pattern of KMT2D in early tooth development and outline the molecular mechanism of KMT2D in dental epithelial cell line. KMT2D and H3K4me mainly expressed in enamel organ and Kmt2d knockdown led to the reduction in cell proliferation activity and cell cycling activity in dental epithelial cell line (LS8). RNA-sequencing (RNA-seq) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis screened out several important pathways affected by Kmt2d knockdown including Wnt signaling. Consistently, Top/Fop assay confirmed the reduction in Wnt signaling activity in Kmt2d knockdown cells. Nuclear translocation of β-catenin was significantly reduced by Kmt2d knockdown, while lithium chloride (LiCl) partially reversed this phenomenon. Moreover, LiCl partially reversed the decrease in cell proliferation activity and G₁ arrest, and the down-regulation of Wnt-related genes in Kmt2d knockdown cells. In summary, the present study uncovered a pivotal role of histone methyltransferase KMT2D in dental epithelium proliferation and cell cycle homeostasis partially through regulating Wnt/β-catenin signaling. The findings are important for understanding the role of KMT2D and histone methylation in tooth development.

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.

The Role of Wnt Signaling in Postnatal Tooth Root Development

Appropriate tooth root formation and tooth eruption are critical for achieving and maintaining good oral health and quality of life. Tooth eruption is the process through which teeth emerge from their intraosseous position to their functional position in the oral cavity. This temporospatial process occurs simultaneously with tooth root formation through a cascade of interactions between the epithelial and adjoining mesenchymal cells. Here, we will review the role of the Wnt system in postnatal tooth root development. This signaling pathway orchestrates the process of tooth root formation and tooth eruption in conjunction with several other major signaling pathways. The Wnt signaling pathway is comprised of the canonical, or Wnt/β-catenin, and the non-Canonical signaling pathway. The expression of multiple Wnt ligands and their downstream transcription factors including β-catenin is found in the cells in the epithelia and mesenchyme starting from the initiation stage of tooth development. The inhibition of canonical Wnt signaling in an early stage arrests odontogenesis. Wnt transcription factors continue to be present in dental follicle cells, the progenitor cells responsible for differentiation into cells constituting the tooth root and the periodontal tissue apparatus. This expression occurs concurrently with osteogenesis and cementogenesis. The conditional ablation of β-catenin in osteoblast and odontoblast causes the malformation of the root dentin and cementum. On the contrary, the overexpression of β-catenin led to shorter molar roots with thin and hypo-mineralized dentin, along with the failure of tooth eruption. Therefore, the proper expression of Wnt signaling during dental development is crucial for regulating the proliferation, differentiation, as well as epithelial-mesenchymal interaction essential for tooth root formation and tooth eruption.

The concurrent stimulation of Wnt and FGF8 signaling induce differentiation of dental mesenchymal cells into odontoblast-like cells

Fibroblast growth factor 8 (FGF8) is known to be a potent stimulator of canonical Wnt/β-catenin activity, an essential factor for tooth development. In this study, we analyzed the effects of co-administration of FGF8 and a CHIR99021 (GSK3β inhibitor) on differentiation of dental mesenchymal cells into odontoblasts. Utilizing Cre-mediated EGFP reporter mice, dentin matrix protein 1 (Dmp1) expression was examined in mouse neonatal molar tooth germs. At birth, expression of Dmp1-EGFP was not found in mesenchymal cells but rather epithelial cells, after which Dmp1-positive cells gradually emerged in the mesenchymal area along with disappearance in the epithelial area. Primary cultured mesenchymal cells from neonatal tooth germ specimens showed loss of Dmp1-EGFP positive signals, whereas addition of Wnt3a or the CHIR99021 significantly regained Dmp1 positivity within approximately 2 weeks. Other odontoblast markers such as dentin sialophosphoprotein (Dspp) could not be clearly detected. Concurrent stimulation of primary cultured mesenchymal cells with the CHIR99021 and FGF8 resulted in significant upregulation of odonto/osteoblast proteins. Furthermore, increased expression levels of runt-related transcription factor 2 (Runx2), osterix, and osteocalcin were also observed. The present findings indicate that coordinated action of canonical Wnt/β-catenin and FGF8 signals is essential for odontoblast differentiation of tooth germs in mice.

Noggin Overexpression Impairs the Development of Muscles, Tendons, and Aponeurosis in Soft Palates by Disrupting BMP-Smad and Shh-Gli1 Signaling

The roles of bone morphogenetic protein (BMP) signaling in palatogenesis were well documented in the developing hard palate; however, little is known about how BMP signaling regulates the development of soft palate. In this study, we overexpressed Noggin transgene via Osr2-cre ^KI allele to suppress BMP signaling in the developing soft palate. We found that BMP-Smad signaling was detected in the palatal muscles and surrounding mesenchyme. When BMP-Smad signaling was suppressed by the overexpressed Noggin, the soft palatal shelves were reduced in size with the hypoplastic muscles and the extroversive hypophosphatasia (HPP). The downregulated cell proliferation and survival in the Osr2-cre ^KI ;pMes-Noggin soft palates were suggested to result from the repressed Shh transcription and Gli1 activity, implicating that the BMP-Shh-Gli1 network played a similar role in soft palate development as in the hard palate. The downregulated Sox9, Tenascin-C (TnC), and Col1 expression in Osr2-cre ^KI ;pMes-Noggin soft palate indicated the impaired differentiation of the aponeurosis and tendons, which was suggested to result in the hypoplasia of palatal muscles. Intriguingly, in the Myf5-cre ^KI ;pMes-Noggin and the Myf5-cre ^KI ;Rosa26R-DTA soft palates, the hypoplastic or abrogated muscles affected little the fusion of soft palate. Although the Scx, Tnc, and Co1 transcription was significantly repressed in the tenogenic mesenchyme of the Myf5-cre ^KI ;pMes-Noggin soft palate, the Sox9 expression, and the Tnc and Col1 transcription in aponeurosis mesenchyme were almost unaffected. It implicated that the fusion of soft palate was controlled by the mesenchymal clues at the tensor veli palatini (TVP) and levator veli palatini (LVP) levels, but by the myogenic components at the palatopharyngeus (PLP) level.

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.

Sox9CreER-mediated deletion of β-catenin in palatal mesenchyme results in delayed palatal elevation accompanied with repressed canonical Wnt signaling and reduced actin polymerization

Cleft palate is a good model to pushing us toward a deeper understanding of the molecular mechanisms of spatiotemporal patterns in tissues and organisms because of the multiple-step processes such as elevation and fusion. Previous studies have shown that the epithelial β-catenin is crucial for palatal fusion, however, the function of the mesenchymal β-catenin remains elusive. We investigate the role of mesenchymal β-catenin in palatal development by generating a β-catenin conditional knockout mouse (CKO) (Sox9CreER; Ctnnb1^F/F ). We found that the CKO mice exhibited delayed palatal elevation, leading to cleft palate in both in vivo and ex vivo. Abnormal cell proliferation and repressed mesenchymal canonical Wnt signaling were found in the CKO palate. Interestingly, Filamentous actin (F-actin) polymerization was significantly reduced in the palatal mesenchyme of mutant embryos. Furthermore, overexpression of adenovirus-mediated transfection with Acta1 in the mutant could help to elevate the palatal shelves but could not prevent cleft palate in ex vivo. Our results suggest that conditionally knock out β-catenin in the palatal mesenchyme by Sox9CreER leading to delayed palatal elevation, which results in repressed mesenchymal canonical Wnt signaling, decreased cell proliferation, and reduced actin polymerization, finally causes cleft palate.

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.

Early perturbation of Wnt signaling reveals patterning and invagination-evagination control points in molar tooth development

Tooth formation requires complex signaling interactions both within the oral epithelium and between the epithelium and the underlying mesenchyme. Previous studies of the Wnt/β-catenin pathway have shown that tooth formation is partly inhibited in loss-of-function mutants, and gain-of-function mutants have perturbed tooth morphology. However, the stage at which Wnt signaling is first important in tooth formation remains unclear. Here, using an Fgf8-promoter-driven, and therefore early, deletion of β-catenin in mouse molar epithelium, we found that loss of Wnt/β-catenin signaling completely deletes the molar tooth, demonstrating that this pathway is central to the earliest stages of tooth formation. Early expression of a dominant-active β-catenin protein also perturbs tooth formation, producing a large domed evagination at early stages and supernumerary teeth later on. The early evaginations are associated with premature mesenchymal condensation marker, and are reduced by inhibition of condensation-associated collagen synthesis. We propose that invagination versus evagination morphogenesis is regulated by the relative timing of epithelial versus mesenchymal cell convergence regulated by canonical Wnt signaling. Together, these studies reveal new aspects of Wnt/β-catenin signaling in tooth formation and in epithelial morphogenesis more broadly.

Wnt1 Promotes Cementum and Alveolar Bone Growth in a Time-Dependent Manner

The WNT/β-catenin signaling pathway plays a central role in the biology of the periodontium, yet the function of specific extracellular WNT ligands remains poorly understood. By using a Wnt1-inducible transgenic mouse model targeting Col1a1-expressing alveolar osteoblasts, odontoblasts, and cementoblasts, we demonstrate that the WNT ligand WNT1 is a strong promoter of cementum and alveolar bone formation in vivo. We induced Wnt1 expression for 1, 3, or 9 wk in Wnt1Tg mice and analyzed them at the age of 6 wk and 12 wk. Micro–computed tomography (CT) analyses of the mandibles revealed a 1.8-fold increased bone volume after 1 and 3 wk of Wnt1 expression and a 3-fold increased bone volume after 9 wk of Wnt1 expression compared to controls. In addition, the alveolar ridges were higher in Wnt1Tg mice as compared to controls. Nondecalcified histology demonstrated increased acellular cementum thickness and cellular cementum volume after 3 and 9 wk of Wnt1 expression. However, 9 wk of Wnt1 expression was also associated with periodontal breakdown and ectopic mineralization of the pulp. The composition of this ectopic matrix was comparable to those of cellular cementum as demonstrated by quantitative backscattered electron imaging and immunohistochemistry for noncollagenous proteins. Our analyses of 52-wk-old mice after 9 wk of Wnt1 expression revealed that Wnt1 expression affects mandibular bone and growing incisors but not molar teeth, indicating that Wnt1 influences only growing tissues. To further investigate the effect of Wnt1 on cementoblasts, we stably transfected the cementoblast cell line (OCCM-30) with a vector expressing Wnt1-HA and performed proliferation as well as differentiation experiments. These experiments demonstrated that Wnt1 promotes proliferation but not differentiation of cementoblasts. Taken together, our findings identify, for the first time, Wnt1 as a critical regulator of alveolar bone and cementum formation, as well as provide important insights for harnessing the WNT signal pathway in regenerative dentistry.

The role of canonical Wnt signaling in dentin bridge formation

OBJECTIVE

Wnt signaling has been reported to be involved in dentin bridge formation. However, the detailed mechanism has not yet been clarified. We elucidated the localization of canonical Wnt signaling molecules during dentin bridge formation.

METHODS

Pulp of the maxillary first molar in mice was exposed and directly capped with MTA cement. Maxillae were collected on the 1st, 4th, 7th, 14th, and 28th days after treatment. After μCT analysis, immunohistochemistry for Wnt3a, Wnt10a, β-catenin, F4/80, and osterix was performed in paraffin-embedded sections.

RESULTS

On the 4th and 7th days after pulp capping, odontoblasts and dental pulp cells expressed Wnt3a, Wnt10a, and β-catenin. On the 14th day, reactionary dentin was formed around the pulp exposure area. Odontoblasts and dental pulp cells express Wnt3a, Wnt10a, and β-catenin. Additionally, F4/80- and Wnt10a-positive macrophages were observed at the center of the dental pulp. When the dentin bridge was formed on the 28th day, reparative odontoblasts expressed Wnt3a, β-catenin and osterix.

CONCLUSION

Wnt ligands derived from odontoblasts and dental pulp cells are important for the activation of odontoblasts and the differentiation of reparative odontoblasts during dentin bridge formation. Macrophage-derived Wnts are also involved in reparative odontoblast differentiation.

Tissue-specific analysis of Fgf18 gene function in palate development

BACKGROUND

Previous studies showed that mice lacking Fgf18 function had cleft palate defects and that the FGF18 locus was associated with cleft lip and palate in humans, but what specific roles Fgf18 plays during palatogenesis are unclear.

RESULTS

We show that Fgf18 exhibits regionally restricted expression in developing palatal shelves, mandible, and tongue, during palatal outgrowth and fusion in mouse embryos. Tissue-specific inactivation of Fgf18 throughout neural crest-derived craniofacial mesenchyme caused shortened mandible and reduction in ossification of the frontal, nasal, and anterior cranial base skeletal elements in Fgf18c/c ;Wnt1-Cre mutant mice. About 64% of Fgf18c/c ;Wnt1-Cre mice exhibited cleft palate. Whereas palatal shelf elevation was impaired in many Fgf18c/c ;Wnt1-Cre embryos, no significant difference in palatal cell proliferation was detected between Fgf18c/c ;Wnt1-Cre embryos and their control littermates. Embryonic maxillary explants from Fgf18c/c ;Wnt1-Cre embryos showed successful palatal shelf elevation and fusion in organ culture similar to the maxillary explants from control embryos. Furthermore, tissue-specific inactivation of Fgf18 in the early palatal mesenchyme did not cause cleft palate.

CONCLUSION

These results demonstrate a critical role for Fgf18 expression in the neural crest-derived mesenchyme for the development of the mandible and multiple craniofacial bones but Fgf18 expression in the palatal mesenchyme is dispensable for palatogenesis.

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.