Hedgehog pathway gene expression during early development of the molar tooth root in the mouse

The oligodontia phenotype in a X-linked hypohidrotic ectodermal dysplasia patient with a novel EVC2 variant

Objectives

To analyse the pathogenic genes in a patient with hypohidrotic ectodermal dysplasia (HED) and explore the relationship between pathogenic genes and the oligodontia phenotype.

Methods

Clinical data and peripheral blood were collected from a patient with HED. Pathogenic genes were analysed by whole-exon sequencing (WES) and verified by Singer sequencing. The secondary and tertiary structures of the variant proteins were predicted to analyse their toxicity.

Results

The patient exhibited a severe oligodontia phenotype, wherein only two deciduous canines were left in the upper jaw. WES revealed a hemizygous EDA variant c.466C > T p.(Arg156Cys) and a novel heterozygous EVC2 variant c.1772T > C p.(Leu591Ser). Prediction of the secondary and tertiary structures of the EDA variant p.(Arg156Cys) and EVC2 variant p.(Leu591Ser) indicated impaired function of both molecules.

Conclusion

The patient demonstrated a more severe oligodontia phenotype when compared with the other patients caused by the EDA variant c.466C > T. Since Evc2 is a positive regulator of the Sonic Hedgehog (Shh) signal pathway, we speculated that the EVC2 variant p.(Leu591Ser) may play a synergistic role in the oligodontia phenotype of HED, thereby exacerbating the oligodontia phenotype. Knowledge of oligodontia caused by multiple gene variants is of great significance for understanding individual differences in oligodontia phenotypes.

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 Sonic Hedgehog–Patched–Gli Signaling Pathway Maintains Dental Epithelial and Pulp Stem/Progenitor Cells and Regulates the Function of Odontoblasts

This study aimed to elucidate the role of the Sonic hedgehog (Shh)–Patched (Ptch)–Gli signaling pathway in maintaining dental epithelial and pulp stem/progenitor cells and regulating the function of odontoblasts. Doxycycline (dox)-inducible histone 2B (H2B)–green fluorescent protein (GFP) transgenic mice ingested dox at prenatal embryonic days 14.5 or 15.5 and their offspring were collected from postnatal day 1 (P1) to week 3 (P3W). Immunohistochemistry for Gli1, Ptch1, and Ptch2 andin situhybridization forShhandPtch1were conducted. Mandibular incisors of postnatal day 2 H2B-GFP transgenic and wild-type mice were cultivated in a nutrient medium with Shh antibody for 4 days and subsequently processed for immunohistochemistry for Sox2. In molars, dense H2B-GFP-label-retaining cells (H2B-GFP-LRCs) were densely distributed throughout the dental pulp during P1 to postnatal week 2 (P2W) and decreased in number by postnatal P3W, whereas the number of dense H2B-GFP-LRCs in the subodontoblastic layer increased in number at P2W. Gli1+and Pthc1+cells were distributed throughout the enamel organ and dental pulp, including the odontoblast and subodontoblastic layers.ShhmRNA was expressed in the inner enamel epithelium and shifted into odontoblasts after dentin deposition.Ptch1mRNA was expressed in the inner enamel epithelium and cuspal pulpal tissue on P1 and decreased in intensity from postnatal week 1 to P3W. In incisors, the apical bud contained H2B-GFP-LRCs, Gli1+cells, and Ptch1+cells. The addition of Shh antibody to explants induced a decrease in the number of Sox2+cells due to the increase in apoptotic cells in the apical bud. Thus, the Shh–Ptch–Gli signaling pathway plays a role in maintaining quiescent adult stem cells and regulating the function of odontoblasts.

Sonic Hedgehog Signaling and Tooth Development

Sonic hedgehog (Shh) is a secreted protein with important roles in mammalian embryogenesis. During tooth development, Shh is primarily expressed in the dental epithelium, from initiation to the root formation stages. A number of studies have analyzed the function of Shh signaling at different stages of tooth development and have revealed that Shh signaling regulates the formation of various tooth components, including enamel, dentin, cementum, and other soft tissues. In addition, dental mesenchymal cells positive for Gli1, a downstream transcription factor of Shh signaling, have been found to have stem cell properties, including multipotency and the ability to self-renew. Indeed, Gli1-positive cells in mature teeth appear to contribute to the regeneration of dental pulp and periodontal tissues. In this review, we provide an overview of recent advances related to the role of Shh signaling in tooth development, as well as the contribution of this pathway to tooth homeostasis and regeneration.

Mesenchymal Progenitor Regulation of Tooth Eruption: A View from PTHrP

Tooth eruption is a unique biological process by which highly mineralized tissues emerge into the outer world, and it occurs concomitantly with tooth root formation. These 2 processes have been considered independent phenomena; however, recent studies support the theory that they are indeed intertwined. Dental mesenchymal progenitor cells in the dental follicle lie at the heart of the coupling of these 2 processes, providing a source for diverse mesenchymal cells that support formation of the highly functional tooth root and the periodontal attachment apparatus, while facilitating formation of osteoclasts. These cells are regulated by autocrine signaling by parathyroid hormone-related protein (PTHrP) and its parathyroid hormone/PTHrP receptor PPR. This PTHrP-PPR signaling appears to crosstalk with other signaling pathways and regulates proper cell fates of mesenchymal progenitor cell populations. Disruption of this autocrine PTHrP-PPR signaling in these cells leads to defective formation of the periodontal attachment apparatus, tooth root malformation, and failure of tooth eruption in molars, which essentially recapitulate primary failure of eruption in humans, a rare genetic disorder exclusively affecting tooth eruption. Diversity and distinct functionality of these mesenchymal progenitor cell populations that regulate tooth eruption and tooth root formation are beginning to be unraveled.

The Impact of the Eda Pathway on Tooth Root Development

The Eda pathway ( Eda, Edar, Edaradd) plays an important role in tooth development, determining tooth number, crown shape, and enamel formation. Here we show that the Eda pathway also plays a key role in root development. Edar (the receptor) is expressed in Hertwig's epithelial root sheath (HERS) during root development, with mutant mice showing a high incidence of taurodontism: large pulp chambers lacking or showing delayed bifurcation or trifurcation of the roots. The mouse upper second molars in the Eda pathway mutants show the highest incidence of taurodontism, this enhanced susceptibility being matched in human patients with mutations in EDA-A1. These taurodont teeth form due to defects in the direction of extension of the HERS from the crown, associated with a more extensive area of proliferation of the neighboring root mesenchyme. In those teeth where the angle at which the HERS extends from the crown is very wide and therefore more vertical, the mutant HERSs fail to reach toward the center of the tooth in the normal furcation region, and taurodont teeth are created. The phenotype is variable, however, with milder changes in angle and proliferation leading to normal or delayed furcation. This is the first analysis of the role of Eda in the root, showing a direct role for this pathway during postnatal mouse development, and it suggests that changes in proliferation and angle of HERS may underlie taurodontism in a range of syndromes.

Sonic Hedgehog Signaling and Development of the Dentition

Sonic hedgehog (Shh) is an essential signaling peptide required for normal embryonic development. It represents a highly-conserved marker of odontogenesis amongst the toothed vertebrates. Signal transduction is involved in early specification of the tooth-forming epithelium in the oral cavity, and, ultimately, in defining tooth number within the established dentition. Shh also promotes the morphogenetic movement of epithelial cells in the early tooth bud, and influences cell cycle regulation, morphogenesis, and differentiation in the tooth germ. More recently, Shh has been identified as a stem cell regulator in the continuously erupting incisors of mice. Here, we review contemporary data relating to the role of Shh in odontogenesis, focusing on tooth development in mammals and cartilaginous fishes. We also describe the multiple actions of this signaling protein at the cellular level.

Hypodontia: An Update on Its Etiology, Classification, and Clinical Management

Hypodontia, or tooth agenesis, is the most prevalent craniofacial malformation in humans. It may occur as part of a recognised genetic syndrome or as a nonsyndromic isolated trait. Excluding third molars, the reported prevalence of hypodontia ranges from 1.6 to 6.9%, depending on the population studied. Most affected individuals lack only one or two teeth, with permanent second premolars and upper lateral incisors the most likely to be missing. Both environmental and genetic factors are involved in the aetiology of hypodontia, with the latter playing a more significant role. Hypodontia individuals often present a significant clinical challenge for orthodontists because, in a number of cases, the treatment time is prolonged and the treatment outcome may be compromised. Hence, the identification of genetic and environmental factors may be particularly useful in the early prediction of this condition and the development of prevention strategies and novel treatments in the future.

Dickkopf-related protein 3 negatively regulates the osteogenic differentiation of rat dental follicle cells

The present study aimed to investigate the effect of Dickkopf-related protein 3 (DKK3) on osteogenic differentiation of rat dental follicle cells (DFCs). A PCR array analysis of Wnt pathway activation in DFCs identified genes dysregulated by mineral induction. Among them, DKK3expression levels were decreased, and further experiments were conducted to investigate its role in DFC osteogenesis. By comparing DFCs grown in normal growth and mineral-induction media for 4 weeks, the present study confirmed that DKK3 was a potential target gene of osteogenesis through reverse transcription-quantitative polymerase chain reaction (RT-qPCR) and western blotting (WB). A short hairpin RNA (shRNA) was introduced into DFCs using a lentiviral vector to inhibit DKK3 expression. An alkaline phosphatase (ALP) activity assay and Alizarin Red staining were performed to observe the DKK3-shRNA DFCs. In addition, the osteogenic differentiation of DKK3-shRNA DFCs was analyzed by RT-qPCR and WB. In vivo, DKK3-shRNA DFCs seeded on hydroxyapatite/β-tricalcium phosphate (HA/TCP) scaffolds were transplanted into the subcutaneous tissue of mice with severe combined immunodeficiency, followed by hematoxylin-eosin and Masson staining. The results confirmed that DKK3 expression was downregulated during mineral induction in rat DFCs. Lentivirus-mediated expression of DKK3 shRNA in DFCs promoted calcified-nodule formation, ALP activity and the expression of β-catenin, runt-related transcription factor 2 and osteocalcin, compared with control cells. In vivo, the implanted section presented the majority of newly formed osteoid matrices and collagen, with limited space between the HA/TCP scaffolds and matrices. In conclusion, DKK3 expression negatively regulates the osteogenic differentiation of DFCs and, conversely, downregulation of DKK3 may enhance DFC osteogenesis.

Sonic Hedgehog Promotes Cementoblastic Differentiation via Activating the BMP Pathways

Although sonic hedgehog (SHH), an essential molecule in embryogenesis and organogenesis, stimulates proliferation of human periodontal ligament (PDL) stem cells, the effects of recombinant human SHH (rh-SHH) on osteoblastic differentiation are unclear. To reveal the role of SHH in periodontal regeneration, expression of SHH in mouse periodontal tissues and its effects on the osteoblastic/cementoblastic differentiation in human cementoblasts were investigated. SHH is immunolocalized to differentiating cementoblasts, PDL cells, and osteoblasts of the developing mouse periodontium. Addition of rh-SHH increased cell growth, ALP activity, and mineralization nodule formation, and upregulated mRNA expression of osteoblastic and cementoblastic markers. The osteoblastic/cementoblastic differentiation of rh-SHH was abolished by the SHH inhibitor cyclopamine (Cy) and the BMP antagonist noggin. rh-SHH increased the expression of BMP-2 and -4 mRNA, as well as levels of phosphorylated Akt, ERK, p38, and JNK, and of MAPK and NF-κB activation, which were reversed by noggin, Cy, and BMP-2 siRNA. Collectively, this study is the first to demonstrate that SHH can promote cell growth and cell osteoblastic/cementoblastic differentiation via BMP pathway. Thus, SHH plays important roles in the development of periodontal tissue, and might represent a new therapeutic target for periodontitis and periodontal regeneration.

The role of the wnt/β-catenin signaling pathway in formation and maintenance of bone and teeth

The Wnt signaling pathway is known as one of the important molecular cascades that regulate cell fate throughout lifespan. The Wnt signaling pathway is further separated into the canonical signaling pathway that depends on the function of β-catenin (Wnt/β-catenin pathway) and the noncanonical pathways that operate independently of β-catenin (planar cell polarity pathway and Wnt/Ca(2+) pathway). The Wnt/β-catenin signaling pathway is complex and consists of numerous receptors, inhibitors, activators, modulators, phosphatases, kinases and other components. However, there is one central, critical molecule to this pathway, β-catenin. While there are at least 3 receptors, LRP 4, 5 and 6, and over twenty activators known as the wnts, and several inhibitors such as sclerostin, dickkopf and secreted frizzled-related protein, these all target β-catenin. These regulators/modulators function to target β-catenin either to the proteasome for degradation or to the nucleus to regulate gene expression. Therefore, the interaction of β-catenin with different factors and Wnt/β-catenin signaling pathway will be the subject of this review with a focus on how this pathway relates to and functions in the formation and maintenance of bone and teeth based on mainly basic and pre-clinical research. Also in this review, the role of this pathway in osteocytes, bone cells embedded in the mineralized matrix, is covered in depth. This pathway is not only important in mineralized tissue growth and development, but for modulation of the skeleton in response to loading and unloading and the viability and health of the adult and aging skeleton.

An Nfic-hedgehog signaling cascade regulates tooth root development

Coordination between the Hertwig's epithelial root sheath (HERS) and apical papilla (AP) is crucial for proper tooth root development. The hedgehog (Hh) signaling pathway and Nfic are both involved in tooth root development; however, their relationship has yet to be elucidated. Here, we establish a timecourse of mouse molar root development by histological staining of sections, and we demonstrate that Hh signaling is active before and during root development in the AP and HERS using Gli1 reporter mice. The proper pattern of Hh signaling activity in the AP is crucial for the proliferation of dental mesenchymal cells, because either inhibition with Hh inhibitors or constitutive activation of Hh signaling activity in transgenic mice leads to decreased proliferation in the AP and shorter roots. Moreover, Hh activity is elevated in Nfic(-/-) mice, a root defect model, whereas RNA sequencing and in situ hybridization show that the Hh attenuator Hhip is downregulated. ChIP and RNAscope analyses suggest that Nfic binds to the promoter region of Hhip. Treatment of Nfic(-/-) mice with Hh inhibitor partially restores cell proliferation, AP growth and root development. Taken together, our results demonstrate that an Nfic-Hhip-Hh signaling pathway is crucial for apical papilla growth and proper root formation. This discovery provides insight into the molecular mechanisms regulating tooth root development.

Wnt/β-catenin signaling regulates Dental Pulp Stem Cells' responses to pulp injury by resinous monomers

OBJECTIVES

Aim of this study was to investigate whether Dental Pulp Stem Cells-DPSCs responses to pulp injury caused by resinous monomers is be mediated through activation of Wnt/β-catenin signaling.

METHODS

DPSCs cultures were established from third molars of healthy donors and characterized for stem cell markers with flow cytometry. Cells were exposed to TEGDMA (T: 0.5-2mM) with or without presence of the Wnt-1 ligand (W:25-100ng/ml) or the GSK3β inhibitor Lithium (L:1-10mM), used both as activators of Wnt/β-catenin signaling. Cell viability was evaluated by MTT assay, cell cycle profiles by flow cytometry and expression of key molecules of Wnt/β-catenin signaling by Real-time PCR and Western Blot.

RESULTS

DPSC exposure to TEGDMA caused a concentration-dependent cytotoxicity, accompanied by G1 arrest at lower and G2/M arrest at higher concentrations or after prolonged exposure. Lithium caused a dual effect, by stimulating/inhibiting cell proliferation at lower/higher concentrations respectively and causing a G2/M arrest in a concentration-dependent manner. Wnt signaling could be activated in DPSCs after Lithium or Wnt-1 treatment, as shown by accumulation of β-catenin, its translocation into the nucleus and enhanced expression of key pathway players, like LEF1 and Cyclin D1. Importantly, exposure to TEGDMA caused a more pronounced activation of the pathway, whereas cumulative effects were observed after T/L or T/W co-treatment, indicating a very strong activation of Wnt signaling after treatment of already "activated" (by Lithium or Wnt-1) cells with TEGDMA.

SIGNIFICANCE

These findings highlight the important role of Wnt canonical signaling in pulp repair responses to common injuries.

Tooth bioengineering leads the next generation of dentistry

BACKGROUND

As a result of numerous rapid and exciting developments in tissue engineering technology, scientists are able to regenerate a fully functional tooth in animal models, from a bioengineered tooth germ. Advances in technology, together with our understanding of the mechanisms of tooth development and studies dealing with dentally derived stem cells, have led to significant progress in the field of tooth regeneration.

AIM AND DESIGN

This review focuses on some of the recent advances in tooth bioengineering technology, the signalling pathways in tooth development, and in dental stem cell biology. These factors are highlighted in respect of our current knowledge of tooth regeneration.

RESULTS AND CONCLUSION

An understanding of these new approaches in tooth regeneration should help to prepare clinicians to use this new and somewhat revolutionary therapy while also enabling them to partake in future clinical trials. Tooth bioengineering promises to be at the forefront of the next generation of dental treatments.

Developmental basis of toothlessness in turtles: insight into convergent evolution of vertebrate morphology

The tooth is a major component of the vertebrate feeding apparatus and plays a crucial role in species survival, thus subjecting tooth developmental programs to strong selective constraints. However, irrespective of their functional importance, teeth have been lost in multiple lineages of tetrapod vertebrates independently. To understand both the generality and the diversity of developmental mechanisms that cause tooth agenesis in tetrapods, we investigated expression patterns of a series of tooth developmental genes in the lower jaw of toothless turtles and compared them to that of toothed crocodiles and the chicken as a representative of toothless modern birds. In turtle embryos, we found impairment of Shh signaling in the oral epithelium and early-stage arrest of odontoblast development caused by termination of Msx2 expression in the dental mesenchyme. Our data indicate that such changes underlie tooth agenesis in turtles and suggest that the mechanism that leads to early-stage odontogenic arrest differs between birds and turtles. Our results demonstrate that the cellular and molecular mechanisms that regulate early-stage arrest of tooth development are diverse in tetrapod lineages, and odontogenic developmental programs may respond to changes in upstream molecules similarly thereby evolving convergently with feeding morphology.

Future dentistry: cell therapy meets tooth and periodontal repair and regeneration

Cell-based tissue repair of the tooth and – tooth-supporting – periodontal ligament (PDL) is a new attractive approach that complements traditional restorative or surgical techniques for replacement of injured or pathologically damaged tissues. In such therapeutic approaches, stem cells and/or progenitor cells are manipulated in vitro and administered to patients as living and dynamic biological agents. In this review, we discuss the clonogenic potential of human dental and periodontal tissues such as the dental pulp and the PDL and their potential for tooth and periodontal repair and/or regeneration. We propose novel therapeutic approaches using stem cells or progenitor cells, which are targeted to regenerate the lost dental or periodontal tissue.

Bone resorption control of tooth eruption and root morphogenesis: Involvement of the receptor activator of NF-κB (RANK)

Activation of the receptor activator of NF-κB (RANK) is a crucial step in osteoclastogenesis. Loss- and gain-of-function mutations in the Rank gene cause, respectively, osteopetrosis and several forms of extensive osteolysis. Tooth and alveolar bone alterations are associated with these pathologies but remain to be better characterized. The aim of the present study was to establish the tooth and alveolar bone phenotype of a transgenic mouse model of RANK over-expression in osteoclast precursors. Early tooth eruption and accelerated tooth root elongation were observed subsequent to an increase in osteoclast numbers surrounding the tooth. The final root length appeared not to be affected by RANK over-expression, but a significant reduction in root diameter occurred in both control and root-morphogenesis-defective Msx2 null mutant mice. These results indicate that root length is independent of the surrounding bone resorption activity. In contrast, root diameter is sensitive to the activity of alveolar bone osteoclasts. These data suggest that early eruption and thin root are phenotypic features that could be associated with extensive osteolytic pathologies.

Smad4-Shh-Nfic signaling cascade-mediated epithelial-mesenchymal interaction is crucial in regulating tooth root development

Transforming growth factor beta (TGF-beta)/bone morphogenetic protein (BMP) signaling is crucial for regulating epithelial-mesenchymal interaction during organogenesis, and the canonical Smad pathway-mediated TGF-beta/BMP signaling plays important roles during development and disease. During tooth development, dental epithelial cells, known as Hertwig's epithelial root sheath (HERS), participate in root formation following crown development. However, the functional significance of HERS in regulating root development remains unknown. In this study we investigated the signaling mechanism of Smad4, the common Smad for TGF-beta/BMP signaling, in HERS in regulating root development. Tissue-specific inactivation of Smad4 in HERS results in abnormal enamel and dentin formation in K14-Cre;Smad4(fl/fl) mice. HERS enlarges but cannot elongate to guide root development without Smad4. At the molecular level, Smad4-mediated TGF-beta/BMP signaling is required for Shh expression in HERS and Nfic (nuclear factor Ic) expression in the cranial neural crest (CNC)-derived dental mesenchyme. Nfic is crucial for root development, and loss of Nfic results in a CNC-derived dentin defect similar to the one of K14-Cre;Smad4(fl/fl) mice. Significantly, we show that ectopic Shh induces Nfic expression in dental mesenchyme and partially rescues root development in K14-Cre;Smad4(fl/fl) mice. Taken together, our study has revealed an important signaling mechanism in which TGF-beta/BMP signaling relies on a Smad-dependent mechanism in regulating Nfic expression via Shh signaling to control root development. The interaction between HERS and the CNC-derived dental mesenchyme may guide the size, shape, and number of tooth roots.

Formation of the tooth-bone interface

Not only are teeth essential for mastication, but also missing teeth are considered a social handicap due to speech and aesthetic problems, with a resulting high impact on emotional well-being. Several treatment procedures are currently available for tooth replacement with mostly inert prosthetic materials and implants. Natural tooth substitution based on copying the developmental process of tooth formation is particularly challenging and creates a rapidly developing area of molecular dentistry. In any approach, functional interactions among the tooth, the surrounding bone, and the periodontium must be established. Therefore, recent research in craniofacial genetics searches for mechanisms responsible for correct cell and tissue interactions, not only within a specific structure, but also in the context of supporting structures. A tooth crown that is not functionally anchored to roots and bone is useless. This review aims to summarize the developmental and tissue homeostatic aspects of the tooth-bone interface, from the initial patterning toward tooth eruption and lifelong interactions between the tooth and its surrounding alveolar bone.

Cell fate determination during tooth development and regeneration

Teeth arise from sequential and reciprocal interactions between the oral epithelium and the underlying cranial neural crest-derived mesenchyme. Their formation involves a precisely orchestrated series of molecular and morphogenetic events, and gives us the opportunity to discover and understand the nature of the signals that direct cell fates and patterning. For that reason, it is important to elucidate how signaling factors work together in a defined number of cells to generate the diverse and precise patterned structures of the mature functional teeth. Over the last decade, substantial research efforts have been directed toward elucidating the molecular mechanisms that control cell fate decisions during tooth development. These efforts have contributed toward the increased knowledge on dental stem cells, and observation of the molecular similarities that exist between tooth development and regeneration.

Fate of HERS during tooth root development

Tooth root development begins after the completion of crown formation in mammals. Previous studies have shown that Hertwig's epithelial root sheath (HERS) plays an important role in root development, but the fate of HERS has remained unknown. In order to investigate the morphological fate and analyze the dynamic movement of HERS cells in vivo, we generated K14-Cre;R26R mice. HERS cells are detectable on the surface of the root throughout root formation and do not disappear. Most of the HERS cells are attached to the surface of the cementum, and others separate to become the epithelial rest of Malassez. HERS cells secrete extracellular matrix components onto the surface of the dentin before dental follicle cells penetrate the HERS network to contact dentin. HERS cells also participate in the cementum development and may differentiate into cementocytes. During root development, the HERS is not interrupted, and instead the HERS cells continue to communicate with each other through the network structure. Furthermore, HERS cells interact with cranial neural crest derived mesenchyme to guide root development. Taken together, the network of HERS cells is crucial for tooth root development.