The Experts below are selected from a list of 1041 Experts worldwide ranked by ideXlab platform
Frederic Michon - One of the best experts on this subject based on the ideXlab platform.
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plasticity within the niche ensures the maintenance of a sox2 stem cell population in the mouse incisor
Development, 2018Co-Authors: Kerstin Seidel, Ophir D. Klein, Frederic Michon, Maria Sanznavarro, Ludivine Bertonnierbrouty, Brad A AmendtAbstract:ABSTRACT In mice, the incisors grow throughout the animal9s life, and this continuous renewal is driven by dental epithelial and mesenchymal stem cells. Sox2 is a principal marker of the epithelial stem cells that reside in the mouse incisor stem cell niche, called the labial Cervical Loop, but relatively little is known about the role of the Sox2 + stem cell population. In this study, we show that conditional deletion of Sox2 in the embryonic incisor epithelium leads to growth defects and impairment of ameloblast lineage commitment. Deletion of Sox2 specifically in Sox2 + cells during incisor renewal revealed cellular plasticity that leads to the relatively rapid restoration of a Sox2 -expressing cell population. Furthermore, we show that Lgr5 -expressing cells are a subpopulation of dental Sox2 + cells that also arise from Sox2 + cells during tooth formation. Finally, we show that the embryonic and adult Sox2 + populations are regulated by distinct signalling pathways, which is reflected in their distinct transcriptomic signatures. Together, our findings demonstrate that a Sox2 + stem cell population can be regenerated from Sox2 − cells, reinforcing its importance for incisor homeostasis.
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Sox2+ Stem Cells Contribute to All Epithelial Lineages of the Tooth via Sfrp5+ Progenitors
Developmental Cell, 2012Co-Authors: Emma Juuri, Irma Thesleff, Mark Tummers, Kan Saito, Laura Ahtiainen, Kerstin Seidel, Konrad Hochedlinger, Ophir D. Klein, Frederic MichonAbstract:The continuously growing mouse incisor serves as a valuable model to study stem cell regulation during organ renewal. Epithelial stem cells are localized in the proximal end of the incisor in the labial Cervical Loop. Here, we show that the transcription factor Sox2 is a specific marker for these stem cells. Sox2+ cells became restricted to the labial Cervical Loop during tooth morphogenesis, and they contributed to the renewal of enamel-producing ameloblasts as well as all other epithelial cell lineages of the tooth. The early progeny of Sox2-positive stem cells transiently expressed the Wnt inhibitor Sfrp5. Sox2 expression was regulated by the tooth initiation marker FGF8 and specific miRNAs, suggesting a fine-tuning to maintain homeostasis of the dental epithelium. The identification of Sox2 as a marker for the dental epithelial stem cells will facilitate further studies on their lineage segregation and differentiation during tooth renewal.
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Tooth morphogenesis and ameloblast differentiation are regulated by micro-RNAs.
Developmental Biology, 2010Co-Authors: Frederic Michon, Mark Tummers, Marika Kyyrönen, Mikko J. Frilander, Irma ThesleffAbstract:Abstract Teeth form as appendages of the ectoderm and their morphogenesis is regulated by tissue interactions mediated by networks of conserved signal pathways. Micro-RNA (miRNA) pathway has emerged as important regulator of various aspects of embryonic development, but its function in odontogenesis has not been elucidated. We show that the expression of RNAi pathway effectors is dynamic during tooth morphogenesis and differentiation of dental cells. Based on microarray profiling we selected 8 miRNAs expressed during morphogenesis and 7 miRNAs in the incisor Cervical Loop containing the stem cell niche. These miRNAs were mainly expressed in the dental epithelium. Conditional deletion of Dicer-1 in the epithelium (DcrK14−/−) resulted in rather mild but significant aberrations in tooth shape and enamel formation. The cusp patterns of the DcrK14−/− molar crowns resembled the patterns of both ancestral muroid rodents and mouse mutants with modulated signal pathways. In the DcrK14−/− incisors, longitudinal grooves formed on the labial surface and these were shown to result from ectopic budding of the progenitor epithelium in the Cervical Loop. In addition, ameloblast differentiation was impaired and resulted in deficient enamel formation in molars and incisors. To help the identification of candidate target genes of the selected tooth enriched miRNAs, we constructed a new ectodermal organ oriented database, miRTooth. The predicted targets of the selected miRNAs included several components of the main morphogenetic signal pathways regulating tooth development. Based on our findings we suggest that miRNAs modulate tooth morphogenesis largely by fine tuning conserved signaling networks and that miRNAs may have played important roles during tooth evolution.
Hidemitsu Harada - One of the best experts on this subject based on the ideXlab platform.
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Establishment of dental epithelial cell line (HAT-7) and the cell differentiation dependent on Notch signaling pathway.
Connective tissue research, 2020Co-Authors: Shintaro Kawano, Takahiko Morotomi, Takashi Toyono, Norifumi Nakamura, Takashi Uchida, Kuniaki Toyoshima, Masamichi Ohishi, Hidemitsu HaradaAbstract:Rat incisors grow continuously throughout life. Producing a variety of dental epithelial cells is performed by stem cells located in the Cervical Loop of the incisor apex. To study the mechanisms for cell differentiation, we established a dental epithelial cell line (HAT-7) originating from a Cervical Loop epithelium of a rat incisor. Immunochemical studies showed that HAT-7 produced the cells expressing amelogenin, ameloblastin, or alkaline phosphatase (ALP). To illustrate a role of Notch signaling in the determinant of the cell fate, we examined expression patterns of Notch1 and Jagged1 in HAT-7 density dependently. At lower cell density, Notch1- or Jagged1-expressing cells were not seen. However, when they were fully confluent, cells began to express Notch1 or Jagged1 strongly. Some ALP-positive cells were almost consistent with Notch1-expressing cells but not Jagged1-expressing cells. These results suggested that the determinant of direction of differentiation was associated with Notch signaling pathway.
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cell dynamics in Cervical Loop epithelium during transition from crown to root implications for hertwig s epithelial root sheath formation
Journal of Periodontal Research, 2013Co-Authors: M Sakano, Keishi Otsu, Naoki Fujiwara, Satoshi Fukumoto, Aya Yamada, Hidemitsu HaradaAbstract:Background and Objective Some clinical cases of hypoplastic tooth root are congenital. Because the formation of Hertwig's epithelial root sheath (HERS) is an important event for root development and growth, we have considered that understanding the HERS developmental mechanism contributes to elucidate the causal factors of the disease. To find integrant factors and phenomenon for HERS development and growth, we studied the proliferation and mobility of the Cervical Loop (CL). Material and Methods We observed the cell movement of CL by the DiI labeling and organ culture system. To examine cell proliferation, we carried out immunostaining of CL and HERS using anti-Ki67 antibody. Cell motility in CL was observed by tooth germ slice organ culture using green fluorescent protein mouse. We also examined the expression of paxillin associated with cell movement. Results Imaging using DiI labeling showed that, at the apex of CL, the epithelium elongated in tandem with the growth of outer enamel epithelium (OEE). Cell proliferation assay using Ki67 immunostaining showed that OEE divided more actively than inner enamel epithelium (IEE) at the onset of HERS formation. Live imaging suggested that mobility of the OEE and cells in the apex of CL were more active than in IEE. The expression of paxillin was observed strongly in OEE and the apex of CL. Conclusion The more active growth and movement of OEE cells contributed to HERS formation after reduction of the growth of IEE. The expression pattern of paxillin was involved in the active movement of OEE and HERS. The results will contribute to understand the HERS formation mechanism and elucidate the cause of anomaly root.
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Core Binding Factor Beta Functions in the Maintenance of Stem Cells and Orchestrates Continuous Proliferation and Differentiation in Mouse Incisors
Stem Cells, 2011Co-Authors: Hiroshi Kurosaka, Hidemitsu Harada, Koh-ichi Kuremoto, Nurul Islam, Satoru Hayano, Masahiro Nakamura, Noriaki Kawanabe, Takeshi Yanagita, David P. Rice, Ichiro TaniuchiAbstract:Rodent incisors grow continuously throughout life, and epithelial progenitor cells are supplied from stem cells in the Cervical Loop. We report that epithelial Runx genes are involved in the maintenance of epithelial stem cells and their subsequent continuous differentiation and therefore growth of the incisors. Core binding factor b (Cbfb) acts as a binding partner for all Runx proteins, and targeted inactivation of this molecule abrogates the activity of all Runx complexes. Mice deficient in epithelial Cbfb produce short incisors and display marked underdevelopment of the Cervical Loop and suppressed epithelial Fgf9 expression and mesenchymal Fgf3 and Fgf10 expression in the Cervical Loop. In culture, FGF9 protein rescues these phenotypes. These findings indicate that epithelial Runx functions to maintain epithelial stem cells and that Fgf9 may be a target gene of Runx signaling. Cbfb mutants also lack enamel formation and display downregulated Shh mRNA expression in cells differentiating into ameloblasts. Furthermore, Fgf9 deficiency results in a proximal shift of the Shh expressing cell population and ectopic FGF9 protein suppresses Shh expression. These findings indicate that Shh as well as Fgf9 expression is maintained by Runx/Cbfb but that Fgf9 antagonizes Shh expression. The present results provide the first genetic evidence that Runx/Cbfb genes function in the maintenance of stem cells in developing incisors by activating Fgf signaling Loops between the epithelium and mesenchyme. In addition, Runx genes also orchestrate continuous proliferation and differentiation by maintaining the expression of Fgf9 and Shh mRNA. STEM CELLS 2011;29:1792–1803
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Reduction of Egf signaling decides transition from crown to root in the development of mouse molars.
Journal of experimental zoology. Part B Molecular and developmental evolution, 2009Co-Authors: Naoki Fujiwara, Keishi Otsu, Tadashi Akimoto, Tadayoshi Kagiya, Kiyoto Ishizeki, Hidemitsu HaradaAbstract:Mouse, rat, and human molars begin to form their roots after the completion of crown morphogenesis. Though several signaling pathways and transcription factors have been implicated in the regulation of molar crown development, relatively little is known about the regulatory mechanisms involved in the transition from crown to root development. Tooth root formation is initiated by the development of Hertwig's epithelial root sheath (HERS) from the Cervical Loop in the enamel organ. In this study we examined the change in epidermal growth factor (Egf) signaling during this transition process. Immunohistochemical studies showed that the expression of Egf receptors in the enamel organ disappear gradually in the process and are not observed in HERS. Here, to examine the effect of Egf on the transition, we used the organ culture method to examine the root development. In the presence of Egf, stellate reticulum (SR) cells between the inner and outer epithelial layers in the enamel organ actively proliferated and maintained the enamel organ, and the formation of HERS was not observed. On the other hand, in either the absence of Egf or the presence of the inhibitor of Egf receptors, the SR cells disappeared and HERS formation started. Subsequently, root formation proceeded in the culture period. Therefore, disappearance of SR area may be a key event that controls the timing of onset of HERS formation, and Egf may be one of regulatory factors involved in the change from Cervical Loop epithelium to HERS during root development.
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Reduction of Egf signaling decides transition from crown to root in the development of mouse molars.
Journal of Experimental Zoology, 2009Co-Authors: Naoki Fujiwara, Keishi Otsu, Tadashi Akimoto, Tadayoshi Kagiya, Kiyoto Ishizeki, Hidemitsu HaradaAbstract:Mouse, rat, and human molars begin to form their roots after the completion of crown morphogenesis. Though several signaling pathways and transcription factors have been implicated in the regulation of molar crown development, relatively little is known about the regulatory mechanisms involved in the transition from crown to root development. Tooth root formation is initiated by the development of Hertwig's epithelial root sheath (HERS) from the Cervical Loop in the enamel organ. In this study we examined the change in epidermal growth factor (Egf) signaling during this transition process. Immunohistochemical studies showed that the expression of Egf receptors in the enamel organ disappear gradually in the process and are not observed in HERS. Here, to examine the effect of Egf on the transition, we used the organ culture method to examine the root development. In the presence of Egf, stellate reticulum (SR) cells between the inner and outer epithelial layers in the enamel organ actively proliferated and maintained the enamel organ, and the formation of HERS was not observed. On the other hand, in either the absence of Egf or the presence of the inhibitor of Egf receptors, the SR cells disappeared and HERS formation started. Subsequently, root formation proceeded in the culture period. Therefore, disappearance of SR area may be a key event that controls the timing of onset of HERS formation, and Egf may be one of regulatory factors involved in the change from Cervical Loop epithelium to HERS during root development. J. Exp. Zool. (Mol. Dev. Evol.) 312B:486–494, 2009. © 2008 Wiley-Liss, Inc.
Joo-cheol Park - One of the best experts on this subject based on the ideXlab platform.
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The Nfic-osterix pathway regulates ameloblast differentiation and enamel formation
Cell and Tissue Research, 2018Co-Authors: Joo-cheol ParkAbstract:Enamel makes up the outermost layer of the crown and its hardness protects other dental tissues from various stimuli. Enamel cannot be regenerated once damaged because ameloblasts are lost during the tooth eruption. Since the ameloblast differentiation mechanism is still unknown, further research is essential for developing treatments for defective or damaged enamel. Previously, we have reported that osteoblast differentiation and bone formation were regulated through the runt-related transcription factor 2 (Runx2)-nuclear factor 1-C (Nfic)-osterix (Osx) pathway where Nfic directly controls Osx expression. This pathway regulates odontoblast differentiation and dentin formation as well. The aim of this study was to investigate if the same pathway is applicable for ameloblast differentiation. Structural enamel defects with disorganized ameloblasts and decreased proliferation activity of the Cervical Loop were observed in Nfic ^−/− mice incisors. Expression of the ameloblast differentiation markers was also downregulated significantly in Nfic ^−/− mice. Real-time PCR analyses suggested that Runx2, Nfic, and Osx regulate the expression of ameloblast differentiation markers, where Runx2 is upstream of Nfic, and Nfic controls Osx expression. Therefore, we suggest the Runx2-Nfic-Osx pathway as one of the key factors that regulate ameloblast differentiation.
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Regulation of root patterns in mammalian teeth
Scientific Reports, 2017Co-Authors: Jae Joon Hwang, Joo-cheol Park, Ho-gul Jeong, Wonse Park, Hong Seomun, Kyung-seok HuAbstract:Mammalian teeth have diverse pattern of the crown and root. The patterning mechanism of the root position and number is relatively unknown compared to that of the crown. The root number does not always match to the cusp number, which has prevented the complete understanding of root patterning. In the present study, to elucidate the mechanism of root pattern formation, we examined (1) the pattern of Cervical tongues, which are tongue-like epithelial processes extending from Cervical Loops, (2) factors influencing the Cervical tongue pattern and (3) the relationship among patterns of cusp, Cervical tongue and root in multi-rooted teeth. We found a simple mechanism of Cervical tongue formation in which the lateral growth of dental mesenchyme in the cuspal region pushes the Cervical Loop outward, and the Cervical tongue appears in the intercuspal region subsequently. In contrast, when lateral growth was physically inhibited, Cervical tongue formation was suppressed. Furthermore, by building simple formulas to predict the maximum number of Cervical tongues and roots based on the cusp pattern, we demonstrated a positive relationship among cusp, Cervical tongue and root numbers. These results suggest that the cusp pattern and the lateral growth of cusps are important in the regulation of the root pattern.
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nuclear factor i c is essential for odontogenic cell proliferation and odontoblast differentiation during tooth root development
Journal of Biological Chemistry, 2009Co-Authors: Jongtae Park, Jea Seung Ko, Richard M Gronostajski, Pillhoon Choung, Joo-cheol ParkAbstract:Our previous studies have demonstrated that nuclear factor I-C (NFI-C) null mice developed short molar roots that contain aberrant odontoblasts and abnormal dentin formation. Based on these findings, we performed studies to elucidate the function of NFI-C in odontoblasts. Initial studies demonstrated that aberrant odontoblasts become dissociated and trapped in an osteodentin-like mineralized tissue. Abnormal odontoblasts exhibit strong bone sialoprotein expression but a decreased level of dentin sialophosphoprotein expression when compared with wild type odontoblasts. Loss of Nfic results in an increase in p-Smad2/3 expression in aberrant odontoblasts and pulp cells in the subodontoblastic layer in vivo and primary pulp cells from Nfic-deficient mice in vitro. Cell proliferation analysis of both Cervical Loop and ectomesenchymal cells of the Nfic-deficient mice revealed significantly decreased proliferative activity compared with wild type mice. In addition, Nfic-deficient primary pulp cells showed increased expression of p21 and p16 but decreased expression of cyclin D1 and cyclin B1, strongly suggesting cell growth arrest caused by a lack of Nfic activity. Analysis of the pulp and abnormal dentin in Nfic-deficient mice revealed an increase in apoptotic activity. Further, Nfic-deficient primary pulp cells exhibited an increase in caspase-8 and -3 activation, whereas the cleaved form of Bid was hardly detected. These results indicate that the loss of Nfic leads to the suppression of odontogenic cell proliferation and differentiation and induces apoptosis of aberrant odontoblasts during root formation, thereby contributing to the formation of short roots.
Weidong Tian - One of the best experts on this subject based on the ideXlab platform.
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Comparative study on differentiation of Cervical-Loop cells and Hertwig’s epithelial root sheath cells under the induction of dental follicle cells in rat
Scientific Reports, 2018Co-Authors: Jie Chen, Ye Tian, Guoqing Chen, Weidong TianAbstract:Cervical Loop cells (CLC) and Hertwig’s epithelial root sheath (HERS) cells are believed to play critical roles in distinct developmental patterns between rodent incisors and molars, respectively. However, the differences in differentiation between CLC and HERS cells, and their response to inductions from dental follicle cells, remain largely unknown. In present study, CLC and HERS cells, as well as incisor dental follicle (IF) cells and molar dental follicle (MF) cells were isolated from post-natal 7-day rats. IF and MF cell derived conditioned medium (CM) was obtained for induction of CLC and HERS cells. In vitro experiments, we found that, under the induction of dental follicle cell derived CM, CLC cells maintained the epithelial polygonal-shapes and formed massive minerals, while part of HERS cells underwent shape transformation and generated granular minerals. CLC cells expressed higher enamel-forming and mineralization related genes, while HERS cells showed opposite expression patterns of BMP2, BMP4, AMBN and AMGN. In vivo, CLC cells generated enamel-like tissues while HERS cells formed cementum-periodontal ligament-like structures. Taken together, CLC and HERS cells present distinct differentiation patterns under the inductions from dental follicle cells.
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comparative study on differentiation of Cervical Loop cells and hertwig s epithelial root sheath cells under the induction of dental follicle cells in rat
Scientific Reports, 2018Co-Authors: Jie Chen, Ye Tian, Guoqing Chen, Weidong TianAbstract:Cervical Loop cells (CLC) and Hertwig’s epithelial root sheath (HERS) cells are believed to play critical roles in distinct developmental patterns between rodent incisors and molars, respectively. However, the differences in differentiation between CLC and HERS cells, and their response to inductions from dental follicle cells, remain largely unknown. In present study, CLC and HERS cells, as well as incisor dental follicle (IF) cells and molar dental follicle (MF) cells were isolated from post-natal 7-day rats. IF and MF cell derived conditioned medium (CM) was obtained for induction of CLC and HERS cells. In vitro experiments, we found that, under the induction of dental follicle cell derived CM, CLC cells maintained the epithelial polygonal-shapes and formed massive minerals, while part of HERS cells underwent shape transformation and generated granular minerals. CLC cells expressed higher enamel-forming and mineralization related genes, while HERS cells showed opposite expression patterns of BMP2, BMP4, AMBN and AMGN. In vivo, CLC cells generated enamel-like tissues while HERS cells formed cementum-periodontal ligament-like structures. Taken together, CLC and HERS cells present distinct differentiation patterns under the inductions from dental follicle cells.
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The role of odontogenic genes and proteins in tooth epithelial cells and their niche cells during rat tooth root development
Archives of Oral Biology, 2012Co-Authors: Yaneng Ge, Ziren Kong, Wei Tang, Weidong TianAbstract:Abstract Objectives The rodent incisor Cervical Loop and molar Hertwig's epithelial root sheath (HERS) are common models used for investigating tooth root generation. The purpose of the present study was to gain a better understanding of the molecular mechanisms mediating root development by determining the distinctive gene and protein expression profiles of each during different stages of development. Methods In this study, we used quantitative real time reverse transcription-PCR and immunohistochemistry to analyse the expression levels of high mobility group AT-hook 2, ameloblastin, amelogenin, dentine sialoprotein, dentine matrix protein 1, osteocalcin, and bone sialoprotein in rat epithelial and mesenchymal cells isolated at postnatal days 4 and 8. Results Results showed that the expression of these genes and proteins was up-regulated in Cervical Loop epithelial cells, but decreased or unchanged in other cells during development. This increase in expression in the incisor Cervical Loop may be due to the interaction of the inner incisor dental papilla cells, which are the niche cells of Cervical Loop epithelial cells and demonstrated up-regulated expression of the corresponding proteins, revealing a complex and dynamic interplay of these molecules during neonatal tooth development. Conclusion These findings provide novel insights into the molecular processes underlying crown development of rodent incisors, and contribute to our overall understanding of the pathogenic processes of tooth root dysontogenesis.
Irma Thesleff - One of the best experts on this subject based on the ideXlab platform.
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Functionally Distinctive Ptch Receptors Establish Multimodal Hedgehog Signaling in the Tooth Epithelial Stem Cell Niche
Stem Cells, 2019Co-Authors: Martin Binder, Piotr Chmielarz, Leah C. Biggs, Peter J Mckinnon, Irma Thesleff, Anamaria BalicAbstract:Continuous growth of the mouse incisor teeth is due to the life-long maintenance of epithelial stem cells (SCs) in their niche called Cervical Loop (CL). Several signaling factors regulate SC maintenance and/or their differentiation to achieve organ homeostasis. Previous studies indicated that Hedgehog signaling is crucial for both the maintenance of the SCs in the niche, as well as for their differentiation. How Hedgehog signaling regulates these two opposing cellular behaviors within the confinement of the CL remains elusive. In this study, we used in vitro organ and cell cultures to pharmacologically attenuate Hedgehog signaling. We analyzed expression of various genes expressed in the SC niche to determine the effect of altered Hedgehog signaling on the cellular hierarchy within the niche. These genes include markers of SCs (Sox2 and Lgr5) and transit-amplifying cells (P-cadherin, Sonic Hedgehog, and Yap). Our results show that Hedgehog signaling is a critical survival factor for SCs in the niche, and that the architecture and the diversity of the SC niche are regulated by multiple Hedgehog ligands. We demonstrated the presence of an additional Hedgehog ligand, nerve-derived Desert Hedgehog, secreted in the proximity of the CL. In addition, we provide evidence that Hedgehog receptors Ptch1 and Ptch2 elicit independent responses, which enable multimodal Hedgehog signaling to simultaneously regulate SC maintenance and differentiation. Our study indicates that the cellular hierarchy in the continuously growing incisor is a result of complex interplay of two Hedgehog ligands with functionally distinct Ptch receptors. Stem Cells 2019;37:1238-1248.
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Sox2+ Stem Cells Contribute to All Epithelial Lineages of the Tooth via Sfrp5+ Progenitors
Developmental Cell, 2012Co-Authors: Emma Juuri, Irma Thesleff, Mark Tummers, Kan Saito, Laura Ahtiainen, Kerstin Seidel, Konrad Hochedlinger, Ophir D. Klein, Frederic MichonAbstract:The continuously growing mouse incisor serves as a valuable model to study stem cell regulation during organ renewal. Epithelial stem cells are localized in the proximal end of the incisor in the labial Cervical Loop. Here, we show that the transcription factor Sox2 is a specific marker for these stem cells. Sox2+ cells became restricted to the labial Cervical Loop during tooth morphogenesis, and they contributed to the renewal of enamel-producing ameloblasts as well as all other epithelial cell lineages of the tooth. The early progeny of Sox2-positive stem cells transiently expressed the Wnt inhibitor Sfrp5. Sox2 expression was regulated by the tooth initiation marker FGF8 and specific miRNAs, suggesting a fine-tuning to maintain homeostasis of the dental epithelium. The identification of Sox2 as a marker for the dental epithelial stem cells will facilitate further studies on their lineage segregation and differentiation during tooth renewal.
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Notch signalling is required for the survival of epithelial stem cells in the continuously growing mouse incisor.
Differentiation; research in biological diversity, 2010Co-Authors: Szabolcs Felszeghy, Marika Suomalainen, Irma ThesleffAbstract:The Notch pathway regulates the renewal and fate decisions of stem cells in multiple tissues. Notch1, -2, as well as the Notch target gene Hes1 are expressed in the putative stem cells in the continuously growing mouse incisors, but so far there has not been any evidence for a function of the Notch pathway in the regulation of the incisor stem cells. We have analysed the effects of the Notch pathway inhibitor DAPT on the maintenance, proliferation, and differentiation of the epithelial stem cells in explant cultures of the mouse incisor. The proximal part of the incisor containing the Cervical Loop stem cell niche was dissected from newborn mice and cultured for 2-6 days in vitro. DAPT inhibited the expression of Notch target gene Hes1 in the Cervical Loop indicating that Notch signalling was inhibited in the putative stem cells. The most striking effect of DAPT was a significant reduction in the size of the Cervical Loop. DAPT caused a marked but partially reversible decrease in cell proliferation, as well as massive apoptosis in the epithelial stem cell niche. Interestingly, restricted apoptosis was detected within the Notch expressing putative stem cells also in the control cultures as well as in incisors in vivo, suggesting that apoptosis may be a mechanism regulating the size of the epithelial stem cell pool in the incisor. The differentiation of the epithelial cells into enamel-forming ameloblasts was not affected by DAPT but the number of preameloblasts was progressively decreased during culture period reflecting the depletion of stem and progenitor cells. Our results indicate that Notch signalling is required for epithelial stem cell survival and enamel formation in the continuously growing mouse incisor.
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Tooth morphogenesis and ameloblast differentiation are regulated by micro-RNAs.
Developmental Biology, 2010Co-Authors: Frederic Michon, Mark Tummers, Marika Kyyrönen, Mikko J. Frilander, Irma ThesleffAbstract:Abstract Teeth form as appendages of the ectoderm and their morphogenesis is regulated by tissue interactions mediated by networks of conserved signal pathways. Micro-RNA (miRNA) pathway has emerged as important regulator of various aspects of embryonic development, but its function in odontogenesis has not been elucidated. We show that the expression of RNAi pathway effectors is dynamic during tooth morphogenesis and differentiation of dental cells. Based on microarray profiling we selected 8 miRNAs expressed during morphogenesis and 7 miRNAs in the incisor Cervical Loop containing the stem cell niche. These miRNAs were mainly expressed in the dental epithelium. Conditional deletion of Dicer-1 in the epithelium (DcrK14−/−) resulted in rather mild but significant aberrations in tooth shape and enamel formation. The cusp patterns of the DcrK14−/− molar crowns resembled the patterns of both ancestral muroid rodents and mouse mutants with modulated signal pathways. In the DcrK14−/− incisors, longitudinal grooves formed on the labial surface and these were shown to result from ectopic budding of the progenitor epithelium in the Cervical Loop. In addition, ameloblast differentiation was impaired and resulted in deficient enamel formation in molars and incisors. To help the identification of candidate target genes of the selected tooth enriched miRNAs, we constructed a new ectodermal organ oriented database, miRTooth. The predicted targets of the selected miRNAs included several components of the main morphogenetic signal pathways regulating tooth development. Based on our findings we suggest that miRNAs modulate tooth morphogenesis largely by fine tuning conserved signaling networks and that miRNAs may have played important roles during tooth evolution.
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Observations on continuously growing roots of the sloth and the K14‐Eda transgenic mice indicate that epithelial stem cells can give rise to both the ameloblast and root epithelium cell lineage creating distinct tooth patterns
Evolution & Development, 2008Co-Authors: Mark Tummers, Irma ThesleffAbstract:Current address and address at time of work for both authors: Institute of Biotechnology, P.O. Box 56, FIN-00014 University of Helsinki, Finland. SUMMARY Root development is traditionally associated with the formation of Hertwig’s epithelial root sheath (HERS), whose fragments give rise to the epithelial cell rests of Malassez (ERM). The HERS is formed by depletion of the core of stellate reticulum cells, the putative stem cells, in the Cervical Loop, leaving only a double layer of the basal epithelium with limited growth capacity. The continuously growing incisor of the rodent is subdivided into a crown analog half on the labial side, with a Cervical Loop containing a large core of stellate reticulum, and its progeny gives rise to enamel producing. The lingual side is known as the root analog and gives rise to ERM. We show that the lingual Cervical Loop contains a small core of stellate reticulum cells and suggest that it acts as a functional stem cell niche. Similarly we show that continuously growing roots represented by the sloth molar and K14-Eda transgenic incisor maintain a Cervical Loop with a small core of stellate reticulum cells around the entire circumference of the tooth and do not form a HERS, and still give rise to ERM. We propose that HERS is not a necessary structure to initiate root formation. Moreover, we conclude that crown vs. root formation, i.e. the production of enamel vs. cementum, and the differentiation of the epithelial cells into ameloblasts vs. ERM, can be regulated independently from the regulation of stem cell maintenance. This developmental flexibility may underlie the developmental and evolutionary diversity in tooth patterning.