Epithelial histogenesis during tooth development

Developmental roles of glomerular epithelial protein-1 in mice molar morphogenesis

Glomerular epithelial protein-1 (Glepp1), a R3 subtype family of receptor-type protein tyrosine phosphatases, plays important role in the activation of Src family kinases and regulates cellular processes such as cell proliferation, differentiation, and apoptosis. In this study, we firstly examined the functional evaluation of Glepp1 in tooth development and morphogenesis. The precise expression level and developmental function of Glepp1 were examined by RT-qPCR, in situ hybridization, and loss and gain of functional study using a range of in vitro organ cultivation methods. Expression of Glepp1 was detected in the developing tooth germs in cap and bell stage of tooth development. Knocking down Glepp1 at E13 for 2 days showed the altered expression levels of tooth development-related signaling molecules, including Bmps, Dspp, Fgf4, Lef1, and Shh. Moreover, transient knock down of Glepp1 revealed alterations in cellular physiology, examined by the localization patterns of Ki67 and E-cadherin. Similarly, knocking down of Glepp1 showed disrupted enamel rod and interrod formation in 3-week renal transplanted teeth. In addition, due to attrition of odontoblastic layers, the expression signals of Dspp and the localization of NESTIN were almost not detected after knock down of Glepp1; however, their expressions were increased after Glepp1 overexpression. Thus, our results suggested that Glepp1 plays modulating roles during odontogenesis by regulating the expression levels of signaling molecules and cellular events to achieve the proper structural formation of hard tissue matrices in mice molar development.

Dynamic patterns of histone lactylation during early tooth development in mice

Histone lactylation on its lysine (K) residues has been reported to have indispensable roles in lung fibrosis, embryogenesis, neural development, inflammation, and tumors. However, little is known about the lactylation activity towards histone lysine residue during tooth development. We investigated the dynamic patterns of lactate-derived histone lysine lactylation (Kla) using a pan-Kla antibody during murine tooth development, including lower first molar and lower incisor. The results showed that pan-Kla exhibited temporo-spatial patterns in both dental epithelium and mesenchyme cells during development. Notably, pan-Kla was identified in primary enamel knot (PEK), stratum intermedium (SI), stellate reticulum (SR), dental follicle cells, odontoblasts, ameloblasts, proliferating cells in dental mesenchyme, as well as osteoblasts around the tooth germ. More importantly, pan-Kla was also identified to be co-localized with neurofilament during tooth development, suggesting histone lysine lactylation may be involved in neural invasion during tooth development. These findings suggest that histone lysine lactylation may play important roles in regulating tooth development.

Spatiotemporal expression of fibroblast growth factor 4 and 10 during the morphogenesis of deciduous molars in miniature pigs

OBJECTIVE

Fibroblast growth factors (FGFs) play pivotal roles in mediating interactions between dental epithelium and mesenchyme throughout tooth initiation and morphogenesis. This study aimed to elucidate the roles of FGF4 and FGF10 in the regulation of tooth development.

DESIGN

In this study, we investigated spatiotemporal expression patterns of FGF4 and FGF10 in the third deciduous molars (DM3) of miniature pigs at the cap, early bell, and late bell stages. Pregnant miniature pigs were obtained, and the samples were processed for histological staining. Non-radioactive in situ hybridization, immunohistochemistry, and real-time PCR were used to detect mRNA and protein expression levels of FGF4 and FGF10.

RESULTS

FGF4 was expressed in the dental epithelium and mesenchyme at the cap stage. At the early bell stage, epithelial expression of FGF4 was reduced while mesenchymal expression got stronger. At the late bell stage, the FGF4 expression was restricted to the inner enamel epithelium (IEE) and differentiating odontoblasts. FGF10 was expressed intensely in both epithelium and mesenchyme at the cap stage. The expression of FGF10 was concentrated in the secondary enamel knots and surrounding mesenchyme at the early bell stage. FGF10 was weakly detected in the IEE by the late bell stage.

CONCLUSIONS

Our results indicated that FGF4 and FGF10 might have partially redundant functions in regulating epithelium morphogenesis. FGF4 may be involved in regulatory signaling cascades mediating interactions between the epithelium and mesenchyme. In addition, the downregulation of FGF10 expression may be associated with the cessation of mesenchymal cell proliferation and initiation of preodontoblast polarization.

Immunohistochemical expression of paxillin in ameloblastoma and odontogenic keratocyst: An observational study

Background

Cell adhesion molecules (CAMs) are found on the surface of all cells, where they allow dynamic processes to take place. These include cadherins, integrins, selectins and Immunoglobulin superfamily. Directly associated with β-integrin tails is a multidomain protein known as paxillin. However, CAMs participate in cell-cell and extracellular matrix-cell interactions during histomorphogenesis in the various phases of odontogenesis. Some tumours or cysts like ameloblastoma (AB) or odontogenic keratocyst (OKC) having odontogenic origin show disturbance in the interaction of these CAMs. Hence, the assessment of paxillin expression in AB and OKC was carried out.

Materials and Methods

The present observational study comprised 30 clinically and histologically confirmed cases of AB and OKC. All the slides were stained immunohistochemically using a paxillin antibody.

Results

Upon comparison of staining intensity of paxillin among AB and OKC showed statistically significant result, whereas quantitative staining and final summation showed non-significant result. Gender-wise comparison of paxillin staining intensity, quantitative staining and final summation among OKC showed significant result; however, in AB, staining intensity showed non-significant result, whereas quantitative staining and final summation showed significant result.

Conclusion

Paxillin has the greatest influence on tissue morphogenesis and development. The regulation of cell mobility is aided by the multiple roles that paxillin plays in a range of cells and tissues. However, further studies using a large sample size, along with other molecular analytical methods, may be essential to draw a definite conclusion about the association of paxillin and its exact function in OKC and AB.

Genetic Correlation, Pleiotropy, and Molar Morphology in a Longitudinal Sample of Australian Twins and Families

This study aims to expand our understanding of the genetic architecture of crown morphology in the human diphyodont dentition. Here, we present bivariate genetic correlation estimates for deciduous and permanent molar traits and evaluate the patterns of pleiotropy within (e.g., m1–m2) and between (e.g., m2–M1) dentitions. Morphology was observed and scored from dental models representing participants of an Australian twin and family study (deciduous n = 290, permanent n = 339). Data collection followed Arizona State University Dental Anthropology System standards. Genetic correlation estimates were generated using maximum likelihood variance components analysis in SOLAR v.8.1.1. Approximately 23% of deciduous variance components models and 30% of permanent variance components models yielded significant genetic correlation estimates. By comparison, over half (56%) of deciduous–permanent homologues (e.g., m2 hypocone–M1 hypocone) were significantly genetically correlated. It is generally assumed that the deciduous and permanent molars represent members of a meristic molar field emerging from the primary dental lamina. However, stronger genetic integration among m2–M1/M2 homologues than among paired deciduous traits suggests the m2 represents the anterior-most member of a “true” molar field. The results indicate genetic factors act at distinct points throughout development to generate homologous molar form, starting with the m2, which is later replaced by a permanent premolariform crown.

Cell Differentiation and Replication during Postnatal Development of the Murine First Molar

Simple Summary

Teeth are necessary to prepare food for swallowing. The process of teeth development before and after birth may be studied in normal mice and also by reproducing diseases or genetic conditions. However, mice teeth are different from human teeth, since mice have only permanent teeth. Moreover, their incisors continue to grow for the whole lifespan. Hence, it is important to know how the mouse teeth develop. We studied the development of the first molar in mice from birth to weaning and showed that dividing cells are located in a different part of the developing tooth according to age.

Abstract

Various signaling molecular pathways are involved in odontogenesis to promote cellular replication and differentiation. Tooth formation is controlled mainly by epithelial–mesenchymal interactions. The aim of this work was to investigate how cellular replication and differentiation ensue during the formation of the murine first molar in postnatal ages until eruption, focusing on morphogenesis, odontoblast differentiation and cellular replication. Wild-type CD1 mice were examined from birth to weaning. Morphogenesis and interaction between developing epithelial and mesenchymal tissues were evaluated in hematoxylin–eosin and Gomori trichome stained sections. Immunohistochemistry for nestin, which mediates the differentiation of odontoblasts, especially their polarization and elongation, showed that this intermediate filament was apparent already at postnatal day P1 in the apical region of odontoblasts and progressed apically from cusp tips, while it was not present in epithelial tissues. The expression of nuclear antigen Ki-67 highlighted dividing cells in both epithelial and mesenchymal tissues at P1, while one week later they were restricted to the cementoenamel junction, guiding root elongation. The link between odontoblast maturation and cellular replication in the different tooth tissues is essential to understand the development of tooth shape and dimension, to outline mechanisms of tooth morphogenesis and possibly eruption.

Hypoxia-Responsive Oxygen Nanobubbles for Tissues-Targeted Delivery in Developing Tooth Germs

Hypoxia is a state of inadequate supply of oxygen. Increasing evidence indicates that a hypoxic environment is strongly associated with abnormal organ development. Oxygen nanobubbles (ONBs) are newly developed nanomaterials that can deliver oxygen to developing tissues, including hypoxic cells. However, the mechanisms through which nanobubbles recover hypoxic tissues, such as developing tooth germs remain to be identified. In this study, tooth germs were cultured in various conditions: CO₂ chamber, hypoxic chamber, and with 20% ONBs for 3 h. The target stages were at the cap stage (all soft tissue) and bell stage (hard tissue starts to form). Hypoxic tooth germs were recovered with 20% ONBs in the media, similar to the tooth germs incubated in a CO₂ chamber (normoxic condition). The tooth germs under hypoxic conditions underwent apoptosis both at the cap and bell stages, and ONBs rescued the damaged tooth germs in both the cap and bell stages. Using kidney transplantation for hard tissue formation in vivo, amelogenesis and dentinogenesis imperfecta in hypoxic conditions at the bell stage were rescued with ONBs. Furthermore, glucose uptake by tooth germs was highly upregulated under hypoxic conditions, and was restored with ONBs to normoxia levels. Our findings indicate that the strategies to make use of ONBs for efficient oxygen targeted delivery can restore cellular processes, such as cell proliferation and apoptosis, glucose uptake, and hypomineralization in hypoxic environments.

The involvement of genes related to bile secretion pathway in rat tooth germ development

Tooth formation is accomplished under strict genetic control procedures. Therefore, exploring the gene network system of tooth development has a very positive practical significance for the study of tooth tissue regeneration and the prevention and treatment of tooth abnormalities. Early bell stage is the initial phase of odontoblast formation and dentin matrix deposition in the process of tooth development. Through RNA sequencing and differential gene analysis of the rat tooth germ samples at cap stage and early bell stage, we found that the bile secretion pathway was the most significant difference signal pathway during the development between cap stage and bell stage, which mainly included ABCC3, AQP4, SLC10A1, SLC2A1, SLC4A4, ADCY5, AQP9, CFTR, ATP1A2, ATP1B1 and ATP1A1, totally 11genes. Immunostaining revealed that SLC2A1, SLC4A4, ADCY5 and ATP1B1were mainly expressed in epithelium in bud stage and inner and outer enamel epithelium during the embryonic phase. In the postnatal 1 and postnatal 7, SLC2A1, SLC4A4 and ABCC3 were highly expressed in ameloblasts and odontoblasts while ADCY5, ATP1B1 and SLC10A1was expressed moderately only in odontoblasts. This finding illustrated that the bile secretion pathway related genes may participate in the development of tooth germ.

Pointing on the early stages of maxillary bone and tooth development - histological findings

Although the morphological stages of tooth development, in parallel with maxillary bone construction, are known for decades, the intimate mechanisms of early development of the oral cavity structures and tooth's proper and associated tissues are still incompletely elucidated. Nowadays, the research in embryology was shifted from the morphological to the molecular and genetic approach. This new approach is accomplished by using in vivo and in vitro experimental studies performed on animal models and cell lines. The interest in the knowledge of these events at gene and molecular level is still current, aiming to sustain the progress in the endorsement of novel regenerative and restorative therapies. However, the morphological standpoint maintains its interest, because the extrapolation of the results of experimental studies in humans requires a strong confirmation. Within this context, our work aims to analyze the histological characteristics of the maxillary bone and integrated tooth germs during the early stages of embryonic development. The study group consisted in mandible fragments obtained by dissection of the cephalic extremities collected from fetuses aged from 10 to 24 weeks, after medical or spontaneous abortions. The tissue specimens were processed for the histological exam. The histoarchitectonic traits of the initial stages of mandibular bone tissue and tooth development were assessed. The results revealed the dynamics of the ossification stages, from stages of early-dispersed intramembranous ossification to the organization of the dental alveoli, incorporated step-by-step in the maxillary body, and the simultaneous presence of tooth germs with different sizes and shapes, in accordance with the development stage. Our study complements the existing data regarding the embryonic period, bringing an important contribution for the enlargement of existing morphological, visual information for maxillary bone and tooth development.

Expression of Different Isoforms of Versican During the Development of Mouse Mandibular First Molars

Versican is a large chondroitin sulfate proteoglycan enriched in the extracellular matrix, and it has at least four different isoforms, termed V0, V1, V2, and V3. Although several studies have demonstrated that versican is stably expressed in various developing organs, the expression of versican isoforms during tooth development has not been elucidated yet. Therefore, the present study was to investigate the expression of versican isoforms in the developing mouse molars. The mandibular first molars from embryonic day (E) 11.5 to postnatal day (PN) 21 were used to investigate the expression of versican isoforms by immunohistochemistry, and the gene expressions of versican (Vcan) isoforms from E13.5 to PN7 were analyzed by quantitative real-time PCR. The results exhibited different expressing patterns of versican isoforms-the stellate reticulum (SR) and the dental mesenchymal cells adjacent to Hertwig's Epithelial Root Sheath (HERS) only expressed V1 and the mature odontoblasts mainly expressed V2, while the dental papilla and the ameloblasts might both express V0/V1/V2. These results suggested that different versican isoforms may act different roles in the tooth development, and we speculated that V0/V1 might be intimately involved in the cell proliferation while V2 was associated in the cytodifferentiation.

Enamel: Molecular identity of its transepithelial ion transport system

Enamel is the most calcified tissue in vertebrates. It differs from bone in a number of characteristics including its origin from ectodermal epithelium, lack of remodeling capacity by the enamel forming cells, and absence of collagen. The enamel-forming cells known as ameloblasts, choreograph first the synthesis of a unique protein-rich matrix, followed by the mineralization of this matrix into a tissue that is ∼95% mineral. To do this, ameloblasts arrange the coordinated movement of ions across a cell barrier while removing matrix proteins and monitoring extracellular pH using a variety of buffering systems to enable the growth of carbonated apatite crystals. Although our knowledge of these processes and the molecular identity of the proteins involved in transepithelial ion transport has increased in the last decade, it remains limited compared to other cells. Here we present an overview of the evolution and development of enamel, its differences with bone, and describe the ion transport systems associated with ameloblasts.

Comparative histological and immunohistochemical study of ameloblastomas and ameloblastic carcinomas

BACKGROUND

This study aimed to compare the histological and immunohistochemical characteristics of ameloblastomas (AM) and ameloblastic carcinomas (AC).

MATERIAL AND METHODS

Fifteen cases of AM and 9 AC were submitted to hematoxilin and eosin (H&E) and immunohistochemical analysis with the following antibodies: cytokeratins 5,7,8,14 and 19, Ki-67, p53, p63 and the cellular adhesion molecules CD138 (Syndecan-1), E-cadherin and β-catenin. The mean score of the expression of Ki-67 and p53 labelling index (LIs) were compared between the groups using the t test. A value of p<0.05 was considered to be statistically significant.

RESULTS

All cases were positive for CKs 5, 14 and 19, but negative for CKs 7 and 8. CKs 5 and 19 were positive mainly in the central regions of the ameloblastic islands, while the expression in AC was variable in intensity and localization. CK14 was also variably expressed in both AM and AC. Ki-67 (P=.001) and p53 (P=.004) immunoexpression was higher in AC. All cases were positive for p63, but values were higher in AC. CD138 was mainly expressed in peripheral cells of AM, with a weak positivity in the central areas, while it was positive in most areas of ACs, except in less differentiated regions, where expression was decreased or lost. E-cadherin and β-catenin were weakly positive in both AM and AC.

CONCLUSIONS

These results shows that Ki-67, p53 and p63 expression was higher in AC as compared to AM, suggesting that these markers can be useful when considering diagnosis of malignancy, and perhaps could play a role in malignant transformation of AM. Pattern of expression of CKs 5 and 19 in AC were different to those found in AM, suggesting genetic alterations of these proteins in malignant cells. It was confirmed that CK19 is a good marker for benign odontogenic tumors, such as AM, but it is variably expressed in malignant cases.

Epiprofin Regulates Enamel Formation and Tooth Morphogenesis by Controlling Epithelial-Mesenchymal Interactions During Tooth Development

The synchronization of cell proliferation and cytodifferentiation between dental epithelial and mesenchymal cells is required for the morphogenesis of teeth with the correct functional shapes and optimum sizes. Epiprofin (Epfn), a transcription factor belonging to the Sp family, regulates dental epithelial cell proliferation and is essential for ameloblast and odontoblast differentiation. Epfn deficiency results in the lack of enamel and ironically the formation of extra teeth. We investigated the mechanism underlying the functions of Epfn in tooth development through the creation of transgenic mice expressing Epfn under the control of an epithelial cell-specific K5 promoter (K5-Epfn). We found that these K5-Epfn mice developed abnormally shaped incisors and molars and formed fewer molars in the mandible. Remarkably, ameloblasts differentiated ectopically and enamel was formed on the lingual side of the K5-Epfn incisors. By contrast, ameloblasts and enamel were found only on the labial side in wild-type mice, as Follistatin (Fst) expressed in the lingual side inhibits BMP4 signaling necessary for ameloblast differentiation. We showed that Epfn transfection into the dental epithelial cell line SF2 abrogated the inhibitory activity of Fst and promoted ameloblast differentiation of SF2 cells. We found that Epfn induced FGF9 in dental epithelial cells and this dental epithelial cell-derived FGF9 promoted dental mesenchymal cell proliferation via the FGF receptor 1c (FGFR1c). Taken together, these results suggest that Epfn preserves the balance between cell proliferation and cytodifferentiation in dental epithelial and mesenchymal cells during normal tooth development and morphogenesis. © 2016 American Society for Bone and Mineral Research.

Regeneration of a Tooth in a Tissue-Engineered Mandible After Resection of a Central Giant Cell Tumor. Demonstrating Evidence of Functional Matrix Theory and Ectodermal Origin of Teeth in a Human Model-A Case Report

Central giant cell tumors (CGCTs) are uncommon lesions occurring in the jaw. They are benign but locally destructive osteolytic lesions. They usually occur in pediatric patients 5 to 15 years of age. Multiple noninvasive modalities of treatment (intralesional steroids, interferon, calcitonin, and denosumab) have been described for those lesions, but for those that are refractory to treatment, enucleation and curettage or resection is a curative surgery. This case report describes a pediatric patient who was diagnosed with an aggressive CGCT of the left mandible encompassing the right angle to the condyle. The lesion became refractory to noninvasive treatments and immediate resection and reconstruction was performed using principles of tissue engineering. After 5 years of close observation, the patient showed normal morphology and growth of his mandible, but surprisingly developed a left mandibular third molar (tooth 17) in the site of the mandibular resection and reconstruction. This is the first case report in the literature to show the spontaneous development of teeth in a human reconstructed mandible, contributing evidence toward the functional matrix theory of mandibular growth and ectodermal origin of teeth.

Test Tube Tooth: The Next Big Thing

Unlike some vertebrates and fishes, humans do not have the capacity for tooth regeneration after the loss of permanent teeth. Although artificial replacement with removable dentures, fixed prosthesis and implants is possible through advances in the field of prosthetic dentistry, it would be ideal to recreate a third set of natural teeth to replace lost dentition. For many years now, researchers in the field of tissue engineering have been trying to bioengineer dental tissues as well as whole teeth. In order to attain a whole tooth through dental engineering, that has the same or nearly same biological, mechanical and physical properties of a natural tooth, it's necessary to deal with all the cells and tissues which are concerned with the formation, maintenance and repair of the tooth. In this article we review the steps involved in odontogenesis or organogenesis of a tooth and progress in the bioengineering of a whole tooth.

Integration of tooth morphogenesis and innervation by local tissue interactions, signaling networks, and semaphorin 3A

The tooth, like many other organs, develops from both epithelial and mesenchymal tissues, and has proven to be a valuable tool with which to investigate organ formation and peripheral innervation. Tooth formation is regulated by local epithelial-mesenchymal tissue interactions, and is closely integrated with stereotypic dental nerve navigation and patterning. Recent analyses of the function and regulation of semaphorin 3A (SEMA3A) have shed light on the regulatory mechanisms that coordinate organogenesis and innervation at the tissue and molecular levels. In the tooth, SEM3A acts as a developmentally regulated secretory chemo-repellent, that controls tooth innervation during embryonic and postnatal development. The tooth germ governs its own innervation by a combination of local tissue interactions and SEMA3A expression. SEMA3A signaling, in turn, is controlled by a number of conserved signaling effectors, including TGF-β superfamily members, FGF, and WNT; all function in embryo and organ development, and are essential for tooth histo-morphogenesis. Thus, SEMA3A driven axon guidance is integrated into key odontogenic signaling networks, establishing this protein as a critical molecular tether between 2 distinct developmental processes (morphogenesis and sensory innervation), both of which are required to obtain a functional tooth.

Involvement of IGF-2, IGF-1R, IGF-2R and PTEN in development of human tooth germ - an immunohistochemical study

Insulin-Like Growth Factor 2 (IGF-2) is a peptide hormone essential for prenatal growth and development. IGF-2 exerts its mitogenic effects via Insulin-Like Growth Factor 1 Receptor (IGF-1R), and is eliminated by binding to Insulin-Like Growth Receptor 2 (IGF-2R). IGF-2 is also negatively regulated by Phosphatase and Tensin Homolog (PTEN), a phosphatase mutated in various tumors. Not much is known about the interplay between these factors during human odontogenesis. In this study, expression patterns of IGF-2, IGF-1R, IGF-2R and PTEN were analyzed by double immunofluorescence in incisor human tooth germs during the foetal period of development between the 7^th and 20^th gestational week. Throughout the investigated period, IGF-2 was mostly expressed in enamel organ, whereas mild to moderate expression of PTEN could be seen in dental papilla and parts of enamel organ. Expression of IGF-1R was ubiquitous and displayed strong intensity throughout the entire enamel organ. In contrast, expression of IGF-2R had rather erratic pattern in enamel organ and dental papilla alike. Expression patterns of IGF-2, IGF-1R, IGF-2R and PTEN in highly proliferative cervical loops, as well as in differentiating pre-ameloblasts and pre-odontoblasts of cusp tip region during the early and late bell stages when enamel organ acquires definitive shape, indicate importance of these factors in crown morphogenesis of human incisor. Taken together, our data suggest the involvement of IGF-2, IGF-1R, IGF-2R and PTEN in temporo-spatial patterning of basic cellular processes (proliferation, differentiation) during normal tooth development. They are also relevant for improving knowledge of molecular basis of human odontogenesis.

Hyaluronan and hyaluronan synthases expression and localization in embryonic mouse molars

Hyaluronan (HA) and hyaluronan synthases (HASs) have been shown to play critical roles in embryogenesis and organ development. However, there have not been any studies examining HA and HAS expression and localization during tooth development. The present study was designed to investigate the expression of HA and three isoforms of HASs (HAS1, 2, 3) in embryonic mouse molars. The first mandibular embryonic mouse molars were examined by immunohistochemistry at E11.5, E13.5, E14.5, E16.5, and E18.5. PCR and western blot analyses were performed on RNA and proteins samples from E13.5 to E18.5 tooth germs. At the initial stage (E11.5), HA and HASs were expressed in the dental epithelium but not the underlying dental mesenchyme. HA immunostaining gradually increased in the enamel organ from the bud stage (E13.5) to the late bell stage (E18.5), and HA and HASs were highly expressed in the stellate reticulum and stratum intermedium. HA immunostaining was also enhanced in the dental mesenchyme and its derived tissues, but it was not expressed in the ameloblast and odontoblast regions. The three HAS isoforms had distinct expression patterns, and they were expressed in the dental mesenchyme and odontoblast at various levels. Furthermore, HAS1 and HAS2 expression decreased, while HAS3 expression increased from E13.5 to E18.5. These results suggested that HA synthesized by different HASs is involved in embryonic mouse molar morphogenesis and cytodifferentiation.

Modification of tooth development by heat shock protein 60

Although several heat shock proteins have been investigated in relation to tooth development, no available information is available about the spatial and temporal expression pattern of heat shock protein 60 (Hsp 60). To characterize Hsp 60 expression in the structures of the developing tooth germ, we used Western blotting, immunohistochemistry and in situ hybridization. Hsp 60 was present in high amounts in the inner and outer enamel epithelia, enamel knot (EK) and stratum intermedium (SI). Hsp 60 also appeared in odontoblasts beginning in the bell stage. To obtain data on the possible effect of Hsp 60 on isolated lower incisors from mice, we performed in vitro culturing. To investigate the effect of exogenous Hsp 60 on the cell cycle during culturing, we used the 5-bromo-2-deoxyuridine (BrdU) incorporation test on dental cells. Exogenously administered Hsp 60 caused bluntness at the apical part of the 16.5-day-old tooth germs, but it did not influence the proliferation rate of dental cells. We identified the expression of Hsp 60 in the developing tooth germ, which was present in high concentrations in the inner and outer enamel epithelia, EK, SI and odontoblasts. High concentration of exogenous Hsp 60 can cause abnormal morphology of the tooth germ, but it did not influence the proliferation rate of the dental cells. Our results suggest that increased levels of Hsp 60 may cause abnormalities in the morphological development of the tooth germ and support the data on the significance of Hsp during the developmental processes.

Beta-Catenin and Plakoglobin Expression during Zebrafish Tooth Development and Replacement

We analyzed the protein distribution of two cadherin-associated molecules, plakoglobin and β-catenin, during the different stages of tooth development and tooth replacement in zebrafish. Plakoglobin was detected at the plasma membrane already at the onset of tooth development in the epithelial cells of the tooth. This pattern remained unaltered during further tooth development. The mesenchymal cells only showed plakoglobin from cytodifferentiation onwards. Plakoglobin 1a morpholino-injected embryos showed normal tooth development with proper initiation and differentiation. Although plakoglobin is clearly present during normal odontogenesis, the loss of plakoglobin 1a does not influence tooth development. β-catenin was found at the cell borders of all cells of the successional lamina but also in the nuclei of surrounding mesenchymal cells. Only membranous, not nuclear, β-catenin, was found during morphogenesis stage. However, during cytodifferentiation stage, both nuclear and membrane-bound β-catenin was detected in the layers of the enamel organ as well as in the differentiating odontoblasts. Nuclear β-catenin is an indication of an activated Wnt pathway, therefore suggesting a possible role for Wnt signalling during zebrafish tooth development and replacement.

Primary tooth size asymmetry in twins and singletons

OBJECTIVES

To explore asymmetry values of antimeric deciduous tooth crown dimensions in three types of twins: monozygotic (MZ), dizygotic same-sex (DZ) and opposite-sex (OS) vs. single-born controls.

SETTING AND SAMPLE POPULATION

Mesiodistal and labio-lingual crown dimensions of second deciduous molars and mesiodistal canine and first molar crown dimensions of 2159 children at 6-12 years of age were evaluated, originating from the US cross-sectional Collaborative Perinatal Study from the 1970s, including altogether MZ (n = 28), DZ same-sex (n = 33) and OS (n = 39) pairs. Single born (n = 1959) were used as controls.

MATERIAL AND METHODS

Dental casts were measured for comparison of variance relationships calculated from antimeric teeth, exhibiting fluctuating (FA), and directional (DA) asymmetry using anova.

RESULTS

Significant differences appeared in MZ and OS girls in DA of deciduous canines, which gain size in the first and second trimester, and deciduous second molars, which finally stop crown growth during the early post-natal period. Significantly, increased FA values appeared for lower deciduous canines and second molars, indicating greatest environmental stress in OS girls, MZ girls and DZ boys. Twin girls had more fluctuating and directional crown asymmetry than twin boys, but in some dimensions, the twins were more symmetric than controls.

CONCLUSIONS

Transmembrane hormonal influence between opposite-sex twins, and late gestational stress factors, caused by placental malfunction and/or monochorionicity, may be involved in asymmetric growth of antimers, during critical periods of crown size gain.

Klf10 regulates odontoblast differentiation and mineralization via promoting expression of dentin matrix protein 1 and dentin sialophosphoprotein genes

Klf10, a member of the Krüppel-like family of transcription factors, is critical for osteoblast differentiation, bone formation and mineralization. However, whether Klf10 is involved in odontoblastic differentiation and tooth development has not been determined. In this study, we investigate the expression patterns of Klf10 during murine tooth development in vivo and its role in odontoblastic differentiation in vitro. Klf10 protein was expressed in the enamel organ and the underlying mesenchyme, ameloblasts and odontoblasts at early and later stages of murine molar formation. Furthermore, the expression of Klf10, Dmp1, Dspp and Runx2 was significantly elevated during the process of mouse dental papilla mesenchymal differentiation and mineralization. The overexpression of Klf10 induced dental papilla mesenchymal cell differentiation and mineralization as detected by alkaline phosphatase staining and alizarin red S assay. Klf10 additionally up-regulated the expression of odontoblastic differentiation marker genes Dmp1, Dspp and Runx2 in mouse dental papilla mesenchymal cells. The molecular mechanism of Klf10 in controlling Dmp1 and Dspp expression is thus to activate their regulatory regions in a dosage-dependent manner. Our results suggest that Klf10 is involved in tooth development and promotes odontoblastic differentiation via the up-regulation of Dmp1 and Dspp transcription.

Tooth germ invagination from cell-cell interaction: Working hypothesis on mechanical instability

In the early stage of tooth germ development, the bud of the dental epithelium is invaginated by the underlying mesenchyme, resulting in the formation of a cap-like folded shape. This bud-to-cap transition plays a critical role in determining the steric design of the tooth. The epithelial-mesenchymal interaction within a tooth germ is essential for mediating the bud-to-cap transition. Here, we present a theoretical model to describe the autonomous process of the morphological transition, in which we introduce mechanical interactions among cells. Based on our observations, we assumed that peripheral cells of the dental epithelium bound tightly to each other to form an elastic sheet, and mesenchymal cells that covered the tooth germ would restrict its growth. By considering the time-dependent growth of cells, we were able to numerically show that the epithelium within the tooth germ buckled spontaneously, which is reminiscent of the cap-stage form. The difference in growth rates between the peripheral and interior parts of the dental epithelium, together with the steric size of the tooth germ, were determining factors for the number of invaginations. Our theoretical results provide a new hypothesis to explain the histological features of the tooth germ.

Analysis of expression patterns of IGF-1, caspase-3 and HSP-70 in developing human tooth germs

AIMS

To analyze expression patterns of IGF-1, caspase-3 and HSP-70 in human incisor and canine tooth germs during the late bud, cap and bell stages of odontogenesis.

MATERIALS AND METHODS

Head areas or parts of jaw containing teeth from 10 human fetuses aged between 9th and 20th developmental weeks were immunohistochemically analyzed using IGF-1, active caspase-3 and HSP-70 markers. Semi-quantitative analysis of each marker's expression pattern was also performed.

RESULTS

During the analyzed period, IGF-1 and HSP-70 were mostly expressed in enamel organ. As development progressed, expression of IGF-1 and HSP-70 became more confined to differentiating tissues in the future cusp tip area, as well as in highly proliferating cervical loops. Few apoptotic bodies highly positive to active caspase-3 were observed in enamel organ and dental papilla from the cap stage onward. However, both enamel epithelia moderately expressed active caspase-3 throughout the investigated period.

CONCLUSIONS

Expression patterns of IGF-1, active caspase-3 and HSP-70 imply importance of these factors for early human tooth development. IGF-1 and HSP-70 have versatile functions in control of proliferation, differentiation and anti-apoptotic protection of epithelial parts of human enamel organ. Active caspase-3 is partially involved in formation and apoptotic removal of primary enamel knot, although present findings might reflect its ability to perform other non-death functions such as differentiation of hard dental tissues secreting cells and guidance of ingrowth of proliferating cervical loops.

Cell-to-cell communication--periodontal regeneration

BACKGROUND

Although regenerative treatment options are available, periodontal regeneration is still regarded as insufficient and unpredictable.

AIM

This review article provides scientific background information on the animated 3D film Cell-to-Cell Communication - Periodontal Regeneration.

RESULTS

Periodontal regeneration is understood as a recapitulation of embryonic mechanisms. Therefore, a thorough understanding of cellular and molecular mechanisms regulating normal tooth root development is imperative to improve existing and develop new periodontal regenerative therapies. However, compared to tooth crown and earlier stages of tooth development, much less is known about the development of the tooth root. The formation of root cementum is considered the critical element in periodontal regeneration. Therefore, much research in recent years has focused on the origin and differentiation of cementoblasts. Evidence is accumulating that the Hertwig's epithelial root sheath (HERS) has a pivotal role in root formation and cementogenesis. Traditionally, ectomesenchymal cells in the dental follicle were thought to differentiate into cementoblasts. According to an alternative theory, however, cementoblasts originate from the HERS. What happens when the periodontal attachment system is traumatically compromised? Minor mechanical insults to the periodontium may spontaneously heal, and the tissues can structurally and functionally be restored. But what happens to the periodontium in case of periodontitis, an infectious disease, after periodontal treatment? A non-regenerative treatment of periodontitis normally results in periodontal repair (i.e., the formation of a long junctional epithelium) rather than regeneration. Thus, a regenerative treatment is indicated to restore the original architecture and function of the periodontium. Guided tissue regeneration or enamel matrix proteins are such regenerative therapies, but further improvement is required. As remnants of HERS persist as epithelial cell rests of Malassez in the periodontal ligament, these epithelial cells are regarded as a stem cell niche that can give rise to new cementoblasts. Enamel matrix proteins and members of the transforming growth factor beta (TGF-ß) superfamily have been implicated in cementoblast differentiation.

CONCLUSION

A better knowledge of cell-to-cell communication leading to cementoblast differentiation may be used to develop improved regenerative therapies to reconstitute periodontal tissues that were lost due to periodontitis.

Comparative studies between mice molars and incisors are required to draw an overview of enamel structural complexity

In the field of dentistry, the murine incisor has long been considered as an outstanding model to study amelogenesis. However, it clearly appears that enamel from wild type mouse incisors and molars presents several structural differences. In incisor, exclusively radial enamel is observed. In molars, enamel displays a high level of complexity since the inner part is lamellar whereas the outer enamel shows radial and tangential structures. Recently, the serotonin 2B receptor (5-HT_2BR) was shown to be involved in ameloblast function and enamel mineralization. The incisors from 5HT_2BR knockout (KO) mice exhibit mineralization defects mostly in the outer maturation zone and porous matrix network in the inner zone. In the molars, the mutation affects both secretory and maturation stages of amelogenesis since pronounced alterations concern overall enamel structures. Molars from 5HT_2BR KO mice display reduction in enamel thickness, alterations of inner enamel architecture including defects in Hunter-Schreger Bands arrangements, and altered maturation of the outer radial enamel. Differences of enamel structure were also observed between incisor and molar from other KO mice depleted for genes encoding enamel extracellular matrix proteins. Thus, upon mutation, enamel analysis based exclusively on incisor defects would be biased. In view of the functional relationship between enamel structure and tooth morphogenesis, identification of molecular actors involved in amelogenesis requires comparative studies between mice molars and incisors.

Expression of cytokeratin 8, vimentin, syndecan-1 and Ki-67 during human tooth development

Spatio-temporal immunolocalizations of cytokeratin 8 (CK8), vimentin, syndecan-1 and Ki-67 were analyzed in ten human incisors and canine tooth germs between the 7th and 20th developmental weeks. CK8 expression was mild to moderate in the epithelial tooth parts, while it shifted from absent or mild in its mesenchymal parts, but few cells, sparsely distributed throughout the tooth germ, strongly expressed CK8. As development progressed, CK8 expression increased to strong in preameloblasts, while expression of vimentin increased to moderate in the epithelial and mesenchymal tooth parts, particularly in the dental papilla and sac. Co-expression of CK8 and vimentin was observed in some parts of the tooth germ, and was increasing in the differentiating preameloblasts and preodontoblasts. Syndecan-1 showed characteristic shift of expression from epithelial to mesenchymal tooth parts, being particularly strong in dental papilla, sac and cervical loops, while co-expression of Ki-67/syndecan-1 was strong in the dental papilla. Our study demonstrated spatio-temporal expression and restricted co-expression of the investigated markers, indicating participation of CK8 and vimentin in cell proliferation and migration, and differentiation of preodontoblasts and preameloblasts. Our data also suggest involvement of syndecan-1 in morphogenesis of the developing tooth crown and cervical loops, and together with CK8 and vimentin in differentiation of preameloblasts and preodontoblasts.

Effect of radiotherapy on the eruption rate and morphology of the odontogenic region of rat incisors

OBJECTIVE

The goal in this study was to evaluate the results of doses of 5 and 15 Gy of radiation in odontogenic region of the rats inferior mandibular-incisors by a histological analysis and the rate of eruptions.

DESIGN

Animals were divided into three groups: control, radiotherapy 5 Gy and radiotherapy 15 Gy. In which tooth-eruption-rate was measured every two days.

RESULTS

Animals in Group 5 Gy presented values similar to those of the control group. Animals in Group 15 Gy presented reduction in tooth-eruption-rate as of the sixth day of the experiment, vast disorganization of odontoblasts and ameloblasts, apparent reduction in cell population in the follicle region and alterations in cervical loop formation of the dental organ.

CONCLUSIONS

It was concluded that there was a difference between the researched doses, and histological alteration at 15 Gy lead to statistical reduction in tooth-eruption-rate.

Malformation of Incisor Teeth in Grem2-/- Mice

GREMLIN 2 ( GREM2)—formerly, protein related to Dan and cerberus ( PRDC)—is a potent antagonist of the bone morphogenetic proteins 2 and 4, but little else in known about its functions. We found that Grem2-/- mice developed small deformed mandibular and maxillary incisors, indicating that GREMLIN2 is required for normal tooth morphogenesis. Although DEXA scans suggested that bone mineral density might be increased in Grem2-/- mice, histology did not reveal any evident bone phenotype. Grem2-/- mice did not display any other notable phenotypes evaluated in a high-throughput screening process that encompassed a range of immunologic, metabolic, ophthalmic, and behavioral parameters. Our findings indicate that Grem2 can be added to the growing list of genes that affect tooth development in mice.

Three-dimensional analysis of the early development of the dentition

Tooth development has attracted the attention of researchers since the 19th century. It became obvious even then that morphogenesis could not fully be appreciated from two-dimensional histological sections. Therefore, methods of three-dimensional (3D) reconstructions were employed to visualize the surface morphology of developing structures and to help appreciate the complexity of early tooth morphogenesis. The present review surveys the data provided by computer-aided 3D analyses to update classical knowledge of early odontogenesis in the laboratory mouse and in humans. 3D reconstructions have demonstrated that odontogenesis in the early stages is a complex process which also includes the development of rudimentary odontogenic structures with different fates. Their developmental, evolutionary, and pathological aspects are discussed. The combination of in situ hybridization and 3D reconstruction have demonstrated the temporo-spatial dynamics of the signalling centres that reflect transient existence of rudimentary tooth primordia at loci where teeth were present in ancestors. The rudiments can rescue their suppressed development and revitalize, and then their subsequent autonomous development can give rise to oral pathologies. This shows that tooth-forming potential in mammals can be greater than that observed from their functional dentitions. From this perspective, the mouse rudimentary tooth primordia represent a natural model to test possibilities of tooth regeneration.

Molecular markers of cell adhesion in ameloblastomas. An update

Ameloblastoma is the most common odontogenic tumor of epithelial origin, and though it is of a benign nature, it frequently infiltrates the bone, has a high rate of recurrence and could potentially become malignant. Cellular adhesion potentially plays an important role in the manifestation of these characteristics and in the tumor biology of ameloblastomas. Losses of cell-cell and extracellular matrix adhesion and cohesion are among the first events that occur in the invasion and growth of tumors of epithelial origin. The present review includes a description of the molecules that are involved in cell adhesion as reported for various types of ameloblastomas and discusses the possible roles of these molecules in the biological behaviors of this odontogenic tumor. Knowledge of the complex mechanisms in which these molecules play a role is critical for the research and discovery of future therapeutic targets.

Induction of dental epithelial cell differentiation marker gene expression in non-odontogenic human keratinocytes by transfection with thymosin beta 4

Previous studies have shown that the recombination of cells liberated from developing tooth germs develop into teeth. However, it is difficult to use human developing tooth germ as a source of cells because of ethical issues. Previous studies have reported that thymosin beta 4 (Tmsb4x) is closely related to the initiation and development of the tooth germ. We herein attempted to establish odontogenic epithelial cells from non-odontogenic HaCaT cells by transfection with TMSB4X. TMSB4X-transfected cells formed nodules that were positive for Alizarin-red S (ALZ) and von Kossa staining (calcium phosphate deposits) when cultured in calcification-inducing medium. Three selected clones showing larger amounts of calcium deposits than the other clones, expressed PITX2, Cytokeratin 14, and Sonic Hedgehog. The upregulation of odontogenesis-related genes, such as runt-related transcription factor 2 (RUNX2), Amelogenin (AMELX), Ameloblastin (AMBN) and Enamelin (ENAM) was also detected. These proteins were immunohistochemically observed in nodules positive for the ALZ and von Kossa staining. RUNX2-positive selected TMSB4X-transfected cells implanted into the dorsal subcutaneous tissue of nude mice formed matrix deposits. Immunohistochemically, AMELX, AMBN and ENAM were observed in the matrix deposits. This study demonstrated the possibility of induction of dental epithelial cell differentiation marker gene expression in non-odontogenic HaCaT cells by TMSB4X.

The role of nectins in different types of cell-cell adhesion

Mammalian tissues and organs are composed of different types of cells that adhere to each other homotypically (i.e. interactions between cells of the same cell type) or heterotypically (i.e. interactions between different cell types), forming a variety of cellular patterns, including mosaic patterns. At least three types of cell-cell adhesion have been observed: symmetric homotypic, asymmetric homotypic and heterotypic cell adhesions. Cadherins and nectins, which are known cell-cell adhesion molecules, mediate these cell adhesions. Cadherins comprise a family of more than 100 members, but they are primarily involved in homophilic trans-interactions (i.e. interactions between the same cadherin members) between opposing cells. By contrast, the nectin family comprises only four members, and these proteins form both homophilic and heterophilic trans-interactions (i.e. interactions between the same and different nectin members on opposing cells). In addition, heterophilic trans-interactions between nectins are much stronger than homophilic trans-interactions. Because of these unique properties, nectins have crucial roles in asymmetric homotypic cell-cell adhesion at neuronal synapses and in various types of heterotypic cell-cell adhesions. We summarize recent progress in our understanding of the biology of nectins and discuss their roles in heterotypic cell-cell adhesions, whose formation cannot be solely explained by the action of cadherins.

Intrauterine hormone effects on tooth dimensions

The human dentition is a complex adaptive system that is influenced by genetic, epigenetic, and environmental factors. Within this system, is sexual dimorphism related to the growth promotion of the Y chromosome, or to hormonal influences, or both? This study is the first to investigate both primary and permanent tooth sizes in females from opposite-sex dizygotic (DZOS) twin pairs compared with females from dizygotic same-sex (DZSS) and monozygotic (MZ) twin pairs to indicate the influence of intrauterine male hormone, including the initial testosterone surge, on dental development. Serial dental models of the primary, mixed, and permanent dentitions of 134 females from DZOS, DZSS, and MZ twins were examined. Mesiodistal, buccolingual, crown height, and intercuspal dimensions of all primary teeth and selected permanent teeth were determined by image analysis. Univariate and multivariate analyses showed statistically significantly larger crown size in DZOS females in both dentitions, with the crown height dimensions displaying the greatest increase in size. These findings strongly support the Twin Testosterone Transfer (TTT) hypothesis. We propose that the growth-promoting effects of the Y chromosome and intrauterine male hormone levels influence different tooth dimensions and contribute differentially to the sexual dimorphism of human teeth.

Adult human gingival epithelial cells as a source for whole-tooth bioengineering

Teeth develop from interactions between embryonic oral epithelium and neural-crest-derived mesenchyme. These cells can be separated into single-cell populations and recombined to form normal teeth, providing a basis for bioengineering new teeth if suitable, non-embryonic cell sources can be identified. We show here that cells can be isolated from adult human gingival tissue that can be expanded in vitro and, when combined with mouse embryonic tooth mesenchyme cells, form teeth. Teeth with developing roots can be produced from this cell combination following transplantation into renal capsules. These bioengineered teeth contain dentin and enamel with ameloblast-like cells and rests of Malassez of human origin.

microRNA miR-34a regulates cytodifferentiation and targets multi-signaling pathways in human dental papilla cells

Odontogenesis relies on the reciprocal signaling interactions between dental epithelium and neural crest-derived mesenchyme, which is regulated by several signaling pathways. Subtle changes in the activity of these major signaling pathways can have dramatic effects on tooth development. An important regulator of such subtle changes is the fine tuning function of microRNAs (miRNAs). However, the underlying mechanism by which miRNAs regulate tooth development remains elusive. This study determined the expression of miRNAs during cytodifferentiation in the human tooth germ and studied miR-34a as a regulator of dental papilla cell differentiation. Using microarrays, miRNA expression profiles were established at selected times during development (early bell stage or late bell stage) of the human fetal tooth germ. We identified 29 differentially expressed miRNAs from early bell stage/late bell stage comparisons. Out of 6 miRNAs selected for validation by qPCR, all transcripts were confirmed to be differentially expressed. miR-34a was selected for further investigation because it has been previously reported to regulate organogenesis. miR-34a mimics and inhibitors were transfected into human fetal dental papilla cells, mRNA levels of predicted target genes were detected by quantitative real-time PCR, and levels of putative target proteins were examined by western blotting. ALP and DSPP expression were also tested by qPCR, western blotting, and immunofluorescence. Findings from these studies suggested that miR-34a may play important roles in dental papilla cell differentiation during human tooth development by targeting NOTCH and TGF-beta signaling.

In-vitro regulation of odontogenic gene expression in human embryonic tooth cells and SHED cells

The bud-to-cap stage transition during early tooth development is a time when the tooth-inducing potential becomes restricted to the mesenchyme. Several key genes, expressed in the mesenchyme at this stage, are an absolute requirement for the progression of tooth development. These include the transcription factors Msx1 and Pax9. The inductive potential of tooth mesenchyme cells is a key requisite for whole-tooth bioengineering and thus identification of cells that can retain this property following expansion in culture is an important as yet unresolved, goal. We show here that in-vitro culture of embryonic human tooth mesenchyme cells and SHED cells express low levels of PAX9 and MSX1 and that these levels can be significantly upregulated by activation of different signalling pathways. Such in-vitro manipulation may thus offer a simple way of maintaining/restoring/inducing the odontogenic-inducing capacity in mesenchymal cells.

Developmentally regulated expression of intracellular Fgf11-13, hormone-like Fgf15 and canonical Fgf16, -17 and -20 mRNAs in the developing mouse molar tooth

OBJECTIVE

To investigate and compare the cellular expression of non-secreted Fgf11-14 and secreted Fgf15-18 and -20 mRNAs during tooth formation.

MATERIALS AND METHODS

mRNA expression was analyzed from the morphological initiation of the mouse mandibular first molar development to the onset of crown calcification using sectional in situ hybridization.

RESULTS

This study found distinct, differentially regulated expression patterns for the Fgf11-13, -15-17 and -20, in particular in the epithelial-mesenchymal interface, whereas Fgf14 and 18 mRNAs were not detected. Fgf11, -15, -16, -17 and -20 were seen in the epithelium, whereas Fgf12 and -13 signals were restricted to the mesenchymal tissue component of the tooth. Fgf11 was observed in the putative epithelial signaling areas, the tertiary enamel knots and enamel free areas of the calcifying crown. Fgf15, Fgf17 and -20 were transiently colocalized in the thickened dental epithelium at E11.5. Later Fgf15 and -20 were exclusively expressed in the epithelial enamel knot signaling centers. In contrast, Fgf13 was present in the dental mesenchyme including odontoblasts cell lineage, whereas Fgf12 appeared transiently in the preodontoblasts.

CONCLUSIONS

The expression of the Fgf11-13, -15, -17 and -20 in the epithelial signaling centers and/or epithelial-mesenchymal interfaces at key stages of the tooth formation suggest important functions in odontogenesis. Future analyses of the transgenic mice will help elucidate in vivo functions of the studied Fgfs during odontogenesis and whether any of the functions of the tooth expressed epithelial and mesenchymal Fgfs of different sub-families are redundant.

Expression and characterization of c-Myb in prenatal odontogenesis

The transcription factor c-Myb is involved in the control of cell proliferation, survival and differentiation. As these processes accompany the morphogenesis of developing teeth, this work investigates the possible role of c-Myb during odontogenesis. Analysis of the expression of c-Myb in the monophyodont mouse was followed by similar analysis in a diphyodont species, the pig, which has a dentition more closely resembling that of the human. The distribution of c-Myb was correlated with the pattern of proliferation and apoptosis and the tooth phenotype of c-Myb mutant mice was also assessed. In the mouse, c-Myb expression was detected throughout prenatal development of the first molar tooth. Negative temporospatial correlation was found between c-Myb expression and apoptosis, while c-Myb expression positively correlated with proliferation. c-Myb-positive cells, however, were more abundant than the proliferating cell nuclear antigen positive cells, suggesting other roles of c-Myb in odontogenesis. In the minipig, in contrast to the mouse, there was an asymmetrical arrangement of c-Myb positive cells, with a higher presence on the labial side of the tooth germ and dental lamina. A cluster of negative cells was situated in the mesenchyme close to the tooth bud. At later stages, the number of positive cells decreased and these cells were situated in the upper part of the dental papilla in the areas of future cusp formation. The expression of c-Myb in both species was strong in the odontoblasts and ameloblasts at the stage of dentin and enamel production suggesting a possible novel role of c-Myb during tooth mineralization.

Stem cell-based biological tooth repair and regeneration

Teeth exhibit limited repair in response to damage, and dental pulp stem cells probably provide a source of cells to replace those damaged and to facilitate repair. Stem cells in other parts of the tooth, such as the periodontal ligament and growing roots, play more dynamic roles in tooth function and development. Dental stem cells can be obtained with ease, making them an attractive source of autologous stem cells for use in restoring vital pulp tissue removed because of infection, in regeneration of periodontal ligament lost in periodontal disease, and for generation of complete or partial tooth structures to form biological implants. As dental stem cells share properties with mesenchymal stem cells, there is also considerable interest in their wider potential to treat disorders involving mesenchymal (or indeed non-mesenchymal) cell derivatives, such as in Parkinson's disease.

Cooperation of nectin-1 and nectin-3 is required for normal ameloblast function and crown shape development in mouse teeth

Nectins are immunoglobulin-like cell adhesion proteins and their interactions recruit various cell-cell junctions. Mutations in human NECTIN-1 cause an ectodermal dysplasia syndrome, but Nectin-1 null mice have only slight defects in teeth, suggesting compensation by other nectin(s). We observed overlapping expression of nectin-3 with nectin-1 and enamel abnormality in the nectin-3 mutant. We, therefore, generated nectin-1;nectin-3 compound mutants. However, all teeth developed and no significant dental abnormalities were observed before birth. At postnatal day 10, the upper molars of compound mutants exhibited conical crown shape and retarded enamel maturation. Nectin-1 was expressed in ameloblasts whereas nectin-3 was expressed in neighboring stratum intermedium cells at this stage. The immunohistochemical localization and electron microscopical observations indicated that the desmosomal junctions between stratum intermedium and ameloblasts were significantly reduced. These results suggest that heterophilic interaction between nectin-1 and nectin-3 recruits desmosomal junctions, and that these are required for proper enamel formation.

Dact1-3 mRNAs exhibit distinct expression domains during tooth development

Wnt signaling is essential for tooth formation and Dact proteins modulate Wnt signaling by binding to the intracellular protein Dishevelled (Dvl). Comparison of the three known mouse Dact genes, Dact1-3, from the morphological initiation of mandibular first molar development through the onset of root formation using section in situ hybridization showed distinct, complementary and overlapping expression patterns for these genes. Whereas Dact2 expression was restricted to the dental epithelium, including the enamel knot signaling centers and pre-ameloblasts, Dact1 and Dact3 showed developmentally regulated expression in the dental mesenchyme. Both Dact1 and Dact3 mRNAs were first detected in the presumptive dental mesenchyme. After being downregulated from the condensing dental mesenchyme of the bud stage tooth germ, Dact1 was upregulated in the dental follicle mesenchyme at the cap stage and subsequently also in the dental papilla at the bell stage, where the expression persisted to the postnatal stages. In contrast, Dact3 transcripts persisted throughout the dental mesenchyme, including the preodontoblasts, during embryogenesis before transcripts were largely downregulated from the tooth germ postnatally. Collectively, these results suggest that Dact1 and -3 may contribute to early tooth formation by modulation of Wnt signaling pathways in the mesenchyme, including preodontoblasts, whereas Dact2 may play important signal-modulating roles in the adjacent epithelial cells including the enamel knot signaling centers and pre-ameloblasts. Future loss-of-function studies will help elucidate whether any of these functions are redundant, particularly for Dact1 and Dact3.

Multilevel complex interactions between genetic, epigenetic and environmental factors in the aetiology of anomalies of dental development

Dental anomalies are caused by complex interactions between genetic, epigenetic and environmental factors during the long process of dental development. This process is multifactorial, multilevel, multidimensional and progressive over time. In this paper the evidence from animal models and from human studies is integrated to outline the current position and to construct and evaluate models, as a basis for future work. Dental development is multilevel entailing molecular and cellular interactions which have macroscopic outcomes. It is multidimensional, requiring developments in the three spatial dimensions and the fourth dimension of time. It is progressive, occurring over a long period, yet with critical stages. The series of interactions involving multiple genetic signalling pathways are also influenced by extracellular factors. Interactions, gradients and spatial field effects of multiple genes, epigenetic and environmental factors all influence the development of individual teeth, groups of teeth and the dentition as a whole. The macroscopic, clinically visible result in humans is a complex unit of four different tooth types formed in morphogenetic fields, in which teeth within each field form directionally and erupt at different times, reflecting the spatio-temporal control of development. Even when a specific mutation of a single gene or one major environmental insult has been identified in a patient with a dental anomaly, detailed investigation of the phenotype often reveals variation between affected individuals in the same family, between dentitions in the same individual and even between different teeth in the same dentition. The same, or closely similar phenotypes, whether anomalies of tooth number or structure, may arise from different aetiologies: not only mutations in different genes but also environmental factors may result in similar phenotypes. Related to the action of a number of the developmental regulatory genes active in odontogenesis, in different tissues, mutations can result in syndromes of which dental anomalies are part. Disruption of the antagonistic balance between developmental regulatory genes, acting as activators or inhibitors can result in dental anomalies. There are critical stages in the development of the individual tooth germs and, if progression fails, the germ will not develop further or undergoes apoptosis. The reiterative signalling patterns over time during the sequential process of initiation and morphogenesis are reflected in the clinical association of anomalies of number, size and form and the proposed models. An initial step in future studies is to combine the genetic investigations with accurate recording and measurement of the phenotype. They also need to collate findings at each level and exploit the accurate definition of both human and murine phenotypes now possible.