Immunolocalization of BMP-2/-4, FGF-4, and WNT10b in the developing mouse first lower molar

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.

FGF4 and FGF9 have synergistic effects on odontoblast differentiation

The purpose of this study was to investigate whether fibroblast growth factor 4 (FGF4) and FGF9 are active in dentin differentiation. Dentin matrix protein 1 (Dmp1) -2A-Cre transgenic mice, which express the Cre-recombinase in Dmp1-expressing cells, were crossed with CAG-tdTomato mice as reporter mouse. The cell proliferation and tdTomato expressions were observed. The mesenchymal cell separated from neonatal molar tooth germ were cultured with or without FGF4, FGF9, and with or without their inhibitors ferulic acid and infigratinib (BGJ398) for 21 days. Their phenotypes were evaluated by cell count, flow cytometry, and real-time PCR. Immunohistochemistry for FGFR1, 2, and 3 expression and the expression of DMP1 were performed. FGF4 treatment of mesenchymal cells obtained promoted the expression of all odontoblast markers. FGF9 failed to enhance dentin sialophosphoprotein (Dspp) expression levels. Runt-related transcription factor 2 (Runx2) was upregulated until day 14 but was downregulated on day 21. Compared to Dmp1-negative cells, Dmp1-positive cells expressed higher levels of all odontoblast markers, except for Runx2. Simultaneous treatment with FGF4 and FGF9 had a synergistic effect on odontoblast differentiation, suggesting that they may play a role in odontoblast maturation.

BMP Signaling Pathway in Dentin Development and Diseases

BMP signaling plays an important role in dentin development. BMPs and antagonists regulate odontoblast differentiation and downstream gene expression via canonical Smad and non-canonical Smad signaling pathways. The interaction of BMPs with their receptors leads to the formation of complexes and the transduction of signals to the canonical Smad signaling pathway (for example, BMP ligands, receptors, and Smads) and the non-canonical Smad signaling pathway (for example, MAPKs, p38, Erk, JNK, and PI3K/Akt) to regulate dental mesenchymal stem cell/progenitor proliferation and differentiation during dentin development and homeostasis. Both the canonical Smad and non-canonical Smad signaling pathways converge at transcription factors, such as Dlx3, Osx, Runx2, and others, to promote the differentiation of dental pulp mesenchymal cells into odontoblasts and downregulated gene expressions, such as those of DSPP and DMP1. Dysregulated BMP signaling causes a number of tooth disorders in humans. Mutation or knockout of BMP signaling-associated genes in mice results in dentin defects which enable a better understanding of the BMP signaling networks underlying odontoblast differentiation and dentin formation. This review summarizes the recent advances in our understanding of BMP signaling in odontoblast differentiation and dentin formation. It includes discussion of the expression of BMPs, their receptors, and the implicated downstream genes during dentinogenesis. In addition, the structures of BMPs, BMP receptors, antagonists, and dysregulation of BMP signaling pathways associated with dentin defects are described.

Differential lncRNA/mRNA Expression Profiling and Functional Network Analyses in Bmp2 Deletion of Mouse Dental Papilla Cells

Bmp2 is essential for dentin development and formation. Bmp2 conditional knock-out (KO) mice display a similar tooth phenotype of dentinogenesis imperfecta (DGI). To elucidate a foundation for subsequent functional studies of cross talk between mRNAs and lncRNAs in Bmp2-mediated dentinogenesis, we investigated the profiling of lncRNAs and mRNAs using immortalized mouse dental Bmp2 flox/flox (iBmp2^fx/fx) and Bmp2 knock-out (iBmp2^ko/ko) papilla cells. RNA sequencing was implemented to study the expression of the lncRNAs and mRNAs. Quantitative real-time PCR (RT-qPCR) was used to validate expressions of lncRNAs and mRNAs. The Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) databases were used to predict functions of differentially expressed genes (DEGs). Protein-protein interaction (PPI) and lncRNA-mRNA co-expression network were analyzed by using bioinformatics methods. As a result, a total of 22 differentially expressed lncRNAs (16 downregulated vs 6 upregulated) and 227 differentially expressed mRNAs (133 downregulated vs. 94 upregulated) were identified in the iBmp2^ko/ko cells compared with those of the iBmp2^fx/fx cells. RT-qPCR results showed significantly differential expressions of several lncRNAs and mRNAs which were consistent with the RNA-seq data. GO and KEGG analyses showed differentially expressed genes were closely related to cell differentiation, transcriptional regulation, and developmentally relevant signaling pathways. Moreover, network-based bioinformatics analysis depicted the co-expression network between lncRNAs and mRNAs regulated by Bmp2 in mouse dental papilla cells and symmetrically analyzed the effect of Bmp2 during dentinogenesis via coding and non-coding RNA signaling.

Role of Cell Death in Cellular Processes During Odontogenesis

The development of a tooth germ in a precise size, shape, and position in the jaw, involves meticulous regulation of cell proliferation and cell death. Apoptosis, as the most common type of programmed cell death during embryonic development, plays a number of key roles during odontogenesis, ranging from the budding of the oral epithelium during tooth initiation, to later tooth germ morphogenesis and removal of enamel knot signaling center. Here, we summarize recent knowledge about the distribution and function of apoptotic cells during odontogenesis in several vertebrate lineages, with a special focus on amniotes (mammals and reptiles). We discuss the regulatory roles that apoptosis plays on various cellular processes during odontogenesis. We also review apoptosis-associated molecular signaling during tooth development, including its relationship with the autophagic pathway. Lastly, we cover apoptotic pathway disruption, and alterations in apoptotic cell distribution in transgenic mouse models. These studies foster a deeper understanding how apoptotic cells affect cellular processes during normal odontogenesis, and how they contribute to dental disorders, which could lead to new avenues of treatment in the future.

Homeobox protein MSX-1 inhibits expression of bone morphogenetic protein 2, bone morphogenetic protein 4, and lymphoid enhancer-binding factor 1 via Wnt/β-catenin signaling to prevent differentiation of dental mesenchymal cells during the late bell stage

Homeobox protein MSX-1 (hereafter referred to as MSX-1) is essential for early tooth-germ development. Tooth-germ development is arrested at bud stage in Msx1 knockout mice, which prompted us to study the functions of MSX-1 beyond this stage. Here, we investigated the roles of MSX-1 during late bell stage. Mesenchymal cells of the mandibular first molar were isolated from mice at embryonic day (E)17.5 and cultured in vitro. We determined the expression levels of β-catenin, bone morphogenetic protein 2 (Bmp2), Bmp4, and lymphoid enhancer-binding factor 1 (Lef1) after knockdown or overexpression of Msx1. Our findings suggest that knockdown of Msx1 promoted expression of Bmp2, Bmp4, and Lef1, resulting in elevated differentiation of odontoblasts, which was rescued by blocking the expression of these genes. In contrast, overexpression of Msx1 decreased the expression of Bmp2, Bmp4, and Lef1, leading to a reduction in odontoblast differentiation. The regulation of Bmp2, Bmp4, and Lef1 by Msx1 was mediated by the Wnt/β-catenin signaling pathway. Additionally, knockdown of Msx1 impaired cell proliferation and slowed S-phase progression, while overexpression of Msx1 also impaired cell proliferation and prolonged G1-phase progression. We therefore conclude that MSX-1 maintains cell proliferation by regulating transition of cells from G1-phase to S-phase and prevents odontoblast differentiation by inhibiting expression of Bmp2, Bmp4, and Lef1 at the late bell stage via the Wnt/β-catenin signaling pathway.

Tooth Morphogenesis and FGF4 Expression During Development of Molar Tooth in Three Muroid Rodents: Calomyscus elburzensis (Calomyscidae), Mesocricetus auratus (Cricetidae) and Mus musculus (Muridae)

To date, no studies have examined the tooth formation during developmental stages of brush-tailed mice (Calomyscidae) and true hamsters (Cricetidae). Herein, we compared the timing of tooth morphogenesis and FGF4 expression pattern during development of the first lower molar in Goodwin's brush-tailed mouse, Calomyscus elburzensis with two other muroid rodents; the house mouse, Mus musculus (Muridae), model organism for tooth morphogenesis, and the golden hamster, Mesocricetus auratus which shares great similarities in cusp pattern with brush-tailed mice. All three species were bred in captivity and developing embryos were isolated at different embryonic days (E). Histological evaluation of lower molars was performed and spatiotemporal pattern of FGF4 expression was determined by immunohistochemistry. Results indicated that morphogenesis of the tooth cusps starts at the beginning of the cap stage of the first lower molar (E14 in house mouse, about E11.5 in golden hamster and E22 in Goodwin's brush-tailed mouse). During the cap to bell stage (E15 in house mouse, E12 in golden hamster and at about E24 in Goodwin's brush-tailed mouse), a decrease in the expression of FGF4 was observed in the mesenchyme, except for the cusp tips. According to our observations, the developmental process of the first lower molar formation in Goodwin's brush-tailed mouse began much later as compared with the other two species. Despite the differences in the temporal pattern of molar development between these three members of the same superfamily (Muroidea), the correlation in the expression of FGF4 with specific stages of tooth morphogenesis supported its regulatory function. Anat Rec, 300:2138-2149, 2017. © 2017 Wiley Periodicals, Inc.

Cytokine and hormonal regulation of bone marrow immune cell Wnt10b expression

Background & aims

Wnt10b is a crucial regulator of bone density through its ability to promote osteoblastogenesis. Parathyroid hormone has been shown to regulate Wnt10b expression in CD8⁺ T cells. However, the relative expression and other source(s) of Wnt10b in the bone marrow immune cells (BMICs) is unknown. Sex hormones and cytokines such as, estrogen and TNFα are critical regulators of bone physiology but whether they regulate BMIC Wnt10b expression is unclear. To determine the potential regulation of Wnt10b by estrogen and TNFα, we assessed Wnt10b expression by flow cytometry under estrogen- and TNFα-deficient conditions.

Methods

Effects of TNFα was determined in male and female C57BL/6 wildtype and TNFα knockout mice. Effect of estrogen was investigated 4, 6 and 8 weeks post-surgery in ovariectomized Balb/c mice. Intracellular Wnt10b was detected using goat anti-mouse Wnt10b and a conjugated secondary antibody and analyzed by flow cytometry.

Results

Wnt10b expression was sex- and lineage-specific. Females had 1.8-fold higher Wnt10b signal compared to males. Percent of Wnt10b⁺ myeloid cells was higher in females than males (8.9% Vs 5.4%) but Wnt10b⁺ lymphoid cells was higher in males than females (6.3% Vs 2.5%). TNFα ablation in males increased total BM Wnt10b expression 1.5-fold but significantly reduced the percentage of BM Wnt10b⁺ CD4⁺ T cells (65%), CD8⁺ T cells (59%), dendritic cells (59%), macrophages (56%) and granulocytes (52%). These effects of TNFα on Wnt10b were observed only in males. In contrast to TNFα, estrogen-deficiency had indirect effects on BMIC Wnt10b levels; reducing the average percentage of BM Wnt10b⁺ CD8⁺ T cells (25%) and granulocytes (26%) across an 8-week time course.

Conclusion

Our results demonstrate unique cell type- and sex-dependent effects on BMIC Wnt10b expression. Together, our results reveal myeloid cells in the bone marrow as an important source of Wnt10b under complex hormonal and cytokine regulation.

FGF2, FGF3 and FGF4 expression pattern during molars odontogenesis in Didelphis albiventris

Odontogenesis is guided by a complex signaling cascade in which several molecules, including FGF2-4, ensure all dental groups development and specificity. Most of the data on odontogenesis derives from rodents, which does not have all dental groups. Didelphis albiventris is an opossum with the closest dentition to humans, and the main odontogenesis stages occur when the newborns are in the pouch. In this study, D. albiventris postnatals were used to characterize the main stages of their molars development; and also to establish FGF2, FGF3 and FGF4 expression pattern. D. albiventris postnatals were processed for histological and indirect immunoperoxidase analysis of the tooth germs. Our results revealed similar dental structures between D. albiventris and mice. However, FGF2, FGF3 and FGF4 expression patterns were observed in a larger number of dental structures, suggesting broader functions for these molecules in this opossum species. The knowledge of the signaling that determinates odontogenesis in an animal model with complete dentition may contribute to the development of therapies for the replacement of lost teeth in humans. This study may also contribute to the implementation of D. albiventris as model for Developmental Biology studies.

Evolution of gremlin 2 in cetartiodactyl mammals: gene loss coincides with lack of upper jaw incisors in ruminants

Understanding the processes that give rise to genomic variability in extant species is an active area of research within evolutionary biology. With the availability of whole genome sequences, it is possible to quantify different forms of variability such as variation in gene copy number, which has been described as an important source of genetic variability and in consequence of phenotypic variability. Most of the research on this topic has been focused on understanding the biological significance of gene duplication, and less attention has been given to the evolutionary role of gene loss. Gremlin 2 is a member of the DAN gene family and plays a significant role in tooth development by blocking the ligand-signaling pathway of BMP2 and BMP4. The goal of this study was to investigate the evolutionary history of gremlin 2 in cetartiodactyl mammals, a group that possesses highly divergent teeth morphology. Results from our analyses indicate that gremlin 2 has experienced a mixture of gene loss, gene duplication, and rate acceleration. Although the last common ancestor of cetartiodactyls possessed a single gene copy, pigs and camels are the only cetartiodactyl groups that have retained gremlin 2. According to the phyletic distribution of this gene and synteny analyses, we propose that gremlin 2 was lost in the common ancestor of ruminants and cetaceans between 56.3 and 63.5 million years ago as a product of a chromosomal rearrangement. Our analyses also indicate that the rate of evolution of gremlin 2 has been accelerated in the two groups that have retained this gene. Additionally, the lack of this gene could explain the high diversity of teeth among cetartiodactyl mammals; specifically, the presence of this gene could act as a biological constraint. Thus, our results support the notions that gene loss is a way to increase phenotypic diversity and that gremlin 2 is a dispensable gene, at least in cetartiodactyl mammals.

Evolutionary aspects of the development of teeth and baleen in the bowhead whale

In utero, baleen whales initiate the development of several dozens of teeth in upper and lower jaws. These tooth germs reach the bell stage and are sometimes mineralized, but toward the end of prenatal life they are resorbed and no trace remains after birth. Around the time that the germs disappear, the keratinous baleen plates start to form in the upper jaw, and these form the food-collecting mechanism. Baleen whale ancestors had two generations of teeth and never developed baleen, and the prenatal teeth of modern fetuses are usually interpreted as an evolutionary leftover. We investigated the development of teeth and baleen in bowhead whale fetuses using histological and immunohistochemical evidence. We found that upper and lower dentition initially follow similar developmental pathways. As development proceeds, upper and lower tooth germs diverge developmentally. Lower tooth germs differ along the length of the jaw, reminiscent of a heterodont dentition of cetacean ancestors, and lingual processes of the dental lamina represent initiation of tooth bud formation of replacement teeth. Upper tooth germs remain homodont and there is no evidence of a secondary dentition. After these germs disappear, the oral epithelium thickens to form the baleen plates, and the protein FGF-4 displays a signaling pattern reminiscent of baleen plates. In laboratory mammals, FGF-4 is not involved in the formation of hair or palatal rugae, but it is involved in tooth development. This leads us to propose that the signaling cascade that forms teeth in most mammals has been exapted to be involved in baleen plate ontogeny in mysticetes.

Role of the Wnt signaling molecules in the tooth

Wnt signaling plays a central role in many processes during embryonic development and adult homeostasis. At least 19 types of Wnt ligands, receptors, transducers, transcription factors, and antagonists have been identified in mammals. Two distinct Wnt signaling pathways, the canonical signaling pathway and the noncanonical signaling pathway, have been described. Some Wnt signaling pathway components are expressed in the dental epithelium and mesenchyme during tooth development in humans and mice. Functional studies and experimental analysis of relevant animal models confirm the effects of Wnt signaling pathway on the regulation of developing tooth formation and adult tooth homeostasis. Mutations in some Wnt signaling pathway components have been identified in syndromic and non-syndromic tooth agenesis. This review provides an overview of progress in elucidating the role of Wnt signaling pathway components in the tooth and the resulting possibilities for therapeutic development.

A systematic analysis for localization of predominant growth factors and their receptors involved in murine tooth germ differentiation using in situ hybridization technique

Tooth development is regulated by various growth factors and their receptors. However, the overall mechanism of growth factor-mediated odontogenesis remains to be elucidated. The present study examined expression sites and intensities of major growth factors and receptors in the tooth germ of murine fetuses and neonates. Signals of TGF-β and CTGF in fetuses were released from the enamel epithelium, while their neonatal signals arose in odontoblasts. Moreover, BMP/Smad signaling may affect the differentiation of ameloblasts, in contrast to PDGFα whose signals may cause odontoblast differentiation. Growth factors associated with the formation of the periodontium were IGF1, IGF2, IGFBP3, CTGF, and PDGFα. Concerning cusp formation, the enamel knot selectively expressed FGF4, BMP2, and BMP4 with an expression of PDGFα in the enamel-free area. It is concluded that many molecules play critical roles in the epithelium-mesenchyme interaction of tooth germ differentiation, and their expressions are precisely controlled.

Differentiation of mouse iPS cells into ameloblast-like cells in cultures using medium conditioned by epithelial cell rests of Malassez and gelatin-coated dishes

Induced pluripotent stem (iPS) cells are generated from adult cells and are potentially of great value in regenerative medicine. Recently, it was shown that iPS cells can differentiate into ameloblast-like cells in cultures using feeder cells. In the present study, we sought to induce differentiation of ameloblast-like cells from iPS cells under feeder-free conditions using medium conditioned by cultured epithelial cell rests of Malassez (ERM) cells and gelatin-coated dishes. Two culture conditions were compared: co-cultures of iPS cells and ERM cells; and, culture of iPS cells in ERM cell-conditioned medium. Differentiation of ameloblast-like cells in the cultures was assessed using real-time RT-PCR assays of expression of the marker genes keratin 14, amelogenin, and ameloblastin and by immunocytochemical staining for amelogenin. We found greater evidence of ameloblast-like cell differentiation in the cultures using the conditioned medium. In the latter, the level of amelogenin expression increased daily and was significantly higher than controls on the 7th, 10th, and 14th days. Expression of ameloblastin also increased daily and was significantly higher than controls on the 14th day. The present study demonstrates that mouse iPS cells can be induced to differentiate into ameloblast-like cells in feeder-free cell cultures using ERM cell-conditioned medium and gelatin-coated dishes.

Interactions between BMP-7 and USAG-1 (uterine sensitization-associated gene-1) regulate supernumerary organ formations

Bone morphogenetic proteins (BMPs) are highly conserved signaling molecules that are part of the transforming growth factor (TGF)-beta superfamily, and function in the patterning and morphogenesis of many organs including development of the dentition. The functions of the BMPs are controlled by certain classes of molecules that are recognized as BMP antagonists that inhibit BMP binding to their cognate receptors. In this study we tested the hypothesis that USAG-1 (uterine sensitization-associated gene-1) suppresses deciduous incisors by inhibition of BMP-7 function. We learned that USAG-1 and BMP-7 were expressed within odontogenic epithelium as well as mesenchyme during the late bud and early cap stages of tooth development. USAG-1 is a BMP antagonist, and also modulates Wnt signaling. USAG-1 abrogation rescued apoptotic elimination of odontogenic mesenchymal cells. BMP signaling in the rudimentary maxillary incisor, assessed by expressions of Msx1 and Dlx2 and the phosphorylation of Smad protein, was significantly enhanced. Using explant culture and subsequent subrenal capsule transplantation of E15 USAG-1 mutant maxillary incisor tooth primordia supplemented with BMP-7 demonstrated in USAG-1+/- as well as USAG-1-/- rescue and supernumerary tooth development. Based upon these results, we conclude that USAG-1 functions as an antagonist of BMP-7 in this model system. These results further suggest that the phenotypes of USAG-1 and BMP-7 mutant mice reported provide opportunities for regenerative medicine and dentistry.

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.

Gene expression and cytokine release during odontogenic differentiation of human dental pulp stem cells induced by 2 endodontic biomaterials

INTRODUCTION

Mineral trioxide aggregate (MTA) and calcium-enriched mixture (CEM) have shown osteogenic/cementogenic/dentinogenic activities; however, their mechanism of action is not fully understood. We aimed to evaluate the effect of these biomaterials on odontogenic differentiation of human dental pulp stem cells (DPSCs).

METHODS

Flow cytometry with stem cell markers for the confirmation of stemness and homogeneity was first performed. Then isolated DPSCs were seeded on prepared discs of MTA, CEM, differentiation medium (DM), and growth medium (GM) and incubated up to 14 days. Concentrations of transforming growth factor-β1, bone morphogenetic protein (BMP)2, BMP4, and fibroblast growth factor 4 were measured at each interval using an enzyme-linked immunosorbent assay reader. Gene expression of dentin sialophosphoprotein, dentin matrix protein 1, and the cytokines were evaluated by reverse-transcription polymerase chain reaction. To evaluate the cell morphology, scanning electron micrographs were taken; mineralization potential was evaluated using alizarin red S staining.

RESULTS

Scanning electron micrographs showed that DPSCs spread/adhered/proliferated similarly on MTA and CEM. On day 14, alizarin red S staining confirmed that mineralization occurred in all groups except GM. Expressions of dentin matrix protein 1 and dentin sialophosphoprotein genes were similar in the CEM, MTA, and DM groups; they were significantly higher compared with the GM group (P < .05). A greater amount of transforming growth factor-β1 gene was expressed in MTA compared with the other groups (P < .05). However, the expression of fibroblast growth factor 4 and BMP2 genes was significantly greater in the CEM group (P < .05). In all the tested groups, the expression of BMP4 was less than GM (P < .01); however, CEM and DM were similar but more than MTA (P < .05). Concentrations of protein product detected using an enzyme-linked immunosorbent assay reader confirmed these gene expressions.

CONCLUSIONS

MTA and CEM can induce osteo-/odontogenic-like phenotype differentiation of human DPSCs; however, they stimulate different gene expressions and growth factor release.

Regulation of CCN2 gene expression and possible roles in developing tooth germs

CCN proteins are extracellular and cell-associated molecules involved in several developmental processes, but their expression patterns and regulation in tooth development remain unclear. Here we first determined the expression patterns of CCN genes in mouse tooth germs. We found that at early stages CCN2 was detected in dental lamina, dental mesenchyme, and primary enamel knot, while other CCN family members were expressed broadly. By the bell stage, all members were expressed in differentiating odontoblasts and ameloblasts, but CCN1 and CCN2 transcripts were conspicuous in differentiating osteoblasts in dental follicle. Next, we asked what signalling molecules regulate CCN2 expression and what roles CCN2 may have. We found that upon surgical removal of dental epithelium CCN2 was not longer expressed in dental mesenchyme in cultured bud stage germs. Implantation of beads pre-coated with BMPs and FGFs onto E12-13 mandibular explants induced CCN2 expression in dental mesenchyme. There was a dose-dependent effect of BMP-4 on CCN2 induction; a concentration of 100 ng/μl was able to induce strong CCN2 expression while a minimum concentration of 25 ng/μl was needed to elicit appreciable expression. Importantly, Noggin treatment inhibited endogenous and BMP-induced CCN2 expression, verifying that CCN2 expression in developing tooth germs requires BMP signalling. Lastly, we found that rCCN2 stimulated proliferation in dental mesenchyme in a dose-dependent manner. Together, the data indicate that expression of CCN genes is spatio-temporally regulated in developing tooth germs. CCN2 expression appears to depend on epithelial and mesenchymal-derived signalling factors, and CCN2 can elicit strong proliferation in dental mesenchyme.

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

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

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.

Canonical Wnt signaling induces skin fibrosis and subcutaneous lipoatrophy: a novel mouse model for scleroderma?

OBJECTIVE

Because aberrant Wnt signaling has been linked with systemic sclerosis (SSc) and pulmonary fibrosis, we sought to investigate the effect of Wnt-10b on skin homeostasis and differentiation in transgenic mice and in explanted mesenchymal cells.

METHODS

The expression of Wnt-10b in patients with SSc and in a mouse model of fibrosis was investigated. The skin phenotype and biochemical characteristics of Wnt-10b-transgenic mice were evaluated. The in vitro effects of ectopic Wnt-10b were examined in explanted skin fibroblasts and preadipocytes.

RESULTS

The expression of Wnt-10b was increased in lesional skin biopsy specimens from patients with SSc and in those obtained from mice with bleomycin-induced fibrosis. Transgenic mice expressing Wnt-10b showed progressive loss of subcutaneous adipose tissue accompanied by dermal fibrosis, increased collagen deposition, fibroblast activation, and myofibroblast accumulation. Wnt activity correlated with collagen gene expression in these biopsy specimens. Explanted skin fibroblasts from transgenic mice demonstrated persistent Wnt/β-catenin signaling and elevated collagen and α-smooth muscle actin gene expression. Wnt-10b infection of normal fibroblasts and preadipocytes resulted in blockade of adipogenesis and transforming growth factor β (TGFβ)-independent up-regulation of fibrotic gene expression.

CONCLUSION

SSc is associated with increased Wnt-10b expression in the skin. Ectopic Wnt-10b causes loss of subcutaneous adipose tissue and TGFβ-independent dermal fibrosis in transgenic mice. These findings suggest that Wnt-10b switches differentiation of mesenchymal cells toward myofibroblasts by inducing a fibrogenic transcriptional program while suppressing adipogenesis. Wnt-10b-transgenic mice represent a novel animal model for investigating Wnt signaling in the setting of fibrosis.

Cell differentiation and matrix organization in engineered teeth

Embryonic dental cells were used to check a series of criteria to be achieved for tooth engineering. Implantation of cultured cell-cell re-associations led to crown morphogenesis, epithelial histogenesis, organ vascularization, and root and periodontium development. The present work aimed to investigate the organization of predentin/dentin, enamel, and cementum which formed and mineralized after implantation. These implants were processed for histology, transmission electron microscopy, x-ray microanalysis, and electron diffraction. After two weeks of implantation, the re-associations showed gradients of differentiating odontoblasts. There were ciliated, polarized, and extended cell processes in predentin/dentin. Ameloblasts became functional. Enamel crystals showed a typical oriented arrangement in the inner and outer enamel. In the developing root, odontoblasts differentiated, cementogenesis occurred, and periodontal ligament fibroblasts interacted with the root surface and newly formed bone. The implantation of cultured dental cell re-associations allows for reproduction of complete functional differentiation at the cell, matrix, and mineral levels.

Wnt5a promotes differentiation of human dental papilla cells

AIM

To investigate the role of Wnt5a in the process of differentiation of human dental papilla cells (HDPCs).

METHODOLOGY

Recombinant adenovirus encoding full-length Wnt5a cDNA was constructed to investigate the biological role of Wnt5a on the differentiation of HDPCs. The effect of Wnt5a on HDPCs differentiation was determined by ALP activity assay, ALP staining and mineral induction assay. Mineralization-related gene expressions were assessed by RT-PCR.

RESULTS

Immunostaining revealed Wnt5a expression in the odontoblast layer and dental papilla tissue. Over-expression of Wnt5a by transfecting HDPCs with an Wnt5a-carrying construct increased ALPase activity and the formation of mineralized nodules of HDPCs. RT-PCR analysis showed that the expressions of mineralization-related genes, such as bone sialoprotein, collagen type I, osteonectin, osteopontin (OCN), dentine matrix protein-1 were up-regulated by Wnt5a.

CONCLUSIONS

Wnt5a promoted differentiation of HDPCs.

The canonical BMP signaling pathway plays a crucial part in stimulation of dentin sialophosphoprotein expression by BMP-2

Dentin sialophosphoprotein (DSPP), a typical dentin-specific protein, is mainly expressed in the dentin extracellular matrix and plays a role in dentin mineralization. BMP-2 provides a strong signal for differentiation and mineralization of odontoblasts and osteoblasts. Previously, BMP-2 treatment is reported to stimulate Dspp expression in the MD10-F2 pre-odontoblast cells through activation of the heterotrimeric transcription factor Y (NF-Y). The canonical BMP signaling pathway is known to contribute greatly to biomineralization, however, it is not known whether it is involved in Dspp expression. Here, we investigated this question. Activation of the canonical BMP-2 signaling pathway in MDPC-23, preodontoblast cell, by overexpression of constitutively active Smad1/5 or downstream transcription factors Dlx5 and Runx2 stimulated Dspp expression. Conversely, knockdown of each element with siRNA significantly blocked the BMP-2-induced Dspp expression. To test whether these transcription factors downstream of BMP-2 are directly involved in regulating Dspp, we analyzed the mouse Dspp promoter. There are 5 well conserved homeodomain binding elements, H1 to H5, in Dspp proximal promoter regions (-791 to +54). A serial deletion of H1 and H2 greatly changed basal promoter activity and responsiveness to Dlx5 or Msx2. However, further deletions did not change the responsiveness to Dlx5 or Msx2. H1 and H2 sites can be suggested as specific response elements of Dlx5 and Msx2, respectively, based on their promoter activity modulation. Thus, the canonical BMP-2 signaling pathway plays a crucial part in the regulation of Dspp expression through the action of Smads, Dlx5, Runx2, and Msx2.

Toxicity of Flow Line, Durafill VS, and Dycal to dental pulp cells: effects of growth factors

INTRODUCTION

The objective was to determine the effects of growth factor treatment on dental pulp cell sensitivity to toxicity of 2 composite restoration materials, Flow Line and Durafill VS, and a calcium hydroxide pulp capping material, Dycal.

METHODS

Toxicity of the dental materials to cultures of primary dental pulp cells was determined by the MTT metabolism assay. The ability of 6 different growth factors to influence the toxicity was tested.

RESULTS

A 24-hour exposure to either Flow Line or Durafill VS caused approximately 40% cell death, whereas Dycal exposure caused approximately 80% cell death. The toxicity of Flow Line and Durafill VS was mediated by oxidative stress. Four of the growth factors tested (bone morphogenetic protein [BMP]-2, BMP-7, epidermal growth factor [EGF], and transforming growth factor [TGF]-beta) decreased the basal MTT values while making the cells resistant to Flow Line and Durafill VS toxicity except BMP-2, which made the cells more sensitive to Flow Line. Treatment with fibroblast growth factor-2 caused no change in basal MTT metabolism, prevented the toxicity of Durafill VS, but increased the toxicity of Flow Line. Treatment with insulin-like growth factor-I (IGF-I) increased basal MTT metabolism and made the cells resistant to Flow Line and Durafill VS toxicity. None of the growth factors made the cells resistant to Dycal toxicity.

CONCLUSIONS

The results indicated that growth factors can be used to alter the sensitivity of dental pulp cells to commonly used restoration materials. The growth factors BMP-7, EGF, TGF-beta, and IGF-I provided the best profile of effects, making the cells resistant to both Flow Line and Durafill VS toxicity.

Cell fate determination during tooth development and regeneration

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

Temporal analysis of ectopic enamel production in incisors from sprouty mutant mice

The mouse incisor has two unusual features: it grows continuously and it is covered by enamel exclusively on the labial side. The continuous growth is driven in part by epithelial stem cells in the cervical loop region that can both self-renew and give rise to ameloblasts. We have previously reported that ectopic enamel is found on the lingual side of the incisor in mice with loss-of-function of sprouty (spry) genes. Spry2(+/-); Spry4(-/-) mice, in which three sprouty alleles have been inactivated, have ectopic enamel as a result of upregulation of epithelial-mesenchymal FGF signaling in the lingual part of the cervical loop. Interestingly, lingual enamel is also present in the early postnatal period in Spry4(-/-) mice, in which only two sprouty alleles have been inactivated, but ectopic enamel is not found in adults of this genotype. To explore the mechanisms underlying the disappearance of lingual enamel in Spry4(-/-) adults, we studied the fate of the lingual enamel in Spry4(-/-) mice by comparing the morphology and growth of their lower incisors with wild type and Spry2(+/-); Spry4(-/-) mice at several timepoints between the perinatal period and adulthood. Ameloblasts and enamel were detected on the lingual side in postnatal Spry2(+/-); Spry4(+/-) incisors. By contrast, new ectopic ameloblasts ceased to differentiate after postnatal day 3 in Spry4(-/-) incisors, which was followed by a progressive loss of lingual enamel. Both the posterior extent of lingual enamel and the time of its last deposition were variable early postnatally in Spry4(-/-) incisors, but in all Spry4(-/-) adult incisors the lingual enamel was ultimately lost through continuous growth and abrasion of the incisor.

Revitalization of a diastemal tooth primordium in Spry2 null mice results from increased proliferation and decreased apoptosis

An understanding of the factors that promote or inhibit tooth development is essential for designing biological tooth replacements. The embryonic mouse dentition provides an ideal system for studying such factors because it consists of two types of tooth primordia. One type of primordium will go on to form a functional tooth, whereas the other initiates development but arrests at or before the bud stage. This developmental arrest contributes to the formation of the toothless mouse diastema. It is accompanied by the apoptosis of the rudimentary diastemal buds, which presumably results from the insufficient activity of anti-apoptotic signals such as fibroblast growth factors (FGFs). We have previously shown that the arrest of a rudimentary tooth bud can be rescued by inactivating Spry2, an antagonist of FGF signaling. Here, we studied the role of the epithelial cell death and proliferation in this process by comparing the development of a rudimentary diastemal tooth bud (R(2)) and the first molar in the mandibles of Spry2(-/-) and wild-type (WT) embryos using histological sections, image analysis and 3D reconstructions. In the WT R(2) at embryonic day 13.5, significantly increased apoptosis and decreased proliferation were found compared with the first molar. In contrast, increased levels of FGF signaling in Spry2(-/-) embryos led to significantly decreased apoptosis and increased proliferation in the R(2) bud. Consequently, the R(2) was involved in the formation of a supernumerary tooth primordium. Studies of the revitalization of rudimentary tooth primordia in mutant mice can help to lay the foundation for tooth regeneration by enhancing our knowledge of mechanisms that regulate tooth formation.

Phosphate: known and potential roles during development and regeneration of teeth and supporting structures

Inorganic phosphate (P(i)) is abundant in cells and tissues as an important component of nucleic acids and phospholipids, a source of high-energy bonds in nucleoside triphosphates, a substrate for kinases and phosphatases, and a regulator of intracellular signaling. The majority of the body's P(i) exists in the mineralized matrix of bones and teeth. Systemic P(i) metabolism is regulated by a cast of hormones, phosphatonins, and other factors via the bone-kidney-intestine axis. Mineralization in bones and teeth is in turn affected by homeostasis of P(i) and inorganic pyrophosphate (PPi), with further regulation of the P(i)/PP(i) ratio by cellular enzymes and transporters. Much has been learned by analyzing the molecular basis for changes in mineralized tissue development in mutant and knock-out mice with altered P(i) metabolism. This review focuses on factors regulating systemic and local P(i) homeostasis and their known and putative effects on the hard tissues of the oral cavity. By understanding the role of P(i) metabolism in the development and maintenance of the oral mineralized tissues, it will be possible to develop improved regenerative approaches.

Epithelial histogenesis during tooth development

This paper reviews the current understanding of the progressive changes mediating dental epithelial histogenesis as a basis for future collaborative studies. Tooth development involves morphogenesis, epithelial histogenesis and cell differentiation. The consecutive morphological stages of lamina, bud, cap and bell are also characterized by changes in epithelial histogenesis. Differential cell proliferation rates, apoptosis, and alterations in adhesion and shape lead to the positioning of groups of cells with different functions. During tooth histo-morphogenesis changes occur in basement membrane composition, expression of signalling molecules and the localization of cell surface components. Cell positional identity may be related to cell history. Another important parameter is cell plasticity. Independently of signalling molecules, which play a major role in inducing or modulating specific steps, cell-cell and cell-matrix interactions regulate the plasticity/rigidity of particular domains of the enamel organ. This involves specifying in space the differential growth and influences the progressive tooth morphogenesis by shaping the epithelial-mesenchymal junction. Deposition of a mineralized matrix determines the final shape of the crown. All data reviewed in this paper were investigated in the mouse.

Localization of Bmp-4, Shh and Wnt-5a transcripts during early mice tooth development by in situ hybridization

A comparative nonisotopic in situ hybridization (ISH) analysis was carried out for the detection of Bmp-4, Shh and Wnt-5a transcripts during mice odontogenesis from initiation to cap stage. Bmp-4 was expressed early in the epithelium and then in the underlying mesenchyme. Shh expression was seen in the odontogenic epithelial lining thickening, being stronger in the enamel knot area, during the cap stage. Wnt-5a transcripts were expressed only in the mesenchyme during the initiation, bud and cap stages, with strong expression in the dental mesenchyme during the bud stage. The present results showed that Bmp-4, Shh and Wnt-5a are expressed since the very early stages of tooth development, and they suggest that the Wnt-5a gene is expressed in different cell populations than Bmp-4 and Shh.

A distinct cohort of the TGFbeta superfamily members expressed in human endometrium regulate decidualization

BACKGROUND

Successful blastocyst implantation requires the differentiation of human endometrial stromal cells (HESC), a process known as decidualization. Activin A, a transforming growth factor beta (TGFbeta) superfamily member, enhances HESC decidualization and localizes to decidual cells in human endometrium. Other TGFbeta superfamily members, including BMP2, BMP4, BMP7, GDF5, GDF8, GDF11, TGFbetas and Nodal, may also play a role during decidualization. This study aimed to identify these TGFbeta family members in human endometrium, and to determine whether they are involved in human decidualization.

METHODS

Protein localization of TGFbeta family members was examined in secretory phase human endometrium and first trimester decidua by immunohistochemistry. mRNA expression was examined in HESC. Activin inhibitors (Activin-M108A/SB431542) with differing specificities for the other TGFbeta members under consideration were applied during HESC decidualization in vitro. The secretion levels of potential TGFbeta superfamily members were measured during decidualization, and recombinant proteins added to examine their effect.

RESULTS

This study has identified BMP2, BMP4, BMP7, GDF5, GDF8 and GDF11 but not Nodal in secretory phase human endometrium, but only BMP2, GDF5 and TGFbeta1 protein were detected in decidual cells. All ligands except Nodal were expressed by cultured HESC. Both inhibitors significantly reduced decidualization validating the role of activin, but potentially also other TGFbeta members, during decidualization. BMP2 and TGFbeta1 secretion increased during HESC decidualisation and exogenous administration of these proteins significantly enhanced decidualization in vitro.

CONCLUSIONS

Like activin, BMP2 and TGFbeta1 are likely to be involved in HESC decidualization. This is the first study to identify and localize BMP4, BMP7, GDF5, GDF8 and GDF11 in secretory phase human endometrium. Understanding the factors critical for the implantation process is needed for improving fertility and pregnancy outcomes.

Transgenic overexpression of gremlin results in developmental defects in enamel and dentin in mice

Bone morphogenetic proteins (BMPs) and BMP antagonists play a crucial role in the regulation of tooth development. One of the BMP extracellular antagonists, gremlin, is a highly conserved 20.7-kDa glycoprotein. Previously, researchers reported that transgenic mice overexpressing gremlin under the control of the osteocalcin promoter (gremlin OE) exhibit a skeletal phenotype and tooth fragility. To further define the tooth phenotype, teeth and surrounding supporting tissues, obtained from gremlin OE at ages of 4 weeks, 2 months, and 4 months, were examined. The histological results demonstrate that gremlin OE exhibit an enlarged pulp chamber with ectopic calcification and thinner dentin and enamel compared with wild-type control. In vitro studies using murine pulp cells revealed that gremlin inhibited BMP-4 mediated induction of Dspp. These data provide evidence that balanced interactions between BMP agonists/antagonists are required for proper development of teeth and surrounding tissues. It is clear that these interactions require further investigation to better define the mechanisms controlling tooth root formation (pulp, dentin, cementum, and surrounding tissue) to provide the information needed to successfully regenerate these tissues.

Cell apoptosis control using BMP4 and noggin embedded in a polyelectrolyte multilayer film

Programmed cell death (apoptosis) is a genetically regulated process of cell elimination essential during development. During development, programmed cell death is involved in the specific shaping of organs, in the elimination of cells having achieved their program, and in regulating the number of cells to differentiate. Tooth development includes these three aspects and was used here as a model to study the control of apoptosis. Bone morphogenetic proteins (BMPs) are currently considered as playing a major role in signaling apoptosis. This apoptosis could be stopped by treatments with a BMP antagonist ("Noggin"). We selected a model system made by a layer-by-layer approach using poly-L-glutamic acid (PlGA) and poly-L-lysine (PlL) films into which BMP4 and/or Noggin have been embedded. Our results indicate that in situ control of apoptosis during tooth differentiation mediated by both BMP4 and Noggin embedded in a polyelectrolyte multilayer film is possible. We show here for the first time that in the presence of BMP4 and Noggin embedded in a multilayered film, we can induce or inhibit cell death in tooth differentiation, and conserve their biological effects.

Conservation and divergence of Bmp2a, Bmp2b, and Bmp4 expression patterns within and between dentitions of teleost fishes

The diversity of tooth location in teleost fishes provides an excellent system for comparing genetic divergence between teeth in different species (phylogenetic homologs) with divergence between teeth within one species (iterative homologs). We have chosen to examine the expression of three members of the bone morphogenetic protein (Bmp) family because they are known to play multiple roles in tooth development and evolution in tetrapod vertebrates. We characterized expression of Bmp2a, Bmp2b, and Bmp4 during the development of oral and pharyngeal dentitions in three species of teleost fishes, the zebrafish (Danio rerio), Mexican tetra (Astyanax mexicanus), and Japanese medaka (Oryzias latipes). We found that expression in teleosts is generally highly conserved, with minor differences found among both iteratively homologous and phylogenetically homologous teeth. Expression of orthologous genes differs in several ways between the teeth of teleost fishes and those of the mouse, but between these vertebrate groups the summed expression pattern of Bmp genes is highly conserved. Significantly, the toothless oral region of the zebrafish lacks Bmp expression domains found in teleosts with oral teeth, implicating these genes in evolutionary tooth loss. We conclude that Bmp expression has been largely conserved in vertebrate tooth development over evolutionary time, and that loss of Bmp expression is correlated with region-specific loss of the dentition in a major group of fishes.

Advances in defining regulators of cementum development and periodontal regeneration

Substantial advancements have been made in defining the cells and molecular signals that guide tooth crown morphogenesis and development. As a result, very encouraging progress has been made in regenerating crown tissues by using dental stem cells and recombining epithelial and mesenchymal tissues of specific developmental ages. To date, attempts to regenerate a complete tooth, including the critical periodontal tissues of the tooth root, have not been successful. This may be in part due to a lesser degree of understanding of the events leading to the initiation and development of root and periodontal tissues. Controversies still exist regarding the formation of periodontal tissues, including the origins and contributions of cells, the cues that direct root development, and the potential of these factors to direct regeneration of periodontal tissues when they are lost to disease. In recent years, great strides have been made in beginning to identify and characterize factors contributing to formation of the root and surrounding tissues, that is, cementum, periodontal ligament, and alveolar bone. This review focuses on the most exciting and important developments over the last 5 years toward defining the regulators of tooth root and periodontal tissue development, with special focus on cementogenesis and the potential for applying this knowledge toward developing regenerative therapies. Cells, genes, and proteins regulating root development are reviewed in a question-answer format in order to highlight areas of progress as well as areas of remaining uncertainty that warrant further study.

Tissue engineering of tooth crown, root, and periodontium

Tissue engineering of teeth requires the coordinated formation of correctly shaped crowns, roots, and periodontal ligament. Previous studies have shown that the dental mesenchyme controls crown morphogenesis and epithelial histogenesis during tooth development in vivo, but little is known about the inductive potential of dissociated mesenchymal cells used in ex vivo cultures. A 2-step method is described in which, by using different types of reassociations between epithelial and mesenchymal tissues and/or cells from mouse embryos, reassociations were cultured in vitro before in vivo implantation. In vitro, the reassociated tissues developed and resulted in tooth-like structures that exhibited normal epithelial histogenesis and allowed the functional differentiation of odontoblasts and ameloblasts. After implantation, the reassociations formed roots and periodontal ligament, the latter connected to developing bone. The shape of the crown, initially suspected to depend on the integrity of the mesenchyme, could be modulated by adjusting the number of dissociated mesenchymal cells reassociated with the epithelial compartment. Based on these results, we propose a refined strategy for tooth tissue engineering that may help to eventually generate morphologically defined teeth.

Expression patterns of BMPRs in the developing mouse molar

During development, Bone Morphogenetic Proteins (BMPs) can induce apoptosis, cell growth or differentiation. These different effects are mediated by dimers of two types of BMP-receptors (BMPRs). To identify the responding cells during tooth development and search for possible tissue-or stage-specificities in the receptors involved, the distribution patterns of BMPR-IA, -IB and -II were investigated in the mouse molar, from bud to bell stage. At the bud stage, BMP-2 was suggested to be involved in the formation of an epithelial signaling center, the primary enamel knot (PEK), while BMP-4 would mediate the condensation of the mesenchyme. Immunostaining showed the presence of BMPR-IA and -II in the epithelium instead of BMPR-IB and -II in the mesenchyme. At the cap stage, BMPR-IB was detected in the epithelium but not BMPR-II, suggesting the existence of another type II receptor to form a functional dimer. At the late cap stage in the epithelium, BMP-4, BMPR-IA and -II were restricted to the internal part of the PEK and the stalk: two apoptotic areas. The three proteins were detected in the mesenchyme, showing a strong staining where cusps were about to form. At the late bell stage, BMP-2 or -4 may induce cell differentiation. BMPR-IB and -II were detected in odontoblasts instead of BMPR-IA and -II in ameloblasts. These results provide the first evidence of multiple type I and type II BMP-receptors, expressed in the dental epithelium and mesenchyme at different stages of development, to signal different cellular activities in a time- and tissue-specific way.

Molecular mechanisms of cytodifferentiation in mammalian tooth development

The morphological stages of tooth development--bud, cap, bell, and terminal differentiation--have been known for decades. The past 10 years have seen the elucidation of many of the molecular events driving these morphogenetic stages. Signaling via the fibroblast growth factor (FGF), bone morphogenetic protein (BMP), hedgehog, and wingless protein families and their downstream transcription factors have been identified as key players in the epithelial-mesenchymal signaling loops driving tooth development. Currently the most complete description of the mechanisms in tooth development extends only through the cap stage. The body of work concerning the mechanisms directing the bell and cytodifferentiation stages is growing. This has mainly, but not exclusively, focused on the expression and effects of FGFs and BMPs in these latter stages, and is reviewed here. Additionally, recent results suggest that phenotypic proteins of both ameloblasts and odontoblasts, such as amelogenin and dentin matrix protein 2 may act as the final instructive signals in cytodifferentiation.