Altered miRNA expression profiling in enamel organ of fluoride affected rat embryos

Evidence has shown that miRNAs could play a role in dental fluorosis, but there is no study has investigated the global expression miRNA profiles of fluoride-exposed enamel organ. In this study, we analysed the differentially expressed (DE) miRNAs between fluoride-treated and control enamel organ for the first time and found several candidate miRNAs and signaling pathways worthy of further research. Thirty Wistar rats were randomly distributed into three groups and exposed to drinking water with different fluoride contents for 10 weeks and during the gestation. The three groups were a control group (distilled water), medium fluoride group (75 mg/L NaF), and high fluoride group (150 mg/L NaF). On the embryonic day 19.5, the mandible was dissected for histological analysis, and the enamel organ of the mandibular first molar tooth germ was collected for miRNA sequencing (miRNA-seq) and quantitative real-time PCR analysis (qRT-PCR). Typical dental fluorosis was observed in the incisors of the prepregnant rats. In addition to the disorganized structure of enamel organ cells, 39 DE miRNAs were identified in the fluoride groups compared with the control group, and good agreement between the miRNA-seq data and qRT-PCR data was found. The functional annotation of the target genes of 39 DE miRNAs showed significant enrichment in metabolic process, cell differentiation, calcium signaling pathway, and mitogen-activated protein kinase(MAPK) signaling pathway terms. This study provides a theoretical reference for an extensive understanding of the mechanism of fluorosis and potential valuable miRNAs as therapeutic targets in fluorosis.

Epithelial Cdc42 Deletion Induced Enamel Organ Defects and Cystogenesis

Cdc42, a Rho family small GTPase, regulates cytoskeleton organization, vesicle trafficking, and other cellular processes in development and homeostasis. However, Cdc42's roles in prenatal tooth development remain elusive. Here, we investigated Cdc42 functions in mouse enamel organ. Cdc42 showed highly dynamic temporospatial patterns in the developing enamel organ, with robust expression in the outer enamel epithelium, stellate reticulum (SR), and stratum intermedium layers. Strikingly, epithelium-specific Cdc42 deletion resulted in cystic lesions in the enamel organ. Cystic lesions were first noted at embryonic day 15.5 and progressively enlarged during gestation. At birth, cystic lesions occupied the bulk of the entire enamel organ, with intracystic erythrocyte accumulation. Ameloblast differentiation was retarded upon epithelial Cdc42 deletion. Apoptosis occurred in the Cdc42 mutant enamel organ prior to and synchronously with cystogenesis. Transmission electron microscopy examination showed disrupted actin assemblies, aberrant desmosomes, and significantly fewer cell junctions in the SR cells of Cdc42 mutants than littermate controls. Autophagosomes were present in the SR cells of Cdc42 mutants relative to the virtual absence of autophagosome in the SR cells of littermate controls. Epithelium-specific Cdc42 deletion attenuated Wnt/β-catenin and Shh signaling in dental epithelium and induced aberrant Sox2 expression in the secondary enamel knot. These findings suggest that excessive cell death and disrupted cell-cell connections may be among multiple factors responsible for the observed cystic lesions in Cdc42 mutant enamel organs. Taken together, Cdc42 exerts multidimensional and pivotal roles in enamel organ development and is particularly required for cell survival and tooth morphogenesis.

Signaling Pathways Critical for Tooth Root Formation

Tooth is made of an enamel-covered crown and a cementum-covered root. Studies on crown dentin formation have been a major focus in tooth development for several decades. Interestingly, the population prevalence for genetic short root anomaly (SRA) with no apparent defects in crown is close to 1.3%. Furthermore, people with SRA itself are predisposed to root resorption during orthodontic treatment. The discovery of the unique role of Nfic (nuclear factor I C; a transcriptional factor) in controlling root but not crown dentin formation points to a new concept: tooth crown and root have different control mechanisms. Further genetic mechanism studies have identified more key molecules (including Osterix, β-catenin, and sonic hedgehog) that play a critical role in root formation. Extensive studies have also revealed the critical role of Hertwig's epithelial root sheath in tooth root formation. In addition, Wnt10a has recently been found to be linked to multirooted tooth furcation formation. These exciting findings not only fill the critical gaps in our understanding about tooth root formation but will aid future research regarding the identifying factors controlling tooth root size and the generation of a whole "bio-tooth" for therapeutic purposes. This review starts with human SRA and mainly focuses on recent progress on the roles of NFIC-dependent and NFIC-independent signaling pathways in tooth root formation. Finally, this review includes a list of the various Cre transgenic mouse lines used to achieve tooth root formation-related gene deletion or overexpression, as well as strengths and limitations of each line.

Perlecan, a Basement Membrane-type Heparan Sulfate Proteoglycan, in the Enamel Organ: Its Intraepithelial Localization in the Stellate Reticulum

The localization and biosynthesis of perlecan, a basement membrane-type heparan sulfate proteoglycan, were studied in developing tooth germs by using murine molars in neonatal and postnatal stages and primary cultured cells of the enamel organ and dental papilla to demonstrate the role of perlecan in normal odontogenesis. Perlecan was immunolocalized mainly in the intercellular spaces of the enamel organ as well as in the dental papilla/pulp or in the dental follicle. By in situ hybridization, mRNA signals for perlecan core protein were intensely demonstrated in the cytoplasm of stellate reticulum cells and in dental papilla/pulp cells, including odontoblasts and fibroblastic cells in the dental follicle. Furthermore, the in vitro biosyntheses of perlecan core protein by the enamel organ and dental papilla/pulp cells were confirmed by immunofluorescence, immunoprecipitation, and reverse transcriptase-polymerase chain reaction. The results indicate that perlecan is synthesized by the dental epithelial cells and is accumulated in their intercellular spaces to form the characteristic stellate reticulum, whose function is still unknown.

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.

Morphogenetic roles of perlecan in the tooth enamel organ: an analysis of overexpression using transgenic mice

Perlecan, a heparan sulfate proteoglycan, is enriched in the intercellular space of the enamel organ. To understand the role of perlecan in tooth morphogenesis, we used a keratin 5 promoter to generate transgenic (Tg) mice that over-express perlecan in epithelial cells, and examined their tooth germs at tissue and cellular levels. Immunohistochemistry showed that perlecan was more strongly expressed in the enamel organ cells of Tg mice than in wild-type mice. Histopathology showed wider intercellular spaces in the stellate reticulum of the Tg molars and loss of cellular polarity in the enamel organ, especially in its cervical region. Hertwig's epithelial root sheath (HERS) cells in Tg mice were irregularly aligned due to excessive deposits of perlecan along the inner, as well as on the outer sides of the HERS. Tg molars had dull-ended crowns and outward-curved tooth roots and their enamel was poorly crystallized, resulting in pronounced attrition of molar cusp areas. In Tg mice, expression of integrin β1 mRNA was remarkably higher at E18, while expression of bFGF, TGF-β1, DSPP and Shh was more elevated at P1. The overexpression of perlecan in the enamel organ resulted in irregular morphology of teeth, suggesting that the expression of perlecan regulates growth factor signaling in a stage-dependent manner during each step of the interaction between ameloblast-lineage cells and mesenchymal cells.

Immunolocalization of hepatocyte growth factor receptor, c-Met, in human fetal tooth germ

To evaluate c-Met expression in human tooth germ development. An immunohistochemical study on c-Met expression in tooth germs of 8 human fetus between the 7th and 9th week. In the Bud stage C-Met immunopositivity was, moderately to strongly, detected both in the inner and the outer epithelium of the enamel organ. In particular, moderate staining was detected in a specific portion of tooth germs that corresponds to apical portion of the enamel organ. In the bell stage tooth germs were characterized by much stronger c-Met immunopositivity in cytoplasm, inner enamel epithelium, bilateral cusps and above all in the plasma apical membrane on the mesenchymal side. In conclusion because enamel organ cells can interact with mesenchymal cells directly, and c-Met is expressed in the stages at which mesenchymal induction is guided by the dental epithelium, it is conceivable that c-Met is related to tooth germ morphogenesis and cell differentiation.

[If the chicks would have teeth?]

130 millions years ago, birds have diverged from other archosaurs. Except the most primitive birds of the cretaceous, they lost the property to produce teeth. Tooth development requires complex epithelialmesenchymal interactions, which imply the expression of numerous genes, which begin to be well known. Four different experiments have permitted to obtain tooth rudiments in chick embryos. The association of oral chick ectoderm with mouse molar mesenchyme, the exposition of oral chick ectoderm to BMP's and FGF's, the transposition of mouse neural crest in young chick embryos, and the use of a Talpid mutation lead to tooth anlage development in the chick embryo.

Tooth Development: 1. Generating Teeth in the Embryo

Teeth are organs that develop in the embryo via a series of interactions between oral epithelium and neural crest-derived ectomesenchyme of the early jaws. These interactions are initiated by the regional production of signalling molecules in the oral epithelium and the transfer of information to the underlying mesenchyme via homeobox gene transcription. This article describes how these interactions are co-ordinated in the embryo during development of the dentition and provides a theoretical basis for the second article in this series; understanding how biologists are attempting to generate teeth artificially in the laboratory.

Expression of p63 transcription factor in ectoderm-derived oral tissues

The p63 gene encodes six splice variants expressed with transactivating or dominant-negative activities. Animal studies with p63 -/- mutants have suggested that p63 is important for proper development of several organs, including tooth and salivary gland. Moreover, mutations of p63 have been detected in patients affected by ectrodactyly, ectodermal dysplasia and facial clefts. To clarify the role of p63 in craniofacial development, we have studied the localization of p63 protein in human and rat oral tissues using immunohistochemistry. p63 immunostaining was identified in the enamel organ, oral epithelium and developing salivary glands. All compartments of the enamel organ were immunolabelled, whereas only basal and some suprabasal cells of the oral epithelium were stained. Ectomesenchyme-derived cells, including pulp cells, odontoblasts, bone cells and chondrocytes, were negative. The staining pattern was identical in human and rat tissues. These data lend support to the hypothesis that p63 is involved in growth and differentation of ectoderm-derived oral tissues and may be useful to clarify molecular and developmental aspects observed in animal knockout experiments and human syndromes related to p63 gene alteration.

Dens Evaginatus: Literature Review, Pathophysiology, and Comprehensive Treatment Regimen

Dens evaginatus (DE) is an uncommon dental anomaly, having been well documented since 1925. It occurs primarily in people of Asian descent and is exhibited by protrusion of a tubercle from occlusal surfaces of posterior teeth, and lingual surfaces of anterior teeth. Tubercles have an enamel layer covering a dentin core containing a thin extension of pulp. These cusp-like protrusions are susceptible to pulp exposure from wear or fracture because of malocclusion, leading to pulpal complications soon after eruption. Endodontic intervention of permanent teeth with immature roots is unpredic for inflamed pulps, and leaves a tooth with compromised root structure when treating necrotic pulps. Efforts to ensure root maturity have involved preventive or prophylactic treatment with varying degrees of pulp invasiveness. Treatment options have changed as technology and materials have improved. The goal is to review the literature and pathophysiology regarding DE, and present a new comprehensive treatment regimen, including a truly prophylactic approach without pulpal invasiveness. A case study of a mestizo with DE is documented. Treatment of four affected mandibular premolars exhibiting three distinct diagnostic categories will illustrate various aspects of the treatment protocol presented, and tooth morphology of the anomaly is shown to aid clinical recognition.

Dental Epithelial Histomorphogenesis in vitro

Recent developments in tooth-tissue engineering require that we understand the regulatory processes to be preserved to achieve histomorphogenesis and cell differentiation, especially for enamel tissue engineering. Using mouse first lower molars, our objectives were: (1) to determine whether the cap-stage dental mesenchyme can control dental epithelial histogenesis, (2) to test the role of the primary enamel knot (PEK) in specifying the potentialities of the dental mesenchyme, and (3) to evaluate the importance of positional information in epithelial cells. After tissue dissociation, the dental epithelium was further dissociated into individual cells, re-associated with dental mesenchyme, and cultured. Epithelial cells showed a high plasticity: Despite a complete loss of positional information, they rapidly underwent typical dental epithelial histogenesis. This was stimulated by the mesenchyme. Experiments performed at E13 demonstrated that the initial potentialities of the mesenchyme are not specified by the PEK. Positional information of dental epithelial cells does not require the memorization of their history.

Increased apoptosis during morphogenesis of the lower cheek teeth in tabby/EDA mice

In wild-type (WT) mice, epithelial apoptosis is involved in reducing the embryonic tooth number and the mesial delimitation of the first molar. We investigated whether apoptosis could also be involved in the reduction of tooth number and the determination of anomalous tooth boundaries in tabby (Ta)/EDA mice. Using serial histological sections and computer-aided 3D reconstructions, we investigated epithelial apoptosis in the lower cheek dentition at embryonic days 14.5-17.5. In comparison with WT mice, apoptosis was increased mainly mesially in Ta dental epithelium from day 15.5. This apoptosis showed a similar mesio-distal extent in all 5 morphotypes (Ia,b,c and IIa,b) of Ta dentition and eliminated the first cheek tooth in morphotypes IIa,b. Apoptosis did not appear to play any causal role in positioning inter-dental gaps. Analysis of the present data suggests that the increased apoptosis in Ta mice is a consequence of impaired tooth development caused by a defect in segmentation of dental epithelium.

Apoptosis distribution in the first molar tooth germ of the field vole (Microtus agrestis)

Apoptosis represents an important process in organ and tissue morphogenesis and remodeling during embryonic development. A role for apoptosis in shape formation of developing teeth has been suggested. The field vole is a useful model for comparative studies in odontogenesis, particularly because of its contrasting molar morphogenesis when compared to the mouse. However, little is known concerning apoptosis in tooth development of this species. Morphological (cellular and nuclear alterations) and biochemical (specific DNA breaks--TUNEL staining) characteristics of apoptotic cells were used to evaluate the temporal and spatial occurrence of apoptosis in epithelial and mesenchymal tissues of the developing first molar tooth germs of the field vole. Apoptotic cells were found in non-proliferating areas (identified previously) throughout bud to bell stages, particularly in the epithelium, however, scattered also in the mesenchyme. A high concentration of TUNEL positive cells was evident in primary enamel knots at late bud stage with increasing density of apoptotic cells until ED 16 when the primary enamel knot in the field vole disappears and mesenchyme becomes protruded in the middle axes of the bell forming two shallow areas with zig-zag located secondary enamel knots. Distribution of TUNEL positive cells corresponded with localisation of secondary enamel knots as shown using histological and 3D analysis. Apoptosis was shown to be involved in the first molar development of the field vole, however, exact mechanisms and roles of this process in tooth morphogenesis require further investigation.

Expression of the neural cell adhesion molecule during mouse tooth development

The neural cell adhesion molecule (NCAM) is a molecule that mediates calcium-independent cell-cell adhesion, and is expressed during development in a variety of tissues including nonneuronal ones. Expression of NCAM during molar tooth development was analyzed in this present study. In the enamel organ, NCAM expression was detected from the embryonic day 13 onward in restricted areas, including a part of the stratum intermedium of the tooth germ at the bell stage. Mesenchymal cells also expressed NCAM from the bud stage of the first molar onward. In addition to the strong expression in the dental follicle, weak and transient expression was detected in the dental papilla. The second and the third molars appeared to initiate their development by the interaction between the epithelial cells and NCAM-expressing mesenchymal cells. Comparison of the expression patterns of NCAM and transcription factor Barx1 revealed a possibility that Barx1 negatively regulates NCAM expression.

Cell-cell and cell-matrix interactions during initial enamel organ histomorphogenesis in the mouse

Relationships between cell-cell/cell-matrix interactions and enamel organ histomorphogenesis were examined by immunostaining and electron microscopy. During the cap-bell transition in the mouse molar, laminin-5 (LN5) disappeared from the basement membrane (BM) associated with the inner dental epithelium (IDE), and nondividing IDE cells from the enamel knot (EK) underwent a tooth-specific segregation in as many subpopulations as cusps develop. In the incisor, the basement membrane (BM) in contact with EK cells showed strong staining for LN5 and integrin alpha 6 beta 4. LN5 seems to provide stable adhesion, while its proteolytic processing might facilitate cell segregation. In both teeth, immunostaining for antigens associated with desmosomes or adherens junctions was similar for EK cells and neighboring IDE cells. Outside the EK, IDE cell-BM interactions changed locally during the initial molar cusp delimitation and on the labial part of the incisor cervical loop. Conversely, cell-cell junctions stabilized the anterior part of the incisor during completion of morphogenesis. Time and space regulation of cell-matrix and cell-cell interactions might thus play complementary roles in allowing plasticity during tooth morphogenesis and stabilization at later stages of epithelial histogenesis.

Expression of neural cell-adhesion molecule mRNA during mouse molar tooth development

This study employed in situ hybridisation using a probe recognising all isoforms of the molecule. Expression of the molecule in tooth germs started at embryonic day 13, when they were at the bud stage. Both inner cells of the epithelial bud and peripheral cells of the dental mesenchyme were positive. At the cap stage, positive cells were found in the inner part of the enamel organ but only in a limited area near the outer enamel epithelium. In the mesenchyme at the cap stage, expression was weak in the dental papilla and strong in the follicle. From the bell stage onward, epithelial cells in the enamel organ were negative except for the cells of the stratum intermedium, which were transiently positive at early and late bell stages. In the dental papilla, expression had mostly ceased during and after the bell stage, although transient expression was found in cuspal areas at the early bell stage. The dental follicle strongly expressed neural cell-adhesion molecule (NCAM) to the end of the experimental period, at post-natal day 4. In contrast to the first molar at its earliest stage of appearance, in which both the thickened epithelium and surrounding mesenchyme were negative for the expression of the molecule, the second molar appeared as a combination of extending epithelial thickenings and mesenchymal cells strongly positive for its expression. This study newly identifies the dental papilla and the stratum intermedium as NCAM-expressing sites.

Spatiotemporal expression of NGFR during pre-natal human tooth development

OBJECTIVES

The relation between nerve growth factor receptor (NGFR) in the human pre-natal tooth buds and the dental follicle was investigated. In particular, we sought to determine if there is a specific pattern of p75NGFR expression in developing human tooth buds and their surrounding tissue.

SETTING AND SAMPLE POPULATION

The Department of Orthodontics at Copenhagen University, Denmark. Histological sections from 11 fetuses, aged 11-21 gestational weeks.

METHOD

The sections were studied by conventional immunohistochemistry.

RESULTS

Specific spatiotemporal patterns of p75NGFR reactions were observed in the tooth buds and dental follicle: Before matrix production by the ameloblasts, the entire inner enamel epithelium and the entire dental follicle display p75NGFR immunoreactivity; after matrix production is initiated, the immunoreactivity of the matrix producing cells is lost, as is that of the dental follicle adjacent to these matrix-producing cells.

CONCLUSION

A unique spatiotemporal distribution of NGFR in the pre-eruptive human tooth bud was demonstrated.

Cell-matrix interactions and cell-cell junctions during epithelial histo-morphogenesis in the developing mouse incisor

The continuously growing rodent incisor develops mainly along its antero-posterior axis. The labio-lingual asymmetry which characterizes this tooth is initiated at the cap stage and increases further during the cap to bell transition (ED14 to ED16) when histogenesis of the enamel organ proceeds. Histology, transmission electron microscopy (TEM), and immunostaining were used to document the changes in the basement membrane (BM) as well as the modifications of epithelial cell-matrix and cell-cell interactions during this period. The expression of plakoglobin, desmoglein and E-cadherin at ED14 suggested that the main cell-cell junctional complexes were adherens junctions. The expression of desmoglein and TEM observations suggested a progressive antero-posterior stabilization of the enamel organ by means of desmosomes from ED14 to ED18. alpha6 integrin, BP 230 and laminin gamma2 chain were all expressed in the developing incisor but were not always co-distributed. Immunostaining and TEM suggested that only primitive type II hemidesmosomes were present. At ED14, cells of the enamel knot (EK) did not show any specific expression for antigens involved in cell-cell interaction. However, strong staining for the laminin gamma2 chain characterized the BM in contact with EK cells. The BM in the labial part of the cervical loop demonstrated ultrastructural changes: the presence of loops of the lamina densa in this region preceeded the differential expression of the integrin alpha6 subunit and that of the laminin gamma2 chain in the labial/lingual parts of the cervical loop. Apoptosis was transiently observed in the contiguous mesenchyme. This affected osteoblasts and also nerve cells close to the labial part of the cervical loop.

Ultrastructural immunolocalization of enamel matrix proteins during early stages of ameloblast differentiation

Enamel matrix proteins (EMP) represent specific molecular markers of ameloblast secretion. In order to study early differentiation stages of the cells of the inner enamel epithelium, we have investigated the ultrastructural localization of EMP-immunoreactivity in rat tooth germ. Pre-secretory stages of ameloblast differentiation were identified by the absence of EMP-immunoreactivity within epithelial cells as well as adjoining extra-cellular matrix. During subsequent secretory stages EMP-like immunoreactive material could be detected both within epithelial cells as well as within the adjoining extra-cellular matrix. The intensity of the immunoreactivity increased while advancing with the differentiation of epithelial cells. Intracellularly, EMP-immunoreactivity was detectable in cytoplasmic compartments involved in exocrine secretion pathway. During the early secretory stage, EMP-immunoreactive material was also detectable in the basement membrane of the epithelial-mesenchymal interface and within the pre-dentine, close to odontoblast plasma membranes and processes. It is thus suggested that EMP may cross the basement membrane between epithelial and mesenchymal cells. Our study suggests that this aspect might be important in molecular mechanisms that regulate epithelial-mesenchymal interactions during odontogenesis.

Evidence for reactive nitrogen species formation in the gingivomucosal tissue

An increase in nitric oxide production has been demonstrated in periodontitis. Here we investigated the potential role of nitric-oxide-derived nitrating species (such as peroxynitrite) in a rat model of ligature-induced periodontitis. Formation of 3-nitrotyrosine, the stable product formed from tyrosine reacting with nitric-oxide-derived nitrating species, was detected in the gingivomucosal tissue. 3-Nitrotyrosine immunohistochemical analysis revealed a significant elevation in the number of immunopositive leukocytes, and higher immunoreactivity of the gingival ligaments and epithelium in the ligated than in the contralateral (control) side. On both sides, several 3-nitrotyrosine-positive bands and, on the ligated side, a unique 52-kDa 3-nitrotyrosine-positive band were detected by Western blot. However, in the sterile gingivomucosal tissue of rat pups, no 3-nitrotyrosine or inducible nitric oxide synthase immunoreactivity was found. Analysis of these data suggests that resident bacteria of the gingivomucosal tissue induce an increase in reactive nitrogen species, which is greatly enhanced by plaque formation in periodontitis.

Quantitative image analysis of hyaluronan expression in human tooth germs

The expression of hyaluronan in human tooth germs was studied by using a biotinylated hyaluronan-binding complex and quantitative digital image analysis. At the cap stage, dental papilla exhibited a moderate staining, while intense reaction was observed in the apical portion of presecretory ameloblasts, stellate reticulum, and in dental basement membrane. When the enamel and dentine matrices started to develop, a strong hyaluronan reaction was evident in the young enamel and the apical portion of secretory ameloblasts. No hyaluronan could be detected in the secretory ameloblasts and enamel matrix of the early (9-wk-old) post-natal stage. It is concluded that hyaluronan may play a transitory role in the early phase of the development of the enamel matrix organization. A very weak signal was observed in the wall of dentin tubules, whereas the rest of the dentine matrix was not stained. The odontoblasts and the pulp were also moderately stained, and these reactions gradually decreased with age, suggesting that hyaluronan may also contribute to the development of dentine matrix and pulp.

Expression of cytokeratins in human enamel organ

Cytokeratin (CK) is a filament which plays a central role in epithelial tissue and, like the polypeptides of intermediate filaments in general, shows a high degree of tissue specificity. The CK expression patterns of odontogenic epithelia are still poorly described. We studied the distribution of individual CK polypeptides in the human enamel organ at bell stage and in remnants of the dental lamina. Our immunohistochemical study showed that epithelial cells stained for CKs 7, 13, 14 and 19 with slight changes in their pattern during the differentiation phase of odontogenesis. There was negative staining for all other CK polypeptides tested (CKs 8, 10, 16, 17 and 18). Most of the CKs in the enamel organ epithelia did not show differences related to the stage-specific state of differentiation, except for CKs 14 and 19 at the inner enamel epithelium. A strong label for CK 14 was present at the inner dental epithelium at early bell stage, and this was substituted by CK 19 at the late bell stage when the ameloblasts were fully differentiated.

Effects of apatite, transforming growth factor beta-1, bone morphogenetic protein-2 and interleukin-7 on ameloblast differentiation in vitro

Preameloblasts overtly differentiate in vitro when recombined with cell-free predentin-dentin. The predentin-dentin components able to trigger ameloblast terminal differentiation have not been identified, but some growth factors (members of the TGFbeta superfamily) have been demonstrated to be associated with this matrix, and in situ hybridization has demonstrated the presence of transcripts for TGFbetas and BMP-2 in odontoblasts facing differentiating ameloblasts. Moreover, intense expression of receptors for the cytokine interleukin-7 (IL-7) in polarizing ameloblasts has been reported. In this study, isolated E-18 and E-19 mouse molar enamel organs were cultured in vitro in presence of BMP-2, TGFbeta-1, IL-7 or synthetic apatite. TGFbeta-1 and BMP-2 combined with heparin induced cytodifferentiation of ameloblasts. IL-7 maintained the polarized state of ameloblasts. BMP-2-soaked apatite induced functional differentiation of ameloblasts (secretion of amelogenin). Integrating these data with previous work, a working hypothesis concerning the control of ameloblast terminal differentiation is presented: members of the TGFbeta superfamily secreted by odontoblasts might be trapped by predentin components first and then by dentin apatites, and these growth factors might trigger the cytological-functional sequence of ameloblast terminal differentiation.

Inhibition of transforming growth factor-beta type II receptor signaling accelerates tooth formation in mouse first branchial arch explants

Members of the transforming growth factor-beta (TGF-beta) superfamily signal through their cognate receptors to determine cell phenotypes during embryogenesis. Our previous studies on the regulation of first branchial arch morphogenesis have identified critical components of a hierarchy of different TGF-beta isoforms and their possible functions in regulating tooth and cartilage formation during mandibular morphogenesis. Here we tested the hypothesis that TGF-beta type II receptor (TGF-beta IIR) is a critical component in the TGF-beta signaling pathway regulating tooth formation. To establish the precise location of TGF-beta ligand and its cognate receptor, we first performed detailed analyses of the localization of both TGF-beta2 and TGF-beta IIR during initiation and subsequent morphogenesis of developing embryonic mouse tooth organs. A possible autocrine functional role for TGF-beta and its cognate receptor (TGF-beta IIR) was inferred due to the temporal and spatial localization patterns during the early inductive stages of tooth morphogenesis. Second, loss of function of TGF-beta IIR in a mandibular explant culture model resulted in the acceleration of tooth formation to the cap stage while the mandibular explants in the control group only showed bud stage tooth formation. In addition, there was a significant increase in odontogenic epithelial cell proliferation following TGF-beta IIR abrogation. These results demonstrate, for the first time, that abrogation of the TGF-beta IIR stimulates embryonic tooth morphogenesis in culture and reverses the negative regulation of endogenous TGF-beta signaling upon enamel organ epithelial cell proliferation.

Hyaluronan in human deciduous tooth germs in the bell stage. Histochemistry and immunohistochemistry

The primary aim of the present study was a localization of hyaluronan (HA) in human deciduous tooth germs in the bell stage. HA was compared to the content of chondroitin sulfates (CSs). HA was detected with a biotin-labeled HA-binding protein (HABP) and CS with a monoclonal antibody. As controls, enzyme digestions were carried out. Furthermore, the glycosaminoglycans were investigated histochemically with enzyme digestions followed by alcian blue staining. The investigation showed a considerable content of HA in the stellate reticulum, although CS was also found, primarily when treatment with protease was omitted. The dental papilla contained both HA and CS, while the predentin and the dentin contained only CS. The enamel did not contain any CS, but some staining with HABP was observed along the borderline between the ameloblasts and the enamel. The significance of HA in the stellate reticulum is discussed. The importance of carrying out investigations with and without protease digestions is stressed.

Mouse molar morphogenesis revisited by three-dimensional reconstruction. III. Spatial distribution of mitoses and apoptoses up to bell-staged first lower molar teeth

Computer-assisted 3D reconstructions were used to follow the development of the embryonic mouse first lower molar (M1). At ED 12.5, the thickening of the oral epithelium, which was thought to correspond to the molar dental lamina, regressed in its anterior part as a result of apoptosis. Only the posterior part later gave rise to molars. The transition to the cap stage entailed medial and lateral extensions of the dental epithelium. The growth and histo-morphogenesis of the enamel organ as well as cervical loop formation proceeded more rapidly in the anterior part of the M1 during the cap and early bell stages producing significant morphological differences along the antero-posterior axis. Apoptosis was temporarily intensive in the anterior part of the bud- and cap-shaped epithelium and thus pointed domains which do not participate in the formation of the final M1 enamel organ. In the well-formed cap, apoptoses displayed maximum concentration in the enamel knot (EK). No increase in the number of metaphases could be detected in the vicinity of the EK. Mitoses were distributed throughout the epithelial compartment until cap stage and then mainly concentrated in the inner dental epithelium at the early bell stage. At this later stage, either lateral views or thick virtual sections performed in the reconstruction demonstrated a clear cut distribution of mitoses and apoptoses in the enamel organ. At the early bell stage, mitoses in the mesenchyme demonstrated an increasing postero-anterior gradient.

Distribution of the neural cell adhesion molecule (NCAM) during pre- and postnatal development of mouse incisors

Developmental changes in the distribution of the neural cell adhesion molecule (NCAM) were investigated in mouse incisors by means of the indirect immunofluorescence method. During the prenatal stages of development, NCAM was predominantly found in the dental follicle, but not in the dental papilla; the results were analogous to the distribution of NCAM during molar development. After birth, the expression of NCAM continued in the tissue between the enamel organ and the alveolar bone on the labial aspect. In contrast, the follicular tissue covering the lingual aspect of the incisor gradually lost NCAM immunoreactivity from its outer zone as it differentiated into the highly organized periodontal ligament. The intermediate zone of the ligament continued to express NCAM-immunoreactivity even in mice of 6 weeks of age. This pattern of NCAM expression was different from that found in molar teeth, where the organized peridontal ligament was NCAM-negative. The dental pulp, in which we previously reported that an NCAM-positive area appeared at later stages of molar tooth development, did not express NCAM immunoreactivity even at the latest stage of development covered in this study. These differences in the distribution of NCAM between the incisors and the molars might be related to the fact that rodent incisors continue to grow throughout the life of the animal.

Tooth morphogenesis and the differentiation of ameloblasts

All vertebrate organs are formed from several cell types, and it is currently believed that interactions between the different components constitute the most important mechanism in the regulation of organ morphogenesis. In developing teeth morphogenetic interactions occur between the epithelium covering the facial processes and the underlying neural crest-derived mesenchyme. Morphogenesis is accompanied by differentiation of the various dental cell types, including the ameloblasts. Although ameloblasts differentiate terminally and start the deposition of enamel matrix only after the completion of crown morphogenesis, there is increasing evidence suggesting that the segregation of the ameloblast cell lineage may start much earlier. For example, the down-regulation of the North receptor, which in some other developmental system is associated with cell fate determination, is already seen in the dental epithelium prior to the bud stage. It is not known to what extent the differentiation of ameloblasts depends on tooth morphogenesis, and whether the same mesenchymal signals regulate morphogenesis and cell differentiation. There is evidence that growth factors act as morphogenetic signals. Bone morphogenetic proteins and fibroblast growth factors appear to regulate the initiation of tooth development, as well as the morphogenesis of the crown shape. However, the molecular nature of the signals regulating the advancing specialization of the cells in the ameloblast cell lineage remains unknown.

Polarizing activity, Sonic hedgehog, and tooth development in embryonic and postnatal mouse

Tooth development involves reciprocal epithelial-mesenchymal interactions, polarized growth, mesenchyme condensation, and complex morphogenetic events. Because these processes bear similarities to those occurring in the developing limb, we asked whether morphogenetic signals found in the limb also occur in the developing tooth. We grafted mouse embryo tooth germs to the anterior margin of host chick embryo wing buds and determined whether the dental tissues had polarizing activity. Indeed, the grafts induced supernumerary digits. Activity of both molar and incisor tooth germs increased from bud to cap stages and was maximal at late bell stage in newborn. With further development the polarizing activity began to decrease, became undetectable in adult molar mesenchyme but persisted in incisor mesenchyme, correlating with the fact that incisors grow throughout postnatal life while molars do not. When different portions of neonatal incisors were assayed, a clear proximo-distal gradient of activity was apparent, with maximal activity restricted to the most proximal portion where undifferentiated mesenchyme and enamel organ reside. In situ hybridizations demonstrated that prior to induction of supernumerary digits, the tooth germ grafts induced expression in host tissue of Hoxd-12 and Hoxd-13. In addition, whole-mount in situ hybridizations and immunohistochemistry showed that developing tooth germs express Sonic hedgehog (Shh). Shh expression was first detected in bud stage tooth germs; at later stages Shh transcripts were prominent in enamel knot and differentiating ameloblasts at the cuspal region. We concluded that tooth germs possess polarizing activity and produce polarizing factors such as Shh. As in the limb, these factor(s) and activity probably play key roles in establishing polarity and regulating morphogenesis during early tooth development. Given its subsequent association with differentiating ameloblasts, Shh probably participates also in cytogenetic events during odontogenesis.

Development and cell fate in interspecific (Mus musculus/Mus caroli) intraocular transplants of mouse molar tooth-germ tissues detected by in situ hybridization

Mandibular first molar tooth germs were dissected from Mus musculus (CDI) and Mus caroli (age range: 14-day embryo to 1-day postnatal). Most of the tooth germs were separated enzymically into epithelial and mesenchymal components. Interspecific tissue recombinations and intact M. caroli tooth germs were grown in the anterior chamber of the eye of adult CDI mice for 24 weeks. Recombinations of M. caroli enamel-organ epithelium with M. musculus, dental papilla and follicle mesenchyme developed into normal teeth with advanced root, periodontal ligament and bone formation, thereby confirming extensive epithelial-mesenchymal interactions across the species barrier. Labelling sections by in situ hybridization with a M. musculus-specific DNA probe (pMSat5) showed that almost all cells in the pulp, periodontal ligament and bone were M. musculus, including cementoblasts. Reduced enamel epithelium and epithelial cell rests derived from donor M. caroli enamel organ were unlabelled. This indicates that any cementogenic role of Hertwig's epithelial root sheath must be short-lived. The immunological privilege of the intraocular transplantation site in M. musculus CDI mice did not extend to grafts including xenogeneic M. caroli dental mesenchyme. Thus, intact M. caroli tooth germs and recombinations of M. musculus enamel organ with M. caroli dental papilla and follicle showed limited development, with no root formation, and were populated almost exclusively with labelled host M. musculus lymphocytes.

The expression of the gene coding for parathyroid hormone-related protein (PTHrP) during tooth development in the rat

By means of in situ hybridisation studies, it is shown that parathyroid hormone-related protein (PTHrP) mRNA is strongly expressed in the developing enamel organs of rat teeth. In particular, the cervical loop hybridises strongly with the PTHrP probe and expression is maintained at this site throughout life in the permanently erupting incisor teeth. In mature molar teeth, expression is downregulated to low levels and confined to the epithelial cell rests of Malassez and/or cementoblasts which may derive from these. The gene is also expressed at low levels in the tissue overlying the erupting molars and, thereafter, in the junctional epithelia and connective tissue cells of the epithelial attachment on all tooth surfaces. The premise that PTHrP may undergo post-translational processing and that the resultant products could act in different ways raises the possibility of its exerting multiple paracrine actions during tooth development. These could include the control of cell division and local vascular dilation during development.

Immunohistochemical demonstration of bcl-2 protein in human tooth germs

This study sought to detect patterns of bcl-2 protein expression that could provide more insight into the cellular dynamics of tooth development. As bcl-2 serves to prevent cell death, its occurrence in odontogenic tissues might be helpful in identifying cell populations from which odontogenic tumours may arise. The bcl-2 protein was found only in the epithelial part of the tooth germ and was present in all parts of the enamel organ except the ameloblast. This suggests that bcl-2 protein plays a part in maintaining the viability of the enamel organ. The presence of bcl-2 in the fully matured tooth germ and adjacent dental lamina might indicate that epithelial odontogenic tumours may originate from various parts of the enamel organ.

Early determination and permissive expression of amelogenin transcription during mouse mandibular first molar development

The expression of tissue-specific enamel matrix genes is believed to require both instructive and permissive interactions of enamel organ epithelium with dental papilla mesenchyme and/or extracellular matrix during a restricted period of development. Biosynthesis of amelogenin gene products has been found to be associated with the terminal differentiation of inner enamel organ epithelium. The developing mouse first mandibular molar was used for a detailed examination of the temporal initiation and developmental pattern of amelogenin transcription. These studies define temporally instructive versus permissive influences on amelogenin transcription. During in vivo development, amelogenin transcripts were detected in late cap (15 days in utero; E15) through bell stage (E16 through E19) mouse molar tooth formation utilizing reverse transcription coupled to polymerase chain reaction amplification. Alternatively spliced amelogenin transcripts were detected in late bell stage (E18) molars. Amelogenin transcripts were also detected in isolated late cap stage (E15) enamel organ epithelium dissected free of dental papilla mesenchyme and cultured within a substitute basement membrane gel, but not in identical cap stage enamel organ epithelium cultured on plastic or a laminin-coated filter. Amelogenin transcripts were also found in early cap stage (E14) isolated enamel organ epithelium cultured within a basement membrane gel, but were not detected in enamel organ epithelium isolated from earlier stages of odontogenesis and cultured within a basement membrane gel. The results of these experiments indicate that a basement membrane gel is a useful extracellular substrate which provides permissive interactions required for the expression of amelogenin transcripts by enamel organ epithelium and that instructive interactions which determine enamel organ epithelium to become committed to amelogenin transcription occur prior to the early cap stage (E14) of odontogenesis. The results also suggest that continued interactions of enamel organ epithelium with dental papilla mesenchyme serve to regulate amelogenin transcription and post-transcriptional amelogenin RNA splicing in a complex manner during odontogenesis.

Microcirculation of human fetal tooth buds: a SEM study of corrosion casts

The microcirculation of tooth buds at the bell stage obtained from 5-month-old human fetuses was studied using corrosion casting and scanning electron microscopy. Each tooth bud has two independent vascular networks: one of the enamel organ and one of the dental papilla. Both systems are supplied by vertical branches of the inferior alveolar artery. The vascular bed of the enamel organ consists of capillaries relatively uniform in shape, forming a moderately dense network with irregular meshes. In contrast, the vasculature of dental papilla is extremely dense and its vessels show a sinusoidal character and signs of a vivid angiogenesis. The cast surfaces of capillaries in both vascular systems show the presence of tiny blebs probably representing extravasations of the casting medium through endothelial fenestrations.

Transforming growth factor-beta 1 mRNA in neonatal ovine molars visualized by in situ hybridization: potential role for the stratum intermedium

Human dentine contains relatively large amounts of transforming growth factor-beta (TGF-beta), which might originate from odontoblasts. The expression of the TGF-beta 1 message in developing teeth was examined by in situ hybridization. The analysis was made on 5-microns serial sections of mandibular third molars of neonatal sheep cut from tissues that had been fixed in glutaraldehyde and paraffin-embedded. A 35S-labelled cRNA probe, complementary to TGF-beta 1 mRNA, was constructed from human TGF-beta 1 cDNA. Northern analysis of total RNA from sheep placenta and neonatal third molars demonstrated hybridization to a single 2.4 kb TGF-beta 1 transcript from both tissues, indicating cross-reactivity of the human probe in the sheep. In the neonatal molars, in situ hybridization was observed in cells of the inner enamel epithelium, mature ameloblasts and mature odontoblasts, but not within preodontoblasts before dentine matrix formation. TGF-beta 1 mRNA expression was also evident in the cells of the dental papilla but scarcely so in the stellate reticulum. The most striking feature was the appearance of hybridization signal in the cells of the stratum intermedium before hybridization was evident in the inner enamel epithelium. Control sections incubated with RNAase before incubation with probe did not show evidence of hybridization. These findings suggest that TGF-beta 1 may have an important regulatory role in the differentiation of ameloblasts and odontoblasts, perhaps by modulating matrix formation during amelogenesis or odontogenesis. They also suggest a potential novel regulatory role for the cells of the stratum intermedium.

An immunocytochemical study of keratin reactivity during rat odontogenesis

The cytokeratin distribution in the developing rat enamel organ from day 15 of gestation through to 11 days post partum was examined immunohistochemically using a panel of monoclonal antibodies. A temporo-spatial programme of keratin expression was observed during odontogenesis and positive reactivity of the enamel organ was seen with the pan keratin antibodies CK1 (clone LP34 - reacts with a number of keratins including 6 and 18) and AE1-3 (reacts with most acidic and basic keratins). No reactivity was observed in the enamel organ with the other antibodies examined (Ks 8.12 [reacts with keratins 13 and 16], Ks 8.60 [reacts with keratins 10 and 11) and MCA157 [reacts with rat liver antigen]), although these antibodies did stain other epithelial tissues. This study supports the view that the epithelial cells of the enamel organ synthesize a tissue-specific subset of keratins which are related to the differentiation of the cells.

[Embryonic development of the teeth]

Embrionary evolution of the tooth in the rat continues four times very clearly. The first of them or "dentair esbozo" it takes place about the 16th day. The second one of them or "caperuza time" between the 17th and 18th day. The third one of them or "bell time" it takes place 19th, 20th and 21th days. At the end the fourth one of then or "of resolution" it takes place the 22 and first of the afterborn days.

Distribution of intermediate-filament proteins in the human enamel organ: unusually complex pattern of coexpression of cytokeratin polypeptides and vimentin

We applied immunohistochemical techniques and gel electrophoresis to examine the distribution of intermediate filaments in human fetal oral epithelium and the epithelia of the human enamel organ. Both methods demonstrated that human enamel epithelia contain cytokeratins 5, 14, and 17, which are typical of the basal cells of stratified epithelia, as well as smaller quantities of cytokeratins 7, 8, 19, and in trace amounts 18, which are characteristic components of simple epithelial cells. In the external enamel epithelium and stellate-reticulum cells, most of these components appeared to be simultaneously expressed. In contrast, the parental oral epithelium was negative for cytokeratin 7, thus indicating possible "neoexpression" during the course of tooth formation. Immunohistochemical procedures using various monoclonal antibodies against vimentin revealed the transient coexpression of vimentin and cytokeratins in the external enamel epithelium and in stellate-reticulum cells during enamel development. The significance of the coexpression of cytokeratins and vimentin is discussed in relation to previous findings obtained in other normal tissues and in the light of the functional processes characteristic of these epithelia.

Hertwig's epithelial root sheath differentiation and initial cementum and bone formation during long-term organ culture of mouse mandibular first molars using serumless, chemically-defined medium

Studies were designed to test the hypothesis that Hertwig's epithelial root sheath (HERS) synthesizes and secretes enamel-related proteins that participate in the process of acellular cementum formation. Our experimental strategy was to examine sequential root development of the mouse mandibular first molar in vivo and in long-term organ culture in vitro using serumless, chemically-defined medium. Using anti-amelogenin, anti-enamelin and anti-peptide antibodies, enamel-related antigens were localized within intermediate cementum during HERS differentiation and root formation in vivo. Cap stage molars maintained for periods of up to 31 days in organ culture expressed morphogenesis and cytodifferentiation as identified by tooth crown and initial root, cementum and bone formation. Metabolically-labeled HERS products were analyzed by immunodetection using enamel-related antibodies and one- and two-dimensional SDS gel electrophoresis. A 72 kDa and 26 kDa polypeptide were identified in forming mouse cementum. Both of these root putative cementum proteins yield similar (identical) amino acid compositions; however, both proteins differed from the compositions of either mouse crown enamelin or amelogenin proteins. This approach provides a new and novel in vitro model towards understanding HERS differentiation and functions related to root and bone formation. The data support the hypothesis that HERS cells synthesize polypeptides related to but also different from canonical crown enamel proteins.

1987 Kreshover lecture. Gene regulation in the development of oral tissues

The regulatory processes associated with tooth formation are being investigated by the identification of when, where, and how cell adhesion molecules (CAMs), substrate adhesion molecules (SAMs), dentin phosphoprotein, enamel gene products, and intermediate cementum products are expressed during sequential developmental stages of morphogenesis, cytodifferentiation, dentin, enamel and cementum extracellular matrix (ECM) formation, and biomineralization. Instructive and permissive signaling is required for both morphogenesis and cytodifferentiation based upon in vitro organotypic culture studies in serumless, chemically-defined medium. Intrinsic developmental instructions, independent of exogenous growth factors, mediate tooth morphogenesis from the initiation of the dental lamina through crown and initial root development. Recent progress using recombinant DNA methods has advanced descriptions of several dental structural genes. The complete nucleic acid sequence for mouse amelogenin has been defined. This sequence is located on the mouse X chromosome and on the human X and Y chromosomes. This discussion summarizes recent results using experimental embryology, recombinant DNA technology, and immunocytology in the context of instructive epithelial-mesenchymal interactions associated with epithelial differentiation into ameloblasts, ectomesenchyme differentiation into odontoblasts, and dentin and enamel ECM biomineralization. The tooth organ provides opportunities at several levels of biological organization to investigate cellular, molecular, and developmental processes.

Dental cell interaction with extracellular-matrix constituents: type-I collagen and fibronectin

It has been suggested that, during odontoblast differentiation, the extracellular matrix present at the epitheliomesenchymal junction modulates the activity of the cytoskeleton by means of membrane constituents (proteins, proteoglycans or gangliosides). To investigate this, we studied the interaction of iodinated fibronectin and type-I collagen with dissociated dental tissues and with membrane proteins prepared from these tissues. Isolated dental papillae and enamel organs were cultured for increasing periods of time in the presence of iodinated proteins. Fibronectin and type-I collagen were preferentially bound to dental papillae; however, after 6 h of incubation, fibronectin no longer interacted with the dental papillae, and the bound radioactivity was released. In the meantime, de novo synthesized fibronectin was deposited in the extracellular matrix of the dental papillae. Membrane proteins were prepared from isolated enamel organs and dental papillae. After sodium dodecyl sulphate (SDS)-polyacrylamide gel electrophoresis, these proteins were transferred to nitrocellulose by electroblotting and then incubated in the presence of either 125I-labelled fibronectin or 125I-labelled type-I collagen. Autoradiography confirmed the preferential interaction of fibronectin with the dental papilla. Fibronectin interacted with three high-molecular-weight proteins (Mr, 145,000, 154,000 and 185,000), which were not detected when membranes were prepared from enamel organs. Under the same conditions, type-I collagen did not interact with membrane proteins. The known interaction of type-I collagen with the plasma membrane of dental-papilla cells might be mediated either by another constituent of the extracellular matrix or by cell-surface-associated proteoglycans.

The basement membrane of the enamel organ in human odontogenesis

This study was designed to record findings of a basement membrane in human fetal jaws relative to the oral epithelium, dental lamina, and developing teeth. With the use of Lillie's allochrome stain, this could be followed from the oral epithelium and dental lamina to the tooth jaws. The basement membrane was easily traced along the inner enamel epithelium, but only after elimination of the counterstaining phases was it visible adjacent to the outer enamel epithelium.