Specific transforming growth factor-beta subtypes regulate embryonic mouse Meckel's cartilage and tooth development

Fate of the mammalian cranial neural crest during tooth and mandibular morphogenesis

Neural crest cells are multipotential stem cells that contribute extensively to vertebrate development and give rise to various cell and tissue types. Determination of the fate of mammalian neural crest has been inhibited by the lack of appropriate markers. Here, we make use of a two-component genetic system for indelibly marking the progeny of the cranial neural crest during tooth and mandible development. In the first mouse line, Cre recombinase is expressed under the control of the Wnt1 promoter as a transgene. Significantly, Wnt1 transgene expression is limited to the migrating neural crest cells that are derived from the dorsal CNS. The second mouse line, the ROSA26 conditional reporter (R26R), serves as a substrate for the Cre-mediated recombination. Using this two-component genetic system, we have systematically followed the migration and differentiation of the cranial neural crest (CNC) cells from E9.5 to 6 weeks after birth. Our results demonstrate, for the first time, that CNC cells contribute to the formation of condensed dental mesenchyme, dental papilla, odontoblasts, dentine matrix, pulp, cementum, periodontal ligaments, chondrocytes in Meckel's cartilage, mandible, the articulating disc of temporomandibular joint and branchial arch nerve ganglia. More importantly, there is a dynamic distribution of CNC- and non-CNC-derived cells during tooth and mandibular morphogenesis. These results are a first step towards a comprehensive understanding of neural crest cell migration and differentiation during mammalian craniofacial development. Furthermore, this transgenic model also provides a new tool for cell lineage analysis and genetic manipulation of neural-crest-derived components in normal and abnormal embryogenesis.

Transforming growth factor beta2, but not beta1 and beta3, is critical for early rat lung branching

Mesenchymal-epithelial tissue interactions are critical for lung branching morphogenesis, and polypeptide growth factors are likely involved in these tissue interactions. Transforming growth factorbetas (TGFbetas) have been implicated in lung development, but their involvement in early lung branching morphogenesis is unclear. In the present study, we investigated the role of the three mammalian TGFbeta subtypes (beta1, beta2, and beta3) and their receptors (type III (TbetaR-III), type II (TbetaR-II), and two types I (TbetaR-I), ALK-1 and ALK-5) in early rat lung organogenesis by using an embryonic rat lung explant culture. Transcripts and proteins for all three TGFbetas and their receptors were detected during the embryonic period of fetal rat lung development. Inhibition of TGFbeta2, but not beta1 and beta3, with antisense oligonucleotides and neutralizing antibodies resulted in significant inhibition of early lung branching in culture. Addition of minute amounts (</=1 ng/ml) of exogenous TGFbeta2, but not beta1 and beta3, restored the branching of TGFbeta2 antisense-treated explants. Higher concentrations of TGFbeta2 were inhibitory. BrdU labeling of lung explants was not altered by antisense TGFbeta2 treatment, but low concentrations of TGFbeta2 increased thymidine uptake by isolated epithelial cells. Fibronectin and metallogelatinase activities of embryonic lung cells were not affected by any TGFbeta isoform but TGFbeta2 specifically decreased mesenchymal hyaluronan synthesis. Antisense inhibition of ALK-5 and TbetaR-II showed a similar reduction in early lung branching as observed with antisense TGFbeta2. Incubation of lung explants with soluble TbetaR-II receptors also abrogated lung branching. ALK-1 antisense treatment did not affect early branching. Administration of neither activin A, which can act via ALK-1, nor follistatin, the natural inhibitor of activin, to the explants cultures had any significant effect on lung branching. Antisense inhibition of the activin receptor-II (Act-RII) also did not affect lung branching. These results are consistent with TGFbeta2, but not beta1 and beta3, regulating pattern formation during early rat lung organogenesis. This TGFbeta signaling in rat lung branching in vitro appears to be predominantly mediated via the TbetaR-I(ALK-5)/TbetaR-II heteromeric complex.

Transforming growth factor-beta isoform expression in mature human healthy and carious molar teeth

Transforming growth factor (TGF)-beta isoforms have been implicated in cellular signalling during tooth development and repair, but little is known of their cellular localisation or distribution within the dental tissues in the mature tooth. This study investigated the presence of TGF-beta1, beta2 and beta3 isoforms in tissues of sound and carious human molar teeth, to understand better the expression of TGF-betas during health and disease. In healthy tissues, odontoblasts, cells of the cell rich layer, pulpal fibroblasts and endothelial cells were stained to varying degrees for all isoforms, with TGF-beta3 showing the greatest intensity and TGF-beta1 the weakest intensity. Similar patterns of staining were observed in carious teeth; however, TGF-beta1 showed significantly increased staining intensity within odontoblasts and pulpal cells of carious teeth (p < 0.001). Biochemical analysis showed greater amounts of TGF-beta1 in tertiary dentine than in primary dentine samples. The expression of TGF-betas in odontoblasts and the increased presence of TGF-beta1 in tertiary dentine suggest that these isoforms may be important in odontoblast behaviour and the modulation of the tissue response to injury.

Enhancement by hepatocyte growth factor of bone and cartilage formation during embryonic mouse mandibular development in vitro

To elucidate the possible roles of hepatocyte growth factor (HGF) in the early development of mouse mandible, HGF was applied to an organ-culture system with chemically defined media. Mandibular arches microdissected from mouse embryos at the 10th day of gestation were cultured for 10 days with or without HGF, HGF plus HGF-receptor (c-met) antisense oligodeoxyribonucleotide, or HGF plus c-met sense oligodeoxyribonucleotide in the media. The cultured mandibles were then analysed, histologically in serial paraffin sections. In the absence of HGF, the tooth organs of bud stage, Meckel's cartilage and the tongue were formed, whereas only a slight amount of bone tissue was formed in the cultured mandible. The expression of intrinsic HGF and c-met in the cultured mandibles was confirmed by reverse transcriptase-polymerase chain reaction. Furthermore, immunohistochemistry demonstrated that both HGF and c-met were localized in areas of the mesenchymal tissue forming bone and cartilage. With HGF in the medium, the volume of both bone and cartilage increased significantly and dose-dependently. HGF also increased the rate of proliferation of osteogenic cells and chondrocytes. Addition of c-met antisense oligodeoxyribonucleotide partially inhibited the HGF-induced enhancement of bone and cartilage formation, whereas addition of c-met sense oligodeoxyribonucleotide had no effect. These results revealed that exogenous HGF enhances bone and cartilage morphogenesis in the cultured mandibles, suggesting physiological roles for intrinsic HGF in the early development of mouse mandible.

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.

Inhibition of TGF-beta 3 restores the invasive capability of extravillous trophoblasts in preeclamptic pregnancies

Preeclampsia, the major cause of maternal morbidity and mortality in developed countries, is associated with abnormalities of placenta function due to shallow invasion of the maternal decidua by trophoblasts. Data suggest that TGF-beta may play a role in inhibiting trophoblast outgrowth or invasion, or both. We report that placental TGF-beta 3 expression is high in early pregnancy but falls at around 9 weeks' gestation. This pattern is inversely correlated with trophoblast outgrowth and fibronectin synthesis, markers of early trophoblast differentiation toward an invasive phenotype. We demonstrate that TGF-beta 3 is overexpressed in preeclamptic placentae. In contrast to control placentae, explants from preeclamptic pregnancies fail to exhibit spontaneous invasion in vitro. Significantly, antisense-induced inhibition of TGF-beta 3 expression, and inhibition of TGF-beta 3 activity with antibodies, induces the formation of columns of trophoblast cells, which migrate out of the explant into the underlying Matrigel. To our knowledge, this is the first demonstration that the hypoinvasive placental phenotype characteristic of preeclampsia can be essentially normalized in vitro by biochemical manipulation. We speculate that a failure to downregulate expression of TGF-beta 3 at around 9 weeks' gestation results in shallow trophoblast invasion and predisposes the pregnancy to preeclampsia.

Teeth. Where and how to make them

Organs have to develop at precisely determined sites to ensure functionality of the whole organism. Organogenesis is typically regulated by a series of interactions between morphologically distinct tissues. The developing tooth of the mouse is an excellent model to study these processes and we are beginning to understand the networks regulating reciprocal tissue interactions at the molecular level. Synergistic and antagonistic effects of signaling molecules including FGFs and BMPs are recursively used to induce localized responses in the adjacent tissue layer (mesenchyme or epithelium). However, at different phases of odontogenesis these secreted growth factors have distinct effects and at the same time they are regulated by different upstream factors. The mesenchymal transcription factors Msx1 and Pax9 are initially regulated by epithelial FGFs and BMPs, but subsequently they function upstream of these signaling molecules. This cascade provides a molecular model by which reciprocal tissue interactions are controlled.

Stimulation of the dentine-pulp complex of rat incisor teeth by transforming growth factor-beta isoforms 1-3 in vitro

Expression of transforming growth factor-beta (TGF-beta) isoforms by odontoblasts leads to their sequestration within dentine matrix. TGF-beta1 and -beta3 stimulate matrix secretion and also initiate odontoblast cytodifferentiation in vitro and in vivo. Using a recently established organ-culture model, the aim here was to examine the effects of TGF-beta isoforms on the response of the dentine-pulp complex during culture. Agarose beads were soaked in isoforms 1-3, and placed on the odontoblast area of slices of 28-day-old rat incisor tooth. The slices were maintained in Trowel-type cultures for 7 days. Both TGF-beta1 and -beta3 stimulated a local increase in predentine secretion at the site of the bead application when compared to control cultures. Mitogenic effects on the cells of the subodontoblast layer were also seen and occasionally small foci of newly differentiated odontoblast-like cells could be observed a little distant from the application site of TGF-beta3. TGF-beta2 had a minimal effect on the cultured tissues. These results demonstrate that TGF-beta1 and -beta3 can stimulate secretion of extracellular matrix by odontoblasts, are mitogenic to pulp cells, and that TGF-beta3 may have inductive effects on pulpal cells. Such activities might be important during reparative processes in the dentine-pulp complex after tissue injury.

PDGF-A and PDGFR-alpha regulate tooth formation via autocrine mechanism during mandibular morphogenesis in vitro

Platelet-derived growth factor A (PDGF-A) binding to the PDGF receptor alpha (PDGFR-alpha) mediates signal transduction processes related to DNA synthesis, cell migrations, cytodifferentiation, and wound healing. Recent studies indicate that PDGFR-alpha functions during cranial neural crest cell migrations and first branchial arch morphogenesis (Stephenson et al. [1991] Proc. Natl. Acad. Sci. USA 88:6-10; Morrison-Graham et al. [1992] Development 115:133-142; Hu et al. [1995] Int. J. Dev. Biol. 39:939-945; Soriano [1997] Development 124:2691-2700). The present studies were designed to test the hypothesis that PDGF-A, interacts with its cognate receptor PDGFR-alpha via an autocrine mechanism that regulates the timing, rates, and size of embryonic mouse tooth morphogenesis. Both PDGF-A and PDGFR-alpha transcripts were coordinately expressed in mandibular prominences prior to and during tooth formation using reverse transcriptase-polymerase chain reaction (RT-PCR). During the dental lamina stage, ligand and receptor were present in both enamel organ epithelium and adjacent mesenchymal cells. During the bud stage, ligand and receptor were localized mainly to the enamel organ epithelium. Exogenous PDGF-A at 20 ng/ml enhanced tooth development to reach the cap stage with increased tooth size (P < 0.05) using embryonic day (E)10 mandibular explants cultured in serumless, chemically defined medium. A significant increase in DNA synthesis was observed within enamel organ epithelium at E10+4 when the mandibular explants were treated with PDGF-A at 20 ng/ml. These data suggest that PDGF-A and its cognate receptor (PDGFR-alpha) regulate the size and stage of tooth development via an autocrine mechanism during odontogenesis in vitro.

Transforming growth factor-beta superfamily members expressed in rat incisor pulp

The transforming growth factor (TGF)-beta superfamily comprises more than 35 structurally related genes that have been implicated in embryonic induction and morphogenesis. Different superfamily members may have distinct regulatory roles in tooth development and maintenance. Degenerate primer sets derived from the highly conserved carboxy terminal region of the TGF-beta superfamily were used for reverse transcriptase polymerase with poly(A)+ RNA from the rat incisor pulp as a template. TGF-beta superfamily members expressed in the pulp with known potential to differentiate into odontoblasts and to form dentine were identified. Nucleotide-sequence analysis of the amplified cDNAs identified those encoding activin-betaB; bone morphogenic protein (BMP)-2, -4, -7 and -8; growth/differentiation factor (GDF)-1, -5 and -6; and glial cell line-derived neurotrophic factor. In addition, Northern blot analysis detected TGF-beta1 -beta2 and -beta3; activin-betaA; BMP-6 and GDF-7 mRNA transcripts in the pulp. Coordinated expression of TGF-beta superfamily members in pulp may be critical in tooth development and repair.

Activin is an essential early mesenchymal signal in tooth development that is required for patterning of the murine dentition

Development of the mammalian tooth has been intensively studied as a model system for epithelial/mesenchymal interactions during organogenesis, and progress has been made in identifying key molecules involved in this signaling. We show that activin betaA is expressed in presumptive tooth-germ mesenchyme and is thus a candidate for a signaling molecule in tooth development. Analysis of tooth development in activin betaA mutant embryos shows that incisor and mandibular molar teeth fail to develop beyond the bud stage. Activin betaA is thus an essential component of tooth development. Development of maxillary molars, however, is unaffected in the mutants. Using tissue recombination experiments we show that activin is required in the mesenchyme prior to bud formation and that although activin signaling from mesenchyme to epithelium takes place, mutant epithelium retains its ability to support tooth development. Implantation of beads soaked in activin A, into developing mandibles, is able to completely rescue tooth development from E11.5, but not E12.5 or E13.5, confirming that activin is an early, essential mesenchyme signal required before tooth bud formation. Normal development of maxillary molars in the absence of activin shows a position specific role for this pathway in development of dentition. Functional redundancy with activin B or other TGFbeta family members that bind to activin receptors cannot explain development of maxillary molars in the mutants since the activin-signaling pathway appears not to be active in these tooth germs. The early requirement for activin signaling in the mesenchyme in incisor and mandibular molar tooth germs must be carried-out in maxillary molar mesenchyme by other independent signaling pathways.

Signalling networks regulating dental development

There has been rapid progress recently in the identification of signalling pathways regulating tooth development. It has become apparent that signalling networks involved in Drosophila development and development of mammalian organs such as the limb are also used in tooth development. Teeth are epithelial appendages formed in the oral region of vertebrates and their early developmental anatomy resembles that of other appendages, such as hairs and glands. The neural crest origin of tooth mesenchyme has been confirmed and recent evidence suggests that specific combinations of homeobox genes expressed in the neural crest cells may regulate the types of teeth and their patterning. Signalling molecules in the Shh, FGF, BMP and Wnt families appear to regulate the early steps of tooth morphogenesis and some transcription factors associated with these pathways have been shown to be necessary for tooth development. Several of the conserved signals are also transiently expressed in the enamel knots in the dental epithelium. The enamel knots are associated with the characteristic epithelial folding morphogenesis which is responsible for the development of tooth shape and it is currently believed that the enamel knots function as signalling centres regulating tooth shape development. The developing tooth has proven to be an excellent model in studies of the molecular basis of patterning and morphogenesis of organs and it can be expected that continuing studies will rapidly increase the understanding of these mechanisms.

The activin-binding protein follistatin is expressed in developing murine molar and induces odontoblast-like cell differentiation in vitro

It has recently been shown that mice deficient in activin-beta A subunits and follistatin exhibit major defects in dentition. To increase understanding of the roles played by these molecules during tooth development, we determined the temporospatial expression of activin-beta A subunit and follistatin messenger RNA and their corresponding proteins in developing murine molars (between day E 14 and 2 days after birth). The effects of recombinant human activin A and its binding protein follistatin on odontoblast differentiation were also studied in cultures of dental papillae (DP) isolated from the mandibular first molars of E-17-day mice. In situ hybridization indicated that transcripts for activin-beta A subunit were abundant in pre-odontoblasts at the tips of forming cusps prior to odontoblast terminal differentiation, and transcripts for follistatin in overlying inner enamel epithelial cells (pre-ameloblasts). Pre-odontoblasts were also weakly immunoreactive in relation to activin-beta A subunit, pre-ameloblasts in relation to follistatin. When follistatin was added at different concentrations to a DP culture model (2-14 nmol/DP) together with heparin at constant concentration, differentiation of odontoblast-like cells was induced, as evidenced by polarization and deposition of extracellular matrix in vitro, to extents depending on the follistatin concentration. In contrast, the addition of activin A (2 nmol/DP) had no effect on the differentiation parameters studied. These findings suggest that the activin-follistatin system regulates odontoblast differentiation during tooth development. In particular, we suggest that binding of endogenous activin A by follistatin may allow odontoblast terminal differentiation to occur.

Specific inhibition of hair follicle formation by epidermal growth factor in an organ culture of developing mouse skin

Embryonic mouse skin undergoes a drastic morphological change from 13 to 16 gestational days, i.e., formation of rudiments of hair follicles and stratification and cornification of interfollicular epidermis. To investigate underlying molecular mechanisms of the morphogenesis, we established an organ culture system that allows skin tissues isolated from 12.5- or 13.5-days postcoitus embryos to develop in a manner that is histologically and temporally similar to the process in vivo. Expression of differentiation markers of epidermal keratinocytes including cholesterol sulfotransferase and cytokeratin K1 was induced in culture, as it occurs also in vivo. The morphogenic process was observed by time-lapse videomicrography. In this culture system, epidermal growth factor (EGF) and transforming growth factor alpha specifically and completely inhibited the hair follicle formation with marginal effects on interfollicular epidermis. The inhibitory action by EGF was reversible and stage specific, i.e., at an early stage of the development of hair rudiments. Among known ligands to the EGF receptor, Schwannoma-derived growth factor and heparin-binding EGF were expressed in in vivo epidermis during the period of the initial formation of hair follicles. EGF receptor is expressed in epidermis throughout the developing period examined. Using an adenovirus vector, we demonstrated that the lacZ gene was transduced into the epidermal and dermal cell layers without appreciable toxicity. These results indicate that the present culture system provides a unique opportunity to investigate molecular mechanisms of skin morphogenesis including the role of EGF signaling under defined experimental conditions.

Matrix metalloproteinases regulate morphogenesis, migration and remodeling of epithelium, tongue skeletal muscle and cartilage in the mandibular arch

We have investigated the role of proteinases in the developmental program of bone, cartilage, tongue muscle and epithelial differentiation and remodeling in the mandibular arch during murine embryogenesis. Expression of matrix metalloproteinases (MMPs) and their tissue inhibitors (TIMPs) was tissue-specific with little or no expression in the epithelium of tooth buds, tongue or oral cavity. Gelatinase A mRNA transcripts were strongly expressed in the perichondrium of Meckel's cartilage and mesenchymal areas of embryonic day 13–15 mandibles, whereas gelatinase B, collagenase-3, TIMP-1 and TIMP-2 mRNA were found primarily in the ossifying areas of the mandibles. The skeletal muscle of the tongue expressed stromelysin-3, TIMP-2 and TIMP-3 mRNA while stromelysin-3, TIMP-2 and gelatinase A were seen in the overlying connective tissue layer. Gelatinase A, gelatinase B, stromelysin-1, urokinase, TIMP-1 and TIMP-2 mRNA and protein activities were also detected in cultured mandibular explants. Culture of day 10 mandibular explants with a hydroxamic acid metalloproteinase inhibitor, but not with inhibitors of metalloendopeptidases (thiorphan and phosphoramidon), serine proteinases (aprotinin), cysteine proteinases (leupeptin) and urokinase (amiloride), altered mandibular morphogenesis dramatically. Development of the tongue (glossogenesis) and cartilage, but not bone or teeth was affected. Formation of the oral sulcus and fusion of the two epithelia of the medial sulcus were inhibited, and number and migration of myoblasts decreased. The resulting ‘tongue-tied phenotype’ indicates that MMPs are involved in epithelial morphogenesis and the migration of myoblasts to the region of the tongue. Development of the anterior segment of Meckel's cartilage was also inhibited and proteoglycan content of the cartilage was reduced by inhibiting MMPs. Our data suggest that matrix metalloproteinases play a pivotal role in the morphogenesis of structures derived from epithelium (oral sulcus), cranial paraxial mesoderm (tongue) and cranial neural crest (Meckel's cartilage).

Characterization of the fate of midline epithelial cells during the fusion of mandibular prominences in vivo

The fusion of the mandibular prominences along the midline is achieved with the absence of medial epithelial cells at the fusion site. Failure of fusion of the mandibular prominences results in median cleft of the lower lip and mandible. Cellular and molecular events controlling mandibular fusion were examined during the fusion process in mouse embryogenesis. Cell lineage analyses at the fusion site revealed that epithelial cells migrated to the surface and oral epithelia. DiI-labeled epithelial cells were not observed within the mandibular mesenchyme at any state of fusion. Examination of the midline region did not reveal cells with ultrastructural changes characteristic of apoptotic cell death. An increase in lysosomal enzymes in the midline epithelial cells, which would be correlated with programmed cell death, was not observed. Mice lacking TGF-beta 3 did not have cleft mandible, but had clefting of the secondary palate as a feature of null mutation phenotype. We interpret our comparisons between wild type and homozygous TGF-beta 3 (-/-) mice to suggest that different developmental processes control palatal vs. mandibular fusion. We hypothesize that medical epithelial cells at the fusion site of mandibular prominences migrate to the surface epithelium during the fusion process and neither transdifferentiate into mesenchyme nor express apoptosis.

Comparative analysis of transforming growth factor-beta isoforms 1-3 in human and rabbit dentine matrices

Previous studies have implicated transforming growth factor-beta (s)(TGF-beta) in both development. Here TGF-beta isoforms in dentine extracellular matrix were analysed because these molecules may participate in dental issue repair. EDTA-soluble and collagenase-released fractions were isolated from human crown and root and rabbit incisor dentine samples and analysed for TGF-beta isoforms. TGF-beta(1) was the major isoform detected in all samples and the only isoform detected in human dentine samples. TGF-beta(2) was detected only in the collagenase-released fraction of rabbit incisor dentine and was present at low levels. TGF-beta(3) was detected in both EDTA-soluble and collagenase-released fractions of rabbit dentine. Greater levels of the TGF-beta(1) isoform were detected in the rabbit than human dentine samples and some differences in distribution amongst the two tissue fractions were observed between these species. The presence of these isoforms of TGF-beta in dentine may provide a reservoir of growth factor in the matrix that could participate in processes leading to tissue repair after injury.

Organ cultures of embryonic rat tongue support tongue and gustatory papilla morphogenesis in vitro without intact sensory ganglia

Taste buds on the mammalian tongue are confined to the epithelium of three types of gustatory papillae: the fungiform, circumvallate, and foliate. The gustatory papillae are composed of an epithelium that covers a broad connective tissue core, with extensive innervation to taste bud and nongustatory epithelial locations. Although the temporal sequence of gustatory papilla development is known for several species, factors that regulate initiation, growth, and maintenance of the papillae are not understood. We tested the hypothesis that sensory innervation is required for the initial formation and early morphogenesis of fungiform papillae in a patterned array. An organ culture of the embryonic rat tongue was developed to provide an in vitro system for studying mechanisms involved in fungiform papilla morphogenesis in patterns on the anterior tongue. Tongues were dissected from embryos at 13 days of gestation (E13), a time when the tongue has not yet fully formed and gustatory papillae have not yet appeared, and at 14 days of gestation (E14), when the tongue is well formed and papillae make their initial morphological appearance. Dissected tongues were maintained at the gas/liquid interface in standard organ culture dishes, fed with DMEM/F12 plus 2% B-27 supplement and 1% fetal bovine serum. After 1, 2, 3, or 6 days in culture, tongues were processed for scanning electron or light microscopy, or immunocytochemistry. Tongues cultured from E13 or E14 underwent extensive morphogenesis and growth in vitro. Furthermore, fungiform papillae developed on these tongues on a culture day equivalent to E15 in vivo; that is, after 2 days for cultures begun at E13 and 1 day for those begun at E14. Because E15 is the characteristic time for gustatory papilla formation in the intact embryo, results demonstrate that the cultured tongues retain important temporal information related to papilla development. In addition, fungiform papillae formed in the tongue cultures in the stereotypic pattern of rows. The papillae were large structures with epithelial and mesenchymal cell integrity, and an intact epithelial basement membrane was indicated with laminin immunoreactivity. The cultures demonstrate that gustatory papilla morphogenesis can progress in the absence of an intact sensory innervation. To exclude a potential developmental role for autonomic ganglion cells that are located in the posterior rat tongue, cultures consisting of only the anterior half of E14 tongues were established. Fungiform papilla development progressed in half tongues in a manner directly comparable to whole tongue cultures. Therefore, robust, reproducible development of fungiform papillae in patterns is supported in rat tongue cultures from E13 or E14, without inclusion of intact sensory or major, posterior tongue autonomic ganglia. This is direct evidence that papillae will form and develop further in vitro without sensory ganglion support. The data also provide the first detailed account of in vitro development of the entire embryonic tongue.

Expression of cytokeratin 14 in ameloblast-lineage cells of the developing tooth of rat, both in vivo and in vitro

In the search for a cell marker useful for studying tooth development, immunohistochemical studies using antibodies against cytokeratin 14 (K14), c-Met/hepatocyte growth factor receptor and amelogenin were carried out in the developing tooth of the newborn rat and in primary cultured cells of the ameloblast lineage, including inner enamel epithelium cells, preameloblasts and ameloblasts, prepared from the mandibular incisors of postnatal 7-day-old rats. The appearance of K14 was cell- and differentiation-stage specific, i.e. there was a weak expression signal within inner enamel epithelial cells that were in the proliferating stage, and there were strong signals within preameloblasts and ameloblasts that were in the post-proliferating and amelogenesis stages, respectively. In the culture system, c-Met appeared in all cells, whereas K14 and amelogenin appeared mainly in clustered cells that were considered to be in the post-proliferating stage. K14 was detected earlier than amelogenin, and it was also confirmed by immunofluorostaining that c-Met, K14 and amelogenin were coexpressed in ameloblasts. These findings indicate that K14 is a good new marker for ameloblast-lineage cells during rat tooth development both in vivo and in vitro.

Expression of the gene for transforming growth factor-beta in avian dyschondroplasia

Previous immunolocalisation studies of dyschondroplasia have indicated that there is a reduction in the number of growth plate chondrocytes containing the protein transforming growth factor beta 3 (TGF-beta 3). The reduction in TGF-beta 3 in dyschondroplasia is likely to be a direct result of a reduction in the expression of the TGF-beta 3 gene. mRNA was extracted from small (0.09 g) samples of growth cartilage from the proximal tibiotarsus of three-week-old broiler chicks. The cartilage samples contained cells from all three zones of the growth plate (proliferative, transitional and upper hypertrophic) and were collected from normal and dyschondroplastic growth plates. The dyschondroplastic growth plates were identified by an accumulation of transitional chondrocytes which were considered to be a result of a failure to differentiate to the hypertrophic phenotype. A semi-quantitative polymerase chain reaction (PCR) was used to estimate the quantity of mRNA specific for glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and for each of the three isoforms of TGF beta (TGF-beta 1, TGF-beta 2 and TGF-beta 3) in each of the cartilage samples. The levels of expression of mRNA for GAPDH, TGF-beta 1 and TGF-beta 2 were similar in the two groups, but the expression of TGF-beta 3 mRNA was significantly reduced in the samples from the dyschondroplastic growth plates. The reduction in TGF-beta 3 levels is thought to be associated with the failure of chondrocyte hypertrophy in dyschondroplasia, and provides in vivo evidence that TGF-beta 3 is part of the cascade of events associated with the differentiation of chondrocytes during endochondral ossification in the chick.

Spatial and temporal distribution of Indian hedgehog mRNA in the embryonic mouse mandible

Hedgehog genes are involved in pattern formation during embryonic development. A recent report showed that Sonic hedgehog is expressed in the mouse mandible in the presumptive incisor region. In the present study, Indian hedgehog (Ihh) transcripts were present from gestational day 9 to 14 in the mouse mandible (reverse transcription/polymerase chain reaction analysis). Ihh mRNA was present in the dental lamina in both incisor and molar regions and in the developing whiskers (in-situ hybridization). Ihh may be involved in the site-specific proliferation of mandibular epithelium during the formation of the dental lamina. This is consistent with the observation that endogenous synthesis of retinoic acid is necessary for the initiation of odontogenesis and that retinoic acid induces hedgehog expression.

Periodontal ligament cell population: the central role of fibroblasts in creating a unique tissue

BACKGROUND

Fibroblasts are the predominant cells of the periodontal ligament (PL) and have important roles in the development, function, and regeneration of the tooth support apparatus. Biological processes initiated during the formation of the PL contribute to the long-lasting homeostasic properties exhibited by PL fibroblast populations.

DEVELOPMENT

The formation of the PL is likely controlled by epithelial-mesenchymal and epithelial hard tissue interactions, but the actual mechanisms that contribute to the development of cellular lineages in the PL are unknown. Fibroblasts in the normally functioning PL migrate through the tissue along collagen fibres to cementum and bone and in an apico-coronal direction during tooth eruption. ADULT TISSUE: Cell kinetic experiments have shown that PL fibroblasts comprise a renewal cell system in steady-state and the progenitors can generate multiple types of more differentiated, specialized cells. Progenitor cell populations of the PL are enriched in locations adjacent to blood vessels and in contiguous endosteal spaces. In normally functioning periodontal tissues, there is a relatively modest turnover of cells in which apoptotic cell death balances proliferation. Large increases of cell formation and cell differentiation occur after application of orthodontic forces or wounding. As PL cells comprise multiple cellular phenotypes, it has been postulated that after wounding, the separate phenotypes repopulating the site will ultimately dictate the tissue form and type.

CONCLUSIONS

PL fibroblasts play an essential role in responses to mechanical force loading of the tooth by remodelling and repairing effete or damaged matrix components. In consideration of the important roles played by fibroblasts in PL homeostasis, they could be described as "the architect, builder, and caretaker" of the periodontal ligament.

Different signalling pathways involved in transforming growth factor-beta 1-induced morphological change and type IV collagen synthesis in simian virus-40-immortalized normal human salivary gland duct and myoepithelial cell clones

To understand the specific cell type responsible for the synthesis of basement membrane components of the salivary gland, the effects of transforming growth factor (TGF)-beta 1 on morphological change, cellular proliferation and collagen synthesis were examined in these immortalized duct and myoepithelial cell clones, and the expression forms of their TGF-beta receptors analysed. When TGF-beta 1 was added to the cell clones in vitro, it induced a morphological alteration, with flattening in myoepithelial but not in duct cells. Although the growth of Mv1Lu mink lung epithelial cells was almost completely inhibited by TGF-beta 1, the duct and myoepithelial cells were partially resistant to such inhibition. By immunoblot analysis of immunoprecipitates, p53 was found bound to the simian virus-40 large T antigen, suggesting a functional loss of p53 in regulation of cell-cycle arrest. In the cloned myoepithelial cells but not the duct cells, TGF-beta 1 stimulated the production of type IV collagen. To attempt to understand the distinct responsiveness of cell clones to TGF-beta 1, the expression forms of TGF-beta receptors were examined by affinity cross-linking. Although the intensities of the cross-linked bands of the TGF-beta type II and type III receptors, particularly the type II, were weaker in the duct than the myoepithelial cell clones, the expression of the type II receptor mRNA was consistently detected in both clones. Accordingly, the reduction of TGF-beta 1 binding may have occurred at the post-transcriptional level. These findings imply that the cloned myoepithelial cells but not the cloned duct cells produce type IV collagen in response to TGF-beta 1 through the receptor-mediated signal transduction pathway, which is presumably disrupted in the cloned duct cells.

Hepatocyte growth factor is involved in the morphogenesis of tooth germ in murine molars

The patterns of gene expression for hepatocyte growth factor (HGF) and its receptor, c-Met, were revealed in the tooth germ of rat mandibular molars using RT-PCR. In situ hybridization demonstrated that the HGF gene was expressed only in the cells of the dental papilla of the tooth germ in vivo. The characteristic temporospatial distribution of HGF and c-Met during germ development was revealed using immunohistochemical studies in vivo. In order to demonstrate the functional role played by HGF in tooth development, HGF translation arrest by antisense phosphorothioate oligodeoxynucleotide (ODN) was carried out in vitro. In the control experiment, explants of tooth germs from embryonic 14 day mice were cultured in a modification of Trowell's system under serum-free and chemically defined conditions for two weeks. Other explants were cultured with 15mer antisense or sense ODN targeted to the HGF mRNA. Both the control and the sense-treated explants showed normal histological structure, as observed in vivo. On the other hand, antisense-treated explants exhibited an abnormal structure in which the enamel organs were surrounded by a thin layer of dentin and dental papilla, appearing ‘inside-out’ compared to the control and sense-treated explants, although the cytodifferentiation of ameloblasts and odontoblasts was not inhibited. The explants treated with recombinant human HGF combined with antisense ODN showed normal development, indicating that exogenous HGF rescued the explants from the abnormal structure caused by antisense ODN. The findings of a BrdU incorporation experiment suggested that the imbalance between the proliferation activity of the inner enamel epithelium and that of the dental papilla caused by HGF translation arrest results in the abnormal structure of the tooth germ. These results indicate that HGF is involved in the morphogenesis of the murine molar.

Transforming growth factor-beta 3 is required for secondary palate fusion

Mice lacking TGF-beta 3 exhibit an incompletely penetrant failure of the palatal shelves to fuse leading to cleft palate. The defect appears to result from impaired adhesion of the apposing medial edge epithelia of the palatal shelves and subsequent elimination of the mid-line epithelial seam. No craniofacial abnormalities were observed. This result demonstrates that TGF-beta 3 affects palatal shelf fusion by an intrinsic, primary mechanism rather than by effects secondary to craniofacial defects.

Spatial and temporal distribution of sonic hedgehog mRNA in the embryonic mouse mandible by reverse transcription/polymerase chain reaction and in situ hybridization analysis

Hedgehog genes have recently been implicated in the control of pattern formation in many developing organ system. Vertebrate homologues of the Drosophila hedgehog have been identified in mouse and rate embryos. The temporal regulation of sonic hedgehog (mouse homologue) has previously been studied by Northern analysis of whole embryos with varying results. Sonic hedgehog transcript expression in the mouse mandibular process was now characterized using polymerase chain reaction (PCR) an in situ hybridization techniques. PCR analysis revealed transcripts at gestational days 10 and 11, before the formation of the dental lamina, but not at days 12-14, after tooth buds have formed. Transcripts were localized to, primarily, the epithelium in the presumptive incisor region of the mandibular midline at gestational day 10. No mRNA was detected by in situ hybridization techniques in the presumptive molar regions of odontogenic epithelium. Sonic hedgehog expression may be involved in the regulation of pattern formation through establishment of an incisor-molar axis of polarity.

A role for mesenchyme-derived tachykinins in tooth and mammary gland morphogenesis

Tachykinin peptides such as substance P (SP) function as neurotransmitters and neuromodulators in the mammalian central and peripheral nervous systems. Here, we provide evidence that they may also play an important role in the morphogenesis of some nonneural organs where epithelial-mesenchymal interactions are involved. We show the following. (1) mRNA encoding tachykinin precursor proteins is expressed transiently in condensing mesenchyme during the development of mouse tooth germ, mammary gland, limb bud, external auditory meatus and genital tubercle. (2) In developing tooth germ and mammary gland; mRNA encoding the neutral endopeptidase (NEP) that degrades secreted tachykinins is spatially and temporally co-expressed with tachykinin precursor mRNA. (3) SP and the mRNA encoding SP receptors are also expressed in the developing tooth germ. (4) Tooth development in explant cultures is blocked both by tachykinin-precursor-specific antisense oligonucleotide and by an SP receptor antagonist: in both cases the block is relieved by exogenous SP. Together, these findings suggest a surprising new role for tachykinins in tooth and mammary gland morphogenesis, and possibly also in limb, ear and external genitalia morphogenesis.

Blockade of the initiation of murine odontogenesis in vitro by citral, an inhibitor of endogenous retinoic acid synthesis

Endogenous retinoids are present in the embryonic mouse mandible and reach a concentration peak immediately before the formation of the dental lamina. Because exogenous retinoids alter the pattern of the dental lamina and the expression of epidermal growth factor mRNA (a transcript necessary for initiation of odontogenesis), the role of retinoic acid in the initiation of odontogenesis was studied here. Citral (3,7-dimethyl-2,6-octadienal), a known inhibitor of retinoic acid synthesis, was used to block the endogenous synthesis of retinoic acid in the mouse embryonic mandible before the formation of the dental lamina (gestational day 9). A 24-h exposure to citral totally blocked tooth formation in 7/10 mandibles. Reductions of endogenous retinoic acid concentrations were confirmed by high-performance liquid chromatography. Tooth formation was restored by simultaneous treatment with all-trans retinoic acid or 9-cis retinoic acid during the citral exposures (first 24 h of culture). Endogenous retinoic acid is necessary for the initiation of odontogenesis.

Instruction by retinoic acid of incisor morphology in the mouse embryonic mandible

Endogenous retinoids are present in the embryonic mouse mandible and reach a peak in concentration at the time of the formation of the dental lamina. All-trans retinoic acid is present in a 10-fold higher concentration in the future incisor region of the mouse embryonic mandible at day 11.5. It was found here that exogenous all-trans retinoic acid has pleiotropic effects on the pattern of odontogenesis when applied before the formation of the dental lamina. These effects include a change in the pattern of the dental lamina, supernumerary buds and incisors in the diastema region, and replacement of molars with incisors in the molar region. Thus retinoic acid appears to instruct incisor morphology in the mouse embryonic mandible.

Molecular biology experimental strategies for craniofacial-oral-dental dysmorphology

This paper is a minireview of molecular biology experimental strategies for problems within craniofacial-oral-dental biology. Many of these strategies have already made remarkable contributions towards understanding the complex developmental processes associated with craniofacial biology. For example, the utilization of these strategies has resulted in the successful mapping of approximately 70 genes related to craniofacial anomalies (e.g., Pax, retinoic acid receptors, cadhedrins, aggrecan, cell adhesion molecules, substrate adhesion molecules, etc.), 30 genes related to dental tissue disorders (e.g., BMPs, bone morphogenetic proteins; dentin phosphoproteins, dentin sialoglycoproteins, enamelins, amelogenins), 20 genes related to facial clefting defects (e.g., Hox genes, transforming growth factor alpha), and 3 genes related to craniosynostosis (e.g., Msx-2). This minireview highlights selected examples of scientific progress derived from the following experimental strategies: (i) molecular approaches to the organization of the mouse and human chromosomes, with the mapping of specific gene sequences linked to human diseases (e.g., amelogenesis imperfecta, Boston type craniosynostosis, Rieger's syndrome, Treacher Collins syndrome); (ii) reverse genetic approaches for studies of gene function; (iii) homologous recombinations and the advances from "knock-out" transgenic mouse models for human craniofacial-oral-dental diseases; (iv) mutational analyses of congenital craniofacial-oral-dental dysmorphogenesis; (v) structural biology studies using computer-assisted molecular modeling for protein-protein, protein-nucleic acid and protein-inorganic interactions; (vi) computer modeling of genetic paradigms; and (vii) a cluster of newer methodologies including computer-assisted morphometry, new microinjection techniques, new cell membrane and intracellular dyes, and a number of new RNA and DNA viral constructs for the delivery of genes to enhance the resolution of cell fate maps, cell lineage studies and gene therapy approaches to human diseases.

Spatial distribution of endogenous retinoids in the murine embryonic mandible

Retinoids play an important part in pattern formation during embryonic development. Exogenous retinoids alter the pattern of skeletal, neural and odontogenic tissues. Endogenous retinoids have been demonstrated previously in the murine embryonic mandible, reaching a concentration peak during the initiation of odontogenesis. It was now found that endogenous retinoids are present in a concentration gradient in the embryonic mouse mandible at the time of the initiation of the dental lamina. All-trans-retinoic acid was more concentrated in the incisor region and retinol in the molar region. These results, and the fact that exogenous retinoids produce supernumerary incisors and missing molars, suggest that all-trans-retinoic acid may instruct incisor morphology.

Induction of dentine in amputated pulp of dogs by recombinant human bone morphogenetic proteins-2 and -4 with collagen matrix

Recombinant human bone morphogenetic protein (BMP)-2, BMP-4 and transforming growth factor (TGF)-beta 1 combined with collagen matrix as a carrier were examined for their effects on pulp regeneration and dentine formation. Seventy days after implantation of 2 micrograms of BMP-2, mineralized osteodentine-like tissue containing embedded osteodentinocytes was seen in the cavity. Unmineralized fibrous tissue and pulp-like loose connective tissue were also found in the same cavity. In teeth implanted with 660 ng of BMP-2 only unmineralized fibrous and pulp tissues were seen. In teeth with 220 ng of BMP-2 or collagen alone, pulp tissue was seen. It is therefore likely that the cavity fills with pulp tissue and that spindle-shaped cells elaborate extracellular matrix that mineralizes to be osteodentine in a dose-dependent manner. Similar osteodentine was seen in teeth implanted with 4 micrograms of BMP-4 and collagen. No distinct tubular dentine was formed, unlike an earlier experiment in which BMP-2 or -4 was implanted with enriched, inactivated dentine matrix. These findings suggest that both BMP-2 and -4 induce osteodentine formation if combined with collagen matrix; some other matrix component present in inactivated dentine matrix might be essential for further differentiation into odontoblasts. In teeth implanted with TGF-beta 1, the carrier collagen remained in the cavity and little pulp tissue proliferation was seen, suggesting a possible inhibitory effect of TGF-beta 1 in pulp regeneration. It is likely that the response to growth and differentiation factors is dependent on the state of differentiation of pulp cells.