The life history of an embryonic signaling center: BMP-4 induces p21 and is associated with apoptosis in the mouse tooth enamel knot

Convergent signalling through Fgfr2 regulates divergent craniofacial morphogenesis

Fibroblast growth factor receptor 2 (Fgfr2) has two splice variants IIIb and IIIc, which are unique in function and localization. Signalling through Fgfr2IIIb controls epithelial-mesenchymal interactions, which regulate morphogenesis during the development of several organs including the palate and tooth. In this study, we confirm that molar tooth development in Fgfr2IIIb(-/-) mice is arrested early in development and that the molar teeth of Fgf10(-/-) mice develop through all the normal stages of morphogenesis. We show that the molar phenotype of Fgfr2IIIb(-/-) mice is, in part, owing to reduced cell proliferation in both epithelial and mesenchymal compartments. We also show that the developing molar teeth of Fgf10(-/-) mice exhibit reduced cell proliferation. However, this reduction is not sufficient to arrest molar development. Recent evidence has indicated that Fgfr2IIIb/Fgf10 signalling is active in the calvaria in some pathological situations as heterozygous deletion of Fgfr2 exon IIIc in mice leads to ectopic expression of Fgfr2IIIb in the calvarial bones and causes craniosynostosis. Here, we investigate the mRNA expression of Fgfr2IIIb and Fgfr2IIIc as well as their ligands Fgf3, -7 and -10 in the developing murine tooth, palate and calvaria. We show that Fgf7 is expressed in the calvarial mesenchyme adjacent to the developing frontal bone and Fgf10 is expressed by osteoprogenitors in the developing frontal bone condensation. Taken together, we highlight the overlapping roles of Fgfr2IIIb/Fgf10 signalling in controlling epithelial-mesenchymal interactions during normal palate and tooth morphogenesis and how elevated signalling through Fgfr2IIIb/Fgf10 solely within the mesenchyme can result in abnormal calvarial morphogenesis.

Msx and dlx homeogene expression in epithelial odontogenic tumors

Epithelial odontogenic tumors are rare jaw pathologies that raise clinical diagnosis and prognosis dilemmas notably between ameloblastomas and clear cell odontogenic carcinomas (CCOCs). In line with previous studies, the molecular determinants of tooth development-amelogenin, Msx1, Msx2, Dlx2, Dlx3, Bmp2, and Bmp4-were analyzed by RT-PCR, ISH, and immunolabeling in 12 recurrent ameloblastomas and in one case of CCOC. Although Msx1 expression imitates normal cell differentiation in these tumors, other genes showed a distinct pattern depending on the type of tumor and the tissue involved. In benign ameloblastomas, ISH localized Dlx3 transcripts and inconstantly detected Msx2 transcripts in epithelial cells. In the CCOC, ISH established a lack of both Dlx3 and Msx2 transcripts but allowed identification of the antisense transcript of Msx1, which imitates the same scheme of distribution between mesenchyme and epithelium as in the cup stage of tooth development. Furthermore, while exploring the expression pattern of signal molecules by RT-PCR, Bmp2 was shown to be completely inactivated in the CCOC and irregularly noticeable in ameloblastomas. Bmp4 was always expressed in all the tumors. Based on the established roles of Msx and Dlx transcription factors in dental cell fates, these data suggest that their altered expression is a proposed trail to explain the genesis and/or the progression of odontogenic tumors.

Dental agenesis: genetic and clinical perspectives

Dental agenesis is the most common developmental anomaly in humans and is frequently associated with several other oral abnormalities. Whereas the incidence of missing teeth may vary considerably depending on dentition, gender, and demographic or geographic profiles, distinct patterns of agenesis have been detected in the permanent dentition. These frequently involve the last teeth of a class to develop (I2, P2, M3) suggesting a possible link with evolutionary trends. Hypodontia can either occur as an isolated condition (non-syndromic hypodontia) involving one (80% of cases), a few (less than 10%) or many teeth (less than 1%), or can be associated with a systemic condition or syndrome (syndromic hypodontia), essentially reflecting the genetically and phenotypically heterogeneity of the condition. Based on our present knowledge of genes and transcription factors that are involved in tooth development, it is assumed that different phenotypic forms are caused by different genes involving different interacting molecular pathways, providing an explanation not only for the wide variety in agenesis patterns but also for associations of dental agenesis with other oral anomalies. At present, the list of genes involved in human non-syndromic hypodontia includes not only those encoding a signaling molecule (TGFA) and transcription factors (MSX1 and PAX9) that play critical roles during early craniofacial development, but also genes coding for a protein involved in canonical Wnt signaling (AXIN2), and a transmembrane receptor of fibroblast growth factors (FGFR1). Our objective was to review the current literature on the molecular mechanisms that are responsible for selective dental agenesis in humans and to present a detailed overview of syndromes with hypodontia and their causative genes. These new perspectives and future challenges in the field of identification of possible candidate genes involved in dental agenesis are discussed.

The large functional spectrum of the heparin-binding cytokines MK and HB-GAM in continuously growing organs: the rodent incisor as a model

The heparin binding molecules MK and HB-GAM are involved in the regulation of growth and differentiation of many tissues and organs. Here we analyzed the expression of MK and HB-GAM in the developing mouse incisors, which are continuously growing organs with a stem cell compartment. Overlapping but distinct expression patterns for MK and HB-GAM were observed during all stages of incisor development (initiation, morphogenesis, cytodifferentiation). Both proteins were detected in the enamel knot, a transient epithelial signaling structure that is important for tooth morphogenesis, and the cervical loop where the stem cell niche is located. The functions of MK and HB-GAM were studied in dental explants and organotypic cultures in vitro. In mesenchymal explants, MK stimulated HB-GAM expression and, vice-versa, HB-GAM upregulated MK expression, thus indicating a regulatory loop between these proteins. BMP and FGF molecules also activated expression of both cytokines in mesenchyme. The proliferative effects of MK and HB-GAM varied according to the mesenchymal or epithelial origin of the tissue. Growth, cytodifferentiation and mineralization were inhibited in incisor germs cultured in the presence of MK neutralizing antibodies. These results demonstrate that MK and HB-GAM are involved in stem cells maintenance, cytodifferentiation and mineralization processes during mouse incisor development.

Ectodermal Smad4 and p38 MAPK are functionally redundant in mediating TGF-beta/BMP signaling during tooth and palate development

Smad4 is a central intracellular effector of TGF-beta signaling. Smad-independent TGF-beta pathways, such as those mediated by p38 MAPK, have been identified in cell culture systems, but their in vivo functional mechanisms remain unclear. In this study, we investigated the role of TGF-beta signaling in tooth and palate development and noted that conditional inactivation of Smad4 in oral epithelium results in much milder phenotypes than those seen with the corresponding receptor mutants, Bmpr1a and Tgfbr2, respectively. Perturbed p38 function in these tissues likewise has no effect by itself; however, when both Smad4 and p38 functions are compromised, dramatic recapitulation of the receptor mutant phenotypes results. Thus, our study demonstrates that p38 and Smad4 are functionally redundant in mediating TGF-beta signaling in diverse contexts during embryonic organogenesis. The ability of epithelium to utilize both pathways illustrates the complicated nature of TGF-beta signaling mechanisms in development and disease.

Epithelial histogenesis during tooth development

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

Inheritance of occlusal topography: a twin study

AIM

This was to determine the relative contribution of genetic factors on the morphology of occlusal surfaces of mandibular primary first molars by employing the twin study model.

METHODS

The occlusal morphology of mandibular primary first molar teeth from dental casts of 9 monozygotic (MZ) twin pairs and 12 dizygotic (DZ) twin pairs 4 to 7 years old, were digitized by contact-type three-dimensional (3D) scanner. To compare the similarity of occlusal morphology between twin sets, each twin pair of occlusal surfaces was superimposed to establish the best fit by using computerized least squared techniques. Heritability was computed using a variance component model, adjusted for age and gender.

RESULTS

DZ pairs demonstrated a greater degree of occlusal morphology variance. The total amount of difference in surface overlap was 0.0508 mm (0.0018 (inches) for the MZ (n=18) sample and 0.095 mm (0.0034 inches) for the DZ (n=24) sample and were not statistically significant (p=0.2203). The transformed mean differences were not statistically significantly different (p=0.2203). Heritability estimates of occlusal surface areas for right and left mandibular primary first molars were 97.5% and 98.2% (p<0.0001), respectively.

CONCLUSIONS

Occlusal morphology of DZ twin pairs was more variable than that of MZ twin pairs. Heritability estimates revealed that genetic factors strongly influence occlusal morphology of mandibular primary first molars.

Apoptosis in developmental and repair-related human tooth remodeling: a view from the inside

Apoptosis is a key phenomenon in the regulation of the life span of odontoblasts, which are responsible for dentin matrix production of the teeth. The mechanism controlling odontoblasts loss in developing, normal, and injured human teeth is largely unknown. A possible correlation between apoptosis and dental pulp volume reduction was examined. Histomorphometric analysis was performed on intact 10 to 14 year-old premolars to follow dentin deposition and evaluate the total number of odontoblasts. Apoptosis in growing healthy teeth as well as in mature irritated human teeth was determined using a modified TUNEL technique and an anti-caspase-3 antibody. In intact growing teeth, the sequential rearrangement of odontoblasts into a multi-layer structure during tooth crown formation was correlated with an apoptotic wave that leads to the massive elimination of odontoblasts. These data suggest that apoptosis, coincident with dentin deposition changes, plays a role in tooth maturation and homeostasis. Massive apoptotic events were observed after dentin irritation. In carious and injured teeth, apoptosis was detected in cells surrounding the lesion sites, as well as in mono-nucleated cells nearby the injury. These results indicate that apoptosis is a part of the mechanism that regulate human dental pulp chamber remodeling during tooth development and pathology.

Wnt/beta-catenin signaling directs multiple stages of tooth morphogenesis

Wnt/beta-catenin signaling plays key roles in tooth development, but how this pathway intersects with the complex interplay of signaling factors regulating dental morphogenesis has been unclear. We demonstrate that Wnt/beta-catenin signaling is active at multiple stages of tooth development. Mutation of beta-catenin to a constitutively active form in oral epithelium causes formation of large, misshapen tooth buds and ectopic teeth, and expanded expression of signaling molecules important for tooth development. Conversely, expression of key morphogenetic regulators including Bmp4, Msx1, and Msx2 is downregulated in embryos expressing the secreted Wnt inhibitor Dkk1 which blocks signaling in epithelial and underlying mesenchymal cells. Similar phenotypes are observed in embryos lacking epithelial beta-catenin, demonstrating a requirement for Wnt signaling within the epithelium. Inducible Dkk1 expression after the bud stage causes formation of blunted molar cusps, downregulation of the enamel knot marker p21, and loss of restricted ectodin expression, revealing requirements for Wnt activity in maintaining secondary enamel knots. These data place Wnt/beta-catenin signaling upstream of key morphogenetic signaling pathways at multiple stages of tooth development and indicate that tight regulation of this pathway is essential both for patterning tooth development in the dental lamina, and for controlling the shape of individual teeth.

Tooth morphogenesis in vivo, in vitro, and in silico

One of the aims of developmental biology is to understand how a single egg cell gives rise to the complex spatial distributions of cell types and extracellular components of the adult phenotype. This review discusses the main genetic and epigenetic interactions known to play a role in tooth development and how they can be integrated into coherent models. Along the same lines, several hypotheses about aspects of tooth development that are currently not well understood are evaluated. This is done from their morphological consequences from the model and how these fit known morphological variation and change during tooth development. Thus the aim of this review is two-fold. On one hand the model and its comparison with experimental evidence will be used to outline our current understanding about tooth morphogenesis. On the other hand these same comparisons will be used to introduce a computational model that makes accurate predictions on three-dimensional morphology and patterns of gene expression by implementing cell signaling, proliferation and mechanical interactions between cells. In comparison with many other models of development this model includes reaction-diffusion-like dynamics confined to a diffusion chamber (the developing tooth) that changes in shape in three-dimensions over time. These changes are due to mechanical interactions between cells triggered by the proliferation enhancing effect of the reactants (growth factors). In general, tooth morphogenesis can be understood from the indirect cross-regulation between extracellular signals, the local regulation of proliferation and differentiation rates by these signals and the effect of intermediate developing morphology on the diffusion, dilution, and spatial distribution of these signals. Overall, this review should be interesting to either readers interested in the mechanistic bases of tooth morphogenesis, without necessarily being interested in modeling per se, and readers interested in development modeling in general.

Zebrafish dentition in comparative context

Studies of the zebrafish (Danio rerio) promise to contribute much to an understanding of the developmental genetic mechanisms underlying diversification of the vertebrate dentition. Tooth development, structure, and replacement in the zebrafish largely reflect the primitive condition of jawed vertebrates, providing a basis for comparison with features of the more extensively studied mammalian dentition. A distinctive derived feature of the zebrafish dentition is restriction of teeth to a single pair of pharyngeal bones. Such reduction of the dentition, characteristic of the order Cypriniformes, has never been reversed, despite subsequent and extensive diversification of the group in numbers of species and variety of feeding modes. Studies of the developmental genetic mechanism of dentition reduction in the zebrafish suggest a potential explanation for irreversibility in that tooth loss seems to be associated with loss of developmental activators rather than gain of repressors. The zebrafish and other members of the family Cyprinidae exhibit species-specific numbers and arrangements of pharyngeal teeth, and extensive variation in tooth shape also occurs within the family. Mutant screens and experimental alteration of gene expression in the zebrafish are likely to yield variant tooth number and shape phenotypes that can be compared with those occurring naturally within the Cyprinidae. Such studies may reveal the relative contribution to trends in dental evolution of biases in the generation of variation and sorting of this variation by selection or drift.

TGF-β superfamily signaling is essential for tooth and hair morphogenesis and differentiation

Members of the transforming growth factor beta (TGF-beta) superfamily of signaling molecules are involved in the regulation of many developmental processes that involve the interaction between mesenchymal and epithelial tissues. Smad7 is a potent inhibitor of many members of the TGF-beta family, notably TGF-beta and activin. In this study, we show that embryonic overexpression of Smad7 in stratified epithelia using a keratin 5 promoter, results in severe morphogenetic defects in skin and teeth and leads to embryonic and perinatal lethality. To further analyze the functions of Smad7 in epithelial tissues of adult mice, we used an expression system that allowed a controlled overexpression of Smad7 in terms of both space and time. Skin defects in adult mice overexpressing Smad7 were characterized by hyper-proliferation and missing expression of early markers of keratinocyte differentiation. Upon Smad7-mediated blockade of TGF-beta superfamily signaling, ameloblasts failed to produce an enamel layer in incisor teeth. In addition, TGF-beta blockade in adult mice altered the pattern of thymic T cell differentiation and the number of thymic T cells was significantly reduced. This study shows that TGF-beta superfamily signaling is essential for development of hair, tooth and T-cells as well as differentiation and proliferation control in adult tissues.

Intercusp relationships of the permanent maxillary first and second molars in American whites

Much of a human molar's morphology is concentrated on its occlusal surface. In view of embryologists' recent attention on the determination of crown morphology by enamel knots that initiate cusp formation, we were interested in the arrangement of cusp apices in the definitive tooth. Computer-assisted image analysis was used to measure intercusp distances and angles on permanent maxillary M1 and M2 in a sample of 160 contemporary North American whites. The intent was to generate normative data and to compare the size and variability gradients from M1 to M2. There is little sexual dimorphism in intercusp distances or angles, even though the conventional mesiodistal (MD) and buccolingual (BL) crown size is 2.0% and 4.0% larger in males, respectively, in these same teeth. Dimensions decreased in size and increased in variability from M1 to M2, but differentially. Cusps of the trigon were more stable between teeth, especially the paracone-protocone relationship. Principal components analysis on the six M1 distances disclosed only one eigenvalue above 1.0, indicating that overall crown size itself is the paramount controlling factor in this tooth that almost invariably exhibits a hypocone. In contrast, four components were extracted from among the 12 angular cusp relationships in M1. These axes of variation may prove useful in studies of intergroup differences. A shape difference occurs in M2, depending on whether the hypocone is present; when absent, the metacone is moved lingually, creating more of an isosceles arrangement for the cusps of the trigon. Statistically, correlations are low between occlusal intercusp relationships and conventional crown diameters measured at the margins of the crowns that form later. Weak statistical dependence between cusp relationships and traditional MD and BL diameters suggest that separate stage- and location-specific molecular signals control these different parts (and different stages) of crown formation.

Making a tooth: growth factors, transcription factors, and stem cells

Mammalian tooth development is largely dependent on sequential and reciprocal epithelial-mesenchymal interactions. These processes involve a series of inductive and permissive interactions that result in the determination, differentiation, and organization of odontogenic tissues. Multiple signaling molecules, including BMPs, FGFs, Shh, and Wnt proteins, have been implicated in mediating these tissue interactions. Transcription factors participate in epithelial-mesenchymal interactions via linking the signaling loops between tissue layers by responding to inductive signals and regulating the expression of other signaling molecules. Adult stem cells are highly plastic and multipotent. These cells including dental pulp stem cells and bone marrow stromal cells could be reprogrammed into odontogenic fate and participated in tooth formation. Recent progress in the studies of molecular basis of tooth development, adult stem cell biology, and regeneration will provide fundamental knowledge for the realization of human tooth regeneration in the near future.

Genetische Störungen der Zahnentwicklung und Dentition

Die Zähne sind Organe, die aus ektodermalen epithelialen Aussackungen im Bereich des 1. Kiemenbogens entstehen, gesteuert von epitheliomesenchymalen Interaktionen. Dabei spielen zahlreiche Signalmoleküle speziell der 4 großen Familien TGF-β, FGF, Hedgehog und WNT sowie diverse Transkriptionsfaktoren eine Rolle. Eine Beteiligung der Retinoide an der Odontogenese ist durch umfangreiche Befunde belegt, auch wenn die Inaktivierung relevanter Gene in Mausmodellen meist keine Zahnanomalien verursacht. Die Zahnentwicklung wird klassischerweise in verschiedene Stadien eingeteilt: Entstehung der Zahnleiste, der Zahnknospe, der Schmelzkappe, der Schmelzglocke, die Wurzelbildung und der Zahndurchbruch. Anomalien der Zahnentwicklung können isoliert oder gemeinsam mit anderen Symptomen im Zusammenhang mit Syndromen auftreten. Sie können genetisch bedingt sein oder unter Einwirkung teratogener Stoffe während der Bildung und Mineralisierung der Zahnkeime zustande kommen. Dentibukkale Entwicklungsanomalien treten im Kontext seltener Erkrankungen auf und finden zunehmend Beachtung, da sie bei bestimmten Erkrankungen in der Diagnostik und als prädikative Faktoren wichtige Anhaltspunkte geben können. Allerdings ist hierfür eine interdisziplinäre und internationale Kooperation notwendig, die bislang erst in Ansätzen verwirklicht wurde.

Predicting evolutionary patterns of mammalian teeth from development

One motivation in the study of development is the discovery of mechanisms that may guide evolutionary change. Here we report how development governs relative size and number of cheek teeth, or molars, in the mouse. We constructed an inhibitory cascade model by experimentally uncovering the activator-inhibitor logic of sequential tooth development. The inhibitory cascade acts as a ratchet that determines molar size differences along the jaw, one effect being that the second molar always makes up one-third of total molar area. By using a macroevolutionary test, we demonstrate the success of the model in predicting dentition patterns found among murine rodent species with various diets, thereby providing an example of ecologically driven evolution along a developmentally favoured trajectory. In general, our work demonstrates how to construct and test developmental rules with evolutionary predictability in natural systems.

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

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

Embryonic development of Python sebae – II: Craniofacial microscopic anatomy, cell proliferation and apoptosis

This study explores the microscopic craniofacial morphogenesis of the oviparous African rock python (Python sebae) spanning the first two-thirds of the post-oviposition period. At the time of laying, the python embryo consists of largely undifferentiated mesenchyme and epithelium with the exception of the cranial base and trabeculae cranii, which are undergoing chondrogenesis. The facial prominences are well defined and are at a late stage, close to the time when lip fusion begins. Later (11-12d), specializations in the epithelia begin to differentiate (vomeronasal and olfactory epithelia, teeth). Dental development in snakes is different from that of mammals in several aspects including an extended dental lamina with the capacity to form 4 sets of generational teeth. In addition, the ophidian olfactory system is very different from the mammalian. There is a large vomeronasal organ, a nasal cavity proper and an extraconchal space. All of these areas are lined with a greatly expanded olfactory epithelium. Intramembranous bone differentiation is taking place at stage 3 with some bones already ossifying whereas most are only represented as mesenchymal condensations. In addition to routine histological staining, PCNA immunohistochemistry reveals relatively higher levels of proliferation in the extending dental laminae, in osseous mesenchymal condensations and in the olfactory epithelia. Areas undergoing apoptosis were noted in the enamel organs of the teeth and osseous mesenchymal condensations. We propose that localized apoptosis helps to divide a single condensation into multiple ossification centres and this is a mechanism whereby novel morphology can be selected in response to evolutionary pressures. Several additional differences in head morphology between snakes and other amniotes were noted including a palatal groove separating the inner and outer row of teeth in the upper jaw, a tracheal opening within the tongue and a pharyngeal adhesion that closes off the pharynx from the oral cavity between stages 1 and 4. Our studies on these and other differences in the python will provide valuable insights into in developmental, molecular and evolutionary mechanisms of patterning.

The primary enamel knot determines the position of the first buccal cusp in developing mice molars

The enamel knot (EK), which is located in the center of bud and cap stage tooth germs, is a transitory cluster of non-dividing epithelial cells. The EK acts as a signaling center that provides positional information for tooth morphogenesis and regulates the growth of tooth cusps by inducing secondary EKs. The morphological, cellular, and molecular events leading to the relationship between the primary and secondary EKs have not been described clearly. This study investigated the relationship between the primary and secondary EKs in the maxillary and mandibular first molars of mice. The location of the primary EK and secondary EKs was investigated by chasing Fgf4 expression patterns in tooth germ at some intervals of in vitro culture, and the relationship between the primary EK and secondary EK was examined by tracing the primary EK cells in the E13.5 tooth germs which were frontally half sliced to expose the primary EK. After 48 hr, the primary EK cells in the sliced tooth germs were located on the buccal secondary EKs, which correspond to the future paracone in maxilla and protoconid in mandible. The Bmp4 expression in buccal part of the dental mesenchyme might be related with the lower growth in buccal epithelium than in lingual epithelium, and the Msx2 expressing area in epithelium was overlapped with the enamel cord (or septum) and cell dense area. The enamel cord might connect the primary EK with enamel navel to fix the location of the primary EK in the buccal side during the cap to bell stages. Overall, these results suggest that primary EK cells strictly contribute to form the paracone or protoconid, which are the main cusps of the tooth in the maxilla or mandible.

Regulation of Twist, Snail, and Id1 is conserved between the developing murine palate and tooth

The development of both the tooth and palate requires coordinated bone morphogenetic protein (BMP) and fibroblast growth factor (FGF) signalling between epithelial and mesenchymal tissues. Here, we demonstrate that transcription factors Twist and Snail are downstream targets of FGF signalling, that Id1 and Msx2 are downstream targets of BMP signalling, and that Msx1 is regulated by both signalling pathways during tooth and palate development. We show that Twist and Snail expression in the mesenchyme is regulated by the overlying epithelium and that exogenous FGF4 in tooth and FGF2 in palate can mimic this regulation in isolated mesenchymal explants. Ids act in a dominant-negative manner to inhibit the function of other transcription factors such as Twist and Snail. FGF and BMP signalling can regulate development antagonistically, and we suggest that FGF-regulated Twist and Snail and BMP-regulated Id1 may mediate these antagonistic effects during both tooth and palate development.

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

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

Continuous tooth generation in mouse is induced by activated epithelial Wnt/beta-catenin signaling

The single replacement from milk teeth to permanent teeth makes mammalian teeth different from teeth of most nonmammalian vertebrates and other epithelial organs such as hair and feathers, whose continuous replacement has been linked to Wnt signaling. Here we show that mouse tooth buds expressing stabilized beta-catenin in epithelium give rise to dozens of teeth. The molar crowns, however, are typically simplified unicusped cones. We demonstrate that the supernumerary teeth develop by a renewal process where new signaling centers, the enamel knots, bud off from the existing dental epithelium. The basic aspects of the unlocked tooth renewal can be reproduced with a computer model on tooth development by increasing the intrinsic level of activator production, supporting the role of beta-catenin pathway as an upstream activator of enamel knot formation. These results may implicate Wnt signaling in tooth renewal, a capacity that was all but lost when mammals evolved progressively more complicated tooth shapes.

The relationship between the termination of cell proliferation and expression of heat-shock protein-25 in the rat developing tooth germ

Odontoblast- and ameloblast-lineage cells acquire heat-shock protein (HSP)-25 immunoreactivity after they complete cell division during postnatal odontogenesis in rat molars. However, there are no data available concerning the relationship between the termination of cell proliferation and HSP-25 immunoreactivity during tooth morphogenesis. We compared the expression of HSP-25 in tooth germs with their proliferative activity in the rat prenatal to perinatal molar and postnatal incisor to clarify the functional significance of HSP-25 during tooth morphogenesis by immunohistochemistry using anti-HSP-25 and anti-Ki67/5-bromo-2'-deoxyuridine (BrdU). Numerous proliferating cells in developing molars were distributed throughout the tooth germ and HSP-25 immunoreactivity was recognizable in the dental epithelial and mesenchymal cells after they completed cell division. However, both cell proliferation and immunoreaction for HSP-25 are absent in the enamel knots. The distribution pattern of the proliferating cells in the incisors was basically identical to that in the prenatal molars except for the lack of non-proliferating secondary enamel knots and the sparse distribution of proliferating cells in the apical bud. Thus, HSP-25 protein is suggested to act as a switch between cell proliferation and terminal cyto-differentiation during odontogenesis.

Primary enamel knot cell death in Apaf-1 and caspase-9 deficient mice

During molar development, apoptosis occurs in a well-characterised pattern suggesting several roles for cell death in odontogenesis. However, molecular mechanisms of dental apoptosis are only poorly understood. In this study, Apaf-1 and caspase-9 knockouts were used to uncover the engagement of these members of the apoptotic machinery during early tooth development, concentrating primarily on their function in the apoptotic elimination of primary enamel knot cells. Molar tooth germ morphology, proliferation and apoptosis were investigated on frontal histological sections of murine heads at embryonic days (ED) 15.5, the stage when the primary enamel knot is eliminated apoptotically. In molar tooth germs of both knockouts, no apoptosis was observed according to morphological (haematoxylin-eosin) as well as biochemical criteria (TUNEL). Morphology of the mutant tooth germs, however, was not changed. Additionally, knockout mice showed no changes in proliferation compared to wild type mice. According to our findings on knockout embryos, Apaf-1 and caspase-9 are involved in apoptosis during tooth development; however, they seem dispensable and not necessary for proper tooth shaping. Compensatory or other mechanisms of cell death may act to eliminate the primary enamel knot cells in the absence of Apaf-1 and caspase-9.

The mammary bud as a skin appendage: unique and shared aspects of development

Like other skin appendages, the embryonic mammary gland develops via extensive epithelial-mesenchymal interactions. Early stages in embryonic mammary development strikingly resemble analogous steps in the development of hair follicles and teeth. In each case the first morphological sign of development is a localized thickening in the surface epithelium that subsequently invaginates to form a mammary, hair follicle or tooth bud. Similar sets of intersecting signaling pathways are involved in patterning the mammary, hair follicle and dental epithelium, directing placode formation, and controlling bud invagination. Despite these similarities, subsequent events in the formation of these appendages are diverse. The mammary bud extends to form a sprout that begins to branch upon contact with the mammary fat pad. Hair follicles also extend into the underlying mesenchyme, but instead of branching, hair follicle epithelium folds around a condensation of dermal cells. In contrast, teeth undergo a more complex folding morphogenesis. Here, we review what is known of the molecular and cellular mechanisms controlling early steps in the development of these organs, attempt to unravel both common themes and unique aspects that can begin to explain the diversity of appendage formation, and discuss human genetic diseases that affect appendage morphogenesis.

CD 95 mediated apoptosis in embryogenesis: implication in tooth development

INTRODUCTION

Understanding of apoptotic mechanisms involved in tissue shaping is of particular interest because of possible targeted modulation of the development of organ structures such as teeth. Research of CD 95 mediated apoptosis has been focused particularly on cell death in the immune system and related disorders. However, CD 95 mediated apoptosis is also involved in embryogenesis of many organs as the kidney, the lung, the intestine and tissue networks such as the nervous system.

DESIGN

Narrative review.

RESULTS

This review briefly summarizes the current knowledge of CD 95 mediated apoptosis in embryogenesis with possible implication in tooth development. CD 95 receptor and CD 95 ligand are found at early stages of tooth development. The data suggest some positive correlations with dental apoptosis distribution, particularly in the primary enamel knot where apoptosis occurs during elimination of this structure. CD 95 deficient (lpr) adult mouse tooth phenotype, however, did not show any alterations in final tooth pattern and morphology.

CONCLUSION

To date studies of apoptotic machinery during tooth development show spatial localization of many of the components together with precise and localized timing of cell death. There is still much to be learned about the regulation and importance of apoptosis in tooth development. Nevertheless, the involvement of apoptotic regulatory mechanisms interplaying with other molecules participates to the cellular cross-talk in developing tissues, which opens possible targeted modulations as suggested, e.g. for future molecular dentistry.

Evolution of novelty in the cichlid dentition

The shape of teeth occupies a central position in various biological disciplines, from paleo-ecology to molecular biology to cosmetic and reconstructive dentistry. Despite a long tradition of study in mammals, important questions remain regarding the genetic and developmental basis of differences in tooth shape. Here, we use natural mutants of cichlid fish from East Africa, which exhibit tremendous dental diversity, to help fill the gaps in our understanding of vertebrate odontogenesis. We employ an expanded genetic linkage map to demonstrate that cusp number segregates as a gene of major effect, which explains approximately 40% of the phenotypic variance, on cichlid chromosome 5. Furthermore, we examine patterns of Bmp4 expression in early odontogenesis to address and refine predictions of models linking tooth shape and tooth number. Mutations in the Bmp4 cistron do not control tooth shape in this mapping cross. Our data suggest that the evolution of novelty in the cichlid dentition is galvanized by a small number of genetic changes, echoing similar conclusions from recent studies of other vertebrate adaptive morphologies.

An immunohistochemical study of the expression of heat-shock protein-25 and cell proliferation in the dental pulp and enamel organ during odontogenesis in rat molars

OBJECTIVES

The aim of this study is to clarify the functional significance of heat-shock protein (HSP)-25 during tooth development.

DESIGN

We compared the expression of HSP-25 in the dental epithelial and mesenchymal cells with their proliferative activity during odontogenesis in rat molars on postnatal days 1-100 by immunohistochemistry using anti-HSP-25 and anti-5-bromo-2'-deoxyuridine (BrdU) for cell proliferation assay.

RESULTS

On day 1, BrdU-immunoreactive cells were densely located in the inner enamel epithelium in the cervical loop and intercusped areas and the dental pulp adjacent to them, whereas HSP-25-immunoractivity (IR) was restricted to the cusped area where odontoblasts and ameloblasts had already differentiated. Subsequently, BrdU-IR shifted in the apical direction to be localized around Hertwig's epithelial root sheath during days 5-30, never overlapping with concomitantly apically-shifted HSP-25-IR. On days 60-100, BrdU-immunoreactive cells were hardly recognizable in the dental pulp, where HSP-25-IR was exclusively localized in the odontoblast layer. Furthermore, the odontoblast- and ameloblast-lineage cells exhibited two steps in the expression of HSP-25 throughout the postnatal stages: first, dental epithelial and pulpal mesenchymal cells showed a weak IR for HSP-25 after the cessation of their proliferative activity, and subsequently odontoblasts and ameloblasts consistently expressed an intense HSP-25-IR.

CONCLUSION

Odontoblast- and ameloblast-lineage cells acquire HSP-25-IR after they complete their cell division, suggesting that this protein acts as a switch between cell proliferation and differentiation during tooth development. The consistent expression of HSP-25-IR in the formative cells may be involved in the maintenance of their functional integrity.

Fibroblast growth factor 9: Cloning and immunolocalisation during tooth development in Didelphis albiventris

There are no reports in literature about functional roles of fibroblast growth factor 9 (FGF-9) in tooth development in animals with complete tooth pattern. The classical model for studying tooth development is the mouse, which has small number of teeth and distinctive incisor and molar patterns. The opossum Didelphis albiventris with five upper and four lower incisors, one canine, three premolars, and four molars, on each side of the jaw, seems to be a convenient model to test results obtained in the mouse. Molecular expression studies indicate that FGF-9 participates in murine tooth initiation and regulation of morphogenesis. Searching for similarities and differences in FGF-9 expression between the opossum and the mouse, amino acid sequence and expression pattern of FGF-9 in the developing first molars of D. albiventris were characterised. FGF-9 cDNA sequence was obtained using RT-PCR and expressed in bacterial system for recombinant protein production and analysis of immunoreactivity. FGF-9 expression during tooth development was investigated by immunoperoxidase method. FGF-9 protein consists in a 209-residue polypeptide with a predicted molecular mass of 23.5 kDa. FGF-9 amino acid sequence has 98% of sequence identity to human and 97% to rodents. During tooth development, epithelial FGF-9 expression was seen at the dental lamina stage. Mesenchymal expression was seen at the bud stage and at the cap stage. No significant expression was found in the enamel knot. While in rodents FGF-9 is involved in initiation and regulation of tooth shape, it is suggested that it is only involved in tooth initiation in D. albiventris.

Immunohistochemical detection of retinoblastoma protein and E2 promoter-binding factor-1 in ameloblastomas

BACKGROUND

To clarify the roles of cell cycle regulation in oncogenesis and cytodifferentiation of odontogenic tumors, expression of retinoblastoma protein (RB) and E2 promoter-binding factor-1 (E2F-1) was analyzed in ameloblastomas as well as in tooth germs.

METHODS

Tissue specimens of 10 tooth germs, 40 benign ameloblastomas, and five malignant ameloblastomas were examined immunohistochemically with the use of antibodies against RB, E2F-1, and phosphorylated RB. Ki-67 antigen immunostaining was made as a marker of cell proliferation.

RESULTS

Immunohistochemical reactivity for RB, E2F-1, phosphorylated RB, and Ki-67 was detected in the nuclei of odontogenic epithelial cells near the basement membrane in tooth germs and benign and malignant ameloblastomas. The number of cells positive for phosphorylated RB was nearly equal to or slightly less than the number of cells positive for RB or E2F-1. The number of Ki-67-positive cells was slightly more than the numbers of cell positive for RB, E2F-1, or phosphorylated RB. The levels of immunoreactivity for RB, E2F-1, phosphorylated RB, and Ki-67 were slightly higher in benign and malignant ameloblastomas than in tooth germs. Plexiform ameloblastomas showed significantly higher expression of RB than follicular ameloblastomas. Ki-67 immunoreactivity was significantly higher in ameloblastic carcinomas than in metastasizing ameloblastomas.

CONCLUSION

Similar immunoreactivity for RB, E2F-1, phosphorylated RB, and Ki-67 in tooth germs and ameloblastomas indicated cellular expression of phosphorylated RB and active-free E2F-1 in both normal and neoplastic odontogenic tissues. Expression of RB, E2F-1, and phosphorylated RB was considered to be involved in cell proliferation and differentiation of odontogenic epithelium via control of the cell cycle.

From conical to spatulate: Intra- and interspecific changes in tooth shape in closely related cichlids (Teleostei; Cichlidae: Eretmodini)

The Eretmodini are closely related cichlids endemic to Lake Tanganyika with very divergent oral tooth shapes, ranging from spatulate in Eretmodus to conical in Tanganicodus. To study how closely related cichlids can generate such divergent tooth shapes, we investigated how the enamel organ directs the development of spatulate teeth in Eretmodus cf. cyanostictus (lineage A), both in ontogeny and in adults, and of conical teeth in adult Tanganicodus cf. irsacae, using 3D-reconstructions from serially sectioned tooth germs. The spatulate oral tooth shape that characterizes adult E. cf. cyanostictus (lineage A) is preceded early in ontogeny by a conical tooth shape. We propose two possible hypotheses to account for changes in the folding of the enamel organ (in particular its epithelio-mesenchymal boundary) capable of generating such distinct tooth shapes. Different arguments lead us to favor the hypothesis of an asymmetric growth and differentiation of the enamel organ, such that the tip of a conical tooth corresponds to one "corner" of a spatulate tooth. Applying current molecular models of tooth shape variation, this would imply the existence of asymmetric fields of inhibition. Whether such asymmetric growth reflects the reutilization of a simple mechanism operating in ontogeny has to be clarified.

The genetic basis of tooth development and dental defects

More than 300 genes have so far been associated with tooth development, mainly in mouse embryos. The majority of them are associated with conserved signaling pathways mediating cellular communication, in particular between epithelial and mesenchymal tissues. Necessary functions of many signals, receptors and transcription factors have been demonstrated in mice, and mutations causing dental defects in humans have been identified in several genes.

Laminin alpha5 is required for dental epithelium growth and polarity and the development of tooth bud and shape

In tooth development, the oral ectoderm and mesenchyme coordinately and reciprocally interact through the basement membrane for their growth and differentiation to form the proper shape and size of the tooth. Laminin alpha5 subunit-containing laminin-10/11 (LM-511/521) is the major laminin in the tooth germ basement membrane. Here, we have examined the role of laminin alpha5 (Lama5) in tooth development using laminin alpha5-null mouse primary dental epithelium and tooth germ organ cultures. Lama5-null mice develop a small tooth germ with defective cusp formation and have reduced proliferation of dental epithelium. Also, cell polarity and formation of the monolayer of the inner dental epithelium are disturbed. The enamel knot, a signaling center for tooth germ development, is defective, and there is a significant reduction of Shh and Fgf4 expression in the dental epithelium. In the absence of laminin alpha5, the basement membrane in the inner dental epithelium becomes discontinuous. In normal mice, integrin alpha6beta4, a receptor for laminin alpha5, is strongly localized at the basal layer of the epithelium, whereas in mutant mice, integrin alpha6beta4 is expressed around the cell surface. In primary dental epithelium culture, laminin-10/11 promotes cell growth, spreading, and filopodia-like microspike formation. This promotion is inhibited by anti-integrin alpha6 and beta4 antibodies and by phosphatidylinositol 3-kinase inhibitors and dominant negative Rho-GTPase family proteins Cdc42 and Rac. In organ culture, anti-integrin alpha6 antibody and wortmannin reduce tooth germ size and shape. Our studies demonstrate that laminin alpha5 is required for the proliferation and polarity of basal epithelial cells and suggest that the interaction between laminin-10/11-integrin alpha6beta4 and the phosphatidylinositol 3-kinase-Cdc42/Rac pathways play an important role in determining the size and shape of tooth germ.

Smad protein mediated transforming growth factor β1 induction of apoptosis in the MDPC-23 odontoblast-like cell line

OBJECTIVE

The function of apoptosis and its regulation in odontoblasts remain unclear. In this study, we characterize the possible role of transforming growth factor (TGF)-beta 1 in the induction of apoptosis and the molecular mechanisms that mediate TGF-beta1-induced apoptosis in odontoblasts.

METHODS

Annexin V/propidium iodide staining, cell Death Detection ELISA and DNA ladder were used to examine the effect of TGF-beta1 on apoptosis in a mouse odontoblast-like cell line, MDPC-23. Stable cell clones expressing Smad2 or Smad3 dominant negative mutants, or wild-type Smad7 were constructed to investigate the role of Smad proteins in the mediation of apoptosis by TGF-beta1 in MDPC-23 cells. The TGF-beta1-induced transcriptional activity in stable cell clones expressing Smad proteins was analyzed by a transient transfected TGF-beta-responsive reporter gene, p3TP-Lux.

RESULTS

TGF-beta1 can induce apoptotic cell death in MDPC-23 cells in a dose-dependent manner. Transfection of dominant negative mutant forms of Smad2 or Smad3 blocked TGF-beta1-induced apoptosis; moreover, the Smad3 mutant was more efficient than the Smad2 mutant. Transfection of Smad7, an inhibitory Smad, also significantly inhibited TGF-beta1-induced apoptosis of these cells. Over-expression of Smad3 dominant negative mutant or Smad7 significantly inhibited TGF-beta1-induced transcriptional activity.

CONCLUSION

These results suggest that Smad proteins are involved in TGF-beta1-induced apoptosis of odontoblast cells.

Reduction of Pax9 gene dosage in an allelic series of mouse mutants causes hypodontia and oligodontia

Missing teeth (hypodontia and oligodontia) are a common developmental abnormality in humans and heterozygous mutations of PAX9 have recently been shown to underlie a number of familial, non-syndromic cases. Whereas PAX9 haploinsufficiency has been suggested as the underlying genetic mechanism, it is not known how this affects tooth development. Here we describe a novel, hypomorphic Pax9 mutant allele (Pax9neo) producing decreased levels of Pax9 wild-type mRNA and show that this causes oligodontia in mice. Homozygous Pax9neo mutants (Pax9neo/neo) exhibit hypoplastic or missing lower incisors and third molars, and when combined with the null allele Pax9lacZ, the compound mutants (Pax9neo/lacZ) develop severe forms of oligodontia. The missing molars are arrested at different developmental stages and posterior molars are consistently arrested at an earlier stage, suggesting that a reduction of Pax9 gene dosage affects the dental field as a whole. In addition, hypomorphic Pax9 mutants show defects in enamel formation of the continuously growing incisors, whereas molars exhibit increased attrition and reparative dentin formation. Together, we conclude that changes of Pax9 expression levels have a direct consequence for mammalian dental patterning and that a minimal Pax9 gene dosage is required for normal morphogenesis and differentiation throughout tooth development.

Immunohistochemical expression of p63 in human prenatal tooth primordia

The aim of this study was to investigate the expression of the p63 gene in normal human tooth buds at different gestational stages. This is the first detailed study of p63 expression in normal human prenatal tooth primordia. The material consisted of sections of the midaxial tissue block from the cranial base of three human fetuses of gestational ages (GA) 11, 15, and 21 weeks. The sections included tooth primordia representing cap stages and bell stages of human tooth morphogenesis. In the present study, immunostaining was carried out using the primary antibody, monoclonal mouse anti human p63 protein, clone 4A4. The sections were counterstained with hematoxylin Mayer. p63 immunoreactivity was identified by microscopy. The study showed a positive reaction of p63 in both the cap stage and the bell stage. In both stages, positivity was observed in the cells of the oral mucosa, the inner and outer enamel epithelium, and in the primary and secondary dental lamina. In the early cap stage, there is a strong positive reaction to p63 in the enamel knot, but not in the late cap stage. We suggest that p63 may have an important regulatory function in the enamel knot.

Regulation of Mammalian Tooth Cusp Patterning by Ectodin

Mammalian tooth crowns have precise functional requirements but cannot be substantially remodeled after eruption. In developing teeth, epithelial signaling centers, the enamel knots, form at future cusp positions and are the first signs of cusp patterns that distinguish species. We report that ectodin, a secreted bone morphogenetic protein (BMP) inhibitor, is expressed as a "negative" image of mouse enamel knots. Furthermore, we show that ectodin-deficient mice have enlarged enamel knots, highly altered cusp patterns, and extra teeth. Unlike in normal teeth, excess BMP accelerates patterning in ectodin-deficient teeth. We propose that ectodin is critical for robust spatial delineation of enamel knots and cusps.

Possible functional involvement of thymosin beta 4 in developing tooth germ of mouse lower first molar

We examined the detailed in situ expression pattern of thymosin beta 4 (Tbeta4) in the developing mouse mandibular first molar. Tbeta4 mRNA was expressed in the presumptive dental epithelium at embryonic day 10.5 (E10.5) and in the thickened dental epithelium at E12. An in situ signal was observed in the invaginated epithelial bud at E13, in the enamel organ at E14 and E14.5, and in the primary enamel knot (PEK) at E14.5. The signal was localized in the epithelial cells of the outer layer of the enamel organ at E15 and E15.5. No signal was found in the PEK at these stages. Tbeta4 mRNA was expressed in the inner enamel epithelium, cervical loop and dental lamina at E16 and E17. The expression of Tbeta4 mRNA was observed in the polarized inner epithelial cells at E18, newborn day 1 (N1) and N2. However, the signal intensity decreased markedly at N3. We herein report for the first time that Tbeta4 is distinctly expressed in developing tooth germ, and it may also play functional roles in the initiation, growth and differentiation of tooth germ.

Bone morphogenetic proteins in dentin regeneration for potential use in endodontic therapy

The human dentition is indispensable for nutrition and physiology. The teeth have evolved for mastication of food. Caries is a common dental problem in which the dentin matrix is damaged. When the caries is deep and the dental pulp is exposed, the pulp has to be removed in many cases, resulting ultimately in loss of the tooth. Therefore, the regeneration of dentin-pulp complex is the long-term goal of operative dentistry and endodontics. The key elements of dentin regeneration are stem cells, morphogens such as bone morphogenetic proteins (BMPs) and a scaffold of extracellular matrix. The dental pulp has stem/progenitor cells that have the potential to differentiate into dentin-forming odontoblasts in response to BMPs. Pulpal wound healing consists of stem/progenitor cells release from dental pulp niche after noxious stimuli such as caries, migration to the injured site, proliferation and differentiation into odontoblasts. There are two main strategies for pulp therapy to regenerate dentin: (1) in vivo method of enhancing the natural healing potential of pulp tissue by application of BMP proteins or BMP genes, (2) ex vivo method of isolation of stem/progenitor cells, differentiation with BMP proteins or BMP genes and transplantation to the tooth. This review summarizes recent advances in application of BMPs for dentin regeneration and possible use in endodotic therapy.

FGF-2 signaling is sufficient to induce dermal condensations during feather development

In a previous report, we showed that fibroblast growth factor-2 (FGF-2) is a signal produced by epidermal placode cells during feather development and that this growth factor can induce feathers in scaleless mutant skins that fail to form feathers due to a defective epidermis (Song et al., [1996] Proc Natl Acad Sci USA 93:10246-10249). Here, we test whether FGF-2 is sufficient to induce dermal condensations, structures that normally form under the control of signals from the epidermal placode and are identified on the basis of aggregation of cells, the expression of FGF receptor-1 and bone morphogenetic protein-2 transcripts and the cessation of proliferation of the condensed cells. By using denuded 8-day scaleless dermis as a test system, we have established that FGF-2 is sufficient to induce dermal condensation. We suggest that the primary effect of FGF-2 is an increase in cellular density mediated through cell migration, followed by the expression of dermal condensation-specific genes and cessation of cell proliferation. The FGF-2 effect can be abolished by heparin, suggesting the involvement of heparan sulfate proteoglycans (HSPGs) in growth factor signaling. The spatiotemporal expression of syndecan-3 during feather development suggests that this cell-surface HSPG may be involved in the response of competent embryonic skin dermis to FGF-2.

Msx2 controls ameloblast terminal differentiation

Late tooth morphogenesis is characterized by a series of events that determine cusp morphogenesis and the histodifferentiation of epithelial cells into enamel-secreting ameloblasts. Mice lacking the homeobox gene Msx2 exhibit defects in cusp morphogenesis and in the process of amelogenesis. To better understand the basis of the Msx2 mutant tooth defects, we have investigated the function of Msx2 during late stages of tooth morphogenesis. Cusp formation is thought to be under the control of the enamel knot, which has been proposed to act as an organizing center during this process (Vaahtokari et al. [ 1996] Mech. Dev. 54:39-43). Bone morphogenetic protein-4 (BMP4) has been suggested to mediate termination of enamel knot signaling by means of regulation of programmed cell death (Jernvall et al. [ 1998] Development 125:161-169). Here, we show that Bmp4 expression in the enamel knot is Msx2-dependent. We further show that during amelogenesis Msx2 is required for the expression of the extracellular matrix gene Laminin 5 alpha 3, which is known to play an essential role during ameloblast differentiation. This result thus provides a paradigm for understanding how transcription factors and extracellular matrix can be integrated into a developmental pathway controlling cell differentiation.

Disruption of Smad5 gene induces mitochondria-dependent apoptosis in cardiomyocytes

Our previous studies have shown that SMAD5, an important intracellular mediator of transforming growth factor beta (TGF-beta) family, is required for normal development of the cardiovascular system in vivo. In the current study, we reported that the lack of the Smad5 gene resulted in apoptosis of cardiac myocytes in vivo. To further investigate the mechanism of the Smad5 gene in cardiomyocyte apoptosis, the embryonic stem (ES) cell differentiation system was employed. We found that the myotubes that differentiated from the homozygous Smad5ex6/ex6 mutant ES cells underwent collapse and degeneration during the late stages of in vitro differentiation, mimicking the in vivo observation. By electron microscopy, abnormal swollen mitochondria were observed in cardiomyocytes both from Smad5-deficient embryos and from ES-differentiated cells. There was also a significant reduction in mitochondrial membrane potential (Deltapsi m) and a leakage of cytochrome c from mitochondria into the cytosol of myocytes differentiated from Smad5 mutant ES cells. The expression of p53 and p21 was found to be elevated in the differentiated Smad5 mutant myocytes, and this was accompanied by an up-regulation in caspase 3 expression. These results suggest that the Smad5-mediated TGF-beta signals may protect cardiomyocytes from apoptosis by maintaining the integrity of the mitochondria, probably through suppression of p53 mediated pathways.

Cytokine therapy for craniosynostosis

The birth prevalence of craniosynostosis (premature suture fusion) is 300-500 per 1,000,000 live births. Surgical management involves the release of the synostosed suture. In many cases, however, the suturectomy site rapidly reossifies, further restricts the growing brain and alters craniofacial growth. This resynostosis requires additional surgery, which increases patient morbidity and mortality. New findings in bone biology and molecular pathways involved with suture fusion, combined with novel tissue engineering techniques, may allow the design of targeted and complementary therapies to decrease complications inherent in high-risk surgical procedures. This paper selectively reviews recent advances in i) identifying genetic mutations and the aetiopathogenesis of a number of craniosynostotic conditions; ii) cranial suture biology and molecular biochemical pathways involved in suture fusion; and iii) the design, development and application of various vehicles and tissue engineered constructs to deliver cytokines and genes to cranial sutures. Such biologically based therapies may be used as surgical adjuncts to rescue fusing sutures or help manage postoperative resynostosis.

New approach to quantifying developmental variation in the dentition using serial microtomographic imaging

Dental morphogenesis and cellular differentiation are expressed in the fully formed tooth by the topography of the dentin-enamel junction and outer enamel surface. These boundaries can be differentiated using a variety of imaging systems. In this study, we used serial microCT imaging to provide accurate 3D reconstructions of developing lower human second deciduous molars. These were used to quantify the volume of enamel and dentin of individual cusps in relation to basal crown height. As growth and differentiation proceed apically, the spatial orientation of cusp tips and their bases were used to estimate their order of initiation and coalescence. We found that the order of coalescence differed from the order of initiation. We also found that dentin cusp height and volume as well as rate and quantity of enamel apposition varied along mesio-distal and bucco-lingual axes, and were independent of order of initiation and duration of growth. These results demonstrate that the potential for variation in crown size and form is maintained throughout development. We propose that the microCT model developed in this study constitutes a new approach for the investigation of developmental variation and its contribution to phylogenetic variation expressed in crown form and size.