The enamel knot as a signaling center in the developing mouse tooth

Regulation of hair follicle development by the TNF signal ectodysplasin and its receptor Edar

X-linked and autosomal forms of anhidrotic ectodermal dysplasia syndromes (HED) are characterized by deficient development of several ectodermal organs, including hair, teeth and exocrine glands. The recent cloning of the genes that underlie these syndromes, ectodysplasin (ED1) and the ectodysplasin A receptor (EDAR), and their identification as a novel TNF ligand-receptor pair suggested a role for TNF signaling in embryonic morphogenesis. In the mouse, the genes of the spontaneous mutations Tabby (Ta) and downless (dl) were identified as homologs of ED1 and EDAR, respectively. To gain insight into the function of this signaling pathway in development of skin and hair follicles, we analyzed the expression and regulation of Eda and Edar in wild type as well as Tabby and Lef1 mutant mouse embryos. We show that Eda and Edar expression is confined to the ectoderm and occurs in a pattern that suggests a role of ectodysplasin/Edar signaling in the interactions between the ectodermal compartments and the formation and function of hair placodes. By using skin explant cultures, we further show that this signaling pathway is intimately associated with interactions between the epithelial and mesenchymal tissues. We also find that Ta mutants lack completely the placodes of the first developing tylotrich hairs, and that they do not show patterned expression of placodal genes, including Bmp4, Lef1, Shh, Ptch and Edar, and the genes for β-catenin and activin A. Finally, we identified activin as a mesenchymal signal that stimulates Edar expression and WNT as a signal that induces Eda expression, suggesting a hierarchy of distinct signaling pathways in the development of skin and hair follicles. In conclusion, we suggest that Eda and Edar are associated with the onset of ectodermal patterning and that ectodysplasin/edar signaling also regulates the morphogenesis of hair follicles.

Bone morphogenetic protein 4 is involved in cusp formation in molar tooth germ of mice

In order to clarify the role of BMP4 in the development of the tooth crown, we employed the antisense technique on molar tooth germs removed from the mandibles of embryonic 13.5-d-old mice. In the tooth germ explants incubated for 14 d with antisense oligodeoxynucleotide (AS-ODN) against Bmp4 (a) cusps were not formed, whereas dentin matrix was secreted in the whole region of the crown, (b) inner enamel epithelial (IEE) cells remained in the undifferentiated state in the occlusal region of the crown, though they differentiated in the proximal region (lateral surface region of tooth crown), and (c) insufficient growth of the dental papilla was observed. A 5-bromo-2'-deoxyuridine (BrdU) uptake experiment showed that, although a site-specific proliferation of IEE cells occurred in the occlusal region in the control explants, it was not found in the AS-ODN-treated explants. In the proximal region, however, the proliferation of IEE cells was detected evenly in all explants treated with or without AS-ODNs. These results suggest that AS-ODN against Bmp4 inhibited the differentiation and the site-specific proliferation of IEE cells in the occlusal region of molar tooth germs, resulting in the suppression of cusp formation. Our data thus suggest that BMP4 is involved in cusp formation and differentiation of ameloblasts in the occlusal region of molars.

Unique functions of Sonic hedgehog signaling during external genitalia development

Coordinated growth and differentiation of external genitalia generates a proximodistally elongated structure suitable for copulation and efficient fertilization. The differentiation of external genitalia incorporates a unique process, i.e. the formation of the urethral plate and the urethral tube. Despite significant progress in molecular embryology, few attempts have been made to elucidate the molecular developmental processes for external genitalia. The sonic hedgehog (Shh) gene and its signaling genes have been found to be dynamically expressed during murine external genitalia development. Functional analysis by organ culture revealed that Shh could regulate mesenchymally expressed genes, patched 1 (Ptch1), bone morphogenetic protein 4 (Bmp4), Hoxd13 and fibroblast growth factor 10 (Fgf10), in the anlage: the genital tubercle (GT). Activities of Shh for both GT outgrowth and differentiation were also demonstrated. Shh–/– mice displayed complete GT agenesis, which is compatible with such observations. Furthermore, the regulation of apoptosis during GT formation was revealed for the first time. Increased cell death and reduced cell proliferation of the Shh–/– mice GT were shown. A search for alterations of Shh downstream gene expression identified a dramatic shift of Bmp4 gene expression from the mesenchyme to the epithelium of the Shh mutant before GT outgrowth. Regulation of mesenchymal Fgf10 gene expression by the epithelial Shh was indicated during late GT development. These results suggest a dual mode of Shh function, first by the regulation of initiating GT outgrowth, and second, by subsequent GT differentiation.

Sonic hedgehog regulates epithelial proliferation and cell survival in the developing tooth germ

Shh expression is highly restricted to the future sites of tooth development during the initiation of odontogenesis. This suggests a role for Shh as a proliferative factor, as localized epithelial thickenings invaginate to form a tooth bud. We have investigated this role by blocking Shh signaling between E10.5 and E12.5 in murine mandibular processes using a 5E1 blocking antibody and the PKA activator Forskolin. This results in down-regulation of Ptc, a principle target of Shh signaling. The effects of inhibition varied with developmental time. At E10.5, tooth development was arrested as epithelial thickenings and the numbers of teeth developing were considerably reduced. Inhibition at E12.5 produced localized apoptosis in the epithelium at the tip of the tooth buds, although some teeth were able to develop. Thus, Shh has dual roles in early odontogenesis, first in bud formation by stimulating epithelial proliferation, and second in the development of cap-stage tooth germs by increasing epithelial cell survival.

Signaling and subcellular localization of the TNF receptor Edar

Tabby and downless mutant mice have identical phenotypes characterized by deficient development of several ectodermally derived organs such as teeth, hair, and sweat glands. Edar, encoded by the mouse downless gene and defective in human dominant and recessive forms of autosomal hypohidrotic ectodermal dysplasia (EDA) syndrome, is a new member of the tumor necrosis factor (TNF) receptor superfamily. The ligand of Edar is ectodysplasin, a TNF-like molecule mutated in the X-linked form of EDA and in the spontaneous mouse mutant Tabby. We have analyzed the response of Edar signaling in transfected cells and show that it activates nuclear factor-kappaB (NF-kappaB) in a dose-dependent manner. When Edar was expressed at low levels, the NF-kappaB response was enhanced by coexpression of ectodysplasin. The activation of NF-kappaB was greatly reduced in cells expressing mutant forms of Edar associated with the downless phenotype. Overexpression of Edar did not activate SAPK/JNK nor p38 kinase. Even though Edar harbors a death domain its overexpression did not induce apoptosis in any of the four cell lines analyzed, nor was there any difference in apoptosis in developing teeth of wild-type and Tabby mice. Additionally, we show that the subcellular localization of dominant negative alleles of downless is dramatically different from that of recessive or wild-type alleles. This together with differences in NF-kappaB responses suggests an explanation for the different mode of inheritance of the different downless alleles.

Development and evolution of the mammalian limb: adaptive diversification of nails, hooves, and claws

Paleontological evidence indicates that the evolutionary diversification of mammals early in the Cenozoic era was characterized by an adaptive radiation of distal limb structures. Likewise, neontological data show that morphological variation in distal limb integumentary appendages (e.g., nails, hooves, and claws) can be observed not only among distantly related mammalian taxa but also among closely related species within the same clade. Comparative analysis of nail, claw, and hoof morphogenesis reveals relatively subtle differences in mesenchymal and epithelial patterning underlying these adult differences in distal limb appendage morphology. Furthermore, studies of regulatory gene expression during vertebrate claw development demonstrate that many of the signaling molecules involved in patterning ectodermal derivatives such as teeth, hair, and feathers are also involved in organizing mammalian distal limb appendages. For example, Bmp4 signaling plays an important role during the recruitment of mesenchymal cells into the condensations forming the terminal phalanges, whereas Msx2 affects the length of nails and claws by suppressing proliferation of germinal epidermal cells. Evolutionary changes in the form of distal integumentary appendages may therefore result from changes in gene expression during formation of mesenchymal condensations (Bmp4, posterior Hox genes), induction of the claw fold and germinal matrix (shh), and/or proliferation of epidermal cells in the claw matrix (Msx1, Msx2). The prevalence of convergences and parallelisms in nail and claw structure among mammals underscores the existence of multiple morphogenetic pathways for evolutionary change in distal limb appendages.

The whereabouts of a morphogen: direct evidence for short- and graded long-range activity of hedgehog signaling peptides

Sonic Hedgehog (Shh) and Indian Hedgehog (Ihh) are members of the Hedgehog (Hh) family of signaling molecules known to be involved in embryonic patterning and morphogenesis. The Hh proteins undergo an autocatalytic cleavage to yield an N-terminal and a C-terminal peptide, with the signaling capacities confined to the N peptide. Drosophila Hh-N has been shown to act via both short- and long-range signaling. In vertebrates, however, attempts to directly demonstrate Shh (SHH) or Ihh (IHH) proteins at a distance from producing cells have been largely unsuccessful. Furthermore, the fact that the Hh N peptides occur in a cholesterol-modified, membrane-tethered form is not easily reconciled with long-range signaling. This study used optimized immunohistochemistry combined with tissue separation and biochemical analyses in vivo and in vitro to determine the range of action of SHH and IHH in the mouse embryo. In all embryonic structures studied, we detect signaling peptides in producing cells, but we also find that ligands move over considerable distances depending on the tissue. These data provide direct evidence for the presence of Hedgehog signaling peptides in target compartments, suggesting a direct long-range action without a need for secondary mediators. Visualization of Hedgehog proteins in target tissues was achieved only under conditions that allowed proteoglycan/glycosaminoglycan (PG/GAG) preservation. Furthermore, we show that induced changes of the composition of PG/GAG in the tooth alter SHH signaling. These data suggest a crucial role for PG/GAGs in Hedgehog movement.

Expression of Set-alpha during morphogenesis of mouse lower first molar

The detailed in situ expression pattern of the Set-alpha gene has been studied. Previously we showed that Set-alpha is a differentially expressed gene in the embryonic mouse mandible at day 10.5 (E10.5) gestational age. Cells expressing Set-alpha were widely distributed in both the epithelial and underlying ectomesenchymal cells at E10.5. At E12, they were slightly aggregated in an area where tooth germ of the lower first molar is estimated to be formed. At E13.5, Set-alpha was strongly expressed in the tooth germ. At the cap stage, Set-alpha was expressed in the enamel organ and dental papilla. At the bell stage, Set-alpha was distinctly expressed in the inner enamel epithelial and dental papilla cells facing the inner enamel epithelial layer, which were intended to differentiate into ameloblasts and odontoblasts, respectively. Interestingly, Set-alpha was also expressed in several embryonic craniofacial tissues derived from the ectoderm. This study is the first report that Set-alpha is distinctly expressed in the developing tooth germ, and suggests that Set-alpha plays an important role in both the initiation and the growth of the tooth germ, as well as in the differentiation of ameloblasts and odontoblasts.

Enamel knots as signaling centers linking tooth morphogenesis and odontoblast differentiation

Odontoblasts differentiate from the cells of the dental papilla, and it has been well-established that their differentiation in developing teeth is induced by the dental epithelium. In experimental studies, no other mesenchymal cells have been shown to have the capacity to differentiate into odontoblasts, indicating that the dental papilla cells have been committed to odontoblast cell lineage during earlier developmental stages. We propose that the advancing differentiation within the odontoblast cell lineage is regulated by sequential epithelial signals. The first epithelial signals from the early oral ectoderm induce the odontogenic potential in the cranial neural crest cells. The next step in the determination of the odontogenic cell lineage is the development of the dental papilla from odontogenic mesenchyme. The formation of the dental papilla starts at the onset of the transition from the bud to the cap stage of tooth morphogenesis, and this is regulated by epithelial signals from the primary enamel knot. The primary enamel knot is a signaling center which forms at the tip of the epithelial tooth bud. It becomes fully developed and morphologically discernible in the cap-stage dental epithelium and expresses at least ten different signaling molecules belonging to the BMP, FGF, Hh, and Wnt families. In molar teeth, secondary enamel knots appear in the enamel epithelium at the sites of the future cusps. They also express several signaling molecules, and their formation precedes the folding and growth of the epithelium. The differentiation of odontoblasts always starts from the tips of the cusps, and therefore, it is conceivable that some of the signals expressed in the enamel knots may act as inducers of odontoblast differentiation. The functions of the different signals in enamel knots are not precisely known. We have shown that FGFs stimulate the proliferation of mesenchymal as well as epithelial cells, and they may also regulate the growth of the cusps. We have proposed that the enamel knot signals also have important roles, together with mesenchymal signals, in regulating the patterning of the cusps and hence the shape of the tooth crown. We suggest that the enamel knots are central regulators of tooth development, since they link cell differentiation to morphogenesis.

Developmental genetics and early hominid craniodental evolution

Although features of the dentition figure prominently in discussions of early hominid phylogeny, remarkably little is known of the developmental basis of the variations in occlusal morphology and dental proportions that are observed among taxa. Recent experiments on tooth development in mice have identified some of the genes involved in dental patterning and the control of tooth specification. These findings provide valuable new insight into dental evolution and underscore the strong developmental links that exist among the teeth and the jaws and cranium. The latter has important implications for cladistic studies that traditionally consider features of the skull independently from the dentition.

Localization of activated caspase-3-positive and apoptotic cells in the developing tooth germ of the mouse lower first molar

This study examined the immunohistochemical detection of activated caspase-3, and its association with apoptosis, during tooth morphogenesis of the mouse lower first molar. The distribution of cells positive for caspase-3 closely corresponded with the localization of the terminal deoxynucleotidyl transferase-mediated deoxyuridine-5'-triphosphate-biotin nick end labelling (TUNEL)-positive apoptotic cells through the developmental course of tooth germs from embryo day 12 (E12) to E19, thus showing that the apoptosis occurring in the developing odontogenic tissue was induced by the activation of the caspase family. The specific distribution pattern of apoptotic cells in the developing odontogenic epithelial tissue from the initiation (E12) of tooth germ to the completion of tooth crown morphology (E19) also suggests that apoptotic events are related not only to a deletion of functionally suspended cells, but also participate in initiation and the completion of tooth morphogenesis. Electron microscopic examination revealed that apoptotic cells were present in the primary enamel knot, and these apoptotic cells were phagocytized by neighbouring odontogenic epithelial cells, thus indicating the prompt disposal of any dead cells by epithelial cells.

Homologies of process and modular elements of embryonic construction

There are several signal transduction pathways that integrate embryonic development. We find that both within species and between species, these pathways constitute homologous modules. The processes, themselves, can be considered homologous, just as structures can be considered homologous. Just like vertebrate limbs, these pathways are composed of homologous parts (in this case, the proteins of the pathway) that are organized in homologous ways. These pathways are conserved through evolutionary time, and they undergo descent with modification. Such homologies of processes become critical to the discussion of evolution and development when we consider (1) that evolution depends on heritable changes in development, (2) that development is modular such that different modules can change without affecting other modules, (3) that modules can be co-opted into new functions, and (4) that modules depend on intercellular communication.

3D modelling of gene expression patterns

The current genome-sequencing projects provide "word indices" of the book of life. A central post-genomic question will be how these words are three-dimensionally deployed in the generation of organism form. Gene expression studies of developing organisms contribute an increasing wealth of snapshot data on the activation of individual genes at selected locations and single moments in the developmental process. However, a comprehensive understanding of the dynamic activation of multiple genes and their functional role in controlling the 3D processes of collective cell behaviour, pattern formation and morphogenesis, requires special tools for a systematic description of spatio-temporal patterns of gene activation and the ensuing phenotypic effects. This article concentrates on new, computer-based tools for the 3D analysis of gene expression patterns in embryonic development and their use for the systematic establishment of comprehensive gene expression maps.

TNF signaling via the ligand-receptor pair ectodysplasin and edar controls the function of epithelial signaling centers and is regulated by Wnt and activin during tooth organogenesis

Ectodermal dysplasia syndromes affect the development of several organs, including hair, teeth, and glands. The recent cloning of two genes responsible for these syndromes has led to the identification of a novel TNF family ligand, ectodysplasin, and TNF receptor, edar. This has indicated a developmental regulatory role for TNFs for the first time. Our in situ hybridization analysis of the expression of ectodysplasin (encoded by the Tabby gene) and edar (encoded by the downless gene) during mouse tooth morphogenesis showed that they are expressed in complementary patterns exclusively in ectodermal tissue layer. Edar was expressed reiteratively in signaling centers regulating key steps in morphogenesis. The analysis of the effects of eight signaling molecules in the TGFbeta, FGF, Hh, Wnt, and EGF families in tooth explant cultures revealed that the expression of edar was induced by activinbetaA, whereas Wnt6 induced ectodysplasin expression. Moreover, ectodysplasin expression was downregulated in branchial arch epithelium and in tooth germs of Lef1 mutant mice, suggesting that signaling by ectodysplasin is regulated by LEF-1-mediated Wnt signals. The analysis of the signaling centers in tooth germs of Tabby mice (ectodysplasin null mutants) indicated that in the absence of ectodysplasin the signaling centers were small. However, no downstream targets of ectodysplasin signaling were identified among several genes expressed in the signaling centers. We conclude that ectodysplasin functions as a planar signal between ectodermal compartments and regulates the function, but not the induction, of epithelial signaling centers. This TNF signaling is tightly associated with epithelial-mesenchymal interactions and with other signaling pathways regulating organogenesis. We suggest that activin signaling from mesenchyme induces the expression of the TNF receptor edar in the epithelial signaling centers, thus making them responsive to Wnt-induced ectodysplasin from the nearby ectoderm. This is the first demonstration of integration of the Wnt, activin, and TNF signaling pathways.

Evolutionary modification of development in mammalian teeth: quantifying gene expression patterns and topography

The study of mammalian evolution often relies on detailed analysis of dental morphology. For molecular patterning to play a role in dental evolution, gene expression differences should be linkable to corresponding morphological differences. Because teeth, like many other structures, are complex and evolution of new shapes usually involves subtle changes, we have developed topographic methods by using Geographic Information Systems. We investigated how genetic markers for epithelial signaling centers known as enamel knots are associated with evolutionary divergence of molar teeth in two rodent species, mouse and vole. Our analysis of expression patterns of Fgf4, Lef1, p21, and Shh genes in relation to digital elevation models of developing tooth shapes shows that molecular prepatterns predict the lateral cusp topography more than a day in advance. A heterotopic shift in the molecular prepatterns can be implicated in the evolution of mouse molar, changing locations from which historically homologous cusps form. The subtle but measurable heterotopic shifts may play a large role in the evolution of tooth cusp topographies. However, evolutionary increase in the number of longitudinal cusps in vole molar has involved accelerated longitudinal growth and iterative addition of new cusps without changes in lateral cusp topography. The iterative addition of cusps after the establishment of lateral cusp topography may limit the independence of individual morphological features used in evolutionary studies. The diversity of mammalian molar patterns may largely result from the heterotopic and iterative processes.

Osteoprotegerin and osteoclast differentiation factor in tooth eruption

A critical cellular event in tooth eruption is the formation of osteoclasts that are needed for bone resorption to form an eruption pathway. To analyze molecular regulation of osteoclast formation and activation, we examined the expression of osteoprotegerin (OPG), an inhibitor of osteoclast formation. In vivo, the gene expression of OPG is reduced in the dental follicle of the first mandibular molar of the rat at day 3 post-natally and in the mouse at day 5. This correlates with the days of maximal mononuclear cell influx and osteoclast numbers in the rat and mouse. Thus, inhibition of OPG gene expression on these days might allow osteoclasts to be formed and/or activated. In vitro studies demonstrated that both colony-stimulating factor-1 and parathyroid hormone-related protein reduced OPG gene expression in follicle cells, suggesting that these are candidate molecules for the in vivo inhibition of OPG expression. Osteoclast differentiation factor (ODF) immunolocalizes to the alveolar bone stromal cells adjacent to the follicle, whereby it might act to stimulate fusion of the mononuclear cells in the follicle.

Transgenically ectopic expression of Bmp4 to the Msx1 mutant dental mesenchyme restores downstream gene expression but represses Shh and Bmp2 in the enamel knot of wild type tooth germ

Bmp4 is a downstream gene of Msx1 in early mouse tooth development. In this study, we introduced the Msx1-Bmp4 transgenic allele to the Msx1 mutants in which tooth development is arrested at the bud stage in an effort of rescuing Msx1 mutant tooth phenotype in vivo. Ectopic expression of a Bmp4 transgene driven by the mouse Msx1promoter in the dental mesenchyme restored the expression of Lef-1 and Dlx2 but neither Fgf3 nor syndecan-1 in the Msx1 mutant molar tooth germ. The mutant phenotype of molar but not incisor could be partially rescued to progress to the cap stage. The Msx1-Bmp4 transgene was also able to rescue the alveolar processes and the neonatal lethality of the Msx1 mutants. In contrast, overexpression of Bmp4 in the wild type molar mesenchyme down-regulated Shh and Bmp2 expression in the enamel knot, the putative signaling center for tooth patterning, but did not produce a tooth phenotype. These results indicate that Bmp4 can bypass Msx1 function to partially rescue molar tooth development in vivo, and to support alveolar process formation. Expression of Shh and Bmp2 in the enamel knot may not represent critical signals for tooth patterning.

BMP4 rescues a non-cell-autonomous function of Msx1 in tooth development

The development of many organs depends on sequential epithelial-mesenchymal interactions, and the developing tooth germ provides a powerful model for elucidating the nature of these inductive tissue interactions. In Msx1-deficient mice, tooth development arrests at the bud stage when Msx1 is required for the expression of Bmp4 and Fgf3 in the dental mesenchyme (Bei, M. and Maas, R. (1998) Development 125, 4325–4333). To define the tissue requirements for Msx1 function, we performed tissue recombinations between wild-type and Msx1 mutant dental epithelium and mesenchyme. We show that through the E14.5 cap stage of tooth development, Msx1 is required in the dental mesenchyme for tooth formation. After the cap stage, however, tooth development becomes Msx1 independent, although our experiments identify a further late function of Msx1 in odontoblast and dental pulp survival. These results suggest that prior to the cap stage, the dental epithelium receives an Msx1-dependent signal from the dental mesenchyme that is necessary for tooth formation. To further test this hypothesis, Msx1 mutant tooth germs were first cultured with either BMP4 or with various FGFs for two days in vitro and then grown under the kidney capsule of syngeneic mice to permit completion of organogenesis and terminal differentiation. Previously, using an in vitro culture system, we showed that BMP4 stimulated the growth of Msx1 mutant dental epithelium (Chen, Y., Bei, M. Woo, I., Satokata, I. and Maas, R. (1996). Development 122, 3035–3044). Using the more powerful kidney capsule grafting procedure, we now show that when added to explanted Msx1-deficient tooth germs prior to grafting, BMP4 rescues Msx1 mutant tooth germs all the way to definitive stages of enamel and dentin formation. Collectively, these results establish a transient functional requirement for Msx1 in the dental mesenchyme that is almost fully supplied by BMP4 alone, and not by FGFs. In addition, they formally prove the postulated downstream relationship of BMP4 with respect to Msx1, establish the non-cell-autonomous nature of Msx1 during odontogenesis, and disclose an additional late survival function for Msx1 in odontoblasts and dental pulp.

Edar/Eda interactions regulate enamel knot formation in tooth morphogenesis

tabby and downless mutant mice have apparently identical defects in teeth, hair and sweat glands. Recently, genes responsible for these spontaneous mutations have been identified. downless (Dl) encodes Edar, a novel member of the tumour necrosis factor (TNF) receptor family, containing the characteristic extracellular cysteine rich fold, a single transmembrane region and a death homology domain close to the C terminus. tabby (Ta) encodes ectodysplasin-A (Eda) a type II membrane protein of the TNF ligand family containing an internal collagen-like domain. As predicted by the similarity in adult mutant phenotype and the structure of the proteins, we demonstrate that Eda and Edar specifically interact in vitro. We have compared the expression pattern of Dl and Ta in mouse development, taking the tooth as our model system, and find that they are not expressed in adjacent cells as would have been expected. Teeth develop by a well recorded series of epithelial-mesenchymal interactions, similar to those in hair follicle and sweat gland development, the structures found to be defective in tabby and downless mice. We have analysed the downless mutant teeth in detail, and have traced the defect in cusp morphology back to initial defects in the structure of the tooth enamel knot at E13. Significantly, the defect is distinct from that of the tabby mutant. In the tabby mutant, there is a recognisable but small enamel knot, whereas in the downless mutant the knot is absent, but enamel knot cells are organised into a different shape, the enamel rope, showing altered expression of signalling factors (Shh, Fgf4, Bmp4 and Wnt10b). By adding a soluble form of Edar to tooth germs, we were able to mimic the tabby enamel knot phenotype, demonstrating the involvement of endogenous Eda in tooth development. We could not, however, reproduce the downless phenotype, suggesting the existence of yet another ligand or receptor, or of ligand-independent activation mechanisms for Edar. Changes in the structure of the enamel knot signalling centre in downless tooth germs provide functional data directly linking the enamel knot with tooth cusp morphogenesis. We also show that the Lef1 pathway, thought to be involved in these mutants, functions independently in a parallel pathway.

Sonic hedgehog signaling pathway in vertebrate epithelial appendage morphogenesis: perspectives in development and evolution

Vertebrate epithelial appendages are elaborate topological transformations of flat epithelia into complex organs that either protrude out of external (integument) and internal (oral cavity, gut) epithelia, or invaginate into the surrounding mesenchyme. Although they have specific structures and diverse functions, most epithelial appendages share similar developmental stages, including induction, morphogenesis, differentiation and cycling. The roles of the SHH pathway are analyzed in exemplary organs including feather, hair, tooth, tongue papilla, lung and foregut. SHH is not essential for induction and differentiation, but is involved heavily in morphogenetic processes including cell proliferation (size regulation), branching morphogenesis, mesenchymal condensation, fate determination (segmentation), polarizing activities and so on. Through differential activation of these processes by SHH in a spatiotemporal-specific fashion, organs of different shape and size are laid down. During evolution, new links of developmental pathways may occur and novel forms of epithelial appendages may emerge, upon which evolutionary selections can act. Sites of major variations have progressed from the body plan to the limb plan to the epithelial appendage plan. With its powerful morphogenetic activities, the SHH pathway would likely continue to play a major role in the evolution of novel epithelial appendages.

Associations of FGF-3 and FGF-10 with signaling networks regulating tooth morphogenesis

The morphogenesis and cell differentiation in developing teeth is governed by interactions between the oral epithelium and neural crest-derived ectomesenchyme. The fibroblast growth factors FGF-4, -8, and -9 have been implicated as epithelial signals regulating mesenchymal gene expression and cell proliferation during tooth initiation and later during epithelial folding morphogenesis and the establishment of tooth shape. To further evaluate the roles of FGFs in tooth development, we analyzed the roles of FGF-3, FGF-7, and FGF-10 in developing mouse teeth. In situ hybridization analysis showed developmentally regulated expression during tooth formation for Fgf-3 and Fgf-10 that was mainly restricted to the dental papilla mesenchymal cells. Fgf-7 transcripts were restricted to the developing bone surrounding the developing tooth germ. Fgf-10 expression was observed in the presumptive dental epithelium and mesenchyme during tooth initiation, whereas Fgf-3 expression appeared in the dental mesenchyme at the late bud stage. During the cap and bell stage, both Fgf-3 and Fgf-10 were intensely expressed in the dental papilla mesenchymal cells both in incisors and molars. It is of interest that Fgf-3 expression was also observed in the primary enamel knot, a putative signaling center of the tooth, whereas no transcripts were seen in the secondary enamel knots that appear in the tips of future cusps of the bell stage tooth germs. Down-regulation of Fgf-3 and Fgf-10 expression in postmitotic odontoblasts correlated with the terminal differentiation of the odontoblasts and the neighboring ameloblasts. In the incisors, mesenchymal cells of the cervical loop area showed partially overlapping expression patterns for all studied Fgfs. In vitro analyses showed that expression of Fgf-3 and Fgf-10 in the dental mesenchyme was dependent on dental epithelium and that epithelially expressed FGFs, FGF-4 and -8 induced Fgf-3 but not Fgf-10 expression in the isolated dental mesenchyme. Beads soaked in Shh, BMP-2, and TGF-beta 1 protein did not induce either Fgf-3 or Fgf-10 expression. Cells expressing Wnt-6 did not induce Fgf-10 expression. Furthermore, FGF-10 protein stimulated cell proliferation in the dental epithelium but not in the mesenchyme. These results suggest that FGF-3 and FGF-10 have redundant functions as mesenchymal signals regulating epithelial morphogenesis of the tooth and that their expressions appear to be differentially regulated. In addition, FGF-3 may participate in signaling functions of the primary enamel knot. The dynamic expression patterns of different Fgfs in dental epithelium and mesenchyme and their interactions suggest existence of regulatory signaling cascades between epithelial and mesenchymal FGFs during tooth development.

Nodal and bone morphogenetic protein 5 interact in murine mesoderm formation and implantation

Mice mutant for the TGF-beta family member, nodal, lack mesoderm and die between E8.5 and E9.5. The short ear-lethal (se(l) ) mutation, a deletion that eliminates Bmp-5, causes a strikingly similar gastrulation defect. Here we analyze se(l);nodal compound mutants and find a dosage effect. Embryos homozygous for one mutation show distinct gastrulation stage defects that depend on whether they are heterozygous or homozygous for the other mutation. Embryos mutant for nodal or se(l);nodal compound mutants fail to execute an antigenic shift indicative of mesoderm differentiation and ectoderm cells are shunted into an apoptotic pathway. Furthermore, we find a novel phenotype in se(l);nodal double mutant litters, in which two to four genetically different embryos are contained within the same deciduum. Both the gastrulation and implantation phenotypes can also arise in short ear-viable (se(v) ) and se(v); nodal mutant mice. These data indicate that loss of Bmp-5 may underlie the se(l) gastrulation phenotype and suggest that nodal and Bmp-5 interact during murine mesoderm formation. Our data also reveal an unsuspected role for Bmp-5 in implantation and the decidual response in the mouse.

Conservation of early odontogenic signaling pathways in Aves

Teeth have been missing from birds (Aves) for at least 60 million years. However, in the chick oral cavity a rudiment forms that resembles the lamina stage of the mammalian molar tooth germ. We have addressed the molecular basis for this secondary loss of tooth formation in Aves by analyzing in chick embryos the status of molecular pathways known to regulate mouse tooth development. Similar to the mouse dental lamina, expression of Fgf8, Pitx2, Barx1, and Pax9 defines a potential chick odontogenic region. However, the expression of three molecules involved in tooth initiation, Bmp4, Msx1, and Msx2, are absent from the presumptive chick dental lamina. In chick mandibles, exogenous bone morphogenetic protein (BMP) induces Msx expression and together with fibroblast growth factor promotes the development of Sonic hedgehog expressing epithelial structures. Distinct epithelial appendages also were induced when chick mandibular epithelium was recombined with a tissue source of BMPs and fibroblast growth factors, chick skin mesenchyme. These results show that, although latent, the early signaling pathways involved in odontogenesis remain inducible in Aves and suggest that loss of odontogenic Bmp4 expression may be responsible for the early arrest of tooth development in living birds.

Responsiveness of developing dental tissues to fibroblast growth factors: expression of splicing alternatives of FGFR1, -2, -3, and of FGFR4; and stimulation of cell proliferation by FGF-2, -4, -8, and -9

To elucidate the roles of fibroblast growth factors (FGF) in tooth development, we have analyzed the expression patterns of fibroblast growth factor receptors (FGFR) in mouse teeth by in situ hybridization and studied the effects of FGF-2, -4, -8, and -9 on cell proliferation in vitro by local application with beads on isolated dental mesenchymes. mRNAs of FGFR-1, -2, and -3 were localized by probes specific for the alternative splice variants IIIb and IIIc. The expression patterns of FGFR1 -2, and -3 were completely different, and the two splicing variants of FGFR1 and 2 exhibited different expression domains. FGFR4 was not expressed in the developing teeth. The IIIb splice forms of FGFR1 and -2 were expressed in the dental epithelium during morphogenesis. The IIIc splice form of FGFR1 was expressed both in epithelium and mesenchyme whereas FGFR2 IIIc was confined to the mesenchymal cells of the dental follicle. Both splice forms of FGFR3 were expressed in dental papilla mesenchyme. None of the FGF-receptors was detected in the primary enamel knot, the putative signaling center regulating tooth morphogenesis. This may explain the fact that enamel knot cells do not proliferate, although they express intensely mitogenic FGFs. Beads releasing FGF-2, -4, -8, or -9 proteins stimulated cell proliferation in cultured dental mesenchymes. These data, together with our earlier data on FGF expression [Kettunen and Thesleff (1998): Dev Dyn 211:256-268] suggest that FGF-8 and -9 mediate epithelial-mesenchymal interactions during tooth initiation. During advancing morphogenesis FGF-3, -4, and -9 may act both on mesenchyme and epithelium. Finally, the intense expression of FGFR1 in odontoblasts and ameloblasts and FGFR2 IIIb in ameloblasts suggests that FGFs participate in regulation of their differentiation and/or secretory functions.

BMP receptors in limb and tooth formation

Members of the TGF-beta superfamily signal through receptor complexes comprised of type I and type II receptors. These receptors, which are serine/threonine kinases, form two new classes of transmembrane receptor kinases. The activity of both of the kinases is necessary for signal transduction in response to ligand binding. Bone morphogenetic proteins (BMPs), which are members of the TGF-beta superfamily, bind to multiple type I and type II receptors. There is growing evidence to support the hypothesis that the BMP receptors are differentially regulated during development and that they have both unique and overlapping functions. Thus, the nature and distribution of the BMP receptors, which are reviewed here in the context of the development of limbs and teeth, appear to be critical in the control of the diverse activities of BMPs.

Wnt/Shh interactions regulate ectodermal boundary formation during mammalian tooth development

Interactions between the Wnt (wingless) and hedgehog signaling pathways were first described as playing a role in establishing boundaries between ectodermal cells in Drosophila segmentation. During the initiation of mammalian tooth development, boundaries that distinguish oral from dental ectoderm must be formed to correctly position the sites of tooth formation. We describe a reciprocal relationship between the expression of Wnt-7b in presumptive oral ectoderm and Shh in presumptive dental ectoderm in mouse embryos that mark boundaries between these cells with different developmental fates. By using a murine retrovirus to ectopically express Wnt-7b in presumptive dental ectoderm in mandibular arch explants, we show that Shh expression in the ectoderm and Ptc expression in the underlying ectomesenchyme are down-regulated, and tooth development is subsequently arrested. This suggests that Wnt-7b acts to repress Shh expression in oral ectoderm, thus maintaining the boundaries between oral and dental ectodermal cells. Implantation of beads soaked in Shh protein into Wnt-7b-infected explants resulted in complete rescue of tooth development, confirming that the repressive action of Wnt-7b specifically affects Shh signaling.

A new function of BMP4: dual role for BMP4 in regulation of Sonic hedgehog expression in the mouse tooth germ

The murine tooth development is governed by sequential and reciprocal epithelial-mesenchymal interactions. Multiple signaling molecules are expressed in the developing tooth germ and interact each other to mediate the inductive tissue interactions. Among them are Sonic hedgehog (SHH), Bone Morphogenetic Protein-2 (BMP2) and Bone Morphogenetic Protein-4 (BMP4). We have investigated the interactions between these signaling molecules during early tooth development. We found that the expression of Shh and Bmp2 is downregulated at E12.5 and E13.5 in the dental epithelium of the Msx1 mutant tooth germ where Bmp4 expression is significantly reduced in the dental mesenchyme. Inhibition of BMP4 activity by noggin resulted in repression of Shh and Bmp2 in wild-type dental epithelium. When implanted into the dental mesenchyme of Msx1 mutants, beads soaked with BMP4 protein were able to restore the expression of both Shh and Bmp2 in the Msx1 mutant epithelium. These results demonstrated that mesenchymal BMP4 represents one component of the signal acting on the epithelium to maintain Shh and Bmp2 expression. In contrast, BMP4-soaked beads repressed Shh and Bmp2 expression in the wild-type dental epithelium. TUNEL assay indicated that this suppression of gene expression by exogenous BMP4 was not the result of an increase in programmed cell death in the tooth germ. Ectopic expression of human Bmp4 to the dental mesenchyme driven by the mouse Msx1 promoter restored Shh expression in the Msx1 mutant dental epithelium but repressed Shh in the wild-type tooth germ in vivo. We further demonstrated that this regulation of Shh expression by BMP4 is conserved in the mouse developing limb bud. In addition, Shh expression was unaffected in the developing limb buds of the transgenic mice in which a constitutively active Bmpr-IB is ectopically expressed in the forelimb posterior mesenchyme and throughout the hindlimb mesenchyme, suggesting that the repression of Shh expression by BMP4 may not be mediated by BMP receptor-IB. These results provide evidence for a new function of BMP4. BMP4 can act upstream to Shh by regulating Shh expression in mouse developing tooth germ and limb bud. Taken together, our data provide insight into a new regulatory mechanism for Shh expression, and suggest that this BMP4-mediated pathway in Shh regulation may have a general implication in vertebrate organogenesis.

An in vitro model for the study of taste papillae morphogenesis using branchial arch explants

It is generally accepted that innervation is required for the maintenance of taste papillae and taste buds, but it is not entirely clear what role, if any, innervation plays in papillae and taste bud formation. Events in taste papillae formation and differentiation take place almost entirely in utero and, therefore, the study of the role of innervation in these events requires a suitable in vitro model. In the past, investigators have made use of various culture techniques to study mammalian taste papillae development in vitro and the role of innervation in this process with varying success. All of these models examined papillae development in isolated tongue or tongue fragments and have lacked the ability to manipulate the innervation of developing taste papillae in these explants. We have established a protocol for an in vitro model of taste papillae morphogenesis using branchial arch explants and roller tube culture methodology. Our results demonstrate that this model supports the morphogenesis of the circumvallate papilla with an integrated nerve. In addition, the use of branchial arch explants allows the inclusion or exclusion of geniculate and petrosal ganglia to examine directly the effects of the presence or absence of innervation on papillae formation and maintenance.

Reiterative signaling and patterning during mammalian tooth morphogenesis

Mammalian dentition consists of teeth that develop as discrete organs. From anterior to posterior, the dentition is divided into regions of incisor, canine, premolar and molar tooth types. Particularly teeth in the molar region are very diverse in shape. The development of individual teeth involves epithelial-mesenchymal interactions that are mediated by signals shared with other organs. Parts of the molecular details of signaling networks have been established, particularly in the signal families BMP, FGF, Hh and Wnt, mostly by the analysis of gene expression and signaling responses in knockout mice with arrested tooth development. Recent evidence suggests that largely the same signaling cascade is used reiteratively throughout tooth development. The successional determination of tooth region, tooth type, tooth crown base and individual cusps involves signals that regulate tissue growth and differentiation. Tooth type appears to be determined by epithelial signals and to involve differential activation of homeobox genes in the mesenchyme. This differential signaling could have allowed the evolutionary divergence of tooth shapes among the four tooth types. The advancing tooth morphogenesis is punctuated by transient signaling centers in the epithelium corresponding to the initiation of tooth buds, tooth crowns and individual cusps. The latter two signaling centers, the primary enamel knot and the secondary enamel knot, have been well characterized and are thought to direct the differential growth and subsequent folding of the dental epithelium. Several members of the FGF signal family have been implicated in the control of cell proliferation around the non-dividing enamel knots. Spatiotemporal induction of the secondary enamel knots determines the cusp patterns of individual teeth and is likely to involve repeated activation and inhibition of signaling as suggested for patterning of other epithelial organs.

The effects of bone morphogenetic protein 2 and 4 (BMP2 and BMP4) on gap junctions during neurodevelopment

Nervous system deficits account for the third largest group of fatal birth defects (after heart and respiratory problems) in North America. Although considerable advance has been made in neuroscience research, the early events involved in neurogenesis remain to be elucidated. More specifically, the effects of signaling molecules on intercellular communication during neurodevelopment have not yet been studied. The development of the central nervous system is regulated, at least in part, by signaling molecules such as bone morphogenetic proteins (BMPs). In this study, we have used the embryonal mouse P19 cell line to examine the effects of BMP2 and BMP4 on gap junctional communication as well as neuronal and astrocytic differentiation. The undifferentiated P19 cells show high levels of the gap junction protein, connexin43 (Cx43), and functional intercellular coupling. However, Cx43 expression and dye coupling decrease as these cells differentiate into neurons and astrocytes. In contrast, cells treated with BMP2 or BMP4 lose their capacity to differentiate into neurons but not astrocytes, while they maintain extensive gap junctional communication. The very few neurons that remain in the BMP-treated cultures are coupled (a characteristic not seen in the control neurons). Together, our data suggest that BMPs may play a critical role in morphogenesis of P19 cells while they affect gap junctions.

Antagonistic signals between BMP4 and FGF8 define the expression of Pitx1 and Pitx2 in mouse tooth-forming anlage

Members of the Pitx/RIEG family of homeodomain-containing transcription factors have been implicated in vertebrate organogenesis. In this study, we examined the expression and regulation of Pitx1 and Pitx2 during mouse tooth development. Pitx1 expression is detected in early development in a widespread pattern, in both epithelium and mesenchyme, covering the tooth-forming region in the mandible, and is then maintained in the dental epithelium from the bud stage to the late bell stage. Pitx2 expression, on the other hand, is restricted to the dental epithelium throughout odontogenesis. Interestingly, from E9.5 to E10.5, the expression domains of Pitx1 and Pitx2, in the developing mandible, overlap with that of Fgf8 but are exclusive to the zone of Bmp4 expression. Bead implantation experiments demonstrate that ectopic expression of Fgf8 can induce/maintain the expression of both Pitx1 and Pitx2 at E9.5. In contrast, Bmp4-expressing tissues and BMP4-soaked beads were able to repress Pitx1 expression in mandibular mesenchyme and Pitx2 expression in the presumptive dental epithelium, respectively. However, the effects of FGF8 and BMP4 are transient. It thus appears that the early expression patterns of Pitx1 and Pitx2 in the developing mandible are regulated by the antagonistic effects of FGF8 and BMP4 such that the Pitx1 and Pitx2 expression patterns are defined. These results indicate that the epithelial-derived signaling molecules are responsible not only for restricting specific gene expression in the dental mesenchyme, but also for defining gene expression in the dental epithelium.

Cusp patterning defect in Tabby mouse teeth and its partial rescue by FGF

Tabby is a mouse mutant characterized by deficient development of the ectodermal organs: teeth, hair, and a subset of glands. Ectodysplasin, the protein encoded by the Tabby gene, was recently identified as a novel TNF-like transmembrane protein but little is known about its function. We have examined the Tabby tooth phenotype in detail by analysis of the adult and embryonic teeth. Tabby first molars had an obvious defect in cusp patterning as the number of cusps was reduced and the buccal and lingual cusps were joined. The disturbance in development was first visible morphologically in the bud stage molar. The primary enamel knot in a cap stage Tabby tooth expressed all enamel knot markers analyzed but was smaller than wild type and the first pair of developing secondary enamel knots was fused. We propose that the Tabby tooth phenotype is due to growth retardation during early stages of development which leads to reduced signaling from the primary enamel knot, followed by deficient growth of the dental epithelium and lack of formation of the last developing secondary enamel knots. The ectodysplasin transcripts were expressed in the outer enamel epithelium and dental lamina. When cultured in vitro Tabby bud/cap stage molars formed fewer cusps than wild-type controls. This phenotype was not rescued by exogenously added EGF despite the previously proposed link between Tabby and EGF. Instead FGF-10 partially restored morphogenesis and stimulated the development of additional tooth cusps in cultured Tabby molars.

Effects of BMP-7 on mouse tooth mesenchyme and chick mandibular mesenchyme

BMP-7 is a member of the BMP family of signaling molecules that are thought to play key roles in mediating inductive events during embryogenesis. In the present study the possible roles of BMP-7 in mediating inductive events during the initiation phase of odontogenesis and mandibular morphogenesis were investigated. To do so, we have examined the effects of agarose beads soaked in recombinant BMP-7 on E11 mouse molar-forming mesenchyme and stage 23 chick mandibular mesenchyme, and analyzed the patterns of expression of Bmp-7 in developing mouse and chick first branchial arches. Beads releasing BMP-7 induced a translucent zone, cellular proliferation, and expression of Msx-1, Msx-2, and Bmp-4 in molar-forming mesenchyme after 24 hr. The effects of BMP-7 on molar-forming mesenchyme are similar to the effects of BMP-4 and are consistent with their overlapping patterns of expression in the thickened epithelium of the early developing tooth buds, which is suggestive of cooperative and/or redundant roles of BMPs in mediating the inductive interactions during the early stages of odontogenesis. Our studies in the developing chick mandible showed that Bmp-7 is expressed in the mandibular epithelium. In the absence of mandibular epithelium, BMP-7 beads maintained cell proliferation and Msx expression in the medial mandibular mesenchyme and were able to induce cell proliferation, cell death, and Msx expression in the lateral chick mandibular mesenchyme. The effects of BMP-7 on the expression of Msx genes in lateral chick mandibular mesenchyme, although different from the effects of lateral mandibular epithelium, are similar to the effects of epithelium from the medial region where multiple Bmps are expressed. We also showed that laterally placed BMP-7 beads induced ectopic expression of Msx genes and changes in the development of posterior skeletal elements in the maxillary and mandibular arches. However, despite its proliferative effects on mandibular mesenchyme, BMP-7 did not support the directional outgrowth of the mandible. These observations suggest that epithelial-mesenchymal interactions in the medial region of the mandibular arch regulating directional outgrowth of the mandibular mesenchyme are mediated by cooperative interactions between BMPs and other growth factors. Our observations also indicated that EGF, another growth factor implicated in mediating epithelial-mesenchymal interactions in the initiation phase of odontogenesis and morphogenesis of the developing mandible, induces an extensive translucent zone and cellular proliferation in the E11 mouse molar-forming mesenchyme and stage 23 chick mandibular mesenchyme. However, in contrast to BMPs, EGF did not induce Msx-1, Msx-2, and Bmp-4, but modulated the effects of BMPs on the expression of Msx-1 and Msx-2 in these mesenchymes. Our combined data suggest that BMP-7 is a component of the signaling network mediating epithelial-mesenchymal interactions during the initiation phase of odontogenesis and morphogenesis of the mandibular arch.

Subtilisin-like proprotein convertase PACE4 (SPC4) is a candidate processing enzyme of bone morphogenetic proteins during tooth formation

The temporospatial expression of PACE4, a member of the mammalian subtilisin-like proprotein convertase family, in the developing rat molar tooth was determined by in situ hybridization. At the initiation stage of tooth development, PACE4 mRNA was weakly expressed in the dental lamina, whereas the mesenchymal cells intensely expressed the PACE4 transcript. At the bud stage, high-level expression of PACE4 mRNA was found in the dental epithelium and condensed dental mesenchyme. Its expression became more localized in the differentiating ameloblasts during cap and early bell stages. In the newborn rats, PACE4 mRNA was localized in the ameloblasts and odontoblasts, but its expression became weaker with advancing development, showing apparent association with the differentiation and establishment of functional ameloblasts and odontoblasts. These expression patterns of PACE4 were very similar to those of several bone morphogenetic proteins (BMPs) reported previously. Because BMPs, which are primarily involved in the morphogenesis in tooth formation, are synthesized as inactive precursors and activated by limited proteolysis at the consensus Arg-X-X-Arg maturation site, the present observations suggest that PACE4 is possibly a candidate proBMP convertase that acts during tooth formation.

An in vitro model for the study of the role of innervation in circumvallate papillae morphogenesis

The following study was done to demonstrate the reliability of an in vitro model for use in the study of early events and the role of innervation in mouse circumvallate papillae development. Gestational day (gd)-11 fetuses were partially dissected to produce explants that included the mandibular, hyoid, third and fourth branchial arches and their ganglia. In ganglionectomized explants, the nodose ganglia and either the geniculate, petrosal or both ganglia were removed. Explants were cultivated in roller tube culture for 24, 48, 72, and 96 h of culture and examined for the presence of papillary structures. Innervation was verified by immunostaining for neural cell adhesion molecule (NCAM). In all control explants, circumvallate papillae had formed by 72 h in culture. These papillae were innervated by fibers originating in petrosal or nodose ganglia, although, in a small number, fibers from the geniculate also contributed. Circumvallate papillae also formed in some explants in which either the geniculate or petrosal ganglia had been removed. However, placodal structures failed to mature into papillary structures even by 96 h in explants in which both ganglia had been removed. Our results demonstrate that an in vitro model using branchial arch explants supports the morphogenesis of an epithelial placode through the formation of a definite papillary structure, the circumvallate papilla, with an integrated nerve. Our results also indicate that, whereas the initial stages in gustatory papillae formation, the formation of a placode, are nerve-independent, the maturation of the placodal structure to form a papilla requires the presence of an intact nerve.

The immunohistochemical localization of phospholipase Cgamma and the epidermal growth-factor, platelet-derived growth-factor and fibroblast growth-factor receptors in the cells of the rat molar enamel organ during early amelogenesis

Findings on the localization and possible roles of the major growth factors, epidermal (EGF), platelet-derived (PDGF) and fibroblast (FGF) in early amelogenesis are contradictory and inconclusive. This study sought to localize immunohistochemically phospholipase (PLCgamma) and the EGF, PDGF and FGF receptors in the cells of the enamel organ during the events leading directly to early enamel formation in rat molars. PLCgamma is an immediate, downstream, signal-transduction pathway effector unique to the three receptors. A whole-head, freeze-dried sectioning method was used to reduce the possibilities of false-negative staining. A modification of the avidin/biotin complex method of immunohistochemical localization was used. Anti-PLCgamma and antibodies to each of EGF, PDGF and FGF receptors colocalized in the preameloblasts of the cervical loop, adjacent to the undifferentiated mesenchymal cells of the dental pulp. This staining disappeared shortly after the beginning of dentine mineralization. Staining for all four antibodies appeared on the proximal ends of the differentiating presecretory ameloblasts at the level of the beginning of predentine matrix deposition and continued in the secretory ameloblasts. It appears that EGF, PDGF and FGF have roles in the differentiation of ameloblasts and in control of cellular functions in presecretory and secretory ameloblasts. Their roles may represent redundancy of the kind seen in highly conserved tissues.

Increased tooth crown size in females with twin brothers: Evidence for hormonal diffusion between human twins in utero

In rodents, the position of a fetus in utero is associated with the expression of sexually dimorphic traits. This phenomenon has been explained by prenatal diffusion of sex hormones among litter mates. To test for such effects in humans, female-male twin pairs provide a natural experiment. The size of dental crowns is a sexually dimorphic trait which can be measured with a high degree of reliability. Thus, two crown diameters of 28 permanent teeth were recorded for 56 opposite-sexed (OS) and 242 same-sexed (SS) twin pairs, and 150 singletons. Comparisons of OS twins with SS twins and singletons within each sex reveal that OS females have consistently larger teeth (on average) than other females, while there is no consistent difference between OS and SS twin males. It is proposed that diffusion of sex hormones from male to female co-twins in utero may account for the increased tooth size in OS females. This study is one of the first to report such an effect on a morphological variable in humans. The finding that the maxillary canine, one of the most sexually dimorphic teeth, exhibits the least effect in OS female twins, suggests that prenatal sex hormone levels may have less impact on sexual dimorphism in the maxillary canines than in other permanent teeth. Am. J. Hum. Biol. 11:577-586, 1999. Copyright 1999 Wiley-Liss, Inc.

Tooth-specific expression conferred by the regulatory sequences of rat dentin sialoprotein gene in transgenic mice

We have isolated a 3.8-kb DNA fragment containing the 5' flanking region, 1st exon, and 1st intron of the rat dentin sialoprotein (rDsp) gene and produced transgenic mice carrying a LacZ reporter gene under the control of this fragment. Expression of the transgene transcript and beta-galactosidase activity were restricted to dentin and odontoblasts with spatial and temporal patterns comparable to those of the endogenous mouse Dsp transcript, although beta-galactosidase activity could not be detected visually during embryonal stages. Other tissues tested, such as alveolar bones, ameloblasts and dental pulps, did not express the transgene. This indicates that the regulatory elements necessary for tooth-specific expression are present in the fragment, which contains a TATA box and several consensus sequences for binding sites of transcription factors related to tooth development, such as TCF-1/LEF-1, MSX-1 and Dlx-1. The regulatory sequences and the transgenic mice described here provide useful information for the study of tooth development.

The distribution of BrdU- and TUNEL-positive cells during odontogenesis in mouse lower first molars

This study investigated the minute distribution of both proliferating and non-proliferating cells, and cell death in the developing mouse lower first molars using 5-bromo-2'-deoxyuridine (BrdU) incorporation and the terminal deoxynucleotidyl transferase-mediated deoxyuridine-5'-triphosphate (dUTP)-biotin nick end labeling (TUNEL) double-staining technique. The distribution pattern of the TUNEL-positive cells was more notable than that of the BrdU-positive cells. TUNEL-positive cells were localized in the following six sites: (1) in the most superficial layer of the dental epithelium during the initiation stage, (2) in the dental lamina throughout the period during which tooth germs grow after bud formation, (3) in the dental epithelium in the most anterior part of the antero-posterior axis of the tooth germ after bud formation, (4) in the primary enamel knot from the late bud stage to the late cap stage, (5) in the secondary enamel knots from the late cap stage to the late bell stage, and (6) in the stellate reticulum around the tips of the prospective cusps after the early bell stage. These peculiar distributions of TUNEL-positive cells seemed to have some effect on either the determination of the exact position of the tooth germ in the mandible or on the complicated morphogenesis of the cusps. The distribution of BrdU-negative cells was closely associated with TUNEL-positive cells, which thus suggested cell arrest and the cell death to be essential for the tooth morphogenesis.

Msx1 is required for the induction of Patched by Sonic hedgehog in the mammalian tooth germ

We have used the mouse developing tooth germ as a model system to explore the transmission of Sonic hedgehog (Shh) signal in the induction of Patched (Ptc). In the early developing molar tooth germ, Shh is expressed in the dental epithelium, and the transcripts of Shh downstream target genes Ptc and Gli1 are expressed in dental epithelium as well as adjacent mesenchymal tissue. The homeobox gene Msx1 is also expressed in the dental mesenchyme of the molar tooth germ at this time. We show here that the expression of Ptc, but not Gli1, was downregulated in the dental mesenchyme of Msx1 mutants. In wild-type E11.0 molar tooth mesenchyme SHH-soaked beads induced the expression of Ptc and Gli1. However, in Msx1 mutant dental mesenchyme SHH-soaked beads were able to induce Gli1 but failed to induce Ptc expression, indicating a requirement for Msx1 in the induction of Ptc by SHH. Moreover, we show that another signaling molecule, BMP4, was able to induce Ptc expression in wild-type dental mesenchyme, but induced a distinct expression pattern of Ptc in the Msx1 mutant molar mesenchyme. We conclude that in the context of the tooth germ Msx1 is a component of the Shh signaling pathway that leads to Ptc induction. Our results also suggest that the precise pattern of Ptc expression in the prospective tooth-forming region is controlled and coordinated by at least two inductive signaling pathways.

Expression of sonic hedgehog, patched, and Gli1 in developing taste papillae of the mouse

Lingual taste buds form within taste papillae, which are specialized structures that develop in a characteristic spatial and temporal pattern. To investigate the signaling events responsible for patterning and morphogenesis of taste papillae, the authors examined the time course and distribution of expression of several related developmental signaling genes as well as the time course of innervation of taste papillae in mouse embryos from embryonic day 12 (E12) to E18. Lingual expression of the signaling molecule Sonic hedgehog (Shh), its receptor Patched (Ptc), and the Shh-activated transcription factor Gli1 were assayed by using in situ hybridization. Shh is expressed broadly in the lingual epithelium at E12 but becomes progressively restricted to developing circumvallate and fungiform papillary epithelia. Shh is expressed specifically within the central cells of the papillary epithelium starting at E13.5 and persisting through E18. Ptc and Gli1 expression follow a pattern similar to that of Shh. Compared with Shh, Ptc is expressed in larger regions surrounding the central papillary cells and also in the mesenchyme underlying Shh-expressing epithelium. Innervation of taste papillae was examined by using the panneuronal antibody to ubiquitin carboxyl terminal hydrolase (protein gene product 9.5). Nerves reach the basal lamina of developing taste papillae at E14 to densely innervate the papillary epithelium by E16. Thus, the pattern of Shh expression within developing taste papillae is established prior to innervation, ruling out neuronal induction of papillae. The results suggest that the Shh signaling pathway may be involved in: 1) establishing papillary boundaries in taste papilla morphogenesis, 2) papillary epithelial-mesenchymal interactions, and/or 3) specifying the location or development of taste buds within taste papillae.

Molecular genetics of tooth morphogenesis and patterning: the right shape in the right place

Development of the mammalian tooth has for many years served as a useful model system for the study of cell-cell interactions in organogenesis. Early development of teeth (tooth buds) shows many morphological and molecular similarities with early development of other organs such as the lung, hair, kidney, etc. There has been much progress toward understanding epithelial/mesenchymal cell signaling in tooth germ formation. Advances in understanding the formation of different shapes of teeth (morphogenesis) at their correct positions in the jaws (patterning) has, until recently, been less forthcoming. We review here the latest ideas on the control of odontogenic patterning and morphogenesis. The stages of early tooth development are well-defined histologically and have been described in numerous textbooks. The progression from localized thickenings of oral epithelium to bud, cap, and bell stages provides an adequate description of the gross morphological changes seen in the epithelial cells of early developing tooth germs. Less obvious are the concomitant changes taking place in the dental (ecto)mesenchymal cells which originate from the cranial neural crest and which condense around the tooth bud epithelium. However, it is very clear that these mesenchymal cells are equal partners with epithelium during the early stages of tooth germ formation and undergo complex changes which, although not obvious histologically, are revealed with molecular (gene) probes. Genes identified as being important for the early communication between the epithelial and ectomesenchymal cells mainly comprise those which code for proteins which act as secreted signals between the cells (ligands) and those that code for nuclear proteins that act to control gene expression in response to the signals. Little is presently known about the changes in structural proteins such as cell adhesion molecules which are involved in mediating the physical interactions between cells and generating the morphological changes.

The surface ectoderm is essential for nephric duct formation in intermediate mesoderm

The nephric duct is the first epithelial tubule to differentiate from intermediate mesoderm that is essential for all further urogenital development. In this study we identify the domain of intermediate mesoderm that gives rise to the nephric duct and demonstrate that the surface ectoderm is required for its differentiation. Removal of the surface ectoderm resulted in decreased levels of Sim-1 and Pax-2 mRNA expression in mesenchymal nephric duct progenitors, and caused inhibition of nephric duct formation and subsequent kidney development. The surface ectoderm expresses BMP-4 and we show that it is required for the maintenance of high-level BMP-4 expression in lateral plate mesoderm. Addition of a BMP-4-coated bead to embryos lacking the surface ectoderm restored normal levels of Sim-1 and Pax-2 mRNA expression in nephric duct progenitors, nephric duct formation and the initiation of nephrogenesis. Thus, BMP-4 signaling can substitute for the surface ectoderm in supporting nephric duct morphogenesis. Collectively, these data suggest that inductive interactions between the surface ectoderm, lateral mesoderm and intermediate mesoderm are essential for nephric duct formation and the initiation of urogenital development.

Expression of hepatocyte growth factor/scatter factor and c-Met in human dental papilla and fibroblasts from dental papilla

Hepatocyte growth factor/scatter factor (HGF/SF), a broad-spectrum and multifunctional cytokine, is essential for the development of tissues including tooth. Here it was found that the HGF/SF content of human dental papillae obtained from 8 to 16-year-old individuals decreased significantly with age. Cultured fibroblasts prepared from the dental papillae of individuals of different ages produced HGF/SF at almost the same rate, but the sensitivities of the cells to interleukin-1alpha and tumour necrosis factor-alpha for the production of HGF/SF increased with age. Generally, mesenchymal cells such as fibroblasts produce HGF/SF but do not express c-Met, a receptor for HGF/SF, yet fibroblasts in dental papilla and cultured fibroblasts prepared from dental papilla did express c-Met, as determined by immunohistochemistry, in situ hybridization and reverse transcription-polymerase chain reaction. Recombinant human [125I]iodo-HGF/SF specifically bound to cell-surface macromolecules with a mol. wt of 146,000, which is the same as that of the beta-subunit of c-Met. The physiological role of c-Met on fibroblasts in dental papilla is unknown, but the addition of 2 ng of HGF/SF per ml to the culture medium significantly stimulated DNA synthesis in the cells, as determined by pulse labelling with [3H]thymidine. Exogenous HGF/SF also stimulated secretion by the cells of vascular endothelial growth factor, a cytokine that induces blood vessel-formation. These results suggest that HGF/SF may be involved in tooth development via autocrine mechanisms.

Conserved regulation of mesenchymal gene expression by Fgf-8 in face and limb development

Clim-2 (NLI, Lbd1) is one of two related mouse proteins that interact with Lim-domain homeoproteins. In the mouse, embryonic expression of Clim-2 is particularly pronounced in facial ectomesenchyme and limb bud mesenchyme in association with Lim genes, Lhx-6 and Lmx-1 respectively. We show that in common with both these Lim genes, Clim-2 expression is regulated by signals from overlying epithelium. In both the developing face and the limb buds we identify Fgf-8 as the likely candidate signalling molecule that regulates Clim-2 expression. We show that in the mandibular arch, as in the limb, Fgf-8 functions in combination with CD44, a cell surface binding protein, and that blocking CD44 binding results in inhibition of Fgf8-induced expression of Clim-2 and Lhx-6. Regulation of gene expression by Fgf8 in association with CD44 is thus conserved between limb and mandibular arch development.

Dynamic interactions and the evolutionary genetics of dental patterning

The mammalian dentition is a segmental, or periodically arranged, organ system whose components are arrayed in specific number and in regionally differentiated locations along the linear axes of the jaws. This arrangement evolved from simpler dentitions comprised of many single-cusp teeth of relatively indeterminate number. The different types of mammalian teeth have subsequently evolved as largely independent units. The experimentally documented developmental autonomy of dental primordia shows that the basic dental pattern is established early in embryogenesis. An understanding of how genetic patterning processes may work must be consistent with the different modes of development, and partially independent evolution, of the upper and lower dentition in mammals. The periodic nature of the location, number, and morphological structure of teeth suggests that processes involving the quantitative interaction of diffusible signaling factors may be involved. Several extracellular signaling molecules and their interactions have been identified that may be responsible for locating teeth along the jaws and for the formation of the incisor field. Similarly, the wavelike expression of signaling factors within developing teeth suggests that dynamic interactions among those factors may be responsible for crown patterns. These factors seem to be similar among different tooth types, but the extent to which crown differences can be explained strictly in terms of variation in the parameters of interactions among the same genes, as opposed to tooth-type-specific combinatorial codes of gene expression, is not yet known. There is evidence that combinatorial expression of intracellular transcription factors, including homeobox gene families, may establish domains within the jaws in which different tooth types are able to develop. An evolutionary perspective can be important for our understanding of dental patterning and the designing of appropriate experimental approaches, but dental patterns also raise basic unresolved questions about the nature of the evolutionary assumptions made in developmental genetics.

Involvement of the sonic hedgehog, patched 1 and bmp2 genes in patterning of the zebrafish dermal fin rays

The signaling molecule encoded by Sonic hedgehog (shh) participates in the patterning of several embryonic structures including limbs. During early fin development in zebrafish, a subset of cells in the posterior margin of pectoral fin buds express shh. We have shown that regulation of shh in pectoral fin buds is consistent with a role in mediating the activity of a structure analogous to the zone of polarizing activity (ZPA) (Akimenko and Ekker (1995) Dev. Biol. 170, 243–247). During growth of the bony rays of both paired and unpaired fins, and during fin regeneration, there does not seem to be a region equivalent to the ZPA and one would predict that shh would play a different role, if any, during these processes specific to fish fins. We have examined the expression of shh in the developing fins of 4-week old larvae and in regenerating fins of adults. A subset of cells in the basal layer of the epidermis in close proximity to the newly formed dermal bone structures of the fin rays, the lepidotrichia, express shh, and ptc1 which is thought to encode the receptor of the SHH signal. The expression domain of ptc1 is broader than that of shh and adjacent blastemal cells releasing the dermal bone matrix also express ptc1. Further observations indicate that the bmp2 gene, in addition to being expressed in the same cells of the basal layer of the epidermis as shh, is also expressed in a subset of the ptc1-expressing cells of the blastema. Amputations of caudal fins immediately after the first branching point of the lepidotrichia, and global administration of all-trans-retinoic acid, two procedures known to cause fusion of adjacent rays, result in a transient decrease in the expression of shh, ptc1 and bmp2. The effects of retinoic acid on shh expression occur within minutes after the onset of treatment suggesting direct regulation of shh by retinoic acid. These observations suggest a role for shh, ptc1 and bmp2 in patterning of the dermoskeleton of developing and regenerating teleost fins.

Analysis of epithelial-mesenchymal interactions in the initial morphogenesis of the mammalian tooth

Epithelial-mesenchymal interactions govern the development of epidermal organs such as teeth. During the early stages of tooth development, a local ectodermal thickening which expresses several signaling molecules appears. It is believed that these in turn signal to the underlying mesenchyme triggering mesenchymal condensation and tooth development. For example, epithelially expressed Bmp4 induces Msx1 and Lef1 as well as itself in the underlying mesenchyme. In this paper we have investigated the role of four epithelial signaling molecules, Bmp2, Shh, Wnt10a, and Wnt10b, in the early inductive cascades that govern tooth development. We show that all four genes are specifically expressed in the epithelium between E11.0 and E12.0 when tooth morphogenesis is first apparent. Although Shh, Bmp2, and Wnt10b have similar, if not identical, expression patterns, each signal has a distinct molecular action on the jaw mesenchyme. Whereas Shh and Wnt10b can induce general Hedgehog and Wnt targets, Ptc and Gli for Shh and Lef1 for Wnt10b, only Bmp2 is able to induce tooth-specific expression of Msx1. Thus, there are distinct targets for all three pathways. Interestingly, both Bmp and Wnt signaling activate Lef1, making it a candidate for integrating the two distinct signaling pathways.

Neurturin mRNA expression suggests roles in trigeminal innervation of the first branchial arch and in tooth formation

Neurturin (NTN) is a recently characterized member of the glial cell line-derived neurotrophic factor (GDNF)-family which, like GDNF, can promote the survival of certain populations of neuronal cells in peripheral and central nervous systems. To elucidate the roles of NTN and a novel glycosyl-phosphatidylinositol (GPI)-linked receptor protein GFRalpha-3, a member of GDNF-family receptor alpha, in the regulation of peripheral trigeminal innervation and tooth formation, their expression patterns during mouse embryonic (E) and early postnatal (P) development (E10-P5) of the first branchial arch were analyzed by in situ hybridization. NTN mRNAs were observed in oral and cutaneous epithelia of the mandibular process at all studied stages and expression became gradually restricted to the suprabasal epithelial cells. In addition, transcripts were also detected in the epithelium of whisker follicles. In the developing first molar tooth germ, NTN showed a developmentally regulated, spatiotemporally changing expression pattern, which partially correlated with the development of innervation. During the initiation of tooth formation NTN mRNAs were expressed in dental epithelium and during later embryonic development transcripts appeared in the dental papilla mesenchyme. In addition, some transcripts were seen in the dental follicle. During postnatal development, NTN expression was restricted to the dental follicle of the incisor tooth germs. GFRalpha-3 mRNAs were not detected in teeth, but an intense expression was seen in non-neuronal cells surrounding trigeminal nerve fibers and in the trigeminal ganglia during E11-E15. Ganglion explant cultures showed that trigeminal neurons start to respond to exogenous NTN at E12, which correlates to the earlier reported appearance of the Ret-tyrosine kinase receptor in the trigeminal ganglion. Local application of NTN with beads on isolated dental mesenchyme did not stimulate cell proliferation or prevent apoptotic cell death. In addition, exogenous NTN had no effects on tooth morphogenesis in in vitro cultures. Taken together, because trigeminal neurons respond to NTN after first axons have reached their primary epithelial target fields, NTN is apparently not involved in the guidance of pioneer trigeminal nerves to their peripheral targets. However, our results show that NTN is a potent neuritogenic factor and, therefore, may act as a target-field-derived neurotrophic factor for trigeminal nerves during innervation of the cutaneous and oral epithelia as well as dental follicle surrounding the developing tooth. In addition, although NTN appears not to be directly involved in the regulation of tooth morphogenesis, it may have non-neuronal, organogenetic functions during tooth formation.