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

FGF-16 is required for embryonic heart development

Fibroblast growth factor 16 (FGF-16) expression has previously been detected in mouse heart at mid-gestation in the endocardium and epicardium, suggesting a role in embryonic heart development. More specifically, exogenously applied FGF-16 has been shown to stimulate growth of embryonic myocardial cells in tissue explants. We have generated mice lacking FGF-16 by targeting the Fgf16 locus on the X chromosome. Elimination of Fgf16 expression resulted in embryonic death as early as day 11.5 (E11.5). External abnormalities, including hemorrhage in the heart and ventral body region as well as facial defects, began to appear in null embryos from E11.5. Morphological analysis of FGF-16 null hearts revealed cardiac defects including chamber dilation, thinning of the atrial and ventricular walls, and poor trabeculation, which were visible at E10.5 and more pronounced at E11.5. These findings indicate FGF-16 is required for embryonic heart development in mid-gestation through its positive effect on myocardial growth.

Tweaking the hinge and caps: testing a model of the organization of jaws

Historically, examinations of gnathostome skulls have indicated that for essentially the entirety of their existence, jaws have been characterized by a high degree of fidelity to an initial basic structural design that will then go on to manifest an amazing array of end-point phenotypes. These two traits-bauplan fidelity and elaboration of design-are inter-connected and striking, and beg a number of questions, including: Are all jaws made in the same manner and if not how not? To begin to tackle such questions, we herein operationally define jaws as two appositional, hinged cranial units for which polarity and potential modularity are characteristics, and then address what is necessary for them to form, including delineating both the sources of cells and tissues that will formally yield the jaws as well as what informs their ontogeny (e.g., sources of positional information and factors directing the interpretation of developmental cues). Following on this, we briefly describe a predictive, testable model of jaw development (the "Hinge and Caps" model) and present evidence that the Satb2+cell population in the developing jaw primordia of mice defines a developmentally and evolutionarily significant jaw module such as would be predicted by the model.

Epiblastic Cited2 deficiency results in cardiac phenotypic heterogeneity and provides a mechanism for haploinsufficiency

AIMS

Deletion of the transcription factor Cited2 causes penetrant and phenotypically heterogenous cardiovascular and laterality defects and adrenal agenesis. Heterozygous human CITED2 mutation is associated with congenital heart disease, suggesting haploinsufficiency. Cited2 functions partly via a Nodal-->Pitx2c pathway controlling left-right patterning. In this present study we investigated the primary site of Cited2 function and mechanisms of haploinsufficiency.

METHODS AND RESULTS

A Cited2 conditional allele enabled its deletion in particular cell lineages in mouse development. A lacZ reporter cassette allowed indication of deletion. Congenic Cited2 heterozygous mice were used to investigate haploinsufficiency. Embryos were examined by magnetic resonance imaging, by sectioning and by quantitative real-time polymerase chain reaction (qRT-PCR). Epiblast-specific deletion of Cited2 using Sox2Cre recapitulated penetrant and phenotypically heterogenous cardiovascular and laterality defects. Neural crest-specific deletion using Wnt1Cre affected cranial ganglia but not cardiac development. Mesodermal deletion with Mesp1Cre resulted in low penetrance of septal defect. Mesodermal deletion with T-Cre resulted in adrenal agenesis, but infrequent cardiac septal and laterality defects. beta-Galatactosidase staining and qRT-PCR demonstrated the efficiency and location of Cited2 deletion. Murine Cited2 heterozygosity is itself associated with cardiac malformation, with three of 45 embryos showing ventricular septal defect. Cited2 gene expression in E13.5 hearts was reduced 2.13-fold in Cited2(+/-) compared with wild-type (P = 2.62 x 10(-6)). The Cited2 target gene Pitx2c was reduced 1.5-fold in Cited2(+/-) (P = 0.038) hearts compared with wild-type, and reduced 4.9-fold in Cited2(-/-) hearts (P = 0.00031). Pitx2c levels were reduced two-fold (P = 0.009) in Cited2(+/-) embryos, in comparison with wild-type. Cited2 and Pitx2c expression were strongly correlated in wild-type and Cited2(+/-) hearts (Pearson rank correlation = 0.68, P = 0.0009). Cited2 expression was reduced 7474-fold in Sox2Cre deleted hearts compared with controls (P = 0.00017) and Pitx2c was reduced 3.1-fold (P = 0.013). Deletion of Cited2 with Mesp1Cre resulted in a 130-fold reduction in cardiac Cited2 expression compared with control (P = 0.0002), but Pitx2c expression was not affected.

CONCLUSION

These results indicate that phenotypically heterogenous and penetrant cardiac malformations in Cited2 deficiency arise from a primary requirement in epiblast derivatives for left-right patterning, with a secondary cell-autonomous role in the mesoderm. Cardiac malformation associated with Cited2 haploinsufficiency may occur by reducing expression of key Cited2 targets such as Pitx2c.

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.

An absence of Twist1 results in aberrant cardiac neural crest morphogenesis

The basic helix-loop-helix transcription factor Twist1 plays an essential role in mesenchymal cell populations during embryonic development and in pathological disease. Remodeling of the cardiac outflow tract (OFT) into the functionally separate aortic arch and pulmonary trunk is dependent upon the dynamic, coordinated contribution of multiple mesenchymal cell populations. Here, we report that Twist1(-/-) mice exhibit OFTs that contain amorphic cellular nodules within their OFT endocardial cushions. The nodular mesenchyme expresses the related bHLH factors Hand1 and Hand2, but reduced levels of the normal cushion marker Periostin. Lineage mapping confirms that nodule cells are exclusively of cardiac neural crest origin (cNCC), and are not ectopic cardiomyocytes or smooth muscle cells. These studies also reveal a delay in cNCC colonization of the OFT cushions. Furthermore, these mapping studies uncover nodules in the pharyngeal arches, and identify Twist1(-/-) neural crest cell defects within the dorsal neural tube, which exhibits an expanded domain of Wnt1-Cre-lineage marked cells. Together, these data support a model where Twist1 is required both for proper cNCC delamination, and for emigration from the dorsal neural tube and along cNCC migration pathways. Within the Twist1(-/-) neural crest cell populations that do emigrate to the OFT, a Hand-expressing subpopulation displays defective maturation, tracking, and, presumably, cell-cell adhesion, further compromising cNCC morphogenesis.

TGF-beta type I receptor Alk5 regulates tooth initiation and mandible patterning in a type II receptor-independent manner

TGF-beta superfamily members signal through a heteromeric receptor complex to regulate craniofacial development. TGF-beta type II receptor appears to bind only TGF-beta, whereas TGF-beta type I receptor (ALK5) also binds to ligands in addition to TGF-beta. Our previous work has shown that conditional inactivation of Tgfbr2 in the neural crest cells of mice leads to severe craniofacial bone defects. In this study, we examine and compare the defects of TGF-beta type II receptor (Wnt1-Cre;Tgfbr2(fl/fl)) and TGF-beta type I receptor/Alk5 (Wnt1-Cre;Alk5(fl)(/fl)) conditional knockout mice. Loss of Alk5 in the neural crest tissue resulted in phenotypes not seen in the Tgfbr2 mutant, including delayed tooth initiation and development, defects in early mandible patterning and altered expression of key patterning genes including Msx1, Bmp4, Bmp2, Pax9, Alx4, Lhx6/7 and Gsc. Alk5 controls the survival of CNC cells by regulating expression of Gsc and other genes in the proximal aboral region of the developing mandible. We conclude that ALK5 regulates tooth initiation and early mandible patterning through a pathway independent of Tgfbr2. There is an intrinsic requirement for Alk5 signal in regulating the fate of CNC cells during tooth and mandible development.

Genetic approaches for changing the heart and dissecting complex syndromes

The genetic, biochemical and molecular bases of human cardiac disease have been the focus of extensive research efforts for many years. Early animal models of cardiovascular disease used pharmacologic or surgical interventions, or took advantage of naturally occurring genetic abnormalities and the data obtained were largely correlative. The inability to directly alter an organism's genetic makeup and cellular protein content and accurately measure the results of that manipulation precluded rigorous examination of true cause-effect and structure-function relationships. Directed genetic manipulation in the mouse gave researchers the ability to modify and control the mammalian heart's protein content, resulting in the rational design of models that could provide critical links between the mutated or absent protein and disease. Two techniques that have proven particularly useful are transgenesis, which involves the random insertion of ectopic genetic material of interest into a "host" genome, and gene targeting, which utilizes homologous recombination at a pre-selected locus. Initially, transgenesis and gene targeting were used to examine systemic loss-of-function and gain-of-function, respectively, but further refinements in both techniques have allowed for investigations of organ-specific, cell type-specific, developmental stage-sensitive and dose-dependent effects. Genetically engineered animal models of pediatric and adult cardiac disease have proven that, when used appropriately, these tools have the power to extend mere observation to the establishment of true causative proof. We illustrate the power of the general approach by showing how genetically engineered mouse models can define the precise signaling pathways that are affected by the gain-of-function mutation that underlies Noonan syndrome. Increasingly precise and modifiable animal models of human cardiac disease will allow researchers to determine not only pathogenesis, but also guide treatment and the development of novel therapies.

Shox2-deficiency leads to dysplasia and ankylosis of the temporomandibular joint in mice

The temporomandibular joint (TMJ) is a unique synovial joint whose development differs from the formation of other synovial joints. Mutations have been associated with the developmental defects of the TMJ only in a few genes. In this study, we report the expression of the homeobox gene Shox2 in the cranial neural crest derived mesenchymal cells of the maxilla-mandibular junction and later in the progenitor cells and undifferentiated chondrocytes of the condyle as well as the glenoid fossa of the developing TMJ. A conditional inactivation of Shox2 in the cranial neural crest-derived cells causes developmental abnormalities in the TMJ, including dysplasia of the condyle and glenoid fossa. The articulating disc forms but fuses with the fibrous layers of the condyle and glenoid fossa, clinically known as TMJ ankylosis. Histological examination indicates a delay in development in the mutant TMJ, accompanied by a significantly reduced rate of cell proliferation. In situ hybridization further demonstrates an altered expression of several key osteogenic genes and a delayed expression of the osteogenic differentiation markers. Shox2 appears to regulate the expression of osteogenic genes and is essential for the development and function of the TMJ. The Shox2 conditional mutant thus provides a unique animal model of TMJ ankylosis.

Persistent expression of Pax3 in the neural crest causes cleft palate and defective osteogenesis in mice

Transcription factors regulate tissue patterning and cell fate determination during development; however, expression of early regulators frequently abates upon differentiation, suggesting that they may also play a role in maintaining an undifferentiated phenotype. The transcription factor paired box 3 (Pax3) is expressed by multipotent neural crest precursors and is implicated in neural crest disorders in humans such as Waardenburg syndrome. Pax3 is required for development of multiple neural crest lineages and for activation of lineage-specific programs, yet expression is generally extinguished once neural crest cells migrate from the dorsal neural tube and differentiate. Using a murine Cre-inducible system, we asked whether persistent Pax3 expression in neural crest derivatives would affect development or patterning. We found that persistent expression of Pax3 in cranial neural crest cells resulted in cleft palate, ocular defects, malformation of the sphenoid bone, and perinatal lethality. Furthermore, we demonstrated that Pax3 directly regulates expression of Sostdc1, a soluble inhibitor of bone morphogenetic protein (BMP) signaling. Persistent Pax3 expression renders the cranial crest resistant to BMP-induced osteogenesis. Thus, one mechanism by which Pax3 maintains the undifferentiated state of neural crest mesenchyme may be to block responsiveness to differentiation signals from the environment. These studies provide in vivo evidence for the importance of Pax3 downregulation during differentiation of multipotent neural crest precursors and cranial development.

SHED repair critical-size calvarial defects in mice

OBJECTIVE

Stem cells from human exfoliated deciduous teeth (SHED) are a population of highly proliferative postnatal stem cells capable of differentiating into odontoblasts, adipocytes, neural cells, and osteo-inductive cells. To examine whether SHED-mediated bone regeneration can be utilized for therapeutic purposes, we used SHED to repair critical-size calvarial defects in immunocompromised mice.

MATERIALS AND METHODS

We generated calvarial defects and transplanted SHED with hydroxyapatite/tricalcium phosphate as a carrier into the defect areas.

RESULTS

SHED were able to repair the defects with substantial bone formation. Interestingly, SHED-mediated osteogenesis failed to recruit hematopoietic marrow elements that are commonly seen in bone marrow mesenchymal stem cell-generated bone. Furthermore, SHED were found to co-express mesenchymal stem cell marker, CC9/MUC18/CD146, with an array of growth factor receptors such as transforming growth factor beta receptor I and II, fibroblast growth factor receptor I and III, and vascular endothelial growth factor receptor I, implying their comprehensive differentiation potential.

CONCLUSIONS

Our data indicate that SHED, derived from neural crest cells, may select unique mechanisms to exert osteogenesis. SHED might be a suitable resource for orofacial bone regeneration.

Disparate osteogenic response of mandible and iliac crest bone marrow stromal cells to pamidronate

OBJECTIVE

Long-term administration of intravenous bisphosphonates like pamidronate is associated with jaw osteonecrosis but axial and appendicular bones remain unaffected. Pathogenesis of bisphosphonate-associated jaw osteonecrosis may relate to skeletal site-specific effects of bisphosphonates on osteogenic differentiation of bone marrow stromal cells (BMSCs) of orofacial and axial/appendicular bones. This study evaluated and compared skeletal site-specific osteogenic response of mandible (orofacial bone) and iliac crest (axial bone) human BMSCs to pamidronate.

MATERIALS AND METHODS

Mandible and iliac crest BMSCs from six normal healthy volunteers were established in culture and tested with pamidronate to evaluate and compare cell survival, osteogenic marker alkaline phosphatase, osteoclast differentiation in co-cultures with CD34+ hematopoietic stem cells, gene expression of receptor activator of NFkappaB ligand (RANKL) and osteoprotegerin, and in vivo bone regeneration.

RESULTS

Mandible BMSCs were more susceptible to pamidronate than iliac crest BMSCs based on decreased cell survival, lower alkaline phosphatase production, and structurally less organized in vivo bone regeneration. Pamidronate promoted higher RANKL gene expression and osteoclast recruitment by mandible BMSCs.

CONCLUSION

Mandible and iliac crest BMSC survival and osteogenic differentiation are disparately affected by pamidronate to favor dysregulated mandible bone homeostasis.

Analysis of Zfhx1a mutant mice reveals palatal shelf contact-independent medial edge epithelial differentiation during palate fusion

Cleft palate is a common birth defect that involves disruptions in multiple developmental steps such as growth, differentiation, elevation, and fusion. Medial edge epithelial (MEE) differentiation is essential for palate fusion. An important question is whether the MEE differentiation that occurs during fusion is induced by palate shelf contact or is programmed intrinsically by the palate shelf itself. Here, we report that the loss of Zfhx1a function in mice leads to a cleft palate phenotype that is mainly attributable to a delay in palate elevation. Zfhx1a encodes a transcription regulatory protein that modulates several signaling pathways including those activated by members of the transforming growth factor-beta (TGF-beta) superfamily. Loss of Zfhx1a function in mice leads to a complete cleft palate with 100% penetrance. Zfhx1a mutant palatal shelves display normal cell differentiation and proliferation and are able to fuse in an in vitro culture system. The only defect detected was a delay of 24-48 h in palatal shelf elevation. Using the Zfhx1a mutant as a model, we studied the relationship between MEE differentiation and palate contact/adhesion. We found that down-regulation of Jag2 expression in the MEE cells, a key differentiation event establishing palate fusion competence, was independent of palate contact/adhesion. Moreover, the expression of several key factors essential for fusion, such as TGF-beta3 and MMP13, was also down-regulated at embryonic stage 16.5 in a contact-independent manner, suggesting that differentiation of the medial edge epithelium was largely programmed through an intrinsic mechanism within the palate shelf.

Cell lineage in mammalian craniofacial mesenchyme

We have analysed the contributions of neural crest and mesoderm to mammalian craniofacial mesenchyme and its derivatives by cell lineage tracing experiments in mouse embryos, using the permanent genetic markers Wnt1-cre for neural crest and Mesp1-cre for mesoderm, combined with the Rosa26 reporter. At the end of neural crest cell migration (E9.5) the two patterns are reciprocal, with a mutual boundary just posterior to the eye. Mesodermal cells expressing endothelial markers (angioblasts) are found not to respect this boundary; they are associated with the migrating neural crest from the 5-somite stage, and by E9.5 they form a pre-endothelial meshwork throughout the cranial mesenchyme. Mesodermal cells of the myogenic lineage also migrate with neural crest cells, as the branchial arches form. By E17.5 the neural crest-mesoderm boundary in the subectodermal mesenchyme becomes out of register with that of the underlying skeletogenic layer, which is between the frontal and parietal bones. At E13.5 the primordia of these bones lie basolateral to the brain, extending towards the vertex of the skull during the following 4-5 days. We used DiI labelling of the bone primordia in ex-utero E13.5 embryos to distinguish between two possibilities for the origin of the frontal and parietal bones: (1) recruitment from adjacent connective tissue or (2) proliferation of the original primordia. The results clearly demonstrated that the bone primordia extend vertically by intrinsic growth, without detectable recruitment of adjacent mesenchymal cells.

Platelet-derived growth factor receptor regulates salivary gland morphogenesis via fibroblast growth factor expression

A coordinated reciprocal interaction between epithelium and mesenchyme is involved in salivary gland morphogenesis. The submandibular glands (SMGs) of Wnt1-Cre/R26R mice have been shown positive for mesenchyme, whereas the epithelium is beta-galactosidase-negative, indicating that most mesenchymal cells are derived from cranial neural crest cells. Platelet-derived growth factor (PDGF) receptor alpha is one of the markers of neural crest-derived cells. In this study, we analyzed the roles of PDGFs and their receptors in the morphogenesis of mouse SMGs. PDGF-A was shown to be expressed in SMG epithelium, whereas PDGF-B, PDGFRalpha, and PDGFRbeta were expressed in mesenchyme. Exogenous PDGF-AA and -BB in SMG organ cultures demonstrated increased levels of branching and epithelial proliferation, although their receptors were found to be expressed in mesenchyme. In contrast, short interfering RNA for Pdgfa and -b as well as neutralizing antibodies for PDGF-AB and -BB showed decreased branching. PDGF-AA induced the expression of the fibroblast growth factor genes Fgf3 and -7, and PDGF-BB induced the expression of Fgf1, -3, -7, and -10, whereas short interfering RNA for Pdgfa and Pdgfb inhibited the expression of Fgf3, -7, and -10, indicating that PDGFs regulate Fgf gene expression in SMG mesenchyme. The PDGF receptor inhibitor AG-17 inhibited PDGF-induced branching, whereas exogenous FGF7 and -10 fully recovered. Together, these results indicate that fibroblast growth factors function downstream of PDGF signaling, which regulates Fgf expression in neural crest-derived mesenchymal cells and SMG branching morphogenesis. Thus, PDGF signaling is a possible mechanism involved in the interaction between epithelial and neural crest-derived mesenchyme.

A nonneural epithelial domain of embryonic cranial neural folds gives rise to ectomesenchyme

The neural crest is generally believed to be the embryonic source of skeletogenic mesenchyme (ectomesenchyme) in the vertebrate head and other derivatives, including pigment cells and neurons and glia of the peripheral nervous system. Although classical transplantation experiments leading to this conclusion assumed that embryonic neural folds were homogeneous epithelia, we reported that embryonic cranial neural folds contain spatially and phenotypically distinct domains, including a lateral nonneural domain with cells that coexpress E-cadherin and PDGFRalpha and a thickened mediodorsal neuroepithelial domain where these proteins are reduced or absent. We now show that Wnt1-Cre is expressed in the lateral nonneural epithelium of rostral neural folds and that cells coexpressing Cre-recombinase and PDGFRalpha delaminate precociously from some of this nonneural epithelium. We also show that ectomesenchymal cells exhibit beta-galactosidase activity in embryos heterozygous for an Ecad-lacZ reporter knock- in allele. We conclude that a lateral nonneural domain of the neural fold epithelium, which we call "metablast," is a source of ectomesenchyme distinct from the neural crest. We suggest that closer analysis of the origin of ectomesenchyme might help to understand (i) the molecular-genetic regulation of development of both neural crest and ectomesenchyme lineages; (ii) the early developmental origin of skeletogenic and connective tissue mesenchyme in the vertebrate head; and (iii) the presumed origin of head and branchial arch skeletal and connective tissue structures during vertebrate evolution.

Msx2 exerts bone anabolism via canonical Wnt signaling

Msx2 is a homeodomain transcription factor first identified in craniofacial bone and human femoral osteoblasts. We hypothesized that Msx2 might activate skeletal Wnt signaling. Therefore, we analyzed the effects of CMV-Msx2 transgene (Msx2Tg) expression on skeletal physiology and composition. Skeletal Msx2 expression was increased 2-3-fold by Msx2Tg, with expanded protein accumulation in marrow, secondary ossification centers, and periosteum. Microcomputed tomography established increased bone volume in Msx2Tg mice, with increased numbers of plate-like trabeculae. Histomorphometry revealed increased bone formation in Msx2Tg mice versus non-Tg siblings, arising from increased osteoblast numbers. While decreasing adipogenesis, Msx2Tg increased osteogenic differentiation via mechanisms inhibited by Dkk1, an antagonist of Wnt receptors LRP5 and LRP6. Bone from Msx2Tg mice elaborated higher levels of Wnt7 canonical agonists, with diminished Dkk1, changes that augment canonical signaling. Analysis of non-Tg and Msx2Tg siblings possessing the TOPGAL reporter confirmed this; Msx2Tg up-regulated skeletal beta-galactosidase expression (p </= 0.01), along with Wnt7a and Wnt7b, and reduced circulating Dkk1. To better understand molecular mechanisms, we studied C3H10T1/2 osteoprogenitor cells. As in bone, Msx2 increased Wnt7 genes and down-regulated Dkk1, while inducing the osteoblast gene alkaline phosphatase. Msx2-directed RNA interference increased Dkk1 expression and promoter activity, while reducing Wnt7a, Wnt7b, and alkaline phosphatase. Moreover, Msx2 inhibited Dkk1 promoter activity and reduced RNA polymerase association with Dkk1 chromatin. RNA interference-mediated knockdown of Wnt7a, Wnt7b, and LRP6 significantly reduced Msx2-induced alkaline phosphatase. Msx2 exerts bone anabolism in part by reducing Dkk1 expression and enhancing Wnt signaling, thus promoting osteogenic differentiation of skeletal progenitors.

Fate mapping of neural crest cells during eye development using a protein 0 promoter-driven transgenic technique

PURPOSE

To map neural crest cell fate during eye development.

METHODS

Neural crest cells were tracked in developing mouse eyes using a transgene expressing Cre recombinase controlled by the Protein 0 promoter and a Rosa26 Cre-responsive reporter gene that produced beta-galactosidase after Cre-mediated recombination.

RESULTS

beta-galactosidase-positive cells were detected in the periocular segment on embryonic day (E) 9.5. Several neural crest cell-derived tissues including corneal stroma, corneal endothelium, iridocorneal angle, ciliary body, primary vitreous and eyelid were strongly stained on E13.5-E18.5. The staining decreased in the corneal stroma after birth, but persisted in the presumptive iridocorneal angle.

CONCLUSIONS

Protein 0-Cre transgenic mice offer a conditional knock-out strategy to investigate anterior eye segment differentiation.

Mice with an anterior cleft of the palate survive neonatal lethality

Many genes are known to function in a region-specific manner in the developing secondary palate. We have previously shown that Shox2-deficient embryos die at mid-gestation stage and develop an anterior clefting phenotype. Here, we show that mice carrying a conditional inactivation of Shox2 in the palatal mesenchyme survive the embryonic and neonatal lethality, but develop a wasting syndrome. Phenotypic analyses indicate a delayed closure of the secondary palate at the anterior end, leading to a failed fusion of the primary and secondary palates. Consistent with a role proposed for Shox2 in skeletogenesis, Shox2 inactivation causes a significantly reduced bone formation in the hard palate, probably due to a down-regulation of Runx2 and Osterix. We conclude that the secondary palatal shelves are capable of fusion with each other, but fail to fuse with the primary palate in a developmentally delayed manner. Mice carrying an anterior cleft can survive neonatal lethality.

Cranial sutures: a brief review

Craniosynostosis, or the premature fusion of one or more cranial sutures, is a relatively common congenital defect that causes a number of morphologic and functional abnormalities. With advances in genetics and molecular biology, research of craniosynostosis has progressed from describing gross abnormalities to understanding the molecular interactions that underlie these cranial deformities. Animal models have been extremely valuable in improving our comprehension of human craniofacial morphogenesis, primarily by human genetic linkage analysis and the development of knock-out animals. This article provides a brief review of perisutural tissue interactions, embryonic origins, signaling molecules and their receptors, and transcription factors in maintaining the delicate balance between proliferation and differentiation of cells within the suture complex that determines suture fate. Finally, this article discusses the potential implications for developing novel therapies for craniosynostosis.

Enamel defects and ameloblast-specific expression in Enam knock-out/lacz knock-in mice

Enamelin is critical for proper dental enamel formation, and defects in the human enamelin gene cause autosomal dominant amelogenesis imperfecta. We used gene targeting to generate a knock-in mouse carrying a null allele of enamelin (Enam) that has a lacZ reporter gene replacing the Enam translation initiation site and gene sequences through exon 7. Correct targeting of the transgene was confirmed by Southern blotting and PCR analyses. No enamelin protein could be detected by Western blotting in the Enam-null mice. Histochemical 5-bromo-4-chloro-3-indolyl-beta-d-galactopyranoside (X-gal) staining demonstrated ameloblast-specific expression of enamelin. The enamel of the Enam(+/-) mice was nearly normal in the maxillary incisors, but the mandibular incisors were discolored and tended to wear rapidly where they contacted the maxillary incisors. The Enam(-/-) mice showed no true enamel. Radiography, microcomputed tomography, and light and scanning electron microscopy were used to document changes in the enamel of Enam(-/-) mice but did not discern any perturbations of bone, dentin, or any other tissue besides the enamel layer. Although a thick layer of enamel proteins covered normal-appearing dentin of unerupted teeth, von Kossa staining revealed almost a complete absence of mineral formation in this protein layer. However, a thin, highly irregular, mineralized crust covered the dentin on erupted teeth, apparently arising from the formation and fusion of small mineralization foci (calcospherites) in the deeper part of the accumulated enamel protein layer. These results demonstrate ameloblast-specific expression of enamelin and reveal that enamelin is essential for proper enamel matrix organization and mineralization.

Epithelial fibroblast growth factor receptor 1 regulates enamel formation

The interaction between epithelial and mesenchymal tissues plays a critical role in the development of organs such as teeth, lungs, and hair. During tooth development, fibroblast growth factor (FGF) signaling is critical for regulating reciprocal epithelial and mesenchymal interactions. FGF signaling requires FGF ligands and their receptors (FGFRs). In this study, we investigated the role of epithelial FGF signaling in tooth development, using the Cre-loxp system to create tissue-specific inactivation of Fgfr1 in mice. In K14-Cre;Fgfr1(fl/fl) mice, the apical sides of enamel-secreting ameloblasts failed to adhere properly to each other, although ameloblast differentiation was unaffected at early stages. Prior to eruption, enamel structure was compromised in the K14-Cre;Fgfr1(fl/fl) mice and displayed severe enamel defects that mimic amelogenesis imperfecta (AI), with a rough, irregular enamel surface. These results suggest that there is a cell-autonomous requirement for FGF signaling in the dental epithelium during enamel formation. Loss of Fgfr1 affects ameloblast organization at the enamel-secretory stage and, hence, the formation of enamel.

Cre reporter mouse expressing a nuclear localized fusion of GFP and beta-galactosidase reveals new derivatives of Pax3-expressing precursors

A new Cre-reporter strain of mouse has been developed that expresses a fusion protein derived from the lacZ gene fused to GFP with a nuclear localization signal. This construct is expressed from the ROSA26 locus upon Cre-mediated recombination that removes a loxP-flanked PGK-neo cassette, thus allowing for detection of Cre activity in all tissues. This reporter strain, which is similar to prior R26R and R26EGFP strains, has certain advantages related to the nuclear expression and the combined expression of both beta-galactosidase and GFP activities. We show that the use of this newly developed reporter line allows for enhanced resolution, detection and co-localization. Thus, we report a previously unrecognized subset of venous endothelial cells derived from Pax3 expressing precursors.

Cytomegalovirus induces abnormal chondrogenesis and osteogenesis during embryonic mandibular development

BACKGROUND

Human clinical studies and mouse models clearly demonstrate that cytomegalovirus (CMV) disrupts normal organ and tissue development. Although CMV is one of the most common causes of major birth defects in humans, little is presently known about the mechanism(s) underlying CMV-induced congenital malformations. Our prior studies have demonstrated that CMV infection of first branchial arch derivatives (salivary glands and teeth) induced severely abnormal phenotypes and that CMV has a particular tropism for neural crest-derived mesenchyme (NCM). Since early embryos are barely susceptible to CMV infection, and the extant evidence suggests that the differentiation program needs to be well underway for embryonic tissues to be susceptible to viral infection and viral-induced pathology, the aim of this study was to determine if first branchial arch NCM cells are susceptible to mCMV infection prior to differentiation of NCM derivatives.

RESULTS

E11 mouse mandibular processes (MANs) were infected with mouse CMV (mCMV) for up to 16 days in vitro. mCMV infection of undifferentiated embryonic mouse MANs induced micrognathia consequent to decreased Meckel's cartilage chondrogenesis and mandibular osteogenesis. Specifically, mCMV infection resulted in aberrant stromal cellularity, a smaller, misshapen Meckel's cartilage, and mandibular bone and condylar dysmorphogenesis. Analysis of viral distribution indicates that mCMV primarily infects NCM cells and derivatives. Initial localization studies indicate that mCMV infection changed the cell-specific expression of FN, NF-kappaB2, RelA, RelB, and Shh and Smad7 proteins.

CONCLUSION

Our results indicate that mCMV dysregulation of key signaling pathways in primarily NCM cells and their derivatives severely disrupts mandibular morphogenesis and skeletogenesis. The pathogenesis appears to be centered around the canonical and noncanonical NF-kappaB pathways, and there is unusual juxtaposition of abnormal stromal cells and surrounding matrix. Moreover, since it is critically important that signaling molecules are expressed in appropriate cell populations during development, the aberrant localization of components of relevant signaling pathways may reveal the pathogenic mechanism underlying mandibular malformations.

Neuroepithelial stem cell proliferation requires LIS1 for precise spindle orientation and symmetric division

Mitotic spindle orientation and plane of cleavage in mammals is a determinant of whether division yields progenitor expansion and/or birth of new neurons during radial glial progenitor cell (RGPC) neurogenesis, but its role earlier in neuroepithelial stem cells is poorly understood. Here we report that Lis1 is essential for precise control of mitotic spindle orientation in both neuroepithelial stem cells and radial glial progenitor cells. Controlled gene deletion of Lis1 in vivo in neuroepithelial stem cells, where cleavage is uniformly vertical and symmetrical, provokes rapid apoptosis of those cells, while radial glial progenitors are less affected. Impaired cortical microtubule capture via loss of cortical dynein causes astral and cortical microtubules to be greatly reduced in Lis1-deficient cells. Increased expression of the LIS/dynein binding partner NDEL1 restores cortical microtubule and dynein localization in Lis1-deficient cells. Thus, control of symmetric division, essential for neuroepithelial stem cell proliferation, is mediated through spindle orientation determined via LIS1/NDEL1/dynein-mediated cortical microtubule capture.

A highly enriched niche of precursor cells with neuronal and glial potential within the hair follicle dermal papilla of adult skin

Skin-derived precursor cells (SKPs) are multipotent neural crest-related stem cells that grow as self-renewing spheres and are capable of generating neurons and myelinating glial cells. SKPs are of clinical interest because they are accessible and potentially autologous. However, although spheres can be readily isolated from embryonic and neonatal skin, SKP frequency falls away sharply in adulthood, and primary sphere generation from adult human skin is more problematic. In addition, the culture-initiating cell population is undefined and heterogeneous, limiting experimental studies addressing important aspects of these cells such as the behavior of endogenous precursors in vivo and the molecular mechanisms of neural generation. Using a combined fate-mapping and microdissection approach, we identified and characterized a highly enriched niche of neural crest-derived sphere-forming cells within the dermal papilla of the hair follicle of adult skin. We demonstrated that the dermal papilla of the rodent vibrissal follicle is 1,000-fold enriched for sphere-forming neural crest-derived cells compared with whole facial skin. These "papillaspheres" share a phenotypic and developmental profile similar to that of SKPs, can be readily expanded in vitro, and are able to generate both neuronal and glial cells in response to appropriate cues. We demonstrate that papillaspheres can be efficiently generated and expanded from adult human facial skin by microdissection of a single hair follicle. This strategy of targeting a highly enriched niche of sphere-forming cells provides a novel and efficient method for generating neuronal and glial cells from an accessible adult somatic source that is both defined and minimally invasive.

Cranial suture biology and dental development: genetic and clinical perspectives

Premature fusion of the calvarial bones at the sutures, or craniosynostosis (CS), is a relatively common birth defect (1:2000-3000) frequently associated with limb deformity. Patients with CS may present oral defects, such as cleft soft palate, hypodontia, hyperdontia, and delayed tooth eruption, but also unusual associations of major dental anomalies such as taurodontism, microdontia, multiple dens invaginatus, and dentin dysplasia. The list of genes that are involved in CS includes those coding for the different fibroblast growth factor receptors and a ligand of ephrin receptors, but also genes encoding transcription factors, such as MSX2 and TWIST. Most of these genes are equally involved in odontogenesis, providing a pausible explanation for clinical associations of CS with dental agenesis or tooth malformations. On the basis of the present knowledge on genes and transcription factors that are involved in craniofacial morphogenesis, and from dental clinics of CS syndromes, the molecular mechanisms that control suture formation and suture closure are expected to play key roles in patterning events and development of teeth. The purpose of this article is to review and merge the recent advances in the field of suture research at the genetic and cellular levels with those of tooth development, and to apply them to the dental clinics of CS syndromes. These new perspectives and future challenges in the field of both dental clinics and molecular genetics, more in particular the identification of possible candidate genes involved in both CS and dental defects, are discussed.

Smad4 is required to regulate the fate of cranial neural crest cells

Smad4 is the central mediator for TGF-beta/BMP signals, which are involved in regulating cranial neural crest (CNC) cell formation, migration, proliferation and fate determination. It is unclear whether TGF-beta/BMP signals utilize Smad-dependent or -independent pathways to control the development of CNC cells. To investigate the functional significance of Smad4 in regulating CNC cells, we generated mice with neural crest specific inactivation of the Smad4 gene. Our study shows that Smad4 is not required for the migration of CNC cells, but is required in neural crest cells for the development of the cardiac outflow tract. Smad4 is essential in mediating BMP signaling in the CNC-derived ectomesenchyme during early stages of tooth development because conditional inactivation of Smad4 in neural crest derived cells results in incisor and molar development arrested at the dental lamina stage. Furthermore, Smad-mediated TGF-beta/BMP signaling controls the homeobox gene patterning of oral/aboral and proximal/distal domains within the first branchial arch. At the cellular level, a Smad4-mediated downstream target gene(s) is required for the survival of CNC cells in the proximal domain of the first branchial arch. Smad4 mutant mice show underdevelopment of the first branchial arch and midline fusion defects. Taken together, our data show that TGF-beta/BMP signals rely on Smad-dependent pathways in the ectomesenchyme to mediate epithelial-mesenchymal interactions that control craniofacial organogenesis.

The proliferating field of neural crest stem cells

Neural crest stem cells were first isolated from early embryonic neural crest in the early 1990s, but in the past 5 years, there has been a burst of discoveries of neural crest-derived stem cells from diverse locations. Here, we summarize these data, highlighting the characteristics of each stem cell type. These cells vary widely in the markers they express and the variety of cell types they appear to generate. They occupy diverse locations, but in some cases multiple stem cell types apparently occupy physically proximate niches. To date, few molecular similarities can be identified between these stem cells, although a systematic comparison is required. We note other issues worthy of attention, including aspects of the in vivo behavior of these stem cells, their niches, and their lineage relationships. Together, analysis of these issues will clarify this expanding, but still young, field and contribute to exploration of the important therapeutic potential of these cells.

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.

Retrograde BMP signaling regulates trigeminal sensory neuron identities and the formation of precise face maps

Somatosensory information from the face is transmitted to the brain by trigeminal sensory neurons. It was previously unknown whether neurons innervating distinct areas of the face possess molecular differences. We have identified a set of genes differentially expressed along the dorsoventral axis of the embryonic mouse trigeminal ganglion and thus can be considered trigeminal positional identity markers. Interestingly, establishing some of the spatial patterns requires signals from the developing face. We identified bone morphogenetic protein 4 (BMP4) as one of these target-derived factors and showed that spatially defined retrograde BMP signaling controls the differential gene expressions in trigeminal neurons through both Smad4-independent and Smad4-dependent pathways. Mice lacking one of the BMP4-regulated transcription factors, Onecut2 (OC2), have defects in the trigeminal central projections representing the whiskers. Our results provide molecular evidence for both spatial patterning and retrograde regulation of gene expression in sensory neurons during the development of the somatosensory map.

Bone voyage: an expedition into the molecular and cellular parameters affecting bone graft fate

The demand for bone grafts in orthopaedic and craniofacial surgery is steadily increasing. Estimations suggest that about 500,000 are performed annually in the United States that include bone grafting as a component of the surgery, and the majority of these surgeries employ autografts. This perspective focuses on the biological events that occur during osseointegration of such bone grafts. Here, three key factors of graft osseointegration--the embryonic origin, the inclusion of skeletal progenitor cells, and the integrity of the recipient site--are discussed. Altogether, they form the foundation for survival of the bone graft and eventually for a positive clinical outcome of the procedure.

TGF-beta mediated Msx2 expression controls occipital somites-derived caudal region of skull development

Craniofacial development involves cranial neural crest (CNC) and mesoderm-derived cells. TGF-beta signaling plays a critical role in instructing CNC cells to form the craniofacial skeleton. However, it is not known how TGF-beta signaling regulates the fate of mesoderm-derived cells during craniofacial development. In this study, we show that occipital somites contribute to the caudal region of mammalian skull development. Conditional inactivation of Tgfbr2 in mesoderm-derived cells results in defects of the supraoccipital bone with meningoencephalocele and discontinuity of the neural arch of the C1 vertebra. At the cellular level, loss of TGF-beta signaling causes decreased chondrocyte proliferation and premature differentiation of cartilage to bone. Expression of Msx2, a critical factor in the formation of the dorsoventral axis, is diminished in the Tgfbr2 mutant. Significantly, overexpression of Msx2 in Myf5-Cre;Tgfbr2flox/flox mice partially rescues supraoccipital bone development. These results suggest that the TGF-beta/Msx2 signaling cascade is critical for development of the caudal region of the skull.

A unique mouse strain expressing Cre recombinase for tissue-specific analysis of gene function in palate and kidney development

Mammalian palate development is a multistep process, involving initial bilateral downward outgrowth of the palatal shelves from the oral side of the maxillary processes, followed by stage-specific palatal shelf elevation to the horizontal position above the developing tongue and subsequent fusion of the bilateral palatal shelves at the midline to form the intact roof of the oral cavity. While mutations in many genes have been associated with cleft palate pathogenesis, the molecular mechanisms regulating palatal shelf growth, patterning, and elevation are not well understood. Genetic studies of the molecular mechanisms controlling palate development in mutant mouse models are often complicated by early embryonic lethality or gross craniofacial malformation. We report here the development of a mouse strain for tissue-specific analysis of gene function in palate development. We inserted an IresCre bicistronic expression cassette into the 3' untranslated region of the mouse Osr2 gene through gene targeting. We show, upon crossing to the R26R reporter mice, that Cre expression from the Osr2(IresCre) knockin allele activated beta-galactosidase expression specifically throughout the developing palatal mesenchyme from the onset of palatal shelf outgrowth. In addition, the Osr2(IresCre) mice display exclusive Cre-mediated recombination in the glomeruli tissues derived from the metanephric mesenchyme and complete absence of Cre activity in other epithelial and mesenchymal tissues in the developing metanephric kidney. These data indicate that the Osr2(IresCre) knockin mice provide a unique tool for tissue-specific studies of the molecular mechanisms regulating palate and kidney development.

Hand transcription factors cooperatively regulate development of the distal midline mesenchyme

Hand proteins are evolutionally conserved basic helix-loop-helix (bHLH) transcription factors implicated in development of neural crest-derived tissues, heart and limb. Hand1 is expressed in the distal (ventral) zone of the branchial arches, whereas the Hand2 expression domain extends ventrolaterally to occupy two-thirds of the mandibular arch. To circumvent the early embryonic lethality of Hand1 or Hand2-null embryos and to examine their roles in neural crest development, we generated mice with neural crest-specific deletion of Hand1 and various combinations of mutant alleles of Hand2. Ablation of Hand1 alone in neural crest cells did not affect embryonic development, however, further removing one Hand2 allele or deleting the ventrolateral branchial arch expression of Hand2 led to a novel phenotype presumably due to impaired growth of the distal midline mesenchyme. Although we failed to detect changes in proliferation or apoptosis between the distal mandibular arch of wild-type and Hand1/Hand2 compound mutants at embryonic day (E)10.5, dysregulation of Pax9, Msx2 and Prx2 was observed in the distal mesenchyme at E12.5. In addition, the inter-dental mesenchyme and distal symphysis of Meckel's cartilage became hypoplastic, resulting in the formation of a single fused lower incisor within the hypoplastic fused mandible. These findings demonstrate the importance of Hand transcription factors in the transcriptional circuitry of craniofacial and tooth development.

Neural crest cells and patterning of the mammalian dentition

The mammalian dentition is composed of serial groups of teeth, each with a distinctive crown and root morphology, highly adapted to its particular masticatory function. In the embryo, generation of individual teeth within the jaws relies upon interactions between ectoderm of the first branchial arch and the neural crest-derived ectomesenchymal cells that migrate into this region from their site of origin along the neural axis. Classic tissue recombination experiments have provided evidence of an essential role of the ectoderm in initiating tooth development; however, the underlying ectomesenchyme rapidly acquires dominance in establishing shape. A key question is how these cells acquire this positional information. One theory suggests that ectomesenchymal cells are pre-patterned with respect to shape generation. Alternatively, this cell population acquires positional information within the first branchial arch itself, following migration. Recent molecular evidence suggests a high degree of plasticity within these ectomesenchymal cells. In particular, signalling molecules within the ectoderm exert a time-dependent influence upon the ectomesenchyme by establishing specific domains of homeobox gene expression. Initially, these ectomesenchymal cells are plastic and able to respond to signalling from the ectoderm, however, this plasticity is rapidly lost and pattern information becomes fixed. Therefore, in the first branchial arch, local regulation between the ectoderm and neural crest-derived ectomesenchyme is crucial in establishing the appropriate tooth shape in the correct region of the jaw.

Tooth regeneration: implications for the use of bioengineered organs in first-wave organ replacement

Experiments with animal models have shown that the tooth crown structure can be regenerated using tissue engineering techniques that combine tooth bud cells and biodegradable materials, or by using embryonic tissue and adult stem cells. Moreover, tooth roots and periodontal tissues have been reconstructed by grafting dental stem cells, which leads to the recovery of tooth function, suggesting that tooth regeneration will become possible in humans in the near future. The present article reviews current research on tooth regeneration, discusses a model of tooth replacement that could be used clinically, and proposes a new tooth regeneration approach that overcomes the difficulties associated with the tooth replacement model. Tooth regeneration is an important stepping stone in the establishment of engineered organ transplantation, which is one of the ultimate goals of regenerative therapies.

Kinesin-2 controls development and patterning of the vertebrate skeleton by Hedgehog- and Gli3-dependent mechanisms

Hedgehog signaling plays an essential role in patterning of the vertebrate skeleton. Here we demonstrate that conditional inactivation of the Kif3a subunit of the kinesin-2 intraflagellar transport motor in mesenchymal skeletal progenitor cells results in severe patterning defects in the craniofacial area, the formation of split sternum and the development of polydactyly. These deformities are reminiscent of those previously described in mice with deregulated hedgehog signaling. We show that in Kif3a-deficient mesenchymal tissues both the repressor function of Gli3 transcription factor and the activation of the Shh transcriptional targets Ptch and Gli1 are compromised. Quantitative analysis of gene expression demonstrates that the Gli1 transcript level is dramatically reduced, whereas Gli3 expression is not significantly affected by kinesin-2 depletion. However, the motor appears to be required for the efficient cleavage of the full-length Gli3 transcription factor into a repressor form.

Resident endothelial precursors in muscle, adipose, and dermis contribute to postnatal vasculogenesis

A novel population of tissue-resident endothelial precursors (TEPs) was isolated from small blood vessels in dermal, adipose, and skeletal muscle of mouse based on their ability to be grown as spheres. Cellular and molecular analyses of these cells revealed that they were highly related regardless of the tissue of origin and distinct from embryonic neural stem cells. Notably, TEPs did not express hematopoietic markers, but they expressed numerous characteristics of angiogenic precursors and their differentiated progeny, such as CD34, Flk-1, Tie-1, CD31, and vascular endothelial cadherin (VE-cadherin). TEPs readily differentiated into endothelial cells in newly formed vascular networks following transplantation into regenerating skeletal muscle. Taken together, these experiments suggest that TEPs represent a novel class of endothelial precursors that are closely associated with small blood vessels in muscle, adipose, and dermal tissue. This finding is of particular interest since it could bring new insight in cancer angiogenesis and collateral blood vessels developed following ischemia. Disclosure of potential conflicts of interest is found at the end of this article.

Concerted action of Msx1 and Msx2 in regulating cranial neural crest cell differentiation during frontal bone development

The homeobox genes Msx1 and Msx2 function as transcriptional regulators that control cellular proliferation and differentiation during embryonic development. Mutations in the Msx1 and Msx2 genes in mice disrupt tissue-tissue interactions and cause multiple craniofacial malformations. Although Msx1 and Msx2 are both expressed throughout the entire development of the frontal bone, the frontal bone defect in Msx1 or Msx2 null mutants is rather mild, suggesting the possibility of functional compensation between Msx1 and Msx2 during early frontal bone development. To investigate this hypothesis, we generated Msx1(-/-);Msx2(-/-) mice. These double mutant embryos died at E17 to E18 with no formation of the frontal bone. There was no apparent defect in CNC migration into the presumptive frontal bone primordium, but differentiation of the frontal mesenchyme and establishment of the frontal primordium was defective, indicating that Msx1 and Msx2 genes are specifically required for osteogenesis in the cranial neural crest lineage within the frontal bone primordium. Mechanistically, our data suggest that Msx genes are critical for the expression of Runx2 in the frontonasal subpopulation of cranial neural crest cells and for differentiation of the osteogenic lineage. This early function of the Msx genes is likely independent of the Bmp signaling pathway.