Tissue origins and interactions in the mammalian skull vault

Pax7 lineage contributions to the mammalian neural crest

Background

Neural crest cells are vertebrate-specific multipotent cells that contribute to a variety of tissues including the peripheral nervous system, melanocytes, and craniofacial bones and cartilage. Abnormal development of the neural crest is associated with several human maladies including cleft/lip palate, aggressive cancers such as melanoma and neuroblastoma, and rare syndromes, like Waardenburg syndrome, a complex disorder involving hearing loss and pigment defects. We previously identified the transcription factor Pax7 as an early marker, and required component for neural crest development in chick embryos. In mammals, Pax7 is also thought to play a role in neural crest development, yet the precise contribution of Pax7 progenitors to the neural crest lineage has not been determined.

Methodology/Principal Findings

Here we use Cre/loxP technology in double transgenic mice to fate map the Pax7 lineage in neural crest derivates. We find that Pax7 descendants contribute to multiple tissues including the cranial, cardiac and trunk neural crest, which in the cranial cartilage form a distinct regional pattern. The Pax7 lineage, like the Pax3 lineage, is additionally detected in some non-neural crest tissues, including a subset of the epithelial cells in specific organs.

Conclusions/Significance

These results demonstrate a previously unappreciated widespread distribution of Pax7 descendants within and beyond the neural crest. They shed light regarding the regionally distinct phenotypes observed in Pax3 and Pax7 mutants, and provide a unique perspective into the potential roles of Pax7 during disease and development.

Wnt5a can both activate and repress Wnt/β-catenin signaling during mouse embryonic development

Embryonic development is controlled by a small set of signal transduction pathways, with vastly different phenotypic outcomes depending on the time and place of their recruitment. How the same molecular machinery can elicit such specific and distinct responses, remains one of the outstanding questions in developmental biology. Part of the answer may lie in the high inherent genetic complexity of these signaling cascades, as observed for the Wnt-pathway. The mammalian genome encodes multiple Wnt proteins and receptors, each of which show dynamic and tightly controlled expression patterns in the embryo. Yet how these components interact in the context of the whole organism remains unknown. Here we report the generation of a novel, inducible transgenic mouse model that allows spatiotemporal control over the expression of Wnt5a, a protein implicated in many developmental processes and multiple Wnt-signaling responses. We show that ectopic Wnt5a expression from E10.5 onwards results in a variety of developmental defects, including loss of hair follicles and reduced bone formation in the skull. Moreover, we find that Wnt5a can have dual signaling activities during mouse embryonic development. Specifically, Wnt5a is capable of both inducing and repressing β-catenin/TCF signaling in vivo, depending on the time and site of expression and the receptors expressed by receiving cells. These experiments show for the first time that a single mammalian Wnt protein can have multiple signaling activities in vivo, thereby furthering our understanding of how signaling specificity is achieved in a complex developmental context.

Deletion of a conserved regulatory element required for Hmx1 expression in craniofacial mesenchyme in the dumbo rat: a newly identified cause of congenital ear malformation

Hmx1 is a homeodomain transcription factor expressed in the developing eye, peripheral ganglia, and branchial arches of avian and mammalian embryos. Recent studies have identified a loss-of-function allele at the HMX1 locus as the causative mutation in the oculo-auricular syndrome (OAS) in humans, characterized by ear and eye malformations. The mouse dumbo (dmbo) mutation, with similar effects on ear and eye development, also results from a loss-of-function mutation in the Hmx1 gene. A recessive dmbo mutation causing ear malformation in rats has been mapped to the chromosomal region containing the Hmx1 gene, but the nature of the causative allele is unknown. Here we show that dumbo rats and mice exhibit similar neonatal ear and eye phenotypes. In midgestation embryos, dumbo rats show a specific loss of Hmx1 expression in neural-crest-derived craniofacial mesenchyme (CM), whereas Hmx1 is expressed normally in retinal progenitors, sensory ganglia and in CM, which is derived from mesoderm. High-throughput resequencing of 1 Mb of rat chromosome 14 from dmbo/dmbo rats, encompassing the Hmx1 locus, reveals numerous divergences from the rat genomic reference sequence, but no coding changes in Hmx1. Fine genetic mapping narrows the dmbo critical region to an interval of ∼410 kb immediately downstream of the Hmx1 transcription unit. Further sequence analysis of this region reveals a 5777-bp deletion located ∼80 kb downstream in dmbo/dmbo rats that is not apparent in 137 other rat strains. The dmbo deletion region contains a highly conserved domain of ∼500 bp, which is a candidate distal enhancer and which exhibits a similar relationship to Hmx genes in all vertebrate species for which data are available. We conclude that the rat dumbo phenotype is likely to result from loss of function of an ultraconserved enhancer specifically regulating Hmx1 expression in neural-crest-derived CM. Dysregulation of Hmx1 expression is thus a candidate mechanism for congenital ear malformation, most cases of which remain unexplained.

Comparison of the relative occipital bone volume between Cavalier King Charles spaniels with and without syringohydromyelia and French bulldogs

Our aim was to determine the relative volume of the occipital bone of Cavalier King Charles spaniels with and without syringohydromyelia and normal French bulldogs to reappraise the role of a possible insufficiency of the paraxial mesoderm in the pathogenesis of the caudal occipital malformation syndrome. Analysis of the occipital bone volume of 43 dogs based on computed tomography datasets was performed. Volume was determined by means of three-dimensional models. Using manual segmentation of the occipital bone in sagittal, transverse, and dorsal images, the volume of the occipital bone and the rest of the skull was calculated. The absolute occipital bone volume was put in relation to the total skull volume, the occipital bone index. For the Cavalier King Charles spaniels without syringomyelia, the median occipital bone index was 0.0681; for Cavalier King Charles spaniels with syringomyelia, it was 0.0646 and for French bulldogs, it was 0.0676. There was no global difference of the occipital bone index between examined groups (P = 0.4331). A reduced volume of the occipital bone was not found in Cavalier King Charles spaniels in general in comparison to French bulldogs, or in Cavalier King Charles spaniels with syringomyelia compared to Cavalier King Charles spaniels without syringomyelia. These results do not support occipital hypoplasia as a cause for syringomyelia development, challenging the paraxial mesoderm insufficiency theory. This also suggests that the term Chiari-like malformation, a term derived from human studies, is not appropriate in the Cavalier King Charles spaniel.

Mesodermal expression of Fgfr2S252W is necessary and sufficient to induce craniosynostosis in a mouse model of Apert syndrome

Coordinated growth of the skull and brain are vital to normal human development. Craniosynostosis, the premature fusion of the calvarial bones of the skull, is a relatively common pediatric disease, occurring in 1 in 2500 births, and requires significant surgical management, especially in syndromic cases. Syndromic craniosynostosis is caused by a variety of genetic lesions, most commonly by activating mutations of FGFRs 1-3, and inactivating mutations of TWIST1. In a mouse model of TWIST1 haploinsufficiency, cell mixing between the neural crest-derived frontal bone and mesoderm-derived parietal bone accompanies coronal suture fusion during embryonic development. However, the relevance of lineage mixing in craniosynostosis induced by activating FGFR mutations is unknown. Here, we demonstrate a novel mechanism of suture fusion in the Apert Fgfr2(S252W) mouse model. Using Cre/lox recombination we simultaneously induce expression of Fgfr2(S252W) and β-galactosidase in either the neural crest or mesoderm of the skull. We show that mutation of the mesoderm alone is necessary and sufficient to cause craniosynostosis, while mutation of the neural crest is neither. The lineage border is not disrupted by aberrant cell migration during fusion. Instead, the suture mesenchyme itself remains intact and is induced to undergo osteogenesis. We eliminate postulated roles for dura mater or skull base changes in craniosynostosis. The viability of conditionally mutant mice also allows post-natal assessment of other aspects of Apert syndrome.

The BMP ligand Gdf6 prevents differentiation of coronal suture mesenchyme in early cranial development

Growth Differentiation Factor-6 (Gdf6) is a member of the Bone Morphogenetic Protein (BMP) family of secreted signaling molecules. Previous studies have shown that Gdf6 plays a role in formation of a diverse subset of skeletal joints. In mice, loss of Gdf6 results in fusion of the coronal suture, the intramembranous joint that separates the frontal and parietal bones. Although the role of GDFs in the development of cartilaginous limb joints has been studied, limb joints are developmentally quite distinct from cranial sutures and how Gdf6 controls suture formation has remained unclear. In this study we show that coronal suture fusion in the Gdf6-/- mouse is due to accelerated differentiation of suture mesenchyme, prior to the onset of calvarial ossification. Gdf6 is expressed in the mouse frontal bone primordia from embryonic day (E) 10.5 through 12.5. In the Gdf6-/- embryo, the coronal suture fuses prematurely and concurrently with the initiation of osteogenesis in the cranial bones. Alkaline phosphatase (ALP) activity and Runx2 expression assays both showed that the suture width is reduced in Gdf6+/- embryos and is completely absent in Gdf6-/- embryos by E12.5. ALP activity is also increased in the suture mesenchyme of Gdf6+/- embryos compared to wild-type. This suggests Gdf6 delays differentiation of the mesenchyme occupying the suture, prior to the onset of ossification. Therefore, although BMPs are known to promote bone formation, Gdf6 plays an inhibitory role to prevent the osteogenic differentiation of the coronal suture mesenchyme.

Regulation of cranial morphogenesis and cell fate at the neural crest-mesoderm boundary by engrailed 1

The characterization of mesenchymal progenitors is central to understanding development, postnatal pathology and evolutionary adaptability. The precise identity of the mesenchymal precursors that generate the coronal suture, an important structural boundary in mammalian skull development, remains unclear. We show in mouse that coronal suture progenitors originate from hedgehog-responsive cephalic paraxial mesoderm (Mes) cells, which migrate rapidly to a supraorbital domain and establish a unidirectional lineage boundary with neural crest (NeuC) mesenchyme. Lineage tracing reveals clonal and stereotypical expansion of supraorbital mesenchymal cells to form the coronal suture between E11.0 and E13.5. We identify engrailed 1 (En1) as a necessary regulator of cell movement and NeuC/Mes lineage boundary positioning during coronal suture formation. In addition, we provide genetic evidence that En1 functions upstream of fibroblast growth factor receptor 2 (Fgfr2) in regulating early calvarial osteogenic differentiation, and postulate that it plays an additional role in precluding premature osteogenic conversion of the sutural mesenchyme.

Loss-of-Function of Gli3 in Mice Causes Abnormal Frontal Bone Morphology and Premature Synostosis of the Interfrontal Suture

Greig cephalopolysyndactyly syndrome (GCPS) is an autosomal dominant disorder with polydactyly and syndactyly of the limbs and a broad spectrum of craniofacial abnormalities. Craniosynostosis of the metopic suture (interfrontal suture in mice) is an important but rare feature associated with GCPS. GCPS is caused by mutations in the transcription factor GLI3, which regulates Hedgehog signaling. The Gli3 loss-of-function (Gli3^Xt-J/Xt-J) mouse largely phenocopies the human syndrome with the mice exhibiting polydactyly and several craniofacial abnormalities. Here we show that Gli3^Xt-J/Xt-J mice exhibit ectopic ossification in the interfrontal suture and in the most severe cases the suture fuses already prior to birth. We show that abnormalities in frontal bones occur early in calvarial development, before the establishment of the interfrontal suture. It provides a model for the metopic suture pathology that can occur in GCPS.

Human neural crest stem cells derived from human ESCs and induced pluripotent stem cells: induction, maintenance, and differentiation into functional schwann cells

The neural crest (NC) is a transient, multipotent, migratory cell population unique to vertebrates that gives rise to diverse cell lineages. Much of our knowledge of NC development comes from studies of organisms such as chicken and zebrafish because human NC is difficult to obtain because of its transient nature and the limited availability of human fetal cells. Here we examined the process of NC induction from human pluripotent stem cells, including human embryonic stem cells (hESCs) and induced pluripotent stem cells (iPSCs). We showed that NC cells could be efficiently induced from hESCs by a combination of growth factors in medium conditioned on stromal cells and that NC stem cells (NCSCs) could be purified by p75 using fluorescence-activated cell sorting (FACS). FACS-isolated NCSCs could be propagated in vitro in five passages and cryopreserved while maintaining NCSC identity characterized by the expression of a panel of NC markers such as p75, Sox9, Sox10, CD44, and HNK1. In vitro-expanded NCSCs were able to differentiate into neurons and glia (Schwann cells) of the peripheral nervous system, as well as mesenchymal derivatives. hESC-derived NCSCs appeared to behave similarly to endogenous embryonic NC cells when injected in chicken embryos. Using a defined medium, we were able to generate and propagate a nearly pure population of Schwann cells that uniformly expressed glial fibrillary acidic protein, S100, and p75. Schwann cells generated by our protocol myelinated rat dorsal root ganglia neurons in vitro. To our knowledge, this is the first report on myelination by hESC- or iPSC-derived Schwann cells.

Sensory nerve induced inflammation contributes to heterotopic ossification

Heterotopic ossification (HO), or bone formation in soft tissues, is often the result of traumatic injury. Much evidence has linked the release of BMPs (bone morphogenetic proteins) upon injury to this process. HO was once thought to be a rare occurrence, but recent statistics from the military suggest that as many as 60% of traumatic injuries, resulting from bomb blasts, have associated HO. In this study, we attempt to define the role of peripheral nerves in this process. Since BMP2 has been shown previously to induce release of the neuroinflammatory molecules, substance P (SP) and calcitonin gene related peptide (CGRP), from peripheral, sensory neurons, we examined this process in vivo. SP and CGRP are rapidly expressed upon delivery of BMP2 and remain elevated throughout bone formation. In animals lacking functional sensory neurons (TRPV1(-/-) ), BMP2-mediated increases in SP and CGRP were suppressed as compared to the normal animals, and HO was dramatically inhibited in these deficient mice, suggesting that neuroinflammation plays a functional role. Mast cells, known to be recruited by SP and CGRP, were elevated after BMP2 induction. These mast cells were localized to the nerve structures and underwent degranulation. When degranulation was inhibited using cromolyn, HO was again reduced significantly. Immunohistochemical analysis revealed nerves expressing the stem cell markers nanog and Klf4, as well as the osteoblast marker osterix, after BMP2 induction, in mice treated with cromolyn. The data collectively suggest that BMP2 can act directly on sensory neurons to induce neurogenic inflammation, resulting in nerve remodeling and the migration/release of osteogenic and other stem cells from the nerve. Further, blocking this process significantly reduces HO, suggesting that the stem cell population contributes to bone formation.

Human epidermal neural crest stem cells (hEPI-NCSC)--characterization and directed differentiation into osteocytes and melanocytes

Here we describe the isolation, characterisation and ex-vivo expansion of human epidermal neural crest stem cells (hEPI-NCSC) and we provide protocols for their directed differentiation into osteocytes and melanocytes. hEPI-NCSC are neural crest-derived multipotent stem cells that persist into adulthood in the bulge of hair follicles. Multipotency and self-renewal were determined by in vitro clonal analyses. hEPI-NCSC generate all major neural crest derivatives, including bone/cartilage cells, neurons, Schwann cells, myofibroblasts and melanocytes. Furthermore, hEPI-NCSC express additional neural crest stem cell markers and global stem cell genes. To variable degrees and in a donor-dependent manner, hEPI-NCSC express the six essential pluripotency genes C-MYC, KLF4, SOX2, LIN28, OCT-4/POU5F1 and NANOG. hEPI-NCSC can be expanded ex vivo into millions of stem cells that remain mulitpotent and continue to express stem cell genes. The novelty of hEPI-NCSC lies in the combination of their highly desirable traits. hEPI-NCSC are embryonic remnants in a postnatal location, the bulge of hair follicles. Therefore they are readily accessible in the hairy skin by minimal invasive procedure. hEPI-NCSC are multipotent somatic stem cells that can be isolated reproducibly and with high yield. By taking advantage of their migratory ability, hEPI-NCSC can be isolated as a highly pure population of stem cells. hEPI-NCSC can undergo robust ex vivo expansion and directed differentiation. As somatic stem cells, hEPI-NCSC are conducive to autologous transplantation, which avoids graft rejection. Together, these traits make hEPI-NCSC novel and attractive candidates for future cell-based therapies and regenerative medicine.

Neural crest progenitors and stem cells: from early development to adulthood

In the vertebrate embryo, the neural crest forms transiently in the dorsal neural primordium to yield migratory cells that will invade nearly all tissues and later, will differentiate into bones and cartilages, neurons and glia, endocrine cells, vascular smooth muscle cells and melanocytes. Due to the amazingly diversified array of cell types it produces, the neural crest is an attractive model system in the stem cell field. We present here in vivo and in vitro studies of single cell fate, which led to the discovery and the characterization of stem cells in the neural crest of avian and mammalian embryos. Some of the key issues in neural crest cell diversification are discussed, such as the time of segregation of mesenchymal vs. neural/melanocytic lineages, and the origin and close relationships between the glial and melanocytic lineages. An overview is also provided of the diverse types of neural crest-like stem cells and progenitors, recently identified in a growing number of adult tissues in animals and humans. Current and future work, in which in vivo lineage studies and the use of injury models will complement the in vitro culture analysis, should help in unraveling the properties and function of neural crest-derived progenitors in development and disease.

Stem cell interactions in a bone marrow niche

Stem cells differ from other cells of the body in their potential for multilineage differentiation and their continued proliferation without substantial loss of potential (so-called self-renewal). These properties are maintained and regulated by a specific microenvironment referred to as "niche." This term has been used to indicate the specific location of stem cells within tissues, as well as the cellular and molecular components that critically determine stem cell behavior. Whereas other, perhaps less complex, stem cell niches (e.g., Drosophila germarium) have been more clearly dissected in the 30 years that have passed since these observations, the hematopoietic stem cell (HSC) niche has proven challenging due to the difficulty to detect HSCs under normal conditions and the dynamism of HSCs and other cells of the bone marrow that influence HSC behavior. This article reviews the recent development of the HSC niche field with emphasis on prospective integrative mechanisms within bone marrow homeostasis and multisystem physiology. For that purpose, we will first highlight anatomical and histological features of the bone marrow of relevance for HSC behavior; then, we will summarize the principal findings concerning different cell types and potential mechanisms by which they critically regulate HSC function.

Dura mater stimulates human adipose-derived stromal cells to undergo bone formation in mouse calvarial defects

Human adipose-derived stromal cells (hASCs) have a proven capacity to aid in osseous repair of calvarial defects. However, the bone defect microenvironment necessary for osseous healing is not fully understood. In this study, we postulated that the cell-cell interaction between engrafted ASCs and host dura mater (DM) cells is critical for the healing of calvarial defects. hASCs were engrafted into critical sized calvarial mouse defects. The DM-hASC interaction was manipulated surgically by DM removal or by insertion of a semipermeable or nonpermeable membrane between DM and hASCs. Radiographic, histologic, and gene expression analyses were performed. Next, the hASC-DM interaction is assessed by conditioned media (CM) and coculture assays. Finally, bone morphogenetic protein (BMP) signaling from DM was investigated in vivo using novel BMP-2 and anti-BMP-2/4 slow releasing scaffolds. With intact DM, osseous healing occurs both from host DM and engrafted hASCs. Interference with the DM-hASC interaction dramatically reduced calvarial healing with abrogated BMP-2-Smad-1/5 signaling. Using CM and coculture assays, mouse DM cells stimulated hASC osteogenesis via BMP signaling. Through in vivo manipulation of the BMP-2 pathway, we found that BMP-2 plays an important role in DM stimulation of hASC osteogenesis in the context of calvarial bone healing. BMP-2 supplementation to a defect with disrupted DM allowed for bone formation in a nonhealing defect. DM is an osteogenic cell type that both participates in and stimulates osseous healing in a hASC-engrafted calvarial defect. Furthermore, DM-derived BMP-2 paracrine stimulation appears to play a key role for hASC mediated repair.

Differential expression of extracellular matrix-mediated pathways in single-suture craniosynostosis

Craniosynostosis is a disease defined by premature fusion of one or more cranial sutures. The mechanistic pathology of single-suture craniosynostosis is complex and while a number of genetic biomarkers and environmental predispositions have been identified, in many cases the causes remain controversial and inconclusive. In this study, gene expression data from 199 patients with isolated sagittal (n = 100), unilateral coronal (n = 50), and metopic (n = 49) synostosis are compared against both a control population (n = 50), as well as each other. After controlling for variables contributing to potential bias, FGF7, SFRP4, and VCAM1 emerged as genes associated with single-suture craniosynostosis due to their significantly large changes in gene expression compared to the control population. Pathway analysis implicated focal adhesion and extracellular matrix (ECM)-receptor interaction as differentially regulated gene networks when comparing all cases of single-suture synostosis and controls. Lastly, overall gene expression was found to be highly conserved between coronal and metopic cases, as evidenced by the fact that WNT2 and IGFBP2 were the only genes differentially regulated to a significantly large extent in a direct comparison. The identification of genes and gene networks associated with Fgf/Igf/Wnt signaling and ECM-mediated focal adhesion not only support the involvement of biomarkers previously reported to be related to craniosynostosis, but also introduce novel transcripts and pathways that may play critical roles in its pathogenesis.

Disintegration of the medial epithelial seam: is cell death important in palatogenesis?

During palatogenesis, the palatal medial edge epithelium (MEE) forms the medial epithelial seam (MES) on adhesion of the opposing palatal shelves. The MES eventually disappears, leading to mesenchymal confluence of the palate and completion of palatogenesis. Failure of these processes results in cleft palate, one of the most common congenital anomalies in human affecting around one case in 500-2500 live births. The cell fate of MEE has been controversial for more than 20 years. Recent studies suggest that the disappearance of MES is a complex process involving cell death, epithelial-mesenchymal transition (EMT) and epithelial migration. Interestingly, transforming growth factor-β3 (Tgf β3) expression in MEE and the tip epithelium of the nasal septum begins just before palatal shelf reorientation and lasts until MES disruption, and several works including targeted disruption of the gene have indicated that the process appears to be regulated mainly by the TGFβ3-TGFβR signaling. However, how MEE cells choose their fate and how the cell fate is altered in response to cellular environment remains to be elucidated.

Craniosynostosis of coronal suture in twist1 mice occurs through endochondral ossification recapitulating the physiological closure of posterior frontal suture

Craniosynostosis, the premature closure of cranial suture, is a pathologic condition that affects 1/2000 live births. Saethre-Chotzen syndrome is a genetic condition characterized by craniosynostosis. The Saethre-Chotzen syndrome, which is defined by loss-of-function mutations in the TWIST gene, is the second most prevalent craniosynostosis. Although much of the genetics and phenotypes in craniosynostosis syndromes is understood, less is known about the underlying ossification mechanism during suture closure. We have previously demonstrated that physiological closure of the posterior frontal suture occurs through endochondral ossification. Moreover, we revealed that antagonizing canonical Wnt-signaling in the sagittal suture leads to endochondral ossification of the suture mesenchyme and sagittal synostosis, presumably by inhibiting Twist1. Classic Saethre-Chotzen syndrome is characterized by coronal synostosis, and the haploinsufficient Twist1^+/− mice represents a suitable model for studying this syndrome. Thus, we seeked to understand the underlying ossification process in coronal craniosynostosis in Twist1^+/− mice. Our data indicate that coronal suture closure in Twist1^+/− mice occurs between postnatal day 9 and 13 by endochondral ossification, as shown by histology, gene expression analysis, and immunohistochemistry. In conclusion, this study reveals that coronal craniosynostosis in Twist1^+/− mice occurs through endochondral ossification. Moreover, it suggests that haploinsufficiency of Twist1 gene, a target of canonical Wnt-signaling, and inhibitor of chondrogenesis, mimics conditions of inactive canonical Wnt-signaling leading to craniosynostosis.

Facial suture synostosis of newborn Fgfr1(P250R/+) and Fgfr2(S252W/+) mouse models of Pfeiffer and Apert syndromes

Apert and Pfeiffer syndromes are hereditary forms of craniosynostosis characterized by midfacial hypoplasia and malformations of the limbs and skull. A serious consequence of midfacial hypoplasia in these syndromes is respiratory compromise due to airway obstruction. In this study, we have evaluated Fgfr1(P250R/+) and Fgfr2(S252W/+) mouse models of these human conditions to study the pathogenesis of midfacial hypoplasia. Our histologic and micro-CT evaluation revealed premature synostosis of the premaxillary-maxillary, nasal-frontal, and maxillary-palatine sutures of the face and dysplasia of the premaxilla, maxilla, and palatine bones. These midfacial abnormalities were detected in the absence of premature ossification of the cranial base at postnatal day 0. Our results indicate that midfacial hypoplasia is not secondary to premature cranial base ossification but rather primary synostosis of facial sutures. Birth Defects Research (Part A), 2011.

Hes1 is required for the development of craniofacial structures derived from ectomesenchymal neural crest cells

The cranial neural crest cells contribute extensively to the formation of skeletogenic mesenchyme in the head and neck. Hes1 functions as a repressor of basic helix-loop-helix transcription factors and is implicated in controlling the maintenance of undifferentiated cells and the timing of cell differentiation. We show here that Hes1 homozygous null mutant mice exhibit multiple craniofacial malformations including calvaria agenesis, defective anterior cranial base, shortened maxilla and mandible, and abnormal palate and tongue. In the null mutant cranium, the calvarial bones, meninges including the dura mater and skin were not formed, and the brain was therefore exposed without the outer cover. The defective anterior cranial base in the mutants was attributable to the lack of presphenoid bone and the flexed cranial base angle, which was in contrast with the flat cranial base of wild-type mice. Furthermore, in the null mutants, palatal shelf growth was impaired because of the early elevation of the palatal shelves, resulting in a narrow palate and oral cavity, which were consistently associated with a small size of the tongue. These craniofacial anomalies could be the result of the defective development of neural crest cells. Taken together, it is supposed that Hes1 signaling plays an essential role in regulating the development of various craniofacial structures derived from the cranial neural crest cells.

Effects of benzo(a)pyrene on the skeletal development of Sebastiscus marmoratus embryos and the molecular mechanism involved

Polycyclic aromatic hydrocarbons (PAHs) are widespread environmental contaminants, which have been known to be carcinogenic and teratogenic. However, the skeletal development toxicity of PAHs and the mechanism involved remain unclear. In fishes, the neurocranial and craniofacial skeleton develop as cartilage. The signaling molecules of hedgehog (Hh) family play crucial roles in regulating skeletal development. In the present study, rockfish (Sebastiscus marmoratus) embryos were exposed to benzo(a)pyrene (BaP) for 7 days at environmental levels (0.05, 0.5 and 5nmol/L) which resulted in craniofacial skeleton deformities. BaP exposure reduced the cell proliferation activity in the craniofacial skeleton as detected by quantitative PCR and in situ hybridization. The expression of Sonic hedgehog (Shh), rather than Indian hedgehog (Ihh), was down-regulated in the craniofacial skeleton in the 0.5 and 5nmol/L groups. Consistent with the Shh results, the expression of Ptch1 and Gli2 was decreased by BaP exposure and BMP4 was presented on changes in the 0.5 and 5nmol/L groups. These results suggested that BaP could impair the expression and function of Shh signaling pathway, perturbing the proliferation of chondrocytes and so disturbing craniofacial skeletal development.

We have got you 'covered': how the meninges control brain development

The meninges have traditionally been viewed as specialized membranes surrounding and protecting the adult brain from injury. However, there is increasing evidence that the fetal meninges play important roles during brain development. Through the release of diffusible factors, the meninges influence the proliferative and migratory behaviors of neural progenitors and neurons in the forebrain and hindbrain. Meningeal cells also secrete and organize the pial basement membrane (BM), a critical anchor point for the radially oriented fibers of neuroepithelial stem cells. With its emerging role in brain development, the potential that defects in meningeal development may underlie certain congenital brain abnormalities in humans should be considered. In this review, we will discuss what is known about assembly of the fetal meninges and review the role of meningeal-derived proteins in mouse and human brain development.

β-catenin/cyclin D1 mediated development of suture mesenchyme in calvarial morphogenesis

Background

Mouse genetic study has demonstrated that Axin2 is essential for calvarial development and disease. Haploid deficiency of β-catenin alleviates the calvarial phenotype caused by Axin2 deficiency. This loss-of-function study provides evidence for the requirement of β-catenin in exerting the downstream effects of Axin2.

Results

Here we utilize a gain-of-function analysis to further assess the role of β-catenin. A transgenic expression system permitting conditional activation of β-catenin in a spatiotemporal specific manner has been developed. Aberrant stimulation of β-catenin leads to increases in expansion of skeletogenic precursors and the enhancement of bone ossification reminiscent to the loss of Axin2. The constitutively active signal promotes specification of osteoprogenitors, but prevents their maturation into terminally differentiated osteoblasts, along the osteoblast lineage. However, the prevention does not interfere with bone synthesis, suggesting that mineralization occurs without the presence of mature osteoblasts. β-catenin signaling apparently plays a key role in suture development through modulation of calvarial morphogenetic signaling pathways. Furthermore, genetic inactivation of the β-catenin transcriptional target, cyclin D1, impairs expansion of the skeletogenic precursors contributing to deficiencies in calvarial ossification. There is a specific requirement for cyclin D1 in populating osteoprogenitor cell types at various developmental stages.

Conclusion

These findings advance our knowledge base of Wnt signaling in calvarial morphogenesis, suggesting a key regulatory pathway of Axin2/β-catenin/cyclin D1 in development of the suture mesenchyme.

Activation of FGF signaling mediates proliferative and osteogenic differences between neural crest derived frontal and mesoderm parietal derived bone

Background

As a culmination of efforts over the last years, our knowledge of the embryonic origins of the mammalian frontal and parietal cranial bones is unambiguous. Progenitor cells that subsequently give rise to frontal bone are of neural crest origin, while parietal bone progenitors arise from paraxial mesoderm. Given the unique qualities of neural crest cells and the clear delineation of the embryonic origins of the calvarial bones, we sought to determine whether mouse neural crest derived frontal bone differs in biology from mesoderm derived parietal bone.

Methods

BrdU incorporation, immunoblotting and osteogenic differentiation assays were performed to investigate the proliferative rate and osteogenic potential of embryonic and postnatal osteoblasts derived from mouse frontal and parietal bones. Co-culture experiments and treatment with conditioned medium harvested from both types of osteoblasts were performed to investigate potential interactions between the two different tissue origin osteoblasts. Immunoblotting techniques were used to investigate the endogenous level of FGF-2 and the activation of three major FGF signaling pathways. Knockdown of FGF Receptor 1 (FgfR1) was employed to inactivate the FGF signaling.

Results

Our results demonstrated that striking differences in cell proliferation and osteogenic differentiation between the frontal and parietal bone can be detected already at embryonic stages. The greater proliferation rate, as well as osteogenic capacity of frontal bone derived osteoblasts, were paralleled by an elevated level of FGF-2 protein synthesis. Moreover, an enhanced activation of FGF-signaling pathways was observed in frontal bone derived osteoblasts. Finally, the greater osteogenic potential of frontal derived osteoblasts was dramatically impaired by knocking down FgfR1.

Conclusions

Osteoblasts from mouse neural crest derived frontal bone displayed a greater proliferative and osteogenic potential and endogenous enhanced activation of FGF signaling compared to osteoblasts from mesoderm derived parietal bone. FGF signaling plays a key role in determining biological differences between the two types of osteoblasts.

Msx1 and Dlx5 function synergistically to regulate frontal bone development

The Msx and Dlx families of homeobox proteins are important regulators for embryogenesis. Loss of Msx1 in mice results in multiple developmental defects including craniofacial malformations. Although Dlx5 is widely expressed during embryonic development, targeted null mutation of Dlx5 mainly affects the development of craniofacial bones. Msx1 and Dlx5 show overlapping expression patterns during frontal bone development. To investigate the functional significance of Msx1/Dlx5 interaction in regulating frontal bone development, we generated Msx1 and Dlx5 double null mutant mice. In Msx1(-/-) ;Dlx5(-/-) mice, the frontal bones defect was more severe than that of either Msx1(-/-) or Dlx5(-/-) mice. This aggravated frontal bone defect suggests that Msx1 and Dlx5 function synergistically to regulate osteogenesis. This synergistic effect of Msx1 and Dlx5 on the frontal bone represents a tissue specific mode of interaction of the Msx and Dlx genes. Furthermore, Dlx5 requires Msx1 for its expression in the context of frontal bone development. Our study shows that Msx1/Dlx5 interaction is crucial for osteogenic induction during frontal bone development.

Bioengineering strategies for regeneration of craniofacial bone: a review of emerging technologies

Although advances in surgical techniques and bone grafting have significantly improved the functional and cosmetic restoration of craniofacial structures lost because of trauma or disease, there are still significant limitations in our ability to regenerate these tissues. The regeneration of oral and craniofacial tissues presents a formidable challenge that requires synthesis of basic science, clinical science, and engineering technology. Tissue engineering is an interdisciplinary field of study that addresses this challenge by applying the principles of engineering to biology and medicine toward the development of biological substitutes that restore, maintain, and improve normal function. This review will explore the impact of biomaterials design, stem cell biology and gene therapy on craniofacial tissue engineering.

Beyond the closed suture in apert syndrome mouse models: evidence of primary effects of FGFR2 signaling on facial shape at birth

Apert syndrome is a congenital disorder caused mainly by two neighboring mutations on fibroblast growth factor receptor 2 (FGFR2). Premature closure of the coronal suture is commonly considered the identifying and primary defect triggering or preceding the additional cranial malformations of Apert phenotype. Here we use two transgenic mouse models of Apert syndrome, Fgfr2(+/S252W) and Fgfr2(+/P253R), to explore variation in cranial phenotypes in newborn (P0) mice. Results show that the facial skeleton is the most affected region of the cranium. Coronal suture patency shows marked variation that is not strongly correlated with skull dysmorphology. The craniofacial effects of the FGFR2 mutations are similar, but Fgfr2(+/S252W) mutant mice display significantly more severe dysmorphology localized to the posterior palate. Our results demonstrate that coronal suture closure is neither the primary nor the sole locus of skull dysmorphology in these mouse models for Apert syndrome, but that the face is also primarily affected.

Role of canonical Wnt signaling/ß-catenin via Dermo1 in cranial dermal cell development

Cranial dermis develops from cephalic mesoderm and neural crest cells, but what signal(s) specifies the dermal lineage is unclear. Using genetic tools to fate map and manipulate a cranial mesenchymal progenitor population in the supraorbital region, we show that the dermal progenitor cells beneath the surface ectoderm process canonical Wnt signaling at the time of specification. We show that Wnt signaling/β-catenin is absolutely required and sufficient for Dermo1 expression and dermal cell identity in the cranium. The absence of the Wnt signaling cue leads to formation of cartilage in craniofacial and ventral trunk regions at the expense of dermal and bone lineages. Dermo1 can be a direct transcription target and may mediate the functional role of Wnt signaling in dermal precursors. This study reveals a lineage-specific role of canonical Wnt signaling/β-catenin in promoting dermal cell fate in distinct precursor populations.

Origins of gliogenic stem cell populations within adult skin and bone marrow

The generation of Schwann cells from precursors within adult skin and bone marrow is of significant clinical interest because of the opportunities for disease modelling and strategies for remyelination. Recent evidence has suggested that glial cells can be generated from (i) mesenchymal stem cells (MSCs) within adult bone marrow and (ii) skin-derived precursor cells (SKPs) within adult skin. However, there is a need to clarify the developmental mechanism whereby such multipotent adult stem cell populations generate glia. We used Wnt1-Cre/Rosa26R(LacZ) and Wnt1-Cre/Rosa26R(YFP) neural crest reporter mice to test the hypothesis that (i) MSCs and (ii) SKPs represent adult gliogenic precursor cells of neural crest origin. We demonstrate that, although labeled cells can be identified within long bone preparation, such cells are rarely found in marrow plugs. Moreover, we did not find evidence of a neural crest origin of bone marrow-derived MSCs and were not able to provide a developmental rationale for the derivation of glial cells from MSCs using this approach. In contrast, we provide robust evidence for the neural crest origin of SKPs derived from adult skin. These precursor cells reliably generate cells with a Schwann cell phenotype, expressing appropriate transcription factors and Schwann cell markers. We demonstrate multiple anatomical origins of gliogenic SKPs within adult skin. We conclude that SKPs, rather than bone marrow-derived MSCs, represent a more defined and developmentally rational source for the study and generation of Schwann cells from readily accessible adult tissues.

Paracrine interaction between adipose-derived stromal cells and cranial suture-derived mesenchymal cells

BACKGROUND

Adipose-derived stromal cells are a potential cell source for the successful healing of skeletal defects. In this study, the authors sought to investigate the potential for cranial suture-derived mesenchymal cells to promote the osteogenic differentiation of adipose-derived stromal cells. Various reports have previously examined the unique in vitro attributes of suture-derived mesenchymal cells; this study sought to extend those findings.

METHODS

Suture-derived mesenchymal cells were isolated from wild-type mice (n = 30) from both fusing posterofrontal and patent sagittal sutures. Cells were placed in Transwell inserts with human adipose-derived stromal cells (n = 5 patients) with osteogenic differentiation medium with or without recombinant Noggin (10 to 400 ng/ml). Specific gene expression of osteogenic markers and Hedgehog pathway were assayed; standard osteogenic assays (alkaline phosphatase and alizarin red staining) were performed. All assays were performed in triplicate.

RESULTS

Both posterofrontal and sagittal suture-derived mesenchymal cells induced osteogenic differentiation of adipose-derived stromal cells (p < 0.05). Posterofrontal suture-derived mesenchymal cells induced adipose-derived stromal cell osteogenesis to a greater degree than sagittal suture-derived mesenchymal cells (p < 0.05). This was accompanied by an increase in bone morphogenetic protein expression (p < 0.05). Finally, recombinant Noggin mitigated the pro-osteogenic effects of co-culture accompanied by a reduction in Hedgehog signaling (p < 0.05).

CONCLUSIONS

Suture-derived mesenchymal cells secrete paracrine factors that induce osteogenic differentiation of multipotent stromal cells (human adipose-derived stromal cells). Cells derived from the fusing posterofrontal suture do this to a significantly greater degree than cells from the patent sagittal suture. Enhanced bone morphogenetic protein and Hedgehog signaling may underlie this paracrine effect.

Incidence and development of the human supracochlear cartilage

The supracochlear cartilage is known as an accessory cartilage of the chondrocranium situated between the otic capsule and the trigeminal ganglion. Although claimed to appear regularly during human development, its incidence and development have been reported only scarcely in the literature. The aim of this study was to describe the position and relationships of the supracochlear cartilage during its development. This study was made in 96 human specimens of 7-17 weeks of development, belonging to a collection of the Embryology Institute of Complutense University of Madrid. In addition, three-dimensional reconstruction of the supracochlear cartilage was made from 1 specimen. This cartilage, spherical in shape, appeared bilaterally in 23 specimens and unilaterally (left side) in 5. In our results, the supracochlear cartilage was found in 26.5% of the cases and was related to the trigeminal ganglion, the dura mater of the trigeminal cavity and the otic capsule. In 4 specimens, bilaterally, the supracochlear cartilage was continuous with the otic capsule. This work suggests that, based on the structures to which the supracochlear cartilage is related, it could be derived from the cranial neural crest.

Craniofacial birth defects: The role of neural crest cells in the etiology and pathogenesis of Treacher Collins syndrome and the potential for prevention

Of all the babies born with birth defects, approximately one-third display anomalies of the head and face [Gorlin et al., 1990] including cleft lip, cleft palate, small or absent facial and skull bones and improperly formed nose, eyes, ears, and teeth. Craniofacial disorders are a primary cause of infant mortality and have serious lifetime functional, esthetic, and social consequences that are devastating to both children and parents alike. Comprehensive surgery, dental care, psychological counseling, and rehabilitation can help ameliorate-specific problems but at great cost over many years which dramatically affects national health care budgets. For example, the Center for Disease Control and Prevention estimates that the lifetime cost of treating the children born each year with cleft lip and/or cleft palate alone to be US$697 million. Treating craniofacial malformations, of which in excess of 700 distinct syndromes have been described, through comprehensive, well-coordinated and integrated strategies can provide satisfactory management of individual conditions, however, the results are often variable and rarely fully corrective. Therefore, better techniques for tissue repair and regeneration need to be developed and therapeutic avenues of prevention need to be explored in order to eliminate the devastating consequences of head and facial birth defects. To do this requires a thorough understanding of the normal events that control craniofacial development during embryogenesis. This review therefore focuses on recent advances in our understanding of the basic etiology and pathogenesis of a rare craniofacial disorder known as Treacher Collins syndrome and emerging prospects for prevention that may have broad application to congenital craniofacial birth defects.

Jagged1 functions downstream of Twist1 in the specification of the coronal suture and the formation of a boundary between osteogenic and non-osteogenic cells

The Notch pathway is crucial for a wide variety of developmental processes including the formation of tissue boundaries. That it may function in calvarial suture development and figure in the pathophysiology of craniosynostosis was suggested by the demonstration that heterozygous loss of function of JAGGED1 in humans can cause Alagille syndrome, which has craniosynostosis as a feature. We used conditional gene targeting to examine the role of Jagged1 in the development of the skull vault. We demonstrate that Jagged1 is expressed in a layer of mesoderm-derived sutural cells that lie along the osteogenic-non-osteogenic boundary. We show that inactivation of Jagged1 in the mesodermal compartment of the coronal suture, but not in the neural crest compartment, results in craniosynostosis. Mesodermal inactivation of Jagged1 also results in changes in the identity of sutural cells prior to overt osteogenic differentiation, as well as defects in the boundary between osteogenic and non-osteogenic compartments at the coronal suture. These changes, surprisingly, are associated with increased expression of Notch2 and the Notch effector, Hes1, in the sutural mesenchyme. They are also associated with an increase in nuclear β-catenin. In Twist1 mutants, Jagged1 expression in the suture is reduced substantially, suggesting an epistatic relationship between Twist1 and Jagged1. Consistent with such a relationship, Twist1-Jagged1 double heterozygotes exhibit a substantial increase in the severity of craniosynostosis over individual heterozygotes. Our results thus suggest that Jagged1 is an effector of Twist1 in coronal suture development.

Different endogenous threshold levels of Fibroblast Growth Factor-ligands determine the healing potential of frontal and parietal bones

In the skull vault, neural crest derived frontal bones have an increased healing capacity and higher expression levels of Fibroblast Growth Factor-ligands as compared to mesoderm-derived parietal bones. Thus, we asked whether Fibroblast Growth Factor-ligands are responsible for the superior healing potential of frontal bones. Parietal defects in juvenile and adult mice treated with Fibroblast Growth Factor-2, -9 and -18 showed increased bone regeneration, comparable to frontal defects. Immunohistochemistry revealed increased recruitment of osteoprogenitors and activation of FGF-signaling pathways in FGF-treated parietal defects. Conversely, calvarial defects in Fgf-9(+/-) and Fgf-18(+/-) mice showed impaired calvarial healing which could be rescued by exogenous Fibroblast Growth Factor-ligands. Moreover, by utilizing Wnt1Cre/R26R mice, the migration and contribution of dura mater and pericranium cells to calvarial healing could be demonstrated. Taken together our results demonstrated that different endogenous threshold levels of Fibroblast Growth Factor-ligands in frontal and parietal bones have a profound impact on calvarial regeneration. The present study thereby opens new avenues for translational medicine.

Origin matters: differences in embryonic tissue origin and Wnt signaling determine the osteogenic potential and healing capacity of frontal and parietal calvarial bones

Calvarial bones arise from two embryonic tissues, namely, the neural crest and the mesoderm. In this study we have addressed the important question of whether disparate embryonic tissue origins impart variable osteogenic potential and regenerative capacity to calvarial bones, as well as what the underlying molecular mechanism(s). Thus, by performing in vitro and in vivo studies, we have investigated whether differences exist between neural crest-derived frontal and paraxial mesodermal-derived parietal bone. Of interest, our data indicate that calvarial bone osteoblasts of neural crest origin have superior potential for osteogenic differentiation. Furthermore, neural crest-derived frontal bone displays a superior capacity to undergo osseous healing compared with calvarial bone of paraxial mesoderm origin. Our study identified both in vitro and in vivo enhanced endogenous canonical Wnt signaling in frontal bone compared with parietal bone. In addition, we demonstrate that constitutive activation of canonical Wnt signaling in paraxial mesodermal-derived parietal osteoblasts mimics the osteogenic potential of frontal osteoblasts, whereas knockdown of canonical Wnt signaling dramatically impairs the greater osteogenic potential of neural crest-derived frontal osteoblasts. Moreover, fibroblast growth factor 2 (FGF-2) treatment induces phosphorylation of GSK-3beta and increases the nuclear levels of beta-catenin in osteoblasts, suggesting that enhanced activation of Wnt signaling might be mediated by FGF. Taken together, our data provide compelling evidence that indeed embryonic tissue origin makes a difference and that active canonical Wnt signaling plays a major role in contributing to the superior intrinsic osteogenic potential and tissue regeneration observed in neural crest-derived frontal bone.

Primary melanocytic neoplasms of the central nervous system

Primary melanocytic neoplasms of the central nervous system (CNS) are rare lesions arising from melanocytes of the leptomeninges. They include diffuse leptomeningeal melanocytosis or melanomatosis, melanocytoma and primary malignant melanoma. We have reviewed the English literature regarding these lesions, which consists of case reports and a small number of larger case series. The presenting features, radiological, surgical and histological findings are reviewed, as are current management options and prognosis. We also present illustrative case reports of diffuse leptomeningeal melanocytosis and primary melanoma of the CNS.

Differential activation of canonical Wnt signaling determines cranial sutures fate: a novel mechanism for sagittal suture craniosynostosis

Premature closure of cranial sutures, which serve as growth centers for the skull vault, result in craniosynostosis. In the mouse posterior frontal (PF) suture closes by endochondral ossification, whereas sagittal (SAG) remain patent life time, although both are neural crest tissue derived. We therefore, investigated why cranial sutures of same tissue origin adopt a different fate. We demonstrated that closure of the PF suture is tightly regulated by canonical Wnt signaling, whereas patency of the SAG suture is achieved by constantly activated canonical Wnt signaling. Importantly, the fate of PF and SAG sutures can be reversed by manipulating Wnt signaling. Continuous activation of canonical Wnt signaling in the PF suture inhibits endochondral ossification and therefore, suture closure, In contrast, inhibition of canonical Wnt signaling in the SAG suture, upon treatment with Wnt antagonists results in endochondral ossification and suture closure. Thus, inhibition of canonical Wnt signaling in the SAG suture phenocopies craniosynostosis. Moreover, mice haploinsufficient for Twist1, a target gene of canonical Wnt signaling which inhibits chondrogenesis, have sagittal craniosynostosis. We propose that regulation of canonical Wnt signaling is of crucial importance during the physiological patterning of PF and SAG sutures. Importantly, dysregulation of this pathway may lead to craniosynostosis.

Prenatal alcohol exposure triggers ceramide-induced apoptosis in neural crest-derived tissues concurrent with defective cranial development

Fetal alcohol syndrome (FAS) is caused by maternal alcohol consumption during pregnancy. The reason why specific embryonic tissues are sensitive toward ethanol is not understood. We found that in neural crest-derived cell (NCC) cultures from the first branchial arch of E10 mouse embryos, incubation with ethanol increases the number of apoptotic cells by fivefold. Apoptotic cells stain intensely for ceramide, suggesting that ceramide-induced apoptosis mediates ethanol damage to NCCs. Apoptosis is reduced by incubation with CDP-choline (citicoline), a precursor for the conversion of ceramide to sphingomyelin. Consistent with NCC cultures, ethanol intubation of pregnant mice results in ceramide elevation and increased apoptosis of NCCs in vivo. Ethanol also increases the protein level of prostate apoptosis response 4 (PAR-4), a sensitizer to ceramide-induced apoptosis. Prenatal ethanol exposure is concurrent with malformation of parietal bones in 20% of embryos at day E18. Meninges, a tissue complex derived from NCCs, is disrupted and generates reduced levels of TGF-β1, a growth factor critical for bone and brain development. Ethanol-induced apoptosis of NCCs leading to defects in the meninges may explain the simultaneous presence of cranial bone malformation and cognitive retardation in FAS. In addition, our data suggest that treatment with CDP-choline may alleviate the tissue damage caused by alcohol.

Retinoic acid enhances osteogenesis in cranial suture-derived mesenchymal cells: potential mechanisms of retinoid-induced craniosynostosis

BACKGROUND

In utero retinoid exposure results in numerous craniofacial malformations, including craniosynostosis. Although many malformations associated with retinoic acid syndrome are associated with neural crest defects, the specific mechanisms of retinoid-induced craniosynostosis remain unclear. The authors used the culture of mouse cranial suture-derived mesenchymal cells to probe the potential cellular mechanisms of this teratogen to better elucidate mechanisms of retinoid-induced suture fusion.

METHODS

Genes associated with retinoid signaling were assayed in fusing (posterofrontal) and patent (sagittal, coronal) sutures by quantitative real-time polymerase chain reaction. Cultures of mouse suture-derived mesenchymal cells from the posterofrontal suture were established from 4-day-old mice. Cells were cultured with all-trans retinoic acid (1 and 5 muM). Proliferation, osteogenic differentiation, and specific gene expression were assessed.

RESULTS

Mouse sutures were found to express genes necessary for retinoic acid synthesis, binding, and signal transduction, demonstrated by quantitative real-time polymerase chain reaction (Raldh1, Raldh2, Raldh3, and Rbp4). These genes were not found to be differentially expressed in fusing as compared with patent cranial sutures in vivo. Addition of retinoic acid enhanced the osteogenic differentiation of suture-derived mesenchymal cells in vitro, including up-regulation of alkaline phosphatase activity and Runx2 expression. Contemporaneously, cellular proliferation was repressed, as shown by proliferative cell nuclear antigen expression. The pro-osteogenic effect of retinoic acid was accompanied by increased gene expression of several hedgehog and bone morphogenetic protein ligands.

CONCLUSIONS

Retinoic acid represses proliferation and enhances osteogenic differentiation of suture-derived mesenchymal cells. These in vitro data suggest that retinoid exposure may lead to premature cranial suture fusion by means of enhanced osteogenesis and hedgehog and bone morphogenetic protein signaling.

Inactivation of Msx1 and Msx2 in neural crest reveals an unexpected role in suppressing heterotopic bone formation in the head

In an effort to understand the morphogenetic forces that shape the bones of the skull, we inactivated Msx1 and Msx2 conditionally in neural crest. We show that Wnt1-Cre inactivation of up to three Msx1/2 alleles results in a progressively larger defect in the neural crest-derived frontal bone. Unexpectedly, in embryos lacking all four Msx1/2 alleles, the large defect is filled in with mispatterned bone consisting of ectopic islands of bone between the reduced frontal bones, just anterior to the parietal bones. The bone is derived from neural crest, not mesoderm, and, from DiI cell marking experiments, originates in a normally non-osteogenic layer of cells through which the rudiment elongates apically. Associated with the heterotopic osteogenesis is an upregulation of Bmp signaling in this cell layer. Prevention of this upregulation by implantation of noggin-soaked beads in head explants also prevented heterotopic bone formation. These results suggest that Msx genes have a dual role in calvarial development: They are required for the differentiation and proliferation of osteogenic cells within rudiments, and they are also required to suppress an osteogenic program in a cell layer within which the rudiments grow. We suggest that the inactivation of this repressive activity may be one cause of Wormian bones, ectopic bones that are a feature of a variety of pathological conditions in which calvarial bone development is compromised.

Role of Wnt signaling in the biology of the periodontium

Continuously erupting teeth have associated with them a continuously regenerating periodontal ligament, but the factors that control this amazing regenerative potential are unknown. We used genetic strategies to show that the periodontal ligament arises from the cranial neural crest. Despite their histological similarity, the periodontal ligament of continuously erupting incisor teeth differs dramatically from the periodontal ligament of molar teeth. The most notable difference was in the distribution of Wnt responsive cells in the incisor periodontal ligament, which coincided with regions of periodontal ligament cell proliferation. We discuss these findings in the context of dental tissue regeneration.

Arginylation-dependent neural crest cell migration is essential for mouse development

Coordinated cell migration during development is crucial for morphogenesis and largely relies on cells of the neural crest lineage that migrate over long distances to give rise to organs and tissues throughout the body. Recent studies of protein arginylation implicated this poorly understood posttranslational modification in the functioning of actin cytoskeleton and in cell migration in culture. Knockout of arginyltransferase (Ate1) in mice leads to embryonic lethality and severe heart defects that are reminiscent of cell migration-dependent phenotypes seen in other mouse models. To test the hypothesis that arginylation regulates cell migration during morphogenesis, we produced Wnt1-Cre Ate1 conditional knockout mice (Wnt1-Ate1), with Ate1 deletion in the neural crest cells driven by Wnt1 promoter. Wnt1-Ate1 mice die at birth and in the first 2-3 weeks after birth with severe breathing problems and with growth and behavioral retardation. Wnt1-Ate1 pups have prominent defects, including short palate and altered opening to the nasopharynx, and cranial defects that likely contribute to the abnormal breathing and early death. Analysis of neural crest cell movement patterns in situ and cell motility in culture shows an overall delay in the migration of Ate1 knockout cells that is likely regulated by intracellular mechanisms rather than extracellular signaling events. Taken together, our data suggest that arginylation plays a general role in the migration of the neural crest cells in development by regulating the molecular machinery that underlies cell migration through tissues and organs during morphogenesis.