Tissue origins and interactions in the mammalian skull vault

Syndromic craniosynostosis: from history to hydrogen bonds

The syndromic craniosynostoses, usually involving multiple sutures, are hereditary forms of craniosynostosis associated with extracranial phenotypes such as limb, cardiac, CNS and tracheal malformations. The genetic etiology of syndromic craniosynostosis in humans is only partially understood. Syndromic synostosis has been found to be associated with mutations of the fibroblast growth factor receptor family (FGFR1, -R2, -R3), TWIST1, MSX2, and EFNB1. Apert, Pfeiffer, Crouzon, and Jackson-Weiss syndromes are due to gain-of-function mutations of FGFR2 in either the Ig II-III linker region (Apert) or Ig III domain. Loss of function mutations of TWIST1 and gain-of-function mutations of MSX2 lead to Saethre-Chotzen and Boston-type syndromes, respectively. The mutations in Pfeiffer (FGFR1), Muenke (FGFR3), and Apert syndrome (FGFR2) are caused by the same amino acid substitution in a highly conserved region of the Ig II-III linker region of these proteins, which suggests that these receptor tyrosine kinases have an overlapping function in suture biology. In this review we will discuss the historical descriptions, current phenotypes and molecular causes of the more common forms of syndromic craniosynostosis.

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.

Endoglin is required for myogenic differentiation potential of neural crest stem cells

Genetic studies show that TGFbeta signaling is essential for vascular development, although the mechanism through which this pathway operates is incompletely understood. Here we demonstrate that the TGFbeta auxiliary coreceptor endoglin (eng, CD105) is expressed in a subset of neural crest stem cells (NCSCs) in vivo and is required for their myogenic differentiation. Overexpression of endoglin in the neural crest caused pericardial hemorrhaging, correlating with altered vascular smooth muscle cell investment in the walls of major vessels and upregulation of smooth muscle alpha-actin protein levels. Clonogenic differentiation assay of NCSCs derived from neural tube explants demonstrated that only NCSC expressing high levels of endoglin (NCSC(CD105+)) had myogenic differentiation potential. Furthermore, myogenic potential was deficient in NCSCs obtained from endoglin null embryos. Expression of endoglin in NCSCs declined with age, coinciding with a reduction in both smooth muscle differentiation potential and TGFbeta1 responsiveness. These findings demonstrate a cell autonomous role for endoglin in smooth muscle cell specification contributing to vascular integrity.

Neural crest progenitors and stem cells

In the vertebrate embryo, multiple cell types originate from a common structure, the neural crest (NC), which forms at the dorsal tips of the neural epithelium. The NC gives rise to migratory cells that colonise a wide range of embryonic tissues and later differentiate into neurones and glial cells of the peripheral nervous system (PNS), pigment cells (melanocytes) in the skin and endocrine cells in the adrenal and thyroid glands. In the head and the neck, the NC also yields mesenchymal cells that form craniofacial cartilages, bones, dermis, adipose tissue, and vascular smooth muscle cells. The NC is therefore a model system to study cell diversification during embryogenesis and phenotype maintenance in the adult. By analysing the developmental potentials of quail NC cells in clonal cultures, we have shown that the migratory NC is a collection of heterogeneous progenitors, including various types of intermediate precursors and highly multipotent cells, some of which being endowed of self-renewal capacity. We also have identified common progenitors for mesenchymal derivatives and neural/melanocytic cells in the cephalic NC. These results are consistent with a hierarchical model of lineage segregation wherein environmental cytokines control the fate of progenitors and stem cells. One of these cytokines, the endothelin3 peptide, promotes the survival, proliferation, and self-renewal capacity of common progenitors for glial cells and melanocytes. At post-migratory stages, when they have already differentiated, NC-derived cells exhibit phenotypic plasticity. Epidermal pigment cells and Schwann cells from peripheral nerves in single-cell culture are able to reverse into multipotent NC-like progenitors endowed with self-renewal. Therefore, stem cell properties are expressed by a variety of NC progenitors and can be re-acquired by differentiated cells of NC origin, suggesting potential function for repair.

Evidence that a late-emerging population of trunk neural crest cells forms the plastron bones in the turtle Trachemys scripta

The origin of the turtle plastron is not known, but these nine bones have been homologized to the exoskeletal components of the clavicles, the interclavicular bone, and gastralia. Earlier evidence from our laboratory showed that the bone-forming cells of the plastron were positive for HNK-1 and PDGFRalpha, two markers of the skeletogenic neural crest. This study looks at the embryonic origin of these plastron-forming cells. We show that the HNK-1+ cells are also positive for p75 and FoxD3, confirming their neural crest identity, and that they originate from the dorsal neural tube of stage 17 turtle embryos, several days after the original wave of neural crest cells have migrated and differentiated. DiI studies show that these are migratory cells, and they can be observed in the lateral regions of the embryo and can be seen forming intramembranous bone in the ventral (plastron) regions. Before migrating ventrally, these late-emerging neural crest cells reside for over a week in a carapacial staging area above the neural tube and vertebrae. It is speculated that this staging area is where they lose the inability to form skeletal cells.

Neural crest-specific removal of Zfhx1b in mouse leads to a wide range of neurocristopathies reminiscent of Mowat-Wilson syndrome

Mowat-Wilson syndrome is a recently delineated autosomal dominant developmental anomaly, whereby heterozygous mutations in the ZFHX1B gene cause mental retardation, delayed motor development, epilepsy and a wide spectrum of clinically heterogeneous features, suggestive of neurocristopathies at the cephalic, cardiac and vagal levels. However, our understanding of the etiology of this condition at the cellular level remains vague. This study presents the Zfhx1b protein expression domain in mouse embryos and correlates this with a novel mouse model involving a conditional mutation in the Zfhx1b gene in neural crest precursor cells. These mutant mice display craniofacial and gastrointestinal malformations that show resemblance to those found in human patients with Mowat-Wilson syndrome. In addition to these clinically recognized alterations, we document developmental defects in the heart, melanoblasts and sympathetic and parasympathetic anlagen. The latter observations in our mouse model for Mowat-Wilson suggest a hitherto unknown role for Zfhx1b in the development of these particular neural crest derivatives, which is a set of observations that should be acknowledged in the clinical management of this genetic disorder.

Neural crest and mesoderm lineage-dependent gene expression in orofacial development

The present study utilizes a combination of genetic labeling/selective isolation of pluripotent embryonic progenitor cells, and oligonucleotide-based microarray technology, to delineate and compare the "molecular fingerprint" of two mesenchymal cell populations from distinct lineages in the developing embryonic orofacial region. The first branchial arches-bi-lateral tissue primordia that flank the primitive oral cavity-are populated by pluripotent mesenchymal cells from two different lineages: neural crest (neuroectoderm)- and mesoderm-derived mesenchymal cells. These cells give rise to all of the connective tissue elements (bone, cartilage, smooth and skeletal muscle, dentin) of the orofacial region (maxillary and mandibular portion), as well as neurons and glia associated with the cranial ganglia, among other tissues. In the present study, neural crest- and mesoderm-derived mesenchymal cells were selectively isolated from the first branchial arch of gestational day 9.5 mouse embryos using laser capture microdissection (LCM). The two different embryonic cell lineages were distinguished through utilization of a novel two component transgenic mouse model (Wnt1Cre/ZEG) in which the neural crest cells and their derivatives are indelibly marked (i.e., expressing enhanced green fluorescent protein, EGFP) throughout the pre- and post-natal lifespan of the organism. EGFP-labeled neural crest-derived, and non-fluorescent mesoderm-derived mesenchymal cells from the first branchial arch were visualized in frozen tissue sections from gestational day 9.5 mouse embryos and independently isolated by LCM under epifluorescence optics. RNA was extracted from the two populations of LCM-procured cells, and amplified by double-stranded cDNA synthesis and in vitro transcription. Gene expression profiles of the two progenitor cell populations were generated via hybridization of the cell-type specific cRNA samples to oligo-based GeneChip microarrays. Comparison of gene expression profiles of neural crest- and mesoderm-derived mesenchymal cells from the first branchial arch revealed over 140 genes that exhibited statistically significant differential levels of expression. The gene products of many of these differentially expressed genes have previously been linked to the development of mesoderm- or neural crest-derived tissues in the embryo. Interestingly, however, hitherto uncharacterized coding sequences with highly significant differences in expression between the two embryonic progenitor cell types were also identified. These lineage-dependent mesenchymal cell molecular fingerprints offer the opportunity to elucidate additional mechanisms governing cellular growth, differentiation, and morphogenesis of the embryonic orofacial region. The chemokine stromal cell-derived factor 1, (SDF-1), was found to exhibit greater expression in mesoderm-derived mesenchyme in the branchial arch when compared with neurectoderm, suggesting a possible chemotactic role for SDF-1 in guiding the migratory neural crest cells to their destination. The novel combination of genetic labeling of the neural crest cell population by EGFP coupled with isolation of cells by LCM for gene expression analysis has enabled, for the first time, the generation of gene expression profiles of distinct embryonic cell lineages.

MULTIPLE DIFFERENTIATION POTENTIALS OF NEONATAL DURA MATER-DERIVED CELLS

OBJECTIVE

The involvement of the dura mater in calvarial development and bone healing lead to a hypothesis that progenitor cells with multiple differentiation potentials exist within this tissue. The present study investigated the differentiation potentials of dura mater-derived cells by driving them into several cell-restricted lineages.

METHODS

Dissected dura mater tissue of neonatal rats was washed, finely minced, and enzymatically digested. The harvested cells were exposed to different differentiation (osteogenic, adipogenic, and chondrogenic) and basic media.

RESULTS

At defined time points, dura mater-derived cells were observed to differentiate into osteoblastic, adipoblastic, and chondroblastic cells, evidenced by specific biochemical staining. In addition, gene expressions of osteogenesis (alkaline phosphatase, osteocalcin, and osteopontin), chondrogenesis (collagen Type II and aggrecan core protein) and adipogenesis (peroxisome proliferator activated receptor gamma-2) were up-regulated in the differentiated dura mater-derived cells, confirmed by polymerase chain reaction.

CONCLUSION

Preliminarily, it was concluded that a subpopulation of multiple potential mesenchymal cells exists in neonatal dura mater, which explains the function of the dura mater on neurocranium development and calvarial bone healing.

Craniofacial tissue engineering by stem cells

Craniofacial tissue engineering promises the regeneration or de novo formation of dental, oral, and craniofacial structures lost to congenital anomalies, trauma, and diseases. Virtually all craniofacial structures are derivatives of mesenchymal cells. Mesenchymal stem cells are the offspring of mesenchymal cells following asymmetrical division, and reside in various craniofacial structures in the adult. Cells with characteristics of adult stem cells have been isolated from the dental pulp, the deciduous tooth, and the periodontium. Several craniofacial structures--such as the mandibular condyle, calvarial bone, cranial suture, and subcutaneous adipose tissue--have been engineered from mesenchymal stem cells, growth factor, and/or gene therapy approaches. As a departure from the reliance of current clinical practice on durable materials such as amalgam, composites, and metallic alloys, biological therapies utilize mesenchymal stem cells, delivered or internally recruited, to generate craniofacial structures in temporary scaffolding biomaterials. Craniofacial tissue engineering is likely to be realized in the foreseeable future, and represents an opportunity that dentistry cannot afford to miss.

Effects of developmental and functional interactions on mouse cranial variability through late ontogeny

The mammalian skull performs a variety of functions and its growth and development mirrors this complexity. Cranial growth and development have been actively studied for many years. Despite this interest, the variation in the patterns and processes of skull growth has attracted little attention. An important and unanswered question is the extent to which patterns of cranial covariation and variation are dynamically reworked throughout postnatal growth. To address this question, we examine patterns of variability in random-bred mouse skulls aged 35, 90, and 150 days. Using a battery of both Procrustes coordinate and Euclidean distance-based methods, we measure mean shape, canalization, developmental stability, and morphological integration in these skulls. We predict that the patterns of variability are dynamic, particularly between the youngest and the two oldest age groups due to the influence of functional effects such as postweaning mastication. We also hypothesize that patterns of variability are structured by the same functional and developmental factors that have been shown to influence cranial growth in primates. Our results indicate that contrary to our predictions, patterns of canalization, developmental stability, and morphological integration are stabilized before 35 days. The mean shape, however, changed significantly with growth. We found that only the facial region showed significant integration as predicted by the functional matrix model used in other studies of integration. These results indicate that phenotypic integration in these mice does not closely match those found for primate species, suggesting that comparisons between species should be made with care.

Scanning electron microscope and micro-CT evaluation of cranial sutures in health and disease

Current knowledge of suture biology has been ascertained as a result of morphological studies of normal cranial sutures (and rarely those undergoing craniosynostosis). These were initially undertaken often using histological investigations, or more recently using CT scans, as investigative tools, but have often used animal models. However, recent technological advances have provided the potential to refine our understanding of the ultrastructure by the use of new advanced scanning technology, which offers the possibility of more detailed resolution. Our aim was to undertake detailed scans of normal, fusing and fused sutures from patients with craniosynosotosis affecting different sutures, to study the detailed structure at different stages of the fusion process using a modern micro-CT scanner and a microanalytical scanning electron microscope. We wished to include in our study all the human sutures because previous studies have mostly been undertaken using the sagittal suture. Ten sutures from seven patients have revealed a complex ultra-structural arrangement. The different patterns of bone ridging seen on the ectocranial and endocranial surfaces of the fused sagittal suture were not repeated on closer inspection of either fused coronal or lambdoid sutures. Elemental analysis confirmed that the amount of calcium increased and the amount of carbon decreased as sampled areas moved away from the suture margin. We conclude that scanning allowed detailed assessment and revealed the complex arrangement of the structure of the human cranial sutures and those undergoing the process of craniosynostosis, with some differences in final structure depending on the affected suture.

Inhibition of gap junction communication at ectopic Eph/ephrin boundaries underlies craniofrontonasal syndrome

Mutations in X-linked ephrin-B1 in humans cause craniofrontonasal syndrome (CFNS), a disease that affects female patients more severely than males. Sorting of ephrin-B1–positive and –negative cells following X-inactivation has been observed in ephrin-B1^+/− mice; however, the mechanisms by which mosaic ephrin-B1 expression leads to cell sorting and phenotypic defects remain unknown. Here we show that ephrin-B1^+/− mice exhibit calvarial defects, a phenotype autonomous to neural crest cells that correlates with cell sorting. We have traced the causes of calvarial defects to impaired differentiation of osteogenic precursors. We show that gap junction communication (GJC) is inhibited at ectopic ephrin boundaries and that ephrin-B1 interacts with connexin43 and regulates its distribution. Moreover, we provide genetic evidence that GJC is implicated in the calvarial defects observed in ephrin-B1^+/− embryos. Our results uncover a novel role for Eph/ephrins in regulating GJC in vivo and suggest that the pleiotropic defects seen in CFNS patients are due to improper regulation of GJC in affected tissues.

Mutations inephrin-B1 associated with X-linked CFNS result in aberrant formation of facial bone structures.Ephrin-B1 interacts with gap junctions to influence the cell sorting events that underlie the CFNS deformations.

Deer antler regeneration: Cells, concepts, and controversies

The periodic replacement of antlers is an exceptional regenerative process in mammals, which in general are unable to regenerate complete body appendages. Antler regeneration has traditionally been viewed as an epimorphic process closely resembling limb regeneration in urodele amphibians, and the terminology of the latter process has also been applied to antler regeneration. More recent studies, however, showed that, unlike urodele limb regeneration, antler regeneration does not involve cell dedifferentiation and the formation of a blastema from these dedifferentiated cells. Rather, these studies suggest that antler regeneration is a stem-cell-based process that depends on the periodic activation of, presumably neural-crest-derived, periosteal stem cells of the distal pedicle. The evidence for this hypothesis is reviewed and as a result, a new concept of antler regeneration as a process of stem-cell-based epimorphic regeneration is proposed that does not involve cell dedifferentiation or transdifferentiation. Antler regeneration illustrates that extensive appendage regeneration in a postnatal mammal can be achieved by a developmental process that differs in several fundamental aspects from limb regeneration in urodeles.

Ephrin-B2 forward signaling regulates somite patterning and neural crest cell development

Genetic studies in the mouse have implicated ephrin-B2 (encoded by the gene Efnb2) in blood vessel formation, cardiac development and remodeling of the lymphatic vasculature. Here we report that loss of ephrin-B2 leads to defects in populations of cranial and trunk neural crest cells (NCC) and to defective somite development. In addition, we show that Efnb1/Efnb2 double heterozygous embryos exhibit phenotypes in a number of NCC derivatives. Expression of one copy of a mutant version of Efnb2 that lacks tyrosine phosphorylation sites was sufficient to rescue the embryonic phenotypes associated with loss of Efnb2. Our results uncover an important role for ephrin-B2 in NCC and somites during embryogenesis and suggest that ephrin-B2 exerts many of its embryonic function via activation of forward signaling.

Endogenous bone morphogenetic protein antagonists regulate mammalian neural crest generation and survival

We demonstrate here that Chordin and Noggin function as bone morphogenetic protein (BMP) antagonists in vivo to promote mammalian neural crest development. Using Chrd and Nog single and compound mutants, we find that Noggin has a major role in promoting neural crest formation, in which Chordin is partially redundant. BMP signaling is increased in dorsal tissues lacking Noggin and is further increased when Chordin is also absent. The early neural crest domain is expanded with decreased BMP antagonism in vivo. Noggin and Chordin also regulate subsequent neural crest cell emigration from the neural tube. However, reduced levels of these BMP antagonists ultimately result in perturbation of neural crest cell derived peripheral nervous system and craniofacial skeletal elements. Such defects reflect, at least in part, a function to limit apoptosis in neural crest cells. Noggin and Chordin, therefore, function together to regulate both the generation and survival of neural crest cells in mammalian development.

Neural Crest Stem and Progenitor Cells

Neural crest cells are a multipotent, migratory cell population that generates an astonishingly diverse array of cell types during vertebrate development. These include bones; tendons; neurons; glia; melanocytes; and connective, endocrine, and adipose tissue. With a limited capacity for self-renewal and a wide range of differentiation fates, neural crest cells bear many of the hallmarks of stem cells and persist throughout embryonic and adult development. But are all neural crest cells true stem cells, or do the majority of neural crest cells more closely resemble progenitor cells? In this review we discuss recent advances in characterizing the properties of neural crest cells, together with their potential for tissue-specific repair.

Craniosynostosis caused by Axin2 deficiency is mediated through distinct functions of beta-catenin in proliferation and differentiation

Targeted disruption of Axin2 in mice induces skeletal defects, a phenotype resembling craniosynostosis in humans. Premature fusion of cranial sutures, caused by deficiency in intramembranous ossification, occurs at early postnatal stages. Axin2 negatively regulates both expansion of osteoprogenitors and maturation of osteoblasts through its modulation on Wnt/beta-catenin signaling. We investigate the dual role of beta-catenin to gain further insights into the skull morphogenetic circuitry. We show that as a transcriptional co-activator, beta-catenin promotes cell division by stimulating its target cyclin D1 in osteoprogenitors. Upon differentiation of osteoprogenitors, BMP signaling is elevated to accelerate the process in a positive feedback mechanism. This Wnt-dependent BMP signal dictates cellular distribution of beta-catenin. As an adhesion molecule, beta-catenin promotes cell-cell interaction mediated by adherens junctions in mature osteoblasts. Finally, haploid deficiency of beta-catenin alleviates the Axin2-null skeletal phenotypes. These findings support a model for disparate roles of beta-catenin in osteoblast proliferation and differentiation.

Phenotypic integration of neurocranium and brain

Evolutionary history of Mammalia provides strong evidence that the morphology of skull and brain change jointly in evolution. Formation and development of brain and skull co-occur and are dependent upon a series of morphogenetic and patterning processes driven by genes and their regulatory programs. Our current concept of skull and brain as separate tissues results in distinct analyses of these tissues by most researchers. In this study, we use 3D computed tomography and magnetic resonance images of pediatric individuals diagnosed with premature closure of cranial sutures (craniosynostosis) to investigate phenotypic relationships between the brain and skull. It has been demonstrated previously that the skull and brain acquire characteristic dysmorphologies in isolated craniosynostosis, but relatively little is known of the developmental interactions that produce these anomalies. Our comparative analysis of phenotypic integration of brain and skull in premature closure of the sagittal and the right coronal sutures demonstrates that brain and skull are strongly integrated and that the significant differences in patterns of association do not occur local to the prematurely closed suture. We posit that the current focus on the suture as the basis for this condition may identify a proximate, but not the ultimate cause for these conditions. Given that premature suture closure reduces the number of cranial bones, and that a persistent loss of skull bones is demonstrated over the approximately 150 million years of synapsid evolution, craniosynostosis may serve as an informative model for evolution of the mammalian skull.

Use of a ROSA26:GFP transgenic line for long-term Xenopus fate-mapping studies

Widespread and persistent marker expression is a prerequisite for many transgenic applications, including chimeric transplantation studies. Although existing transgenic tools for the clawed frog, Xenopus laevis, offer a number of promoters that drive widespread expression during embryonic stages, obtaining transgene expression through metamorphosis and into differentiated adult tissues has been difficult to achieve with this species. Here we report the application of the murine ROSA26 promoter in Xenopus. GFP is expressed in every transgenic tissue and cell type examined at post-metamorphic stages. Furthermore, transgenic ROSA26:GFP frogs develop normally, with no apparent differences in growth or morphology relative to wild-type frogs. ROSA26 transgenes may be used as a reliable marker for embryonic fate-mapping of adult structures in Xenopus laevis. Utility of this transgenic line is illustrated by its use in a chimeric grafting study that demonstrates the derivation of the adult bony jaw from embryonic cranial neural crest.

Normal skull suture variant mimicking intentional injury

A presumed skull fracture might be a variant of a normal skull suture, especially when bilateral

TGFbeta-mediated FGF signaling is crucial for regulating cranial neural crest cell proliferation during frontal bone development

The murine frontal bone derives entirely from the cranial neural crest (CNC) and consists of the calvarial (lateral) aspect that covers the frontal lobe of brain and the orbital aspect that forms the roof of bony orbit. TGFbeta and FGF signaling have important regulatory roles in postnatal calvarial development. Our previous study has demonstrated that conditional inactivation of Tgfbr2 in the neural crest results in severe defects in calvarial development, although the cellular and molecular mechanisms by which TGFbeta signaling regulates the fate of CNC cells during frontal bone development remain unknown. Here, we show that TGFbeta IIR is required for proliferation of osteoprogenitor cells in the CNC-derived frontal bone anlagen. FGF acts downstream of TGFbeta signaling in regulating CNC cell proliferation, and exogenous FGF2 rescues the cell proliferation defect in the frontal primordium of Tgfbr2 mutant. Furthermore, the CNC-derived frontal primordium requires TGFbeta IIR to undergo terminal differentiation. However, this requirement is restricted to the developing calvarial aspect of the frontal bone, whereas the orbital aspect forms despite the ablation of Tgfbr2 gene, implying a differential requirement for TGFbeta signaling during the development of various regions of the frontal bone. This study demonstrates the biological significance of TGFbeta-mediated FGF signaling cascade in regulating frontal bone development, suggests that TGFbeta functions as a morphogen in regulating the fate of the CNC-derived osteoblast and provides a model for investigating abnormal craniofacial development.

Cell mixing at a neural crest-mesoderm boundary and deficient ephrin-Eph signaling in the pathogenesis of craniosynostosis

Boundaries between cellular compartments often serve as signaling interfaces during embryogenesis. The coronal suture is a major growth center of the skull vault and develops at a boundary between cells derived from neural crest and mesodermal origin, forming the frontal and parietal bones, respectively. Premature fusion of these bones, termed coronal synostosis, is a common human developmental anomaly. Known causes of coronal synostosis include haploinsufficiency of TWIST1 and a gain of function mutation in MSX2. In Twist1(+/-) mice with coronal synostosis, we found that the frontal-parietal boundary is defective. Specifically, neural crest cells invade the undifferentiated mesoderm of the Twist1(+/-) mutant coronal suture. This boundary defect is accompanied by an expansion in Msx2 expression and reduction in ephrin-A4 distribution. Reduced dosage of Msx2 in the Twist1 mutant background restores the expression of ephrin-A4, rescues the suture boundary and inhibits craniosynostosis. Underlining the importance of ephrin-A4, we identified heterozygous mutations in the human orthologue, EFNA4, in three of 81 patients with non-syndromic coronal synostosis. This provides genetic evidence that Twist1, Msx2 and Efna4 function together in boundary formation and the pathogenesis of coronal synostosis.

Patterns of infantile hemangiomas: new clues to hemangioma pathogenesis and embryonic facial development

OBJECTIVES

Large facial infantile hemangiomas have higher rates of complications than small localized hemangiomas, more often require treatment, and can be associated with neurological, ophthalmologic, and cardiac anomalies (PHACE syndrome). The anatomic patterns of these hemangiomas are often referred to as "segmental" despite a lack of precise anatomic definitions. Our study aims to define "segmental" hemangiomas based on clinically observed patterns. Our secondary goal is to relate the observed patterns to currently accepted developmental patterns to gain insight into hemangioma pathogenesis and craniofacial development.

METHODS

Photographic data were extracted from a large cohort of patients with infantile hemangiomas. We mapped 294 hemangiomas and recorded common morphologic patterns. Anatomic descriptions of the most common patterns were described and compared with accepted concepts of craniofacial development.

RESULTS

Four primary segments were identified (Seg1-Seg4). Seg2 and Seg3 correspond with the previously recognized maxillary and mandibular prominences. Seg1 and Seg4 differ from standard human embryology texts. The frontotemporal segment, Seg1, encompasses the lateral forehead, anterior temporal scalp, and lateral frontal scalp. The segment Seg4, encompassing the medial frontal scalp, nasal bridge, nasal tip, ala, and philtrum, is substantially narrower on the forehead than the previously described frontonasal prominence.

CONCLUSIONS

The patterns provide new clues regarding facial development. The observed patterns resemble previously described facial developmental units on the lower face but are distinctly different on the upper face. The patterns suggest that neural crest derivatives may play a role in the development of facial hemangiomas. Finally, these patterns (Seg1-Seg4) help standardize the nomenclature of facial segmental hemangiomas to analyze more effectively hemangioma risks and behavior.

FGF9 can induce endochondral ossification in cranial mesenchyme

Background

The flat bones of the skull (i.e., the frontal and parietal bones) normally form through intramembranous ossification. At these sites cranial mesenchymal cells directly differentiate into osteoblasts without the formation of a cartilage intermediate. This type of ossification is distinct from endochondral ossification, a process that involves initial formation of cartilage and later replacement by bone.

Results

We have analyzed a line of transgenic mice that expresses FGF9, a member of the fibroblast growth factor family (FGF), in cranial mesenchymal cells. The parietal bones in these mice show a switch from intramembranous to endochondral ossification. Cranial cartilage precursors are induced to proliferate, then hypertrophy and are later replaced by bone. These changes are accompanied by upregulation of Sox9, Ihh, Col2a1, Col10a1 and downregulation of CbfaI and Osteocalcin. Fate mapping studies show that the cranial mesenchymal cells in the parietal region that show a switch in cell fate are likely to be derived from the mesoderm.

Conclusion

These results demonstrate that FGF9 expression is sufficient to convert the differentiation program of (at least a subset of) mesoderm-derived cranial mesenchyme cells from intramembranous to endochondral ossification.

Neurulation in the cranial region--normal and abnormal

Cranial neurulation is the embryonic process responsible for formation of the brain primordium. In the mouse embryo, cranial neurulation is a piecemeal process with several initiation sites and two neuropores. Variation in the pattern of cranial neurulation occurs in different mouse strains, and a simpler version of this morphogenetic scheme has been described in human embryos. Exencephaly is more common in females than in males, an unexplained phenomenon seen in both mice and humans. As the cranial neural tube closes, a critical morphogenetic event is the formation of dorsolateral bending points near the neural fold tips, which enables subsequent midline fusion of the neural folds. Many mutant and gene-targeted mouse strains develop cranial neural tube defects, and analysis of the underlying molecular defects identifies several requirements for normal dorsolateral bending. These include a functional actin cytoskeleton, emigration of the cranial neural crest, spatio-temporally regulated apoptosis, and a balance between cell proliferation and the onset of neuronal differentiation. A small number of mouse mutants exhibit craniorachischisis, a combined brain and spine neurulation defect. Recent studies show that disturbance of a single molecular signalling cascade, the planar cell polarity pathway, is implicated in mutants with this defect.

Relations and interactions between cranial mesoderm and neural crest populations

The embryonic head is populated by two robust mesenchymal populations, paraxial mesoderm and neural crest cells. Although the developmental histories of each are distinct and separate, they quickly establish intimate relations that are variably important for the histogenesis and morphogenesis of musculoskeletal components of the calvaria, midface and branchial regions. This review will focus first on the genesis and organization within nascent mesodermal and crest populations, emphasizing interactions that probably initiate or augment the establishment of lineages within each. The principal goal is an analysis of the interactions between crest and mesoderm populations, from their first contacts through their concerted movements into peripheral domains, particularly the branchial arches, and continuing to stages at which both the differentiation and the integrated three-dimensional assembly of vascular, connective and muscular tissues is evident. Current views on unresolved or contentious issues, including the relevance of head somitomeres, the processes by which crest cells change locations and constancy of cell-cell relations at the crest-mesoderm interface, are addressed.

Growth of the normal skull vault and its alteration in craniosynostosis: insights from human genetics and experimental studies

The mammalian skull vault is constructed principally from five bones: the paired frontals and parietals, and the unpaired interparietal. These bones abut at sutures, where most growth of the skull vault takes place. Sutural growth involves maintenance of a population of proliferating osteoprogenitor cells which differentiate into bone matrix-secreting osteoblasts. Sustained function of the sutures as growth centres is essential for continuous expansion of the skull vault to accommodate the growing brain. Craniosynostosis, the premature fusion of the cranial sutures, occurs in 1 in 2500 children and often presents challenging clinical problems. Until a dozen years ago, little was known about the causes of craniosynostosis but the discovery of mutations in the MSX2, FGFR1, FGFR2, FGFR3, TWIST1 and EFNB1 genes in both syndromic and non-syndromic cases has led to considerable insights into the aetiology, classification and developmental pathology of these disorders. Investigations of the biological roles of these genes in cranial development and growth have been carried out in normal and mutant mice, elucidating their individual and interdependent roles in normal sutures and in sutures undergoing synostosis. Mouse studies have also revealed a significant correspondence between the neural crest-mesoderm boundary in the early embryonic head and the position of cranial sutures, suggesting roles for tissue interaction in suture formation, including initiation of the signalling system that characterizes the functionally active suture.

Craniofacial dermoids: an embryological theory unifying nasal dermoid sinus cysts

OBJECTIVE

The nasal dermoid sinus cyst (NDSC) is an uncommon congenital lesion presenting as a large panel of midline craniofacial anomalies. The objective of this study was to review and reanalyze embryological hypotheses concerning NDSCs and to propose an embryological theory unifying the various anatomical characteristics of these lesions. The first case of frontal localization of a NDSC extending within the diploetic bone in a 9-month-old boy, presenting as a median frontal fistula with recurrent frontal swelling, 6 months after a mild frontal trauma is presented.

RESULTS

Complete surgical removal was performed, and there was no evidence of either persistent or recurrent disease 2 years after his surgery. The embryological and anatomical origins of NDSCs are reviewed. This article reexamines and discusses major embryological theories on NDSC pathogenesis and proposes to refute the "prenasal space" theory of Grunwald and rehabilitate a forgotten embryological hypothesis, which unifies the main various clinical presentations of NDSCs.

Twist1 dimer selection regulates cranial suture patterning and fusion

Saethre-Chotzen syndrome is associated with haploinsufficiency of the basic-helix-loop-helix (bHLH) transcription factor TWIST1 and is characterized by premature closure of the cranial sutures, termed craniosynostosis; however, the mechanisms underlying this defect are unclear. Twist1 has been shown to play both positive and negative roles in mesenchymal specification and differentiation, and here we show that the activity of Twist1 is dependent on its dimer partner. Twist1 forms both homodimers (T/T) and heterodimers with E2A E proteins (T/E) and the relative level of Twist1 to the HLH inhibitor Id proteins determines which dimer forms. On the basis of the expression patterns of Twist1 and Id1 within the cranial sutures, we hypothesized that Twist1 forms homodimers in the osteogenic fronts and T/E heterodimers in the mid-sutures. In support of this hypothesis, we have found that genes regulated by T/T homodimers, such as FGFR2 and periostin, are expressed in the osteogenic fronts, whereas genes regulated by T/E heterodimers, such as thrombospondin-1, are expressed in the mid-sutures. The ratio between these dimers is altered in the sutures of Twist1+/- mice, favoring an increase in homodimers and an expansion of the osteogenic fronts. Of interest, the T/T to T/E ratio is greater in the coronal versus the sagittal suture, and this finding may contribute to making the coronal suture more susceptible to fusion due to TWIST haploinsufficiency. Importantly, we were able to inhibit suture fusion in Twist1+/- mice by modulating the balance between these dimers toward T/E formation, by either increasing the expression of E2A E12 or by decreasing Id expression. Therefore, we have identified dimer partner selection as an important mediator of Twist1 function and provide a mechanistic understanding of craniosynostosis due to TWIST haploinsufficiency.

Recent advances in craniofacial morphogenesis

Craniofacial malformations are involved in three fourths of all congenital birth defects in humans, affecting the development of head, face, or neck. Tremendous progress in the study of craniofacial development has been made that places this field at the forefront of biomedical research. A concerted effort among evolutionary and developmental biologists, human geneticists, and tissue engineers has revealed important information on the molecular mechanisms that are crucial for the patterning and formation of craniofacial structures. Here, we highlight recent advances in our understanding of evo-devo as it relates to craniofacial morphogenesis, fate determination of cranial neural crest cells, and specific signaling pathways in regulating tissue-tissue interactions during patterning of craniofacial apparatus and the morphogenesis of tooth, mandible, and palate. Together, these findings will be beneficial for the understanding, treatment, and prevention of human congenital malformations and establish the foundation for craniofacial tissue regeneration.

The contribution of the neural crest to the vertebrate body

As a transitory structure providing adult tissues of the vertebrates with very diverse cell types, the neural crest (NC) has attracted for long the interest of developmental biologists and is still the subject of ongoing research in a variety of animal models. Here we review a number of data from in vivo cell tracing and in vitro single cell culture experiments, which gained new insights on the mechanisms of cell migration, proliferation and differentiation during NC ontogeny. We put emphasis on the role of Hox genes, morphogens and interactions with neighbouring tissues in specifying and patterning the skeletogenic NC cells in the head. We also include advances made towards characterizing multipotent stem cells in the early NC as well as in various NC derivatives in embryos and even in adult.

Study ofPax6 mutant rat revealed the association between upper incisor formation and midface formation

In this study, we investigated the process of supernumerary upper incisor formation in the Pax6 mutant rat, rSey(2)/rSey(2), which exhibits a facial cleft between the medial nasal and maxillary processes. Histological investigation and epithelial labeling studies of wild type rat embryos indicated that the upper incisor develops by fusion of two primary dental placodes (PDPs) in the medial nasal process with a contribution from the epithelium of the maxillary process. In the rSey(2)/rSey(2) embryo, both PDPs are formed but they stay apart, then subsequently these PDPs independently develop into upper incisor tooth buds. In order to examine if the failure of the two placodes to fuse is due to the cleft between the maxillary and medial nasal processes, maxillary and medial nasal process fusion was inhibited with a barrier in wild type embryos. This resulted in the maintenance of the two distinct PDPs. These results demonstrate that fusion of the facial processes reduces the number of odontogenic placodes and is required to assemble all components at one site for rat upper incisor formation. The results also provide further insight into the mechanism of supernumerary incisor formation in human cleft lip conditions.

Spatial relations between avian craniofacial neural crest and paraxial mesoderm cells

Fate maps based on quail-chick grafting of avian cephalic neural crest precursors and paraxial mesoderm cells have identified the majority of derivatives from each population but have not unequivocally resolved the precise locations of and population dynamics at the interface between them. The relation between these two mesenchymal tissues is especially critical for the development of skeletal muscles, because crest cells play an essential role in their differentiation and subsequent spatial organization. It is not known whether myogenic mesoderm and skeletogenic neural crest cells establish permanent relations while en route to their final destinations, or later at the sites where musculoskeletal morphogenesis is completed. We applied beta-galactosidase-encoding, replication-incompetent retroviruses to paraxial mesoderm, to crest progenitors, or at the interface between mesodermal and overlying neural crest as both were en route to branchial or periocular regions in chick embryos. With respect to skeletal structures, the results identify the avian neural crest:mesoderm boundary at the junction of the supraorbital and calvarial regions of the frontal bone, lateral to the hypophyseal foramen, and rostral to laryngeal cartilages. Therefore, in the chick embryo, most of the frontal and the entire parietal bone are of mesodermal, not neural crest, origin. Within paraxial mesoderm, the progenitors of each lineage display different behaviors. Chondrogenic cells are relatively stationary and intramembranous osteogenic cells move only in transverse planes around the brain. Angioblasts migrate invasively in all directions. Extraocular muscle precursors form tightly aggregated masses that en masse cross the crest:mesoderm interface to enter periocular territories, while branchial myogenic lineages shift ventrally coincidental with the movements of corresponding neural crest cells. En route to the branchial arches, myogenic mesoderm cells do not maintain constant, nearest-neighbor relations with adjacent, overlying neural crest cells. Thus, progenitors of individual muscles do not establish stable, permanent relations with their connective tissues until both populations reach the sites of their morphogenesis within branchial arches or orbital regions.

The differentiation and morphogenesis of craniofacial muscles

Unraveling the complex tissue interactions necessary to generate the structural and functional diversity present among craniofacial muscles is challenging. These muscles initiate their development within a mesenchymal population bounded by the brain, pharyngeal endoderm, surface ectoderm, and neural crest cells. This set of spatial relations, and in particular the segmental properties of these adjacent tissues, are unique to the head. Additionally, the lack of early epithelialization in head mesoderm necessitates strategies for generating discrete myogenic foci that may differ from those operating in the trunk. Molecular data indeed indicate dissimilar methods of regulation, yet transplantation studies suggest that some head and trunk myogenic populations are interchangeable. The first goal of this review is to present key features of these diversities, identifying and comparing tissue and molecular interactions regulating myogenesis in the head and trunk. Our second focus is on the diverse morphogenetic movements exhibited by craniofacial muscles. Precursors of tongue muscles partly mimic migrations of appendicular myoblasts, whereas myoblasts destined to form extraocular muscles condense within paraxial mesoderm, then as large cohorts they cross the mesoderm:neural crest interface en route to periocular regions. Branchial muscle precursors exhibit yet another strategy, establishing contacts with neural crest populations before branchial arch formation and maintaining these relations through subsequent stages of morphogenesis. With many of the prerequisite stepping-stones in our knowledge of craniofacial myogenesis now in place, discovering the cellular and molecular interactions necessary to initiate and sustain the differentiation and morphogenesis of these neglected craniofacial muscles is now an attainable goal.

The new bone biology: Pathologic, molecular, and clinical correlates

Bone and cartilage and their disorders are addressed under the following headings: functions of bone; normal and abnormal bone remodeling; osteopetrosis and osteoporosis; epithelial-mesenchymal interaction, condensation and differentiation; osteoblasts, markers of bone formation, osteoclasts, components of bone, and pathology of bone; chondroblasts, markers of cartilage formation, secondary cartilage, components of cartilage, and pathology of cartilage; intramembranous and endochondral bone formation; RUNX genes and cleidocranial dysplasia (CCD); osterix; histone deacetylase 4 and Runx2; Ligand to receptor activator of NFkappaB (RANKL), RANK, osteoprotegerin, and osteoimmunology; WNT signaling, LRP5 mutations, and beta-catenin; the role of leptin in bone remodeling; collagens, collagenopathies, and osteogenesis imperfecta; FGFs/FGFRs, FGFR3 skeletal dysplasias, craniosynostosis, and other disorders; short limb chondrodysplasias; molecular control of the growth plate in endochondral bone formation and genetic disorders of IHH and PTHR1; ANKH, craniometaphyseal dysplasia, and chondrocalcinosis; transforming growth factor beta, Camurati-Engelmann disease (CED), and Marfan syndrome, types I and II; an ACVR1 mutation and fibrodysplasia ossificans progressiva; MSX1 and MSX2: biology, mutations, and associated disorders; G protein, activation of adenylyl cyclase, GNAS1 mutations, McCune-Albright syndrome, fibrous dysplasia, and Albright hereditary osteodystrophy; FLNA and associated disorders; and morphological development of teeth and their genetic mutations.

The homeoprotein engrailed 1 has pleiotropic functions in calvarial intramembranous bone formation and remodeling

The membranous bones of the mammalian skull vault arise from discrete condensations of neural crest- and mesodermally-derived cells. Recently, a number of homeodomain transcription factors have been identified as critical regulators of this process. Here, we show that the homeoprotein engrailed 1(EN1) is expressed during embryonic and perinatal craniofacial bone development, where it localizes to the skeletogenic mesenchyme, and,subsequently, to calvarial osteoblasts and osteoprogenitors. Mice lacking En1 exhibit generalized calvarial bone hypoplasia and persistent widening of the sutural joints. A reduction in calvarial membranous bone deposition and mineralization (osteopenia) is coupled to enhanced osteolytic resorption in En1 mutants. Consistent with these observations,expression of established osteoblast differentiation markers reveals that En1 function is required for both early and late phases of calvarial osteogenesis. Further analysis shows that EN1 regulates FGF signaling in calvarial osteoblasts. Moreover, EN1 indirectly influences calvarial osteoclast recruitment and bone resorption by regulating the expression of receptor activator of NFκB ligand (RANKL) in osteoblasts. Thus, during intramembranous bone formation, EN1 acts both cell autonomously and non-cell autonomously. In summary, this study identifies EN1 as a novel modulator of calvarial osteoblast differentiation and proliferation, processes that must be exquisitely balanced to ensure proper skull vault formation.

Patterning the neural crest derivatives during development of the vertebrate head: insights from avian studies

Studies carried out in the avian embryo and based on the construction of quail-chick chimeras have shown that most of the skull and all the facial and visceral skeleton are derived from the cephalic neural crest (NC). Contribution of the mesoderm is limited to its occipital and (partly) to its otic domains. NC cells (NCCs) participating in membrane bones and cartilages of the vertebrate head arise from the diencephalon (posterior half only), the mesencephalon and the rhombencephalon. They can be divided into an anterior domain (extending down to r2 included) in which genes of the Hox clusters are not expressed (Hox-negative skeletogenic NC) and a posterior domain including r4 to r8 in which Hox genes of the four first paraloguous groups are expressed. The NCCs that form the facial skeleton belong exclusively to the anterior Hox-negative domain and develop from the first branchial arch (BA1). This rostral domain of the crest is designated as FSNC for facial skeletogenic neural crest. Rhombomere 3 (r3) participates modestly to both BA1 and BA2. Forced expression of Hox genes (Hoxa2, Hoxa3 and Hoxb4) in the neural fold of the anterior domain inhibits facial skeleton development. Similarly, surgical excision of these anterior Hox-negative NCCs results in the absence of facial skeleton, showing that Hox-positive NCCs cannot replace the Hox-negative domain for facial skeletogenesis. We also show that excision of the FSNC results in dramatic down-regulation of Fgf8 expression in the head, namely in ventral forebrain and in BA1 ectoderm. We have further demonstrated that exogenous FGF8 applied to the presumptive BA1 territory at the 5-6-somite stage (5-6ss) restores to a large extent facial skeleton development. The source of the cells responsible for this regeneration was shown to be r3, which is at the limit between the Hox-positive and Hox-negative domain. NCCs that respond to FGF8 by survival and proliferation are in turn necessary for the expression/maintenance of Fgf8 expression in the ectoderm. These results strongly support the emerging picture according to which the processes underlying morphogenesis of the craniofacial skeleton are regulated by epithelial-mesenchymal bidirectional crosstalk.

Evolution of cranial development and the role of neural crest: insights from amphibians

Contemporary studies of vertebrate cranial development document the essential role played by the embryonic neural crest as both a source of adult tissues and a locus of cranial form and patterning. Yet corresponding and basic features of cranial evolution, such as the extent of conservation vs. variation among species in the contribution of the neural crest to specific structures, remain to be adequately resolved. Investigation of these features requires comparable data from species that are both phylogenetically appropriate and taxonomically diverse. One key group are amphibians, which are uniquely able to inform our understanding of the ancestral patterns of ontogeny in fishes and tetrapods as well as the evolution of presumably derived patterns reported for amniotes. Recent data support the hypothesis that a prominent contribution of the neural crest to cranial skeletal and muscular connective tissues is a fundamental property that evolved early in vertebrate history and is retained in living forms. The contribution of the neural crest to skull bones appears to be more evolutionarily labile than that of cartilages, although significance of the limited comparative data is difficult to establish at present. Results underline the importance of accurate and reliable homology assessments for evaluating the contrasting patterns of derivation reported for the three principal tetrapod models: mouse, chicken and frog.

Pluripotent neural crest stem cells in the adult hair follicle

We report the presence of pluripotent neural crest stem cells in the adult mammalian hair follicle. Numerous neural crest cells reside in the outer root sheath from the bulge to the matrix at the base of the follicle. Bulge explants from adult mouse whisker follicles yield migratory neural crest cells, which in clonal culture form colonies consisting of over a thousand cells. Clones contain neurons, smooth muscle cells, rare Schwann cells and melanocytes, demonstrating pluripotency of the clone-forming cell. Targeted differentiation into Schwann cells and chondrocytes was achieved with neuregulin-1 and bone morphogenetic protein-2, respectively. Serial cloning in vitro demonstrated self-renewal capability. Together, the data show that the adult mouse whisker follicle contains pluripotent neural crest stem cells, termed epidermal neural crest cells (eNCSC). eNCSC are promising candidates for diverse cell therapy paradigms because of their high degree of inherent plasticity and due to their easy accessibility in the skin.

Malformations of the axial skeleton in the museum Vrolik: II: craniosynostoses and suture-related conditions

The Museum Vrolik collection of anatomical specimens in Amsterdam, The Netherlands, comprises over 5,000 specimens of human and animal anatomy, embryology, pathology, and congenital anomalies. Recently, we rediagnosed a subset of the collection comprising dried infantile, juvenile, and adult human skulls with congenital and acquired conditions. On external examination and additional radiography, we found 58 skulls with craniosynostosis (CS) involving one or more sutures and 40 skulls with a presumed suture related condition. Most of these were part of the material collected and described by Louis Bolk (1866-1930). Analysis of his observations suggests that skull deformation because of premature suture closure depends not only on the identity of the sutures involved but also on the timing and progression of their closure and the extent of their involvement. Moreover, premature closure of the sagittal suture after 3-6 years of age appeared to be much more common than expected because it is not accompanied by skull deformation. Many of the skulls with single-suture CS were microcephalic, which may be the cause of the premature synostosis. By contrast, microcephaly may be a resulting phenomenon in multi-suture CS. We noticed that the quotient between height of the cranial vault (vertex-porion distance) and head circumference, multiplied by 100, was 26 or higher only in those CS cases with multi-suture involvement. We therefore consider this parameter, which we named "acrocephalic index", to be an indicator of multi-suture involvement in individual CS cases. In two adult skulls, the skull had a quadrangular shape, which we assumed to be correlated to the presence of an unusually interdigitated open metopic suture. We propose to name this anomaly: tetragonocephaly. Another presumed suture-related condition, bathrocephaly, was found concomitantly with basilar invagination in several cases. We hypothesize that the chronically raised intracranial pressure in these cases caused the still open lambdoidal sutures to distend and the occipital bone to protrude.

Abnormalities in cartilage and bone development in the Apert syndrome FGFR2+/S252W mouse

Apert syndrome is an autosomal dominant disorder characterized by malformations of the skull, limbs and viscera. Two-thirds of affected individuals have a S252W mutation in fibroblast growth factor receptor 2(FGFR2). To study the pathogenesis of this condition, we generated a knock-in mouse model with this mutation. The Fgfr2+/S252W mutant mice have abnormalities of the skeleton, as well as of other organs including the brain, thymus, lungs, heart and intestines. In the mutant neurocranium, we found a midline sutural defect and craniosynostosis with abnormal osteoblastic proliferation and differentiation. We noted ectopic cartilage at the midline sagittal suture, and cartilage abnormalities in the basicranium, nasal turbinates and trachea. In addition, from the mutant long bones, in vitro cell cultures grown in osteogenic medium revealed chondrocytes, which were absent in the controls. Our results suggest that altered cartilage and bone development play a significant role in the pathogenesis of the Apert syndrome phenotype.

Specification of neural crest cell formation and migration in mouse embryos

Of all the model organisms used to study human development, rodents such as mice most accurately reflect human craniofacial development. Collective advances in mouse embryology and mouse genetics continue to shape our understanding of neural crest cell development and by extrapolation the etiology of human congenital head and facial birth defects. The aim of this review is to highlight the considerable progress being made in our understanding of cranial neural crest cell patterning in mouse embryos.

Neural crest origins of the neck and shoulder

The neck and shoulder region of vertebrates has undergone a complex evolutionary history. To identify its underlying mechanisms we map the destinations of embryonic neural crest and mesodermal stem cells using Cre-recombinase-mediated transgenesis. The single-cell resolution of this genetic labelling reveals cryptic cell boundaries traversing the seemingly homogeneous skeleton of the neck and shoulders. Within this assembly of bones and muscles we discern a precise code of connectivity that mesenchymal stem cells of both neural crest and mesodermal origin obey as they form muscle scaffolds. The neural crest anchors the head onto the anterior lining of the shoulder girdle, while a Hox-gene-controlled mesoderm links trunk muscles to the posterior neck and shoulder skeleton. The skeleton that we identify as neural crest-derived is specifically affected in human Klippel-Feil syndrome, Sprengel's deformity and Arnold-Chiari I/II malformation, providing insights into their likely aetiology. We identify genes involved in the cellular modularity of the neck and shoulder skeleton and propose a new method for determining skeletal homologies that is based on muscle attachments. This has allowed us to trace the whereabouts of the cleithrum, the major shoulder bone of extinct land vertebrate ancestors, which seems to survive as the scapular spine in living mammals.

The role of Axin2 in calvarial morphogenesis and craniosynostosis

Axin1 and its homolog Axin2/conductin/Axil are negative regulators of the canonical Wnt pathway that suppress signal transduction by promoting degradation of beta-catenin. Mice with deletion of Axin1 exhibit defects in axis determination and brain patterning during early embryonic development. We show that Axin2 is expressed in the osteogenic fronts and periosteum of developing sutures during skull morphogenesis. Targeted disruption of Axin2 in mice induces malformations of skull structures, a phenotype resembling craniosynostosis in humans. In the mutants, premature fusion of cranial sutures occurs at early postnatal stages. To elucidate the mechanism of craniosynostosis, we studied intramembranous ossification in Axin2-null mice. The calvarial osteoblast development is significantly affected by the Axin2 mutation. The Axin2 mutant displays enhanced expansion of osteoprogenitors, accelerated ossification, stimulated expression of osteogenic markers and increases in mineralization. Inactivation of Axin2 promotes osteoblast proliferation and differentiation in vivo and in vitro. Furthermore, as the mammalian skull is formed from cranial skeletogenic mesenchyme, which is derived from mesoderm and neural crest, our data argue for a region-specific effect of Axin2 on neural crest dependent skeletogenesis. The craniofacial anomalies caused by the Axin2 mutation are mediated through activation of beta-catenin signaling, suggesting a novel role for the Wnt pathway in skull morphogenesis.

Targeted disruption of the Mn1 oncogene results in severe defects in development of membranous bones of the cranial skeleton

Fusion of the MN1 gene to TEL (ETV6) results in myeloid leukemia. The fusion protein combines the transcription activating domain of MN1 and the DNA binding domain of TEL and is thought to act as a deranged transcription factor. In addition, disruption of the large first exon of the MN1 gene is thought to inactivate MN1 function in a meningioma. To further investigate the role of MN1 in cancer, we generated Mn1 knockout mice. Mn1(+/-) animals were followed for 30 months, but they had no higher incidence of tumor formation than wild-type littermates. Mn1 null mice, however, were found to die at birth or shortly thereafter as the result of a cleft palate. Investigation of newborn or embryonic day 15.5 (E15.5) to E17.5 null mice revealed that the development of several bones in the skull was abnormal. The affected bones are almost exclusively formed by intramembranous ossification. They are either completely agenic at birth (alisphenoid and squamosal bones and vomer), hypoplastic, deformed (basisphenoid, pterygoid, and presphenoid), or substantially thinner (frontal, parietal, and interparietal bones). In heterozygous mice hypoplastic membranous bones and incomplete penetrance of the cleft palate were observed. We conclude that Mn1 is an important factor in development of membranous bones.