Unexpected role of SIX1 variants in craniosynostosis: expanding the phenotype of SIX1-related disorders

BACKGROUND

Pathogenic heterozygous SIX1 variants (predominantly missense) occur in branchio-otic syndrome (BOS), but an association with craniosynostosis has not been reported.

METHODS

We investigated probands with craniosynostosis of unknown cause using whole exome/genome (n=628) or RNA (n=386) sequencing, and performed targeted resequencing of SIX1 in 615 additional patients. Expression of SIX1 protein in embryonic cranial sutures was examined in the Six1nLacZ/+ reporter mouse.

RESULTS

From 1629 unrelated cases with craniosynostosis we identified seven different SIX1 variants (three missense, including two de novo mutations, and four nonsense, one of which was also present in an affected twin). Compared with population data, enrichment of SIX1 loss-of-function variants was highly significant (p=0.00003). All individuals with craniosynostosis had sagittal suture fusion; additionally four had bilambdoid synostosis. Associated BOS features were often attenuated; some carrier relatives appeared non-penetrant. SIX1 is expressed in a layer basal to the calvaria, likely corresponding to the dura mater, and in the mid-sagittal mesenchyme.

CONCLUSION

Craniosynostosis is associated with heterozygous SIX1 variants, with possible enrichment of loss-of-function variants compared with classical BOS. We recommend screening of SIX1 in craniosynostosis, particularly when sagittal±lambdoid synostosis and/or any BOS phenotypes are present. These findings highlight the role of SIX1 in cranial suture homeostasis.

The developing mouse coronal suture at single-cell resolution

Sutures separate the flat bones of the skull and enable coordinated growth of the brain and overlying cranium. The coronal suture is most commonly fused in monogenic craniosynostosis, yet the unique aspects of its development remain incompletely understood. To uncover the cellular diversity within the murine embryonic coronal suture, we generated single-cell transcriptomes and performed extensive expression validation. We find distinct pre-osteoblast signatures between the bone fronts and periosteum, a ligament-like population above the suture that persists into adulthood, and a chondrogenic-like population in the dura mater underlying the suture. Lineage tracing reveals an embryonic Six2+ osteoprogenitor population that contributes to the postnatal suture mesenchyme, with these progenitors being preferentially affected in a Twist1+/-; Tcf12+/- mouse model of Saethre-Chotzen Syndrome. This single-cell atlas provides a resource for understanding the development of the coronal suture and the mechanisms for its loss in craniosynostosis.

Integrated Transcriptome and Network Analysis Reveals Spatiotemporal Dynamics of Calvarial Suturogenesis

Craniofacial abnormalities often involve sutures, the growth centers of the skull. To characterize the organization and processes governing their development, we profile the murine frontal suture, a model for sutural growth and fusion, at the tissue- and single-cell level on embryonic days (E)16.5 and E18.5. For the wild-type suture, bulk RNA sequencing (RNA-seq) analysis identifies mesenchyme-, osteogenic front-, and stage-enriched genes and biological processes, as well as alternative splicing events modifying the extracellular matrix. Single-cell RNA-seq analysis distinguishes multiple subpopulations, of which five define a mesenchyme-osteoblast differentiation trajectory and show variation along the anteroposterior axis. Similar analyses of in vivo mouse models of impaired frontal suturogenesis in Saethre-Chotzen and Apert syndromes, Twist1+/- and Fgfr2+/S252W, demonstrate distinct transcriptional changes involving angiogenesis and ribogenesis, respectively. Co-expression network analysis reveals gene expression modules from which we validate key driver genes regulating osteoblast differentiation. Our study provides a global approach to gain insights into suturogenesis.

Recent Advances in Craniosynostosis

Craniosynostosis is a pathologic craniofacial disorder and is defined as the premature fusion of one or more cranial (calvarial) sutures. Cranial sutures are fibrous joints consisting of nonossified mesenchymal cells that play an important role in the development of healthy craniofacial skeletons. Early fusion of these sutures results in incomplete brain development that may lead to complications of several severe medical conditions including seizures, brain damage, mental delay, complex deformities, strabismus, and visual and breathing problems. As a congenital disease, craniosynostosis has a heterogeneous origin that can be affected by genetic and epigenetic alterations, teratogens, and environmental factors and make the syndrome highly complex. To date, approximately 200 syndromes have been linked to craniosynostosis. In addition to being part of a syndrome, craniosynostosis can be nonsyndromic, formed without any additional anomalies. More than 50 nuclear genes that relate to craniosynostosis have been identified. Besides genetic factors, epigenetic factors like microRNAs and mechanical forces also play important roles in suture fusion. As craniosynostosis is a multifactorial disorder, evaluating the craniosynostosis syndrome requires and depends on all the information obtained from clinical findings, genetic analysis, epigenetic or environmental factors, or gene modulators. In this review, we will focus on embryologic and genetic studies, as well as epigenetic and environmental studies. We will discuss published studies and correlate the findings with unknown aspects of craniofacial disorders.

A Genetic-Pathophysiological Framework for Craniosynostosis

Craniosynostosis, the premature fusion of one or more cranial sutures of the skull, provides a paradigm for investigating the interplay of genetic and environmental factors leading to malformation. Over the past 20 years molecular genetic techniques have provided a new approach to dissect the underlying causes; success has mostly come from investigation of clinical samples, and recent advances in high-throughput DNA sequencing have dramatically enhanced the study of the human as the preferred "model organism." In parallel, however, we need a pathogenetic classification to describe the pathways and processes that lead to cranial suture fusion. Given the prenatal onset of most craniosynostosis, investigation of mechanisms requires more conventional model organisms; principally the mouse, because of similarities in cranial suture development. We present a framework for classifying genetic causes of craniosynostosis based on current understanding of cranial suture biology and molecular and developmental pathogenesis. Of note, few pathologies result from complete loss of gene function. Instead, biochemical mechanisms involving haploinsufficiency, dominant gain-of-function and recessive hypomorphic mutations, and an unusual X-linked cellular interference process have all been implicated. Although few of the genes involved could have been predicted based on expression patterns alone (because the genes play much wider roles in embryonic development or cellular homeostasis), we argue that they fit into a limited number of functional modules active at different stages of cranial suture development. This provides a useful approach both when defining the potential role of new candidate genes in craniosynostosis and, potentially, for devising pharmacological approaches to therapy.

Metopic suture and RUNX2, a key transcription factor in osseous morphogenesis with possible important implications for human brain evolution

BACKGROUND

Overall, the comparative data available on the timing of metopic suture closure in present-day and fossil members of human lineage, as well as great apes, seem to indicate that human brain evolution occurred within a complex network of fetopelvic constraints, which required modification of frontal neurocranial ossification patterns, involving delayed fusion of the metopic suture. It is very interesting that the recent sequencing of the Neanderthal genome has revealed signs of positive selection in the modern human variant of the RUNX2 gene, which is known to affect metopic suture fusion in addition to being essential for osteoblast development and proper bone formation. It is possible that an evolutionary change in RUNX2, affecting aspects of the morphology of the upper body and cranium, was of importance in the origin of modern humans. Thus, to contribute to a better understanding of the molecular evolution of this gene probably implicated in human evolution, we performed a comparative bioinformatic analysis of the coding sequences of RUNX2 in Homo sapiens and other non-human Primates.

RESULTS

We found amino-acid sequence differences between RUNX2 protein isoforms of Homo sapiens and the other Primates examined, that might have important implications for the timing of metopic suture closure.

CONCLUSIONS

Further studies are needed to clear the potential distinct developmental roles of different species-specific RUNX2 N-terminal isoforms. Meantime, our bioinformatic analysis, regarding expression of the RUNX2 gene in Homo sapiens and other non-human Primates, has provided a contribution to this important issue of human evolution.

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.

Craniosynostosis and multiple skeletal anomalies in humans and zebrafish result from a defect in the localized degradation of retinoic acid

Excess exogenous retinoic acid (RA) has been well documented to have teratogenic effects in the limb and craniofacial skeleton. Malformations that have been observed in this context include craniosynostosis, a common developmental defect of the skull that occurs in 1 in 2500 individuals and results from premature fusion of the cranial sutures. Despite these observations, a physiological role for RA during suture formation has not been demonstrated. Here, we present evidence that genetically based alterations in RA signaling interfere with human development. We have identified human null and hypomorphic mutations in the gene encoding the RA-degrading enzyme CYP26B1 that lead to skeletal and craniofacial anomalies, including fusions of long bones, calvarial bone hypoplasia, and craniosynostosis. Analyses of murine embryos exposed to a chemical inhibitor of Cyp26 enzymes and zebrafish lines with mutations in cyp26b1 suggest that the endochondral bone fusions are due to unrestricted chondrogenesis at the presumptive sites of joint formation within cartilaginous templates, whereas craniosynostosis is induced by a defect in osteoblastic differentiation. Ultrastructural analysis, in situ expression studies, and in vitro quantitative RT-PCR experiments of cellular markers of osseous differentiation indicate that the most likely cause for these phenomena is aberrant osteoblast-osteocyte transitioning. This work reveals a physiological role for RA in partitioning skeletal elements and in the maintenance of cranial suture patency.

Growth of cranial synchondroses and sutures requires polycystin-1

In vertebrates, coordinated embryonic and postnatal growth of the craniofacial bones and the skull base is essential during the expansion of the rostrum and the brain. Identification of molecules that regulate skull growth is important for understanding the nature of craniofacial defects and for development of non-invasive biologically based diagnostics and therapies. Here we report on spatially restricted growth defects at the skull base and in craniofacial sutures of mice deficient for polycystin-1 (Pkd1). Mutant animals reveal a premature closure of both presphenoid and sphenooccipital synchondroses at the cranial base. Furthermore, knockout mice lacking Pkd1 in neural crest cells are characterized by impaired postnatal growth at the osteogenic fronts in craniofacial sutures that are subjected to tensile forces. Our data suggest that polycystin-1 is required for proliferation of subpopulations of cranial osteochondroprogenitor cells of both mesodermal and neural crest origin during skull growth. However, the Erk1/2 signalling pathway is up-regulated in the Pkd1-deficient skeletal tissue, similarly to that previously reported for polycystic kidney.

[Genetics of craniofacial development]

Congenital craniofacial malformations vary widely in both expression and gravity. To understand congenital craniofacial malformations, knowledge of embryonic development is of essential importance. Craniosynostosis has its origin in the failure of suture development between 2 bone centres or in early closure of the suture by bone centre tissue fusion. Hereditary craniosynostosis phenotypes predominantly arise by autosomal dominant inheritance. So far, the majority of mutations have been found in fibroblast growth-factor receptor genes (FGFR-genes). Different phenotypes are not primarily created by disparities of the receptors, but particularly by tissue-specific expressions.

Effects of induced premaxillary suture fusion on the craniofacial morphology in growing rats

OBJECTIVE

Due to premaxillary rapid development and fusion with the maxilla at the fetus stage, the functions of the premaxillary suture still remain unclear. This study was designed to explore the effect of artificial induced premaxillary suture fusion on craniofacial morphology.

METHODS

Thirty Sprague Dawley rats were divided into control and experimental groups, with 3 week, 5 week and 8 week subgroups of five animals each. An incision was made in each rat along the premaxillary suture and cyanoacrylate was administered to immobilize the exposed premaxillary suture for experimental rats. No glue was applied to controls. Weights, dental impressions and radiographs were taken before and after surgery until sacrifice and used to determine the differences between groups using the one-way ANOVA test.

RESULTS

After immobilizing the premaxillary suture, significant changes in the craniofacial morphology were measured at the different time points. In the experimental groups, local changes occurred at the 3rd week. A global alteration in craniofacial morphology was apparent at the 8th week in the experimental group compared to the control. At each successive time point, craniofacial morphological alterations increased in rats with fused premaxillary sutures.

CONCLUSIONS

Induced premaxillary suture fusion can inhibit the growth of the premaxilla and cause extensive craniofacial morphological changes. These findings suggest that premaxillary suture fusion may be related to craniofacial malformation or malocclusion and to the formation of the flattened craniofacial profile in humans.

Dihydrotestosterone Stimulates Proliferation and Differentiation of Fetal Calvarial Osteoblasts and Dural Cells and Induces Cranial Suture Fusion

BACKGROUND

The higher prevalence of metopic and sagittal suture synostosis in male infants suggests a role for androgens in early craniofacial development. These experiments characterize the influence of androgen stimulation on growth and differentiation of fetal dural and calvarial bone cells and on cranial suture fusion.

METHODS

Primary murine fetal (E18) dural cells and calvarial osteoblasts were isolated and cultured. Cells were treated for 48 hours with 5alpha-dihydrotestosterone (0 to 1000 nM). Cell proliferation was examined by nonradioactive proliferation assay; mRNA expression of alkaline phosphatase, transforming growth factor (TGF)-beta1, and the bone matrix proteins osteopontin, osteocalcin, and type 1 collagen was determined by reverse-transcriptase polymerase chain reaction. In separate experiments, intact fetal calvariae were grown in tissue culture with 10 nM 5alpha-dihydrotestosterone for 7 and 14 days and then examined histologically.

RESULTS

Androgen stimulation at 5 nM increased proliferation of fetal dural cells by 46.0 percent and of fetal calvarial osteoblasts by 20.5 percent. Dural expression of osteopontin, osteocalcin, and type 1 collagen was enhanced by 5alpha-dihydrotestosterone, as was that of TGF-beta1 and alkaline phosphatase. Androgen stimulation increased calvarial osteoblast expression of alkaline phosphatase and TGF-beta1 but induced little change in expression of osteocalcin, osteopontin, and type 1 collagen. In tissue culture, 5alpha-dihydrotestosterone stimulated osteoid formation and fusion of sagittal sutures.

CONCLUSIONS

Androgen stimulation of dural cells and osteoblasts isolated from fetal calvaria promotes cell proliferation and osteoblastic differentiation and can induce cranial suture fusion. These results suggest that sex steroid hormone signaling may stimulate sutural osteogenesis by means of osteodifferentiation of dural cells, thus explaining the male prevalence of nonsyndromic craniosynostosis.

The mendosal suture of the occipital bone: occurrence in Indian population, embryology and clinical significance

The occipital bone is ontogenetically and functionally unique when compared to the other bones of the skull in humans and other mammalian cousins. The occipital bone develops from six ossification centers; any defect in the ossification process will give rise to mendosal suture (accessory suture) and conditions like posterior plagiocephaly. There is a paucity of literature regarding the mendosal suture and further more, its report in Indian population is not known. The present study was conducted to find out the occurrence of mendosal suture in the Indian dry skulls. Fifteen specimens (3%) were found to have these sutures out of five hundred (500) skulls examined. Nine (3.1%) samples are of male skulls out of two hundred ninety (290) and six (2.85%) samples are that of female skulls out of two hundred ten (210). The mendosal suture ran close to the superior nuchal line in all specimens and traveled medially from the lambdoidal suture. The length of the sutures ranged from 0.8 cm to 2.6 cm (1.88 cm) on the right side and 1.4 cm to 2.9 cm (1.94 cm) on the left side respectively in male skulls; and 0.7 cm to 2.8 cm (1.55 cm) on the right side and 1 cm to 2.4 cm (1.42 cm) on the left side, respectively, in female skulls. The origin of mendosal suture from the lambdoidal suture was 5.7 cm to -6.3 cm (5.98 cm) from the tip of the mastoid process on the right side and 5.6 to -6.3 cm (6 cm) on the left side, respectively, in male skulls; and 5.4 cm to -5.8 cm (5.58 cm) on the right side and 5.4 cm to -5.6 cm on the left side respectively in female skulls. The occurrence and clinical significance of the present study is discussed.

Impaired posterior frontal sutural fusion in the biglycan/decorin double deficient mice

Biglycan (Bgn) and decorin (Dcn) are highly expressed in numerous tissues in the craniofacial complex. However, their expression and function in the cranial sutures are unknown. In order to study this, we first examined the expression of biglycan and decorin in the posterior frontal suture (PFS), which predictably fuses between 21 and 45 days post-natal and in the non-fusing sagittal (S) suture from wild-type (Wt) mice. Our data showed that Bgn and Dcn were expressed in both cranial sutures. We then characterized the cranial suture phenotype in Bgn deficient, Dcn deficient, Bgn/Dcn double deficient, and Wt mice. At embryonic day 18.5, alizarin red/alcian blue staining showed that the Bgn/Dcn double deficient mice had hypomineralization of the frontal and parietal craniofacial bones. Histological analysis of adult mice (45-60 days post-natal) showed that the Bgn or Dcn deficient mice had no cranial suture abnormalities and immunohistochemistry staining showed increased production of Dcn in the PFS from Bgn deficient mice. To test possible compensation of Dcn in the Bgn deficient sutures, we examined the Bgn/Dcn double deficient mice and found that they had impaired fusion of the PFS. Semi-quantitative RT-PCR analysis of RNA from 35 day-old mice revealed increased expression of Bmp-4 and Dlx-5 in the PFS compared to their non-fusing S suture in Wt tissues and decreased expression of Dlx-5 in both PF and S sutures in the Bgn/Dcn double deficient mice compared to the Wt mice. Failure of PFS fusion and hypomineralization of the calvaria in the Bgn/Dcn double deficient mice demonstrates that these extracellular matrix proteoglycans could have a role in controlling the formation and growth of the cranial vault.

Dlx5 drives Runx2 expression and osteogenic differentiation in developing cranial suture mesenchyme

Craniofacial bones derive from cephalic neural crest, by endochondral or intramembranous ossification. Here, we address the role of the homeobox transcription factor Dlx5 during the initial steps of calvaria membranous differentiation and we show that Dlx5 elicits Runx2 induction and full osteoblast differentiation in embryonic suture mesenchyme grown "in vitro". First, we compare Dlx5 expression to bone-related gene expression in the developing skull and mandibular bones. We classify genes into three groups related to consecutive steps of ossification. Secondly, we study Dlx5 activity in osteoblast precursors, by transfecting Dlx5 into skull mesenchyme dissected prior to the onset of either Dlx5 and Runx2 expression or osteogenesis. We find that Dlx5 does not modify the proliferation rate or the expression of suture markers in the immature calvaria cells. Rather, Dlx5 initiates a complete osteogenic differentiation in these early primary cells, by triggering Runx2, osteopontin, alkaline phosphatase, and other gene expression according to the sequential temporal sequence observed during skull osteogenesis "in vivo". Thirdly, we show that BMP signaling activates Dlx5, Runx2, and alkaline phosphatase in those primary cultures and that a dominant-negative Dlx factor interferes with the ability of the BMP pathway to activate Runx2 expression. Together, these data suggest a pivotal role of Dlx5 and related Dlx factors in the onset of differentiation of chick calvaria osteoblasts.

Raman spectroscopic evidence for octacalcium phosphate and other transient mineral species deposited during intramembranous mineralization

To understand early mineralization events, we studied living murine calvarial tissue by Raman spectroscopy using fibroblast growth factor 2 (FGF2)-soaked porous beads. We detected increased levels of a transient phase resembling octacalcium phosphate in sutures undergoing premature suture closure.

INTRODUCTION

Several calcium phosphates have been postulated as the earliest inorganic precursors to bone mineral. They are unstable and have not been previously detected in tissue specimens. Whether the same intermediates are formed in sutures undergoing premature closure is also unknown.

METHODS

Six coronal suture tissue specimens from fetal day 18.5 B6CBA F1/J wild-type mice were studied. Three sutures specimens were treated with FGF2-soaked heparin acrylic beads to induce accelerated mineralization and premature suture closure. Three control specimens were treated with empty heparin acrylic beads. All sutures were maintained as organ cultures to permit repeated spectral analyses at 12-24 h intervals over a 72-h period.

RESULTS

During the first 24 h, the spectra contained bands of octacalcium phosphate (OCP) or an OCP-like mineral. The main phosphorus-oxygen stretch was at 955 cm(-1), instead of the 957-959 cm(-1) seen in bone mineral, and there was an additional band at 1010-1014 cm(-1), as expected for OCP. A broad band was found at 945 cm(-1), characteristic of a highly disordered or amorphous calcium phosphate. An increased amount of mineral was observed in FGF2-treated sutures, but no qualitative differences in Raman spectra were observed between experimental and control specimens.

CONCLUSIONS

Inorganic mineral deposition proceeds through transient intermediates, including an OCP-like phase. Although this transient phase has been observed in purely inorganic model systems, this study is the first to report OCP or an OCP-like intermediate in living tissue. Raman microspectroscopy allows observation of this transient mineral and may allow observation of other precursors as well.

A model for the pharmacological treatment of crouzon syndrome

OBJECTIVE

Crouzon syndrome is caused by mutations in fibroblast growth factor receptor 2 (FGFR2) leading to constitutive activation of receptors in the absence of ligand binding. The syndrome is characterized by premature fusion of the cranial sutures that leads to abnormal cranium shape, restricted brain growth, and increased intracranial pressure. Surgical remodeling of the cranial vault is currently used to treat affected infants. The purpose of this study was to develop a pharmacological strategy using tyrosine kinase inhibition as a novel treatment for craniosynostotic syndromes caused by constitutive FGFR activation.

METHODS

Characterization of cranial suture fusion in Fgfr2 mutant mice, which carry the most common Crouzon mutation, was performed using micro-computed tomographic analysis from embryogenesis through maturation. Whole calvarial cultures from wild-type and Fgfr2 mice were established and cultured for 2 weeks in the presence of dimethyl sulfoxide control or PD173074, an FGFR tyrosine kinase inhibitor. Paraffin sections were prepared to show suture morphology and calcium deposition.

RESULTS

In untreated Fgfr2 cultures, the coronal suture fused bilaterally with loss of overlap between the frontal bone and parietal bone. Calvaria treated with PD173074 (2 micromol/L) showed patency of the coronal suture and were without evidence of any synostosis.

CONCLUSION

We report the successful use of PD173074 to prevent in vitro suture fusion in a model for Crouzon syndrome. Further studies are underway to develop an in vivo treatment protocol as a novel therapeutic modality for FGFR associated craniosynostotic syndromes.

Noggin Underexpression and Runx-2 Overexpression in a Craniosynostosis Rabbit Model

INTRODUCTION

Normal suture fusion has been shown to be driven by the molecular signals elucidated by the underlying dura. However, the pathogenesis of suture fusion in craniosynostosis is not well described. The purpose of our study was to examine the expression patterns of 2 important molecular signals (Noggin and Runx-2) in a cohort of congenital craniosynostotic rabbits to gain a better understanding of suture behavior in craniosynostosis.

METHODS

Coronal (fusing) and sagittal (patent) rabbit cranial sutures from a colony of congenitally synostosed rabbits and wild-type (control) rabbits were harvested at a neonatal time point. These sections were then grown in organ culture and harvested for histology at 0, 7, or 14 days of culture. Fusion percentage was then assessed and an overall fusion score was calculated. Expression of Noggin and Runx-2 was then localized by immunohistochemistry and quantified by Western blot analysis.

RESULTS

Histology of the wild-type cranial sutures (control) showed suture patency (score of 0%) for all coronal and sagittal sutures at 0 days, 7 days, and 14 days of organ culture. Sagittal sutures of craniosynostotic animals also showed suture patency (score of 0%) at all culture times (0, 7, and 14 days). Of the 18 coronal sutures from the craniosynostotic animals, 8 remained patent and 10 fused. For the coronal sutures that fused, fusion scores of 14%, 41%, and 84% were documented at 0, 7, and 14 days of organ culture, respectively. With immunolocalization, Noggin was found to be expressed in both the dura and suture cells underlying patent sutures, but not in fusing sutures in vitro. Runx-2 was found to be expressed in the dura beneath the suture and suture cells of fusing sutures, not patent sutures. Western blot densitometry confirmed these findings.

CONCLUSIONS

Our results suggest that pathologic rabbit coronal sutures progressed toward complete suture fusion in vitro, and expression patterns of Noggin and Runx-2 paralleled that of a well-studied normal suture fusion model.

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.

Metopic suture in fetuses with holoprosencephaly at 11 + 0 to 13 + 6 weeks of gestation

OBJECTIVE

To investigate the development of the metopic suture in fetuses with holoprosencephaly at 11 + 0 to 13 + 6 weeks of gestation.

METHODS

Three-dimensional (3D) ultrasound was used to measure the height and gap between the frontal bones in 200 normal fetuses and in nine fetuses with holoprosencephaly at 11 + 0 to 13 + 6 (median, 12) weeks of gestation.

RESULTS

In the 200 normal fetuses, the height of the frontal bones increased significantly with gestation from a mean of 2.5 mm (5(th) and 95(th) centiles: 1.9 mm and 3.3 mm) at 11 weeks to 6.1 mm (5(th) and 95(th) centiles: 4.6 mm and 8.1 mm) at 13 + 6 weeks. The gap between the two frontal bones did not change significantly with gestation (mean: 1.5 mm; 5(th) centile: 1.0 mm; 95(th) centile: 2.0 mm). In fetuses with holoprosencephaly, the height of the frontal bones was significantly larger (mean difference, 5.6 SDs; range, 3.9-7.7 SDs; P < 0.0001) and the gap was significantly smaller (mean 0.2 mm, range 0-0.8 mm; P < 0.0001) than those in normal fetuses.

CONCLUSIONS

Holoprosencephaly is associated with an accelerated development of the frontal bones and premature closure of the metopic suture.

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.

Metopic suture in fetuses with Apert syndrome at 22-27 weeks of gestation

OBJECTIVES

To examine the possible association of skull deformity and the development of the cranial sutures in fetuses with Apert syndrome.

METHODS

Three-dimensional (3D) ultrasound was used to examine the metopic and coronal sutures in seven fetuses with Apert syndrome at 22-27 weeks of gestation. The gap between the frontal bones in the transverse plane of the head at the level of the cavum septi pellucidi was measured and compared to findings in 120 anatomically normal fetuses undergoing routine ultrasound examination at 16-32 weeks.

RESULTS

In the normal group, the gap between the frontal bones in the metopic suture at the level of the cavum septi pellucidi, decreased significantly with gestation from a mean of 2.2 mm (5th and 95th centiles: 1.5 mm and 2.9 mm) at 16 weeks to 0.9 mm (5th and 95th centiles: 0.3 mm and 1.6 mm) at 32 weeks. In the seven cases with Apert syndrome, two-dimensional ultrasound examination demonstrated the characteristic features of frontal bossing, depressed nasal bridge and bilateral syndactyly. On 3D examination there was complete closure of the coronal suture and a wide gap in the metopic suture (15-23 mm).

CONCLUSION

In normal fetuses, cranial bones are believed to grow in response to the centrifugal pressure from the expanding brain and proximity of the dura to the suture is critical in maintaining its patency. In Apert syndrome, the frontal bossing may be a mere consequence of a genetically predetermined premature closure of the coronal suture. Alternatively, there is a genetically predetermined deformation of the brain, which in turn, through differential stretch of the dura in the temporal and frontal regions, causes premature closure of the coronal suture and impaired closure of the metopic suture.

TGF-β2 stimulates cranial suture closure through activation of the Erk-MAPK pathway

Cranial sutures are important growth sites of the skull. During suture closure, the dura mater is one of the most important sources of various positive and negative regulatory signals. Previous results indicate that TGF-beta2 from dura mater strongly accelerates suture closure, however, its exact regulatory mechanism is still unclear. In this study, we confirmed that removal of dura mater in calvarial organ culture strongly accelerates sagittal suture closure and that this effect is further enhanced by TGF-beta2 treatment. TGF-beta2 stimulated cell proliferation in the MC3T3-E1 cell line. Similarly, it stimulated the proliferation of cells in the sutural space in calvarial organ culture. Furthermore, TGF-beta2-mediated enhanced cell proliferation and suture closure were almost completely inhibited by an Erk-MAPK blocker, PD98059. These results indicate that TGF-beta2-induced activation of Erk-MAPK is an important signaling component that stimulates cell proliferation to enrich osteoprogenitor cells, thereby promoting their differentiation into osteoblasts to achieve a rapid calvarial bone expansion.

Erk1/2 signaling is required for Tgf-β2–induced suture closure

Transforming growth factors beta (Tgf-betas) act by means of Smad signaling pathways and may also interact with the mitogen-activated protein kinase pathway. The hypothesis was tested that Erk1/2 signaling is required for Tgf-beta2-induced suture closure, by culturing embryonic mouse calvariae in the presence of Tgf-beta2 with or without Erk1/2 inhibitor PD98059 (PD). Suture widths were measured daily, and microdissected sutures and bones were homogenized and protein analyzed by Western blots. Tgf-beta2 induced narrowing of the sutures after 72 hr, an effect inhibited by treatment with PD. Erk1/2 and Egf but not Smad2/3 protein expression was up-regulated by Tgf-beta2 calvarial tissues at 72 hr. PD inhibited endogenous and Tgf-beta2-stimulated Erk1/2 protein as well as Tgf-beta2-stimulated Egf, but increased Smad2/3 protein expression. In tissues harvested 0, 15, and 30 min after exposure to Tgf-beta2, Erk1/2 phosphorylation was up-regulated after 15 min, an effect abrogated by the simultaneous addition of PD. In summary, Tgf-beta2 stimulated Erk1/2 phosphorylation and induced Egf and Erk1/2 expression, associated with suture closure after 72 hr. Blocking Erk1/2 activity with PD inhibited these effects but increased Smad2/3 expression. We postulate that Tgf-beta2 regulates suture closure directly by means of phosphorylation of Erk1/2 and indirectly by up-regulating Erk1/2, a substrate for Fgf receptor signaling required for Fgf induction of premature suture obliteration.

Three-dimensional sonographic description of abnormal metopic suture in second- and third-trimester fetuses

OBJECTIVE

To describe patterns of abnormal development of the metopic suture in association with fetal malformations during the second and third trimesters of pregnancy.

METHODS

This was a cross-sectional study of the frontal bones and metopic suture in 11 fetuses at 17-32 weeks of gestation. Cases were selected because there were obvious abnormalities in the metopic sutures. In each case, a malformation was detected by two-dimensional (2D) ultrasound and the abnormality of the metopic suture was detected and evaluated on three-dimensional (3D) ultrasound, using transparent maximum mode.

RESULTS

There were essentially four patterns of abnormality in the metopic suture: firstly, delayed development with a V- or Y-shaped open suture, which is found in normal fetuses at 12-16 weeks; secondly, a U-shaped open suture, presumably due to upward growth of the frontal bones with delayed closure; thirdly, premature closure of the suture, which is normally observed after 32 weeks; fourthly, the presence of additional bone between the frontal bones. Premature closure of the suture or additional bone between the frontal bones was observed in fetuses with holoprosencephaly and abnormalities of the corpus callosum, whereas the V-, Y- and U-shaped metopic sutures were observed in fetuses with facial defects involving the orbits, nasal bones, lip, palate and mandible, in the absence of holoprosencephaly and abnormal corpus callosum.

CONCLUSIONS

This preliminary study describes the pattern of possible abnormalities of the metopic suture and should stimulate further investigation to establish the prevalence and evolution of abnormal sutures as well as the incidence and pattern of other associated defects.

Three-dimensional sonographic description of the fetal frontal bones and metopic suture

OBJECTIVE

To describe the morphology of the frontal bones and metopic suture at 9-34 weeks of gestation using three-dimensional (3D) ultrasonography.

METHODS

This was a cross-sectional study of the frontal bones and metopic suture in 16 fetuses at 9-34 weeks of gestation. 3D ultrasonography was used to obtain volumes of the whole fetal skull in the mid-sagittal plane. The volumes were analyzed using the rendering mode and transparent maximum mode in order to achieve a clear view of the frontal bones and the metopic suture.

RESULTS

At 9 weeks, a small ossification center was visible in the middle of each supraorbital part of the frontal bones and by 11 weeks the frontal bones appeared as 'thick eyebrows'. In the second trimester there was progressive radial bone expansion and delineation of the metopic suture and in the third trimester there was closure of the metopic suture, starting from the glabella and moving upwards towards the anterior fontanelle.

CONCLUSIONS

This study provides 3D ultrasound images that illustrate the process of ossification of the frontal bones and the subsequent development of the metopic suture during prenatal life.

Twist is required for establishment of the mouse coronal suture

Cranial sutures are the growth centres of the skull, enabling expansion of the skull to accommodate rapid growth of the brain. Haploinsufficiency of the human TWIST gene function causes the craniosynostosis syndrome, Saethre-Chotzen syndrome (SCS), in which premature fusion of the coronal suture is a characteristic feature. Previous studies have indicated that Twist is expressed in the coronal suture during development, and therefore that it may play an important role in development and maintenance of the suture. The Twist-null mouse is lethal before the onset of osteogenesis, and the heterozygote exhibits coronal suture synostosis postnatally. In this study we investigated the function of Twist in the development of the mouse coronal suture, by inhibiting Twist synthesis using morpholino antisense oligonucleotides in calvarial organ culture. Decreased Twist production resulted in a narrow sutural space and fusion of bone domains within 48 h after the addition of the morpholino oligonucleotides. Proliferation activity in the sutural cells was decreased, and the expression of osteogenic marker genes such as Runx2 and Fgfr2 was up-regulated in the developing bone domain within 4 h. These results suggest that during establishment of the suture area, Twist is required for the regulation of sutural cell proliferation and osteoblast differentiation.

Mutations in snail family genes enhance craniosynostosis of Twist1 haplo-insufficient mice: implications for Saethre-Chotzen Syndrome

In Drosophila, mutations in the Twist gene interact with mutations in the Snail gene. We show that the mouse Twist1 mutation interacts with Snai1 and Snai2 mutations to enhance aberrant cranial suture fusion, demonstrating that genetic interactions between genes of the Twist and Snail families have been conserved during evolution.

Immunolocalization of androgen receptor in the developing craniofacial skeleton

Male predominance in metopic and sagittal craniosynostosis and in nonsynostotic plagiocephaly suggests a role for circulating androgens in early craniofacial development. Androgens have been documented to play an important role in postnatal skeletal growth, and the androgen receptor has been recently demonstrated in human and rat osteoblast-like cell lines and in human long bones. The purpose of this study was to describe the expression of androgen receptor in the fetal craniofacial skeleton. The heads of E18 fetal CD-1 male and female mice were fixed in 10% formalin, decalcified, and embedded in paraffin. Four- to 6-mum coronal and sagittal sections were stained with a monoclonal antibody specific to androgen receptor, which was detected by an avidinbiotin conjugate and peroxidase system. The sections were then examined for androgen receptor expression patterns. Strong androgen receptor immunoreactivity was observed in the dura mater of developing fetuses. Androgen receptor expression was also noted in cells lining the osteogenic fronts and in calvarial osteoblasts. Similar androgen receptor expression patterns were found in male and female mice. Androgen receptor is abundantly expressed in fetal dura mater and calvarial bone. This study confirms the presence of androgen receptor in the murine fetal craniofacial skeleton, suggesting a potential role for the anabolic effects of androgens in the developing craniofacial skeleton.

Regulation of cranial suture morphogenesis

The cranial sutures are the primary sites of bone formation during skull growth. Morphogenesis and phenotypic maintenance of the cranial sutures are regulated by tissue interactions, especially those with the underlying dura mater. Removal of the dura mater in fetuses causes abnormal suture development and premature suture obliteration. The dura mater interacts with overlying tissues of the cranial vault by providing: (1) intercellular signals, (2) mechanical signals and (3) cells, which undergo transformation and migrate to the suture. The intercellular signaling governing suture development employs the fibroblast growth factors (FGFs). In rats during formation of the sutures in the fetus, FGF-1 is localized mainly in the dura mater, while other FGFs are expressed in the overlying tissues. By birth, FGF-2 largely replaces FGF-1 in the dura mater. FGFs present in the calvaria bind either the IIIb or IIIc mRNA splice variants of the FGF receptors (FGFRs) 1, 2, or 3. Monoclonal antibodies to the b variant of FGFR2 were used to determine the distribution of FGFR2IIIb during suture development and its extracellular localization. FGFR2IIIb is present in association with mature osteoblasts and osteogenic precursor cells of the suture in the fetus. Ectodomains of FGFR2IIIb, the products of proteolytic cleavage of the receptors, were present throughout the extracellular matrix of sutures resisting obliteration (coronal and sagittal), but absent from the core of sutures undergoing normal fusion (posterior intrafrontal). This observation is consistent with a possible mechanism, in which truncated receptors bind FGFs, thus regulating free FGF available to nearby cells. Mechanical signaling in the calvaria results from tensional forces in the dura mater generated during rapid expansion of the neurocranium. Posterior intrafrontal sutures of rats, which fuse between days 16 and 24, were subjected to cyclical tensional forces in vitro. Significant delay in the timing of suture fusion and increases in the expression domains of FGFR1 and 2 were observed, demonstrating the sensitivity of suture patency to mechanical signals and a possible role of the FGF system in mediating such stimuli. Finally, cells of the dura mater beneath the intrafrontal and sagittal sutures were observed to undergo a morphological transformation to a dendritic morphology and migrate into the suture mesenchyme between days 10 and 16 of development. This process may participate in suture and bone morphogenesis and influence the patency of the sutures along the anterior-posterior axis.

The role of three-dimensional ultrasound in visualizing the fetal cranial sutures and fontanels during the second half of pregnancy

OBJECTIVES

The aim of this study was to evaluate the significance of three-dimensional (3D) ultrasound in visualizing fetal cranial sutures and fontanels and to determine factors that could influence visualization and image quality.

METHODS

Serial 3D ultrasound examinations were evaluated for visibility of fetal cranial sutures and fontanels, image quality and possible influencing parameters in the second half of pregnancy. Thirty fetuses were scanned at four different gestational ages providing a data set of 120 cases.

RESULTS

Most (82-100%) cranial sutures and fontanels could be visualized with 3D ultrasound. However, the sagittal suture and posterior fontanel were visualized in only 47% and 42%, respectively. Gestational age significantly influenced the visibility of the sutures and fontanels, image quality decreasing with advancing gestational age.

CONCLUSIONS

3D ultrasound can be a reliable technique for visualizing most fetal cranial sutures and fontanels. By performing a sagittal and a transverse scan, most of the sutures and fontanels can be made visible during the second half of pregnancy. Visualization depends on gestational age.

Demonstration of cranial sutures and fontanelles at 15 to 16 weeks of gestation: a comparison between two-dimensional and three-dimensional ultrasonography

OBJECTIVE

The purpose of this study was to evaluate the feasibility of obtaining good quality three-dimensional ultrasound pictures of the cranial sutures and fontanelles and to compare between two-dimensional and three-dimensional ultrasound in identifying the normal appearance of cranial sutures and fontanelles by the transvaginal approach at 15 to 16 weeks of gestation.

METHODS

Fifty fetuses were prospectively evaluated by two-dimensional and three-dimensional transvaginal sonography between 15 and 16 weeks of gestation. The sagittal, coronal, lambdoidal, and metopic sutures, as well as anterior and posterior fontanelles, were inspected.

RESULTS

Three-dimensional ultrasound enabled visualization of all sutures in 37 (74%) fetuses compared to 28 (56%) fetuses examined by two-dimensional ultrasonography (p = NS). The visualization of the sagittal suture was significantly superior by three-dimensional ultrasonography compared to two-dimensional ultrasonography (50 (100%) vs 35 (70%), p < 0.001). No significant difference between the two modalities was found in visualization of the fontanelles.

CONCLUSION

Sutures and fontanelles are usually satisfactorily demonstrated by two-dimensional and three-dimensional ultrasound at 15 to 16 weeks of gestation. The sagittal suture is difficult to visualize using two-dimensional ultrasound, and three-dimensional ultrasound appears to be the best method for its demonstration.

Prenatal Organization and Morphogenesis of the Sphenofrontal Suture in Humans

OBJECTIVE

The aim of this study was to describe the prenatal structure and morphogenesis of the sphenofrontal suture.

METHODS

Eleven human specimens, two embryos and nine fetuses, were prepared for light microscopy study of the sphenofrontal suture. Ten-micrometer sections were made with the microtome in the sagittal plane from the midline to the sphenoidal fontanelle.

RESULTS

At the end of the fetal period, the sphenofrontal suture had a five-layer structure like the cranial sutures, and was formed by two different morphogenetic unities. The orbitosphenofrontal suture was formed between the membranous ossification of the orbital part of the frontal bone and the endochondral ossification of the lesser wing of the sphenoid bone, i.e. the ala orbitalis. In the early stage, a transient sphenoethmoidal cartilage was inserted between these two ossifications. The second unit, the lateral sphenofrontal suture, was formed between the frontal bone and the greater wing of the sphenoid, and the ossification was membranous in this portion. It is formed like the cranial suture, directly from the mesenchyme.

CONCLUSION

The sphenofrontal suture is a typical fibrous suture arising from two morphogenetic unities.

A Ser252Trp [corrected] substitution in mouse fibroblast growth factor receptor 2 (Fgfr2) results in craniosynostosis

Apert syndrome (AS) is one of the most severe craniosynostoses and is characterized by premature fusion of craniofacial sutures. Mutations of either Ser252Trp or Pro253Arg in fibroblast growth factor receptor 2 (FGFR2) are responsible for nearly all known cases of AS. Here we show that mutant mice carrying the activation mutation, Ser252Trp [corrected] which corresponds to Ser252Trp in human FGFR2, have malformations mimicking the skull abnormalities found in AS patients. Mutant mice (Fgfr2(250/+)) are smaller in body size with brachycephaly and exhibit distorted skulls with widely spaced eyes. Unexpectedly, the premature closure of the coronal suture is accompanied by decreased, rather than increased, bone formation. We demonstrate that the Fgfr2-Ser252Trp [corrected] mutation does not cause obvious alterations in cell proliferation and differentiation; however, it results in increased Bax expression and apoptosis of osteogenic cells in mutant coronal suture. The accelerated cell death possibly reduces the space between osteogenic fronts of flat bones and results in the physical contact of these bones. Thus, our data reveal that dysregulated apoptosis plays an important role in the pathogenesis of AS related phenotypes.

A fractal analysis of human cranial sutures

OBJECTIVES

Many biological structures are products of repeated iteration functions. As such, they demonstrate characteristic, scale-invariant features. Fractal analysis of these features elucidates the mechanism of their formation. The objectives of this project were to determine whether human cranial sutures demonstrate self-similarity and measure their exponents of similarity (fractal dimensions).

DESIGN

One hundred three documented human skulls from the Terry Collection of the Smithsonian Institution were used. Their sagittal sutures were digitized and the data converted to bitmap images for analysis using box-counting method of fractal software.

RESULTS

The log-log plots of the number of boxes containing the sutural pattern, N(r), and the size of the boxes, r, were all linear, indicating that human sagittal sutures possess scale-invariant features and thus are fractals. The linear portion of these log-log plots has limits because of the finite resolution used for data acquisition. The mean box dimension, D(b), was 1.29289 +/- 0.078457 with a 95% confidence interval of 1.27634 to 1.30944.

CONCLUSIONS

Human sagittal sutures are self-similar and have a fractal dimension of 1.29 by the box-counting method. The significance of these findings includes: sutural morphogenesis can be described as a repeated iteration function, and mathematical models can be constructed to produce self-similar curves with such D(b). This elucidates the mechanism of actual pattern formation. Whatever the mechanisms at the cellular and molecular levels, human sagittal suture follows the equation log N(r) = 1.29 log 1/r, where N(r) is the number of square boxes with sides r that are needed to contain the sutural pattern and r equals the length of the sides of the boxes.

Normal palatal sutures in newborns and fetuses: a critical fact for successful palatal distraction

Distraction osteogenesis (DO) has recently been applied to the palate. Successful posterior lengthening and medial advancement of the palates was continuously reported. Based on these studies, it is obvious that DO will play a major role in the management of problems related to palatal defects in the near future. Although the results are appealing, they may not be applicable for humans due to anatomic differences. All experimental studies used normal palatal sutures of young dogs for size expansion. Therefore, it is necessary to know normal palatal sutures in infants before one can clinically apply this new technique. With consent, palates of fetuses and neonates who died of various causes were examined. Eight fresh cadavers were available for the dissection, with two being skeletonized using the boiling process. There were three fetal deaths in utero (33-41 weeks of gestational age) and five postnatal deaths (aged between 5 hours and 6 months). All specimens were grossly normal in shape and size except for one with a unilateral complete cleft of lip and palate. A midline palatal suture was found in every noncleft specimen, while premaxillary and transverse palatomaxillary sutures were present in every specimen. Laterally, there was no true suture except for the most posterior portion, which was contiguous with the greater palatine foramen. The palatal sutures of third-trimester fetuses and neonates are not different from adult ones. There is no lateral suture that will allow distraction in the medial direction. It is only the posterior hard palate (palatine bones) that can potentially be moved medially and posteriorly by sutural expansion with DO.

The role of fibroblast growth factor (FGF) and type beta transforming growth factor (TGF-beta 1-beta 2-beta 3) during rat craniofacial development

Growth factors seem to be part of a complex cellular signalling language, in which individual growth factors are the equivalents of the letters that compose words. According to this analogy, informational content lies, not in an individual growth factor, but in the entire set of growth factors and others signals to which a cell is exposed. The ways in which growth factors exert their combinatorial effects are becoming clearer as the molecular mechanisms of growth factors actions are being investigated. A number of related extracellular signalling molecules that play widespread roles in regulating development in both invertebrates and vertebrates constitute the Fibroblast Growth Factor (FGF) and type beta Transforming Growth Factor (TGF beta). The latest research literature about the role and fate of these Growth factors and their influence in the craniofacial bone growth ad development is reviewed.

Molecular mechanisms in calvarial bone and suture development, and their relation to craniosynostosis

The development and growth of the skull is a co-ordinated process involving many different tissues that interact with each other to form a complex end result. When normal development is disrupted, debilitating pathological conditions, such as craniosynostosis (premature calvarial suture fusion) and cleidocranial dysplasia (delayed suture closure), can result. It is known that mutations in the fibroblast growth factor receptors 1, 2, and 3(FGFR1, 2, and 3), as well as the transcription factors MSX2 and TWIST cause craniosynostosis, and that mutations in the transcription factor RUNX2 (CBFA1) cause cleidocranial dysplasia. However, relatively little is known about the development of the calvaria: where and when these genes are active during normal calvarial development, how these genes may interact in the developing calvaria, and the disturbances that may occur to cause these disorders. In this work an attempt has been made to address some of these questions from a basic biological perspective. The expression patterns of the above-mentioned genes in the developing mouse skull are detailed. The microdissection and in vitro culture techniques have begun the task of identifying Fgfrs, Msx2, and Twist interacting in intricate signalling pathways that if disrupted could lead to craniosynostosis.

[A new cephalometric analysis of the profile. 1. Anatomic bases, cephalometry]

Through a revision of the classical anatomy of the exocranial skull base, the authors present of a new exobasicranial plan in cephalometry, the Nasion-Glenion (or Porion plane). The median part of the base devoted to ventilation goes from the Nasion to the spheno-occipital synchondrosis, joining two essentially cartilaginous sites. The lateral parts, with manducatory functions, extend from supra-orbital rim to glenoid fossa and are mainly membranous. Pterygoid processes belong to the facial squeleton. Dental and growth axes are projected on the new exobasicranial base. Facial structures are analyzed in four planes, perpendicular to Francfort plane (facial cutaneous plane, premaxillary plane, postmaxillary plane, craniocervical plane).

Tgf-beta1, Tgf-beta2, Tgf-beta3 and Msx2 expression is elevated during frontonasal suture morphogenesis and during active postnatal facial growth

OBJECTIVES

It is hypothesized that regulation of facial suture morphogenesis is similar to that of cranial sutures, with expression of similar regulatory molecules, governing suture formation and patency. The present study was designed to characterize the morphology of the frontonasal (FN) suture of the rat at different developmental stages and to investigate the presence and temporal-spatial expression of transforming growth factor-beta 1 (Tgf-beta1), Tgf-beta2, Tgf-beta3 and Msx2 mRNA within these structures.

SETTING AND SAMPLE POPULATION

The Department of Biomedical Sciences at Texas A&M University System Health Science Center, Baylor College of Dentistry, Dallas, TX USA. Histological sections and RNA isolated from FN suture tissues of Sprague-Dawley rats, aged embryonic day 16 through postnatal day 20.

METHOD

Sections were examined after immunohistochemical staining. Gene expression was determined by densitometric analysis of RT-PCR products run on agarose gels.

RESULTS

FN sutures develop slightly later than cranial sutures and show increased complexity over time when compared to cranial sutures. FN sutures were closely associated with the nasal capsular cartilage, with intervening layers of perichondrium and periosteum. The pattern of expression of Tgf-betas within the FN suture tissues was similar to that seen in the cranial sutures. However, mRNA and protein of the Tgf-betas were differentially expressed over time compared to cranial sutures. In FN sutures, Tgf-beta mRNA levels were elevated both during the period of suture morphogenesis and during active bone growth from the suture in the early postnatal period. Msx2 mRNA expression was elevated in both the prenatal and postnatal periods, similar to Tgf-beta mRNA expression.

CONCLUSION

Tgf-beta and Msx2 are present in facial sutures similar to cranial sutures, but are differentially expressed over time, perhaps reflecting different bone growth rates from these sutures.

Transforming growth factor-beta3 (Tgf-beta3) down-regulates Tgf-beta3 receptor type I (Tbetar-I) during rescue of cranial sutures from osseous obliteration

Appropriate biochemical regulation of intramembranous bone growth from sutures is necessary to achieve correct craniofacial morphology. Failure to form sutures (agenesis) or to maintain sutures in their unossified state (craniosynostosis) can result in severe facial dysmorphology. Several factors such as Twist, Msx2, fibroblast growth factors (Fgfs), bone morphogenetic proteins (Bmps) and transforming growth factors-beta (Tgf-betas) regulate suture patency, likely by interacting with one another. Tgf-beta2 and Tgf-beta3 use the same cell surface receptors, yet have opposite effects on suture patency, cellular proliferation and apoptosis within the suture. One possible mechanism by which Tgf-beta3 rescues sutures from obliteration is by regulating the ability of suture cells to respond to Tgf-beta2. As Tgf-beta3 does not regulate protein levels of Tgf-beta2 in sutures, Tgf-beta3 could regulate tissue responsiveness to Tgf-beta2 by regulating Tgf-beta2 access to receptors. Tgf-beta3 is a more potent competitor than Tgf-beta2 for cell surface receptors, so it is proposed that Tgf-beta3 binds to and down-regulates Tgf-beta receptor type I (Tbetar-I) expression by suture cells. This down-regulation would limit the ability of cells to respond to all Tgf-betas, including Tgf-beta2. To test this hypothesis, an in vitro culture model was used in which fetal rat sutures either remain patent or are induced to fuse when cultured in the presence or absence of dura mater, respectively. Tgf-beta3 was added to cultured calvaria and changes in the number of receptor positive cells within the suture were established. Data were compared with that seen in control sutures and in normal sutures in vivo. It was found that the numbers of cells expressing Tbetar-I within the suture matrix increased over time in sutures remaining patent. Osteoblastic cells lining the bone fronts on either side of sutures were Tbetar-I positive during early morphogenesis, but these numbers declined as sutures fused, both in vivo and in vitro. Addition of Tgf-beta3 to calvaria in culture decreased the number of Tbetar-I expressing cells in both fusing and non-fusing sutures, with dramatic decreases in the numbers of osteoblasts expressing Tbetar-I.

Ultrasound diagnosis of craniosynostosis

OBJECTIVE

To retrospectively study prenatal ultrasound images of patients with craniosynostosis to determine the extent to which prenatal diagnosis is possible.

METHOD

Prenatal ultrasound images of 19 patients with postnatally diagnosed metopic or coronal suture craniosynostosis were retrospectively reviewed. The 26 ultrasound examinations obtained were compared with normal images and tables of gestation.

RESULTS

It was not possible to diagnose craniosynostosis in the first trimester. In the second trimester, Kleeblattschädel was diagnosed at 20.5 weeks. A multilobular shape to the skull and diastasis of the frontotemporal suture was identified. In a second child with Kleeblattschädel, the cephalic index was above normal 86.4 (normal range 70 to 86), and the head circumference to abdominal circumference was increased. In the third trimester, the head shape deformation was more obvious. Brachycephaly diagnosis was made in the second trimester. In Crouzon syndrome the hypertelorism was identified at 19.9 weeks. Plagiocephaly was diagnosed at 21.4 weeks. In trigonocephaly the reduced cephalic index was noted at 18.8 weeks. In the third trimester, the deformity was diagnosed in all cases.

CONCLUSION

No diagnosis of craniosynostosis was made in the first trimester. In the second trimester, it was possible to diagnose Kleeblattschädel, trigonocephaly, brachycephaly (bilateral coronal suture craniosynostosis), and plagiocephaly (unilateral coronal suture craniosynostosis) in nine of the examinations. In the third trimester and at term, it was possible to diagnose previously listed conditions from six of the seven examinations obtained. Kleeblattschädel was suspected during original examination. A total of 15 examinations of 26 were correctly diagnosed during this investigation.

Tissue origins and interactions in the mammalian skull vault

During mammalian evolution, expansion of the cerebral hemispheres was accompanied by expansion of the frontal and parietal bones of the skull vault and deployment of the coronal (fronto-parietal) and sagittal (parietal-parietal) sutures as major growth centres. Using a transgenic mouse with a permanent neural crest cell lineage marker, Wnt1-Cre/R26R, we show that both sutures are formed at a neural crest-mesoderm interface: the frontal bones are neural crest-derived and the parietal bones mesodermal, with a tongue of neural crest between the two parietal bones. By detailed analysis of neural crest migration pathways using X-gal staining, and mesodermal tracing by DiI labelling, we show that the neural crest-mesodermal tissue juxtaposition that later forms the coronal suture is established at E9.5 as the caudal boundary of the frontonasal mesenchyme. As the cerebral hemispheres expand, they extend caudally, passing beneath the neural crest-mesodermal interface within the dermis, carrying with them a layer of neural crest cells that forms their meningeal covering. Exposure of embryos to retinoic acid at E10.0 reduces this meningeal neural crest and inhibits parietal ossification, suggesting that intramembranous ossification of this mesodermal bone requires interaction with neural crest-derived meninges, whereas ossification of the neural crest-derived frontal bone is autonomous. These observations provide new perspectives on skull evolution and on human genetic abnormalities of skull growth and ossification.

Cranial base growth and morphology in second-trimester normal human fetuses and fetuses with cleft lip

OBJECTIVE

The present radiographic study describes the size and shape of the cranial base from the sagittal aspect for a sample of 77 second-trimester "normal" control fetuses (n = 61) and fetuses (n = 16) exhibiting isolated, unilateral clefts of the lip (CL), ranging in fertilization age from 10 to 22 weeks.

METHODS

Fetuses were placed in a cephalostat, and standardized, lateral head radiographs were taken. The radiographs were traced, and 15 cephalometric landmarks were identified and digitized for analysis. Growth curves for cranial base lengths, angles, and areas were compared between control and CL groups. Also, cranial base triangles were constructed and shape comparisons were made using tensor biometric analysis.

RESULTS

No significant differences (p >.05) in regression line slopes were noted for any comparisons between the control and CL samples. Tensor biometric analysis also revealed no significant differences in the shapes of various cranial base triangles between the control and CL samples.

CONCLUSION

This report presents second-trimester baseline growth curves for various cranial base components in CL human fetal specimens, and these data suggest that CL fetuses may also be used as an appropriate control sample for prenatal growth comparison studies of cleft lip and palate and cleft palate.

Histological investigation of the palatine bone in prenatal trisomy 21

OBJECTIVE

The purpose of the present study was to investigate the horizontal part of the palatine bone in palates from human fetuses with trisomy 21 to improve the phenotypic classification of the genotypic anomaly.

METHODS

Material from 23 human trisomy 21 fetuses was included in the study. The crown rump lengths of the fetuses ranged from 80 mm to 190 mm, corresponding to about 12 to 21 weeks of gestational age. The material was examined histologically.

RESULTS AND CONCLUSIONS

Histological examination demonstrated four different palatal phenotypes on the basis of the development of the horizontal part of the palatine bone: type I, palatine bone complete; type II, the mesial region of the horizontal part of the palatine bone is lacking; type III, complete absence of the horizontal part of the palatine bone; and type IV, auxiliary bones in the region of the transpalatine suture. This finding shows that different types of malformations may occur in the horizontal part of the palatine bone in human trisomy 21 fetuses.