Studies in cranial suture biology: in vitro cranial suture fusion

Incidence of persistent metopic suture in Australia: findings from 1034 three-dimensional computed tomography scans

PURPOSE

To investigate the incidence of persistent, open metopic sutures in contemporary Australians aged 24 months and older.

METHODS

Metopic suture evaluation was conducted on retrospective cranial/cervical computed tomography scans of patients aged 24 to 252 months who presented to the Women's and Children's Hospital in Adelaide, Australia, between 2010 and 2020. Suture ossification was graded according to Lottering scoring system based on 4 stages, on three-dimensional volume-rendered reconstructions (stage 1: fibrous tissue interface, stage 2: commenced fusion, stage 3: complete fusion and stage 4: obliterated suture). The complete persistent sutures were classified as stage 1. Partially closed sutures were classified into stages 2 and 3, while completely closed sutures were defined as stage 4.

RESULTS

One thousand thirty-four patients (61.2% male and 38.8% female) were included, with a mean age at scan of 66 months. More than half of patients were subject to scanning due to closed-head injuries. The incidence of persistent (completely open) metopic suture was 4.8% (2.3% in males and 2.5% in females). In comparison, a partially closed metopic suture was found in 6.3% of the study cohort, with the remaining sutures located along the metopic suture line, at the glabella, mid-part of the suture, bregma and glabella-bregma areas.

CONCLUSION

The prevalence of persistent metopic sutures in our study of the Australian population is 4.8%, and it is equally distributed between the genders. The pattern of suture closure can commence from any location along the suture line, which is in contrast to the existing literature.

The Intertwined Evolution and Development of Sutures and Cranial Morphology

Phenotypic variation across mammals is extensive and reflects their ecological diversification into a remarkable range of habitats on every continent and in every ocean. The skull performs many functions to enable each species to thrive within its unique ecological niche, from prey acquisition, feeding, sensory capture (supporting vision and hearing) to brain protection. Diversity of skull function is reflected by its complex and highly variable morphology. Cranial morphology can be quantified using geometric morphometric techniques to offer invaluable insights into evolutionary patterns, ecomorphology, development, taxonomy, and phylogenetics. Therefore, the skull is one of the best suited skeletal elements for developmental and evolutionary analyses. In contrast, less attention is dedicated to the fibrous sutural joints separating the cranial bones. Throughout postnatal craniofacial development, sutures function as sites of bone growth, accommodating expansion of a growing brain. As growth frontiers, cranial sutures are actively responsible for the size and shape of the cranial bones, with overall skull shape being altered by changes to both the level and time period of activity of a given cranial suture. In keeping with this, pathological premature closure of sutures postnatally causes profound misshaping of the skull (craniosynostosis). Beyond this crucial role, sutures also function postnatally to provide locomotive shock absorption, allow joint mobility during feeding, and, in later postnatal stages, suture fusion acts to protect the developed brain. All these sutural functions have a clear impact on overall cranial function, development and morphology, and highlight the importance that patterns of suture development have in shaping the diversity of cranial morphology across taxa. Here we focus on the mammalian cranial system and review the intrinsic relationship between suture development and morphology and cranial shape from an evolutionary developmental biology perspective, with a view to understanding the influence of sutures on evolutionary diversity. Future work integrating suture development into a comparative evolutionary framework will be instrumental to understanding how developmental mechanisms shaping sutures ultimately influence evolutionary diversity.

Developmental Abnormalities of the Skull Base in Patients with Turner Syndrome

The skull base is one of the most complex anatomic structures of the skeleton that is responsible for protecting and supporting the brain and is also involved in the development of the facial structures. The main objective of our study was to evaluate skull base abnormalities in a group of patients diagnosed with Turner syndrome by assessing lateral cephalometric radiographs. A total of 7 patients diagnosed with Turner syndrome in the Endocrinology Department of the Emergency Clinical County Hospital of Craiova were included in the study. The following cephalometric variables were measured in our study: total skull base (N-Ba): Nasion (N)-Basion (Ba); anterior skull base (N-S): Nasion (N)-Sella (S); posterior skull base (S-Ba): Sella (S)-Basion (Ba). Regarding the investigated cephalometric variables, the mean±standard deviation (SD) recorded values in our study were 86.34±4.26mm for the total skull base (N-Ba), 63.87±2.54mm for the anterior skull base (N-S) and 38.33±4.87mm for the posterior skull base (S-Ba). The results of our study were compared to the ones provided by one of the most representative studies described in the literature. A reduced size of the posterior base of the skull is considered pathognomonic in subjects diagnosed with Turner syndrome. Also, the posterior base of the skull directly influences the maxillomandibular skeletal relationships and it is therefore necessary to calculate this cephalometric variable, which is easily highlighted on a lateral cephalometric radiograph.

Mice lacking the conserved transcription factor Grainyhead-like 3 (Grhl3) display increased apposition of the frontal and parietal bones during embryonic development

BACKGROUND

Increased apposition of the frontal and parietal bones of the skull during embryogenesis may be a risk factor for the subsequent development of premature skull fusion, or craniosynostosis. Human craniosynostosis is a prevalent, and often serious embryological and neonatal pathology. Other than known mutations in a small number of contributing genes, the aetiology of craniosynostosis is largely unknown. Therefore, the identification of novel genes which contribute to normal skull patterning, morphology and premature suture apposition is imperative, in order to fully understand the genetic regulation of cranial development.

RESULTS

Using advanced imaging techniques and quantitative measurement, we show that genetic deletion of the highly-conserved transcription factor Grainyhead-like 3 (Grhl3) in mice (Grhl3 ^-/- ) leads to decreased skull size, aberrant skull morphology and premature apposition of the coronal sutures during embryogenesis. Furthermore, Grhl3 ^-/- mice also present with premature collagen deposition and osteoblast alignment at the sutures, and the physical interaction between the developing skull, and outermost covering of the brain (the dura mater), as well as the overlying dermis and subcutaneous tissue, appears compromised in embryos lacking Grhl3. Although Grhl3 ^-/- mice die at birth, we investigated skull morphology and size in adult animals lacking one Grhl3 allele (heterozygous; Grhl3 ^+/- ), which are viable and fertile. We found that these adult mice also present with a smaller cranial cavity, suggestive of post-natal haploinsufficiency in the context of cranial development.

CONCLUSIONS

Our findings show that our Grhl3 mice present with increased apposition of the frontal and parietal bones, suggesting that Grhl3 may be involved in the developmental pathogenesis of craniosynostosis.

Insights into the development of molecular therapies for craniosynostosis

For the past 2 decades, clinical and basic science researchers have gained significant insights into the molecular and genetic pathways associated with common forms of craniosynostosis. This has led to invaluable information for families and physicians in their attempts to understand the heterogeneity of craniosynostosis. Genetic mutations have been identified in the fibroblast growth factor receptors (FGFRs) as well as in other targets, including TWIST1, BMP, and RUNX2. Greater understanding of these and other pathways has led to the development of innovative approaches for applying medical therapies to the treatment of craniosynostosis, in particular by maintaining suture patency. In this article, the authors discuss the molecular pathophysiological mechanisms underlying various forms of craniosynostosis. They also highlight recent developments in the field of molecular craniosynostosis research with the hope of identifying targets for medical therapies that might augment the results of surgical intervention.

Signaling pathways in osteogenesis and osteoclastogenesis: Lessons from cranial sutures and applications to regenerative medicine

One of the simplest models for examining the interplay between bone formation and resorption is the junction between the cranial bones. Although only roughly a quarter of patients diagnosed with craniosynostosis have been linked to known genetic disturbances, the molecular mechanisms elucidated from these studies have provided basic knowledge of bone homeostasis. This work has translated to methods and advances in bone tissue engineering. In this review, we examine the current knowledge of cranial suture biology derived from human craniosynostosis syndromes and discuss its application to regenerative medicine.

Cranial sutures: a multidisciplinary review

INTRODUCTION

Progress in cranial suture research is shaping our current understanding of the topic; however, emphasis has been placed on individual contributing components rather than the cranial sutural system as a whole. Improving our holistic view helps further guide clinicians who treat cranial sutural abnormalities as well as researchers who study them.

MATERIALS AND METHODS

Information from anatomy, anthropology, surgery, and computed modeling was integrated to provide a perspective to interpret suture formation and variability within the cranial functional and structural system.

RESULTS

Evidence from experimental settings, simulations, and evolution suggest a multifactorial morphogenetic process associated with functions and morphology of the sutures. Despite molecular influences, the biomechanical cranial environment has a main role in both the ontogenetic and phylogenetic suture dynamics.

CONCLUSIONS

Furthering our holistic understanding of the intricate cranial sutural system promises to expand our knowledge and enhance our ability to treat associated anomalies.

Role of RANK-RANKL-OPG axis in cranial suture homeostasis

Craniosynostosis is a significant disorder affecting 1 in 2500 live births worldwide. Although a large body of work has focused on dural regulation and the contributions of molecular mediators such as fibroblast growth factor, bone morphogenetic protein, and transforming growth factor β, minimal attention has been directed toward osteoclast function in cranial suture biology. Receptor activator of nuclear factor κB (RANK) is an essential mediator of osteoclastogenesis and osteoclast activation. In this study, physiologic fusion of posterior frontal sutures in murine development correlated with decreasing protein expression of RANK in comparison to age-matched coronal and sagittal sutures via immunohistochemical survey. However, RANK mRNA did not exhibit a similar pattern suggesting that RANK is regulated at the protein level. Fused cranial sutures in nonsyndromic craniosynostotic children also showed decreased levels of RANK staining in immunohistochemistry in comparison to patent sutures from the same patients. Immunohistochemistry with a RANK ligand antibody did not show differences in fused or patent sutures. Moreover, RANK knockdown in calvarial strip suture cultures displayed increased bone density specifically in the suture line after infection with small interfering RANK viruses. Cranial suture biology, similar to bone biology in general, likely depends on a complex interplay between osteoblasts and osteoclasts. We now report a temporospatial correlation between RANK expression and suture morphology that suggests that osteoclast activity is important in maintenance of cranial suture patency in normal physiology and disease. Furthermore, RANK downregulation promoted suture fusion establishing a causal relationship between the presence of RANK and patency.

Nell-1, a key functional mediator of Runx2, partially rescues calvarial defects in Runx2(+/-) mice

Mesenchymal stem cell commitment to an osteoprogenitor lineage requires the activity of Runx2, a molecule implicated in the etiopathology of multiple congenital craniofacial anomalies. Through promoter analyses, we have recently identified a new direct transcriptional target of Runx2, Nell-1, a craniosynostosis (CS)-associated molecule with potent osteogenic properties. This study investigated the mechanistic and functional relationship between Nell-1 and Runx2 in regulating osteoblast differentiation. The results showed that spatiotemporal distribution and expression levels of Nell-1 correlated closely with those of endogenous Runx2 during craniofacial development. Phenotypically, cross-mating Nell-1 overexpression transgenic (CMV-Nell-1) mice with Runx2 haploinsufficient (Runx2(+/-)) mice partially rescued the calvarial defects in the cleidocranial dysplasia (CCD)-like phenotype of Runx2(+/-) mice, whereas Nell-1 protein induced mineralization and bone formation in Runx2(+/-) but not Runx2(-/-) calvarial explants. Runx2-mediated osteoblastic gene expression and/or mineralization was severely reduced by Nell-1 siRNA oligos transfection into Runx2(+/+) newborn mouse calvarial cells (NMCCs) or in N-ethyl-N-nitrosourea (ENU)-induced Nell-1(-/-) NMCCs. Meanwhile, Nell-1 overexpression partially rescued osteoblastic gene expression but not mineralization in Runx2 null (Runx2(-/-)) NMCCs. Mechanistically, irrespective of Runx2 genotype, Nell-1 signaling activates ERK1/2 and JNK1 mitogen-activated protein kinase (MAPK) pathways in NMCCs and enhances Runx2 phosphorylation and activity when Runx2 is present. Collectively, these data demonstrate that Nell-1 is a critical downstream Runx2 functional mediator insofar as Runx2-regulated Nell-1 promotes osteoblastic differentiation through, in part, activation of MAPK and enhanced phosphorylation of Runx2, and Runx2 activity is significantly reduced when Nell-1 is blocked or absent.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Nonviral transfection of mouse calvarial organ in vitro using Accell-modified siRNA

BACKGROUND

Understanding the biology of cranial suture fusion and the precise role of involved molecules implicated in the process will help to identify key factors involved in regulation of suture fusion. Modulation of these key factors may serve as a tissue-engineering technique to replace the traditional surgical procedures for the correction of premature suture fusion. Modulation of gene expression by RNA interference is a widely used technique with high potential. Because there is no available report of calvarial organ transfection in vitro, the authors studied the development of a successful nonviral delivery technique of small inhibitory RNA (siRNA) to an in vitro calvarial organ culture system.

METHODS

In this study, 19-day-old male CD1 mice were euthanized and parallel craniotomies made through the parietal and frontal calvaria, 2 mm to either side of the sagittal suture, with care taken to preserve the underlying dura mater. Organs grown in vitro in a defined medium were transfected with transforming growth factor-beta1-specific Accell-modified siRNA followed by RNA isolation and quantitative polymerase chain reaction analysis.

RESULTS

Transfection of a calvarial organ with transforming growth factor-beta1-specific Accell-modified siRNA effectively knocks down the mRNA level.

CONCLUSIONS

Observations from this study indicate that in an in vitro calvarial organ culture system, a specific, efficient, and durable RNA interference activity can be achieved when Accell-modified siRNA is used. In addition to bypassing the need for toxic lipid carriers, the modifications introduced in Accell-modified siRNAs make it more stable and less off-target. This technique can potentially be used for in vivo studies once the initial effect of gene-specific siRNA on in vitro suture fusion has been determined.

Sustained platelet-derived growth factor receptor alpha signaling in osteoblasts results in craniosynostosis by overactivating the phospholipase C-gamma pathway

The development and growth of the skull is controlled by cranial sutures, which serve as growth centers for osteogenesis by providing a pool of osteoprogenitors. These osteoprogenitors undergo intramembranous ossification by direct differentiation into osteoblasts, which synthesize the components of the extracellular bone matrix. A dysregulation of osteoblast differentiation can lead to premature fusion of sutures, resulting in an abnormal skull shape, a disease called craniosynostosis. Although several genes could be linked to craniosynostosis, the mechanisms regulating cranial suture development remain largely elusive. We have established transgenic mice conditionally expressing an autoactivated platelet-derived growth factor receptor alpha (PDGFRalpha) in neural crest cells (NCCs) and their derivatives. In these mice, premature fusion of NCC-derived sutures occurred at early postnatal stages. In vivo and in vitro experiments demonstrated enhanced proliferation of osteoprogenitors and accelerated ossification of osteoblasts. Furthermore, in osteoblasts expressing the autoactivated receptor, we detected an upregulation of the phospholipase C-gamma (PLC-gamma) pathway. Treatment of differentiating osteoblasts with a PLC-gamma-specific inhibitor prevented the mineralization of synthesized bone matrix. Thus, we show for the first time that PDGFRalpha signaling stimulates osteogenesis of NCC-derived osteoblasts by activating the PLC-gamma pathway, suggesting an involvement of this pathway in the etiology of human craniosynostosis.

Dissecting the influence of regional dura mater on cranial suture biology

BACKGROUND

Craniosynostosis is a relatively common developmental disorder that leads to a number of serious consequences. Previous studies have shown the influence of dura mater on the overlying cranial suture. This study was conducted to determine the role of regional dura mater versus the intrinsic nature of the suture in directing the overlying suture's fate.

METHODS

The authors examined the effect of regional dura mater on the fate and morphology of the posterofrontal and coronal sutures. In 8-day-old Sprague-Dawley rats, calvarial disks, consisting of the posterofrontal and coronal sutures, were excised and placed in one of three positions: (1) native position (control group), (2) rotated 45 degrees, or (3) rotated 90 degrees (n = 5 animals per group). The animals were euthanized 1 month postoperatively, and the sutures were analyzed histologically.

RESULTS

The control group revealed normal suture morphology (n = 5). In the 45-degree rotation group, which placed the posterofrontal and coronal sutures over non-suture-associated dura mater, the posterofrontal sutures fused with thin morphology, and the coronal sutures remained patent (n = 5). In the 90-degree rotation group, the posterofrontal sutures, which were positioned over coronal suture-associated dura mater, were found to be fused with thinner morphology. The coronal sutures of the 90-degree rotation group, which were placed over posterofrontal suture-associated dura mater, remained patent but had acquired a posterofrontal-like morphology (n = 5).

CONCLUSIONS

This study further elucidates variations in the biology of dura mater, depending on its location. Furthermore, these results illustrate the interplay between regional dura mater and the inherent characteristics of the suture complex in determining suture biology.

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.

Isolation and characterization of posterofrontal/sagittal suture mesenchymal cells in vitro

BACKGROUND

Craniosynostosis, the premature fusion of cranial sutures, affects one in 2500 children. In the mouse, the posterofrontal suture is programed to fuse postnatally, but the adjacent sagittal suture remains patent throughout life. To study the cellular process of suture fusion, the authors isolated and studied suture-derived mesenchymal cells.

METHODS

Skulls were harvested from 80 mice (2 to 5 days old), and posterofrontal and sagittal sutures were dissected meticulously. Suture mesenchymal tissue was separated from the underlying dura mater and overlying pericranium and cultured in growth media. After the cells migrated from the explant tissues, the morphologies of the two cell populations were studied carefully, and quantitative real-time polymerase chain reaction was performed to evaluate gene expression.

RESULTS

Both posterofrontal and sagittal cells exhibited highly heterogeneous morphologies, and the posterofrontal cells migrated faster than the sagittal cells. Accordingly, growth factors such as transforming growth factor-beta1 and fibroblast growth factor (FGF)-2 were expressed significantly more highly in posterofrontal compared with sagittal suture mesenchymal cells. In contrast, FGF receptor 2 and FGF-18 were expressed significantly more in sagittal than in posterofrontal suture cells. Importantly, bone morphogenic protein-3, the only osteogenic inhibitor in the bone morphogenic protein family, and noggin, a bone morphogenic protein antagonist, were expressed significantly more in sagittal than in posterofrontal suture cells, suggesting a possible mechanism of suture patency.

CONCLUSIONS

To the authors' knowledge, this is the first analysis of mouse suture-derived mesenchymal cells. The authors conclude that isolation of suture-derived mesenchymal cells will provide a useful in vitro system with which to study the mechanisms underlying suture biology.

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.

The extracellular matrix environment in suture morphogenesis and growth

Sutures are the major bone growth sites of the craniofacial skeleton and form in response to developmental approximation of and interaction between two opposing osteogenic fronts. Premature obliteration of these craniofacial bone growth sites or craniosynostosis results in compensatory growth at other bone growth sites, with concomitant craniofacial dysmorphology. While much is now known about the growth and transcriptional factor regulation of suture formation and maintenance, little about the nature of the extracellular environment within sutures and their surrounding bones has been described. This review elucidates the nature of the sutural extracellular matrix and its role in mediating suture maintenance and growth through the regulation of cellular and biomechanical signaling.

Quantitative transcriptional analysis of fusing and nonfusing cranial suture complexes in mice

Previous studies have documented the differences in expression of various genes associated with the process of osteogenesis in fusing and nonfusing cranial sutures, including growth factors, growth factor receptors, and extracellular matrix molecules. Most of these studies were performed in rats, and although the biology regulating cranial suture fusion in mice and rats is presumed to be similar, studies are needed to verify these expression patterns as mice become increasingly utilized for scientific inquiry into the molecular biology of suture fusion and patency. The purpose of this study was to determine the differences in expression of several genes known to be critical to osteoblast biology. Posterior frontal and sagittal suture complexes (including the associated dura mater, suture mesenchyme, and osteogenic fronts) were isolated from 5-, 15-, 25-, 35-, and 45-day-old male CD-1 mice (n = 8 per age; n = 40 total). Total cellular RNA was extracted and converted to cDNA. Quantitative real-time reverse transcriptase polymerase chain reaction was then performed for the following genes: transforming growth factor beta1 and beta3, fibroblast growth factor receptor 1, Runx2,Osteopontin, and Osteocalcin. Expression of all genes examined was increased significantly in the posterior frontal suture as compared with the sagittal suture. Peak expression for all genes was observed on day 25. These data demonstrate that the expression of osteogenic growth factors, growth factor receptors, transcription factors, and extracellular matrix molecules is increased in the fusing posterior frontal suture in mice.

Relationship of brain and skull in pre- and postoperative sagittal synostosis

Models of vertebrate skull evolution stress the coordinated developmental relationship between the skull and the brain that it houses. This study investigates the relationship between altered skull morphology and brain morphology in premature fusion of the cranial sagittal suture (isolated sagittal synostosis; ISS), a condition associated with dysmorphology of both neurocranium and brain. Although the skull displays a more normal shape following reconstructive cranial vault surgery, effects of this surgery on the brain have not been investigated. Landmark coordinate data were collected from three-dimensional magnetic resonance imaging reconstructions of the brain in a sample of ISS patients and an age-matched unaffected cohort. These data were analysed using Euclidean distance matrix analysis (EDMA). Results show that the brain in ISS is dysmorphic preoperatively, displaying a posteriorly directed neural expansion that does not 'worsen' with growth. Postoperatively, the brain in ISS displays a more globular shape overall as compared with the preoperative morphology, but differs from normal in its subcortical morphology. These results show that the ISS brain is altered following neurocranial surgery, but does not more closely approximate that of unaffected individuals. This suggests that although the brain is affected by manipulation of the skull, it retains a growth pattern that is, at least in part, independent of the skull.

Assessing age-related ossification of the petro-occipital fissure: laying the foundation for understanding the clinicopathologies of the cranial base

The petro-occipital fissure (POF) lies within a critical interface of cranial growth and development in the posterior cranial fossa. The relationships between skeletal and soft tissues make this region especially important for examining biomechanical and basic biologic forces that may mold the cranial base and contribute to significant clinicopathologies associated with the structures located near the POF. Therefore, this study investigates the POF in adults in both preserved human cadavers and dried crania in order to determine if developmental changes can be observed and, if so, their value in age assessment as a model system for describing normal morphogenesis of the POF. This study demonstrates that tissue within the POF undergoes characteristic changes in ossification with age, the onset of which is considerably later than that of other synchondroses of the cranial base. Statistically, there is a moderate to strong correlation between age and stage of ossification within the POF. Further, male crania were observed to reach greater degrees of ossification at a younger age than female crania and that individual asymmetry in ossification of the tissue within the POF was not uncommon. An understanding of the basic temporal biological processes of the POF may yield insight into the development of clinicopathologies in this region of the cranial base.

Cranial sutures and bones: growth and fusion in relation to masticatory strain

Cranial bones and sutures are mechanically loaded during mastication. Their response to masticatory strain, however, is largely unknown, especially in the context of age change. Using strain gages, this study investigated masticatory strain in the posterior interfrontal and the anterior interparietal sutures and their adjacent bones in 3- and 7-month-old miniature swine (Sus scrofa). Double-fluorochrome labeling of these animals and an additional 5-month group was used to reveal suture and bone growth as well as features of suture morphology and fusion. With increasing age, the posterior interfrontal suture strain decreased in magnitude and changed in pattern from pure compression to both compression and tension, whereas the interparietal suture remained in tension and the magnitude increased unless the suture was fused. Morphologically, the posterior interfrontal suture was highly interdigitated at 3 months and then lost interdigitation ectocranially in older pigs, whereas the anterior interparietal suture remained butt-ended. Mineralization apposition rate (MAR) decreased with age in both sutures and was unrelated to strain. Bone mineralization was most vigorous on the ectocranial surface of the frontal and the parietal bones. Unlike the sutures, with age bone strain remained constant while bone MARs significantly increased and were correlated with bone thickness. Fusion had occurred in the interparietal suture of some pigs. In all cases fusion was ectocranial rather than endocranial. Fusion appeared to be associated with increased suture strain and enhanced bone growth on the ectocranial surface. Collectively, these results indicate that age is an important factor for strain and growth of the cranium. .

Adipose-derived adult stromal cells heal critical-size mouse calvarial defects

In adults and children over two years of age, large cranial defects do not reossify successfully, posing a substantial biomedical burden. The osteogenic potential of bone marrow stromal (BMS) cells has been documented. This study investigates the in vivo osteogenic capability of adipose-derived adult stromal (ADAS) cells, BMS cells, calvarial-derived osteoblasts and dura mater cells to heal critical-size mouse calvarial defects. Implanted, apatite-coated, PLGA scaffolds seeded with ADAS or BMS cells produced significant intramembranous bone formation by 2 weeks and areas of complete bony bridging by 12 weeks as shown by X-ray analysis, histology and live micromolecular imaging. The contribution of implanted cells to new bone formation was 84-99% by chromosomal detection. These data show that ADAS cells heal critical-size skeletal defects without genetic manipulation or the addition of exogenous growth factors.

Age-related changes in the biomolecular mechanisms of calvarial osteoblast biology affect fibroblast growth factor-2 signaling and osteogenesis

The ability of immature animals to orchestrate successful calvarial ossification has been well described. This capacity is markedly attenuated in mature animals and humans greater than 2 years of age. Few studies have investigated biological differences between juvenile and adult osteoblasts that mediate successful osteogenesis. To identify possible mechanisms for this clinical observation, we investigated cellular and molecular differences between primary osteoblasts derived from juvenile (2-day-old) and adult (60-day-old) rat calvaria. Data demonstrated that juvenile osteoblasts contain a subpopulation of less differentiated cells as observed by spindle-like morphology and decreased osteocalcin production. Juvenile, compared with adult, osteoblasts showed increased proliferation and adhesion. Furthermore, following rhFGF-2 stimulation juvenile osteoblasts increased expression of collagen I alpha 1 (5-fold), osteopontin (13-fold), and osteocalcin (16-fold), compared with relatively unchanged adult osteoblasts. Additionally, juvenile osteoblasts organized and produced more matrix proteins and formed 41-fold more bone nodules. Alternatively, adult osteoblasts produced more FGF-2 and preferentially translated the high molecular weight (22 kDa) form. Although adult osteoblasts transcribed more FGF-R1 and juvenile osteoblasts transcribed more FGF-R2 at baseline levels, juvenile osteoblasts translated more FGF-R1 and -R2 and showed increased phosphorylation. Collectively, these findings begin to explain why juvenile, but not adult, osteoblasts successfully heal calvarial defects.

Delivery of transforming growth factor-beta2-perturbing antibody in a collagen vehicle inhibits cranial suture fusion in calvarial organ culture

OBJECTIVE

To determine whether antibody perturbation of Tgf-beta, delivered in a collagen gel, could inhibit cranial suture fusion.

DESIGN

Attachment and proliferation of osteoblasts cultured on a collagen gel with or without anti-Tgf-beta2 antibody were determined by AlamarBlue dye assay and cell morphology by toluidine-blue staining. In rat calvarial organ culture, collagen gel with and without anti-Tgf-beta2 antibody was injected subperiosteally over the posterior frontal suture of postnatal day 15 rat calvariae. A quantitative analysis of suture fusion was used to measure suture bridging in histological serial sections at various time points.

RESULTS

Attachment and proliferation for cells cultured on collagen gel with anti-Tgf-beta2 antibody were similar to collagen gel controls. Although proliferation was lower than on tissue culture plastic, cells treated with anti-Tgf-beta2 antibody maintained an osteoblastic morphology. After 7, 10, and 15 days in organ culture, anti-Tgf-beta2 antibody treatment caused a reduction in the percent bridging of posterior frontal sutures, compared with controls. Sutures exposed to anti-Tgf-beta2 antibody and fibroblast growth factor-2 concurrently did not show an inhibition of bony bridging.

CONCLUSIONS

These results support previous reports suggesting a role for Tgf-beta2 in cranial suture fusion. In cell culture the collagen gel, both with and without anti-Tgf-beta2 antibody, promoted similar osteoblast attachment, proliferation, and osteoblastic morphology. In organ culture anti-Tgf-beta2 antibody was delivered in a bioactive state via a collagen gel to inhibit cranial suture fusion. Also, the results suggest that the inductive effect of fibroblast growth factor-2 is not dependent on Tgf-beta2 activity. Together, these results provide further support for the role of Tgf-beta2 in cranial suture fusion.

The BMP antagonist noggin regulates cranial suture fusion

During skull development, the cranial connective tissue framework undergoes intramembranous ossification to form skull bones (calvaria). As the calvarial bones advance to envelop the brain, fibrous sutures form between the calvarial plates. Expansion of the brain is coupled with calvarial growth through a series of tissue interactions within the cranial suture complex. Craniosynostosis, or premature cranial suture fusion, results in an abnormal skull shape, blindness and mental retardation. Recent studies have demonstrated that gain-of-function mutations in fibroblast growth factor receptors (fgfr) are associated with syndromic forms of craniosynostosis. Noggin, an antagonist of bone morphogenetic proteins (BMPs), is required for embryonic neural tube, somites and skeleton patterning. Here we show that noggin is expressed postnatally in the suture mesenchyme of patent, but not fusing, cranial sutures, and that noggin expression is suppressed by FGF2 and syndromic fgfr signalling. Since noggin misexpression prevents cranial suture fusion in vitro and in vivo, we suggest that syndromic fgfr-mediated craniosynostoses may be the result of inappropriate downregulation of noggin expression.

Fibroblast growth factor 2 induces increased calvarial osteoblast proliferation and cranial suture fusion

OBJECTIVE

Craniosynostosis has been associated with fibroblast growth factors (FGFs) and their receptors. The purpose of this study was to quantitatively determine the effect of FGF2 on rat calvarial osteoblasts and a rat cranial suture formation model.

DESIGN

Fetal rat calvarial osteoblasts were cultured with and without FGF2. Cell attachment and proliferation was determined by alamar Blue dye assay and cell morphology by toluidine-blue staining. In rat calvarial organ culture, postnatal day 15 rat calvariae with dura mater were placed in serum-free media with and without FGF2. A unique quantitative analysis of suture fusion was developed by obtaining measurements of suture bridging in histological serial sections at progressive stages of fusion.

RESULTS

Attachment for cells treated with FGF2 was similar to control. In contrast, proliferation was higher for cells treated with FGF2 while maintaining an osteoblastic morphology. After 5 days in organ culture, FGF2-treated posterior frontal sutures showed a dramatic increase in fusion, compared with untreated controls. This increased fusion was maintained throughout days 7 and 10 in culture. Also, fusion was enhanced on the dural side of the suture, as is normally observed in vivo, and the normal tissue architecture was maintained.

CONCLUSIONS

These results indicate that FGF2 can promote rat osteoblast attachment and normal cell morphology as well as induce cell proliferation. In calvarial organ culture, FGF2 treatment produced an enhanced suture fusion. These results provide further support for a critical role for FGF2 in cranial suture development. These studies also present a new quantitative approach to evaluating the effect of suture-perturbing growth factors on cranial suture fusion.

Correction of coronal suture synostosis using suture and dura mater allografts in rabbits with familial craniosynostosis

OBJECTIVE

Resynostosis following surgical correction of craniosynostosis is a common clinical correlate. Recent studies suggest that the dura mater is necessary to maintain suture patency. It has also been hypothesized that dura mater from synostotic individuals may provide aberrant biochemical signals to the osteogenic fronts of the calvaria, which result in premature suture fusion and subsequent resynostosis following surgery. This study was designed to test this hypothesis by surgically manipulating the coronal suture and dura mater in rabbits with familial craniosynostosis to prevent postsurgical resynostosis.

DESIGN

Craniofacial growth and histomorphometric data were collected from 129 rabbits: 72 normal controls and 57 rabbits with bilateral coronal suture synostosis (15 unoperated on controls; 13 surgical controls; 9 dura mater transplant only; 10 suture transplant only; and 10 suture and dura mater transplant). At 10 days of age, all rabbits had radiopaque amalgam markers placed on either side of the coronal, frontonasal, and anterior lambdoidal sutures. At 25 days of age, 42 synostosed rabbits had a 3 to 5-mm wide coronal suturectomy. Coronal sutures and/or underlying dura mater allografts were harvested from same-aged, wild-type, isohistogenic control rabbits and transplanted onto the dura mater of synostosed host rabbits. Serial radiographs were taken at 10, 25, 42, and 84 days of age, and the suturectomy sites were harvested at 84 days of age in 44 rabbits and serially sectioned for histomorphometric examination.

RESULTS

Results revealed that cranial vault growth was significantly (p < .05) improved following surgical release of the fused coronal suture compared with synostosed rabbits who were not operated on but was still significantly different (p < .05) from that of normal control rabbits. By 84 days of age, significant (p < .05) differences were noted in calvarial suture marker separation, cranial vault shape indices, and cranial base angles between rabbits with and without dura mater allografts, probably as a result of resynostosis of the suturectomy site or suture-only allografts. Qualitative histological examination revealed that at 84 days of age rabbits with suture and dura allografts had patent coronal sutures, suture-only allografts had fused coronal sutures with extensive endosteal hyperostosis, dura mater-only allografts had some new bone in the suturectomy site that resembled rudimentary osteogenic fronts, and suturectomy controls had extensive endosteal bone formation and resynostosis of the suturectomy site. Significantly (p < .05) more bone was found in the suturectomy sites of rabbits without dura mater allografts compared with rabbits with dura mater allografts.

CONCLUSIONS

Results support the initial hypothesis that normal dura mater allografts will maintain suture or suturectomy site patency and allow unrestricted craniofacial growth. However, it is still unclear whether the dura mater from normal rabbits was providing biochemical signals to the transplanted sutures or suturectomy sites or simply acting as a barrier to prevent abnormal biochemical signals from the dura mater of synostosed rabbits from reaching the calvaria. The clinical and therapeutic implications of these procedures are discussed.

Regional differentiation of cranial suture-associated dura mater in vivo and in vitro: implications for suture fusion and patency

Despite its prevalence, the etiopathogenesis of craniosynostosis is poorly understood. To better understand the biomolecular events that occur when normal craniofacial growth development goes awry, we must first investigate the mechanisms of normal suture fusion. Murine models in which the posterior frontal (PF) suture undergoes programmed sutural fusion shortly after birth provide an ideal model to study these mechanisms. In previous studies, our group and others have shown that sutural fate (i.e., fusion vs. patency) is regulated by the dura mater (DM) directly underlying a cranial suture. These studies have led to the hypothesis that calvarial DM is regionally differentiated and that this differentiation guides the development of the overlying suture. To test this hypothesis, we evaluated the messenger RNA (mRNA) expression of osteogenic cytokines (transforming growth factor beta1 [TGF-beta1] and TGF-beta3) and bone-associated extracellular matrix (ECM) molecules (collagen I, collagen III, osteocalcin, and alkaline phosphatase) in freshly isolated, rat dural tissues associated with the PF (programmed to fuse) or sagittal (SAG; remains patent) sutures before histological evidence of sutural fusion (postnatal day 6 [N6]). In addition, osteocalcin protein expression and cellular proliferation were localized using immunohistochemical staining and 5-bromo-2'deoxyuridine (BrdU) incorporation, respectively. We showed that the expression of osteogenic cytokines and bone-associated ECM molecules is potently up-regulated in the DM associated with the PF suture. In addition, we showed that cellular proliferation in the DM associated with the fusing PF suture is significantly less than that found in the patent SAG suture just before the initiation of sutural fusion N6. Interestingly, no differences in cellular proliferation rates were noted in younger animals (embryonic day 18 [E18] and N2). To further analyze regional differentiation of cranial suture-associated dural cells, we established dural cell cultures from fusing and patent rat cranial sutures in N6 rats and evaluated the expression of osteogenic cytokines (TGF-beta1 and fibroblast growth factor 2 [FGF-2]) and collagen I. In addition, we analyzed cellular production of proliferating cell nuclear antigen (PCNA). These studies confirmed our in vivo findings and showed that dural cell cultures derived from the fusing PF suture expressed significantly greater amounts of TGF-beta1, FGF-2, and collagen I. In addition, similar to our in vivo findings, we showed that PF suture-derived dural cells produced significantly less PCNA than SAG suture-derived dural cells. Finally, coculture of dural cells with fetal rat calvarial osteoblastic cells (FRCs) revealed a statistically significant increase in proliferation (*p < 0.001) in FRCs cocultured with SAG suture-derived dural cells as compared with FRCs cocultured alone or with PF suture-derived dural cells. Taken together, these data strongly support the hypothesis that the calvarial DM is regionally differentiated resulting in the up-regulation of osteogenic cytokines and bone ECM molecules in the dural tissues underlying fusing but not patent cranial sutures. Alterations in cytokine expression may govern osteoblastic differentiation and ECM molecule deposition, thus regulating sutural fate. Elucidation of the biomolecular events that occur before normal cranial suture fusion in the rat may increase our understanding of the events that lead to premature cranial suture fusion.

Cranial sutures as intramembranous bone growth sites

Intramembranous bone growth is achieved through bone formation within a periosteum or by bone formation at sutures. Sutures are formed during embryonic development at the sites of approximation of the membranous bones of the craniofacial skeleton. They serve as the major sites of bone expansion during postnatal craniofacial growth. For sutures to function as intramembranous bone growth sites, they need to remain in an unossified state, yet allow new bone to be formed at the edges of the overlapping bone fronts. This process relies on the production of sufficient new bone cells to be recruited into the bone fronts, while ensuring that the cells within the suture remain undifferentiated. Unlike endochondral growth plates, which expand through chondrocyte hypertrophy, sutures do not have intrinsic growth potential. Rather, they produce new bone at the sutural edges of the bone fronts in response to external stimuli, such as signals arising from the expanding neurocranium. This process allows growth of the cranial vault to be coordinated with growth of the neurocranium. Too little or delayed bone growth will result in wide-open fontanels and suture agenesis, whereas too much or accelerated bone growth will result in osseous obliteration of the sutures or craniosynostosis. Craniosynostosis in humans, suture fusion in animals, and induced suture obliteration in vitro has been associated with mutations or alterations in expression of several transcription factors, growth factors, and their receptors. Much of the data concerning signaling within sutures has been garnered from research on cranial sutures; hence, only the cranial sutures will be discussed in detail in this review. This review synthesizes classic descriptions of suture growth and pathology with modern molecular analysis of genetics and cell function in normal and abnormal suture morphogenesis and growth in a unifying hypothesis. At the same time, the reader is reminded of the importance of the suture as an intramembranous bone growth site.

New developments in craniofacial surgery research

The recent explosion in our understanding of developmental biology and genetics has enhanced our understanding of craniofacial biology. While it is not possible to summarize all new developments in craniofacial research, this article will review three areas: fetal models and surgery for craniofacial disorders, the biology of distraction osteogenesis, and the molecular mechanisms of cranial suture fusion. Numerous models of craniofacial disorders have been described, including small, short gestation and large, long gestation. The benefits and shortcomings of each are discussed. In addition, we discuss recent studies investigating the molecular mechanisms of mandibular distraction osteogenesis. Finally, we present a review of recent advances in the understanding of mechanisms of craniosynostosis, with particular emphasis on the biology of programmed cranial suture fusion in rodents.

Brain growth rates in craniosynostotic rabbits

OBJECTIVE

It has been suggested that abnormal brain morphology or growth rates may be a primary causal factor of craniosynostosis due, in part, to a lack of normal growth stretch and tension at the sutural margins. The purpose of the present study was to quantify cerebral hemisphere morphology and growth in a rabbit model of nonsyndromic coronal suture synostosis to determine whether cerebral dysmorphology is primary or secondary to synostosis in this model.

DESIGN

Fifty-seven brains (114 hemispheres) were examined from 40 normal control rabbits and 17 rabbits with bilateral coronal suture synostosis ranging in age from 25 to 450 days postconception (synostosis occurs at approximately 23 days postconception in this model). The calvariae were removed, the brains were fixed in 10% paraformaldehyde, and in situ bilateral measurements of cerebral hemisphere length and cerebral hemisphere width were obtained using a Wild microscope with a camera lucida attachment and digital caliper. Regression analysis was used to compare cerebral cortex growth rates by age between the two groups.

RESULTS

Cerebral hemisphere width and cerebral index regression line slopes had similar y intercepts (23 day postconception) with significantly (p < .05) diverging slopes over time. Normal rabbits increased more rapidly than synostosed rabbits. No significant (p > .05) differences were noted in regression line slopes between groups for cerebral hemisphere length by age or length by width.

CONCLUSIONS

Cerebral dysmorphologies are probably a compensatory, secondary (postsynostotic) event and not a primary causal factor of craniosynostosis in this rabbit model of human familial, nonsyndromic coronal suture synostosis.