Cranial sutures require tissue interactions with dura mater to resist osseous obliteration in vitro

Investigation of direction- and age-dependent prestretch in mouse cranial dura mater

Cranial dura mater is a dense interwoven vascularized connective tissue that helps regulate neurocranial remodeling by responding to strains from the growing brain. Previous ex vivo experimentation has failed to account for the role of prestretch in the mechanical behavior of the dura. Here we aim to estimate the prestretch in mouse cranial dura mater and determine its dependency on direction and age. We performed transverse and longitudinal incisions in parietal dura excised from newborn (day ∼ 4) and mature (12 weeks) mice and calculated the ex vivo normalized incision opening (measured width over length). Then, similar incisions were simulated under isotropic stretching within Abaqus/Standard. Finally, prestretch was estimated by comparing the ex vivo and in silico normalized openings. There were no significant differences between the neonatal and adult mice when comparing cuts in the same direction, but adult mice were found to have significantly greater stretch in the anterior-posterior direction than in the medial-lateral direction, while neonatal dura was essentially isotropic. Additionally, our simulations show that increasing curvature impacts the incision opening, indicating that flat in silico models may overestimate prestretch.

Cranium growth, patterning and homeostasis

Craniofacial development requires precise spatiotemporal regulation of multiple signaling pathways that crosstalk to coordinate the growth and patterning of the skull with surrounding tissues. Recent insights into these signaling pathways and previously uncharacterized progenitor cell populations have refined our understanding of skull patterning, bone mineralization and tissue homeostasis. Here, we touch upon classical studies and recent advances with an emphasis on developmental and signaling mechanisms that regulate the osteoblast lineage for the calvaria, which forms the roof of the skull. We highlight studies that illustrate the roles of osteoprogenitor cells and cranial suture-derived stem cells for proper calvarial growth and homeostasis. We also discuss genes and signaling pathways that control suture patency and highlight how perturbing the molecular regulation of these pathways leads to craniosynostosis. Finally, we discuss the recently discovered tissue and signaling interactions that integrate skull and cerebrovascular development, and the potential implications for both cerebrospinal fluid hydrodynamics and brain waste clearance in craniosynostosis.

EphB3 as a Potential Mediator of Developmental and Reparative Osteogenesis

The ephrin-B family of membrane-bound ligands is involved in skeletal patterning, osteogenesis, and bone homeostasis. Yet, despite the increasing collection of data affirming their importance in bone, the Eph tyrosine kinases that serve as the receptors for these ephrins in osteoblast stem cell niches remain unidentified. Here we report the expression of EphB3 at sites of bone growth in the embryo, especially at the calvaria suture fronts, periosteum, chondrocytes, and trabeculae of developing long bones. Strong EphB3 expression persisted in the adult calvarial sutures and in the proliferative chondrocytes of long bones, both of which are documented niches for osteoblastic stem cells. We observed EphB3-positive cells in the tissue filling a created calvarial injury, further implying EphB3 involvement in bone healing. Genetic knockout of EphB3 caused an increase in the bone tissue volume as a fraction of total volume in 6-week-old calvaria and in femoral trabecular density, compared to wild type controls. This difference resolved by 12 weeks of age, when we instead observed an increase in the bone volume of femoral trabeculae and in trabecular thickness. Our data identify EphB3 as a candidate regulator of osteogenesis either alone or in combination with other bone-expressed Ephs, and indicate that it appears to function as a limiter of bone growth.

Mathematical modeling of palatal suture pattern formation: morphological differences between sagittal and palatal sutures

The median palatal suture serves as a growth center for the maxilla; inadequate growth at this site causes malocclusion and dental crowding. However, the pattern formation mechanism of palatal sutures is poorly understood compared with that of calvarial sutures such as the sagittal suture. In the present study, therefore, we compared the morphological characteristics of sagittal and palatal sutures in human bone specimens. We found that palatal suture width was narrower than sagittal suture width, and the interdigitation amplitude of the palatal suture was lower than that of the sagittal suture. These tendencies were also observed in the neonatal stage. However, such differences were not observed in other animals such as chimpanzees and mice. We also used a mathematical model to reproduce the differences between palatal and sagittal sutures. After an extensive parameter search, we found two conditions that could generate the difference in interdigitation amplitude and suture width: bone differentiation threshold [Formula: see text] and growth speed c. We discuss possible biological interpretations of the observed pattern difference and its cause.

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.

Pediatric Skull Fracture Characteristics Associated with the Development of Leptomeningeal Cysts in Young Children after Trauma: A Single Institution's Experience

BACKGROUND

Currently, the pathogenesis of leptomeningeal cysts, also known as growing skull fractures, is still debated. The purpose of this study was to examine the specific skull fracture characteristics that are associated with the development of growing skull fractures and describe the authors' institutional experience managing this rare entity.

METHODS

A retrospective cohort study was performed that included all patients younger than 5 years presenting to a single institution with skull fractures from 2003 to 2017. Patient demographics, cause of injury, skull fracture characteristics (e.g., amount of diastasis, linear versus comminuted fracture), concomitant neurologic injuries, and management outcomes were recorded. Potential factors contributing to the development of a growing skull fracture and neurologic injuries associated with growing skull fractures were evaluated using univariate logistic regression.

RESULTS

A total of 905 patients met the authors' inclusion criteria. Of these, six (0.66 percent) were diagnosed with a growing skull fracture. Growing skull fractures were more likely to be comminuted (83.3 percent versus 40.7 percent; p = 0.082) and to present with diastasis on imaging (100 percent versus 26.1 percent; p < 0.001; mean amount of diastasis, 7.1 mm versus 3.1 mm; p < 0.001). Univariate logistic regression analysis confirmed the role of a comminuted fracture pattern (OR, 7.572) and the degree of diastasis (OR, 2.081 per mm diastasis) as significant risk factors for the development of growing skull fractures.

CONCLUSIONS

The authors' analysis revealed that fracture comminution and diastasis width are associated with the development of growing skull fractures. The authors recommend dural integrity assessment, close follow-up, and early management in young children who present with these skull fracture characteristics.

CLINICAL QUESTION/LEVEL OF EVIDENCE

Risk, III.

Craniofacial skeletal response to encephalization: How do we know what we think we know?

Dramatic changes in cranial capacity have characterized human evolution. Important evolutionary hypotheses, such as the spatial packing hypothesis, assert that increases in relative brain size (encephalization) have caused alterations to the modern human skull, resulting in a suite of traits unique among extant primates, including a domed cranial vault, highly flexed cranial base, and retracted facial skeleton. Most prior studies have used fossil or comparative primate data to establish correlations between brain size and cranial form, but the mechanistic basis for how changes in brain size impact the overall shape of the skull resulting in these cranial traits remains obscure and has only rarely been investigated critically. We argue that understanding how changes in human skull morphology could have resulted from increased encephalization requires the direct testing of hypotheses relating to interaction of embryonic development of the bones of the skull and the brain. Fossil and comparative primate data have thoroughly described the patterns of association between brain size and skull morphology. Here we suggest complementing such existing datasets with experiments focused on mechanisms responsible for producing the observed patterns to more thoroughly understand the role of encephalization in shaping the modern human skull.

On the traces of tcf12: Investigation of the gene expression pattern during development and cranial suture patterning in zebrafish (Danio rerio)

The transcription factor 12 (tcf12) is a basic Helix-Loop-Helix protein (bHLH) of the E-protein family, proven to play an important role in developmental processes like neurogenesis, mesoderm formation, and cranial vault development. In humans, mutations in TCF12 lead to craniosynostosis, a congenital birth disorder characterized by the premature fusion of one or several of the cranial sutures. Current research has been primarily focused on functional studies of TCF12, hence the cellular expression profile of this gene during embryonic development and early stages of ossification remains poorly understood. Here we present the establishment and detailed analysis of two transgenic tcf12:EGFP fluorescent zebrafish (Danio rerio) reporter lines. Using these transgenic lines, we analyzed the general spatiotemporal expression pattern of tcf12 during different developmental stages and put emphasis on skeletal development and cranial suture patterning. We identified robust tcf12 promoter-driven EGFP expression in the central nervous system (CNS), the heart, the pronephros, and the somites of zebrafish embryos. Additionally, expression was observed inside the muscles and bones of the viscerocranium in juvenile and adult fish. During cranial vault development, the transgenic fish show a high amount of tcf12 expressing cells at the growth fronts of the ossifying frontal and parietal bones and inside the emerging cranial sutures. Subsequently, we tested the transcriptional activity of three evolutionary conserved non-coding elements (CNEs) located in the tcf12 locus by transient transgenic assays and compared their in vivo activity to the expression pattern determined in the transgenic tcf12:EGFP lines. We could validate two of them as tcf12 enhancer elements driving specific gene expression in the CNS during embryogenesis. Our newly established transgenic lines enhance the understanding of tcf12 gene regulation and open up the possibilities for further functional investigation of these novel tcf12 enhancer elements in zebrafish.

Calvarial Suture-Derived Stem Cells and Their Contribution to Cranial Bone Repair

In addition to the natural turnover during life, the bones in the skeleton possess the ability to self-repair in response to injury or disease-related bone loss. Based on studies of bone defect models, both processes are largely supported by resident stem cells. In the long bones, the source of skeletal stem cells has been widely investigated over the years, where the major stem cell population is thought to reside in the perivascular niche of the bone marrow. In contrast, we have very limited knowledge about the stem cells contributing to the repair of calvarial bones. In fact, until recently, the presence of specific stem cells in adult craniofacial bones was uncertain. These flat bones are mainly formed via intramembranous rather than endochondral ossification and thus contain minimal bone marrow space. It has been previously proposed that the overlying periosteum and underlying dura mater provide osteoprogenitors for calvarial bone repair. Nonetheless, recent studies have identified a major stem cell population within the suture mesenchyme with multiple differentiation abilities and intrinsic reparative potential. Here we provide an updated review of calvarial stem cells and potential mechanisms of regulation in the context of skull injury repair.

Cell Mechanics of Craniosynostosis

Craniosynostosis is the premature fusion of the calvarial sutures that is associated with a number of physical and intellectual disabilities spanning from pediatric to adult years. Over the past two decades, techniques in molecular genetics and more recently, advances in high-throughput DNA sequencing have been used to examine the underlying pathogenesis of this disease. To date, mutations in 57 genes have been identified as causing craniosynostosis and the number of newly discovered genes is growing rapidly as a result of the advances in genomic technologies. While contributions from both genetic and environmental factors in this disease are increasingly apparent, there remains a gap in knowledge that bridges the clinical characteristics and genetic markers of craniosynostosis with their signaling pathways and mechanotransduction processes. By linking genotype to phenotype, outlining the role of cell mechanics may further uncover the specific mechanotransduction pathways underlying craniosynostosis. Here, we present a brief overview of the recent findings in craniofacial genetics and cell mechanics, discussing how this information together with animal models is advancing our understanding of craniofacial development.

Review of the Genetic Basis of Jaw Malformations

Genetic etiologies for congenital anomalies of the facial skeleton, namely, the maxilla and mandible, are important to understand and recognize. Malocclusions occur when there exist any significant deviation from what is considered a normal relationship between the upper jaw (maxilla) and the lower jaw (mandible). They may be the result of anomalies of the teeth alone, the bones alone, or both. A number of genes play a role in the facial skeletal development and are regulated by a host of additional regulatory molecules. As such, numerous craniofacial syndromes specifically affect the development of the jaws. The following review discusses several genetic anomalies that specifically affect the bones of the craniofacial skeleton and lead to malocclusion.

Understanding craniosynostosis as a growth disorder

Craniosynostosis is a condition of complex etiology that always involves the premature fusion of one or multiple cranial sutures and includes various anomalies of the soft and hard tissues of the head. Steady progress in the field has resulted in identifying gene mutations that recurrently cause craniosynostosis. There are now scores of mutations on many genes causally related to craniosynostosis syndromes, though the genetic basis for the majority of nonsyndromic cases is unknown. Identification of these genetic mutations has allowed significant progress in understanding the intrinsic properties of cranial sutures, including mechanisms responsible for normal suture patency and for pathogenesis of premature suture closure. An understanding of morphogenesis of cranial vault sutures is critical to understanding the pathophysiology of craniosynostosis conditions, but the field is now poised to recognize the repeated changes in additional skeletal and soft tissues of the head that typically accompany premature suture closure. We review the research that has brought an understanding of premature suture closure within our reach. We then enumerate the less well-studied, but equally challenging, nonsutural phenotypes of craniosynostosis conditions that are well characterized in available mouse models. We consider craniosynostosis as a complex growth disorder of multiple tissues of the developing head, whose growth is also targeted by identified mutations in ways that are poorly understood. Knowledge gained from studies of humans and mouse models for these conditions underscores the diverse, associated developmental anomalies of the head that contribute to the complex phenotypes of craniosynostosis conditions presenting novel challenges for future research. WIREs Dev Biol 2016, 5:429-459. doi: 10.1002/wdev.227 For further resources related to this article, please visit the WIREs website.

Signaling mechanisms implicated in cranial sutures pathophysiology: Craniosynostosis

Normal extension and skull expansion is a synchronized process that prevails along the osteogenic intersections of the cranial sutures. Cranial sutures operate as bone growth sites allowing swift bone generation at the edges of the bone fronts while they remain patent. Premature fusion of one or more cranial sutures can trigger craniosynostosis, a birth defect characterized by dramatic manifestations in appearance and functional impairment. Up until today, surgical correction is the only restorative measure for craniosynostosis associated with considerable mortality. Clinical studies have identified several genes implicated in the pathogenesis of craniosynostosis syndromes with useful insights into the underlying molecular signaling events that determine suture fate. In this review, we exploit the intracellular signal transduction pathways implicated in suture pathobiology, in an attempt to identify key signaling molecules for therapeutic targeting.

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Cranial sutures operate as bone growth sites.

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Premature fusion of one or more cranial sutures can trigger craniosynostosis.

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Several genes are involved in the pathogenesis of craniosynostosis syndromes.

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An array of molecular signaling events determine suture fate.

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Herein, the signal transduction pathways implicated in suture pathobiology are discussed.

Does an elevated bony ridge along the course of the metopic suture equal metopic synostosis? Implications for management

Metopic synostosis represents an increasingly prevalent form of nonsyndromic craniosynostosis. Premature fusion of the metopic suture classically results in trigonocephaly, hypotelorism, temporal narrowing, and a pronounced midline forehead ridge. However, as varying degrees of skull deformity exist, there is confusion regarding the appropriate management for an infant with a metopic ridge. We report on a 2-month-old infant with clinical manifestations of metopic synostosis but with a patent metopic suture documented on computed tomography scan. We examine the implications for management related to fusion of the suture, age of the patient, and severity of the head deformity.

Vertical osteoconductivity of sputtered hydroxyapatite-coated mini titanium implants after dura mater elevation: Rabbit calvarial model

This study evaluated the quantity and quality of newly formed vertical bone induced by sputtered hydroxyapatite-coated titanium implants compared with sandblasted acid-etched implants after dura mater elevation. Hydroxyapatite-coated and non-coated implants (n = 20/group) were used and divided equally into two groups. All implants were randomly placed into rabbit calvarial bone (four implants for each animal) emerging from the inferior cortical layer, displacing the dura mater 3 mm below the original bone. Animals were sacrificed at 4 (n = 5) and 8 (n = 5) weeks post-surgery. Vertical bone height and area were analyzed histologically and radiographically below the original bone. Vertical bone formation was observed in both groups. At 4 and 8 weeks, vertical bone height reached a significantly higher level in the hydroxyapatite compared with the non-coated group (p < 0.05). Vertical bone area was significantly larger in the hydroxyapatite compared with the non-coated group at 4 and 8 weeks (p < 0.05). This study indicates that vertical bone formation can be induced by dura mater elevation and sputtered hydroxyapatite coating can enhance vertical bone formation.

Fibroblast growth factor (FGF) signaling in development and skeletal diseases

Fibroblast growth factors (FGF) and their receptors serve many functions in both the developing and adult organism. Humans contain 18 FGF ligands and four FGF receptors (FGFR). FGF ligands are polypeptide growth factors that regulate several developmental processes including cellular proliferation, differentiation, and migration, morphogenesis, and patterning. FGF-FGFR signaling is also critical to the developing axial and craniofacial skeleton. In particular, the signaling cascade has been implicated in intramembranous ossification of cranial bones as well as cranial suture homeostasis. In the adult, FGFs and FGFRs are crucial for tissue repair. FGF signaling generally follows one of three transduction pathways: RAS/MAP kinase, PI3/AKT, or PLCγ. Each pathway likely regulates specific cellular behaviors. Inappropriate expression of FGF and improper activation of FGFRs are associated with various pathologic conditions, unregulated cell growth, and tumorigenesis. Additionally, aberrant signaling has been implicated in many skeletal abnormalities including achondroplasia and craniosynostosis. The biology and mechanisms of the FGF family have been the subject of significant research over the past 30 years. Recently, work has focused on the therapeutic targeting and potential of FGF ligands and their associated receptors. The majority of FGF-related therapy is aimed at age-related disorders. Increased understanding of FGF signaling and biology may reveal additional therapeutic roles, both in utero and postnatally. This review discusses the role of FGF signaling in general physiologic and pathologic embryogenesis and further explores it within the context of skeletal development.

Effects of growth hormone on the ontogenetic allometry of craniofacial bones

Organism size is controlled by interactions between genetic and environmental factors mediated by hormones with systemic and local effects. As changes in size are usually not isometric, a considerable diversity in shape can be generated through modifications in the patterns of ontogenetic allometry. In this study we evaluated the role of timing and dose of growth hormone (GH) release on growth and correlated shape changes in craniofacial bones. Using a longitudinal study design, we analyzed GH deficient mice treated with GH supplementation commencing pre- and post-puberty. We obtained 3D in vivo micro-CT images of the skull between 21 and 60 days of age and used geometric morphometrics to analyze size and shape changes among control and GH deficient treated and non-treated mice. The variable levels of circulating GH altered the size and shape of the adult skull, and influenced the cranial base, vault, and face differently. While cranial base synchondroses and facial sutures were susceptible to either the direct or indirect effect of GH supplementation, its effect was negligible on the vault. Such different responses support the role of intrinsic growth trajectories of skeletal components in controlling the modifications induced by systemic factors. Contrary to the expected, the timing of GH treatment did not have an effect on catch-up growth. GH levels also altered the ontogenetic trajectories by inducing changes in their location and extension in the shape space, indicating that differences arose before 21 days and were further accentuated by a truncation of the ontogenetic trajectories in GHD groups.

Visualisation of the intact dura mater and brain surface in infant autopsies: a minimally destructive technique for the post-mortem assessment of head injury

During the post-mortem examination of babies and young children, it is important to be able to visualise the brain and its coverings, particularly in cases where a head injury is likely to have occurred. In this paper, we present an improved method for removal of the calvarial bones in infant autopsies to enable viewing of the dura mater and brain. In contrast to the standard post-mortem procedure for observing and removing the brain, this novel technique is minimally disruptive, allowing the dura mater to remain undamaged. Specialised paediatric neurosurgical tools were used to remove the skull bones from 23 neonates, infants and young children during post-mortem examination. In 21 of our 23 cases, the calvarial bones were removed successfully with the dura mater remaining intact. In one case, there was a thickening of the dura mater which created a strong adherence of this membrane to the bone. In another case, the dura mater was slightly damaged due to the inexperience of the operator in using the neurosurgical tools. This method of calvarial bone removal reduces the degree of post-mortem artefact and enhances the ability to observe and photographically document autopsy findings, including the artefact-free detection of signs of injury such as epidural or subdural haematoma, and brain swelling. This technique has now become a routine practise in both of our units to remove the skull bones in infant/young children post-mortem examinations.

Endogenous bone regeneration is dependent upon a dynamic oxygen event

Amputation of the digit tip within the terminal phalangeal bone of rodents, monkeys, and humans results in near-perfect regeneration of bone and surrounding tissues; however, amputations at a more proximal level fail to produce the same regenerative result. Digit regeneration is a coordinated, multifaceted process that incorporates signaling from bioactive growth factors both in the tissue matrix and from several different cell populations. To elucidate the mechanisms involved in bone regeneration we developed a novel multi-tissue slice-culture model that regenerates bone ex vivo via direct ossification. Our study provides an integrated multi-tissue system for bone and digit regeneration and allows us to circumvent experimental limitations that exist in vivo. We used this slice-culture model to evaluate the influence of oxygen on regenerating bone. Micro-computed tomography (µCT) and histological analysis revealed that the regenerative response of the digit is facilitated in part by a dynamic oxygen event, in which mutually exclusive high and low oxygen microenvironments exist and vacillate in a coordinated fashion during regeneration. Areas of increased oxygen are initially seen in the marrow and then surrounding areas of vasculature in the regenerating digit. Major hypoxic events are seen at 7 days postamputation (DPA 7) in the marrow and again at DPA 12 in the blastema, and manipulation of oxygen tensions during these hypoxic phases can shift the dynamics of digit regeneration. Oxygen increased to 21% oxygen tension can either accelerate or attenuate bone mineralization in a stage-specific manner in the regenerative timeline. These studies not only reveal a circumscribed frame of oxygen influence during bone regeneration, but also suggest that oxygen may be one of the primary signaling influences during regeneration.

Evidence-based medicine: Craniosynostosis

LEARNING OBJECTIVES

After studying this article, the participant should be able to: (1) Make the appropriate diagnosis for each of the single-sutural synostoses, based on the physical examination. (2) Explain the functional concerns associated with these synostoses and why surgical correction is indicated. (3) Distinguish between the different types of surgical corrections available, the timing for these various interventions, and in what ways these treatments achieve overall management objectives. (4) Identify the basic goals involved in caring for the syndromic synostoses.

SUMMARY

This article provides an overview of the diagnosis and management of infants with craniosynostosis. This review also incorporates some of the treatment philosophies followed at The Craniofacial Center in Dallas, but is not intended to be an exhaustive treatise on the subject. It is designed to serve as a reference point for further in-depth study by review of the reference articles presented. This information base is then used for self-assessment and benchmarking in parts of the Maintenance of Certification process of the American Board of Plastic Surgery.

Midline craniofacial malformations with a lipomatous cephalocele are associated with insufficient closure of the neural tube in the tuft mouse

BACKGROUND

Genetic variations affecting neural tube closure along the head result in malformations to the face and brain, posing a significant impact on health care costs and the quality of life.

METHODS

We have established a mouse line from a mutation that arose spontaneously in our wild-type colony that we called tuft. Tuft mice have heritable midline craniofacial defects featuring an anterior lipomatous cephalocele.

RESULTS

Whole-mount skeletal stains indicated that affected newborns had a broader interfrontal suture where the cephalocele emerged between the frontal bones. Mice with a cephalocele positioned near the rostrum also presented craniofacial malformations such as ocular hypertelorism and midfacial cleft of the nose. Gross and histological examination revealed that the lipomatous cephalocele originated as a fluid filled cyst no earlier than E14.5 while embryos with a midfacial cleft was evident during craniofacial development at E11.5. Histological sections of embryos with a midfacial cleft revealed the cephalic neuroectoderm remained proximal or fused to the frontonasal ectoderm about the closure site of the anterior neuropore, indicating a defect to neural tube closure. We found the neural folds along the rostrum of E9 to E10.5 embryos curled inward and failed to close as well as embryos with exencephaly and anencephaly at later stages. Whole-mount in situ hybridization of anterior markers Fgf8 and Sonic hedgehog indicated closure of the rostral site was compromised in severe cases.

CONCLUSION

We present a model demonstrating how anterior cranial cephaloceles are generated following a defect to neural tube closure and relevance to subsequent craniofacial morphogenesis in the tuft mouse.

Smad7 modulates TGFβ signaling during cranial suture development to maintain suture patency

Craniosynostosis, the premature fusion of one or more sutures between the calvarial bones, is a common birth defect. Mutations in genes encoding receptors for the transforming growth factor-beta (TGFβ) family of signaling molecules have been associated with craniosynostosis, but how TGFβ signaling is regulated during suture development is not known. In the present study, we found that expression of Smad2 and Smad3, intracellular mediators of canonical TGFβ signaling, gradually increases during early postnatal suture development in rat in both the coronal suture (CS), which remains patent throughout life, and the posterior frontal suture (PFS), which undergoes programmed closure by postnatal day 22. The amounts of phosphorylated Smad2 and Smad3 proteins showed a similar gradual increase in the PFS and CS, but in the CS, Smad2/3 activation was suppressed after neonatal day 10. The suppression of Smad2/3 activation in the CS correlated with upregulation of Smad7 expression. We demonstrate that siRNA-mediated knockdown of Smad7 caused increased phosphorylation of Smad2 and Smad3 and induced osseous obliteration of the CS from postnatal days 10 to 22. The Smad7 siRNA-induced CS closure was associated with significantly increased levels of Fgf10 and phosphorylated ERK1/2 in the suture mesenchyme. Moreover, addition of the Erk1/2 inhibitor U0126 partially blocked Smad7-siRNA-induced CS closure. These findings suggest that canonical TGFβ signaling induces suture closure at least in part through activation of FGF and ERK signaling and that Smad7 plays an important role in maintaining suture patency by suppressing canonical TGFβ signaling during suture development.

A review of hedgehog signaling in cranial bone development

During craniofacial development, the Hedgehog (HH) signaling pathway is essential for mesodermal tissue patterning and differentiation. The HH family consists of three protein ligands: Sonic Hedgehog (SHH), Indian Hedgehog (IHH), and Desert Hedgehog (DHH), of which two are expressed in the craniofacial complex (IHH and SHH). Dysregulations in HH signaling are well documented to result in a wide range of craniofacial abnormalities, including holoprosencephaly (HPE), hypotelorism, and cleft lip/palate. Furthermore, mutations in HH effectors, co-receptors, and ciliary proteins result in skeletal and craniofacial deformities. Cranial suture morphogenesis is a delicate developmental process that requires control of cell commitment, proliferation and differentiation. This review focuses on both what is known and what remains unknown regarding HH signaling in cranial suture morphogenesis and intramembranous ossification. As demonstrated from murine studies, expression of both SHH and IHH is critical to the formation and fusion of the cranial sutures and calvarial ossification. SHH expression has been observed in the cranial suture mesenchyme and its precise function is not fully defined, although some postulate SHH to delay cranial suture fusion. IHH expression is mainly found on the osteogenic fronts of the calvarial bones, and functions to induce cell proliferation and differentiation. Unfortunately, neonatal lethality of IHH deficient mice precludes a detailed examination of their postnatal calvarial phenotype. In summary, a number of basic questions are yet to be answered regarding domains of expression, developmental role, and functional overlap of HH morphogens in the calvaria. Nevertheless, SHH and IHH ligands are integral to cranial suture development and regulation of calvarial ossification. When HH signaling goes awry, the resultant suite of morphologic abnormalities highlights the important roles of HH signaling in cranial development.

Ephrin-B stimulation of calvarial bone formation

INTRODUCTION

Ephrin-B2 on osteoclasts was reported to promote bone formation as part of homeostasis by activating the EphB4 tyrosine kinase receptor on osteoblasts. Little is known about the role of ephrin-B signaling to EphBs in developmental bone formation.

RESULTS

We observed expression of an ephrin-B2 LacZ chimeric allele in the periosteum, sutural bone fronts, and dura mater of embryonic and neonatal mice. Expression in the adult skull was confined to sutures, but was heavily upregulated at sites of bone injury. Culture of embryonic calvariae with soluble recombinant ephrin-B2/Fc doubled their bone content without altering suture width or overall skull morphology. Ephrin-B2/Fc also stimulated osteoblast marker gene expression in cultured MC3T3 preosteoblastic cells without the need for type 1 collagen-induced differentiation. EphB4 was absent in embryonic and adult skulls. However, EphB1 and EphB2, both physiological receptors for ephrin-Bs, were expressed at sites of osteogenesis, and EphB1 knockout mice displayed a reduction in calvarial bone content compared to controls.

CONCLUSIONS

These data support a role for ephrin-B2 in the development and healing of bone through activation of osteoblast-specific gene expression. EphB1 and EphB2 are likely candidates receptors for the ephrin-B2 in bone.

Tissue interactions between craniosynostotic dura mater and bone

BACKGROUND

Cells within the dura mater have been implicated in the determination of suture patency and fusion. Craniosynostosis (CS), the premature fusion of 1 or more of the cranial sutures, could result from abnormal control over the differentiation of osteoprogenitor cells from the dura mater. This study tested whether dura mater cells derived from rabbits with congenital CS were different from cells derived from normal rabbits and investigated the effects that CS dura mater had on osteogenic differentiation in vitro and in vivo.

METHODS

Cells were derived from the dura mater from wild-type rabbits (WT; n = 23) or CS rabbits (n = 16). Cells were stimulated with bone morphogenetic protein 4, and alkaline phosphatase (ALP) expression and cell proliferation were assessed. Dura mater-derived cells were also cocultured with primary rabbit bone-derived cells, and ALP was assessed. Finally, interactions between the dura mater and overlying tissues were manipulated in vivo.

RESULTS

Craniosynostotic dura mater-derived cells proliferated faster than did WT cells but were not more ALP positive. Coculture experiments showed that CS dura mater cells induced increased ALP activity in CS bone-derived cells, but not in WT bone-derived cells. In vivo experiments showed that a physical barrier successfully inhibited dura mater-derived osteogenesis.

CONCLUSIONS

Coculture of CS bone- and CS dura mater-derived cells evoked an abnormal phenotype in vitro. Covering the CS dura mater led to decreased bone formation in vivo. Further investigations will focus on the signaling molecules involved in the communication between these 2 CS tissue types in vitro and in vivo.

Relaxin Does Not Rescue Coronal Suture Fusion in Craniosynostotic Rabbits

Objectives

Craniosynostosis affects 1 in 2000 to 3000 live births and may result in craniofacial and neural growth disturbances. Histological data have shown that thick collagenous bundles are present in the sutural ligament, which may tether the osteogenic fronts, resulting in premature fusion. The hormone relaxin has been shown to disrupt collagen fiber organization, possibly preventing craniosynostosis by relaxing the sutural ligament and allowing osteogenic fronts to separate normally and stay patent. This study tested this hypothesis with a rabbit model of delayed-onset coronal suture synostosis.

Methods

A total of 18 New Zealand White rabbits with craniosynostosis were randomly assigned to one of three groups: sham control, protein control (BSA), relaxin treatment. After initial diagnosis, sham surgery, BSA, or relaxin was delivered to the fusing coronal suture in a slow-release (56-day) collagen vehicle. Longitudinal radiographs and body weights were collected at 10, 25, 42, and 84 days of age, and sutures were harvested for histology.

Results

Relaxin-treated animals had more disorganized intrasuture content than control groups. These specimens also appeared to have relatively wider sutures ectocranially. There were no significant differences in relaxin-treated animals for all craniofacial growth measures, or suture separation compared with controls.

Conclusions

These data do not support our initial hypothesis that the use of relaxin may rescue sutures destined to undergo premature suture fusion. These findings suggest that collagen fiber arrangement may not be important for suture fusion. This protein therapy would not be clinically useful for craniosynostosis.

Changes in biomechanical strain and morphology of rat calvarial sutures and bone after Tgf-β3 inhibition of posterior interfrontal suture fusion

Craniofacial sutures are bone growth fronts that respond and adapt to biomechanical environments. Little is known of the role sutures play in regulating the skull biomechanical environment during patency and fusion conditions, especially how delayed or premature suture fusion will impact skull biomechanics. Tgf-β3 has been shown to prevent or delay suture fusion over the short term in rat skulls, yet the long-term patency or its consequences in treated sutures is not known. It was therefore hypothesized that Tgf-β3 had a long-term impact to prevent suture fusion and thus alter the skull biomechanics. In this study, collagen gels containing 3 ng Tgf-β3 were surgically placed superficial to the posterior interfrontal suture (IFS) and deep to the periosteum in postnatal day 9 (P9) rats. At P9, P24, and P70, biting forces and strains over left parietal bone, posterior IFS, and sagittal suture were measured with masticatory muscles bilaterally stimulated, after which the rats were sacrificed and suture patency analyzed histologically. Results demonstrated that Tgf-β3 treated sutures showed less fusion over time than control groups, and strain patterns in the skulls of the Tgf-β3-treated group were different from that of the control group. Although bite force increased with age, no alterations in bite force were attributable to Tgf-β3 treatment. These findings suggest that the continued presence of patent sutures can affect strain patterns, perhaps when higher bite forces are present as in adult animals.

Craniosynostosis and 3-dimensional computed tomography

Craniosynostosis is when cranial sutures fuse prematurely. It causes cosmetic deformity and may limit brain growth with more serious consequences, such as hydrocephalus. It may involve one or more sutures, may be syndromic, and is thought most likely to result from genetic causes. Specific sutural closures result in reproducible cranial appearances and head shapes. Three-dimensional computed tomography has become the standard for preoperative diagnosis to allow surgical planning as well as postsurgical assessment. Early diagnosis allows early surgical management, which improves outcome. Nonsyndromic and syndromic craniosynostoses are illustrated in this pictorial review.

Craniofacial features resembling frontonasal dysplasia with a tubulonodular interhemispheric lipoma in the adult 3H1 tuft mouse

Intracranial lipomas are rare, but 45% of them occur along the midline cisterns between the hemispheres and are often associated with corpus callosum hypoplasia and craniofacial defects. They are difficult to detect as they are generally asymptomatic and visible by MRI or by postmortem examination. The exact cause of these interhemispheric lipomas is not known, but they arise from a developmental defect resulting in the maldifferentiation of mesenchymal cells into mesodermal derivatives that are not normally present. We have identified a new mouse mutant called tuft, exhibiting a forebrain, intracranial lipoma with midline craniofacial defects resembling frontonasal dysplasia (FND) that arose spontaneously in our wild-type 3H1 colony. The tuft trait seems to be transmitted in recessive fashion, but approximately 80% less frequent than the expected Mendelian 25%, due to either incomplete penetrance or prenatal lethality. MRI and histologic analysis revealed that the intracranial lipoma occurred between the hemispheres and often protruded through the sagittal suture. We also observed a lesion at the lamina terminalis (LT) that may indicate improper closure of the anterior neuropore. We have mapped the tuft trait to within an 18 cM region on mouse chromosome 10 by microsatellite linkage analysis and identified several candidate genes involved with craniofacial development and cellular differentiation of adipose tissue. Tuft is the only known mouse model for midline craniofacial defects with an intracranial lipoma. Identifying the gene(s) and mutation(s) causing this early developmental defect will help us understand the pathogenesis of FND and related craniofacial disorders.

The role of the sutures in biomechanical dynamic simulation of a macaque cranial finite element model: implications for the evolution of craniofacial form

The global biomechanical impact of cranial sutures on the face and cranium during dynamic conditions is not well understood. It is hypothesized that sutures act as energy absorbers protecting skulls subjected to dynamic loads. This hypothesis predicts that sutures have a significant impact on global patterns of strain and cranial structural stiffness when analyzed using dynamic simulations; and that this global impact is influenced by suture material properties. In a finite element model developed from a juvenile Rhesus macaque cranium, five different sets of suture material properties for the zygomaticotemporal sutures were tested. The static and dynamic analyses produced similar results in terms of strain patterns and reaction forces, indicating that the zygomaticotemporal sutures have limited impact on global skull mechanics regardless of loading design. Contrary to the functional hypothesis tested in this study, the zygomaticotemporal sutures did not absorb significant amounts of energy during dynamic simulations regardless of loading speed. It is alternatively hypothesized that sutures are mechanically significant only insofar as they are weak points on the cranium that must be shielded from unduly high stresses so as not to disrupt vitally important growth processes. Thus, sutural and overall cranial form in some vertebrates may be optimized to minimize or otherwise modulate sutural stress and strain.

Craniosynostosis following hemispherectomy in a 2.5-month-old boy with intractable epilepsy

The authors report on the case of a 6-week-old boy who presented with infantile spasms. At 2.5 months of age, the patient underwent a right hemispherectomy. Approximately 3 months postoperatively, the patient presented with left coronal craniosynostosis. Subsequent cranial vault remodeling resulted in satisfactory cosmesis. Four years after surgery, the patient remains seizure free without the need for anticonvulsant medications. The authors believe this to be the first reported case of iatrogenic craniosynostosis due to hemispherectomy, and they describe 2 potential mechanisms for its development. This case suggests that, in the surgical treatment of infants with intractable epilepsy, minimization of brain volume loss through disconnection techniques should be considered, among other factors, when determining the best course of action.

Molecular signaling in pathogenesis of craniosynostosis: the role of fibroblast growth factor and transforming growth factor–β

The interplay of signals between dura mater, suture mesenchyme, and brain is essential in determining the fate of cranial sutures and the pathogenesis of premature suture fusion leading to craniosynostosis. At the forefront of research into suture fusion is the role of fibroblast growth factor and transforming growth factor–β, which have been found to be critical in the cell-signaling cascade involved in aberrant suture fusion. In this review, the authors discuss recent and ongoing research into the role of fibroblast growth factor and transforming growth factor–β in the etiopathogenesis of craniosynostosis.

Interparietal bone (Os Incae) in craniosynostosis

The interparietal bone, Os Incae, is formed in a persistent mendosal suture. This suture is a normal variant in the human skull, well-known in anatomy and radiology textbooks. We report 11 children with craniosynostosis in the presence of an interparietal bone, five from Children's Hospital at Montefiore and six children from Children's Hospital Boston. The true incidence of an interparietal bone in patients with craniosynostosis or craniofacial anomalies is not known; nor are there recognized sequelae of an interparietal bone (bathrocephaly). Hypotheses regarding mechanisms that may contribute to the formation of an interparietal bone are discussed.

Nasal septal and premaxillary developmental integration: implications for facial reduction in Homo

The influence of the chondrocranium in craniofacial development and its role in the reduction of facial size and projection in the genus Homo is incompletely understood. As one component of the chondrocranium, the nasal septum has been argued to play a significant role in human midfacial growth, particularly with respect to its interaction with the premaxilla during prenatal and early postnatal development. Thus, understanding the precise role of nasal septal growth on the facial skeleton is potentially informative with respect to the evolutionary change in craniofacial form. In this study, we assessed the integrative effects of the nasal septum and premaxilla by experimentally reducing facial length in Sus scrofa via circummaxillary suture fixation. Following from the nasal septal-traction model, we tested the following hypotheses: (1) facial growth restriction produces no change in nasal septum length; and (2) restriction of facial length produces compensatory premaxillary growth due to continued nasal septal growth. With respect to hypothesis 1, we found no significant differences in septum length (using the vomer as a proxy) in our experimental (n = 10), control (n = 9) and surgical sham (n = 9) trial groups. With respect to hypothesis 2, the experimental group exhibited a significant increase in premaxilla length. Our hypotheses were further supported by multivariate geometric morphometric analysis and support an integrative relationship between the nasal septum and premaxilla. Thus, continued assessment of the growth and integration of the nasal septum and premaxilla is potentially informative regarding the complex developmental mechanisms that underlie facial reduction in genus Homo evolution.

FTY720 promotes local microvascular network formation and regeneration of cranial bone defects

The calvarial bone microenvironment contains a unique progenitor niche that should be considered for therapeutic manipulation when designing regeneration strategies. Recently, our group demonstrated that cells isolated from the dura are multipotent and exhibit expansion potential and robust mineralization on biodegradable constructs in vitro. In this study, we evaluate the effectiveness of healing critical-sized cranial bone defects by enhancing microvascular network growth and host dura progenitor trafficking to the defect space pharmacologically by delivering drugs targeted to sphingosine 1-phosphate (S1P) receptors. We demonstrate that delivery of pharmacological agonists to (S1P) receptors S1P(1) and S1P(3) significantly increase bone ingrowth, total microvessel density, and smooth muscle cell investment on nascent microvessels within the defect space. Further, in vitro proliferation and migration studies suggest that selective activation of S1P(3) promotes recruitment and growth of osteoblastic progenitors from the meningeal dura mater.

CTGF directs fibroblast differentiation from human mesenchymal stem/stromal cells and defines connective tissue healing in a rodent injury model

Fibroblasts are ubiquitous cells that demonstrate remarkable diversity. However, their origin and pathways of differentiation remain poorly defined. Here, we show that connective tissue growth factor (CTGF; also known as CCN2) is sufficient to induce human bone marrow mesenchymal stem/stromal cells (MSCs) to differentiate into fibroblasts. CTGF-stimulated MSCs lost their surface mesenchymal epitopes, expressed broad fibroblastic hallmarks, and increasingly synthesized collagen type I and tenacin-C. After fibroblastic commitment, the ability of MSCs to differentiate into nonfibroblastic lineages - including osteoblasts, chondrocytes, and adipocytes - was diminished. To address inherent heterogeneity in MSC culture, we established 18 single MSC-derived clones by limiting dilution. CTGF-treated MSCs were alpha-SMA-, differentiating into alpha-SMA+ myofibroblasts only when stimulated subsequently with TGF-beta1, suggestive of stepwise processes of fibroblast commitment, fibrogenesis, and pathological fibrosis. In rats, in vivo microencapsulated delivery of CTGF prompted postnatal connective tissue to undergo fibrogenesis rather than ectopic mineralization. The knowledge that fibroblasts have a mesenchymal origin may enrich our understanding of organ fibrosis, cancer stroma, ectopic mineralization, scarring, and regeneration.

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

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

Disheveled hair and ear (Dhe), a spontaneous mouse Lmna mutation modeling human laminopathies

Background

Investigations of naturally-occurring mutations in animal models provide important insights and valuable disease models. Lamins A and C, along with lamin B, are type V intermediate filament proteins which constitute the proteinaceous boundary of the nucleus. LMNA mutations in humans cause a wide range of phenotypes, collectively termed laminopathies. To identify the mutation and investigate the phenotype of a spontaneous, semi-dominant mutation that we have named Disheveled hair and ear (Dhe), which causes a sparse coat and small external ears in heterozygotes and lethality in homozygotes by postnatal day 10.

Findings

Genetic mapping identified a point mutation in the Lmna gene, causing a single amino acid change, L52R, in the coiled coil rod domain of lamin A and C proteins. Cranial sutures in Dhe/+ mice failed to close. Gene expression for collagen types I and III in sutures was deficient. Skulls were small and disproportionate. Skeletons of Dhe/+ mice were hypomineralized and total body fat was deficient in males. In homozygotes, skin and oral mucosae were dysplastic and ulcerated. Nuclear morphometry of cultured cells revealed gene dose-dependent blebbing and wrinkling.

Conclusion

Dhe mice should provide a useful new model for investigations of the pathogenesis of laminopathies.

Brain phenotypes in two FGFR2 mouse models for Apert syndrome

Apert syndrome (AS) is one of at least nine disorders considered members of the fibroblast growth factor receptor (FGFR) -1, -2, and -3-related craniosynostosis syndromes. Nearly 100% of individuals diagnosed with AS carry one of two neighboring mutations on Fgfr2. The cranial phenotype associated with these two mutations includes coronal suture synostosis, either unilateral (unicoronal synostosis) or bilateral (bicoronal synostosis). Brain dysmorphology associated with AS is thought to be secondary to cranial vault or base alterations, but the variation in brain phenotypes within Apert syndrome is unexplained. Here, we present novel three-dimensional data on brain phenotypes of inbred mice at postnatal day 0 each carrying one of the two Fgfr2 mutations associated with AS. Our data suggest that the brain is primarily affected, rather than secondarily responding to skull dysmorphogenesis. Our hypothesis is that the skull and brain are both primarily affected in craniosynostosis and that shared phenogenetic developmental processes affect both tissues in craniosynostosis of Apert syndrome.

Force-induced craniosynostosis in the murine sagittal suture

BACKGROUND

The cause of nonsyndromic craniosynostosis remains elusive. Although compressive forces have been implicated in premature suture fusion, conclusive evidence of force-induced craniosynostosis is lacking. The purpose of this study was to determine whether cyclical loading of the murine calvaria could induce suture fusion.

METHODS

Calvarial coupons from postnatal day-21, B6CBA, wild-type mice (n = 18) were harvested and cultured. A custom appliance capable of delivering controlled, cyclical, compressive loads was applied perpendicular to the sagittal suture within the coupon in vitro. Nine coupons were subjected to 0.3 g of force for 30 minutes each day for a total of 14 days. A control group of nine coupons was clamped in the appliance without loading. Analysis of suture phenotype was performed using alkaline phosphatase and hematoxylin and eosin staining techniques and in situ hybridization analysis using bone sialoprotein.

RESULTS

Control group sagittal sutures-which normally remain patent in mice-showed their customary histologic appearance. In contradistinction, sagittal sutures subjected to cyclic loading showed histologic evidence of premature fusion (craniosynostosis). In addition, alkaline phosphatase activity and bone sialoprotein expression were observed to be increased in the experimental group when compared with matched controls.

CONCLUSIONS

An in vitro model of force-induced craniosynostosis has been devised. Premature fusion of the murine sagittal suture was induced with the application of controlled, cyclical, compressive loads. These results implicate abnormal forces in the development of nonsyndromic craniosynostosis, which supports our global hypothesis that epigenetic phenomena play a crucial role in the pathogenesis of craniosynostosis.

Manifold functions of the Nail-Patella Syndrome gene Lmx1b in vertebrate development

The LIM (Lin-1, Isl-1 and Mec-3)-homeodomain transcription factor 1 beta (Lmx1b) is widely expressed in vertebrate embryos, and is implicated in the development of diverse structures such as limbs, kidneys, eyes and brains. LMX1B mutations in humans cause an autosomal dominant inherited disease called nail-patella syndrome (NPS), which is characterized by abnormalities of the arms and legs as well as kidney disease and glaucoma. Expression of Lmx1b in the dorsal compartment of growing limb buds is critical for specification of dorsal limb cell fates and consequently dorsoventral patterning of limbs. In addition, Lmx1b is involved in the differentiation of anterior eye structures, formation of the glomerular basement membrane in kidneys and development of the skeleton, especially calvarial bones. In the central nervous system, Lmx1b controls the inductive activity of isthmic organizer, differentiation and maintenance of central serotonergic neurons, as well as the differentiation and migration of spinal dorsal horn neurons. Although details of the genetic programs involved in these developmental events are largely unknown, it is suggested that Lmx1b plays central roles in fate determination or cell differentiation in these tissues. Sustained expression of Lmx1b in the postnatal and mature mouse brain suggests that it also plays important roles in brain maturation and in the regulation of normal brain functions. This review aims to highlight recent insights into the many activities of Lmx1b in vertebrates.

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.

Operative management of growing skull fractures: a technical note

OBJECTS

Growing skull fractures can be a challenging surgical problem facing the pediatric neurosurgeon. The goal of this manuscript is to clarify effective surgical methods and to provide the rationale for these techniques.

METHODS

We describe the surgical techniques for treatment of growing skull fractures. We clarify the underlying concepts, with respect to dural closure and repair of bony defects, that have led to these techniques.

CONCLUSIONS

With effective surgical technique, the pediatric neurosurgeon can effectively treat growing skull fractures, with excellent outcomes in terms of bony coverage and cosmesis.