Mechanical influences on suture development and patency

Three-dimensional morphometric analysis of cranial sutures - A novel approach to quantitative analysis

Objective

Differences in complexity of cranial suture forms on the endocranial (i.e., deep) and ectocranial (i.e., superficial) skull surfaces have been noted in the literature, indicating through thickness three-dimensional (3D) suture variability depending on the chosen section and necessity for considering the complete 3D structure in many cases. This study aims to evaluate the variability of suture morphology through the skull thickness using a rat model, and to provide more robust metrics and methodologies to analyze suture morphology.

Design

X-ray micro-computed tomographic (μCT) imaging methods were utilized in order to provide internal structure information. Methods were developed to isolate and analyze sutures widths and linear interdigitation index (LII) values on each adjacent offset transverse plane of the μCT datasets. LII was defined as the curved path length of the suture divided by the linear length between the ends of the region of interest. Scans were obtained on 15 female rats at ages of 16, 20, and 24 weeks (n = 5/age). Samples were imaged at 18 μm resolutions with 90 kV source voltage, 278 μA source amperage, and 0.7° increments. Suture widths and LII values were compared using a Kruskal-Wallis test.

Results

3D variability in local suture widths within individuals, as well as through thickness variabilities in planar widths and LII was observed. Kruskal-Wallis tests for bulk through thickness averaged suture widths and LII were found to be statistically insignificant, despite clear geometric differences through suture thicknesses.

Conclusion

Although the bulk morphometric variability between age groups was found to be statistically insignificant, the 3D variability within individuals point to the importance of analyzing suture form using 3D metrics when studying suture development, response to functional activity, or morphometry in general.

Cranial Bone Changes Induced by Mild Traumatic Brain Injuries: A Neglected Player in Concussion Outcomes?

Mild traumatic brain injuries (TBIs), particularly when repetitive in nature, are increasingly recognized to have a range of significant negative implications for brain health. Much of the ongoing research in the field is focused on the neurological consequences of these injuries and the relationship between TBIs and long-term neurodegenerative conditions such as chronic traumatic encephalopathy and Alzheimer's disease. However, our understanding of the complex relationship between applied mechanical force at impact, brain pathophysiology, and neurological function remains incomplete. Past research has shown that mild TBIs, even below the threshold that results in cranial fracture, induce changes in cranial bone structure and morphology. These structural and physiological changes likely have implications for the transmission of mechanical force into the underlying brain parenchyma. Here, we review this evidence in the context of the current understanding of bone mechanosensitivity and the consequences of TBIs or concussions. We postulate that heterogeneity of the calvarium, including differing bone thickness attributable to past impacts, age, or individual variability, may be a modulator of outcomes after subsequent TBIs. We advocate for greater consideration of cranial responses to TBI in both experimental and computer modeling of impact biomechanics, and raise the hypothesis that calvarial bone thickness represents a novel biomarker of brain injury vulnerability post-TBI.

FGF signaling in cranial suture development and related diseases

Suture mesenchymal stem cells (SMSCs) are a heterogeneous stem cell population with the ability to self-renew and differentiate into multiple cell lineages. The cranial suture provides a niche for SMSCs to maintain suture patency, allowing for cranial bone repair and regeneration. In addition, the cranial suture functions as an intramembranous bone growth site during craniofacial bone development. Defects in suture development have been implicated in various congenital diseases, such as sutural agenesis and craniosynostosis. However, it remains largely unknown how intricate signaling pathways orchestrate suture and SMSC function in craniofacial bone development, homeostasis, repair and diseases. Studies in patients with syndromic craniosynostosis identified fibroblast growth factor (FGF) signaling as an important signaling pathway that regulates cranial vault development. A series of in vitro and in vivo studies have since revealed the critical roles of FGF signaling in SMSCs, cranial suture and cranial skeleton development, and the pathogenesis of related diseases. Here, we summarize the characteristics of cranial sutures and SMSCs, and the important functions of the FGF signaling pathway in SMSC and cranial suture development as well as diseases caused by suture dysfunction. We also discuss emerging current and future studies of signaling regulation in SMSCs.

Mechanical loading of cranial joints minimizes the craniofacial phenotype in Crouzon syndrome

Children with syndromic forms of craniosynostosis undergo a plethora of surgical interventions to resolve the clinical features caused by the premature fusion of cranial sutures. While surgical correction is reliable, the need for repeated rounds of invasive treatment puts a heavy burden on the child and their family. This study explores a non-surgical alternative using mechanical loading of the cranial joints to prevent or delay craniofacial phenotypes associated with Crouzon syndrome. We treated Crouzon syndrome mice before the onset of craniosynostosis by cyclical mechanical loading of cranial joints using a custom designed set-up. Cranial loading applied to the frontal bone partially restores normal skull morphology, significantly reducing the typical brachycephalic appearance. This is underpinned by the delayed closure of the coronal suture and of the intersphenoidal synchondrosis. This study provides a novel treatment alternative for syndromic craniosynostosis which has the potential to be an important step towards replacing, reducing or refining the surgical treatment of all craniosynostosis patients.

A Computational Framework to Predict Calvarial Growth: Optimising Management of Sagittal Craniosynostosis

The neonate skull consists of several bony plates, connected by fibrous soft tissue called sutures. Premature fusion of sutures is a medical condition known as craniosynostosis. Sagittal synostosis, caused by premature fusion of the sagittal suture, is the most common form of this condition. The optimum management of this condition is an ongoing debate in the craniofacial community while aspects of the biomechanics and mechanobiology are not well understood. Here, we describe a computational framework that enables us to predict and compare the calvarial growth following different reconstruction techniques for the management of sagittal synostosis. Our results demonstrate how different reconstruction techniques interact with the increasing intracranial volume. The framework proposed here can be used to inform optimum management of different forms of craniosynostosis, minimising the risk of functional consequences and secondary surgery.

[A cone-beam computed tomography evaluation of three-dimensional changes of circummaxillary sutures following maxillary protraction with alternate rapid palatal expansions and constrictions]

OBJECTIVE

To assess three-dimensional (3D) changes of circummaxillary sutures following maxillary protraction with alternate rapid palatal expansions and constrictions (RPE/C) facemask protocol in maxillary retrusive children, and to investigate the relationship between the changes of circum-maxillary sutures and zygomaticomaxillary suture (ZMS) maturation, and to explore the factors of maxilla forward movement with RPE/C and facemask.

METHODS

In the study (clinical trial registration No: ChiCTR2000034909), 36 maxillary retrusive patients were recruited and block randomized to either the rapid palatal expansion (RPE) group or the RPE/C group. Patients aged 7 to 13 years, Class Ⅲ malocclusion, anterior crossbite, ANB less than 0°, Wits appraisal less than -2 mm, and A-Np less than 0 mm were included in the study. The RPE group received rapid palatal expansion, whereas the RPE/C group received alternate rapid palatal expansions and constrictions, and both with facemask protraction. Head orientations of cone-beam computed tomography (CBCT) images were implemented by Dolphin 11.7. 3D measurements of circummaxillary sutures on CBCT images were evaluated using Mimics 10.01 before (T0) and after treatment (T1). The changes were analyzed with independent t test, two-way ANOVA, Pearson correlation and regression analysis.

RESULTS

Two subjects in the RPE/C group were lost to follow-up. A total of 34 patients reached the completion criteria and were analyzed. Compared with the RPE group, sagittal changes of circummaxillary sutures were significantly increased in the RPE/C group with 1.21 mm advancement of zygomaticotemporal suture, 2.20 mm of ZMS, 1.43 mm of zygoma-ticofrontal suture (P < 0.05, respectively). Except for the zygomaticotemporal suture, the rest forward sagittal changes of other circummaxillary sutures showed no major difference in terms of the ZMS maturation. The Spearman's correlation in RPE/C indicated a strong positive correlation of sagittal changes between ZMS and point A (P < 0.01) with a regression analysis R2=42.5%.

CONCLUSION

RPE/C might be more effective on the treatment of maxillary retrusive children. As one of the major mechanical loading sutures during orthopedic therapy, ZMS showed a strong positive correlation with point A on sagittal changes.

Closure times of neurocranial sutures and synchondroses in Persian compared to Domestic Shorthair cats

Human-directed selective breeding has modified the phenotype of the modern Persian cat towards an extreme brachycephalic phenotype (‘peke-face’ Persian), which originates from a spontaneous mutation that first appeared in the 1950s in traditional Persian types. It was suggested that the peke-face phenotype results from pathologic skull development and might represent a craniosynostosis of the coronal sutures. We followed this hypothesis and investigated the time dependent status of the neurocranial sutures and synchondroses in an ontogenetic series of doll-faced and peke-faced Persian cats compared to Domestic Shorthair cats (DSHs). Cranial suture closure was assessed by examining an ontogenetic series of formalin-fixed head specimens (n = 55) and dry skulls (n = 32) using micro-computed tomography. Sagittal, metopic, coronal and lambdoid sutures as well as intersphenoidal, spheno-occipital and spheno-ethmoid synchondroses were examined. Logistic regression analysis was performed to test the global effect of age on suture closure within a group of peke-face Persians, doll-face Persians and DSHs and the 50% probability of having a closed suture was calculated and compared between groups. Age was a perfect predictor for the condition of the coronal sutures in peke-face Persians. Coronal sutures were found to be closed at 0–0.3 months. In doll-face and DSHs, coronal sutures were open throughout the lifetime with the exception of a few very old cats. Results of this study confirmed a coronal craniosynostosis that likely causes the extreme brachycephalic skull morphology in the peke-face Persian.

Ciliary Signalling and Mechanotransduction in the Pathophysiology of Craniosynostosis

Craniosynostosis (CS) is the second most prevalent inborn craniofacial malformation; it results from the premature fusion of cranial sutures and leads to dimorphisms of variable severity. CS is clinically heterogeneous, as it can be either a sporadic isolated defect, more frequently, or part of a syndromic phenotype with mendelian inheritance. The genetic basis of CS is also extremely heterogeneous, with nearly a hundred genes associated so far, mostly mutated in syndromic forms. Several genes can be categorised within partially overlapping pathways, including those causing defects of the primary cilium. The primary cilium is a cellular antenna serving as a signalling hub implicated in mechanotransduction, housing key molecular signals expressed on the ciliary membrane and in the cilioplasm. This mechanical property mediated by the primary cilium may also represent a cue to understand the pathophysiology of non-syndromic CS. In this review, we aimed to highlight the implication of the primary cilium components and active signalling in CS pathophysiology, dissecting their biological functions in craniofacial development and in suture biomechanics. Through an in-depth revision of the literature and computational annotation of disease-associated genes we categorised 18 ciliary genes involved in CS aetiology. Interestingly, a prevalent implication of midline sutures is observed in CS ciliopathies, possibly explained by the specific neural crest origin of the frontal bone.

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.

Description and rediagnosis of the crested hadrosaurid (Ornithopoda) dinosaur Parasaurolophus cyrtocristatus on the basis of new cranial remains

For nearly 60 years, skulls of Parasaurolophus species have been differentiated primarily on the basis of crest shape rather than on unique morphologic characters of other cranial elements. Complicating matters is the fact that crests dramatically change shape throughout ontogeny. Without a complete growth series, it has become difficult to assess the taxonomic distinctness of each species through the lens of allometric growth. Parasaurolophus cyrtocristatus has proven to be especially troublesome to assess because of the poorly preserved nature of the type and only skull. A new, partial skull from the Fossil Forest Member of the Fruitland Formation—the same geologic unit as the type specimen—is the first opportunity to re-diagnose this species as well as redefine the genus with many new traits. An undescribed, short-crested subadult skull from the Kaiparowits Formation of Utah previously assigned to cf. P. cyrtocristatus allows detailed comparisons to be made between the unnamed Utah taxon and the material of this species from the type locality. We find that several characteristics of the squamosal, supraoccipital, and premaxilla shared between the referred skull and the type skull are unique to P. cyrtocristatus (senso stricto) within the genus, irrespective of the overall crest shape. A phylogenetic analysis that includes six new characters posits that P. cyrtocristatus and P. tubicen are sister taxa, and that the latter does not share a closest common ancestor with the long-crested P. walkeri as previously hypothesized. This result helps to explain why both taxa are found in northeastern New Mexico, USA and in sequential geologic units (Fruitland Formation and Kirtland Formation, respectively). Additionally, the exquisitely preserved new skull provides the first opportunity to unequivocally identify the osteological make-up of the Parasaurolophus cranial crest. Unlike in previous reconstructions, the crest composition in Parasaurolophus follows what is seen in other lambeosaurines such as Corythosaurus, where the dorsal process of the premaxilla dominates the crest, with the nasal forming 80% of the ventral paired tubes, and the lateral premaxillary process acting a lateral cover between the dorsal and ventral tubes. The skull of P. cyrtocristatus is still incompletely known, so more complete material will likely reveal new features that further differentiate this species and aid in determining the pace of ornamental crest evolution.

Morphometric Variation at Different Spatial Scales: Coordination and Compensation in the Emergence of Organismal Form

It is a classic aim of quantitative and evolutionary biology to infer genetic architecture and potential evolutionary responses to selection from the variance–covariance structure of measured traits. But a meaningful genetic or developmental interpretation of raw covariances is difficult, and classic concepts of morphological integration do not directly apply to modern morphometric data. Here, we present a new morphometric strategy based on the comparison of morphological variation across different spatial scales. If anatomical elements vary completely independently, then their variance accumulates at larger scales or for structures composed of multiple elements: morphological variance would be a power function of spatial scale. Deviations from this pattern of “variational self-similarity” (serving as a null model of completely uncoordinated growth) indicate genetic or developmental coregulation of anatomical components. We present biometric strategies and R scripts for identifying patterns of coordination and compensation in the size and shape of composite anatomical structures. In an application to human cranial variation, we found that coordinated variation and positive correlations are prevalent for the size of cranial components, whereas their shape was dominated by compensatory variation, leading to strong canalization of cranial shape at larger scales. We propose that mechanically induced bone formation and remodeling are key mechanisms underlying compensatory variation in cranial shape. Such epigenetic coordination and compensation of growth are indispensable for stable, canalized development and may also foster the evolvability of complex anatomical structures by preserving spatial and functional integrity during genetic responses to selection.[Cranial shape; developmental canalization; evolvability; morphological integration; morphometrics; phenotypic variation; self-similarity.]

Integrated Transcriptome and Network Analysis Reveals Spatiotemporal Dynamics of Calvarial Suturogenesis

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

Stimulatory effects of simvastatin on bone regeneration of the expanded suture in rats

Simvastatin belongs to the family of statins and is found to have some osteopromotive properties in recent years. The aim of the present study was to investigate the potential effects of simvastatin on bone formation of the expanded mid-palatal suture of rats. Forty-five Wistar rats were randomly divided into three groups: control (C), expansion (EP), and expansion plus simvastatin (ES) groups. Rats in the ES group were administrated with simvastatin (20 mg/kg/d body weight). According to the schedule of sacrifice (days 3, 7 and 14), the suture width and bone volume changes of the region of interest (ROI) were detected by micro-computed tomography during RME. Besides, morphological changes and bone morphogenetic protein 2 (BMP-2) expression in the mid-palatal suture were observed by hematoxylin and eosin (HE) and immunohistochemical staining. Kruskal-Wallis one-way analysis of variance (ANOVA) and LSD method were applied to analyze the data at P<0.05 level. By the RME appliance, the suture was successfully widened. On days 7, 14, the bone volume of ROI in the ES group was more than that in the EP group (P<0.05). Besides, histological examinations also demonstrated that more bone regeneration and capillaries in the suture in the ES group were observed than that in the EP group. The BMP-2 expression in the ES group was more (P<0.05) than that in the EP and C groups on days 3, 7, 14. Consequently, those findings showed that simvastatin can induce a favorable effect on bone regeneration in the mid-palatal suture of rats during RME.

A comparative finite element analysis of maxillary expansion with and without midpalatal suture viscoelasticity using a representative skeletal geometry

The goal of this investigation was to adapt and incorporate a nonlinear viscoelastic material model representative of the midpalatal suture’s viscoelastic nature into finite element analysis simulations of maxillary expansion treatment. Step-wise displacements were applied to a partial skull geometry to simulate treatment using an expansion screw appliance. Four simulation cases were considered for the midpalatal and intermaxillary sutures: 1. Neglecting suture tissue; 2. Linear elastic properties; 3. Viscoelastic properties; 4. A fused intermaxillary and viscoelastic midpalatal suture. Results from simulations indicated that removal of suture tissue and inclusion of viscoelastic properties resulted in the same maxillary displacement following 29 activations of 0.125 mm applied directly to the maxilla; however, assuming a fused intermaxillary suture significantly changed maxillary displacement patterns. Initial stress results within the suture complex were significantly influenced by the inclusion of suture viscoelasticity as compared to linear elastic properties. The presented study demonstrates successful incorporation of suture viscoelasticity into finite element analysis simulations of maxillary expansion treatment, and elucidates the appropriateness of various suture material property assumptions depending desired research outcomes.

Mechanobiology of bone and suture - Results from a pig model

OBJECTIVE

To compare the morphology and mechanical function of sutures in normal pigs and minipigs to those of Yucatan minipigs, a natural model for midfacial hypoplasia.

SETTING AND SAMPLE POPULATION

Research took place at the Department of Orthodontics at the University of Washington and used varying sample sizes of normal-snouted pigs and Yucatan minipigs.

MATERIAL AND METHODS

Skulls and heads were examined for morphology of the nasofrontal suture using computed tomography and histology. Strain gauge recordings were made of sutural strain during mastication and during cyclic tensile loading of the nasofrontal suture.

RESULTS

Sutures in Yucatans had narrower gaps than same-age normal pigs. The nasofrontal suture was simpler in construction and had more active osteoblasts on the bone fronts in Yucatans. The sutural ligament was less well organized, and based on a small sample, masticatory strain appeared to be lower than in normal minipigs. However, sutures were not fused and showed similar strains in response to the cyclic loading procedure.

CONCLUSION

Midfacial hypoplasia in Yucatan pigs has the likely proximate cause of hyperossification. Yet prior to fusion, the sutures appear to be amenable to treatment that would promote their growth rate.

An Overview of Modelling Craniosynostosis Using the Finite Element Method

Craniosynostosis is a medical condition caused by the early fusion of the cranial joint. The finite element method (FEM) is a computational technique that can answer a variety of "what if" questions in relation to the biomechanics of this condition. The aim of this study was to review the current literature that has used FEM to investigate the biomechanics of any aspect of craniosynostosis, being its development or its reconstruction. This review highlights that a relatively small number of studies (n = 10) has used FEM to investigate the biomechanics of craniosynostosis. Current studies set a good foundation for the future to take advantage of this method and optimize reconstruction of various forms of craniosynostosis.

Characterizing and Modeling Bone Formation during Mouse Calvarial Development

The newborn mammalian cranial vault consists of five flat bones that are joined together along their edges by soft fibrous tissues called sutures. Early fusion of these sutures leads to a medical condition known as craniosynostosis. The mechanobiology of normal and craniosynostotic skull growth is not well understood. In a series of previous studies, we characterized and modeled radial expansion of normal and craniosynostotic (Crouzon) mice. Here, we describe a new modeling algorithm to simulate bone formation at the sutures in normal and craniosynostotic mice. Our results demonstrate that our modeling approach is capable of predicting the observed ex vivo pattern of bone formation at the sutures in the aforementioned mice. The same approach can be used to model different calvarial reconstruction in children with craniosynostosis to assist in the management of this complex condition.

Anatomical and mechanical properties of swine midpalatal suture in the premaxillary, maxillary, and palatine region

The mechanical properties of the midpalatal suture and their relationship with anatomical parameters are relevant for both tissue engineering and clinical treatments, such as in sutural distraction osteogenesis. Soft tissues were dissected from ten swine heads and the hard palate was sliced perpendicularly to the midpalatal suture. Thirteen specimens were collected from each animal and analysed with micro-computed tomography and 4-point-bending for sutural width (Sw), interdigitation (LII), obliteration (LOI), failure stress (σ _f ), elastic modulus (E), and bone mineral density (BMD). Values of the premaxillary, maxillary, and palatine region were compared with Kruskal-Wallis one-way ANOVA and Spearman's rank coefficient was used to analyse the correlation between parameters and their position along the suture (α = 0.05). LII had values of 1.0, 2.9, and 4.3, LOI had values of 0.0%, 2.5%, and 4.5%, and E had values of 12.5 MPa, 31.3 MPa, and 98.5 MPa, in the premaxillary, maxillary, and palatine region, respectively (p < 0.05). Failure stress and rigidity of the midpalatal suture increased from rostral to caudal, due to greater interdigitation and obliteration. These anatomical and mechanical findings contribute to characterise maxillary growth, and may help to understand its mechanical reaction during loading, and in virtual simulations.

Modelling human skull growth: a validated computational model

During the first year of life, the brain grows rapidly and the neurocranium increases to about 65% of its adult size. Our understanding of the relationship between the biomechanical forces, especially from the growing brain, the craniofacial soft tissue structures and the individual bone plates of the skull vault is still limited. This basic knowledge could help in the future planning of craniofacial surgical operations. The aim of this study was to develop a validated computational model of skull growth, based on the finite-element (FE) method, to help understand the biomechanics of skull growth. To do this, a two-step validation study was carried out. First, an in vitro physical three-dimensional printed model and an in silico FE model were created from the same micro-CT scan of an infant skull and loaded with forces from the growing brain from zero to two months of age. The results from the in vitro model validated the FE model before it was further developed to expand from 0 to 12 months of age. This second FE model was compared directly with in vivo clinical CT scans of infants without craniofacial conditions (n = 56). The various models were compared in terms of predicted skull width, length and circumference, while the overall shape was quantified using three-dimensional distance plots. Statistical analysis yielded no significant differences between the male skull models. All size measurements from the FE model versus the in vitro physical model were within 5%, with one exception showing a 7.6% difference. The FE model and in vivo data also correlated well, with the largest percentage difference in size being 8.3%. Overall, the FE model results matched well with both the in vitro and in vivo data. With further development and model refinement, this modelling method could be used to assist in preoperative planning of craniofacial surgery procedures and could help to reduce reoperation rates.

Predicting calvarial growth in normal and craniosynostotic mice using a computational approach

During postnatal calvarial growth the brain grows gradually and the overlying bones and sutures accommodate that growth until the later juvenile stages. The whole process is coordinated through a complex series of biological, chemical and perhaps mechanical signals between various elements of the craniofacial system. The aim of this study was to investigate to what extent a computational model can accurately predict the calvarial growth in wild-type (WT) and mutant type (MT) Fgfr2C342Y/+ mice displaying bicoronal suture fusion. A series of morphological studies were carried out to quantify the calvarial growth at P3, P10 and P20 in both mouse types. MicroCT images of a P3 specimen were used to develop a finite element model of skull growth to predict the calvarial shape of WT and MT mice at P10. Sensitivity tests were performed and the results compared with ex vivo P10 data. Although the models were sensitive to the choice of input parameters, they predicted the overall skull growth in the WT and MT mice. The models also captured the difference between the ex vivoWT and MT mice. This modelling approach has the potential to be translated to human skull growth and to enhance our understanding of the different reconstruction methods used to manage clinically the different forms of craniosynostosis, and in the long term possibly reduce the number of re-operations in children displaying this condition and thereby enhance their quality of life.

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.

Identification of stiffness-induced signalling mechanisms in cells from patent and fused sutures associated with craniosynostosis

Craniosynostosis is a bone developmental disease where premature ossification of the cranial sutures occurs leading to fused sutures. While biomechanical forces have been implicated in craniosynostosis, evidence of the effect of microenvironmental stiffness changes in the osteogenic commitment of cells from the sutures is lacking. Our aim was to identify the differential genetic expression and osteogenic capability between cells from patent and fused sutures of children with craniosynostosis and whether these differences are driven by changes in the stiffness of the microenvironment. Cells from both sutures demonstrated enhanced mineralisation with increasing substrate stiffness showing that stiffness is a stimulus capable of triggering the accelerated osteogenic commitment of the cells from patent to fused stages. The differences in the mechanoresponse of these cells were further investigated with a PCR array showing stiffness-dependent upregulation of genes mediating growth and bone development (TSHZ2, IGF1), involved in the breakdown of extracellular matrix (MMP9), mediating the activation of inflammation (IL1β) and controlling osteogenic differentiation (WIF1, BMP6, NOX1) in cells from fused sutures. In summary, this study indicates that stiffer substrates lead to greater osteogenic commitment and accelerated bone formation, suggesting that stiffening of the extracellular environment may trigger the premature ossification of the sutures.

Comparative cranial ontogeny of Tapirus (Mammalia: Perissodactyla: Tapiridae)

Skull morphology in tapirs is particularly interesting due to the presence of a proboscis with important trophic, sensory and behavioral functions. Several studies have dealt with tapir skull osteology but chiefly in a comparative framework between fossil and recent species of tapirs. Only one study examined an aspect of cranial ontogeny, development of the sagittal crest (Holbrook. J Zool Soc Lond 2002; 256; 215). Our goal is to describe in detail the morphological changes that occur during the postnatal ontogeny of the skull in two representative tapir species, Tapirus terrestris and Tapirus indicus, and to explore possible functional consequences of their developmental trajectories. We compared qualitative features of the skull on a growth series of 46 specimens of T. terrestris ordered on the basis of the sequence of eruption and tooth wear, dividing the sample into three age classes: class Y (very young juvenile), class J (from young juvenile to young adult) and class A (full and old adult). The qualitative morphological analysis consisted of describing changes in the series in each skull bone and major skull structure, including the type and degree of transformation (e.g. appearance, fusion) of cranial features (e.g. processes, foramina) and articulations (sutures, synchondroses, and synovial joints). We then measured 23 cranial variables in 46 specimens of T. terrestris that included the entire ontogenetic series from newborn to old adults. We applied statistical multivariate techniques to describe allometric growth, and compared the results with the allometric trends calculated for a sample of 25 specimens of T. indicus. Results show that the skull structure was largely conserved throughout the postnatal ontogeny in T. terrestris, so class Y was remarkably similar to class A in overall shape, with the most significant changes localized in the masticatory apparatus, specifically the maxillary tuber as a support of the large-sized permanent postcanine dentition, and correlated changes in diastemata, mandibular body, and sagittal and nuchal crests. In the nasal region, ontogenetic remodeling affected the space for the meatal diverticulum and the surfaces for the origin of the proboscis musculature. Overall, ontogenetic trajectories exhibited more negative allometric components in T. indicus than in T. terrestris, and they shared 47.83% of allometric trends. Tapirus indicus differed most significantly from T. terrestris in the allometry of postcanine toothrows, diastemata and mandibular body. Thus, some allometric trends seem to be highly conserved among the species studied, and the changes observed showed a strong functional and likely adaptive basis in this lineage of ungulates.

Hiding Within the Cracks: Case Report of Rare Sutural Bone Found at the Nasion

Pathology such as skull fractures can be misdiagnosed in the presence of anatomical variations. One variant that has had little description in the literature are the sutural bones associated with the nasal bones. Herein, we describe a case of a rare sutural bone at the nasion, between the bones of the right nasal, frontal, and maxillary frontal process. To our knowledge, this is the first report of such a variant bone in this location, and such it should be considered by clinicians when evaluating patients for pathology in this region.

Zygomatic bone shape in intentional cranial deformations: a model for the study of the interactions between skull growth and facial morphology

Intentional cranial deformations (ICD) were obtained by exerting external mechanical constraints on the skull vault during the first years of life to permanently modify head shape. The repercussions of ICD on the face are not well described in the midfacial region. Here we assessed the shape of the zygomatic bone in different types of ICDs. We considered 14 non-deformed skulls, 19 skulls with antero-posterior deformation, nine skulls with circumferential deformation and seven skulls with Toulouse deformation. The shape of the zygomatic bone was assessed using a statistical shape model after mesh registration. Euclidian distances between mean models and Mahalanobis distances after canonical variate analysis were computed. Classification accuracy was computed using a cross-validation approach. Different ICDs cause specific zygomatic shape modifications corresponding to different degrees of retrusion but the shape of the zygomatic bone alone is not a sufficient parameter for classifying populations into ICD groups defined by deformation types. We illustrate the fact that external mechanical constraints on the skull vault influence midfacial growth. ICDs are a model for the study of the influence of epigenetic factors on craniofacial growth and can help to understand the facial effects of congenital skull malformations such as single or multi-suture synostoses, or of external orthopedic devices such as helmets used to correct deformational plagiocephaly.

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.

Changes in Rat Brain Tissue Microstructure and Stiffness during the Development of Experimental Obstructive Hydrocephalus

Understanding neural injury in hydrocephalus and how the brain changes during the course of the disease in-vivo remain unclear. This study describes brain deformation, microstructural and mechanical properties changes during obstructive hydrocephalus development in a rat model using multimodal magnetic resonance (MR) imaging. Hydrocephalus was induced in eight Sprague-Dawley rats (4 weeks old) by injecting a kaolin suspension into the cisterna magna. Six sham-injected rats were used as controls. MR imaging (9.4T, Bruker) was performed 1 day before, and at 3, 7 and 16 days post injection. T2-weighted MR images were collected to quantify brain deformation. MR elastography was used to measure brain stiffness, and diffusion tensor imaging (DTI) was conducted to observe brain tissue microstructure. Results showed that the enlargement of the ventricular system was associated with a decrease in the cortical gray matter thickness and caudate-putamen cross-sectional area (P < 0.001, for both), an alteration of the corpus callosum and periventricular white matter microstructure (CC+PVWM) and rearrangement of the cortical gray matter microstructure (P < 0.001, for both), while compression without gross microstructural alteration was evident in the caudate-putamen and ventral internal capsule (P < 0.001, for both). During hydrocephalus development, increased space between the white matter tracts was observed in the CC+PVWM (P < 0.001), while a decrease in space was observed for the ventral internal capsule (P < 0.001). For the cortical gray matter, an increase in extracellular tissue water was significantly associated with a decrease in tissue stiffness (P = 0.001). To conclude, this study characterizes the temporal changes in tissue microstructure, water content and stiffness in different brain regions and their association with ventricular enlargement. In summary, whilst diffusion changes were larger and statistically significant for majority of the brain regions studied, the changes in mechanical properties were modest. Moreover, the effect of ventricular enlargement is not limited to the CC+PVWM and ventral internal capsule, the extent of microstructural changes vary between brain regions, and there is regional and temporal variation in brain tissue stiffness during hydrocephalus development.

The Role of the Craniocervical Junction in Craniospinal Hydrodynamics and Neurodegenerative Conditions

The craniocervical junction (CCJ) is a potential choke point for craniospinal hydrodynamics and may play a causative or contributory role in the pathogenesis and progression of neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, MS, and ALS, as well as many other neurological conditions including hydrocephalus, idiopathic intracranial hypertension, migraines, seizures, silent-strokes, affective disorders, schizophrenia, and psychosis. The purpose of this paper is to provide an overview of the critical role of the CCJ in craniospinal hydrodynamics and to stimulate further research that may lead to new approaches for the prevention and treatment of the above neurodegenerative and neurological conditions.

A Genetic-Pathophysiological Framework for Craniosynostosis

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

Anatomical network analysis shows decoupling of modular lability and complexity in the evolution of the primate skull

Modularity and complexity go hand in hand in the evolution of the skull of primates. Because analyses of these two parameters often use different approaches, we do not know yet how modularity evolves within, or as a consequence of, an also-evolving complex organization. Here we use a novel network theory-based approach (Anatomical Network Analysis) to assess how the organization of skull bones constrains the co-evolution of modularity and complexity among primates. We used the pattern of bone contacts modeled as networks to identify connectivity modules and quantify morphological complexity. We analyzed whether modularity and complexity evolved coordinately in the skull of primates. Specifically, we tested Herbert Simon’s general theory of near-decomposability, which states that modularity promotes the evolution of complexity. We found that the skulls of extant primates divide into one conserved cranial module and up to three labile facial modules, whose composition varies among primates. Despite changes in modularity, statistical analyses reject a positive feedback between modularity and complexity. Our results suggest a decoupling of complexity and modularity that translates to varying levels of constraint on the morphological evolvability of the primate skull. This study has methodological and conceptual implications for grasping the constraints that underlie the developmental and functional integration of the skull of humans and other primates.

Mechanical properties of calvarial bones in a mouse model for craniosynostosis

The mammalian cranial vault largely consists of five flat bones that are joined together along their edges by soft fibrous tissues called sutures. Premature closure of the cranial sutures, craniosynostosis, can lead to serious clinical pathology unless there is surgical intervention. Research into the genetic basis of the disease has led to the development of various animal models that display this condition, e.g. mutant type Fgfr2^C342Y/+ mice which display early fusion of the coronal suture (joining the parietal and frontal bones). However, whether the biomechanical properties of the mutant and wild type bones are affected has not been investigated before. Therefore, nanoindentation was used to compare the elastic modulus of cranial bone and sutures in wild type (WT) and Fgfr2^C342Y/+mutant type (MT) mice during their postnatal development. Further, the variations in properties with indentation position and plane were assessed. No difference was observed in the elastic modulus of parietal bone between the WT and MT mice at postnatal (P) day 10 and 20. However, the modulus of frontal bone in the MT group was lower than the WT group at both P10 (1.39±0.30 vs. 5.32±0.68 GPa; p<0.05) and P20 (5.57±0.33 vs. 7.14±0.79 GPa; p<0.05). A wide range of values was measured along the coronal sutures for both the WT and MT samples, with no significant difference between the two groups. Findings of this study suggest that the inherent mechanical properties of the frontal bone in the mutant mice were different to the wild type mice from the same genetic background. These differences may reflect variations in the degree of biomechanical adaptation during skull growth, which could have implications for the surgical management of craniosynostosis patients.

Biomechanical Dynamics of Cranial Sutures during Simulated Impulsive Loading

Background. Cranial sutures are deformable joints between the bones of the skull, bridged by collagen fibres. They function to hold the bones of the skull together while allowing for mechanical stress transmission and deformation. Objective. The aim of this study is to investigate how cranial suture morphology, suture material property, and the arrangement of sutural collagen fibres influence the dynamic responses of the suture and surrounding bone under impulsive loads. Methods. An idealized bone-suture-bone complex was analyzed using a two-dimensional finite element model. A uniform impulsive loading was applied to the complex. Outcome variables of von Mises stress and strain energy were evaluated to characterize the sutures' biomechanical behavior. Results. Parametric studies revealed that the suture strain energy and the patterns of Mises stress in both the suture and surrounding bone were strongly dependent on the suture morphologies. Conclusions. It was concluded that the higher order hierarchical suture morphology, lower suture elastic modulus, and the better collagen fiber orientation must benefit the stress attenuation and energy absorption.

Beyond the functional matrix hypothesis: a network null model of human skull growth for the formation of bone articulations

Craniofacial sutures and synchondroses form the boundaries among bones in the human skull, providing functional, developmental and evolutionary information. Bone articulations in the skull arise due to interactions between genetic regulatory mechanisms and epigenetic factors such as functional matrices (soft tissues and cranial cavities), which mediate bone growth. These matrices are largely acknowledged for their influence on shaping the bones of the skull; however, it is not fully understood to what extent functional matrices mediate the formation of bone articulations. Aiming to identify whether or not functional matrices are key developmental factors guiding the formation of bone articulations, we have built a network null model of the skull that simulates unconstrained bone growth. This null model predicts bone articulations that arise due to a process of bone growth that is uniform in rate, direction and timing. By comparing predicted articulations with the actual bone articulations of the human skull, we have identified which boundaries specifically need the presence of functional matrices for their formation. We show that functional matrices are necessary to connect facial bones, whereas an unconstrained bone growth is sufficient to connect non-facial bones. This finding challenges the role of the brain in the formation of boundaries between bones in the braincase without neglecting its effect on skull shape. Ultimately, our null model suggests where to look for modified developmental mechanisms promoting changes in bone growth patterns that could affect the development and evolution of the head skeleton.

A new ultrasound method for assessment of head shape change in infants with plagiocephaly

Objective

To compare a new ultrasound measurement method with calliper cephalometry in infants with deformational plagiocephaly (DP) and to assess the differences of two methods according to the severity of DP.

Methods

Fifty-two infants with DP were divided into two groups according to the degree of cranial vault asymmetry (CVA); group 1 included 42 infants with CVA over 10 mm, and group 2 included 10 infants with CVA under 10 mm. Cranial vault asymmetry index (CVAI) and occipital angle ratio (OAR) were measured by using calliper and ultrasound measurements, respectively. The occipital angle was defined as the angle between the lines projected along the lambdoid sutures of the skull.

Results

The occipital angles of the affected sides were significantly greater than those of unaffected sides in both groups. The CVAI and OAR were significantly greater in group 1 than in group 2 (CVAI, 9.3%±2.3% vs. 4.6%±1.5%; OAR, 1.05±0.4 vs. 1.01±0.0; p<0.05). The OAR was positively correlated with the CVAI in all infants (r=0.789) and in group 1 (r=0.784; p<0.05).

Conclusion

Our study revealed that OAR using the new ultrasound measurement was positively correlated with the CVAI in infants with DP. Therefore, the occipital angle measurement using ultrasound combined with cephalometry could provide better understanding about the characteristics of the overall cranial bone and lambdoid suture complex in infants with DP.

Cranial sutures work collectively to distribute strain throughout the reptile skull

The skull is composed of many bones that come together at sutures. These sutures are important sites of growth, and as growth ceases some become fused while others remain patent. Their mechanical behaviour and how they interact with changing form and loadings to ensure balanced craniofacial development is still poorly understood. Early suture fusion often leads to disfiguring syndromes, thus is it imperative that we understand the function of sutures more clearly. By applying advanced engineering modelling techniques, we reveal for the first time that patent sutures generate a more widely distributed, high level of strain throughout the reptile skull. Without patent sutures, large regions of the skull are only subjected to infrequent low-level strains that could weaken the bone and result in abnormal development. Sutures are therefore not only sites of bone growth, but could also be essential for the modulation of strains necessary for normal growth and development in reptiles.

A mathematical model for mechanotransduction at the early steps of suture formation

Growth and patterning of craniofacial sutures is subjected to the effects of mechanical stress. Mechanotransduction processes occurring at the margins of the sutures are not precisely understood. Here, we propose a simple theoretical model based on the orientation of collagen fibres within the suture in response to local stress. We demonstrate that fibre alignment generates an instability leading to the emergence of interdigitations. We confirm the appearance of this instability both analytically and numerically. To support our model, we use histology and synchrotron X-ray microtomography and reveal the fine structure of fibres within the sutural mesenchyme and their insertion into the bone. Furthermore, using a mouse model with impaired mechanotransduction, we show that the architecture of sutures is disturbed when forces are not interpreted properly. Finally, by studying the structure of sutures in the mouse, the rat, an actinopterygian (Polypterus bichir) and a placoderm (Compagopiscis croucheri), we show that bone deposition patterns during dermal bone growth are conserved within jawed vertebrates. In total, these results support the role of mechanical constraints in the growth and patterning of craniofacial sutures, a process that was probably effective at the emergence of gnathostomes, and provide new directions for the understanding of normal and pathological suture fusion.

Hand in glove: brain and skull in development and dysmorphogenesis

The brain originates relatively early in development from differentiated ectoderm that forms a hollow tube and takes on an exceedingly complex shape with development. The skull is made up of individual bony elements that form from neural crest- and mesoderm-derived mesenchyme that unite to provide support and protection for soft tissues and spaces of the head. The meninges provide a protective and permeable membrane between brain and skull. Across evolutionary and developmental time, dynamic changes in brain and skull shape track one another so that their integration is evidenced in two structures that fit soundly regardless of changes in biomechanical and physiologic functions. Evidence for this tight correspondence is also seen in diseases of the craniofacial complex that are often classified as diseases of the skull (e.g., craniosynostosis) or diseases of the brain (e.g., holoprosencephaly) even when both tissues are affected. Our review suggests a model that links brain and skull morphogenesis through coordinated integration of signaling pathways (e.g., FGF, TGFβ, Wnt) via processes that are not currently understood, perhaps involving the meninges. Differences in the earliest signaling of biological structure establish divergent designs that will be enhanced during morphogenesis. Signaling systems that pattern the developing brain are also active in patterning required for growth and assembly of the skull and some members of these signaling families have been indicated as causal for craniofacial diseases. Because cells of early brain and skull are sensitive to similar signaling families, variation in the strength or timing of signals or shifts in patterning boundaries that affect one system (neural or skull) could also affect the other system and appropriate co-adjustments in development would be made. Interactions of these signaling systems and of the tissues that they pattern are fundamental to the consistent but labile functional and structural association of brain and skull conserved over evolutionary time obvious in the study of development and disease.

The mouse palate and its cellular responses to midpalatal suture expansion forces

OBJECTIVES

To investigate the anatomy of the mouse palate, the midpalatal suture, and the cellular characteristics in the sutures before and immediately after midpalatal suture expansion.

MATERIALS AND METHODS

Wild-type C57BL/6 male mice, aged between 6 weeks and 12 months, were chosen for all the experiments. The complete palate of the non-operated group and the midpalatal suture-expanded group at different ages was used for histological, micro-CT, immunohistochemistry, and sutural cell analyses.

RESULTS

This study documents precise morphological and histological characteristics of the mouse palatal sutures. In addition to the opening of the midpalatal suture caused by expansion, both transverse and interpalatine sutures were also seen to be affected. Cellular density was decreased in different types of sutures following the application of mechanical force.

CONCLUSIONS

The detailed morphology and histology of the mouse palate and the cellular changes that occur following midpalatal suture expansion, as described here, will be helpful as a basis for further investigations of palatal suture tissue responses to mechanical force.

Discontinuities in the human bone-PDL-cementum complex

A naturally graded interface due to functional demands can deviate toward a discontinuous interface, eventually decreasing the functional efficiency of a dynamic joint. It is this characteristic feature in a human bone-tooth fibrous joint bone-PDL-tooth complex that will be discussed through histochemistry, and site-specific high resolution microscopy, micro tomography(Micro XCT™), X-ray fluorescence imaging and wet nanoindentation techniques. Results demonstrated two causes for the occurrence of 5-50 μm narrowed PDL-space: 1) microscopic scalloped regions at the PDL-insertion sites and macro-scale stratified layers of bone with rich basophilic lines, and 2) macroscopic bony protrusions. Narrowed PDL-complexes illustrated patchy appearance of asporin, and when imaged under wet conditions using an atomic force microscope (AFM), demonstrated structural reorganization of the PDL, collagen periodicity, organic-dominant areas at the PDL-cementum and PDL-bone entheses and within cementum and bone. Scanning electron microscopy (SEM) results confirmed AFM results. Despite the narrowed PDL, continuity between PDL and vasculature in endosteal spaces of bone was demonstrated using a Micro XCT™. The higher levels of Ca and P X-ray fluorescence using a microprobe were correlated with higher elastic modulus values of 0.1-1.4 and 0.1-1.2 GPa for PDL-bone and PDL-cementum using wet nanoindentation. The ranges in elastic modulus values for PDL-bone and PDL-cementum entheses in 150-380 μm wide PDL-complex were 0.1-1.0 and 0.1-0.6 GPa. Based on these results we propose that strain amplification at the entheses could be minimized with a gradual change in modulus profile, a characteristic of 150-380 μm wide functional PDL-space. However, a discontinuity in modulus profile, a characteristic of 5-50 μm wide narrowed PDL-space would cause compromised mechanotransduction. The constrictions or narrowed sites within the bone-tooth fibrous joint will become the new "load bearing sites" that eventually could cause direct local fusion of bone with cementum.

Masticatory hypermuscularity is not related to reduced cranial volume in myostatin-knockout mice

It has been suggested recently that masticatory muscle size reduction in humans resulted in greater encephalization through decreased compressive forces on the cranial vault. Following this logic, if masticatory muscle size were increased, then a reduction in brain growth should also occur. This study was designed to test this hypothesis using a myostatin (GDF-8) knockout mouse model. Myostatin is a negative regulator of skeletal muscle growth, and individuals lacking this gene show significant hypermuscularity. Sixty-two [32 wild-type (WT) and 30 GDF-8 -/- knockout], 1, 28, 56, and 180-day-old CD-1 mice were used. Body and masseter muscle weights were collected following dissection and standardized lateral and dorsoventral cephalographs were obtained. Cephalometric landmarks were identified on the radiographs and cranial volume was calculated. Mean differences were assessed using a two-way ANOVA. KO mice had significantly greater body and masseter weights beginning at 28 days compared with WT controls. No significant differences in cranial volumes were noted between KO and WT. Muscle weight was not significantly correlated with cranial volume in 1, 28, or 180-day-old mice. Muscle weights exhibited a positive correlation with cranial volume at 56 days. Results demonstrate that masticatory hypermuscularity is not associated with reduced cranial volume. In contrast, there is abundant data demonstrating the opposite, brain growth determines cranial vault growth and masticatory apparatus only affects ectocranial morphology. The results presented here do not support the hypothesis that a reduction in masticatory musculature relaxed compressive forces on the cranial vault allowing for greater encephalization.

Inkjet-based biopatterning of bone morphogenetic protein-2 to spatially control calvarial bone formation

The purpose of this study was to demonstrate spatial control of osteoblast differentiation in vitro and bone formation in vivo using inkjet bioprinting technology and to create three-dimensional persistent bio-ink patterns of bone morphogenetic protein-2 (BMP-2) and its modifiers immobilized within microporous scaffolds. Semicircular patterns of BMP-2 were printed within circular DermaMatrix human allograft scaffold constructs. The contralateral halves of the constructs were unprinted or printed with BMP-2 modifiers, including the BMP-2 inhibitor, noggin. Printed bio-ink pattern retention was validated using fluorescent or (125)I-labeled bio-inks. Mouse C2C12 progenitor cells cultured on patterned constructs differentiated in a dose-dependent fashion toward an osteoblastic fate in register to BMP-2 patterns. The fidelity of spatial restriction of osteoblastic differentiation at the boundary between neighboring BMP-2 and noggin patterns improved in comparison with patterns without noggin. Acellular DermaMatrix constructs similarly patterned with BMP-2 and noggin were then implanted into a mouse calvarial defect model. Patterns of bone formation in vivo were comparable with patterned responses of osteoblastic differentiation in vitro. These results demonstrate that three-dimensional biopatterning of a growth factor and growth factor modifier within a construct can direct cell differentiation in vitro and tissue formation in vivo in register to printed patterns.

Assessment of the role of sutures in a lizard skull: a computer modelling study

Sutures form an integral part of the functioning skull, but their role has long been debated among vertebrate morphologists and palaeontologists. Furthermore, the relationship between typical skull sutures, and those involved in cranial kinesis, is poorly understood. In a series of computational modelling studies, complex loading conditions obtained through multibody dynamics analysis were imposed on a finite element model of the skull of Uromastyx hardwickii, an akinetic herbivorous lizard. A finite element analysis (FEA) of a skull with no sutures revealed higher patterns of strain in regions where cranial sutures are located in the skull. From these findings, FEAs were performed on skulls with sutures (individual and groups of sutures) to investigate their role and function more thoroughly. Our results showed that individual sutures relieved strain locally, but only at the expense of elevated strain in other regions of the skull. These findings provide an insight into the behaviour of sutures and show how they are adapted to work together to distribute strain around the skull. Premature fusion of one suture could therefore lead to increased abnormal loading on other regions of the skull causing irregular bone growth and deformities. This detailed investigation also revealed that the frontal-parietal suture of the Uromastyx skull played a substantial role in relieving strain compared with the other sutures. This raises questions about the original role of mesokinesis in squamate evolution.