The Intertwined Evolution and Development of Sutures and Cranial Morphology

Geometric growth of the normal human craniocervical junction from 0 to 18 years old

The craniocervical junction (CCJ) forms the bridge between the skull and the spine, a highly mobile group of joints that allows the mobility of the head in every direction. The CCJ plays a major role in protecting the inferior brainstem (bulb) and spinal cord, therefore also requiring some stability. Children are subjected to multiple constitutive or acquired diseases involving the CCJ: primary bone diseases such as in FGFR-related craniosynostoses or acquired conditions such as congenital torticollis, cervical spine luxation, and neurological disorders. To design efficient treatment plans, it is crucial to understand the relationship between abnormalities of the craniofacial region and abnormalities of the CCJ. This can be approached by the study of control and abnormal growth patterns. Here we report a model of normal skull base growth by compiling a collection of geometric models in control children. Focused analyses highlighted specific developmental patterns for each CCJ bone, emphasizing rapid growth during infancy, followed by varying rates of growth and maturation during childhood and adolescence until reaching stability by 18 years of age. The focus was on the closure patterns of synchondroses and sutures in the occipital bone, revealing distinct closure trajectories for the anterior intra-occipital synchondroses and the occipitomastoid suture. The findings, although based on a limited dataset, showcased specific age-related changes in width and closure percentages, providing valuable insights into growth dynamics within the first 2 years of life. Integration analyses revealed intricate relationships between skull and neck structures, emphasizing coordinated growth at different stages. Specific bone covariation patterns, as found between the first and second cervical vertebrae (C1 and C2), indicated synchronized morphological changes. Our results provide initial data for designing inclusive CCJ geometric models to predict normal and abnormal growth dynamics.

A shared pattern of midfacial bone modelling in hominids suggests deep evolutionary roots for human facial morphogenesis

Midfacial morphology varies between hominoids, in particular between great apes and humans for which the face is small and retracted. The underlying developmental processes for these morphological differences are still largely unknown. Here, we investigate the cellular mechanism of maxillary development (bone modelling, BM), and how potential changes in this process may have shaped facial evolution. We analysed cross-sectional developmental series of gibbons, orangutans, gorillas, chimpanzees and present-day humans (n = 183). Individuals were organized into five age groups according to their dental development. To visualize each species's BM pattern and corresponding morphology during ontogeny, maps based on microscopic data were mapped onto species-specific age group average shapes obtained using geometric morphometrics. The amount of bone resorption was quantified and compared between species. Great apes share a highly similar BM pattern, whereas gibbons have a distinctive resorption pattern. This suggests a change in cellular activity on the hominid branch. Humans possess most of the great ape pattern, but bone resorption is high in the canine area from birth on, suggesting a key role of canine reduction in facial evolution. We also observed that humans have high levels of bone resorption during childhood, a feature not shared with other apes.

Infigratinib, a selective FGFR1-3 tyrosine kinase inhibitor, alters dentoalveolar development at high doses

BACKGROUND

Fibroblast growth factor receptor-3 (FGFR3) gain-of-function mutations are linked to achondroplasia. Infigratinib, a FGFR1-3 tyrosine kinase inhibitor, improves skeletal growth in an achondroplasia mouse model. FGFs and their receptors have critical roles in developing teeth, yet effects of infigratinib on tooth development have not been assessed. Dentoalveolar and craniofacial phenotype of Wistar rats dosed with low (0.1 mg/kg) and high (1.0 mg/kg) dose infigratinib were evaluated using micro-computed tomography, histology, and immunohistochemistry.

RESULTS

Mandibular third molars were reduced in size and exhibited aberrant crown and root morphology in 100% of female rats and 80% of male rats at high doses. FGFR3 and FGF18 immunolocalization and extracellular matrix protein expression were unaffected, but cathepsin K (CTSK) was altered by infigratinib. Cranial vault bones exhibited alterations in dimension, volume, and density that were more pronounced in females. In both sexes, interfrontal sutures were significantly more patent with high dose vs vehicle.

CONCLUSIONS

High dose infigratinib administered to rats during early stages affects dental and craniofacial development. Changes in CTSK from infigratinib in female rats suggest FGFR roles in bone homeostasis. While dental and craniofacial disruptions are not expected at therapeutic doses, our findings confirm the importance of dental monitoring in clinical studies.

Dissecting calvarial bones and sutures at single-cell resolution

Cranial bones constitute a protective shield for the vulnerable brain tissue, bound together as a rigid entity by unique immovable joints known as sutures. Cranial sutures serve as major growth centres for calvarial morphogenesis and have been identified as a niche for mesenchymal stem cells (MSCs) and/or skeletal stem cells (SSCs) in the craniofacial skeleton. Despite the established dogma of cranial bone and suture biology, technological advancements now allow us to investigate these tissues and structures at unprecedented resolution and embrace multiple novel biological insights. For instance, a decrease or imbalance of representation of SSCs within sutures might underlie craniosynostosis; dural sinuses enable neuroimmune crosstalk and are newly defined as immune hubs; skull bone marrow acts as a myeloid cell reservoir for the meninges and central nervous system (CNS) parenchyma in mediating immune surveillance, etc. In this review, we revisit a growing body of recent studies that explored cranial bone and suture biology using cutting-edge techniques and have expanded our current understanding of this research field, especially from the perspective of development, homeostasis, injury repair, resident MSCs/SSCs, immunosurveillance at the brain's border, and beyond.

A New Concept of Sustainable Wind Turbine Blades: Bio-Inspired Design with Engineered Adhesives

In this paper, a new concept of extra-durable and sustainable wind turbine blades is presented. The two critical materials science challenges of the development of wind energy now are the necessity to prevent the degradation of wind turbine blades for several decades, and, on the other side, to provide a solution for the recyclability and sustainability of blades. In preliminary studies by DTU Wind, it was demonstrated that practically all typical wind turbine blade degradation mechanisms (e.g., coating detachment, buckling, spar cap/shell adhesive joint degradation, trailing edge failure, etc.) have their roots in interface degradation. The concept presented in this work includes the development of bio-inspired dual-mechanism-based interface adhesives (combining mechanical interlocking of fibers and chemical adhesion), which ensures, on the one side, extra-strong attachment during the operation time, and on the other side, possible adhesive joint separation for re-use of the blade parts. The general approach and physical mechanisms of adhesive strengthening and separation are described.

White matter microstructural integrity continues to develop from adolescence to young adulthood in mice and humans: Same phenotype, different mechanism

As direct evaluation of a mouse model of human neurodevelopment, adolescent and young adult mice and humans underwent MR diffusion tensor imaging to quantify age-related differences in microstructural integrity of brain white matter fibers. Fractional anisotropy (FA) was greater in older than younger mice and humans. Despite the cross-species commonality, the underlying developmental mechanism differed: whereas evidence for greater axonal extension contributed to higher FA in older mice, evidence for continuing myelination contributed to higher FA in human adolescent development. These differences occurred in the context of species distinctions in overall brain growth: whereas the continued growth of the brain and skull in the murine model can accommodate volume expansion into adulthood, human white matter volume and myelination continue growth into adulthood within a fixed intracranial volume. Appreciation of the similarities and differences in developmental mechanism can enhance the utility of animal models of brain white matter structure, function, and response to exogenous manipulation.

Commensal Microbiota Effects on Craniofacial Skeletal Growth and Morphology

Microbes colonize anatomical sites in health to form commensal microbial communities (e.g., commensal gut microbiota, commensal skin microbiota, commensal oral microbiota). Commensal microbiota has indirect effects on host growth and maturation through interactions with the host immune system. The commensal microbiota was recently introduced as a novel regulator of skeletal growth and morphology at noncraniofacial sites. Further, we and others have shown that commensal gut microbes, such as segmented filamentous bacteria (SFB), contribute to noncraniofacial skeletal growth and maturation. However, commensal microbiota effects on craniofacial skeletal growth and morphology are unclear. To determine the commensal microbiota's role in craniofacial skeletal growth and morphology, we performed craniometric and bone mineral density analyses on skulls from 9-week-old female C57BL/6T germ-free (GF) mice (no microbes), excluded-flora (EF) specific-pathogen-free mice (commensal microbiota), and murine-pathogen-free (MPF) specific-pathogen-free mice (commensal microbiota with SFB). Investigations comparing EF and GF mice revealed that commensal microbiota impacted the size and shape of the craniofacial skeleton. EF versus GF mice exhibited an elongated gross skull length. Cranial bone length analyses normalized to skull length showed that EF versus GF mice had enhanced frontal bone length and reduced cranial base length. The shortened cranial base in EF mice was attributed to decreased presphenoid, basisphenoid, and basioccipital bone lengths. Investigations comparing MPF mice and EF mice demonstrated that commensal gut microbes played a role in craniofacial skeletal morphology. Cranial bone length analyses normalized to skull length showed that MPF versus EF mice had reduced frontal bone length and increased cranial base length. The elongated cranial base in MPF mice was due to enhanced presphenoid bone length. This work, which introduces the commensal microbiota as a contributor to craniofacial skeletal growth, underscores that noninvasive interventions in the gut microbiome could potentially be employed to modify craniofacial skeletal morphology. © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

A novel method for assessment of human midpalatal sutures using CBCT-based geometric morphometrics and complexity scores

INTRODUCTION

Management of dentofacial deficiencies requires knowledge about sutural morphology and complexity. The present study assesses midpalatal sutural morphology based on human cone-beam computed tomography (CBCT) using geometric morphometrics (GMM) and complexity scores. The study is the first to apply a sutural complexity score to human CBCT datasets and demonstrates the potential such a score has to improve objectiveness and comparability when analysing the midpalatal suture.

MATERIALS AND METHODS

CBCTs of various age and sex groups were analysed retrospectively (n = 48). For the geometric morphometric analysis, landmark acquisition and generalised Procrustes superimposition were combined with principal component analysis to detect variability in sutural shape patterns. For complexity analysis, a windowed short-time Fourier transform with a power spectrum density (PSD) calculation was applied to resampled superimposed semi-landmarks.

RESULTS

According to the GMM, younger patients exhibited comparable sutural patterns. With increasing age, the shape variation increased among the samples. The principal components did not sufficiently capture complexity patterns, so an additional methodology was applied to assess characteristics such as sutural interdigitation. According to the complexity analysis, the average PSD complexity score was 1.465 (standard deviation = 0.010). Suture complexity increased with patient age (p < 0.0001), but was not influenced by sex (p = 0.588). The intra-class correlation coefficient exceeded 0.9, indicating intra-rater reliability.

CONCLUSION

Our study demonstrated that GMM applied to human CBCTs can reveal shape variations and allow the comparison of sutural morphologies across samples. We demonstrate that complexity scores can be applied to study human sutures captured in CBCTs and complement GMM for a comprehensive sutural analysis.

Lineage-specific mutation of Lmx1b provides new insights into distinct regulation of suture development in different areas of the calvaria

The calvaria (top part of the skull) is made of pieces of bone as well as multiple soft tissue joints called sutures. The latter is crucial to the growth and morphogenesis of the skull, and thus a loss of calvarial sutures can lead to severe congenital defects in humans. During embryogenesis, the calvaria develops from the cranial mesenchyme covering the brain, which contains cells originating from the neural crest and the mesoderm. While the mechanism that patterns the cranial mesenchyme into bone and sutures is not well understood, function of Lmx1b, a gene encoding a LIM-domain homeodomain transcription factor, plays a key role in this process. In the current study, we investigated a difference in the function of Lmx1b in different parts of the calvaria using neural crest-specific and mesoderm-specific Lmx1b mutants. We found that Lmx1b was obligatory for development of the interfrontal suture and the anterior fontanel along the dorsal midline of the skull, but not for the posterior fontanel over the midbrain. Also, Lmx1b mutation in the neural crest-derived mesenchyme, but not the mesoderm-derived mesenchyme, had a non-cell autonomous effect on coronal suture development. Furthermore, overexpression of Lmx1b in the neural crest lineage had different effects on the position of the coronal suture on the apical part and the basal part. Other unexpected phenotypes of Lmx1b mutants led to an additional finding that the coronal suture and the sagittal suture are of dual embryonic origin. Together, our data reveal a remarkable level of regional specificity in regulation of calvarial development.

Evolution and development of the cetacean skull roof: a case study in novelty and homology

Skulls of living whales and dolphins (cetaceans) are telescoped-bones of the skull roof are overlapped by expanded facial bones and/or anteriorly extended occipital bones. Evolution of the underlying skull roof (calvarium), which lies between the telescoped regions, is relatively unstudied. We explore the evolution and development of the calvarium of toothed whales (odontocetes) by integrating fetal data with Oligocene odontocete fossils from North America, including eight neonatal and juvenile skulls of Olympicetus†. We identified two potential synapomorphies of crown Cetacea: contact of interparietals with frontals, and a single anterior median interparietal (AMI) element. Within Odontoceti, loss of contact between the parietals diagnoses the clade including Delphinida, Ziphiidae and Platanistidae (=Synrhina). Delphinida is characterized by a greatly enlarged interparietal. New fetal series of delphinoids reveal a consistent developmental pattern with three elements: the AMI and bilateral posterior interparietals (PIs). The PIs most resemble the medial interparietal elements of terrestrial artiodactyls, suggesting that the AMI of cetaceans could be a unique ossification. More broadly, the paucity of conserved anatomical relationships of the interparietals, as well as the fact that the elements often do not coalesce into a single bone, demonstrates that assessing homology of the interparietals across mammals remains challenging. This article is part of the theme issue 'The mammalian skull: development, structure and function'.

Developmental origin underlies evolutionary rate variation across the placental skull

The placental skull has evolved into myriad forms, from longirostrine whales to globular primates, and with a diverse array of appendages from antlers to tusks. This disparity has recently been studied from the perspective of the whole skull, but the skull is composed of numerous elements that have distinct developmental origins and varied functions. Here, we assess the evolution of the skull's major skeletal elements, decomposed into 17 individual regions. Using a high-dimensional morphometric approach for a dataset of 322 living and extinct eutherians (placental mammals and their stem relatives), we quantify patterns of variation and estimate phylogenetic, allometric and ecological signal across the skull. We further compare rates of evolution across ecological categories and ordinal-level clades and reconstruct rates of evolution along lineages and through time to assess whether developmental origin or function discriminate the evolutionary trajectories of individual cranial elements. Our results demonstrate distinct macroevolutionary patterns across cranial elements that reflect the ecological adaptations of major clades. Elements derived from neural crest show the fastest rates of evolution, but ecological signal is equally pronounced in bones derived from neural crest and paraxial mesoderm, suggesting that developmental origin may influence evolutionary tempo, but not capacity for specialisation. This article is part of the theme issue 'The mammalian skull: development, structure and function'.

Dissection of insular cortex layer 5 reveals two sublayers with opposing modulatory roles in appetitive drinking behavior

The insular cortex (insula) is known to play a modulatory role in feeding and drinking. Previous studies have revealed anterior-posterior differences of subcortical projections and roles for the insula, yet the anatomical and functional heterogeneity among the cortical layers remains poorly understood. Here, we show that layer 5 of the mouse dysgranular insula has two distinct neuronal subpopulations along the entire anterior-posterior axis: The L5a population, expressing NECAB1, projects bilaterally to the lateral and capsular divisions of the central amygdala, and the L5b population, expressing CTIP2, projects ipsilaterally to the parasubthalamic nucleus and the medial division of the central amygdala. Optogenetically activating L5a and L5b neuronal populations in thirsty male mice led to suppressed and facilitated water spout licking, respectively, without avoidance against or preference for the spout paired with the opto-stimulation. Our results suggest sublayer-specific bidirectional modulatory roles of insula layer 5 in the motivational aspect of appetitive behavior.

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.

Development of Erf-Mediated Craniosynostosis and Pharmacological Amelioration

ETS2 repressor factor (ERF) insufficiency causes craniosynostosis (CRS4) in humans and mice. ERF is an ETS domain transcriptional repressor regulated by Erk1/2 phosphorylation via nucleo-cytoplasmic shuttling. Here, we analyze the onset and development of the craniosynostosis phenotype in an Erf-insufficient mouse model and evaluate the potential of the residual Erf activity augmented by pharmacological compounds to ameliorate the disease. Erf insufficiency appears to cause an initially compromised frontal bone formation and subsequent multisuture synostosis, reflecting distinct roles of Erf on the cells that give rise to skull and facial bones. We treated animals with Mek1/2 and nuclear export inhibitors, U0126 and KPT-330, respectively, to increase Erf activity by two independent pathways. We implemented both a low dosage locally over the calvaria and a systemic drug administration scheme to evaluate the possible indirect effects from other systems and minimize toxicity. The treatment of mice with either the inhibitors or the administration scheme alleviated the synostosis phenotype with minimal adverse effects. Our data suggest that the ERF level is an important regulator of cranial bone development and that pharmacological modulation of its activity may represent a valid intervention approach both in CRS4 and in other syndromic forms of craniosynostosis mediated by the FGFR-RAS-ERK-ERF pathway.

Pedomorphosis in the ancestry of marsupial mammals

Within mammals, different reproductive strategies (e.g., egg laying, live birth of extremely underdeveloped young, and live birth of well-developed young) have been linked to divergent evolutionary histories. How and when developmental variation across mammals arose is unclear. While egg laying is unquestionably considered the ancestral state for all mammals, many long-standing biases treat the extreme underdeveloped state of marsupial young as the ancestral state for therian mammals (clade including both marsupials and placentals), with the well-developed young of placentals often considered the derived mode of development. Here, we quantify mammalian cranial morphological development and estimate ancestral patterns of cranial shape development using geometric morphometric analysis of the largest comparative ontogenetic dataset of mammals to date (165 specimens, 22 species). We identify a conserved region of cranial morphospace for fetal specimens, after which cranial morphology diversified through ontogeny in a cone-shaped pattern. This cone-shaped pattern of development distinctively reflected the upper half of the developmental hourglass model. Moreover, cranial morphological variation was found to be significantly associated with the level of development (position on the altricial-precocial spectrum) exhibited at birth. Estimation of ancestral state allometry (size-related shape change) reconstructs marsupials as pedomorphic relative to the ancestral therian mammal. In contrast, the estimated allometries for the ancestral placental and ancestral therian were indistinguishable. Thus, from our results, we hypothesize that placental mammal cranial development most closely reflects that of the ancestral therian mammal, while marsupial cranial development represents a more derived mode of mammalian development, in stark contrast to many interpretations of mammalian evolution.

Bone structure and composition in a hyperglycemic, obese, and leptin receptor-deficient rat: Microscale characterization of femur and calvarium

Metabolic abnormalities, such as diabetes mellitus and obesity, can impact bone quantity and/or bone quality. In this work, we characterize bone material properties, in terms of structure and composition, in a novel rat model with congenic leptin receptor (LepR) deficiency, severe obesity, and hyperglycemia (type 2 diabetes-like condition). Femurs and calvaria (parietal region) from 20-week-old male rats are examined to probe bones formed both by endochondral and intramembranous ossification. Compared to the healthy controls, the LepR-deficient animals display significant alterations in femur microarchitecture and in calvarium morphology when analyzed by micro-computed X-ray tomography (micro-CT). In particular, shorter femurs with reduced bone volume, combined with thinner parietal bones and shorter sagittal suture, point towards a delay in the skeletal development of the LepR-deficient rodents. On the other hand, LepR-deficient animals and healthy controls display analogous bone matrix composition, which is assessed in terms of tissue mineral density by micro-CT, degree of mineralization by quantitative backscattered electron imaging, and various metrics extrapolated from Raman hyperspectral images. Some specific microstructural features, i.e., mineralized cartilage islands in the femurs and hyper-mineralized areas in the parietal bones, also show comparable distribution and characteristics in both groups. Overall, the altered bone microarchitecture in the LepR-deficient animals indicates compromised bone quality, despite the normal bone matrix composition. The delayed development is also consistent with observations in humans with congenic Lep/LepR deficiency, making this animal model a suitable candidate for translational research.

Bone-targeted bortezomib increases bone formation within Calvarial trans-sutural distraction osteogenesis

The high rate of relapse in craniofacial disharmony treatment via trans-sutural distraction osteogenesis (TSDO) is due to the failure to form a stable bone bridge in the suture gap. Bisphosphonates (BP) have a high propensity to localize to hydroxyapatite in the bone matrix and are commonly used as targeting ligands for local delivery of therapeutics into bone microenvironment. Bone-targeted Bortezomib (BP-Btz) is chemosynthetic by linking Btz (Bortezomib) to a BP residue and could target bone tissue to promote osteoblast differentiation and inhibit osteoclastogenesis. Here, suture-derived mesenchymal stem cells (SuSCs) and osteoclasts were treated with Btz and BP-Btz. Aforesaid drugs were injected locally into the sagittal sutures to explore their effects in TSDO. Further, pharmacological properties of BP-Btz in the suture expansion model were assessed by fluorescent BP analogs and levels of total ubiquitinated (Ub)-proteins. The results showed that BP-Btz could stimulate osteogenic differentiation of SuSCs, bind to bone matrix and inhibit osteoclastogenesis. Biological effects of BP-Btz were similar with those of Btz in osteoblast differentiation and osteoclastogenesis inhibition in vitro. Activated bone metabolism were detected after 14 days in the sagittal suture expansion model. Increased osteoid area, remarkably decreased osteoclast surface and enhanced osteogenesis were detected in vivo after treatment with BP-Btz. Green fluorescence signal detection and pharmacodynamic studies revealed that BP-Btz bound to suture edge, released Btz in remodeling conditions, had a higher local concentration and sustained longer than free Btz. This study delineated the clinical potential of bone-targeted Btz conjugate as an efficacious strategy to promote trans-sutural distraction osteogenesis.

Assessment of the mechanical role of cranial sutures in the mammalian skull: Computational biomechanical modelling of the rat skull

Cranial sutures are fibrocellular joints between the skull bones that are progressively replaced with bone throughout ontogeny, facilitating growth and cranial shape change. This transition from soft tissue to bone is reflected in the biomechanical properties of the craniofacial complex. However, the mechanical significance of cranial sutures has only been explored at a few localised areas within the mammalian skull, and as such our understanding of suture function in overall skull biomechanics is still limited. Here, we sought to determine how the overall strain environment is affected by the complex network of cranial sutures in the mammal skull. We combined two computational biomechanical methods, multibody dynamics analysis and finite element analysis, to simulate biting in a rat skull and compared models with and without cranial sutures. Our results show that including complex sutures in the rat model does not substantially change overall strain gradients across the cranium, particularly strain magnitudes in the bones overlying the brain. However, local variations in strain magnitudes and patterns can be observed in areas close to the sutures. These results show that, during feeding, sutures may be more important in some regions than others. Sutures should therefore be included in models that require accurate local strain magnitudes and patterns of cranial strain, particularly if models are developed for analysis of specific regions, such as the temporomandibular joint or zygomatic arch. Our results suggest that, for mammalian skulls, cranial sutures might be more important for allowing brain expansion during growth than redistributing biting loads across the cranium in adults.

Age estimation in the harbour seal (Phoca vitulina) based on the closure of skull sutures and synchondroses

Age estimation of individuals is an important tool for the management of marine mammals and is mostly done by microscopic analysis of growth layer groups (GLGs) in teeth (cementum and dentin) and bone. The present study evaluated the use of sutural and synchondrosal closure in the skull of the harbour seal (Phoca vitulina) as a potential non-destructive alternative for age estimation. For that, we scored the ectocranial closure of 12 selected sutures and synchondroses in a sample of 303 Eastern Atlantic harbour seals (P. v. vitulina; 138 males, 165 females), for which data on age at death were available, based on determined gross-morphological criteria in individuals younger than 1 year and cement layer analysis in canines in older individuals. A strong positive relationship between the sum of closure level scores (SCS) and age (males: r = 0.8797, females: r = 0.8825) was recorded, which was stronger than that for the relationship between age and condylobasal length (CBL, males: r = 0.7085, females: 0.7086) (all p-values <0.0001). In adult individuals (≥5 years), CBL was higher in males than females (p < 0.0001), while SCS did not significantly differ between the two sexes (p = 0.148). Our findings show that the analysis of sutural/synchondrosal closure is a valid alternative for age estimation in harbour seals, thereby confirming results of previous studies on other pinniped species.

A rare case of the scaphocephalic skull of an adult male

Background

Scaphocephaly is a craniofacial anomaly where the skull is disproportionately long and narrow. Premature closure of the sagittal suture is thought to be at the core of the etiopathology for scaphocephaly.

Case presentation

The skull is a well-preserved skull found in an antiquated graveyard at Parkand village Daregaz, Iran, with no visible signs of surgical manipulation. The craniofacial indices were measured. The maximum cranial length was 200 mm, and the measured length of nasion-bregma was 140mm, whereas most of the measurements were less than the normal scaphocephalic indices. Another morphological finding was the flattening of the frontal bone.

Conclusions

The cranial vault morphometry and morphology of the presented case fit the scalp. In addition to the characteristics of scaphocephaly, the presented skull was of a rare trait, frontal flattening, which has not been reported before.

Skeletal stem cells: a game changer of skeletal biology and regenerative medicine?

Skeletal stem cells (SSCs) were originally discovered in the bone marrow stroma. They are capable of self-renewal and multilineage differentiation into osteoblasts, chondrocytes, adipocytes, and stromal cells. Importantly, these bone marrow SSCs localize in the perivascular region and highly express hematopoietic growth factors to create the hematopoietic stem cell (HSC) niche. Thus, bone marrow SSCs play pivotal roles in orchestrating osteogenesis and hematopoiesis. Besides the bone marrow, recent studies have uncovered diverse SSC populations in the growth plate, perichondrium, periosteum, and calvarial suture at different developmental stages, which exhibit distinct differentiation potential under homeostatic and stress conditions. Therefore, the current consensus is that a panel of region-specific SSCs collaborate to regulate skeletal development, maintenance, and regeneration. Here, we will summarize recent advances of SSCs in long bones and calvaria, with a special emphasis on the evolving concept and methodology in the field. We will also look into the future of this fascinating research area that may ultimately lead to effective treatment of skeletal disorders.

The clinical manifestations, molecular mechanisms and treatment of craniosynostosis

Craniosynostosis is a major congenital craniofacial disorder characterized by the premature fusion of cranial suture(s). Patients with severe craniosynostosis often have impairments in hearing, vision, intracranial pressure and/or neurocognitive functions. Craniosynostosis can result from mutations, chromosomal abnormalities or adverse environmental effects, and can occur in isolation or in association with numerous syndromes. To date, surgical correction remains the primary treatment for craniosynostosis, but it is associated with complications and with the potential for re-synostosis. There is, therefore, a strong unmet need for new therapies. Here, we provide a comprehensive review of our current understanding of craniosynostosis, including typical craniosynostosis types, their clinical manifestations, cranial suture development, and genetic and environmental causes. Based on studies from animal models, we present a framework for understanding the pathogenesis of craniosynostosis, with an emphasis on the loss of postnatal suture mesenchymal stem cells as an emerging disease-driving mechanism. We evaluate emerging treatment options and highlight the potential of mesenchymal stem cell-based suture regeneration as a therapeutic approach for craniosynostosis.

Postnatal Progressive Craniosynostosis in Syndromic Conditions: Two Patients With Saethre-Chotzen Due to TWIST1 Gene Deletions and Review of the Literature

Saethre-Chotzen syndrome (SCS) is a known craniosynostosis syndrome with a variable presentation of craniofacial and somatic involvement. Congenital coronal craniosynostosis is most commonly observed in SCS; however, progressive postnatal craniosynostosis of other sutures has been reported. The authors present 2 infants with progressive postnatal craniosynostosis and SCS caused by chromosome 7p deletions including the TWIST1 gene. The evolution of their clinical features and a literature review of patients with syndromic, postnatal progressive craniosynostosis illustrate the importance of longitudinal observation and management of these patients.

Cytokines in the Brain and Neuroinflammation: We Didn’t Starve the Fire!

In spite of the brain-protecting tissues of the skull, meninges, and blood-brain barrier, some forms of injury to or infection of the CNS can give rise to cerebral cytokine production and action and result in drastic changes in brain function and behavior. Interestingly, peripheral infection-induced systemic inflammation can also be accompanied by increased cerebral cytokine production. Furthermore, it has been recently proposed that some forms of psychological stress may have similar CNS effects. Different conditions of cerebral cytokine production and action will be reviewed here against the background of neuroinflammation. Within this context, it is important to both deepen our understanding along already taken paths as well as to explore new ways in which neural functioning can be modified by cytokines. This, in turn, should enable us to put forward different modes of cerebral cytokine production and action in relation to distinct forms of neuroinflammation.

Cranial Suture Mesenchymal Stem Cells: Insights and Advances

The cranial bones constitute the protective structures of the skull, which surround and protect the brain. Due to the limited repair capacity, the reconstruction and regeneration of skull defects are considered as an unmet clinical need and challenge. Previously, it has been proposed that the periosteum and dura mater provide reparative progenitors for cranial bones homeostasis and injury repair. In addition, it has also been speculated that the cranial mesenchymal stem cells reside in the perivascular niche of the diploe, namely, the soft spongy cancellous bone between the interior and exterior layers of cortical bone of the skull, which resembles the skeletal stem cells’ distribution pattern of the long bone within the bone marrow. Not until recent years have several studies unraveled and validated that the major mesenchymal stem cell population of the cranial region is primarily located within the suture mesenchyme of the skull, and hence, they are termed suture mesenchymal stem cells (SuSCs). Here, we summarized the characteristics of SuSCs, this newly discovered stem cell population of cranial bones, including the temporospatial distribution pattern, self-renewal, and multipotent properties, contribution to injury repair, as well as the signaling pathways and molecular mechanisms associated with the regulation of SuSCs.

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.

Quantifying maxillary development in chimpanzees and humans: An analysis of prognathism and orthognathism at the morphological and microscopic scales

Facial orientation (projection and degree of prognathism) and form in hominins is highly variable, likely related to evolutionary modifications of the microscopic process of bone modeling (the simultaneous cellular activities of bone formation and resorption) during ontogeny. However, in anteriorly projected faces such as those of early hominins, little is known about the link between bone modeling and facial developmental patterns. Similarly, these aspects have been infrequently investigated in extant great apes. In this study, quantitative methods were applied to a cross-sectional ontogenetic sample of 33 chimpanzees (Pan troglodytes verus) and 59 modern humans (Homo sapiens) to compare the development of maxillary prognathism to orthognathism at both microscopic and macroscopic (or morphological) scales using surface histology and geometric morphometric techniques. Chimpanzees express on average lower amounts of bone resorption than humans on the maxillary periosteum throughout ontogeny; however, the premaxilla is consistently resorbed from early stages on. The presence of bone resorption in the chimpanzee premaxilla, such as that seen in some early hominins, suggests a more ape-like pattern of maxillary bone modeling in these specimens. However, this shows that similarities in bone modeling patterns can lead to variations in shape, suggesting that other aspects of facial growth (such as modifications of rates and timings of development, as well as sutural growth) also played a crucial role in facial evolution.