Changes to the cell, tissue and architecture levels in cranial suture synostosis reveal a problem of timing in bone development

Craniofacial anomalies in a murine model of heterozygous fibroblast growth factor 10 gene mutation

OBJECTIVE

Dysregulation of Fibroblast Growth Factor 10 (FGF10), a member of the family of Fibroblast Growth Factor (FGF) proteins, has been implicated in craniofacial and dental anomalies, including craniosynostosis, cleft palate, and Lacrimo-Auriculo-Dento-Digital Syndrome. The aim of this murine study was to assess the craniofacial and dental phenotypes associated with a heterozygous FGF10 gene (FGF10+/- ) mutation at skeletal maturity.

METHODS

Skulls of 40 skeletally mature mice, comprising two genotypes (heterozygous FGF10+/- mutation, n = 22; wildtype, n = 18) and two sexes (male, n = 23; female, n = 17), were subjected to micro-computed tomography. Landmark-based linear dimensions were measured for the cranial vault, maxilla, mandible, and first molar teeth. Multivariate analysis of variance was performed to assess whether there were significant differences in the craniofacial and dental structures between genotypes and sexes.

RESULTS

The craniomaxillary skeleton and the first molar teeth were smaller in the FGF10+/- mice (P < .05), but the mandible was unaffected. Sex did not have a significant effect on these structures (P > .05). Cranial sutural defects were noted in 5/22 (22.7%) mutant versus 2/18 (11.1%) wildtype mice, and cleft palate in only one (4.5%) mutant mouse. None of the mice displayed craniosynostosis, expansive bony lesions, bifid condyles, or impacted teeth.

CONCLUSION

The FGF10+/- mutation was associated with craniomaxillary skeletal hypoplasia that probably arose from deficient (delayed) intramembranous ossification of the sutured bones. Overall, the skeletal and dental data suggest that the FGF10 gene plays an important role in the aetiology of craniofacial dysmorphology and malocclusion.

Scaffold Pore Curvature Influences ΜSC Fate through Differential Cellular Organization and YAP/TAZ Activity

Tissue engineering aims to repair, restore, and/or replace tissues in the human body as an alternative to grafts and prostheses. Biomaterial scaffolds can be utilized to provide a three-dimensional microenvironment to facilitate tissue regeneration. Previously, we reported that scaffold pore size influences vascularization and extracellular matrix composition both in vivo and in vitro, to ultimately influence tissue phenotype for regenerating cranial suture and bone tissues, which have markedly different tissue properties despite similar multipotent stem cell populations. To rationally design biomaterials for specific cell and tissue fate specification, it is critical to understand the molecular processes governed by cell-biomaterial interactions, which guide cell fate specification. Building on our previous work, in this report we investigated the hypothesis that scaffold pore curvature, the direct consequence of pore size, modulates the differentiation trajectory of mesenchymal stem cells (MSCs) through alterations in the cytoskeleton. First, we demonstrated that sufficiently small pores facilitate cell clustering in subcutaneous explants cultured in vivo, which we previously reported to demonstrate stem tissue phenotype both in vivo and in vitro. Based on this observation, we cultured cell-scaffold constructs in vitro to assess early time point interactions between cells and the matrix as a function of pore size. We demonstrate that principle curvature directly influences nuclear aspect and cell aggregation in vitro. Scaffold pores with a sufficiently low degree of principle curvature enables cell differentiation; pharmacologic inhibition of actin cytoskeleton polymerization in these scaffolds decreased differentiation, indicating a critical role of the cytoskeleton in transducing cues from the scaffold pore microenvironment to the cell nucleus. We fabricated a macropore model, which allows for three-dimensional confocal imaging and demonstrates that a higher principle curvature facilitates cell aggregation and the formation of a potentially protective niche within scaffold macropores which prevents MSC differentiation and retains their stemness. Sufficiently high principle curvature upregulates yes-associated protein (YAP) phosphorylation while decreased principle curvature downregulates YAP phosphorylation and increases YAP nuclear translocation with subsequent transcriptional activation towards an osteogenic differentiation fate. Finally, we demonstrate that the inhibition of the YAP/TAZ pathway causes a defect in differentiation, while YAP/TAZ activation causes premature differentiation in a curvature-dependent way when modulated by verteporfin (VP) and 1-oleyl-lysophosphatidic acid (LPA), respectively, confirming the critical role of biomaterials-mediated YAP/TAZ signaling in cell differentiation and fate specification. Our data support that the principle curvature of scaffold macropores is a critical design criterion which guides the differentiation trajectory of mesenchymal stem cells’ scaffolds. Biomaterial-mediated regulation of YAP/TAZ may significantly contribute to influencing the regenerative outcomes of biomaterials-based tissue engineering strategies through their specific pore design.

Large osteocyte lacunae in iliac crest infantile bone are not associated with impaired mineral distribution or signs of osteocytic osteolysis

The enlargement of osteocyte lacunae via osteocytic osteolysis was previously detected in situations of increased calcium demand (e.g., lactation, vitamin D deficiency). However, it is unclear whether similar processes occur also in the growing infantile skeleton and how this is linked to the mineral distribution within the bone matrix. Human iliac crest biopsies of 30 subjects (0-6 months, n = 14; 2-8 years, n = 6 and 18-25 years, n = 10) were acquired. Bone microarchitecture was assessed by micro-CT, while cellular bone histomorphometry was performed on undecalcified histological sections. Quantitative backscattered electron imaging (qBEI) was conducted to determine the bone mineral density distribution (BMDD) as well as osteocyte lacunar size and density. We additionally evaluated cathepsin K positive osteocytes using immunohistochemistry. Infantile bone was characterized by various signs of ongoing bone development such as higher bone (re)modeling, lower cortical and trabecular thickness compared to young adults. Importantly, a significantly higher osteocyte lacunar density and increased lacunar area were detected. Large osteocyte lacunae were associated with a more heterogeneous bone mineral density distribution of the trabecular bone matrix due to the presence of hypermineralized cartilage remnants, whereas the mean mineralization (i.e., CaMean) was not different in infantile bone. Absence of cathepsin K expression in osteocyte lacunae indicated nonexistent osteocytic osteolysis. Taken together, we demonstrated that the overall mineralization distribution in infantile bone is not altered compared to young adults besides high trabecular mineralization heterogeneity. Our study also provides important reference values for bone microstructure, BMDD and osteocyte characteristics in infants, children and young adults. Infantile bone displays large osteocyte lacunae indicating a developmental phenomenon rather than osteocytic osteolysis. Larger osteocytes may have superior mechanosensory abilities to enable bone adaption during growth.

Inter-site Variability of the Human Osteocyte Lacunar Network: Implications for Bone Quality

PURPOSE OF REVIEW

This article provides a review on the variability of the osteocyte lacunar network in the human skeleton. It highlights characteristics of the osteocyte lacunar network in relation to different skeletal sites and fracture susceptibility.

RECENT FINDINGS

Application of 2D analyses (quantitative backscattered electron microscopy, histology, confocal laser scanning microscopy) and 3D reconstructions (microcomputed tomography and synchrotron radiation microcomputed tomography) provides extended high-resolution information on osteocyte lacunar properties in individuals of various age (fetal, children's growth, elderly), sex, and disease states with increased fracture risk. Recent findings on the distribution of osteocytes in the human skeleton are reviewed. Quantitative data highlighting the variability of the osteocyte lacunar network is presented with special emphasis on site specificity and maintenance of bone health. The causes and consequences of heterogeneous distribution of osteocyte lacunae both within specific regions of interest and on the skeletal level are reviewed and linked to differential bone quality factors and fracture susceptibility.

50 years of scanning electron microscopy of bone-a comprehensive overview of the important discoveries made and insights gained into bone material properties in health, disease, and taphonomy

Bone is an architecturally complex system that constantly undergoes structural and functional optimisation through renewal and repair. The scanning electron microscope (SEM) is among the most frequently used instruments for examining bone. It offers the key advantage of very high spatial resolution coupled with a large depth of field and wide field of view. Interactions between incident electrons and atoms on the sample surface generate backscattered electrons, secondary electrons, and various other signals including X-rays that relay compositional and topographical information. Through selective removal or preservation of specific tissue components (organic, inorganic, cellular, vascular), their individual contribution(s) to the overall functional competence can be elucidated. With few restrictions on sample geometry and a variety of applicable sample-processing routes, a given sample may be conveniently adapted for multiple analytical methods. While a conventional SEM operates at high vacuum conditions that demand clean, dry, and electrically conductive samples, non-conductive materials (e.g., bone) can be imaged without significant modification from the natural state using an environmental scanning electron microscope. This review highlights important insights gained into bone microstructure and pathophysiology, bone response to implanted biomaterials, elemental analysis, SEM in paleoarchaeology, 3D imaging using focused ion beam techniques, correlative microscopy and in situ experiments. The capacity to image seamlessly across multiple length scales within the meso-micro-nano-continuum, the SEM lends itself to many unique and diverse applications, which attest to the versatility and user-friendly nature of this instrument for studying bone. Significant technological developments are anticipated for analysing bone using the SEM.

The paracrine effects of human induced pluripotent stem cells promote bone-like structures via the upregulation of BMP expression in a mouse ectopic model

Use of human induced pluripotent stem cells (h-iPSCs) for bone tissue engineering is most appealing, because h-iPSCs are an inexhaustible source of osteocompetent cells. The present study investigated the contribution of undifferentiated h-iPSCs and elucidated aspects of the underlying mechanism(s) of the involvement of these cells to new bone formation. Implantation of undifferentiated h-iPSCs seeded on coral particles in ectopic sites of mice resulted in expression of osteocalcin and DMP-1, and in mineral content similar to that of the murine bone. The number of the implanted h-iPSCs decreased with time and disappeared by 30 days post-implantation. In contrast, expression of the murine osteogenic genes at day 15 and 30 post-implantation provided, for the first time, evidence that the implanted h-iPSCs affected the observed outcomes via paracrine mechanisms. Supporting evidence was provided because supernatant conditioned media from h-iPSCs (h-iPSC CM), promoted the osteogenic differentiation of human mesenchymal stem cells (h-MSCs) in vitro. Specifically, h-iPSC CM induced upregulation of the BMP-2, BMP-4 and BMP-6 genes, and promoted mineralization of the extracellular matrix. Given the current interest in the use of h-iPSCs for regenerative medicine applications, our study contributes new insights into aspects of the mechanism underlying the bone promoting capability of h-iPSCs.

Increased mechanical loading through controlled swimming exercise induces bone formation and mineralization in adult zebrafish

Exercise promotes gain in bone mass through adaptive responses of the vertebrate skeleton. This mechanism counteracts age- and disease-related skeletal degradation, but remains to be fully understood. In life sciences, zebrafish emerged as a vertebrate model that can provide new insights into the complex mechanisms governing bone quality. To test the hypothesis that musculoskeletal exercise induces bone adaptation in adult zebrafish and to characterize bone reorganization, animals were subjected to increased physical exercise for four weeks in a swim tunnel experiment. Cellular, structural and compositional changes of loaded vertebrae were quantified using integrated high-resolution analyses. Exercise triggered rapid bone adaptation with substantial increases in bone-forming osteoblasts, bone volume and mineralization. Clearly, modeling processes in zebrafish bone resemble processes in human bone. This study highlights how exercise experiments in adult zebrafish foster in-depth insight into aging-related bone diseases and can thus catalyze the search for appropriate prevention and new treatment options.

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

BACKGROUND

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

RESULTS

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

CONCLUSIONS

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

Sonographic Analysis of Changes in Skull Shape After Cranial Molding Helmet Therapy in Infants With Deformational Plagiocephaly

OBJECTIVES

-The purpose of this study was to investigate the changes in skull shape on sonography after cranial molding helmet therapy in infants with deformational plagiocephaly.

METHODS

-Twenty-six infants who were treated with cranial molding helmet therapy were recruited. Caliper and sonographic measurements were performed. The lateral length of the affected and unaffected sides of the skull and cranial vault asymmetry index were measured with calipers. The occipital angle, defined as the angle between lines projected along the lambdoid sutures of the skull, was calculated by sonography. The occipital angle difference and occipital angle ratio were also measured. All caliper and sonographic measurements were performed in each infant twice before and twice after treatment.

RESULTS

-The study group included 12 male and 14 female infants with a mean age ± SD of 6.2 ± 3.5 months. The mean treatment duration was 6.0 ± 2.5 months. The difference in lateral length before and after helmet therapy was significantly greater on the affected skull than the unaffected skull (16.7 ± 12.7 versus 9.0 ± 13.4 mm; P < .01). The difference in the occipital angle before and after helmet therapy was significantly greater on the affected skull than the unaffected skull (-5.7° ± 7.3° versus 4.2° ± 7.9°; P < .01). The cranial vault asymmetry index and occipital angle ratio were significantly reduced after helmet therapy (cranial vault asymmetry index, 9.3% ± 2.3% versus 3.5% ± 3.0%; occipital angle ratio, 1.07 ± 0.05 versus 1.01 ± 0.01; P < .05).

CONCLUSIONS

-These results suggest that occipital angle measurements using sonography, combined with cephalometry, could provide a better understanding of the therapeutic effects of cranial molding helmet therapy in infants with deformational plagiocephaly.

Modifications to nano- and microstructural quality and the effects on mechanical integrity in Paget's disease of bone

Paget's disease of bone (PDB) is the second most common bone disease mostly developing after 50 years of age at one or more localized skeletal sites; it is associated with severely high bone turnover, bone enlargement, bowing/deformity, cracking, and pain. Here, to specifically address the origins of the deteriorated mechanical integrity, we use a cohort of control and PDB human biopsies to investigate multiscale architectural and compositional modifications to the bone structure (ie, bone quality) and relate these changes to mechanical property measurements to provide further insight into the clinical manifestations (ie, deformities and bowing) and fracture risk caused by PDB. Here, at the level of the collagen and mineral (ie, nanometer-length scale), we find a 19% lower mineral content and lower carbonate-to-phosphate ratio in PDB, which accounts for the 14% lower stiffness and 19% lower hardness promoting plastic deformation in pathological bone. At the microstructural scale, trabecular regions are known to become densified, whereas cortical bone loses its characteristic parallel-aligned osteonal pattern, which is replaced with a mosaic of lamellar and woven bone. Although we find this loss of anisotropic alignment produces a straighter crack path in mechanically-loaded PDB cases, cortical fracture toughness appears to be maintained due to increased plastic deformation. Clearly, the altered quality of the bone structure in PDB affects the mechanical integrity leading to complications such as bowing, deformities, and stable cracks called fissure fractures associated with this disease. Although the lower mineralization and loss of aligned Haversian structures do produce a lower modulus tissue, which is susceptible to deformities, our results indicate that the higher levels of plasticity may compensate for the lost microstructural features and maintain the resistance to crack growth.

Multi-level characterization of human femoral cortices and their underlying osteocyte network reveal trends in quality of young, aged, osteoporotic and antiresorptive-treated bone

Characterization of bone's hierarchical structure in aging, disease and treatment conditions is imperative to understand the architectural and compositional modifications to the material and its mechanical integrity. Here, cortical bone sections from 30 female proximal femurs - a frequent fracture site - were rigorously assessed to characterize the osteocyte lacunar network, osteon density and patterns of bone matrix mineralization by backscatter-electron imaging and Fourier-transform infrared spectroscopy in relation to mechanical properties obtained by reference-point indentation. We show that young, healthy bone revealed the highest resistance to mechanical loading (indentation) along with higher mineralization and preserved osteocyte-lacunar characteristics. In contrast, aging and osteoporosis significantly alter bone material properties, where impairment of the osteocyte-lacunar network was evident through accumulation of hypermineralized osteocyte lacunae with aging and even more in osteoporosis, highlighting increased osteocyte apoptosis and reduced mechanical competence. But antiresorptive treatment led to fewer mineralized lacunae and fewer but larger osteons signifying rejuvenated bone. In summary, multiple structural and compositional changes to the bone material were identified leading to decay or maintenance of bone quality in disease, health and treatment conditions. Clearly, antiresorptive treatment reflected favorable effects on the multifunctional osteocytic cells that are a prerequisite for bone's structural, metabolic and mechanosensory integrity.

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.

High fluoride and low calcium levels in drinking water is associated with low bone mass, reduced bone quality and fragility fractures in sheep

Chronic environmental fluoride exposure under calcium stress causes fragility fractures due to osteoporosis and bone quality deterioration, at least in sheep. Proof of skeletal fluorosis, presenting without increased bone density, calls for a review of fracture incidence in areas with fluoridated groundwater, including an analysis of patients with low bone mass.

INTRODUCTION

Understanding the skeletal effects of environmental fluoride exposure especially under calcium stress remains an unmet need of critical importance. Therefore, we studied the skeletal phenotype of sheep chronically exposed to highly fluoridated water in the Kalahari Desert, where livestock is known to present with fragility fractures.

METHODS

Dorper ewes from two flocks in Namibia were studied. Chemical analyses of water, blood and urine were executed for both cohorts. Skeletal phenotyping comprised micro-computer tomography (μCT), histological, histomorphometric, biomechanical, quantitative backscattered electron imaging (qBEI) and energy-dispersive X-ray (EDX) analysis. Analysis was performed in direct comparison with undecalcified human iliac crest bone biopsies of patients with fluoride-induced osteopathy.

RESULTS

The fluoride content of water, blood and urine was significantly elevated in the Kalahari group compared to the control. Surprisingly, a significant decrease in both cortical and trabecular bones was found in sheep chronically exposed to fluoride. Furthermore, osteoid parameters and the degree and heterogeneity of mineralization were increased. The latter findings are reminiscent of those found in osteoporotic patients with treatment-induced fluorosis. Mechanical testing revealed a significant decrease in the bending strength, concurrent with the clinical observation of fragility fractures in sheep within an area of environmental fluoride exposure.

CONCLUSIONS

Our data suggest that fluoride exposure with concomitant calcium deficit (i) may aggravate bone loss via reductions in mineralized trabecular and cortical bone mass and (ii) can cause fragility fractures and (iii) that the prevalence of skeletal fluorosis especially due to groundwater exposure should be reviewed in many areas of the world as low bone mass alone does not exclude fluorosis.

Nano-structural, compositional and micro-architectural signs of cortical bone fragility at the superolateral femoral neck in elderly hip fracture patients vs. healthy aged controls

To unravel the origins of decreased bone strength in the superolateral femoral neck, we assessed bone structural features across multiple length scales at this cortical fracture initiating region in postmenopausal women with hip fracture and in aged-matched controls. Our combined methodological approach encompassed atomic force microscopy (AFM) characterization of cortical bone nano-structure, assessment of mineral content/distribution via quantitative backscattered electron imaging (qBEI), measurement of bone material properties by reference point indentation, as well as evaluation of cortical micro-architecture and osteocyte lacunar density. Our findings revealed a wide range of differences between the fracture group and the controls, suggesting a number of detrimental changes at various levels of cortical bone hierarchical organization that may render bone fragile. Namely, mineral crystals at external cortical bone surfaces of the fracture group were larger (65.22nm±41.21nm vs. 36.75nm±18.49nm, p<0.001), and a shift to a higher mineral content and more homogenous mineralization profile as revealed via qBEI were found in the bone matrix of the fracture group. Fracture cases showed nearly 35% higher cortical porosity and showed significantly reduced osteocyte lacunar density compared to controls (226±27 vs. 247±32#/mm(2), p=0.05). Along with increased crystal size, a shift towards higher mineralization and a tendency to increased cortical porosity and reduced osteocyte lacunar number delineate that cortical bone of the superolateral femoral neck bears distinct signs of fragility at various levels of its structural organization. These results contribute to the understanding of hierarchical bone structure changes in age-related fragility.

Pathology in metopic synostosis

Premature closure and subsequent ossification of the metopic suture results in triangular head shape called trigonocephaly and is characterized by a midline metopic ridge, frontotemporal narrowing, and an increased biparietal diameter. Trigonocephaly is the second most frequent type of craniosynostosis. It can be isolated and associated with other congenital anomalies without any known syndrome, or occurs as part of a multiple malformation syndrome. Improvement in treatment is directed by a thorough understanding of the basic pathology of this condition. This review aims to provide an overview of metopic synostosis by correlating what is known about pathogenesis and pathology of this entity.