Mineral tessellation in bone and the stenciling principle for extracellular matrix mineralization

Revisiting the physical and chemical nature of the mineral component of bone

The physico-chemical characteristics of bone mineral remain heavily debated. On the nanoscale, bone mineral resides both inside and outside the collagen fibril as distinct compartments fused together into a cohesive continuum. On the micrometre level, larger aggregates are arranged in a staggered pattern described as crossfibrillar tessellation. Unlike geological and synthetic hydroxy(l)apatite, bone mineral is a unique form of apatite deficient in calcium and hydroxyl ions with distinctive carbonate and acid phosphate substitutions (CHAp), together with a minor contribution of amorphous calcium phosphate as a surface layer around a crystalline core of CHAp. In mammalian bone, an amorphous solid phase has not been observed, though an age-dependent shift in the amorphous-to-crystalline character is observed. Although octacalcium phosphate has been postulated as a bone mineral precursor, there is inconsistent evidence of calcium phosphate phases other than CHAp in the extracellular matrix. In association with micropetrosis, magnesium whitlockite is occasionally detected, indicating pathological calcification rather than a true extracellular matrix component. Therefore, the terms 'biomimetic' or 'bone-like' should be used cautiously in descriptions of synthetic biomaterials. The practice of reporting the calcium-to-phosphorus ratio (Ca/P) as proxy for bone mineral maturity oversimplifies the chemistry since both Ca2+ and PO43- ions are partially substituted. Moreover, non-mineral sources of phosphorus are ignored. Alternative compositional metrics should be considered. In the context of bone tissue and bone mineral, the term 'mature' must be used carefully, with clear criteria that consider both compositional and structural parameters and the potential impact on mechanical properties. STATEMENT OF SIGNIFICANCE: Bone mineral exhibits a unique hierarchical structure and is classified into intrafibrillar and extrafibrillar mineral compartments with distinct physico-chemical characteristics. The dynamic nature of bone mineral, i.e., evolving chemical composition and physical form, is poorly understood. For instance, bone mineral is frequently described as "hydroxy(l)apatite", even though the OH- content of mature bone mineral is negligible. Moreover, the calcium-to-phosphorus ratio is often taken as an indicator of bone mineral maturity without acknowledging substitutions at calcium and phosphate sites. This review takes a comprehensive look at the structure and composition of bone mineral, highlighting how experimental data are misinterpreted and unresolved concerns that warrant further investigation, which have implications for characterisation of bone material properties and development of bone repair biomaterials.

Acral Mesenchymal Tumor Leading to Tumor-Induced Osteomalacia: Case Report and Literature Review

Objective/Background

Tumor-induced osteomalacia (TIO) is a rare paraneoplastic syndrome caused by excessive secretion of fibroblast growth factor 23 (FGF-23) by phosphaturic mesenchymal tumors. This leads to hypophosphatemia, vitamin D deficiency, and impaired bone metabolism. TIO is often misdiagnosed due to its rarity and nonspecific symptoms.

Case Report

We report a 58-year-old male presenting with multiple nontraumatic fractures, muscle weakness, and functional decline. Laboratory evaluation revealed hypophosphatemia, elevated parathyroid hormone, reduced 1,25-dihydroxyvitamin D, and markedly elevated FGF-23 levels. Imaging identified a soft tissue mass in the plantar region of the right foot, which was confirmed as a phosphaturic mesenchymal tumor upon pathological analysis. The patient underwent surgical resection, resulting in rapid normalization of biochemical abnormalities, including serum phosphorus, parathyroid hormone, and 1,25-dihydroxyvitamin D, within 5 days.

Discussion

This case underscores the importance of recognizing TIO in patients with unexplained hypophosphatemia and fractures. The curative potential of tumor resection was demonstrated with rapid biochemical and clinical improvement. Diagnostic challenges often arise due to the rarity and atypical presentation of these tumors, particularly in uncommon locations such as the plantar region. Emerging therapies, such as FGF-23 inhibitors like burosumab, provide alternatives for nonlocalizable or unresectable tumors.

Conclusion

This case emphasizes the need for increased clinician awareness, multidisciplinary approaches, and advances in diagnostic imaging to reduce delays in diagnosing TIO. Further research is necessary to elucidate the pathophysiology, explore genetic associations, and improve treatment options for this debilitating condition.

Understanding the structural biology of osteomalacia through multiscale 3D X-ray and electron tomographic imaging: a review of X-linked hypophosphatemia, the Hyp mouse model, and imaging methods

Biomineralization in bones and teeth is a highly regulated extracellular event. In the skeleton, mineralization at the tissue level is controlled within the collagenous extracellular matrix by both circulating and local factors. While systemic regulation of mineral ion homeostasis has been well-studied over many decades, much less is known about the regulation of mineralization at the local level directly within the extracellular matrix. Some local regulators have been identified, such as tissue-nonspecific alkaline phosphatase (TNAP), phosphate-regulating endopeptidase homolog X-linked (PHEX), pyrophosphate, and osteopontin, and others are currently under investigation. Dysregulation of the actions of enzyme-inhibitor substrate pairs engaged in mineralization (as we describe by the Stenciling Principle for extracellular matrix mineralization) leads to osteomalacic "soft bone" diseases, such as hypophosphatasia (HPP) and X-linked hypophosphatemia (XLH). This review addresses how advances in 3D imaging tools and software now allow contextual and correlative viewing and interpretation of mineralized tissue structure across most length scales. Contextualized and integrated 3D multiscale data obtained from these imaging modalities have afforded an unprecedented structural biology view of bone from the macroscale to the nanoscale. Such correlated volume imaging data is highly quantitative, providing not only an integrated view of the skeleton in health, but also a means to observe alterations that occur in disease. In the context of the many hierarchical levels of skeletal organization, here we summarize structural features of bone over multiple length scales, with a focus on nano- and microscale features as viewed by X-ray and electron tomography imaging methods (submicron μCT and FIB-SEM). We additionally summarize structural changes observed after dysregulation of the mineralization pathway, focusing here on the Hyp mouse model for XLH. More specifically, we summarize how mineral patterns/packs at the microscale (3D crossfibrillar mineral tessellation), and how this is defective in Hyp mouse bone and Hyp enthesis fibrocartilage.

Mineralized collagen plywood contributes to bone autograft performance

Autologous bone (AB) is the gold standard for bone-replacement surgeries1, despite its limited availability and the need for an extra surgical site. Traditionally, competitive biomaterials for bone repair have focused on mimicking the mineral aspect of bone, as evidenced by the widespread clinical use of bioactive ceramics2. However, AB also exhibits hierarchical organic structures that might substantially affect bone regeneration. Here, using a range of cell-free biomimetic-collagen-based materials in murine and ovine bone-defect models, we demonstrate that a hierarchical hybrid microstructure-specifically, the twisted plywood pattern of collagen and its association with poorly crystallized bioapatite-favourably influences bone regeneration. Our study shows that the most structurally biomimetic material has the potential to stimulate bone growth, highlighting the pivotal role of physicochemical properties in supporting bone formation and offering promising prospects as a competitive bone-graft material.

Growth of a Tessellation: Geometric rules for the Development of Stingray Skeletal Patterns

The skeletons of sharks and rays, fashioned from cartilage, and armored by a veneer of mineralized tiles (tesserae) present a mathematical challenge: How can the continuous covering be maintained as the skeleton expands? This study, using microCT and custom visual data analyses of growing stingray skeletons, systematically examines tessellation patterns and morphologies of the many thousand interacting tesserae covering the hyomandibula (a skeletal element critical to feeding), over a two-fold developmental change in hyomandibula length. The number of tesserae remains surprisingly constant, even as the hyomandibula expands isometrically, with all hyomandibulae displaying self-similar distributions of tesserae shapes/sizes. Although the distribution of tesserae geometries largely agrees with the rules for polyhedra tiling of complex surfaces-dominated by hexagons and a minor fraction of pentagons and heptagons, but very few other polygons-the agreement with Euler's classic mathematical laws is not perfect. Contrary to the assumed uniform growth rate (which is shown would create geometric incompatibilities), larger tesserae grow faster to accommodate skeletal expansion. It is hypothesized that this local regulation of global system complexity is driven by tension (from cartilaginous core expansion) in the fibers connecting tesserae, with strain-responsive cells orchestrating local mineral apposition.

Crossing length scales: X-ray approaches to studying the structure of biological materials

Biological materials have outstanding properties. With ease, challenging mechanical, optical or electrical properties are realised from comparatively `humble' building blocks. The key strategy to realise these properties is through extensive hierarchical structuring of the material from the millimetre to the nanometre scale in 3D. Though hierarchical structuring in biological materials has long been recognized, the 3D characterization of such structures remains a challenge. To understand the behaviour of materials, multimodal and multi-scale characterization approaches are needed. In this review, we outline current X-ray analysis approaches using the structures of bone and shells as examples. We show how recent advances have aided our understanding of hierarchical structures and their functions, and how these could be exploited for future research directions. We also discuss current roadblocks including radiation damage, data quantity and sample preparation, as well as strategies to address them.

The M2 Macrophages Derived Migrasomes From the Surface of Titania Nanotubes Array as a New Concept for Enhancing Osteogenesis

As newly discovered substrate anchored extracellular vesicles, migrasomes (Migs) may bring a new opportunity for manipulating target cells bioactivities. In this study, the M2 macrophages derived Migs are obtained by titania nanotubes surface (NTs). Due to the benefits of nanostructuring, the NTs surface is not only able to induce RAW264.7 for M2 polarization but also to generate more Migs formation, which can be internalized by following seeded mesenchymal stem cells (MSCs). Then, the NTs surface induced Migs are collected by density-gradient centrifugation for MSCs treatment. As indicated by immunofluorescence staining, alkaline phosphatase activity, and alizarin red staining, the osteogenic differentiation capacity of MSCs is significantly enhanced by Migs treatment, in line with the dosage. By RNA-sequence analysis, the enhancement of osteogenic differentiation is correlated with PI3K-AKT pathway activation that may originate from the M2 polarization state of donor cells. Finally, the Migs are coated onto Ti surface for therapeutic application. Both the in vitro and in vivo analysis reveal that the Migs coated Ti implant shows significant enhancement of osteogenesis. In conclusion, this study suggests that the nanosurface may be a favorable platform for Migs production, which may bring a new concept for tissue regeneration.

Enzyme-Activated Biomimetic Vesicles Confining Mineralization for Bone Maturation

Inspired by the crucial role of matrix vesicles (MVs), a series of biomimetic vesicles (BVs) fabricated by calcium glycerophosphate (CaGP) modified polyurethane were designed to mediate the mineralization through in situ enzyme activation for bone therapy. In this study, alkaline phosphatase (ALP) was harbored in the porous BVs by adsorption (Ad-BVs) or entrapment (En-BVs). High encapsulation of ALP on En-BVs was effectively self-activating by calcium ions of CaGP-modified PU that specifically hydrolyzed the organophosphorus (CaGP) to inorganic phosphate, thus promoting the formation of the highly oriented bone-like apatite in vitro. Enzyme-catalyzed kinetics confirms the regulation of apatite crystallization by the synergistic action of self-activated ALP and the confined microcompartments of BVs. This leads to a supersaturated microenvironment, with the En-BVs group exhibiting inorganic phosphate (Pi) levels 4.19 times higher and Ca2+ levels 3.67 times higher than those of simulated body fluid (SBF). Of note, the En-BVs group exhibited excellent osteo-inducing differentiation of BMSCs in vitro and the highest maturity with reduced bone loss in rat femoral defect in vivo. This innovative strategy of biomimetic vesicles is expected to provide valuable insights into the enzyme-activated field of bone therapy.

Ultrastructure and Nanoporosity of Human Bone Shown with Correlative On-Axis Electron and Spectroscopic Tomographies

Mineralized collagen fibrils are the building block units of bone at the nanoscale. While it is known that collagen fibrils are mineralized both inside their gap zones (intra-fibrillar mineralization) and on their outer surfaces (extra-fibrillar mineralization), a clear visualization of this architecture in three dimensions (3D), combining structural and compositional information over large volumes, but without compromising the resolution, remains challenging. In this study, we demonstrate the use of on-axis Z-contrast electron tomography (ET) with correlative energy-dispersive X-ray spectroscopy (EDX) tomography to examine rod-shaped samples with diameters up to 700 nm prepared from individual osteonal lamellae in the human femur. Our work mainly focuses on two aspects: (i) low-contrast nanosized circular spaces ("holes") observed in sections of bone oriented perpendicular to the long axis of a long bone, and (ii) extra-fibrillar mineral, especially in terms of morphology and spatial relationship with respect to intra-fibrillar mineral and collagen fibrils. From our analyses, it emerges quite clearly that most "holes" are cross-sectional views of collagen fibrils. While this had been postulated before, our 3D reconstructions and reslicing along meaningful two-dimensional (2D) cross-sections provide a direct visual confirmation. Extra-fibrillar mineral appears to be composed of thin plates that are interconnected and span over several collagen fibrils, confirming that mineralization is cross-fibrillar, at least for the extra-fibrillar phase. EDX tomography shows mineral signatures (Ca and P) within the gap zones, but the signal appears weaker than that associated with the extra-fibrillar mineral, pointing toward the existence of dissimilarities between the two types of mineralization.

Contributions of increased osteopontin and hypophosphatemia to dentoalveolar defects in osteomalacic Hyp mice

X-linked hypophosphatemia (XLH) is an inherited disorder caused by inactivating mutations in the PHEX gene leading to renal phosphate wasting, rickets and osteomalacia. XLH is also associated with dentoalveolar mineralization defects in tooth enamel, dentin and cementum, and in alveolar bone, which lead to an increased prevalence of dental abscesses, periodontal disease and tooth loss. Genetic mouse experiments, and deficiencies in XLH patient therapies where treatments do not fully ameliorate mineralization defects, suggest that other pathogenic mechanisms may exist in XLH. The mineralization-inhibiting, secreted extracellular matrix phosphoprotein osteopontin (OPN, gene Spp1) is a substrate for the PHEX enzyme whereby extensive and inactivating degradation of inhibitory OPN by PHEX facilitates mineralization. Conversely, excess OPN accumulation in skeletal and dental tissues - for example in XLH where inactivating mutations in the PHEX gene limit degradation of inhibitory OPN, or as occurs in Fgf23-null mice - contributes to mineralization defects. We hypothesized that Spp1/OPN ablation in Hyp mice (a mouse model for XLH) would reduce dentoalveolar mineralization defects. Immunostaining revealed increased OPN in Hyp vs. wild-type (WT) alveolar bone, particularly in osteocyte lacunocanalicular networks where Hyp mice have characteristic hypomineralized peri-osteocytic lesions (POLs). Micro-computed tomography and histology showed that ablation of Spp1 in Hyp mice (Hyp;Spp1-/-) on a normal diet did not ameliorate bulk defects in enamel, dentin, or alveolar bone. On a high-phosphate diet, both Hyp and Hyp;Spp1-/- mice showed improved mineralization of enamel, dentin, and alveolar bone. Silver staining indicated Spp1 ablation did not improve alveolar or mandibular bone osteocyte POLs in Hyp mice; however, they were normalized by a high-phosphate diet in both Hyp and Hyp;Spp1-/- mice, although inducing increased OPN. Collectively, these data indicate that despite changes in OPN content in the dentoalveolar mineralized tissues, there exist other compensatory mineralization mechanisms that arise from knockout of Spp1/OPN in the Hyp background.

Mineral tessellation in mouse enthesis fibrocartilage, Achilles tendon, and Hyp calcifying enthesopathy: A shared 3D mineralization pattern

The hallmark of enthesis architecture is the 3D compositional and structural gradient encompassing four tissue zones - tendon/ligament, uncalcified fibrocartilage, calcified fibrocartilage and bone. This functional gradient accommodates the large stiffness differential between calcified bone and uncalcified tendon/ligament. Here we analyze in 3D the organization of the mouse Achilles enthesis and mineralizing Achilles tendon in comparison to lamellar bone. We use correlative, multiscale high-resolution volume imaging methods including μCT with submicrometer resolution and FIB-SEM tomography (both with deep learning-based image segmentation), and TEM and SEM imaging, to describe ultrastructural features of physiologic, age-related and aberrant mineral patterning. We applied these approaches to murine wildtype (WT) Achilles enthesis tissues to describe in normal calcifying fibrocartilage a crossfibrillar mineral tessellation pattern similar to that observed in lamellar bone, but with greater variance in mineral tesselle morphology and size. We also examined Achilles enthesis structure in Hyp mice, a murine model for the inherited osteomalacic disease X-linked hypophosphatemia (XLH) with calcifying enthesopathy. In Achilles enthesis fibrocartilage of Hyp mice, we show defective crossfibrillar mineral tessellation similar to that which occurs in Hyp lamellar bone. At the cellular level in fibrocartilage, unlike in bone where enlarged osteocyte mineral lacunae are found as peri-osteocytic lesions, mineral lacunar volumes for fibrochondrocytes did not differ between WT and Hyp mice. While both WT and Hyp aged mice demonstrate Achilles tendon midsubstance ectopic mineralization, a consistently defective mineralization pattern was observed in Hyp mice. Strong immunostaining for osteopontin was observed at all mineralization sites examined in both WT and Hyp mice. Taken together, this new 3D ultrastructural information describes details of common mineralization trajectories for enthesis, tendon and bone, which in Hyp/XLH are defective.

Subcanalicular Nanochannel Volume Is Inversely Correlated With Calcium Content in Human Cortical Bone

The spatial distribution of mineralization density is an important signature of bone growth and remodeling processes, and its alterations are often related to disease. The extracellular matrix of some vertebrate mineralized tissues is known to be perfused by a lacunocanalicular network (LCN), a fluid-filled unmineralized structure that harbors osteocytes and their fine processes and transports extracellular fluid and its constituents. The current report provides evidence for structural and compositional heterogeneity at an even smaller, subcanalicular scale. The work reveals an extensive unmineralized three-dimensional (3D) network of nanochannels (~30 nm in diameter) penetrating the mineralized extracellular matrix of human femoral cortical bone and encompassing a greater volume fraction and surface area than these same parameters of the canaliculi comprising the LCN. The present study combines high-resolution focused ion beam-scanning electron microscopy (FIB-SEM) to investigate bone ultrastructure in 3D with quantitative backscattered electron imaging (qBEI) to estimate local bone mineral content. The presence of nanochannels has been found to impact qBEI measurements fundamentally, such that volume percentage (vol%) of nanochannels correlates inversely with weight percentage (wt%) of calcium. This mathematical relationship between nanochannel vol% and calcium wt% suggests that the nanochannels could potentially provide space for ion and small molecule transport throughout the bone matrix. Collectively, these data propose a reinterpretation of qBEI measurements, accounting for nanochannel presence in human bone tissue in addition to collagen and mineral. Further, the results yield insight into bone mineralization processes at the nanometer scale and present the possibility for a potential role of the nanochannel system in permitting ion and small molecule diffusion throughout the extracellular matrix. Such a possible function could thereby lead to the sequestration or occlusion of the ions and small molecules within the extracellular matrix. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Complex geometry and integrated macro-porosity: Clinical applications of electron beam melting to fabricate bespoke bone-anchored implants

The last decade has witnessed rapid advancements in manufacturing technologies for biomedical implants. Additive manufacturing (or 3D printing) has broken down major barriers in the way of producing complex 3D geometries. Electron beam melting (EBM) is one such 3D printing process applicable to metals and alloys. EBM offers build rates up to two orders of magnitude greater than comparable laser-based technologies and a high vacuum environment to prevent accumulation of trace elements. These features make EBM particularly advantageous for materials susceptible to spontaneous oxidation and nitrogen pick-up when exposed to air (e.g., titanium and titanium-based alloys). For skeletal reconstruction(s), anatomical mimickry and integrated macro-porous architecture to facilitate bone ingrowth are undoubtedly the key features of EBM manufactured implants. Using finite element modelling of physiological loading conditions, the design of a prosthesis may be further personalised. This review looks at the many unique clinical applications of EBM in skeletal repair and the ground-breaking innovations in prosthetic rehabilitation. From a simple acetabular cup to the fifth toe, from the hand-wrist complex to the shoulder, and from vertebral replacement to cranio-maxillofacial reconstruction, EBM has experienced it all. While sternocostal reconstructions might be rare, the repair of long bones using EBM manufactured implants is becoming exceedingly frequent. Despite the various merits, several challenges remain yet untackled. Nevertheless, with the capability to produce osseointegrating implants of any conceivable shape/size, and permissive of bone ingrowth and functional loading, EBM can pave the way for numerous fascinating and novel applications in skeletal repair, regeneration, and rehabilitation. STATEMENT OF SIGNIFICANCE: Electron beam melting (EBM) offers unparalleled possibilities in producing contaminant-free, complex and intricate geometries from alloys of biomedical interest, including Ti6Al4V and CoCr. We review the diverse range of clinical applications of EBM in skeletal repair, both as mass produced off-the-shelf implants and personalised, patient-specific prostheses. From replacing large volumes of disease-affected bone to complex, multi-material reconstructions, almost every part of the human skeleton has been replaced with an EBM manufactured analog to achieve macroscopic anatomical-mimickry. However, various questions regarding long-term performance of patient-specific implants remain unaddressed. Directions for further development include designing personalised implants and prostheses based on simulated loading conditions and accounting for trabecular bone microstructure with respect to physiological factors such as patient's age and disease status.

Osteomalacia Is Not a Single Disease

Among bone-material qualities, mineralization is pivotal in conferring stiffness and toughness to the bone. Osteomalacia, a disease ensuing from inadequate mineralization of the skeleton, is caused by different processes leading to decreased available mineral (calcium and/or phosphate) or enzymatic alterations. Vitamin D deficiency, which remains the major cause of altered mineralization leading to inadequate intestinal calcium and phosphate absorption, may be also associated with other conditions primarily responsible for abnormal mineralization. Given the reality of widespread vitamin D inadequacy, a full biochemical assessment of mineral metabolism is always necessary to rule out or confirm other conditions. Both too-high or too-low serum alkaline phosphatase (ALP) levels are important for diagnosis. Osteomalacic syndrome is reversible, at least in part, by specific treatment. Osteomalacia and bone mineralization themselves constitute largely unexplored fields of research. The true prevalence of the different forms of osteomalacia and the recovery after proper therapy have yet to be determined in the real world. Although non-invasive techniques to assess bone mineralization are not available in clinical practice, the systematic assessment of bone quality could help in refining the diagnosis and guiding the treatment. This review summarizes what is known of osteomalacia recent therapeutic developments and highlights the future issues of research in this field.

Spatial metabolomics reveals upregulation of several pyrophosphate-producing pathways in cortical bone of Hyp mice

Patients with the renal phosphate-wasting disease X-linked hypophosphatemia (XLH) and Hyp mice, the murine homolog of XLH, are characterized by loss-of-function mutations in phosphate-regulating endopeptidase homolog X-linked (PHEX), leading to excessive secretion of the bone-derived phosphotropic hormone FGF23. The mineralization defect in patients with XLH and Hyp mice is caused by a combination of hypophosphatemia and local accumulation of mineralization-inhibiting molecules in bone. However, the mechanism by which PHEX deficiency regulates bone cell metabolism remains elusive. Here, we used spatial metabolomics by employing matrix-assisted laser desorption/ionization (MALDI) Fourier-transform ion cyclotron resonance mass spectrometry imaging (MSI) of undecalcified bone cryosections to characterize in situ metabolic changes in bones of Hyp mice in a holistic, unbiased manner. We found complex changes in Hyp bone metabolism, including perturbations in pentose phosphate, purine, pyrimidine, and phospholipid metabolism. Importantly, our study identified an upregulation of several biochemical pathways involved in intra- and extracellular production of the mineralization inhibitor pyrophosphate in the bone matrix of Hyp mice. Our data emphasize the utility of MSI-based spatial metabolomics in bone research and provide holistic in situ insights as to how Phex deficiency-induced changes in biochemical pathways in bone cells are linked to impaired bone mineralization.

Fabrication of succinate-alginate xerogel films for in vitro coupling of osteogenesis and neovascularization

The osseointegration of metallic implants is reliant on a cascade of molecular interactions and the delivery of macromolecules to the implant environment that occurs before substantial bone formation. Early blood vessel formation is a requisite first step in the healing timeline for osteoid formation, where vascular development can be accelerated as a result of controlled hypoxic conditioning. In this study, alginate-derived xerogel films containing varied concentrations of disodium succinate salt which has been shown to induce pseudohypoxia (short-term hypoxic effects while maintaining an oxygenated environment) were developed. Xerogels were characterized for their morphology, succinate release over time and cellular response with osteoblast-mimicking Saos-2 and human umbilical vein endothelial cells (HUVEC). Scanning electron microscopy revealed a multiscale topography that may favour osseointegration and alamarBlue assays indicated no cytotoxic effects during in vitro proliferation of Saos-2 cells. pH measurements of eluted succinate reach 95 % of peak value after 7 h of immersion for all gels containing 10 mM of succinate or less, and 60 % within the first 40 min. In vitro exposure of HUVECs to succinate-conditioned media increased the net concentration of total proteins measured by bicinchoninic acid (BCA) assay and maintains stable vascular endothelial growth factor (VEGF) and extracellular platelet-derived growth factor (PDGF) for vessel formation through comparison of enzyme-linked immunosorbent assays (ELISAs) of the culture media and cell lysate. Tube formation assays also showed a sustained increase in tube diameter across the first 48 h of HUVEC culture when succinate concentrations of 1 and 10 μM in the xerogel. Overall, the succinate-alginate films serve as a prospective organic coating for bone-interfacing implant materials which may induce temporary pseudohypoxic conditions favourable for early angiogenesis and bone regeneration in vivo at succinate concentrations of 1 or 10 μM.

Bone hierarchical structure: spatial variation across length scales

Bone is a complex hierarchical biomineralized material, which is special amongst biominerals because it is replete with cells, namely, osteocytes. While bone has been scrutinized for centuries, many questions remain open and new research hints that the ultrastructure of bone, encompassing both the bone matrix itself and the embedded cell network, is much more heterogeneous than hitherto realized. A number of these new findings have been made thanks to the enormous developments in X-ray imaging that have occurred in recent decades, and there is promise that they will also allow many of the remaining open questions to be addressed. X-ray absorption or phase imaging affords high three-dimensional (3D) resolution and allows traversing the length scales of bone all the way down to the fine details of the lacuno-canalicular network housing the osteocytes. Multimodal X-ray imaging provides combined information covering both the length scales defined by the size of the measured volume and tomographic resolution, as well as those probed by the signal that is measured. In X-ray diffraction computed tomography (XRD-CT), for example, diffraction signals can be reconstructed tomographically, which offers detailed information about the spatial variations in the crystallographic properties of the bone biomineral. Orientational information can be obtained by tensor tomography. The combination of both small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) tensor tomography gives information on the orientation of bone nanostructure and crystals, respectively. These new technical developments promise that great strides towards understanding bone structure can be expected in the near future. In this review, recent findings that have resulted from X-ray imaging are highlighted and speculation is given on what can be expected to follow.

Ontogeny of a tessellated surface: Carapace growth of the longhorn cowfish Lactoria cornuta

Biological armors derive their mechanical integrity in part from their geometric architectures, often involving tessellations: individual structural elements tiled together to form surface shells. The carapace of boxfish, for example, is composed of mineralized polygonal plates, called scutes, arranged in a complex geometric pattern and nearly completely encasing the body. In contrast to artificial armors, the boxfish exoskeleton grows with the fish; the relationship between the tessellation and the gross structure of the armor is therefore critical to sustained protection throughout growth. To clarify whether or how the boxfish tessellation is maintained or altered with age, we quantify architectural aspects of the tessellated carapace of the longhorn cowfish Lactoria cornuta through ontogeny (across nearly an order of magnitude in standard length) and in a high‐throughput fashion, using high‐resolution microCT data and segmentation algorithms to characterize the hundreds of scutes that cover each individual. We show that carapace growth is canalized with little variability across individuals: rather than continually adding scutes to enlarge the carapace surface, the number of scutes is surprisingly constant, with scutes increasing in volume, thickness, and especially width with age. As cowfish and their scutes grow, scutes become comparatively thinner, with the scutes at the edges (weak points in a boxy architecture) being some of the thickest and most reinforced in younger animals and thinning most slowly across ontogeny. In contrast, smaller scutes with more variable curvature were found in the limited areas of more complex topology (e.g., around fin insertions, mouth, and anus). Measurements of Gaussian and mean curvature illustrate that cowfish are essentially tessellated boxes throughout life: predominantly zero curvature surfaces comprised of mostly flat scutes, and with scutes with sharp bends used sparingly to form box edges. Since growth of a curved, tiled surface with a fixed number of tiles would require tile restructuring to accommodate the surface's changing radius of curvature, our results therefore illustrate a previously unappreciated advantage of the odd boxfish morphology: by having predominantly flat surfaces, it is the box‐like body form that in fact permits a relatively straightforward growth system of this tessellated architecture (i.e., where material is added to scute edges). Our characterization of the ontogeny and maintenance of the carapace tessellation provides insights into the potentially conflicting mechanical, geometric, and developmental constraints of this species but also perspectives into natural strategies for constructing mutable tiled architectures.

The carapace of boxfish is composed of mineralized polygonal plates, called scutes, arranged in a complex geometric pattern and nearly completely encasing the body. To clarify whether or how this armor is maintained or altered with age, we quantify architectural aspects of the carapace of the longhorn cowfish Lactoria cornuta through ontogeny, using high‐resolution microCT data and segmentation algorithms to characterize the hundreds of scutes that cover each individual.

Bone mineral organization at the mesoscale: A review of mineral ellipsoids in bone and at bone interfaces

Much debate still revolves around bone architecture, especially at the nano- and microscale. Bone is a remarkable material where high strength and toughness coexist thanks to an optimized composition of mineral and protein and their hierarchical organization across several distinct length scales. At the nanoscale, mineralized collagen fibrils act as building block units. Despite their key role in biological and mechanical functions, the mechanisms of collagen mineralization and the precise arrangement of the organic and inorganic constituents in the fibrils remains not fully elucidated. Advances in three-dimensional (3D) characterization of mineralized bone tissue by focused ion beam-scanning electron microscopy (FIB-SEM) revealed mineral-rich regions geometrically approximated as prolate ellipsoids, much larger than single collagen fibrils. These structures have yet to become prominently recognized, studied, or adopted into biomechanical models of bone. However, they closely resemble the circular to elliptical features previously identified by scanning transmission electron microscopy (STEM) in two-dimensions (2D). Herein, we review the presence of mineral ellipsoids in bone as observed with electron-based imaging techniques in both 2D and 3D with particular focus on different species, anatomical locations, and in proximity to natural and synthetic biomaterial interfaces. This review reveals that mineral ellipsoids are a ubiquitous structure in all the bones and bone-implant interfaces analyzed. This largely overlooked hierarchical level is expected to bring different perspectives to our understanding of bone mineralization and mechanical properties, in turn shedding light on structure-function relationships in bone. STATEMENT OF SIGNIFICANCE: In bone, the hierarchical organization of organic (mainly collagen type I) and inorganic (calcium-phosphate mineral) components across several length scales contributes to a unique combination of strength and toughness. However, aspects related to the collagen-mineral organization and to mineralization mechanisms remain unclear. Here, we review the presence of mineral prolate ellipsoids across a variety of species, anatomical locations, and interfaces, both natural and with synthetic biomaterials. These mineral ellipsoids represent a largely unstudied feature in the organization of bone at the mesoscale, i.e., at a level connecting nano- and microscale. Thorough understanding of their origin, development, and structure can provide valuable insights into bone architecture and mineralization, assisting the treatment of bone diseases and the design of bio-inspired materials.

Hierarchical organization of bone in three dimensions: A twist of twists

Structural hierarchy of bone - observed across multiple scales and in three dimensions (3D) - is essential to its mechanical performance. While the mineralized extracellular matrix of bone consists predominantly of carbonate-substituted hydroxyapatite, type I collagen fibrils, water, and noncollagenous organic constituents (mainly proteins and small proteoglycans), it is largely the 3D arrangement of these inorganic and organic constituents at each length scale that endow bone with its exceptional mechanical properties. Focusing on recent volumetric imaging studies of bone at each of these scales - from the level of individual mineralized collagen fibrils to that of whole bones - this graphical review builds upon and re-emphasizes the original work of James Bell Pettigrew and D'Arcy Thompson who first described the ubiquity of spiral structure in Nature. Here we illustrate and discuss the omnipresence of twisted, curved, sinusoidal, coiled, spiraling, and braided motifs in bone in at least nine of its twelve hierarchical levels - a visualization undertaking that has not been possible until recently with advances in 3D imaging technologies (previous 2D imaging does not provide this information). From this perspective, we hypothesize that the twisting motif occurring across each hierarchical level of bone is directly linked to enhancement of function, rather than being simply an energetically favorable way to assemble mineralized matrix components. We propose that attentive consideration of twists in bone and the skeleton at different scales will likely develop, and will enhance our understanding of structure-function relationships in bone.