Initiation and early control of tissue regeneration - bone healing as a model system for tissue regeneration

Complex Spatio-Temporal Interplay of Distinct Immune and Bone Cell Subsets during Bone Fracture Healing

BACKGROUND

The healing of a bone injury is a highly complex process involving a multitude of different tissue and cell types, including immune cells, which play a major role in the initiation and progression of bone regeneration.

METHODS

We histologically analyzed the spatio-temporal occurrence of cells of the innate immune system (macrophages), the adaptive immune system (B and T lymphocytes), and bone cells (osteoblasts and osteoclasts) in the fracture area of a femoral osteotomy over the healing time. This study was performed in a bone osteotomy gap mouse model. We also investigated two key challenges of successful bone regeneration: hypoxia and revascularization.

RESULTS

Macrophages were present in and around the fracture gap throughout the entire healing period. The switch from initially pro-inflammatory M1 macrophages to the anti-inflammatory M2 phenotype coincided with the revascularization as well as the appearance of osteoblasts in the fracture area. This indicates that M2 macrophages are necessary for the restoration of vessels and that they also play an orchestrating role in osteoblastogenesis during bone healing. The presence of adaptive immune cells throughout the healing process emphasizes their essential role for regenerative processes that exceeds a mere pathogen defense. B and T cells co-localize consistently with bone cells throughout the healing process, consolidating their crucial role in guiding bone formation. These histological data provide, for the first time, comprehensive information about the complex interrelationships of the cellular network during the entire bone healing process in one standardized set up. With this, an overall picture of the spatio-temporal interplay of cellular key players in a bone healing scenario has been created.

CONCLUSIONS

A spatio-temporal distribution of immune cells, bone cells, and factors driving bone healing at time points that are decisive for this process-especially during the initial steps of inflammation and revascularization, as well as the soft and hard callus phases-has been visualized. The results show that the bone healing cascade does not consist of five distinct, consecutive phases but is a rather complex interrelated and continuous process of events, especially at the onset of healing.

Resorbable GBR Scaffolds in Oral and Maxillofacial Tissue Engineering: Design, Fabrication, and Applications

Guided bone regeneration (GBR) is a promising technique in bone tissue engineering that aims to replace lost or injured bone using resorbable scaffolds. The promotion of osteoblast adhesion, migration, and proliferation is greatly aided by GBR materials, and surface changes are critical in imitating the natural bone structure to improve cellular responses. Moreover, the interactions between bioresponsive scaffolds, growth factors (GFs), immune cells, and stromal progenitor cells are essential in promoting bone regeneration. This literature review comprehensively discusses various aspects of resorbable scaffolds in bone tissue engineering, encompassing scaffold design, materials, fabrication techniques, and advanced manufacturing methods, including three-dimensional printing. In addition, this review explores surface modifications to replicate native bone structures and their impact on cellular responses. Moreover, the mechanisms of bone regeneration are described, providing information on how immune cells, GFs, and bioresponsive scaffolds orchestrate tissue healing. Practical applications in clinical settings are presented to underscore the importance of these principles in promoting tissue integration, healing, and regeneration. Furthermore, this literature review delves into emerging areas of metamaterials and artificial intelligence applications in tissue engineering and regenerative medicine. These interdisciplinary approaches hold immense promise for furthering bone tissue engineering and improving therapeutic outcomes, leading to enhanced patient well-being. The potential of combining material science, advanced manufacturing, and cellular biology is showcased as a pathway to advance bone tissue engineering, addressing a variety of clinical needs and challenges. By providing this comprehensive narrative, a detailed, up-to-date account of resorbable scaffolds' role in bone tissue engineering and their transformative potential is offered.

Reduced angiogenesis and delayed endochondral ossification in CD163-/- mice highlights a role of M2 macrophages during bone fracture repair

While recent studies showed that macrophages are critical for bone fracture healing, and lack of M2 macrophages have been implicated in models of delayed union, functional roles for specific M2 receptors have yet to be defined. Moreover, the M2 scavenger receptor CD163 has been identified as a target to inhibit sepsis following implant-associated osteomyelitis, but potential adverse effects on bone healing during blockage therapy have yet to be explored. Thus, we investigated fracture healing in C57BL/6 versus CD163-/- mice using a well-established closed, stabilized, mid-diaphyseal femur fracture model. While gross fracture healing in CD163-/- mice was similar to that of C57BL/6, plain radiographs revealed persistent fracture gaps in the mutant mice on Day 14, which resolved by Day 21. Consistently, 3D vascular micro-CT demonstrated delayed union on Day 21, with reduced bone volume (74%, 61%, and 49%) and vasculature (40%, 40%, and 18%) compared to C57BL/6 on Days 10, 14, and 21 postfracture, respectively (p < 0.01). Histology confirmed large amounts of persistent cartilage in CD163-/- versus C57BL/6 fracture callus on Days 7 and 10 that resolves over time, and immunohistochemistry demonstrated deficiencies in CD206+ M2 macrophages. Torsion testing of the fractures confirmed the delayed early union in CD163-/- femurs, which display decreased yield torque on Day 21, and a decreased rigidity with a commensurate increase in rotation at yield on Day 28 (p < 0.01). Collectively, these results demonstrate that CD163 is required for normal angiogenesis, callus formation, and bone remodeling during fracture healing, and raise potential concerns about CD163 blockade therapy.

Intelligent Vascularized 3D/4D/5D/6D-Printed Tissue Scaffolds

Blood vessels are essential for nutrient and oxygen delivery and waste removal. Scaffold-repairing materials with functional vascular networks are widely used in bone tissue engineering. Additive manufacturing is a manufacturing technology that creates three-dimensional solids by stacking substances layer by layer, mainly including but not limited to 3D printing, but also 4D printing, 5D printing and 6D printing. It can be effectively combined with vascularization to meet the needs of vascularized tissue scaffolds by precisely tuning the mechanical structure and biological properties of smart vascular scaffolds. Herein, the development of neovascularization to vascularization to bone tissue engineering is systematically discussed in terms of the importance of vascularization to the tissue. Additionally, the research progress and future prospects of vascularized 3D printed scaffold materials are highlighted and presented in four categories: functional vascularized 3D printed scaffolds, cell-based vascularized 3D printed scaffolds, vascularized 3D printed scaffolds loaded with specific carriers and bionic vascularized 3D printed scaffolds. Finally, a brief review of vascularized additive manufacturing-tissue scaffolds in related tissues such as the vascular tissue engineering, cardiovascular system, skeletal muscle, soft tissue and a discussion of the challenges and development efforts leading to significant advances in intelligent vascularized tissue regeneration is presented.

A Novel Construct of Coral Granules-Poly-L-Lactic Acid Nanomembrane Sandwich Double Stem Cell Sheet Transplantation as Regenerative Therapy of Bone Defect Model

OBJECTIVES

We examined the use of a new approach in nanotechnology and stem cell research as regenerative therapy for bone tissue defects.

MATERIALS AND METHODS

We compared in vitro osteogenic potential of human Wharton jelly mesenchymal stem cells using coral granules and poly-L-lactic acid nanofiber according to proliferation (by cck-8 kit) and osteogenes (runt-related transcription factor 2, alkaline phosphatase, osteonectin) by quantitative reverse transcription-polymerase chain reaction, alkaline phosphatase assay, calcium measurement, and assessment of mineralization by Alizarin red and von Kossa staining. To overcome the limitations of natural coral, we made a modification by packaging the coral granules-human Wharton jelly mesenchymal stem cells by nanomembrane-human Wharton jelly mesenchymal stem cells to form sandwich double cell sheets and compared this hole with other holes (one was filled by human Wharton jelly mesenchymal stem cell suspension, and the other was filled by coral granules saturated with preinduced mesenchymal stem cells) by radiological and histopathological studies for repairing the bone gap.

RESULTS

Collagen-coated poly-L-lactic acid showed higher mRNA levels for all osteogenes (P < .001), higher alkaline phosphatase and calcium content (P < .001), and greater stainability. Our in vivo experiment showed that the holes implanted with sandwich double cell sheet-poly-L-lactic acid coral were completely filled mature compact bone. The holes implanted with human Wharton jelly mesenchymal stem cells alone were filled with immature compact bone. Holes implanted with coral granules-human Wharton jelly mesenchymal stem cells were filled with condensed connective tissue.

CONCLUSIONS

Poly-L-lactic acid nanofiber has greater osteogenic differentiating effect than the coral granules. The new approach of sDCS-PLLA-coral construct proved success for bone regeneration and repairing the bone gap and this may improve the design of tissue constructs for bone tissue regenerative therapy.

Recent advances in biofunctional guided bone regeneration materials for repairing defective alveolar and maxillofacial bone: A review

The anatomy of the oral and maxillofacial sites is complex, and bone defects caused by trauma, tumors, and inflammation in these zones are extremely difficult to repair. Among the most effective and reliable methods to attain osteogenesis, the guided bone regeneration (GBR) technique is extensively applied in defective oral and maxillofacial GBR. Furthermore, endowing biofunctions is crucial for GBR materials applied in repairing defective alveolar and maxillofacial bones. In this review, recent advances in designing and fabricating GBR materials applied in oral and maxillofacial sites are classified and discussed according to their biofunctions, including maintaining space for bone growth; facilitating the adhesion, migration, and proliferation of osteoblasts; facilitating the migration and differentiation of progenitor cells; promoting vascularization; providing immunoregulation to induce osteogenesis; suppressing infection; and effectively mimicking natural tissues using graded biomimetic materials. In addition, new processing strategies (e.g., 3D printing) and new design concepts (e.g., developing bone mimetic extracellular matrix niches and preparing scaffolds to suppress connective tissue to actively acquire space for bone regeneration), are particularly worthy of further study. In the future, GBR materials with richer biological functions are expected to be developed based on an in-depth understanding of the mechanism of bone-GBR-material interactions.

Engineering immunomodulatory hydrogels and cell-laden systems towards bone regeneration

The well-known synergetic interplay between the skeletal and immune systems has changed the design of advanced bone tissue engineering strategies. The immune system is essential during the bone lifetime, with macrophages playing multiple roles in bone healing and biomaterial integration. If in the past, the most valuable aspect of implants was to avoid immune responses of the host, nowadays, it is well-established how important are the crosstalks between immune cells and bone-engineered niches for an efficient regenerative process to occur. For that, it is essential to recapitulate the multiphenotypic cellular environment of bone tissue when designing new approaches. Indeed, the lack of osteoimmunomodulatory knowledge may be the explanation for the poor translation of biomaterials into clinical practice. Thus, smarter hydrogels incorporating immunomodulatory bioactive factors, stem cells, and immune cells are being proposed to develop a new generation of bone tissue engineering strategies. This review highlights the power of immune cells to upgrade the development of innovative engineered strategies, mainly focusing on orthopaedic and dental applications.

Local immune cell contributions to fracture healing in aged individuals - A novel role for interleukin 22

With increasing age, the risk of bone fractures increases while regenerative capacity decreases. This variation in healing potential appears to be linked to adaptive immunity, but the underlying mechanism is still unknown. This study sheds light on immunoaging/inflammaging, which impacts regenerative processes in aging individuals. In an aged preclinical model system, different levels of immunoaging were analyzed to identify key factors that connect immunoaged/inflammaged conditions with bone formation after long bone fracture. Immunological facets, progenitor cells, the microbiome, and confounders were monitored locally at the injury site and systemically in relation to healing outcomes in 12-month-old mice with distinct individual levels of immunoaging. Bone tissue formation during healing was delayed in the immunoaged group and could be associated with significant changes in cytokine levels. A prolonged and amplified pro-inflammatory reaction was caused by upregulated immune cell activation markers, increased chemokine receptor availability and a lack of inhibitory signaling. In immunoaged mice, interleukin-22 was identified as a core cell signaling protein that played a central role in delayed healing. Therapeutic neutralization of IL-22 reversed this specific immunoaging-related disturbed healing. Immunoaging was found to be an influencing factor of decreased regenerative capacity in aged individuals. Furthermore, a novel therapeutic strategy of neutralizing IL-22 may successfully rejuvenate healing in individuals with advanced immune experiences.

The specialist in regeneration-the Axolotl-a suitable model to study bone healing?

While the axolotl's ability to completely regenerate amputated limbs is well known and studied, the mechanism of axolotl bone fracture healing remains poorly understood. One reason might be the lack of a standardized fracture fixation in axolotl. We present a surgical technique to stabilize the osteotomized axolotl femur with a fixator plate and compare it to a non-stabilized osteotomy and to limb amputation. The healing outcome was evaluated 3 weeks, 3, 6 and 9 months post-surgery by microcomputer tomography, histology and immunohistochemistry. Plate-fixated femurs regained bone integrity more efficiently in comparison to the non-fixated osteotomized bone, where larger callus formed, possibly to compensate for the bone fragment misalignment. The healing of a non-critical osteotomy in axolotl was incomplete after 9 months, while amputated limbs efficiently restored bone length and structure. In axolotl amputated limbs, plate-fixated and non-fixated fractures, we observed accumulation of PCNA+ proliferating cells at 3 weeks post-injury similar to mouse. Additionally, as in mouse, SOX9-expressing cells appeared in the early phase of fracture healing and amputated limb regeneration in axolotl, preceding cartilage formation. This implicates endochondral ossification to be the probable mechanism of bone healing in axolotls. Altogether, the surgery with a standardized fixation technique demonstrated here allows for controlled axolotl bone healing experiments, facilitating their comparison to mammals (mice).

M2 macrophagy-derived exosomal miRNA-26a-5p induces osteogenic differentiation of bone mesenchymal stem cells

BACKGROUND

Bone marrow mesenchymal stem cells have always been a heated research topic in bone tissue regeneration and repair because of their self-renewal and multi-differentiation potential. A large number of studies have been focused on finding the inducing factors that will promote the osteogenic differentiation of bone marrow mesenchymal stem cells. Previous studies have shown that macrophage exosomes or miRNA-26a-5p can make it work, but the function of this kind of substance on cell osteogenic differentiation has not been public.

METHODS

M2 macrophages are obtained from IL-4 polarized bone marrow-derived macrophages. Exosomes were isolated from the supernatant of M2 macrophages and identified via transmission electron microscopy (TEM), western blotting, and DLS. Chondrogenic differentiation potential was detected by Alcian blue staining. Oil red O staining was used to detect the potential for lipogenic differentiation. And MTT would detect the proliferative capacity of cells. Western blot was performed to detect differential expression of osteogenic differentiation-related proteins.

RESULTS

The results showed that M2 macrophage exosomes will promote bone differentiation and at the same time inhibit lipid differentiation. In addition, M2 macrophage-derived exosomes have the function of promoting the expression of SOX and Aggrecan suppressing the level of MMP13. The exosome inhibitor GW4689 suppresses miRNA-26a-5p in M2 macrophage exosomes, and the treated exosomes do not play an important role in promoting bone differentiation. Moreover, miRNA-26a-5p can enable to promote bone differentiation and inhibit lipid differentiation. miRNA-26a-5p can promote the expression of ALP (alkaline phosphatase), RUNX-2 (Runt-related transcription factor 2), OPN(osteopontin), and Col-2(collagen type II). Therefore, it is speculated that exosomal miRNA-26a-5p is indispensable in osteogenic differentiation.

CONCLUSIONS

The present study indicated that M2 macrophage exosomes carrying miRNA-26a-5p can induce osteogenic differentiation of bone marrow-derived stem cells to inhibit lipogenic differentiation, and miRNA-26a-5p will also promote the expression of osteogenic differentiation-related proteins ALP, RUNX-2, OPN, and Col-2.

Functional buffering via cell-specific gene expression promotes tissue homeostasis and cancer robustness

Functional buffering that ensures biological robustness is critical for maintaining tissue homeostasis, organismal survival, and evolution of novelty. However, the mechanism underlying functional buffering, particularly in multicellular organisms, remains largely elusive. Here, we proposed that functional buffering can be mediated via expression of buffering genes in specific cells and tissues, by which we named Cell-specific Expression-BUffering (CEBU). We developed an inference index (C-score) for CEBU by computing C-scores across 684 human cell lines using genome-wide CRISPR screens and transcriptomic RNA-seq. We report that C-score-identified putative buffering gene pairs are enriched for members of the same duplicated gene family, pathway, and protein complex. Furthermore, CEBU is especially prevalent in tissues of low regenerative capacity (e.g., bone and neuronal tissues) and is weakest in highly regenerative blood cells, linking functional buffering to tissue regeneration. Clinically, the buffering capacity enabled by CEBU can help predict patient survival for multiple cancers. Our results suggest CEBU as a potential buffering mechanism contributing to tissue homeostasis and cancer robustness in humans.

Effect of data binning and frame averaging for micro-CT image acquisition on the morphometric outcome of bone repair assessment

Despite the current advances in micro-CT analysis, the influence of some image acquisition parameters on the morphometric assessment outcome have not been fully elucidated. The aim of this study was to determine whether data binning and frame averaging affect the morphometric outcome of bone repair assessment using micro-CT. Four Wistar rats' tibiae with a surgically created bone defect were imaged with micro-CT six times each, frame averaging set to 1 and 2, and data binning set to 1, 2 and 4, for each of the averaging values. Two-way ANOVA followed by Bonferroni tests assessed the significance of frame averaging and data binning on a set of morphometric parameters assessed in the image volumes (p < 0.01). The effect of frame averaging was not significant for any of the assessed parameters. Increased data binning led to larger trabecular thickness. In contrast, smaller bone volume fraction and bone volume were found as data binning increased. Trabeculae number and trabecular separation were not influenced by any of the parameters. In conclusion, the morphometric outcome of bone repair assessment in micro-CT demonstrated dependency upon data binning, but not frame averaging. Therefore, image acquisition of small anatomical structures (e.g., rat trabeculae) should be performed without data binning.

[Microcirculation in fractures of long bones of the lower extremity]

The study objective was to investigate soft tissue microcirculation in the area of a lower limb long bone fracture. We studied fractures of the lower limb long bones from 0 to 7 days of age in 17 males and 37 females. The mean age of patients was 57.33±2.53 years. Histological examination of soft tissues from the area of the lower limb long bone fracture and at the distance from the fracture was performed. Standard hematoxylin and eosin staining was used. The thickness and diameter of arterioles, venules, and capillaries were evaluated by morphometry of the sections made. The observed pattern reflects the response of the microcirculation of the soft tissues to the mechanical impact. The relative ratio of the artery diameter to the small-caliber vein diameter was 1.01±0.13, and the artery wall thickness to the vein wall thickness was 2.03±0.28, which can be considered a natural reaction of the microcirculation blood vessels to the mechanical impact in 1-1.5 weeks after the formed fracture of the lower limb long bone.

The Role of Long Non-Coding RNAs and Circular RNAs in Bone Regeneration: Modulating MiRNAs Function

Although bone is a self-healing organ and is able to repair and restore most fractures, large bone fractures, about 10%, are not repairable. Bone grafting, as a gold standard, and bone tissue engineering using biomaterials, growth factors, and stem cells have been developed to restore large bone defects. Since bone regeneration is a complex and multiple-step process and the majority of the human genome, about 98%, is composed of the non-protein-coding regions, non-coding RNAs (ncRNAs) play essential roles in bone regeneration. Recent studies demonstrated that long ncRNAs (lncRNAs) and circular RNAs (circRNAs), as members of ncRNAs, are widely involved in bone regeneration by interaction with microRNAs (miRNAs) and constructing a lncRNA or circRNA/miRNA/mRNA regulatory network. The constructed network regulates the differentiation of stem cells into osteoblasts and their commitment to osteogenesis. This review will present the structure and biogenesis of lncRNAs and circRNAs, the mechanism of bone repair, and the bone tissue engineering in bone defects. Finally, we will discuss the role of lncRNAs and circRNAs in osteogenesis and bone fracture healing through constructing various lncRNA or circRNA/miRNA/mRNA networks and the involved pathways. This article is protected by copyright. All rights reserved.

Development and validation of a modularized external fixator for generating standardized fracture healing micromotions in rats

Aims

To fully verify the reliability and reproducibility of an experimental method in generating standardized micromotion for the rat femur fracture model.

Methods

A modularized experimental device has been developed that allows rat models to be used instead of large animal models, with the aim of reducing systematic errors and time and money constraints on grouping. The bench test was used to determine the difference between the measured and set values of the micromotion produced by this device under different simulated loading weights. The displacement of the fixator under different loading conditions was measured by compression tests, which was used to simulate the unexpected micromotion caused by the rat’s ambulation. In vivo preliminary experiments with a small sample size were used to test the feasibility and effectiveness of the whole experimental scheme and surgical scheme.

Results

The bench test showed that a weight loading < 500 g did not affect the operation of experimental device. The compression test demonstrated that the stiffness of the device was sufficient to keep the uncontrollable motion between fracture ends, resulting from the rat’s daily activities, within 1% strain. In vivo results on 15 rats prove that the device works reliably, without overburdening the experimental animals, and provides standardized micromotion reproductively at the fracture site according to the set parameters.

Conclusion

Our device was able to investigate the effect of micromotion parameters on fracture healing by generating standardized micromotion to small animal models.

Cite this article: Bone Joint Res 2021;10(11):714–722.

The multifaceted roles of macrophages in bone regeneration: A story of polarization, activation and time

To present knowledge, macrophages are found in all tissues of the human body. They are a cell population with high plasticity which come with a multitude of functions which appear to be adapted to the respective tissue niche and micro-environment in which they reside. Bone harbors multiple macrophage subpopulations, with the osteoclasts as classical representative of a bone resorbing cells and osteomacs as a bone tissue resident macrophage first described by the expression of F4/80. Both subtypes are found throughout all phases in bone healing. In vivo data on bone regeneration have demonstrated their essential role in initiating the healing cascade (inflammatory phase) but also of the later phases of healing (e.g. endochondral and intramembranous bone formation). To participate in such diverse processes macrophages have to be highly plastic in their functionality. Thus, the widely used M1/M2 paradigm to distinguish macrophage subpopulations may not mirror the comprehensive role of the dynamics of macrophage plasticity. From a clinical perspective it is especially relevant to distinguish what drives macrophages in impaired healing scenarios, implant loosening or infections, where their specific role of a misbalanced inflammatory setting is so far only partially known. With this review we aim at illustrating current knowledge and gaps of knowledge on macrophage plasticity and function during the cascades of regeneration and reconstitution of bone tissue. We propose aspects of the known biological mechanisms of macrophages and their specific subsets that might serve as targets to control their function in impaired healing and eventually support a scar-free regeneration. STATEMENT OF SIGNIFICANCE: Macrophages are essential for successful regeneration. In scar-free healing such as in bone, a complete failure of healing was shown if macrophages were depleted; the M1/M2 switch appears to be key to the progression from pro-inflammation to regeneration. However, experimental data illustrate that the classical M1/M2 paradigm does not completely mirror the complexity of observed macrophage functions during bone healing and thus demands a broader perspective. Within this review we discuss the high degree of plasticity of macrophages and the relevant contribution of the different and more specific M2 subtypes (M2a-M2f) during (bone) regeneration. It summarizes the versatile roles of macrophages in skeletal regeneration and thereby highlights potential target points for immunomodulatory approaches to enable or even foster bone repair.

Mice Lacking the Calcitonin Receptor Do Not Display Improved Bone Healing

Despite significant advances in surgical techniques, treatment options for impaired bone healing are still limited. Inadequate bone regeneration is not only associated with pain, prolonged immobilization and often multiple revision surgeries, but also with high socioeconomic costs, underlining the importance of a detailed understanding of the bone healing process. In this regard, we previously showed that mice lacking the calcitonin receptor (CTR) display increased bone formation mediated through the increased osteoclastic secretion of sphingosine-1-phosphate (S1P), an osteoanabolic molecule promoting osteoblast function. Although strong evidence is now available for the crucial role of osteoclast-to-osteoblast coupling in normal bone hemostasis, the relevance of this paracrine crosstalk during bone regeneration is unknown. Therefore, our study was designed to test whether increased osteoclast-to-osteoblast coupling, as observed in CTR-deficient mice, may positively affect bone repair. In a standardized femoral osteotomy model, global CTR-deficient mice displayed no alteration in radiologic callus parameters. Likewise, static histomorphometry demonstrated moderate impairment of callus microstructure and normal osseous bridging of osteotomy ends. In conclusion, bone regeneration is not accelerated in CTR-deficient mice, and contrary to its osteoanabolic action in normal bone turnover, osteoclast-to-osteoblast coupling specifically involving the CTR-S1P axis, may only be of minor relevance during bone healing.

[The association of stromal mesenchymal cells with the bone substitute for the treatment of non-union of long bones, an alternative to autologous spongy grafts: a case report]

Bone is the most transplanted human tissue. Surgical interventions for bone repair are necessary for various pathologies such as nonunion, osteoporosis or osteonecrosis. Although autologous bone grafts remain the benchmark for bone regeneration, they unfortunately have a number of disadvantages: the need for a second intervention and the limited amount of tissue removed. Synthetic bone substitutes overcome some of these drawbacks, but their osteoinductive properties do not make it possible to treat significant bone losses. Cellular therapies based on mesenchymal stromal stem cells (MSC) in combination with bone substitutes may be alternatives to autologous bone grafting. It is in this context that we report the case of a patient with congenital dysplasia treated for non-union of the femur. The association of mesenchymal stem cells with bone substitute allowed us to obtain consolidation after 6 months.

Heterogeneity of the osteocyte lacuno-canalicular network architecture and material characteristics across different tissue types in healing bone

Various tissue types, including fibrous connective tissue, bone marrow, cartilage, woven and lamellar bone, coexist in healing bone. Similar to most bone tissue type, healing bone contains a lacuno-canalicular network (LCN) housing osteocytes. These cells are known to orchestrate bone remodeling in healthy bone by sensing mechanical strains and translating them into biochemical signals. The structure of the LCN is hypothesized to influence mineralization processes. Hence, the aim of the present study was to visualize and match spatial variations in the LCN topology with mineral characteristics, within and at the interfaces of the different tissue types that comprise healing bone. We applied a correlative multi-method approach to visualize the LCN architecture and quantify mineral particle size and orientation within healing femoral bone in a mouse osteotomy model (26 weeks old C57BL/6 mice). This approach revealed structural differences across several length scales during endochondral ossification within the following regions: calcified cartilage, bony callus, cortical bone and a transition zone between the cortical and callus region analyzed 21 days after the osteotomy. In this transition zone, we observed a continuous convergence of mineral characteristics and osteocyte lacunae shape as well as discontinuities in the lacunae volume and LCN connectivity. The bony callus exhibits a 34% higher lacunae number density and 40% larger lacunar volume compared to cortical bone. The presented correlations between LCN architecture and mineral characteristics improves our understanding of how bone develops during healing and may indicate a contribution of osteocytes to bone (re)modeling.

The development and in vivo validation of an external fixation device with standardized micromotion for accelerating fracture healing

Micromotion can accelerate fracture healing, with critical parameters being range of motion, frequency of motion, duration of motion, as well as initial timing of the motion. However, these parameters of micromotion have not been optimized. It is because in previous studies large animals were used. The displacement among fracture fragments caused by animal activity brings a considerable systemic error to experimental data. Also, the sample size is limited by time and cost. Thus, the rat with femur fracture can be a good animal model in investigating this problem as its advantages on high consistency of experimental results, short convalescence, and low maintenance cost. The challenge in using a small animal model in the micromotion study include 1) highly specific stiffness of the fixator; 2) lightweight fixator to bring less interference to animal's activity; 3) high accuracy on measurement method. This study aims to solve this problem by integrating 1) an aluminum fixator with a solid construction; 2) a modularized experimental device with dismountable parts; 3) a non-contact measurement model based on video identification technology. Our preliminary validation results confirmed the reliability and reproducibility of the external fixation device used in the investigation on the effect of applied micromotion on bone healing.

In Vivo Validation of Spray-Dried Mesoporous Bioactive Glass Microspheres Acting as Prolonged Local Release Systems for BMP-2 to Support Bone Regeneration

Bone morphogenetic protein-2 (BMP-2) is a known key mediator of physiological bone regeneration and is clinically approved for selected musculoskeletal interventions. Yet, broad usage of this growth factor is impeded due to side effects that are majorly evoked by high dosages and burst release kinetics. In this study, mesoporous bioactive glass microspheres (MBGs), produced by an aerosol-assisted spray-drying scalable process, were loaded with BMP-2 resulting in prolonged, low-dose BMP-2 release without affecting the material characteristics. In vitro, MBGs were found to be cytocompatible and to induce a pro-osteogenic response in primary human mesenchymal stromal cells (MSCs). In a pre-clinical rodent model, BMP-2 loaded MBGs significantly enhanced bone formation and influenced the microarchitecture of newly formed bone. The MBG carriers alone performed equal to the untreated (empty) control in most parameters tested, while additionally exerting mild pro-angiogenic effects. Using MBGs as a biocompatible, pro-regenerative carrier for local and sustained low dose BMP-2 release could limit side effects, thus enabling a safer usage of BMP-2 as a potent pro-osteogenic growth factor.

Nanostructured Biomaterials for Bone Regeneration

This review article addresses the various aspects of nano-biomaterials used in or being pursued for the purpose of promoting bone regeneration. In the last decade, significant growth in the fields of polymer sciences, nanotechnology, and biotechnology has resulted in the development of new nano-biomaterials. These are extensively explored as drug delivery carriers and as implantable devices. At the interface of nanomaterials and biological systems, the organic and synthetic worlds have merged over the past two decades, forming a new scientific field incorporating nano-material design for biological applications. For this field to evolve, there is a need to understand the dynamic forces and molecular components that shape these interactions and influence function, while also considering safety. While there is still much to learn about the bio-physicochemical interactions at the interface, we are at a point where pockets of accumulated knowledge can provide a conceptual framework to guide further exploration and inform future product development. This review is intended as a resource for academics, scientists, and physicians working in the field of orthopedics and bone repair.

M2 Macrophagy-derived exosomal miRNA-5106 induces bone mesenchymal stem cells towards osteoblastic fate by targeting salt-inducible kinase 2 and 3

BACKGROUND

Osteoblast differentiation is a vital process for fracture healing, and exosomes are nanosized membrane vesicles that can deliver therapeutic drugs easily and safely. Macrophages participate in the regulation of various biological processes in vivo, and macrophage-derived exosomes (MD-Exos) have recently been a topic of increasing research interest. However, few study has explored the link between MD-Exos and osteoblast differentiation. Herein, we sought to identify miRNAs differentially expressed between M1 and M2 macrophage-derived exosomes, and to evaluate their roles in the context of osteoblast differentiation.

RESULTS

We found that microRNA-5106 (miR-5106) was significantly overexpressed in M2 macrophage-derived exosomes (M2D-Exos), while its expression was decreased in M1 macrophage-derived exosomes (M1D-Exos), and we found that this exosomal miRNA can induce bone mesenchymal stem cell (BMSC) osteogenic differentiation via directly targeting the Salt-inducible kinase 2 and 3 (SIK2 and SIK3) genes. In addition, the local injection of both a miR-5106 agonist or M2D-Exos to fracture sites was sufficient to accelerate healing in vivo.

CONCLUSIONS

Our study demonstrates that miR-5106 is highly enriched in M2D-Exos, and that it can be transferred to BMSCs wherein it targets SIK2 and SIK3 genes to promote osteoblast differentiation.

Early pH Changes in Musculoskeletal Tissues upon Injury-Aerobic Catabolic Pathway Activity Linked to Inter-Individual Differences in Local pH

Local pH is stated to acidify after bone fracture. However, the time course and degree of acidification remain unknown. Whether the acidification pattern within a fracture hematoma is applicable to adjacent muscle hematoma or is exclusive to this regenerative tissue has not been studied to date. Thus, in this study, we aimed to unravel the extent and pattern of acidification in vivo during the early phase post musculoskeletal injury. Local pH changes after fracture and muscle trauma were measured simultaneously in two pre-clinical animal models (sheep/rats) immediately after and up to 48 h post injury. The rat fracture hematoma was further analyzed histologically and metabolomically. In vivo pH measurements in bone and muscle hematoma revealed a local acidification in both animal models, yielding mean pH values in rats of 6.69 and 6.89, with pronounced intra- and inter-individual differences. The metabolomic analysis of the hematomas indicated a link between reduction in tricarboxylic acid cycle activity and pH, thus, metabolic activity within the injured tissues could be causative for the different pH values. The significant acidification within the early musculoskeletal hematoma could enable the employment of the pH for novel, sought-after treatments that allow for spatially and temporally controlled drug release.

Limbostomy: Longitudinal Intravital Microendoscopy in Murine Osteotomies

Bone healing involves the interplay of immune cells, mesenchymal cells, and vasculature over the time course of regeneration. Approaches to quantify the spatiotemporal aspects of bone healing at cellular resolution during long bone healing do not yet exist. Here, a novel technique termed Limbostomy is presented, which combines intravital microendoscopy with an osteotomy. This design allows a modular combination of an internal fixator plate with a gradient refractive index (GRIN) lens at various depths in the bone marrow and can be combined with a surgical osteotomy procedure. The field of view (FOV) covers a significant area of the fracture gap and allows monitoring cellular processes in vivo. The GRIN lens causes intrinsic optical aberrations which have to be corrected. The optical system was characterized and a postprocessing algorithm was developed. It corrects for wave front aberration-induced image plane deformation and for background and noise signals, enabling us to observe subcellular processes. Exemplarily, we quantitatively and qualitatively analyze angiogenesis in bone regeneration. We make use of a transgenic reporter mouse strain with nucleargreen fluorescent protein and membrane-bound tdTomato under the Cadherin-5 promoter. We observe two phases of vascularization. First, rapid vessel sprouting pervades the FOV within 3-4 days after osteotomy. Second, the vessel network continues to be dynamically remodeled until the end of our observation time, 14 days after surgery. Limbostomy opens a unique set of opportunities and allows further insight on spatiotemporal aspects of bone marrow biology, for example, hematopoiesis, analysis of cellular niches, immunological memory, and vascularization in the bone marrow during health and disease. © 2020 The Authors. Cytometry Part A published by Wiley Periodicals, Inc. on behalf of International Society for Advancement of Cytometry.

Spatial Distribution of Macrophages During Callus Formation and Maturation Reveals Close Crosstalk Between Macrophages and Newly Forming Vessels

Macrophages are essential players in the process of fracture healing, acting by remodeling of the extracellular matrix and enabling vascularization. Whilst activated macrophages of M1-like phenotype are present in the initial pro-inflammatory phase of hours to days of fracture healing, an anti-inflammatory M2-like macrophage phenotype is supposed to be crucial for the induction of downstream cascades of healing, especially the initiation of vascularization. In a mouse-osteotomy model, we provide a comprehensive characterization of vessel (CD31⁺, Emcn⁺) and macrophage phenotypes (F4/80, CD206, CD80, Mac-2) during the process of fracture healing. To this end, we phenotype the phases of vascular regeneration-the expansion phase (d1-d7 after injury) and the remodeling phase of the endothelial network, until tissue integrity is restored (d14-d21 after injury). Vessels which appear during the bone formation process resemble type H endothelium (CD31^hiEmcn^hi), and are closely connected to osteoprogenitors (Runx2⁺, Osx⁺) and F4/80⁺ macrophages. M1-like macrophages are present in the initial phase of vascularization until day 3 post osteotomy, but they are rare during later regeneration phases. M2-like macrophages localize mainly extramedullary, and CD206⁺ macrophages are found to express Mac-2⁺ during the expansion phase. VEGFA expression is initiated by CD80⁺ cells, including F4/80⁺ macrophages, until day 3, while subsequently osteoblasts and chondrocytes are main contributors to VEGFA production at the fracture site. Using Longitudinal Intravital Microendoscopy of the Bone (LIMB) we observe changes in the motility and organization of CX3CR1⁺ cells, which infiltrate the injury site after an osteotomy. A transient accumulation, resulting in spatial polarization of both, endothelial cells and macrophages, in regions distal to the fracture site, is evident. Immunofluorescence histology followed by histocytometric analysis reveals that F4/80⁺CX3CR1⁺ myeloid cells precede vascularization.

Compromised Bone Healing in Aged Rats Is Associated With Impaired M2 Macrophage Function

Fracture repair is initiated by a multitude of immune cells and induction of an inflammatory cascade. Alterations in the early healing response due to an aged adaptive immune system leads to impaired bone repair, delayed healing or even formation of non-union. However, immuno-senescence is not limited to the adaptive immunity, but is also described for macrophages, main effector cells from the innate immune system. Beside regulation of pro- and anti-inflammatory signaling, macrophages contribute to angiogenesis and granulation tissue maturation. Thus, it seems likely that an altered macrophage function due to aging may affect bone repair at various stages and contribute to age related deficiencies in bone regeneration. To prove this hypothesis, we analyzed the expression of macrophage markers and angiogenic factors in the early bone hematoma derived from young and aged osteotomized Spraque Dawley rats. We detected an overall reduced expression of the monocyte/pan-macrophage markers CD14 and CD68 in aged rats. Furthermore, the analysis revealed an impaired expression of anti-inflammatory M2 macrophage markers in hematoma from aged animals that was connected to a diminished revascularization of the bone callus. To verify that the age related disturbed bone regeneration was due to a compromised macrophage function, CD14+ macrophage precursors were transplanted locally into the osteotomy gap of aged rats. Transplantation rescued bone regeneration partially after 6 weeks, demonstrated by a significantly induced deposition of new bone tissue, reduced fibrosis and significantly improved callus vascularization.

Impact of Bacterial Infections on Osteogenesis: Evidence From In Vivo Studies

The clinical impact of bacterial infections on bone regeneration has been incompletely quantified and documented. As a result, controversy exists about the optimal treatment strategy to maximize healing of a contaminated defect. Animal models are extremely useful in this respect, as they can elucidate how a bacterial burden influences quantitative healing of various types of defects relative to non-infected controls. Moreover, they may demonstrate how antibacterial treatment and/or bone grafting techniques facilitate the osteogenic response in the harsh environment of a bacterial infection. Finally, it a well-known contradiction that osteomyelitis is characterized by uncontrolled bone remodeling and bone loss, but at the same time, it can be associated with excessive new bone apposition. Animal studies can provide a better understanding of how osteolytic and osteogenic responses are related to each other during infection. This review discusses the in vivo impact of bacterial infection on osteogenesis by addressing the following questions (i) How does osteomyelitis affect the radiographic bone appearance? (ii) What is the influence of bacterial infection on histological bone healing? (iii) How do bacterial infections affect quantitative bone healing? (iv) What is the effect of antibacterial treatment on the healing outcome during infection? (v) What is the efficacy of osteoinductive proteins in infected bones? (vi) What is the balance between the osteoclastic and osteoblastic response during bacterial infections? (vii) What is the mechanism of the observed pro-osteogenic response as observed in osteomyelitis? © 2019 The Authors. Journal of Orthopaedic Research^© published by Wiley Periodicals, Inc. on behalf of Orthopaedic Research Society. J Orthop Res 37:2067-2076, 2019.

Individual Effector/Regulator T Cell Ratios Impact Bone Regeneration

There is increasing evidence that T lymphocytes play a key role in controlling endogenous regeneration. Regeneration appears to be impaired in case of local accumulation of CD8+ effector T cells (T_EFF), impairing endogenous regeneration by increasing a primary “useful” inflammation toward a damaging level. Thus, rescuing regeneration by regulating the heightened pro-inflammatory reaction employing regulatory CD4+ T (T_Reg) cells could represent an immunomodulatory option to enhance healing. Hypothesis was that CD4+ T_Reg might counteract undesired effects of CD8+ T_EFF. Using adoptive T_Reg transfer, bone healing was consistently improved in mice possessing an inexperienced immune system with low amounts of CD8+ T_EFF. In contrast, mice with an experienced immune system (high amounts of CD8+ T_EFF) showed heterogeneous bone repair with regeneration being dependent upon the individual T_EFF/T_Reg ratio. Thus, the healing outcome can only be improved by an adoptive T_Reg therapy, if an unfavorable T_EFF/T_Reg ratio can be reshaped; if the individual CD8+ T_EFF percentage, which is dependent on the individual immune experience can be changed toward a favorable ratio by the T_Reg transfer. Remarkably, also in patients with impaired fracture healing the T_EFF/T_Reg ratio was higher compared to uneventful healers, validating our finding in the mouse osteotomy model. Our data demonstrate for the first time the key-role of a balanced T_EFF/T_Reg response following injury needed to reach successful regeneration using bone as a model system. Considering this strategy, novel opportunities for immunotherapy in patients, which are at risk for impaired healing by targeting T_EFF cells and supporting T_Reg cells to enhance healing are possible.

The Addition of High Doses of Hyaluronic Acid to a Biphasic Bone Substitute Decreases the Proinflammatory Tissue Response

Biphasic bone substitutes (BBS) are currently well-established biomaterials. Through their constant development, even natural components like hyaluronic acid (HY) have been added to improve both their handling and also their regenerative properties. However, little knowledge exists regarding the consequences of the addition of HY to their biocompatibility and the inflammatory tissue reactions. Thus, the present study was conducted, aiming to analyze the influence of two different amounts of high molecular weight HY (HMWHY), combined with a BBS, on in vitro biocompatibility and in vivo tissue reaction. Established in vitro procedures, using L929 cells, were used for cytocompatibility analyses under the test conditions of DIN EN:ISO 10993-5. For the in vivo part of the study, calvarial defects were created in 20 Wistar rats and subsequently filled with BBS, and BBS combined with two different HMWHY amounts, i.e., BBS + HY(L) and BBS + HY(H). As controls, empty defects were used. Established histological, immunohistochemical, and histomorphometrical methods were applied to analyze the tissue reactions to the three different materials, including the induction of pro- and anti-inflammatory macrophages and multinucleated giant cells (BMGCs). The in vitro results showed that none of the materials or compositions caused biological damage to the L929 cells and can be considered to be non-toxic. The in vivo results showed that only the addition of high doses of HY to a biphasic bone substitute significantly decreases the occurrence of pro-inflammatory macrophages (* p < 0.05), comparable to the numbers found in the control group, while no significant differences within the three study groups for M2-macrophages nor BMGCs were detected. In conclusion, the addition of different amounts of HMWHY does not seem to affect the inflammation response to BBS, while improving the material handling properties.

Immune Modulation to Enhance Bone Healing-A New Concept to Induce Bone Using Prostacyclin to Locally Modulate Immunity

Within an aging population, fracture incidences will rise and with the augmented risks of impaired healing the overall risk of delayed bone regeneration will substantially increase in elderly patients. Thus, new strategies to rescue fracture healing in the elderly are highly warranted. Modulating the initial inflammatory phase toward a reduced pro-inflammation launches new treatment options for delayed or impaired healing specifically in the elderly. Here, we evaluated the capacity of the prostacyclin analog Iloprost to modulate the inflammatory phase toward a pro-regenerative milieu using in vitro as well as in vivo model systems. In vitro, Iloprost administration led to a downregulation of potential unfavorable CD8+ cytotoxic T cells as well as their pro-inflammatory cytokine secretion profile. Furthermore, Iloprost increased the mineralization capacity of osteogenic induced mesenchymal stromal cells through both direct as well as indirect cues. In an in vivo approach, Iloprost, embedded in a biphasic fibrin scaffold, decreased the pro-inflammatory and simultaneously enhanced the anti-inflammatory phase thereby improving bone healing outcome. Overall, our presented data confirms a possible strategy to modulate the early inflammatory phase in aged individuals toward a physiological healing by a downregulation of an excessive pro-inflammation that otherwise would impair healing. Further confirmation in phase I/II trials, however, is needed to validate the concept in a broader clinical evaluation.

The effect of the new hemostatic agent Ostene® on bone healing: An experimental study in rabbits

Introduction: Ostene® is a water-soluble wax-like alkylene oxide copolymer preparation for use as a mechanical hemostatic agent. This study aims to evaluate the effects of Ostene® on bone healing. Materials and Methods: Twenty albino rabbits were divided into four groups according to post-treatment follow-up (24 hr, 3 days, 7 days, 14 days) with five rabbits in each group. Each rabbit in all groups was treated with two study materials (Ostene® and Gelfoam®). Three holes were made in the mandibular bone of each rabbit using 5mm surgical bur; two holes were made on right side: one for testing Ostene® and another for Gelfoam®. A third hole, on the left side of mandible, was not treated, and was used as a control. Finally, the incision was closed. The specimens were collected at different days post-treatment and examined by histopathology. Result and Discussion: This study showed that there is a significant difference (p-value≤ 0.05) between the Ostene® group and the other groups (Gelfoam® and control). At 24 hr post intervention, there is a significant difference in osteoblast cell formation (p-value=0.03), and osteoclast cell formation (p-value=0.05). New blood vessel formation, osteoblast and osteoclast cell formation for Ostene® group at 3 days post-intervention were also significantly different (p-values = 0.05, 0.03, 0.04, respectively). At 7 days post-intervention p-values were 0.05 for osteoblast formation and 0.04 for osteoclast formation, respectively. After 14 days of healing p-value for osteoblast cell formation in the Ostene® group was 0.05 and 0.04 for osteoclast cell formation. Conclusions: The bone hemostatic agent Ostene® is an effective at enhancing osteogenesis by initiating proliferation of osteoblast and osteoclast cells.

Immune Cell Induced Migration of Osteoprogenitor Cells Is Mediated by TGF-β Dependent Upregulation of NOX4 and Activation of Focal Adhesion Kinase

The cytokines secreted by immune cells have a large impact on the tissue, surrounding a fracture, e.g., by attraction of osteoprogenitor cells. However, the underlying mechanisms are not yet fully understood. Thus, this study aims at investigating molecular mechanisms of the immune cell-mediated migration of immature primary human osteoblasts (phOBs), with transforming growth factor beta (TGF-β), nicotinamide adenine dinucleotide phosphate (NADPH) oxidase 4 (NOX4) and focal adhesion kinase (FAK) as possible regulators. Monocyte- and macrophage (THP-1 cells ± phorbol 12-myristate 13-acetate (PMA) treatment)-conditioned media, other than the granulocyte-conditioned medium (HL-60 cells + dimethyl sulfoxide (DMSO) treatment), induce migration of phOBs. Monocyte- and macrophage (THP-1 cells)-conditioned media activate Smad3-dependent TGF-β signaling in the phOBs. Stimulation with TGF-β promotes migration of phOBs. Furthermore, TGF-β treatment strongly induces NOX4 expression on both mRNA and protein levels. The associated reactive oxygen species (ROS) accumulation results in phosphorylation (Y397) of FAK. Blocking TGF-β signaling, NOX4 activity and FAK signaling effectively inhibits the migration of phOBs towards TGF-β. In summary, our data suggest that monocytic- and macrophage-like cells induce migration of phOBs in a TGF-β-dependent manner, with TGF-β-dependent induction of NOX4, associated production of ROS and resulting activation of FAK as key mediators.

A review of biomaterials in bone defect healing, remaining shortcomings and future opportunities for bone tissue engineering: The unsolved challenge

Despite its intrinsic ability to regenerate form and function after injury, bone tissue can be challenged by a multitude of pathological conditions. While innovative approaches have helped to unravel the cascades of bone healing, this knowledge has so far not improved the clinical outcomes of bone defect treatment. Recent findings have allowed us to gain in-depth knowledge about the physiological conditions and biological principles of bone regeneration. Now it is time to transfer the lessons learned from bone healing to the challenging scenarios in defects and employ innovative technologies to enable biomaterial-based strategies for bone defect healing. This review aims to provide an overview on endogenous cascades of bone material formation and how these are transferred to new perspectives in biomaterial-driven approaches in bone regeneration.

Cite this article: T. Winkler, F. A. Sass, G. N. Duda, K. Schmidt-Bleek. A review of biomaterials in bone defect healing, remaining shortcomings and future opportunities for bone tissue engineering: The unsolved challenge. Bone Joint Res 2018;7:232–243. DOI: 10.1302/2046-3758.73.BJR-2017-0270.R1.

Clinical and Research Approaches to Treat Non-union Fracture

PURPOSE OF REVIEW

Impaired healing outcomes or even non-unions after bone injury are still a highly relevant problem in the daily clinical life. Especially within an aging population, the occurrence of bone fractures increases and thus novel treatment approaches to overcome compromised bone regeneration are needed.

RECENT FINDINGS

The gold standard to treat delayed or non-healing bone injuries is still the use of autologous bone grafts to foster regeneration. Besides its successful treatment outcome, it also has disadvantages: a second surgery is needed in order to harvest the bone material and the material is highly limited. Looking into the recent literature, a multitude of different research approaches were already conducted to identify new possible strategies to treat impaired bone regeneration: application of mesenchymal stromal cells, platelet lysates, growth factors, interference in the immune system, or bone formation stimulation by ultrasound. This review gives an overview of the treatment approaches actually performed in the clinic as well as at the bench in the context of compromised bone healing. It clearly highlights the complexity of the nature of non-healing bone fractures as well as patient-dependent factors influencing the healing process.

Immunology Guides Skeletal Muscle Regeneration

Soft tissue trauma of skeletal muscle is one of the most common side effects in surgery. Muscle injuries are not only caused by accident-related injuries but can also be of an iatrogenic nature as they occur during surgical interventions when the anatomical region of interest is exposed. If the extent of trauma surpasses the intrinsic regenerative capacities, signs of fatty degeneration and formation of fibrotic scar tissue can occur, and, consequentially, muscle function deteriorates or is diminished. Despite research efforts to investigate the physiological healing cascade following trauma, our understanding of the early onset of healing and how it potentially determines success or failure is still only fragmentary. This review focuses on the initial physiological pathways following skeletal muscle trauma in comparison to bone and tendon trauma and what conclusions can be drawn from new scientific insights for the development of novel therapeutic strategies. Strategies to support regeneration of muscle tissue after injury are scarce, even though muscle trauma has a high incidence. Based on tissue specific differences, possible clinical treatment options such as local immune-modulatory and cell therapeutic approaches are suggested that aim to support the endogenous regenerative potential of injured muscle tissues.

The Metabolic Microenvironment Steers Bone Tissue Regeneration

Over the past years, basic findings in cancer research have revealed metabolic symbiosis between different cell types to cope with high energy demands under limited nutrient availability. Although this also applies to regenerating tissues with disrupted physiological nutrient and oxygen supply, the impact of this metabolic cooperation and metabolic reprogramming on cellular development, fate, and function during tissue regeneration has widely been neglected so far. With this review, we aim to provide a schematic overview on metabolic links that have a high potential to drive tissue regeneration. As bone is, aside from liver, the only tissue that can regenerate without excessive scar tissue formation, we will use bone healing as an exemplarily model system.

Osteotomy models - the current status on pain scoring and management in small rodents

Fracture healing is a complex regeneration process which produces new bone tissue without scar formation. However, fracture healing disorders occur in approximately 10% of human patients and cause severe pain and reduced quality of life. Recently, the development of more standardized, sophisticated and commercially available osteosynthesis techniques reflecting clinical approaches has increased the use of small rodents such as rats and mice in bone healing research dramatically. Nevertheless, there is no standard for pain assessment, especially in these species, and consequently limited information regarding the welfare aspects of osteotomy models. Moreover, the selection of analgesics is restricted for osteotomy models since non-steroidal anti-inflammatory drugs (NSAIDs) are known to affect the initial, inflammatory phase of bone healing. Therefore, opioids such as buprenorphine and tramadol are often used. However, dosage data in the literature are varied. Within this review, we clarify the background of osteotomy models, explain the current status and challenges of animal welfare assessment, and provide an example score sheet including model specific parameters. Furthermore, we summarize current refinement options and present a brief outlook on further 3R research.

Macrophages in bone fracture healing: Their essential role in endochondral ossification

In fracture healing, skeletal and immune system are closely interacting through common cell precursors and molecular mediators. It is thought that the initial inflammatory reaction, which involves migration of macrophages into the fracture area, has a major impact on the long term outcome of bone repair. Interestingly, macrophages reside during all stages of fracture healing. Thus, we hypothesized a critical role for macrophages in the subsequent phases of bone regeneration. This study examined the impact of in vivo induced macrophage reduction, using clodronate liposomes, on the different healing phases of bone repair in a murine model of a standard closed femoral fracture. A reduction in macrophages had no obvious effect on the early fracture healing phase, but resulted in a delayed hard callus formation, thus severely altering endochondral ossification. Clodronate treated animals clearly showed delayed bony consolidation of cartilage and enhanced periosteal bone formation. Therefore, we decided to backtrack macrophage distribution during fracture healing in non-treated mice, focusing on the identification of the M1 and M2 subsets. We observed that M2 macrophages were clearly prevalent during the ossification phase. Therefore enhancement of M2 phenotype in macrophages was investigated as a way to further bone healing. Induction of M2 macrophages through interleukin 4 and 13 significantly enhanced bone formation during the 3week investigation period. These cumulative data illustrate their so far unreported highly important role in endochondral ossification and the necessity of a fine balance in M1/M2 macrophage function, which appears mandatory to fracture healing and successful regeneration.

CD31hiEmcnhi Vessels Support New Trabecular Bone Formation at the Frontier Growth Area in the Bone Defect Repair Process

CD31hiEmcnhi vessels were a subtype of vessels in the murine skeletal system, with high levels of platelet and endothelial cell adhesion molecule-1 (PECAM-1/CD31) and endomucin (Emcn). They were reported coupling angiogenesis and osteogenesis during bone development. We investigated the distribution of these vessels in rat tibiae and their temporal and spatial distribution during the bone defect repair process to improve our understanding of the importance of these vessels. We confirmed that CD31hiEmcnhi vessels were specially distributed around the trabecular bones near metaphysis and endosteum in rat tibiae. At 3 days post bone injury, CD31hiEmcnhi vessels proliferated and were extensively distributed across the entire repair area. At 7 and 14 days post-injury, these vessels decreased but were specially distributed around the growing trabecular bones near the frontier growth area, suggesting that these vessels support new bone formation. The distribution of CD31hiEmcnhi vessels and the transcriptions of Hif-1α and VEGFA, as well as BMP2 and Osterix decreased at 7 and 14 days post-injury under osteoporotic conditions, in combination with insufficient osteogenesis. Our research is of great significance to help understand the important role of CD31hiEmcnhi vessels in supporting new trabecular bones formation during bone defect repair process.

The involvement and possible mechanism of pro-inflammatory tumor necrosis factor alpha (TNF-α) in thoracic ossification of the ligamentum flavum

Thoracic ossification of the ligamentum flavum (TOLF) is characterized by ectopic bone formation in the ligamentum flavum and is considered to be a leading cause of thoracic spinal canal stenosis and myelopathy. However, the underlying etiology is not well understood. An iTRAQ proteomics was used to reveal the involvement of inflammation factors in TOLF. TNF-α is a pro-inflammatory cytokine implicated in the pathogenesis of many human diseases. Protein profiling analysis showed that the protein level of TNF-α increased in the ossified ligamentum flavum of TOLF, which was confirmed by western blot. The effects of TNF-α on primary ligamentum flavum cells was examined. Cell proliferation assay demonstrated that primary cells from the ossified ligamentum flavum of TOLF grew faster than the control. Flow cytometry assay indicated that the proportions of cells in S phase of cell cycle of primary cells increased after TNF-α stimulation. To address the effect of TNF-α on gene expression, primary cells were derived from ligamentum flavum of TOLF patients. Culture cells were stimulated by TNF-α. RNA was isolated and analyzed by quantitative RT-PCR. G1/S-specific proteins cyclin D1 and c-Myc were upregulated after TNF-α stimulation. On the other hand, osteoblast differentiation related genes such as Bmp2 and Osterix (Osx) were upregulated in the presence of TNF-α. TNF-α activated Osx expression in a dose-dependent manner. Interestingly, a specific mitogen-activated protein kinase ERK inhibitor U0126, but not JNK kinase inhibitor SP600125, abrogated TNF-α activation of Osx expression. This suggests that TNF-α activates Osx expression through the mitogen-activated protein kinase ERK pathway. Taken together, we provide the evidence to support that TNF-α involves in TOLF probably through regulating cell proliferation via cyclin D1 and c-Myc, and promoting osteoblast differentiation via Osx.

Bone fracture healing in mechanobiological modeling: A review of principles and methods

Bone fracture is a very common body injury. The healing process is physiologically complex, involving both biological and mechanical aspects. Following a fracture, cell migration, cell/tissue differentiation, tissue synthesis, and cytokine and growth factor release occur, regulated by the mechanical environment. Over the past decade, bone healing simulation and modeling has been employed to understand its details and mechanisms, to investigate specific clinical questions, and to design healing strategies. The goal of this effort is to review the history and the most recent work in bone healing simulations with an emphasis on both biological and mechanical properties. Therefore, we provide a brief review of the biology of bone fracture repair, followed by an outline of the key growth factors and mechanical factors influencing it. We then compare different methodologies of bone healing simulation, including conceptual modeling (qualitative modeling of bone healing to understand the general mechanisms), biological modeling (considering only the biological factors and processes), and mechanobiological modeling (considering both biological aspects and mechanical environment). Finally we evaluate different components and clinical applications of bone healing simulation such as mechanical stimuli, phases of bone healing, and angiogenesis.

T Lymphocytes Influence the Mineralization Process of Bone

Bone is a unique organ able to regenerate itself after injuries. This regeneration requires the local interplay between different biological systems such as inflammation and matrix formation. Structural reconstitution is initiated by an inflammatory response orchestrated by the host immune system. However, the individual role of T cells and B cells in regeneration and their relationship to bone tissue reconstitution remain unknown. Comparing bone and fracture healing in animals with and without mature T and B cells revealed the essential role of these immune cells in determining the tissue mineralization and thus the bone quality. Bone without mature T and B cells is stiffer when compared to wild-type bone thus lacking the elasticity that helps to absorb forces, thus preventing fractures. In-depth analysis showed dysregulations in collagen deposition and osteoblast distribution upon lack of mature T and B cells. These changes in matrix deposition have been correlated with T cells rather than B cells within this study. This work presents, for the first time, a direct link between immune cells and matrix formation during bone healing after fracture. It illustrates specifically the role of T cells in the collagen organization process and the lack thereof in the absence of T cells.

Multiscale Modeling of Bone Healing: Toward a Systems Biology Approach

Bone is a living part of the body that can, in most situations, heal itself after fracture. However, in some situations, healing may fail. Compromised conditions, such as large bone defects, aging, immuno-deficiency, or genetic disorders, might lead to delayed or non-unions. Treatment strategies for those conditions remain a clinical challenge, emphasizing the need to better understand the mechanisms behind endogenous bone regeneration. Bone healing is a complex process that involves the coordination of multiple events at different length and time scales. Computer models have been able to provide great insights into the interactions occurring within and across the different scales (organ, tissue, cellular, intracellular) using different modeling approaches [partial differential equations (PDEs), agent-based models, and finite element techniques]. In this review, we summarize the latest advances in computer models of bone healing with a focus on multiscale approaches and how they have contributed to understand the emergence of tissue formation patterns as a result of processes taking place at the lower length scales.

CD31+ Cells From Peripheral Blood Facilitate Bone Regeneration in Biologically Impaired Conditions Through Combined Effects on Immunomodulation and Angiogenesis

Controlled revascularization and inflammation are key elements regulating endogenous regeneration after (bone) tissue trauma. Peripheral blood-derived cell subsets, such as regulatory T-helper cells and circulating (endothelial) progenitor cells, respectively, can support endogenous tissue healing, whereas effector T cells that are associated with an aged immune system can hinder bone regeneration. CD31 is expressed by diverse leukocytes and is well recognized as a marker of circulating endothelial (precursor) cells; however, CD31 is absent from the surface of differentiated effector T cells. Thus, we hypothesized that by separating the inhibitory fractions from the supportive fractions of circulating cells within the peripheral blood (PB) using the CD31 marker, bone regeneration in biologically compromised conditions, such as those observed in aged patients, could be improved. In support of our hypothesis, we detected an inverse correlation between CD31+ cells and effector T cells in the hematomas of human fracture patients, dependent on the age of the patient. Furthermore, we demonstrated the regenerative capacity of human PB-CD31+ cells in vitro. These findings were translated to a clinically relevant rat model of impaired bone healing. The transplantation of rat PB-CD31+ cells advanced bone tissue restoration in vivo and was associated with an early anti-inflammatory response, the stimulation of (re)vascularization, and reduced fibrosis. Interestingly, the depletion or enrichment of the highly abundant CD31+/14+ monocytes from the mixed CD31+ cell population diminished tissue regeneration at different levels, suggesting combined effects within the PB-CD31+ subsets. In summary, an intraoperative enrichment of PB-CD31+ cells might be a novel option to facilitate endogenous regeneration under biologically impaired situations by supporting immunomodulation and vascularization. © 2016 American Society for Bone and Mineral Research.

Bone regeneration strategies with bone marrow stromal cells in orthopaedic surgery

Bone is the most transplanted tissue human with 1 million procedures every year in Europe. Surgical interventions for bone repair are required for varied reasons such as trauma resulting non-union fractures, or diseases including osteoporosis or osteonecrosis. Autologous bone grafting is the gold standard in bone regeneration but it requires a second surgery with associated pain and complications, and is also limited by harvested bone quantity. Synthetic bone substitutes lack the osteoinductive properties to heal large bone defects. Cell therapies based on bone marrow or ex vivo expanded mesenchymal stromal stem cells (MSCs) in association with synthetic calcium phosphate (CaP) bone substitutes may be alternatives to autologous bone grafting. This manuscript reviews the different conventional biological and synthetic bone grafting procedures as well as the more recently introduced cell therapy approaches used in orthopaedic surgery for bone regeneration. Some clinical studies have demonstrated safety and efficacy of these approaches but regeneration of large bone defects remain challenging due to the absence of rapid and adequate vascularisation. Future directions in the field of bone regeneration are presented, such as testing alternative cell sources or in situ fabrication of vascularized bone grafts in patients.

Proinflammatory T cells and IL-17 stimulate osteoblast differentiation

The local immune response is important to consider when the aim is to improve bone regeneration. Recently T lymphocytes and their associated cytokines have been identified as regulators in fracture callus formation, but it is not known whether T cells affect bone progenitor cells directly. The goal of this in vitro study was to investigate the role of different T cell subsets and their secreted factors on the osteogenic differentiation of human mesenchymal stem cells (MSCs). Significant increases in the alkaline phosphatase activity and the subsequent matrix mineralization by MSCs were found after their exposure to activated T cells or activated T cell-derived conditioned medium. Blocking IFN-γ in the conditioned medium abolished its pro-osteogenic effect, while blocking TGF-β further enhanced osteogenesis. The relative contribution of an anti- or proinflammatory T cell phenotype in MSC osteogenic differentiation was studied next. Enrichment of the fraction of anti-inflammatory regulatory T cells had no beneficial osteogenic effect. In contrast, soluble factors derived from enriched T helper 17 cells upregulated the expression of osteogenic markers by MSCs. IL-17A, and IL-17F, their main proinflammatory cytokines, similarly exhibited strong osteogenic effects when exposed directly to MSCs. IL-17A in particular showed a synergistic action together with bone morphogenetic protein 2. These results indicate that individual T cell subsets, following their activation, affect osteoblast maturation in a different manner through the production of soluble factors. From all T cells, the proinflammatory T cells, including the T helper 17 cells, are most stimulatory for osteogenesis.