Healing of a Large Long-Bone Defect through Serum-Free In Vitro Priming of Human Periosteum-Derived Cells

Treatment of Bone Defects and Nonunion via Novel Delivery Mechanisms, Growth Factors, and Stem Cells: A Review

Bone nonunion following a fracture represents a significant global healthcare challenge, with an overall incidence ranging between 2 and 10% of all fractures. The management of nonunion is not only financially prohibitive but often necessitates invasive surgical interventions. This comprehensive manuscript aims to provide an extensive review of the published literature involving growth factors, stem cells, and novel delivery mechanisms for the treatment of fracture nonunion. Key growth factors involved in bone healing have been extensively studied, including bone morphogenic protein (BMP), vascular endothelial growth factor (VEGF), and platelet-derived growth factor. This review includes both preclinical and clinical studies that evaluated the role of growth factors in acute and chronic nonunion. Overall, these studies revealed promising bridging and fracture union rates but also elucidated complications such as heterotopic ossification and inferior mechanical properties associated with chronic nonunion. Stem cells, particularly mesenchymal stem cells (MSCs), are an extensively studied topic in the treatment of nonunion. A literature search identified articles that demonstrated improved healing responses, osteogenic capacity, and vascularization of fractures due to the presence of MSCs. Furthermore, this review addresses novel mechanisms and materials being researched to deliver these growth factors and stem cells to nonunion sites, including natural/synthetic polymers and bioceramics. The specific mechanisms explored in this review include BMP-induced osteoblast differentiation, VEGF-mediated angiogenesis, and the role of MSCs in multilineage differentiation and paracrine signaling. While these therapeutic modalities exhibit substantial preclinical promise in treating fracture nonunion, there remains a need for further research, particularly in chronic nonunion and large animal models. This paper seeks to identify such translational hurdles which must be addressed in order to progress the aforementioned treatments from the lab to the clinical setting.

Upscaling Osteoclast Generation by Enhancing Macrophage Aggregation Using Hollow Microgels

Osteoclasts, the bone resorbing cells of hematopoietic origin formed by macrophage fusion, are essential in bone health and disease. However, in vitro research on osteoclasts remains challenging due to heterogeneous cultures that only contain a few multinucleated osteoclasts. Indeed, a strategy to generate homogeneous populations of multinucleated osteoclasts in a scalable manner has remained elusive. Here, the investigation focuses on whether microencapsulation of human macrophages in microfluidically generated hollow, sacrificial tyramine-conjugated dextran (Dex-TA) microgels could facilitate macrophage precursor aggregation and formation of multinucleated osteoclasts. Therefore, human mononuclear cells are isolated from buffy coats and differentiated toward macrophages. Macrophages are encapsulated in microgels using flow focus microfluidics and outside-in enzymatic oxidative phenolic crosslinking, and differentiated toward osteoclasts. Morphology, viability, and osteoclast fusion of microencapsulated cells are assessed. Furthermore, microgels are degraded to allow cell sorting of released cells based on osteoclastic marker expression. The successful encapsulation and osteoclast formation of human macrophages in Dex-TA microgels are reported for the first time using high-throughput droplet microfluidics. Intriguingly, osteoclast formation within these 3D microenvironments occurs at a significantly higher level compared to the conventional 2D culture system. Furthermore, the feasibility of establishing a pure osteoclast culture from cell transfer and release from degradable microgels is demonstrated.

Role of Adipose-Derived Mesenchymal Stem Cells in Bone Regeneration

Bone regeneration involves multiple factors such as tissue interactions, an inflammatory response, and vessel formation. In the event of diseases, old age, lifestyle, or trauma, bone regeneration can be impaired which could result in a prolonged healing duration or requiring an external intervention for repair. Currently, bone grafts hold the golden standard for bone regeneration. However, several limitations hinder its clinical applications, e.g., donor site morbidity, an insufficient tissue volume, and uncertain post-operative outcomes. Bone tissue engineering, involving stem cells seeded onto scaffolds, has thus been a promising treatment alternative for bone regeneration. Adipose-derived mesenchymal stem cells (AD-MSCs) are known to hold therapeutic value for the treatment of various clinical conditions and have displayed feasibility and significant effectiveness due to their ease of isolation, non-invasive, abundance in quantity, and osteogenic capacity. Notably, in vitro studies showed AD-MSCs holding a high proliferation capacity, multi-differentiation potential through the release of a variety of factors, and extracellular vesicles, allowing them to repair damaged tissues. In vivo and clinical studies showed AD-MSCs favoring better vascularization and the integration of the scaffolds, while the presence of scaffolds has enhanced the osteogenesis potential of AD-MSCs, thus yielding optimal bone formation outcomes. Effective bone regeneration requires the interplay of both AD-MSCs and scaffolds (material, pore size) to improve the osteogenic and vasculogenic capacity. This review presents the advances and applications of AD-MSCs for bone regeneration and bone tissue engineering, focusing on the in vitro, in vivo, and clinical studies involving AD-MSCs for bone tissue engineering.

Towards Stem Cell Therapy for Critical-Sized Segmental Bone Defects: Current Trends and Challenges on the Path to Clinical Translation

The management and reconstruction of critical-sized segmental bone defects remain a major clinical challenge for orthopaedic clinicians and surgeons. In particular, regenerative medicine approaches that involve incorporating stem cells within tissue engineering scaffolds have great promise for fracture management. This narrative review focuses on the primary components of bone tissue engineering-stem cells, scaffolds, the microenvironment, and vascularisation-addressing current advances and translational and regulatory challenges in the current landscape of stem cell therapy for critical-sized bone defects. To comprehensively explore this research area and offer insights for future treatment options in orthopaedic surgery, we have examined the latest developments and advancements in bone tissue engineering, focusing on those of clinical relevance in recent years. Finally, we present a forward-looking perspective on using stem cells in bone tissue engineering for critical-sized segmental bone defects.

Cell membrane vesicles derived from hBMSCs and hUVECs enhance bone regeneration

Bone tissue renewal can be enhanced through co-transplantation of bone mesenchymal stem cells (BMSCs) and vascular endothelial cells (ECs). However, there are apparent limitations in stem cell-based therapy which hinder its clinic translation. Hence, we investigated the potential of alternative stem cell substitutes for facilitating bone regeneration. In this study, we successfully prepared cell membrane vesicles (CMVs) from BMSCs and ECs. The results showed that BMSC-derived cell membrane vesicles (BMSC-CMVs) possessed membrane receptors involved in juxtacrine signaling and growth factors derived from their parental cells. EC-derived cell membrane vesicles (EC-CMVs) also contained BMP2 and VEGF derived from their parental cells. BMSC-CMVs enhanced tube formation and migration ability of hUVECs, while EC-CMVs promoted the osteogenic differentiation of hBMSCs in vitro. Using a rat skull defect model, we found that co-transplantation of BMSC-CMVs and EC-CMVs could stimulate angiogenesis and bone formation in vivo. Therefore, our research might provide an innovative and feasible approach for cell-free therapy in bone tissue regeneration.

Bone Organoids: Recent Advances and Future Challenges

Bone defects stemming from tumorous growths, traumatic events, and diverse conditions present a profound conundrum in clinical practice and research. While bone has the inherent ability to regenerate, substantial bone anomalies require bone regeneration techniques. Bone organoids represent a new concept in this field, involving the 3D self-assembly of bone-associated stem cells guided in vitro with or without extracellular matrix material, resulting in a tissue that mimics the structural, functional, and genetic properties of native bone tissue. Within the scientific panorama, bone organoids ascend to an esteemed status, securing significant experimental endorsement. Through a synthesis of current literature and pioneering studies, this review offers a comprehensive survey of the bone organoid paradigm, delves into the quintessential architecture and ontogeny of bone, and highlights the latest progress in bone organoid fabrication. Further, existing challenges and prospective directions for future research are identified, advocating for interdisciplinary collaboration to fully harness the potential of this burgeoning domain. Conclusively, as bone organoid technology continues to mature, its implications for both clinical and research landscapes are poised to be profound.

Single-Step Biofabrication of In Situ Spheroid-Forming Compartmentalized Hydrogel for Clinical-Sized Cartilage Tissue Formation

3D cellular spheroids offer more biomimetic microenvironments than conventional 2D cell culture technologies, which has proven value for many tissue engineering applications. Despite beneficiary effects of 3D cell culture, clinical translation of spheroid tissue engineering is challenged by limited scalability of current spheroid formation methods. Although recent adoption of droplet microfluidics can provide a continuous production process, use of oils and surfactants, generally low throughput, and requirement of additional biofabrication steps hinder clinical translation of spheroid culture. Here, the use of clean (e.g., oil-free and surfactant-free), ultra-high throughput (e.g., 8.5 mL min-1 , 10 000 spheroids s-1 ), single-step, in-air microfluidic biofabrication of spheroid forming compartmentalized hydrogels is reported. This novel technique can reliably produce 1D fibers, 2D planes, and 3D volumes compartmentalized hydrogel constructs, which each allows for distinct (an)isotropic orientation of hollow spheroid-forming compartments. Spheroids produced within ink-jet bioprinted compartmentalized hydrogels outperform 2D cell cultures in terms of chondrogenic behavior. Moreover, the cellular spheroids can be harvested from compartmentalized hydrogels and used to build shape-stable centimeter-sized biomaterial-free living tissues in a bottom-up manner. Consequently, it is anticipated that in-air microfluidic production of spheroid-forming compartmentalized hydrogels can advance production and use of cellular spheroids for various biomedical applications.

Engineered phalangeal grafts for children with symbrachydactyly: A proof of concept

Tissue engineering approaches hold great promise in the field of regenerative medicine, especially in the context of pediatric applications, where ideal grafts need to restore the function of the targeted tissue and consider growth. In the present study, we aimed to develop a protocol to engineer autologous phalangeal grafts of relevant size for children suffering from symbrachydactyly. This condition results in hands with short fingers and missing bones. A previously-described, developmentally-inspired strategy based on endochondral ossification (ECO)-the main pathway leading to bone and bone marrow development-and adipose derived-stromal cells (ASCs) as the source of chondroprogenitor was used. First, we demonstrated that pediatric ASCs associated with collagen sponges can generate hypertrophic cartilage tissues (HCTs) in vitro that remodel into bone tissue in vivo via ECO. Second, we developed and optimized an in vitro protocol to generate HCTs in the shape of small phalangeal bones (108-390 mm3) using freshly isolated adult cells from the stromal vascular fraction (SVF) of adipose tissue, associated with two commercially available large collagen scaffolds (Zimmer Plug® and Optimaix 3D®). We showed that after 12 weeks of in vivo implantation in an immunocompromised mouse model such upscaled grafts remodeled into bone organs (including bone marrow tissues) retaining the defined shape and size. Finally, we replicated similar outcome (albeit with a slight reduction in cartilage and bone formation) by using minimally expanded pediatric ASCs (3 × 106 cells per grafts) in the same in vitro and in vivo settings, thereby validating the compatibility of our pediatric phalanx engineering strategy with a clinically relevant scenario. Taken together, these results represent a proof of concept of an autologous approach to generate osteogenic phalangeal grafts of pertinent clinical size, using ASCs in children born with symbrachydactyly, despite a limited amount of tissue available from pediatric patients.

Bioengineered cartilaginous grafts for repairing segmental mandibular defects

Reconstructing critical-sized craniofacial bone defects is a global healthcare challenge. Current methods, like autologous bone transplantation, face limitations. Bone tissue engineering offers an alternative to autologous bone, with traditional approaches focusing on stimulating osteogenesis via the intramembranous ossification (IMO) pathway. However, IMO falls short in addressing larger defects, particularly in clinical scenarios where there is insufficient vascularisation. This review explores redirecting bone regeneration through endochondral ossification (ECO), a process observed in long bone healing stimulated by hypoxic conditions. Despite its promise, gaps exist in applying ECO to bone tissue engineering experiments, requiring the elucidation of key aspects such as cell sources, biomaterials and priming protocols. This review discusses various scaffold biomaterials and cellular sources for chondrogenesis and hypertrophic chondrocyte priming, mirroring the ECO pathway. The review highlights challenges in current endochondral priming and proposes alternative approaches. Emphasis is on segmental mandibular defect repair, offering insights for future research and clinical application. This concise review aims to advance bone tissue engineering by addressing critical gaps in ECO strategies.

3D-Printed Osteoinductive Polymeric Scaffolds with Optimized Architecture to Repair a Sheep Metatarsal Critical-Size Bone Defect

The reconstruction of critical-size bone defects in long bones remains a challenge for clinicians. A new osteoinductive medical device is developed here for long bone repair by combining a 3D-printed architectured cylindrical scaffold made of clinical-grade polylactic acid (PLA) with a polyelectrolyte film coating delivering the osteogenic bone morphogenetic protein 2 (BMP-2). This film-coated scaffold is used to repair a sheep metatarsal 25-mm long critical-size bone defect. In vitro and in vivo biocompatibility of the film-coated PLA material is proved according to ISO standards. Scaffold geometry is found to influence BMP-2 incorporation. Bone regeneration is followed using X-ray scans, µCT scans, and histology. It is shown that scaffold internal geometry, notably pore shape, influenced bone regeneration, which is homogenous longitudinally. Scaffolds with cubic pores of ≈870 µm and a low BMP-2 dose of ≈120 µg cm-3 induce the best bone regeneration without any adverse effects. The visual score given by clinicians during animal follow-up is found to be an easy way to predict bone regeneration. This work opens perspectives for a clinical application in personalized bone regeneration.

Potential of periosteal cells in bone and cartilage regeneration: a systematic review

Introduction: The unavailability of adequate human primary cells presents multiple challenges in terms of bone and cartilage regeneration and disease modeling experiments in vitro. Periosteal cells (PCs), which represent promising skeletal stem cell sources, could be a promising strategy in tissue engineering. The present study aimed to summarize the characteristics of PCs to investigate the efficacy of these cells in bone and cartilage regeneration in different models, paying special attention to the comparison of bone marrow stromal cells (BMSCs). Methods: A comprehensive literature search was conducted in Embase, PubMed/MEDLINE, Web of Science, and Scopus for articles published in English until April 2023. Only original researches in which PCs were employed for bone or cartilage regeneration experiments were included. Results: A total of 9140 references were retrieved. After screening the results, 36 publications were considered to be eligible for inclusion in the present literature review. Overall, PCs demonstrated beneficial bone and cartilage regenerative efficacy compared to the bare scaffold since almost all included studies reported positive results. The 9 studies assessing the differences in bone formation capacity between PCs and BMSCs indicated that PCs exhibited stronger in vivo osteogenic differentiation capabilities compared to BMSCs, while the other study demonstrated stronger chondrogenic potential of BMSCs. Discussion: PCs demonstrated beneficial to bone regenerative efficacy compared to the bare scaffold with a low risk of most studies included. However, the cartilage formation capacity of BMSCs still needs to be investigated due to the limited research available and the certain risk of bias. PCs exhibited higher osteogenic capabilities compared to BMSCs in combination with various scaffolds in vivo with good evidence. Further researches are needed to elucidate the comparative benefits of cartilage regeneration. Systematic Review Registration: https://www.crd.york.ac.uk/prospero/display_record.php?ID=CRD42023411522, CRD42023411522.

Stirred culture of cartilaginous microtissues promotes chondrogenic hypertrophy through exposure to intermittent shear stress

Cartilage microtissues are promising tissue modules for bottom up biofabrication of implants leading to bone defect regeneration. Hitherto, most of the protocols for the development of these cartilaginous microtissues have been carried out in static setups, however, for achieving higher scales, dynamic process needs to be investigated. In the present study, we explored the impact of suspension culture on the cartilage microtissues in a novel stirred microbioreactor system. To study the effect of the process shear stress, experiments with three different impeller velocities were carried out. Moreover, we used mathematical modeling to estimate the magnitude of shear stress on the individual microtissues during dynamic culture. Identification of appropriate mixing intensity allowed dynamic bioreactor culture of the microtissues for up to 14 days maintaining microtissue suspension. Dynamic culture did not affect microtissue viability, although lower proliferation was observed as opposed to the statically cultured ones. However, when assessing cell differentiation, gene expression values showed significant upregulation of both Indian Hedgehog (IHH) and collagen type X (COLX), well known markers of chondrogenic hypertrophy, for the dynamically cultured microtissues. Exometabolomics analysis revealed similarly distinct metabolic profiles between static and dynamic conditions. Dynamic cultured microtissues showed a higher glycolytic profile compared with the statically cultured ones while several amino acids such as proline and aspartate exhibited significant differences. Furthermore, in vivo implantations proved that microtissues cultured in dynamic conditions are functional and able to undergo endochondral ossification. Our work demonstrated a suspension differentiation process for the production of cartilaginous microtissues, revealing that shear stress resulted to an acceleration of differentiation towards hypertrophic cartilage.

The synovial environment steers cartilage deterioration and regeneration

Osteoarthritis (OA) was recently defined as an epidemic, and the lack of effective treatment is highly correlated to the limited knowledge regarding the underlying pathophysiology. Failure to regenerate upon trauma is thought to be one of the underlying causes for degenerative diseases, including OA. To investigate why lesions within an OA environment fail to heal, a heterogeneous cell population was isolated from the synovial fluid (SF) of OA patients. The cells' ability to undergo processes required for functional tissue regeneration was evaluated in the presence or absence of autologous SF. The obtained mechanistic findings were then used for the development of an immunomodulatory cell treatment, aimed to restore the pro-regenerative environment. Intra-articular injection in a clinical compassionate use study showed that the treatment restored the articular cartilage and joint homeostasis of OA patients. These findings confirm the role of pro-regenerative immune cells and their targeted influence on progenitor cells for degenerative joint disease therapies.

Periosteal Skeletal Stem and Progenitor Cells in Bone Regeneration

PURPOSE OF REVIEW

The periosteum, the outer layer of bone, is a major source of skeletal stem/progenitor cells (SSPCs) for bone repair. Here, we discuss recent findings on the characterization, role, and regulation of periosteal SSPCs (pSSPCs) during bone regeneration.

RECENT FINDINGS

Several markers have been described for pSSPCs but lack tissue specificity. In vivo lineage tracing and transcriptomic analyses have improved our understanding of pSSPC functions during bone regeneration. Bone injury activates pSSPCs that migrate, proliferate, and have the unique potential to form both bone and cartilage. The injury response of pSSPCs is controlled by many signaling pathways including BMP, FGF, Notch, and Wnt, their metabolic state, and their interactions with the blood clot, nerve fibers, blood vessels, and macrophages in the fracture environment. Periosteal SSPCs are essential for bone regeneration. Despite recent advances, further studies are required to elucidate pSSPC heterogeneity and plasticity that make them a central component of the fracture healing process and a prime target for clinical applications.

Progress of Periosteal Osteogenesis: The Prospect of In Vivo Bioreactor

Repairing large segment bone defects is still a clinical challenge. Bone tissue prefabrication shows great translational potentials and has been gradually accepted clinically. Existing bone reconstruction strategies, including autologous periosteal graft, allogeneic periosteal transplantation, xenogeneic periosteal transplantation, and periosteal cell tissue engineering, are all clinically valuable treatments and have made significant progress in research. Herein, we reviewed the research progress of these techniques and briefly explained the relationship among in vivo microenvironment, mechanical force, and periosteum osteogenesis. Moreover, we also highlighted the importance of the critical role of periosteum in osteogenesis and explained current challenges and future perspective.

Advances in Regenerative Sports Medicine Research

Regenerative sports medicine aims to address sports and aging-related conditions in the locomotor system using techniques that induce tissue regeneration. It also involves the treatment of meniscus and ligament injuries in the knee, Achilles’ tendon ruptures, rotator cuff tears, and cartilage and bone defects in various joints, as well as the regeneration of tendon–bone and cartilage–bone interfaces. There has been considerable progress in this field in recent years, resulting in promising steps toward the development of improved treatments as well as the identification of conundrums that require further targeted research. In this review the regeneration techniques currently considered optimal for each area of regenerative sports medicine have been reviewed and the time required for feasible clinical translation has been assessed. This review also provides insights into the direction of future efforts to minimize the gap between basic research and clinical applications.

Coculture of Endothelial and Stromal Cells to Promote Concurrent Osteogenesis and Vasculogenesis

A key challenge in the treatment of large bone defects is the need to provide an adequate and stable vascular supply as new tissue develops. Bone tissue engineering applies selected biomaterials and cell types to create an environment that promotes tissue formation, maturation, and remodeling. Mesenchymal stromal cells (MSCs) have been widely used in these strategies because of their established effects on bone formation, and their ability to act as stabilizing pericytes that support vascular regeneration by endothelial cells (ECs). However, the creation of vascularized bone tissue in vitro requires coupling of osteogenesis and vasculogenesis in a three-dimensional (3D) biomaterial environment. In the present study, 3D fibrin hydrogels containing MSCs and ECs were prevascularized in vitro for 7 days to create an endothelial network in the matrix, and were subsequently cultured for a further 14 days under either continued vasculogenic stimulus, a combination of vasculogenic and osteogenic (hybrid) stimulus, or only osteogenic stimulus. It was found that ECs produced robust vessel networks in 3D fibrin matrices over 7 days of culture, and these networks continued to expand over the 14-day treatment period under vasculogenic conditions. Culture in hybrid medium resulted in maintenance of vessel networks for 14 days, while osteogenic culture abrogated vessel formation. These trends were mirrored in data representing overall cell viability and cell number in the 3D fibrin constructs. MSCs were found to colocalize with EC networks under vasculogenic and hybrid conditions, suggesting pericyte-like function. The bone marker alkaline phosphatase increased over time in hybrid and osteogenic media, but mineral deposition was evident only under purely osteogenic conditions. These results suggest that hybrid media compositions can support some aspects of multiphase tissue formation, but that alternative strategies are needed to obtain robust, concomitant vascularization, and osteogenesis in engineered tissues in vitro. Impact statement The combined use of mesenchymal stromal cells (MSCs) and endothelial cells to concomitantly produce mature bone and a nourishing vasculature is a promising tissue engineering approach to treating large bone defects. However, it is challenging to create and maintain vascular networks in the presence of osteogenic cues. This study used a 3D fibrin matrix to demonstrate that prevascularization of the construct can lead to maintenance of vessel structures over time, but that osteogenesis is compromised under these conditions. This work illuminates the capacity of MSCs to serve as both supportive pericytes and as osteoprogenitor cells, and motivates new strategies for coupling osteogenesis and vasculogenesis in engineered bone tissues.

Human pluripotent stem cell-derived cartilaginous organoids promote scaffold-free healing of critical size long bone defects

Background

Bones have a remarkable capacity to heal upon fracture. Yet, in large defects or compromised conditions healing processes become impaired, resulting in delayed or non-union. Current therapeutic approaches often utilize autologous or allogeneic bone grafts for bone augmentation. However, limited availability of these tissues and lack of predictive biological response result in limitations for clinical demands. Tissue engineering using viable cell-based implants is a strategic approach to address these unmet medical needs.

Methods

Herein, the in vitro and in vivo cartilage and bone tissue formation potencies of human pluripotent stem cells were investigated. The induced pluripotent stem cells were specified towards the mesodermal lineage and differentiated towards chondrocytes, which subsequently self-assembled into cartilaginous organoids. The tissue formation capacity of these organoids was then challenged in an ectopic and orthotopic bone formation model.

Results

The derived chondrocytes expressed similar levels of collagen type II as primary human articular chondrocytes and produced stable cartilage when implanted ectopically in vivo. Upon targeted promotion towards hypertrophy and priming with a proinflammatory mediator, the organoids mediated successful bridging of critical size long bone defects in immunocompromised mice.

Conclusions

These results highlight the promise of induced pluripotent stem cell technology for the creation of functional cartilage tissue intermediates that can be explored for novel bone healing strategies.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02580-7.

Rat model of an autologous cancellous bone graft

Autologous cancellous bone (ACB) grafting is the "gold standard" treatment for delayed bone union. However, small animal models for such grafts are lacking. Here, we developed an ACB graft rat model. Anatomical information regarding the iliac structure was recorded from five rat cadavers (10 ilia). Additionally, 5 and 25 rats were used as controls and ACB graft models, respectively. A defect was created in rat femurs and filled with ACB. Post-graft neo-osteogenic potential was assessed by radiographic evaluation and histological analysis. Iliac bone harvesting yielded the maximum amount of cancellous bone with minimal invasiveness, considering the position of parailiac nerves and vessels. The mean volume of cancellous bone per rat separated from the cortical bone was 73.8 ± 5.5 mm³. Bone union was evident in all ACB graft groups at 8 weeks, and new bone volume significantly increased every 2 weeks (P < 0.001). Histological analysis demonstrated the ability of ACB grafts to act as a scaffold and promote bone union in the defect. In conclusion, we established a stable rat model of ACB grafts by harvesting the iliac bone. This model can aid in investigating ACB grafts and development of novel therapies for bone injury.

Advanced Strategies of Biomimetic Tissue-Engineered Grafts for Bone Regeneration

The failure to repair critical-sized bone defects often leads to incomplete regeneration or fracture non-union. Tissue-engineered grafts have been recognized as an alternative strategy for bone regeneration due to their potential to repair defects. To design a successful tissue-engineered graft requires the understanding of physicochemical optimization to mimic the composition and structure of native bone, as well as the biological strategies of mimicking the key biological elements during bone regeneration process. This review provides an overview of engineered graft-based strategies focusing on physicochemical properties of materials and graft structure optimization from macroscale to nanoscale to further boost bone regeneration, and it summarizes biological strategies which mainly focus on growth factors following bone regeneration pattern and stem cell-based strategies for more efficient repair. Finally, it discusses the current limitations of existing strategies upon bone repair and highlights a promising strategy for rapid bone regeneration.

A novel autologous bone graft substitute containing rhBMP6 in autologous blood coagulum with synthetic ceramics for reconstruction of a large humerus segmental gunshot defect in a dog: The first veterinary patient to receive a novel osteoinductive therapy

Background

Management of large segmental defects is one of the most challenging issues in bone repair biology. Autologous bone graft substitute (ABGS) containing rhBMP6 within autologous blood coagulum (ABC) with synthetic ceramics is a novel biocompatible therapeutic solution for bone regeneration.

Case presentation

A 2-year old dog was brought to the veterinary clinics due to pain and bleeding from the right front leg after being unintendedly hit by a gunshot. Radiological examination revealed a large, 3 cm long multisegmental defect of the humerus on the right front leg with a loss of anatomical structure in the distal portion of the bone. The defect was treated surgically and an external fixator was inserted to ensure immobilization. Complete lack of bone formation 3 months following surgery required a full reconstruction of the defect site with a novel ABGS (rhBMP6 in ABC with ceramic particles) to avoid front leg amputation. The healing was then followed for the next 16 months. The callus formation was observed on x-ray images 2 months following ABGS implantation. The bone segments progressively fused together leading to the defect rebridgment allowing removal of the external fixator by 4 months after the reconstruction surgery. At the end of the observation period, the function of the leg was almost fully restored while analyses of the humeral CT sections revealed restoration and cortices rebridgment with a renewal of uniform medullary canal including structural reconstruction of the distal humerus.

Conclusion

This large humeral gunshot segmental defect of the front leg in a dog was saved from amputation via inducing bone regeneration using a novel ABGS osteoinductive device containing BMP6 in ABC.

Turning Nature's own processes into design strategies for living bone implant biomanufacturing: a decade of Developmental Engineering

A decade after the term developmental engineering (DE) was coined to indicate the use of developmental processes as blueprints for the design and development of engineered living implants, a myriad of proof-of-concept studies demonstrate the potential of this approach in small animal models. This review provides an overview of DE work, focusing on applications in bone regeneration. Enabling technologies allow to quantify the distance between in vitro processes and their developmental counterpart, as well as to design strategies to reduce that distance. By embedding Nature's robust mechanisms of action in engineered constructs, predictive large animal data and subsequent positive clinical outcomes can be gradually achieved. To this end, the development of next generation biofabrication technologies should provide the necessary scale and precision for robust living bone implant biomanufacturing.

Bone defect reconstruction via endochondral ossification: A developmental engineering strategy

Traditional bone tissue engineering (BTE) strategies induce direct bone-like matrix formation by mimicking the embryological process of intramembranous ossification. However, the clinical translation of these clinical strategies for bone repair is hampered by limited vascularization and poor bone regeneration after implantation in vivo. An alternative strategy for overcoming these drawbacks is engineering cartilaginous constructs by recapitulating the embryonic processes of endochondral ossification (ECO); these constructs have shown a unique ability to survive under hypoxic conditions as well as induce neovascularization and ossification. Such developmentally engineered constructs can act as transient biomimetic templates to facilitate bone regeneration in critical-sized defects. This review introduces the concept and mechanism of developmental BTE, explores the routes of endochondral bone graft engineering, highlights the current state of the art in large bone defect reconstruction via ECO-based strategies, and offers perspectives on the challenges and future directions of translating current knowledge from the bench to the bedside.

3-D macro/microporous-nanofibrous bacterial cellulose scaffolds seeded with BMP-2 preconditioned mesenchymal stem cells exhibit remarkable potential for bone tissue engineering

Bone repair using BMP-2 is a promising therapeutic approach in clinical practices, however, high dosages required to be effective pose issues of cost and safety. The present study explores the potential of low dose BMP-2 treatment via tissue engineering approach, which amalgamates 3-D macro/microporous-nanofibrous bacterial cellulose (mNBC) scaffolds and low dose BMP-2 primed murine mesenchymal stem cells (C3H10T1/2 cells). Initial studies on cell-scaffold interaction using unprimed C3H10T1/2 cells confirmed that scaffolds provided a propitious environment for cell adhesion, growth, and infiltration, owing to its ECM-mimicking nano-micro-macro architecture. Osteogenic studies were conducted by preconditioning the cells with 50 ng/mL BMP-2 for 15 min, followed by culturing on mNBC scaffolds for up to three weeks. The results showed an early onset and significantly enhanced bone matrix secretion and maturation in the scaffolds seeded with BMP-2 primed cells compared to the unprimed ones. Moreover, mNBC scaffolds alone were able to facilitate the mineralization of cells to some extent. These findings suggest that, with the aid of 'osteoinduction' from low dose BMP-2 priming of stem cells and 'osteoconduction' from nano-macro/micro topography of mNBC scaffolds, a cost-effective bone tissue engineering strategy can be designed for quick and excellent in vivo osseointegration.

Bone Microvasculature: Stimulus for Tissue Function and Regeneration

Bone is a highly vascularized organ, providing structural support to the body, and its development, regeneration, and remodeling depend on the microvascular homeostasis. Loss or impairment of vascular function can develop diseases, such as large bone defects, avascular necrosis, osteoporosis, osteoarthritis, and osteopetrosis. In this review, we summarize how vasculature controls bone development and homeostasis in normal and disease cases. A better understanding of this process will facilitate the development of novel disease treatments that promote bone regeneration and remodeling. Specifically, approaches based on tissue engineering components, such as stem cells and growth factors, have demonstrated the capacity to induce bone microvasculature regeneration and mineralization. This knowledge will have relevant clinical implications for the treatment of bone disorders by developing novel pharmaceutical approaches and bone grafts. Finally, the tissue engineering approaches incorporating vascular components may widely be applied to treat other organ diseases by enhancing their regeneration capacity. Impact statement Bone vasculature is imperative in the process of bone development, regeneration, and remodeling. Alterations or disruption of the bone vasculature leads to loss of bone homeostasis and the development of bone diseases. In this study, we review the role of vasculature on bone diseases and how vascular tissue engineering strategies, with a detailed emphasis on the role of stem cells and growth factors, will contribute to bone therapeutics.

In vivo bone regeneration assessment of offset and gradient melt electrowritten (MEW) PCL scaffolds

BACKGROUND

Biomaterial-based bone tissue engineering represents a promising solution to overcome reduced residual bone volume. It has been previously demonstrated that gradient and offset architectures of three-dimensional melt electrowritten poly-caprolactone (PCL) scaffolds could successfully direct osteoblast cells differentiation toward an osteogenic lineage, resulting in mineralization. The aim of this study was therefore to evaluate the in vivo osteoconductive capacity of PCL scaffolds with these different architectures.

METHODS

Five different calcium phosphate (CaP) coated melt electrowritten PCL pore sized scaffolds: 250 μm and 500 μm, 500 μm with 50% fibre offset (offset.50.50), tri layer gradient 250-500-750 μm (grad.250top) and 750-500-250 μm (grad.750top) were implanted into rodent critical-sized calvarial defects. Empty defects were used as a control. After 4 and 8 weeks of healing, the new bone was assessed by micro-computed tomography and immunohistochemistry.

RESULTS

Significantly more newly formed bone was shown in the grad.250top scaffold 8 weeks post-implantation. Histological investigation also showed that soft tissue was replaced with newly formed bone and fully covered the grad.250top scaffold. While, the bone healing did not happen completely in the 250 μm, offset.50.50 scaffolds and blank calvaria defects following 8 weeks of implantation. Immunohistochemical analysis showed the expression of osteogenic markers was present in all scaffold groups at both time points. The mineralization marker Osteocalcin was detected with the highest intensity in the grad.250top and 500 μm scaffolds. Moreover, the expression of the endothelial markers showed that robust angiogenesis was involved in the repair process.

CONCLUSIONS

These results suggest that the gradient pore size structure provides superior conditions for bone regeneration.

Recent Advances in Biomaterials for the Treatment of Bone Defects

Bone defects or fractures generally heal in the absence of major interventions due to the high regenerative capacity of bone tissue. However, in situations of severe/large bone defects, these orchestrated regeneration mechanisms are impaired. With advances in modern medicine, natural and synthetic bio-scaffolds from bioceramics and polymers that support bone growth have emerged and gained intense research interest. In particular, scaffolds that recapitulate the molecular cues of extracellular signals, particularly growth factors, offer potential as therapeutic bone biomaterials. The current challenges for these therapies include the ability to engineer materials that mimic the biological and mechanical properties of the real bone tissue matrix, whilst simultaneously supporting bone vascularization. In this review, we discuss the very recent innovative strategies in bone biomaterial technology, including those of endogenous biomaterials and cell/drug delivery systems that promote bone regeneration. We present our understanding of their current value and efficacy, and the future perspectives for bone regenerative medicine.

Heparan sulfate co-immobilized with cRGD ligands and BMP2 on biomimetic platforms promotes BMP2-mediated osteogenic differentiation

The chemical and physical properties of the extracellular matrix (ECM) are known to be fundamental for regulating growth factor bioactivity. The role of heparan sulfate (HS), a glycosaminoglycan, and of cell adhesion proteins (containing the cyclic RGD (cRGD) ligands) on bone morphogenetic protein 2 (BMP2)-mediated osteogenic differentiation has not been fully explored. In particular, it is not known whether and how their effects can be potentiated when they are presented in controlled close proximity, as in the ECM. Here, we developed streptavidin platforms to mimic selective aspects of the in vivo presentation of cRGD, HS and BMP2, with a nanoscale-control of their surface density and orientation to study cell adhesion and osteogenic differentiation. We showed that whereas a controlled increase in cRGD surface concentration upregulated BMP2 signaling due to β₃ integrin recruitment, silencing either β₁ or β₃ integrins negatively affected BMP2-mediated phosphorylation of SMAD1/5/9 and alkaline phosphatase expression. Furthermore, the presence of adsorbed BMP2 promoted cellular adhesion at very low cRGD concentrations. Finally, we proved that HS co-immobilized with cRGD both sustained BMP2 signaling and enhanced osteogenic differentiation compared to BMP2 directly immobilized on streptavidin, even with a low cRGD surface concentration. Altogether, our results show that HS facilitated and sustained the synergy between BMP2 and integrin pathways and that the co-immobilization of HS and cRGD peptides optimised BMP2-mediated osteogenic differentiation. Statement of significance The growth factor BMP2 is used to treat large bone defects. Previous studies have shown that the presentation of BMP2 via extracellular matrix molecules, such as heparan sulfate (HS), can upregulate BMP2 signaling. The potential advantages of dose reduction and local specificity have stimulated interest in further investigations into biomimetic approaches. We designed a streptavidin model surface eligible for immobilizing tunable amounts of molecules from the extracellular space, such as HS, adhesion motifs (cyclic RGD) and BMP2. By studying cellular adhesion, BMP2 bioactivity and its osteogenic potential we reveal the combined effect of integrins, HS and BMP2, which contribute in answering fundamental questions regarding cell-matrix interaction.

A cell-based combination product for the repair of large bone defects

Regenerative cell-based implants using periosteum-derived stem cells were developed for the treatment of large 3 cm fresh and 4.5 centimeter biological compromised bone gaps in a tibial sheep model and compared with an acellular ceramic-collagen void filler. It was hypothesized that the latter is insufficient to heal large skeletal defects due to reduced endogenous biological potency. To this purpose a comparison was made between the ceramic dicalciumphosphate scaffold (CopiOs®) as such, the same ceramic coated with clinical grade Bone Morphogenetic Protein 2 and 6 (BMP) only or a BMP coated cell-seeded combination product. These implants were evaluated in 2 sheep models, a fresh 3 cm critical size tibial defect and a 4.5 cm biologically exhausted tibial defect. For the groups in which growth factors were applied, BMP-6 was chosen at a dose of 344 μg for 3 cm and 1.500 μg or 3.800 μg for 4.5 cm defects. An additional group in the 4.5 cm defect was tested using BMP-2 in a dose of 1.500 μg. For all the cell based implants autologous periosteum-derived cells were used which were cultured in monolayer during 6 weeks. For the fresh defect 408 million cells and for the biologically exhausted tibial defect 612 million cells were drop-seeded on the BMP coated scaffolds. Bone healing was studied during 16 weeks postimplantation, using standard radiographs. While fresh defects responded to all treatments, regardless the use of cells, the biologically hampered defects responded in half of the cases and only if the BMP-cell combination product was used, supporting the concept that cell-based therapies may become attractive in treating defects with a compromised biological status.

Natural and Synthetic Polymers for Bone Scaffolds Optimization

Bone tissue is the structural component of the body, which allows locomotion, protects vital internal organs, and provides the maintenance of mineral homeostasis. Several bone-related pathologies generate critical-size bone defects that our organism is not able to heal spontaneously and require a therapeutic action. Conventional therapies span from pharmacological to interventional methodologies, all of them characterized by several drawbacks. To circumvent these effects, tissue engineering and regenerative medicine are innovative and promising approaches that exploit the capability of bone progenitors, especially mesenchymal stem cells, to differentiate into functional bone cells. So far, several materials have been tested in order to guarantee the specific requirements for bone tissue regeneration, ranging from the material biocompatibility to the ideal 3D bone-like architectural structure. In this review, we analyse the state-of-the-art of the most widespread polymeric scaffold materials and their application in in vitro and in vivo models, in order to evaluate their usability in the field of bone tissue engineering. Here, we will present several adopted strategies in scaffold production, from the different combination of materials, to chemical factor inclusion, embedding of cells, and manufacturing technology improvement.

Enzymatic outside-in cross-linking enables single-step microcapsule production for high-throughput three-dimensional cell microaggregate formation

Cell-laden hydrogel microcapsules enable the high-throughput production of cell aggregates, which are relevant for three-dimensional tissue engineering and drug screening applications. However, current microcapsule production strategies are limited by their throughput, multistep protocols, and limited amount of compatible biomaterials. We here present a single-step process for the controlled microfluidic production of single-core microcapsules using enzymatic outside-in cross-linking of tyramine-conjugated polymers. It was hypothesized that a physically, instead of the conventionally explored biochemically, controlled enzymatic cross-linking process would improve the reproducibility, operational window, and throughput of shell formation. Droplets were flown through a silicone delay line, which allowed for highly controlled diffusion of the enzymatic cross-linking initiator. The microcapsules' cross-linking density and shell thickness is strictly depended on the droplet's retention time in the delay line, which is predictably controlled by flow rate. The here presented hydrogel cross-linking method allows for facile and cytocompatible production of cell-laden microcapsules compatible with the formation and biorthogonal isolation of long-term viable cellular spheroids for tissue engineering and drug screening applications.

Recent advances toward understanding the role of transplanted stem cells in tissue-engineered regeneration of musculoskeletal tissues

Stem cell-based tissue engineering is poised to revolutionize the treatment of musculoskeletal injuries. However, in order to overcome scientific, practical, and regulatory obstacles and optimize therapeutic strategies, it is essential to better understand the mechanisms underlying the pro-regenerative effects of stem cells. There has been an attempted paradigm shift within the last decade to think of transplanted stem cells as "medicinal" therapies that orchestrate healing on the basis of their secretome and immunomodulatory profiles rather than acting as bona fide stem cells that proliferate, differentiate, and directly produce matrix to form de novo tissues. Yet the majority of current bone and skeletal muscle tissue engineering strategies are still premised on a direct contribution of stem cells as building blocks to tissue regeneration. Our review of the recent literature finds that researchers continue to focus on the quantification of de novo bone/skeletal muscle tissue following treatment and few studies aim to address this mechanistic conundrum directly. The dichotomy of thought is reflected in the diversity of new advances ranging from in situ three-dimensional bioprinting to a focus on exosomes and extracellular vesicles. However, recent findings elucidating the role of the immune system in tissue regeneration combined with novel imaging platform technologies will have a profound impact on our future understanding of how stem cells promote healing following biomaterial-mediated delivery to defect sites.

In vitro and in vivo biocompatibility of calcium-phosphate scaffolds three-dimensional printed by stereolithography for bone regeneration

Stereolithography (SLA) is an interesting manufacturing technology to overcome limitations of commercially available particulated biomaterials dedicated to intra-oral bone regeneration applications. The purpose of this study was to evaluate the in vitro and in vivo biocompatibility and osteoinductive properties of two calcium-phosphate (CaP)-based scaffolds manufactured by SLA three-dimensional (3D) printing. Pellets and macro-porous scaffolds were manufactured in pure hydroxyapatite (HA) and in biphasic CaP (HA:60-TCP:40). Physico-chemical characterization was performed using micro X-ray fluorescence, scanning electron microscopy (SEM), optical interferometry, and microtomography (μCT) analyses. Osteoblast-like MG-63 cells were used to evaluate the biocompatibility of the pellets in vitro with MTS assay and the cell morphology and growth characterized by SEM and DAPI-actin staining showed similar early behavior. For in vivo biocompatibility, newly formed bone and biodegradability of the experimental scaffolds were evaluated in a subperiosteal cranial rat model using μCT and descriptive histology. The histological analysis has not indicated evidences of inflammation but highlighted close contacts between newly formed bone and the experimental biomaterials revealing an excellent scaffold osseointegration. This study emphasizes the relevance of SLA 3D printing of CaP-based biomaterials for intra-oral bone regeneration even if manufacturing accuracy has to be improved and further experiments using biomimetic scaffolds should be conducted.

Single-cell characterization and metabolic profiling of in vitro cultured human skeletal progenitors with enhanced in vivo bone forming capacity

Cell populations and their interplay provide the basis of a cell‐based regenerative construct. Serum‐free preconditioning can overcome the less predictable behavior of serum expanded progenitor cells, but the underlying mechanism and how this is reflected in vivo remains unknown. Herein, the cellular and molecular changes associated with a cellular phenotype shift induced by serum‐free preconditioning of human periosteum‐derived cells were investigated. Following BMP‐2 stimulation, preconditioned cells displayed enhanced in vivo bone forming capacity, associated with an adapted cellular metabolism together with an elevated expression of BMPR2. Single‐cell RNA sequencing confirmed the activation of pathways and transcriptional regulators involved in bone development and fracture healing, providing support for the augmentation of specified skeletal progenitor cell populations. The reported findings illustrate the importance of appropriate in vitro conditions for the in vivo outcome. In addition, BMPR2 represents a promising biomarker for the enrichment of skeletal progenitor cells for in vivo bone regeneration.

Concentrated growth factor promotes proliferation, osteogenic differentiation, and angiogenic potential of rabbit periosteum-derived cells in vitro

OBJECTIVE

The aim of this research is to investigate the effects of concentrated growth factor (CGF) on the proliferation, osteogenic differentiation, and angiogenic potential of rabbit periosteum-derived cells (PDCs) in vitro.

METHODS

PDCs were isolated from the femoral and tibial periosteum of rabbits and cultured with or without CGF membranes or CGF conditioned media. Scanning electron microscopy (SEM) was used for the structural characterization. Cell Counting Kit-8 assay was used to measure cell proliferation. Alkaline phosphatase (ALP) activity of PDCs was also measured. Immunohistochemistry was used to detect the expression of CD34. Enzyme-linked immunosorbent assay (ELISA), quantitative real-time PCR (qPCR), and Western blot were used to evaluate the secretion and expression levels of osteogenic differentiation markers (bone morphogenetic protein-2, type I collagen, osteocalcin) and angiogenesis markers (vascular endothelial growth factor, basic fibroblast growth factor) in supernatants and PDCs at days 3, 7, 14, and 21.

RESULTS

The SEM analysis showed a dense three-dimensional fibrin network in CGF, and CGF membranes were covered by PDCs with elongated or polygonal morphological features. Compared with the control group, CGF significantly promoted the proliferation of PDCs during the experimental period (p < 0.05). Immunohistochemistry revealed that PDCs were dispersedly distributed among the CGF substrates, and CD34-positive cells were also present. Moreover, CGF significantly increased the ALP activity and upregulated the expression and secretion of osteogenic differentiation and angiogenesis markers in PDCs at days 3, 7, 14, and 21 (p < 0.05).

CONCLUSION

CGF can increase the proliferation and promote the osteogenic differentiation and angiogenic potential of PDCs in vitro. These results indicate that CGF can be used as a new therapeutic means for biotechnological and clinical applications.

Human Platelet Lysate Improves Bone Forming Potential of Human Progenitor Cells Expanded in Microcarrier-Based Dynamic Culture

Xenogeneic‐free media are required for translating advanced therapeutic medicinal products to the clinics. In addition, process efficiency is crucial for ensuring cost efficiency, especially when considering large‐scale production of mesenchymal stem cells (MSCs). Human platelet lysate (HPL) has been increasingly adopted as an alternative for fetal bovine serum (FBS) for MSCs. However, its therapeutic and regenerative potential in vivo is largely unexplored. Herein, we compare the effects of FBS and HPL supplementation for a scalable, microcarrier‐based dynamic expansion of human periosteum‐derived cells (hPDCs) while assessing their bone forming capacity by subcutaneous implantation in small animal model. We observed that HPL resulted in faster cell proliferation with a total fold increase of 5.2 ± 0.61 in comparison to 2.7 ± 02.22‐fold in FBS. Cell viability and trilineage differentiation capability were maintained by HPL, although a suppression of adipogenic differentiation potential was observed. Differences in mRNA expression profiles were also observed between the two on several markers. When implanted, we observed a significant difference between the bone forming capacity of cells expanded in FBS and HPL, with HPL supplementation resulting in almost three times more mineralized tissue within calcium phosphate scaffolds. FBS‐expanded cells resulted in a fibrous tissue structure, whereas HPL resulted in mineralized tissue formation, which can be classified as newly formed bone, verified by μCT and histological analysis. We also observed the presence of blood vessels in our explants. In conclusion, we suggest that replacing FBS with HPL in bioreactor‐based expansion of hPDCs is an optimal solution that increases expansion efficiency along with promoting bone forming capacity of these cells. stem cells translational medicine2019;8:810&821

From skeletal development to the creation of pluripotent stem cell-derived bone-forming progenitors

Bone has many functions. It is responsible for protecting the underlying soft organs, it allows locomotion, houses the bone marrow and stores minerals such as calcium and phosphate. Upon damage, bone tissue can efficiently repair itself. However, healing is hampered if the defect exceeds a critical size and/or is in compromised conditions. The isolation or generation of bone-forming progenitors has applicability to skeletal repair and may be used in tissue engineering approaches. Traditionally, bone engineering uses osteochondrogenic stem cells, which are combined with scaffold materials and growth factors. Despite promising preclinical data, limited translation towards the clinic has been observed to date. There may be several reasons for this including the lack of robust cell populations with favourable proliferative and differentiation capacities. However, perhaps the most pertinent reason is the failure to produce an implant that can replicate the developmental programme that is observed during skeletal repair. Pluripotent stem cells (PSCs) can potentially offer a solution for bone tissue engineering by providing unlimited cell sources at various stages of differentiation. In this review, we summarize key embryonic signalling pathways in bone formation coupled with PSC differentiation strategies for the derivation of bone-forming progenitors.This article is part of the theme issue 'Designer human tissue: coming to a lab near you'.

Uncoupling of in-vitro identity of embryonic limb derived skeletal progenitors and their in-vivo bone forming potential

The healing of large bone defects remains a major unmet medical need. Our developmental engineering approach consists of the in vitro manufacturing of a living cartilage tissue construct that upon implantation forms bone by recapitulating an endochondral ossification process. Key to this strategy is the identification of the cells to produce such cartilage intermediates efficiently. We applied a cell selection strategy based on published skeletal stem cell markers using mouse embryonic limb cartilage as cell source and analysed their potential to form bone in an in vivo ectopic assay. FGF2 supplementation to the culture media for expansion blocked dedifferentiation of the embryonic cartilage cells in culture and enriched for stem cells and progenitors as quantified using the recently published CD marker set. However, when the stem cells and progenitors were fractionated from expanded embryonic cartilage cells and assessed in the ectopic assay, a major loss of bone forming potential was observed. We conclude that cell expansion appears to affect the association between cell identity based on CD markers and in vivo bone forming capacity.

Periosteum-derived mesenchymal progenitor cells in engineered implants promote fracture healing in a critical-size defect rat model

An attractive alternative to bone autografts is the use of autologous mesenchymal progenitor cells (MSCs) in combination with biomaterials. We compared the therapeutic potential of different sources of mesenchymal stem cells in combination with biomaterials in a bone nonunion model. A critical-size defect was created in Sprague-Dawley rats. Animals were divided into six groups, depending on the treatment to be applied: bone defect was left empty (CTL); treated with live bone allograft (LBA); hrBMP-2 in collagen scaffold (CS^BMP2 ); acellular polycaprolactone scaffold (PCL group); PCL scaffold containing periosteum-derived MSCs (PCL^PMSCs ) and PCL containing bone marrow-derived MSCs (PCL^BMSCs ). To facilitate cell tracking, both MSCs and bone graft were isolated from green fluorescent protein (GFP)-transgenic rats. CTL group did not show any signs of healing during the radiological follow-up (n = 6). In the LBA group, all the animals showed bone bridging (n = 6) whereas in the CS^BMP2 group, four out of six animals demonstrated healing. In PCL and PCL^PMSCs groups, a reduced number of animals showed radiological healing, whereas no healing was detected in the PCL^BMSCs group. Using microcomputed tomography, the bone volume filling the defect was quantified, showing significant new bone formation in the LBA, CS^BMP2 , and PCL^PMSCs groups when compared with the CTL group. At 10 weeks, GFP positive cells were detected only in the LBA group and restricted to the outer cortical bone in close contact with the periosteum. Tracking of cellular implants demonstrated significant survival of the PMSCs when compared with BMSCs. In conclusion, PMSCs improve bone regeneration being suitable for mimetic autograft design.

Bone regeneration strategies: Engineered scaffolds, bioactive molecules and stem cells current stage and future perspectives

Bone fractures are the most common traumatic injuries in humans. The repair of bone fractures is a regenerative process that recapitulates many of the biological events of embryonic skeletal development. Most of the time it leads to successful healing and the recovery of the damaged bone. Unfortunately, about 5-10% of fractures will lead to delayed healing or non-union, more so in the case of co-morbidities such as diabetes. In this article, we review the different strategies to heal bone defects using synthetic bone graft substitutes, biologically active substances and stem cells. The majority of currently available reviews focus on strategies that are still at the early stages of development and use mostly in vitro experiments with cell lines or stem cells. Here, we focus on what is already implemented in the clinics, what is currently in clinical trials, and what has been tested in animal models. Treatment approaches can be classified in three major categories: i) synthetic bone graft substitutes (BGS) whose architecture and surface can be optimized; ii) BGS combined with bioactive molecules such as growth factors, peptides or small molecules targeting bone precursor cells, bone formation and metabolism; iii) cell-based strategies with progenitor cells combined or not with active molecules that can be injected or seeded on BGS for improved delivery. We review the major types of adult stromal cells (bone marrow, adipose and periosteum derived) that have been used and compare their properties. Finally, we discuss the remaining challenges that need to be addressed to significantly improve the healing of bone defects.

Automated Buildup of Biomimetic Films in Cell Culture Microplates for High-Throughput Screening of Cellular Behaviors

An automatic method is established for layer-by-layer (LbL) assembly of biomimetic coatings in cell culture microplates using a commercial liquid-handling robot. Highly homogeneous thin films are formed at the bottom of each microwell. The LbL film-coated microplates are compatible with common cellular assays, using microplate readers and automated microscopes. Cellular adhesion is screened on crosslinked and peptide-functionalized LbL films and stem cell differentiation in response to increasing doses of bone morphogenetic proteins (2, 4, 7, 9). This method paves the way for future applications of LbL films in cell-based assays for regenerative medicine and high-throughput drug screening.

Anabolic Strategies to Augment Bone Fracture Healing

PURPOSE OF REVIEW

The development of therapeutics that target anabolic pathways involved in skeletogenesis is of great importance with regard to disease resulting in bone loss, or in cases of impaired bone repair. This review aims to summarize recent developments in this area.

RECENT FINDINGS

A greater understanding of how drugs that modulate signaling pathways involved in skeletogenesis exert their efficacy, and the molecular mechanisms resulting in bone formation has led to novel pharmacological bone repair strategies. Furthermore, crosstalk between pathways and molecules has suggested signaling synergies that may be exploited for enhanced tissue formation. The sequential pharmacological stimulation of the molecular cascades resulting in tissue repair is a promising strategy for the treatment of bone fractures. It is proposed that a therapeutic strategy which mimics the natural cascade of events observed during fracture repair may be achieved through temporal targeting of tissue repair pathways.

High-throughput approaches for screening and analysis of cell behaviors

The rapid development of new biomaterials and techniques to modify them challenge our capability to characterize them using conventional methods. In response, numerous high-throughput (HT) strategies are being developed to analyze biomaterials and their interactions with cells using combinatorial approaches. Moreover, these systematic analyses have the power to uncover effects of delivered soluble bioactive molecules on cell responses. In this review, we describe the recent developments in HT approaches that help identify cellular microenvironments affecting cell behaviors and highlight HT screening of biochemical libraries for gene delivery, drug discovery, and toxicological studies. We also discuss HT techniques for the analyses of cell secreted biomolecules and provide perspectives on the future utility of HT approaches in biomedical engineering.

Spatially and Temporally Controlled Hydrogels for Tissue Engineering

Recent years have seen tremendous advances in the field of hydrogel-based biomaterials. One of the most prominent revolutions in this field has been the integration of elements or techniques that enable spatial and temporal control over hydrogels' properties and functions. Here, we critically review the emerging progress of spatiotemporal control over biomaterial properties towards the development of functional engineered tissue constructs. Specifically, we will highlight the main advances in the spatial control of biomaterials, such as surface modification, microfabrication, photo-patterning, and three-dimensional (3D) bioprinting, as well as advances in the temporal control of biomaterials, such as controlled release of molecules, photocleaving of proteins, and controlled hydrogel degradation. We believe that the development and integration of these techniques will drive the engineering of next-generation engineered tissues.