A Developmental Engineering-Based Approach to Bone Repair: Endochondral Priming Enhances Vascularization and New Bone Formation in a Critical Size Defect

Advances in medical polyesters for vascular tissue engineering

Blood vessels are highly dynamic and complex structures with a variety of physiological functions, including the transport of oxygen, nutrients, and metabolic wastes. Their normal functioning involves the close and coordinated cooperation of a variety of cells. However, adverse internal and external environmental factors can lead to vascular damage and the induction of various vascular diseases, including atherosclerosis and thrombosis. This can have serious consequences for patients, and there is an urgent need for innovative techniques to repair damaged blood vessels. Polyesters have been extensively researched and used in the treatment of vascular disease and repair of blood vessels due to their excellent mechanical properties, adjustable biodegradation time, and excellent biocompatibility. Given the high complexity of vascular tissues, it is still challenging to optimize the utilization of polyesters for repairing damaged blood vessels. Nevertheless, they have considerable potential for vascular tissue engineering in a range of applications. This summary reviews the physicochemical properties of polyhydroxyalkanoate (PHA), polycaprolactone (PCL), poly-lactic acid (PLA), and poly(lactide-co-glycolide) (PLGA), focusing on their unique applications in vascular tissue engineering. Polyesters can be prepared not only as 3D scaffolds to repair damage as an alternative to vascular grafts, but also in various forms such as microspheres, fibrous membranes, and nanoparticles to deliver drugs or bioactive ingredients to damaged vessels. Finally, it is anticipated that further developments in polyesters will occur in the near future, with the potential to facilitate the wider application of these materials in vascular tissue engineering.

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.

Biomaterials for Regenerative Cranioplasty: Current State of Clinical Application and Future Challenges

Acquired cranial defects are a prevalent condition in neurosurgery and call for cranioplasty, where the missing or defective cranium is replaced by an implant. Nevertheless, the biomaterials in current clinical applications are hardly exempt from long-term safety and comfort concerns. An appealing solution is regenerative cranioplasty, where biomaterials with/without cells and bioactive molecules are applied to induce the regeneration of the cranium and ultimately repair the cranial defects. This review examines the current state of research, development, and translational application of regenerative cranioplasty biomaterials and discusses the efforts required in future research. The first section briefly introduced the regenerative capacity of the cranium, including the spontaneous bone regeneration bioactivities and the presence of pluripotent skeletal stem cells in the cranial suture. Then, three major types of biomaterials for regenerative cranioplasty, namely the calcium phosphate/titanium (CaP/Ti) composites, mineralised collagen, and 3D-printed polycaprolactone (PCL) composites, are reviewed for their composition, material properties, and findings from clinical trials. The third part discusses perspectives on future research and development of regenerative cranioplasty biomaterials, with a considerable portion based on issues identified in clinical trials. This review aims to facilitate the development of biomaterials that ultimately contribute to a safer and more effective healing of cranial defects.

3D printed strontium-doped calcium phosphate ceramic scaffold enhances early angiogenesis and promotes bone repair through the regulation of macrophage polarization

The vascularization of bone repair materials is one of the key issues that urgently need to be addressed in the process of bone repair. The changes in macrophage phenotype and function play an important role in the process of vascularization, and endowing bone repair materials with immune regulatory characteristics to enhance angiogenesis is undoubtedly a new strategy to improve the effectiveness of bone repair. In order to improve the effect of tricalcium phosphate (TCP) on vascularization and bone repair, we doped strontium ions (Sr) into TCP (SrTCP) and prepared porous 3D printed SrTCP scaffolds using 3D printing technology, and studied the scaffold mediated macrophage polarization and subsequent vascularization and bone regeneration. The results of the interaction between the scaffold and macrophages showed that the SrTCP scaffold can promote the polarization of macrophages from M1 to M2 and secrete high concentrations of VEGF and PDGF-bb cytokines, which shows excellent angiogenic potential. When human umbilical vein endothelial cells (HUVECs) were co-cultured with macrophage-conditioned medium of SrTCP scaffold, HUVECs exhibited excellent early angiogenesis-promoting effects in terms of scratch healing, angiogenic gene expression, and in vitro tube formation performance. The results of in vivo bone repair experiments showed that the SrTCP scaffold formed a vascular network with high density and quantity in the bone defect area, which could increase the rate of new bone formation and advance the period of bone formation, and finally achieved a better bone repair effect. We observed a cascade effect in which Sr-doped SrTCP scaffold regulate macrophage polarization to enhance angiogenesis and promote bone repair, which may provide a new strategy for the repair of clinical bone defects.

Harnessing human adipose-derived stromal cell chondrogenesis in vitro for enhanced endochondral ossification

Endochondral ossification (ECO), the major ossification process during embryogenesis and bone repair, involves the formation of a cartilaginous template remodelled into a functional bone organ. Adipose-derived stromal cells (ASC), non-skeletal multipotent progenitors from the stromal vascular fraction (SVF) of human adipose tissue, were shown to recapitulate ECO and generate bone organs in vivo when primed into a hypertrophic cartilage tissue (HCT) in vitro. However, the reproducibility of ECO was limited and the major triggers remain unknown. We studied the effect of the expansion of cells and maturation of HCT on the induction of the ECO process. SVF cells or expanded ASC were seeded onto collagen sponges, cultured in chondrogenic medium for 3-6 weeks and implanted ectopically in nude mice to evaluate their bone-forming capacities. SVF cells from all tested donors formed mature HCT in 3 weeks whereas ASC needed 4-5 weeks. A longer induction increased the degree of maturation of the HCT, with a gradually denser cartilaginous matrix and increased mineralization. This degree of maturation was highly predictive of their bone-forming capacity in vivo, with ECO achieved only for an intermediate maturation degree. In parallel, expanding ASC also resulted in an enrichment of the stromal fraction characterized by a rapid change of their proteomic profile from a quiescent to a proliferative state. Inducing quiescence rescued their chondrogenic potential. Our findings emphasize the role of monolayer expansion and chondrogenic maturation degree of ASC on ECO and provides a simple, yet reproducible and effective approach for bone formation to be tested in specific clinical models.

Towards Polycaprolactone-Based Scaffolds for Alveolar Bone Tissue Engineering: A Biomimetic Approach in a 3D Printing Technique

The alveolar bone is a unique type of bone, and the goal of bone tissue engineering (BTE) is to develop methods to facilitate its regeneration. Currently, an emerging trend involves the fabrication of polycaprolactone (PCL)-based scaffolds using a three-dimensional (3D) printing technique to enhance an osteoconductive architecture. These scaffolds are further modified with hydroxyapatite (HA), type I collagen (CGI), or chitosan (CS) to impart high osteoinductive potential. In conjunction with cell therapy, these scaffolds may serve as an appealing alternative to bone autografts. This review discusses research gaps in the designing of 3D-printed PCL-based scaffolds from a biomimetic perspective. The article begins with a systematic analysis of biological mineralisation (biomineralisation) and ossification to optimise the scaffold's structural, mechanical, degradation, and surface properties. This scaffold-designing strategy lays the groundwork for developing a research pathway that spans fundamental principles such as molecular dynamics (MD) simulations and fabrication techniques. Ultimately, this paves the way for systematic in vitro and in vivo studies, leading to potential clinical applications.

In Vitro Mineralisation of Tissue-Engineered Cartilage Reduces Endothelial Cell Migration, Proliferation and Tube Formation

Tissue engineering bone via endochondral ossification requires the generation of a cartilage template which undergoes vascularisation and remodelling. While this is a promising route for bone repair, achieving effective cartilage vascularisation remains a challenge. Here, we investigated how mineralisation of tissue-engineered cartilage affects its pro-angiogenic potential. To generate in vitro mineralised cartilage, human mesenchymal stromal cell (hMSC)-derived chondrogenic pellets were treated with β-glycerophosphate (BGP). After optimising this approach, we characterised the changes in matrix components and pro-angiogenic factors by gene expression analysis, histology and ELISA. Human umbilical vein endothelial cells (HUVECs) were exposed to pellet-derived conditioned media, and migration, proliferation and tube formation were assessed. We established a reliable strategy to induce in vitro cartilage mineralisation, whereby hMSC pellets are chondrogenically primed with TGF-β for 2 weeks and BGP is added from week 2 of culture. Cartilage mineralisation determines loss of glycosaminoglycans, reduced expression but not protein abundance of collagen II and X, and decreased VEGFA production. Finally, the conditioned medium from mineralised pellets showed a reduced ability to stimulate endothelial cell migration, proliferation and tube formation. The pro-angiogenic potential of transient cartilage is thus stage-dependent, and this aspect must be carefully considered in the design of bone tissue engineering strategies.

Repair of critical-sized rat cranial defects with RADA16-W9 self-assembled peptide hydrogel

Bone defects are common in orthopaedics and there is an urgent need to explore effective bone repair materials with osteoinductive activity. Peptide self-assembled nanomaterials have a fibrous structure like that of the extracellular matrix and are ideal bionic scaffold materials. In this study, a short peptide WP9QY (W9) with strong osteoinductive effect was tagged to a self-assembled peptide RADA16 molecule through solid phase synthesis to design a RADA16-W9 peptide gel scaffold. A rat cranial defect was used as a research model to explore the effect of this peptide material on the repair of bone defects in vivo. The structure characteristic of the functional self-assembling peptide nanofiber hydrogel scaffold RADA16-W9 was evaluated by atomic force microscopy (AFM). Then adipose stem cells (ASCs) were isolated from Sprague-Dawley (SD) rat and cultured. the cellular compatibility of scaffold was evaluated through Live/Dead assay. Furthermore, we explore the effects of hydrogels in vivo with the critical-sized mouse calvarial defect model. Micro-CT analysis showed that the RADA16-W9 group had higher levels of bone volume/total volume (BV/TV) (P < 0.05)，Trabecular number(TB.N) (P < 0.05)，bone mineral density (BMD)(P < 0.05) and trabecular thickness (Tb. Th) (P < 0.05) compared with the RADA16 and PBS groups. Hematoxylin and eosin (H&E) staining showed that RADA16-W9 group had the highest bone regeneration level. Histochemical staining showed significantly higher expression levels of osteogenic factors such as alkaline phosphatase (ALP) and osteocalcin (OCN) in the RADA16-W9 group than in the other two groups (P < 0.05). Reverse transcription polymerase chain reaction (RT-PCR) quantification showed higher mRNA expression levels of osteogenic-related genes ALP, Runt-related transcription factor 2(Runx2), OCN, Osteopontin (OPN) in the RADA16-W9 group than in the RADA16 and PBS groups (P < 0.05). The live/dead staining results showed that RADA16-W9 is not toxic to rASCs and has good biocompatibility. In vivo experiments show that it accelerates the process of bone reconstruction, significantly promoting bone regeneration and can be used to develop a molecular drug for bone defect repair.

3D bioprinting of cartilaginous templates for large bone defect healing

Damaged or diseased bone can be treated using autografts or a range of different bone grafting biomaterials, however limitations with such approaches has motivated increased interest in developmentally inspired bone tissue engineering (BTE) strategies that seek to recapitulate the process of endochondral ossification (EO) as a means of regenerating critically sized defects. The clinical translation of such strategies will require the engineering of scaled-up, geometrically defined hypertrophic cartilage grafts that can be rapidly vascularised and remodelled into bone in mechanically challenging defect environments. The goal of this study was to 3D bioprint mechanically reinforced cartilaginous templates and to assess their capacity to regenerate critically sized femoral bone defects. Human mesenchymal stem/stromal cells (hMSCs) were incorporated into fibrin based bioinks and bioprinted into polycaprolactone (PCL) frameworks to produce mechanically reinforced constructs. Chondrogenic priming of such hMSC laden constructs was required to support robust vascularisation and graft mineralisation in vivo following their subcutaneous implantation into nude mice. With a view towards maximising their potential to support endochondral bone regeneration, we next explored different in vitro culture regimes to produce chondrogenic and early hypertrophic engineered grafts. Following their implantation into femoral bone defects within transiently immunosuppressed rats, such bioprinted constructs were rapidly remodelled into bone in vivo, with early hypertrophic constructs supporting higher levels of vascularisation and bone formation compared to the chondrogenic constructs. Such early hypertrophic bioprinted constructs also supported higher levels of vascularisation and spatially distinct patterns of new formation compared to BMP-2 loaded collagen scaffolds (here used as a positive control). In conclusion, this study demonstrates that fibrin based bioinks support chondrogenesis of hMSCs in vitro, which enables the bioprinting of mechanically reinforced hypertrophic cartilaginous templates capable of supporting large bone defect regeneration. These results support the use of 3D bioprinting as a strategy to scale-up the engineering of developmentally inspired templates for BTE. STATEMENT OF SIGNIFICANCE: Despite the promise of developmentally inspired tissue engineering strategies for bone regeneration, there are still challenges that need to be addressed to enable clinical translation. This work reports the development and assessment (in vitro and in vivo) of a 3D bioprinting strategy to engineer mechanically-reinforced cartilaginous templates for large bone defect regeneration using human MSCs. Using distinct in vitro priming protocols, it was possible to generate cartilage grafts with altered phenotypes. More hypertrophic grafts, engineered in vitro using TGF-β3 and BMP-2, supported higher levels of blood vessel infiltration and accelerated bone regeneration in vivo. This study also identifies some of the advantages and disadvantages of such endochondral bone TE strategies over the direct delivery of BMP-2 from collagen-based scaffolds.

Promoting endogenous articular cartilage regeneration using extracellular matrix scaffolds

Articular cartilage defects fail to heal spontaneously, typically progressing to osteoarthritis. Bone marrow stimulation techniques such as microfracture (MFX) are the current surgical standard of care; however MFX typically produces an inferior fibro-cartilaginous tissue which provides only temporary symptomatic relief. Here we implanted solubilised articular cartilage extracellular matrix (ECM) derived scaffolds into critically sized chondral defects in goats, securely anchoring these implants to the joint surface using a 3D-printed fixation device that overcame the need for sutures or glues. In vitro these ECM scaffolds were found to be inherently chondro-inductive, while in vivo they promoted superior articular cartilage regeneration compared to microfracture. In an attempt to further improve the quality of repair, we loaded these scaffolds with a known chemotactic factor, transforming growth factor (TGF)-β3. In vivo such TGF-β3 loaded scaffolds promoted superior articular cartilage regeneration. This study demonstrates that ECM derived biomaterials, either alone and particularly when combined with exogenous growth factors, can successfully treat articular cartilage defects in a clinically relevant large animal model.

Close-to-native bone repair via tissue-engineered endochondral ossification approaches

In order to solve the clinical challenges related to bone grafting, several tissue engineering (TE) strategies have been proposed to repair critical-sized defects. Generally, the classical TE approaches are designed to promote bone repair via intramembranous ossification. Although promising, strategies that direct the osteogenic differentiation of mesenchymal stem/stromal cells are usually characterized by a lack of functional vascular supply, often resulting in necrotic cores. A less explored alternative is engineering bone constructs through a cartilage-mediated approach, resembling the embryological process of endochondral ossification. The remodeling of an intermediary hypertrophic cartilaginous template triggers vascular invasion and bone tissue deposition. Thus, employing this knowledge can be a promising direction for the next generation of bone TE constructs. This review highlights the most recent biomimetic strategies for applying endochondral ossification in bone TE while discussing the plethora of cell types, culture conditions, and biomaterials essential to promote a successful bone regeneration process.

Effects of Endochondral and Intramembranous Ossification Pathways on Bone Tissue Formation and Vascularization in Human Tissue-Engineered Grafts

Bone grafts can be engineered by differentiating human mesenchymal stromal cells (MSCs) via the endochondral and intramembranous ossification pathways. We evaluated the effects of each pathway on the properties of engineered bone grafts and their capacity to drive bone regeneration. Bone-marrow-derived MSCs were differentiated on silk scaffolds into either hypertrophic chondrocytes (hyper) or osteoblasts (osteo) over 5 weeks of in vitro cultivation, and were implanted subcutaneously for 12 weeks. The pathways' constructs were evaluated over time with respect to gene expression, composition, histomorphology, microstructure, vascularization and biomechanics. Hypertrophic chondrocytes expressed higher levels of osteogenic genes and deposited significantly more bone mineral and proteins than the osteoblasts. Before implantation, the mineral in the hyper group was less mature than that in the osteo group. Following 12 weeks of implantation, the hyper group had increased mineral density but a similar overall mineral composition compared with the osteo group. The hyper group also displayed significantly more blood vessel infiltration than the osteo group. Both groups contained M2 macrophages, indicating bone regeneration. These data suggest that, similar to the body's repair processes, endochondral pathway might be more advantageous when regenerating large defects, whereas intramembranous ossification could be utilized to guide the tissue formation pattern with a scaffold architecture.

Engineering hypertrophic cartilage grafts from lipoaspirate for critical-sized calvarial bone defect reconstruction: An adipose tissue-based developmental engineering approach

Developmental engineering of living implants from different cell sources capable of stimulating bone regeneration by recapitulating endochondral ossification (ECO) is a promising strategy for large bone defect reconstruction. However, the clinical translation of these cell-based approaches is hampered by complex manufacturing procedures, poor cell differentiation potential, and limited predictive in vivo performance. We developed an adipose tissue-based developmental engineering approach to overcome these hurdles using hypertrophic cartilaginous (HyC) constructs engineered from lipoaspirate to repair large bone defects. The engineered HyC constructs were implanted into 4-mm calvarial defects in nude rats and compared with decellularized bone matrix (DBM) grafts. The DBM grafts induced neo-bone formation via the recruitment of host cells, while the HyC pellets supported bone regeneration via ECO, as evidenced by the presence of remaining cartilage analog and human NuMA-positive cells within the newly formed bone. However, the HyC pellets clearly showed superior regenerative capacity compared with that of the DBM grafts, yielding more new bone formation, higher blood vessel density, and better integration with adjacent native bone. We speculate that this effect arises from vascular endothelial growth factor and bone morphogenetic protein-2 secretion and mineral deposition in the HyC pellets before implantation, promoting increased vascularization and bone formation upon implantation. The results of this study demonstrate that adipose-derived HyC constructs can effectively heal large bone defects and present a translatable therapeutic option for bone defect repair.

Strategies for Enhancing Vascularization of Biomaterial-Based Scaffold in Bone Regeneration

Despite the recent advances in reconstructive orthopedics; fracture union is a challenge to bone regeneration. Concurrent angiogenesis is a complex process governed by events, delicately entwined with osteogenesis. However, poorly perfused scaffolds have lower success rates; necessitating the need for a better vascular component, which is important for the delivery of nutrients, oxygen, waste elimination, recruitment of cells for optimal bone repair. This review highlights the latest strategies to promote biomaterial-based scaffold vascularization by incorporation of cells, growth factors, inorganic ions, etc. into natural or synthetic polymers, ceramic materials, or composites of organic and inorganic compounds. Furthermore, it emphasizes structural modifications, biophysical stimuli, and natural molecules to fabricate scaffolds aiding the genesis of dense vascularization following their implantation to manifest a compatible regenerative microenvironment without graft rejection.

Emerging concepts in bone repair and the premise of soft materials

Human bone has a strong regenerative capacity that allows for restoration of its function and structure after damage. For degenerative bone diseases or large defects, bone regeneration requirements exceed the natural potential for self-healing, so bone grafts or bone substitute materials are required to support the regeneration of bone tissue. Compared to the plethora of endogenous bioactive molecules and cells in native bone grafts, the regenerative capacity of tissue-engineered materials is limited. The modest clinical impact of tissue-engineered strategies in this domain can be attributed to a failure to fully recognize key physical and biological events during bone healing, and to recapitulate the structure and composition of the target tissue to generate truly biomimetic grafts. This limitation has motivated the emergence of new strategies such as immunomodulation, endochondral ossification routes, engineered microtissues and hematoma regulation, and the development of advanced biomaterials including gene-activated matrices, soft microgels and hierarchically designed materials.

Recent Advances on Cell-Based Co-Culture Strategies for Prevascularization in Tissue Engineering

Currently, the fabrication of a functional vascular network to maintain the viability of engineered tissues is a major bottleneck in the way of developing a more advanced engineered construct. Inspired by vasculogenesis during the embryonic period, the in vitro prevascularization strategies have focused on optimizing communications and interactions of cells, biomaterial and culture conditions to develop a capillary-like network to tackle the aforementioned issue. Many of these studies employ a combination of endothelial lineage cells and supporting cells such as mesenchymal stem cells, fibroblasts, and perivascular cells to create a lumenized endothelial network. These supporting cells are necessary for the stabilization of the newly developed endothelial network. Moreover, to optimize endothelial network development without impairing biomechanical properties of scaffolds or differentiation of target tissue cells, several other factors, including target tissue, endothelial cell origins, the choice of supporting cell, culture condition, incorporated pro-angiogenic factors, and choice of biomaterial must be taken into account. The prevascularization method can also influence the endothelial lineage cell/supporting cell co-culture system to vascularize the bioengineered constructs. This review aims to investigate the recent advances on standard cells used in in vitro prevascularization methods, their co-culture systems, and conditions in which they form an organized and functional vascular network.

Skeletal regeneration for segmental bone loss: Vascularised grafts, analogues and surrogates

Massive segmental bone defects (SBD) are mostly treated by removing the fibula and transplanting it complete with blood supply. While revolutionary 50 years ago, this remains the standard treatment. This review considers different strategies to repair SBD and emerging potential replacements for this highly invasive procedure. Prior to the technical breakthrough of microsurgery, researchers in the 1960s and 1970s had begun to make considerable progress in developing non autologous routes to repairing SBD. While the breaktthrough of vascularised bone transplantation solved the immediate problem of a lack of reliable repair strategies, much of their prior work is still relevant today. We challenge the assumption that mimicry is necessary or likely to be successful and instead point to the utility of quite crude (from a materials technology perspective), approaches. Together there are quite compelling indications that the body can regenerate entire bone segments with few or no exogenous factors. This is important, as there is a limit to how expensive a bone repair can be and still be widely available to all patients since cost restraints within healthcare systems are not likely to diminish in the near future. STATEMENT OF SIGNIFICANCE: This review is significant because it is a multidisciplinary view of several surgeons and scientists as to what is driving improvement in segmental bone defect repair, why many approaches to date have not succeeded and why some quite basic approaches can be as effective as they are. While there are many reviews of the literature of grafting and bone repair the relative lack of substantial improvement and slow rate of progress in clinical translation is often overlooked and we seek to challenge the reader to consider the issue more broadly.

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.

Platelet-Rich Plasma for Bone Fracture Treatment: A Systematic Review of Current Evidence in Preclinical and Clinical Studies

Background: Recently, there is an increasing interest in the therapeutic potential of platelet-rich plasma (PRP) for bone fracture treatment. Nevertheless, the effect of PRP for bone fracture treatment remains controversial and is still a matter of discussion. Therefore, we performed a systematic review to evaluate the efficacy and safety of PRP injection for treatment of bone fracture. Methods: The main bibliographic databases, including Medline, PubMed, Embase, Web of Science, and the Cochrane library, were comprehensively searched for studies focusing on the application of platelet-rich plasma (PRP) on bone fracture treatment. All relevant articles were screened for eligibility and subdivided into the preclinical and clinical studies. Data were extracted and presented systematically. Results: Finally, twenty-six in vitro preclinical studies (basic studies), nine in vivo preclinical studies (animal studies), and nine clinical studies, met the selection criteria, and were included in the present systematic review. Preclinical studies showed an overall positive effect of PRP on osteoblast-like cells in vitro and bone healing in animal models. The most used treatment for bone fracture in animal and clinical studies is fixation surgery combined with PRP injection. The clinical studies reported PRP shortened bony healing duration, and had no positive effect on improving the healing rate of closed fractures. However, the results of functional outcomes are controversial. Additionally, compared with control group, PRP would not increase the rate of postoperative wound infection. Conclusion: The present systematic review confirmed the continuing interests of PRP as an additional treatment for bone fracture. Preclinical studies highlighted the potential value of PRP as promising therapy for bone fracture. However, the preclinical evidence did not translate into a similar result in the clinical studies. In addition, types of fractures and procedures of PRP preparation are heterogeneous in enrolled studies, which might result in controversial results. Meanwhile, characteristics of PRP, such as platelet concentration, the numbers of leukocytes, still need to be determined and further research is required.

Printing New Bones: From Print-and-Implant Devices to Bioprinted Bone Organ Precursors

Regenerating large bone defects remains a significant clinical challenge, motivating increased interest in additive manufacturing and 3D bioprinting to engineer superior bone graft substitutes. 3D bioprinting enables different biomaterials, cell types, and growth factors to be combined to develop patient-specific implants capable of directing functional bone regeneration. Current approaches to bioprinting such implants fall into one of two categories, each with their own advantages and limitations. First are those that can be 3D bioprinted and then directly implanted into the body and second those that require further in vitro culture after bioprinting to engineer more mature tissues prior to implantation. This review covers the key concepts, challenges, and applications of both strategies to regenerate damaged and diseased bone.

Coupling Osteogenesis and Vasculogenesis in Engineered Orthopedic Tissues

Inadequate vascularization of engineered tissue constructs is a main challenge in developing a clinically impactful therapy for large, complex, and recalcitrant bone defects. It is well established that bone and blood vessels form concomitantly during development, as well as during repair after injury. Endothelial cells (ECs) and mesenchymal stromal cells (MSCs) are known to be key players in orthopedic tissue regeneration and vascularization, and these cell types have been used widely in tissue engineering strategies to create vascularized bone. Coculture studies have demonstrated that there is crosstalk between ECs and MSCs that can lead to synergistic effects on tissue regeneration. At the same time, the complexity in fabricating, culturing, and characterizing engineered tissue constructs containing multiple cell types presents a challenge in creating multifunctional tissues. In particular, the timing, spatial distribution, and cell phenotypes that are most conducive to promoting concurrent bone and vessel formation are not well understood. This review describes the processes of bone and vascular development, and how these have been harnessed in tissue engineering strategies to create vascularized bone. There is an emphasis on interactions between ECs and MSCs, and the culture systems that can be used to understand and control these interactions within a single engineered construct. Developmental engineering strategies to mimic endochondral ossification are discussed as a means of generating vascularized orthopedic tissues. The field of tissue engineering has made impressive progress in creating tissue replacements. However, the development of larger, more complex, and multifunctional engineered orthopedic tissues will require a better understanding of how osteogenesis and vasculogenesis are coupled in tissue regeneration. Impact statement Vascularization of large engineered tissue volumes remains a challenge in developing new and more biologically functional bone grafts. A better understanding of how blood vessels develop during bone formation and regeneration is needed. This knowledge can then be applied to develop new strategies for promoting both osteogenesis and vasculogenesis during the creation of engineered orthopedic tissues. This article summarizes the processes of bone and blood vessel development, with a focus on how endothelial cells and mesenchymal stromal cells interact to form vascularized bone both during development and growth, as well as tissue healing. It is meant as a resource for tissue engineers who are interested in creating vascularized tissue, and in particular to those developing cell-based therapies for large, complex, and recalcitrant bone defects.

Chondrogenic and BMP-4 primings confer osteogenesis potential to human cord blood mesenchymal stromal cells delivered with biphasic calcium phosphate ceramics

Bone marrow mesenchymal stem/stromal cells (BMSCs) show great promise for bone repair, however they are isolated by an invasive bone marrow harvest and their regenerative potential decreases with age. Conversely, cord blood can be collected non-invasively after birth and contains MSCs (CBMSCs) that can be stored for future use. However, whether CBMSCs can replace BMSCs targeting bone repair is unknown. This study evaluates the in vitro osteogenic potential of unprimed, osteogenically primed, or chondrogenically primed CBMSCs and BMSCs and their in vivo bone forming capacity following ectopic implantation on biphasic calcium phosphate ceramics in nude mice. In vitro, alkaline phosphatase (intracellular, extracellular, and gene expression), and secretion of osteogenic cytokines (osteoprotegerin and osteocalcin) was significantly higher in BMSCs compared with CBMSCs, while CBMSCs demonstrated superior chondrogenic differentiation and secretion of interleukins IL-6 and IL-8. BMSCs yielded significantly more cell engraftment and ectopic bone formation compared to CBMSCs. However, priming of CBMSCs with either chondrogenic or BMP-4 supplements led to bone formation by CBMSCs. This study is the first direct quantification of the bone forming abilities of BMSCs and CBMSCs in vivo and, while revealing the innate superiority of BMSCs for bone repair, it provides avenues to induce osteogenesis by CBMSCs.

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.