Growth plate extracellular matrix-derived scaffolds for large bone defect healing

Unraveling devitalization: its impact on immune response and ectopic bone remodeling from autologous and allogeneic callus mimics

Endochondral bone regeneration is a promising approach in regenerative medicine. Callus mimics (CMs) are engineered and remodeled into bone tissue upon implantation. The long-term objective is to fabricate a sustainable off-the-shelf treatment option for patients. Devitalization was introduced to facilitate storage and using allogeneic (donor) cells would further propel the off-the-shelf approach. However, allogeneic CMs for bone regeneration pose a potential antigenicity concern. Here, we explored the impact of devitalization on antigenicity and osteoinductive bone formation when implanting syngeneic or allogeneic CM in a vital or devitalized state. For this, we implanted chondrogenically differentiated rat-derived mesenchymal stromal cells using an allogeneic immunocompetent ectopic rat model. Vital syngeneic CMs demonstrated the highest bone formation, and vital allogeneic CMs showed the lowest bone formation, while both devitalized CMs showed comparable intermediate levels of bone formation. Preceding bone formation, the level of tartrate-resistant acid phosphatase staining at 7 and 14 days was proportional to the level of eventual bone formation. No differences were observed for local innate immune responses at any time point before or after bone formation. In contrast, allogeneic CMs elicit a mild adaptive immune response, which still permits bone formation in an immunocompetent environment, albeit at a reduced rate compared to the autologous living counterpart. Overall, devitalization delays bone formation when autologous CMs are implanted, whereas it accelerates bone formation in allogeneic CMs, highlighting the potential of this approach for achieving off-the-shelf treatment.

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.

Growing Pains: The Need for Engineered Platforms to Study Growth Plate Biology

Growth plates, or physis, are highly specialized cartilage tissues responsible for longitudinal bone growth in children and adolescents. Chondrocytes that reside in growth plates are organized into three distinct zones essential for proper function. Modeling key features of growth plates may provide an avenue to develop advanced tissue engineering strategies and perspectives for cartilage and bone regenerative medicine applications and a platform to study processes linked to disease progression. In this review, a brief introduction of the growth plates and their role in skeletal development is first provided. Injuries and diseases of the growth plates as well as physiological and pathological mechanisms associated with remodeling and disease progression are discussed. Growth plate biology, namely, its architecture and extracellular matrix organization, resident cell types, and growth factor signaling are then focused. Next, opportunities and challenges for developing 3D biomaterial models to study aspects of growth plate biology and disease in vitro are discussed. Finally, opportunities for increasingly sophisticated in vitro biomaterial models of the growth plate to study spatiotemporal aspects of growth plate remodeling, to investigate multicellular signaling underlying growth plate biology, and to develop platforms that address key roadblocks to in vivo musculoskeletal tissue engineering applications are described.

Bilayered extracellular matrix derived scaffolds with anisotropic pore architecture guide tissue organization during osteochondral defect repair

While some clinical advances in cartilage repair have occurred, osteochondral (OC) defect repair remains a significant challenge, with current scaffold-based approaches failing to recapitulate the complex, hierarchical structure of native articular cartilage (AC). To address this need, we fabricated bilayered extracellular matrix (ECM)-derived scaffolds with aligned pore architectures. By modifying the freeze-drying kinetics and controlling the direction of heat transfer during freezing, it was possible to produce anisotropic scaffolds with larger pores which supported homogenous cellular infiltration and improved sulfated glycosaminoglycan deposition. Neo-tissue organization in vitro could also be controlled by altering scaffold pore architecture, with collagen fibres aligning parallel to the long-axis of the pores within scaffolds containing aligned pore networks. Furthermore, we used in vitro and in vivo assays to demonstrate that AC and bone ECM derived scaffolds could preferentially direct the differentiation of mesenchymal stromal cells (MSCs) towards either a chondrogenic or osteogenic lineage respectively, enabling the development of bilayered ECM scaffolds capable of spatially supporting unique tissue phenotypes. Finally, we implanted these scaffolds into a large animal model of OC defect repair. After 6 months in vivo, scaffold implantation was found to improve cartilage matrix deposition, with collagen fibres preferentially aligning parallel to the long axis of the scaffold pores, resulting in a repair tissue that structurally and compositionally was more hyaline-like in nature. These results demonstrate how scaffold architecture and composition can be spatially modulated to direct the regeneration of complex interfaces such as the osteochondral unit, enabling their use as cell-free, off-the-shelf implants for joint regeneration. STATEMENT OF SIGNIFICANCE: The architecture of the extracellular matrix, while integral to tissue function, is often neglected in the design and evaluation of regenerative biomaterials. In this study we developed a bilayered scaffold for osteochondral defect repair consisting of tissue-specific extracellular matrix (ECM)-derived biomaterials to spatially direct stem/progenitor cell differentiation, with a tailored pore microarchitecture to promote the development of a repair tissue that recapitulates the hierarchical structure of native AC. The use of this bilayered scaffold resulted in improved tissue repair outcomes in a large animal model, specifically the ability to guide neo-tissue organization and therefore recapitulate key aspects of the zonal structure of native articular cartilage. These bilayer scaffolds have the potential to become a new therapeutic option for osteochondral defect repair.

Acceleration of Bone Regeneration Induced by a Soft-Callus Mimetic Material

Clinical implementation of endochondral bone regeneration (EBR) would benefit from the engineering of devitalized cartilaginous constructs of allogeneic origins. Nevertheless, development of effective devitalization strategies that preserves extracellular matrix (ECM) is still challenging. The aim of this study is to investigate EBR induced by devitalized, soft callus-mimetic spheroids. To challenge the translatability of this approach, the constructs are generated using an allogeneic cell source. Neo-bone formation is evaluated in an immunocompetent rat model, subcutaneously and in a critical size femur defect. Living spheroids are used as controls. Also, the effect of spheroid maturation towards hypertrophy is evaluated. The devitalization procedure successfully induces cell death without affecting ECM composition or bioactivity. In vivo, a larger amount of neo-bone formation is observed for the devitalized chondrogenic group both ectopically and orthotopically. In the femur defect, accelerated bone regeneration is observed in the devitalized chondrogenic group, where defect bridging is observed 4 weeks post-implantation. The authors' results show, for the first time, a dramatic increase in the rate of bone formation induced by devitalized soft callus-mimetics. These findings pave the way for the development of a new generation of allogeneic, "off-the-shelf" products for EBR, which are suitable for the treatment of every patient.

Bone Scaffolds: An Incorporation of Biomaterials, Cells, and Biofactors

Large injuries to bones are still one of the most challenging musculoskeletal problems. Tissue engineering can combine stem cells, scaffold biomaterials, and biofactors to aid in resolving this complication. Therefore, this review aims to provide information on the recent advances made to utilize the potential of biomaterials for making bone scaffolds and the assisted stem cell therapy and use of biofactors for bone tissue engineering. The requirements and different types of biomaterials used for making scaffolds are reviewed. Furthermore, the importance of stem cells and biofactors (growth factors and extracellular vesicles) in bone regeneration and their use in bone scaffolds and the key findings are discussed. Lastly, some of the main obstacles in bone tissue engineering and future trends are highlighted.

Enlightenment of Growth Plate Regeneration Based on Cartilage Repair Theory: A Review

The growth plate (GP) is a cartilaginous region situated between the epiphysis and metaphysis at the end of the immature long bone, which is susceptible to mechanical damage because of its vulnerable structure. Due to the limited regeneration ability of the GP, current clinical treatment strategies (e.g., bone bridge resection and fat engraftment) always result in bone bridge formation, which will cause length discrepancy and angular deformity, thus making satisfactory outcomes difficult to achieve. The introduction of cartilage repair theory and cartilage tissue engineering technology may encourage novel therapeutic approaches for GP repair using tissue engineered GPs, including biocompatible scaffolds incorporated with appropriate seed cells and growth factors. In this review, we summarize the physiological structure of GPs, the pathological process, and repair phases of GP injuries, placing greater emphasis on advanced tissue engineering strategies for GP repair. Furthermore, we also propose that three-dimensional printing technology will play a significant role in this field in the future given its advantage of bionic replication of complex structures. We predict that tissue engineering strategies will offer a significant alternative to the management of GP injuries.

3D bioprinting of prevascularised implants for the repair of critically-sized bone defects

For 3D bioprinted tissues to be scaled-up to clinically relevant sizes, effective prevascularisation strategies are required to provide the necessary nutrients for normal metabolism and to remove associated waste by-products. The aim of this study was to develop a bioprinting strategy to engineer prevascularised tissues in vitro and to investigate the capacity of such constructs to enhance the vascularisation and regeneration of large bone defects in vivo. From a screen of different bioinks, a fibrin-based hydrogel was found to best support human umbilical vein endothelial cell (HUVEC) sprouting and the establishment of a microvessel network. When this bioink was combined with HUVECs and supporting human bone marrow stem/stromal cells (hBMSCs), these microvessel networks persisted in vitro. Furthermore, only bioprinted tissues containing both HUVECs and hBMSCs, that were first allowed to mature in vitro, supported robust blood vessel development in vivo. To assess the therapeutic utility of this bioprinting strategy, these bioinks were used to prevascularise 3D printed polycaprolactone (PCL) scaffolds, which were subsequently implanted into critically-sized femoral bone defects in rats. Micro-computed tomography (µCT) angiography revealed increased levels of vascularisation in vivo, which correlated with higher levels of new bone formation. Such prevascularised constructs could be used to enhance the vascularisation of a range of large tissue defects, forming the basis of multiple new bioprinted therapeutics. STATEMENT OF SIGNIFICANCE: This paper demonstrates a versatile 3D bioprinting technique to improve the vascularisation of tissue engineered constructs and further demonstrates how this method can be incorporated into a bone tissue engineering strategy to improve vascularisation in a rat femoral defect model.

Decellularized Cartilage Extracellular Matrix Incorporated Silk Fibroin Hybrid Scaffolds for Endochondral Ossification Mediated Bone Regeneration

Tissue engineering strategies promote bone regeneration for large bone defects by stimulating the osteogenesis route via intramembranous ossification in engineered grafts, which upon implantation are frequently constrained by insufficient integration and functional anastomosis of vasculature from the host tissue. In this study, we developed a hybrid biomaterial incorporating decellularized cartilage extracellular matrix (CD-ECM) as a template and silk fibroin (SF) as a carrier to assess the bone regeneration capacity of bone marrow-derived mesenchymal stem cells (hBMSC’s) via the endochondral ossification (ECO) route. hBMSC’s were primed two weeks for chondrogenesis, followed by six weeks for hypertrophy onto hybrid CD-ECM/SF or SF alone scaffolds and evaluated for the mineralized matrix formation in vitro. Calcium deposition biochemically determined increased significantly from 4-8 weeks in both SF and CD-ECM/SF constructs, and retention of sGAG’s were observed only in CD-ECM/SF constructs. SEM/EDX revealed calcium and phosphate crystal localization by hBMSC’s under all conditions. Compressive modulus reached a maximum of 40 KPa after eight weeks of hypertrophic induction. μCT scanning at eight weeks indicated a cloud of denser minerals in groups after hypertrophic induction in CD-ECM/SF constructs than SF constructs. Gene expression by RT-qPCR revealed that hBMSC’s expressed hypertrophic markers VEGF, COL10, RUNX2, but the absence of early hypertrophic marker ChM1 and later hypertrophic marker TSBS1 and the presence of osteogenic markers ALPL, IBSP, OSX under all conditions. Our data indicate a new method to prime hBMSC’S into the late hypertrophic stage in vitro in mechanically stable constructs for ECO-mediated bone tissue regeneration.

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.

Injectable Disc-Derived ECM Hydrogel Functionalised with Chondroitin Sulfate for Intervertebral Disc Regeneration

Low back pain resulting from intervertebral disc (IVD) degeneration is a significant socioeconomic burden. The main effect of the degeneration process involves the alteration of the nucleus pulposus (NP) via cell-mediated enzymatic breakdown of key extracellular matrix (ECM) components. Thus, the development of injectable and biomimetic biomaterials that can instruct the regenerative cell component to produce tissue-specific ECM is pivotal for IVD repair. Chondroitin sulfate (CS) and type II collagen are the primary components of NP tissue and together create the ideal environment for cells to deposit de-novo matrix. Given their high matrix synthesis capacity potential post-expansion, nasal chondrocytes (NC) have been proposed as a potential cell source to promote NP repair. The overall goal of this study was to assess the effects of CS incorporation into disc derived self-assembled ECM hydrogels on the matrix deposition of NCs. Results showed an increased sGAG production with higher amounts of CS in the gel composition and that its presence was found to be critical for the synthesis of collagen type II. Taken together, our results demonstrate how the inclusion of CS into the composition of the material aids the preservation of a rounded cell morphology for NCs in 3D culture and enhances their ability to synthesise NP-like matrix.

[Preparation and osteogenic effect study of small intestinal submucosa sponge]

OBJECTIVE

To prepare and evaluate the basic properties in vitro of a novel small intestinal submucosa (SIS) sponge, and to describe the bone formation ability of the SIS sponge in vivo.

METHODS

The SIS sponge was prepared by freeze-drying method. To evaluate the physicochemical properties of the sponge, electron microscope observation, porosity test, water absorption ability and mechanical property were conducted in vitro. The cytotoxicity of the SIS sponge was performed by cell counting kit-8 method. In vivo experiments, eighteen extraction sockets of premolar of three Beagle dogs were randomly divided into three groups: SIS sponge group (SIS sponge), positive control group (Bio-Oss granules and Bio-Gide membrane) and control group(no treatment). The animals were sacrificed 4 weeks and 12 weeks after operation, and micro computed tomography (Micro-CT) was applied to measure the bone volume fraction (BV/TV) and bone mineralized density (BMD). The data were analyzed with one-way ANOVA.

RESULTS

The average pore diameter of the SIS sponge was (194.90±30.39) μm, the porosity was 92.31%±0.24%, the water absorption rate was 771.50%±40.90%, and the compressive elastic modulus was (2.20±0.19) kPa. There was no significant difference in cell proliferation ability between SIS sponge and control group (P>0.05). Micro-CT quantitative results showed that BV/TV of SIS sponge group (52.81%±3.21%) and positive control group (58.30%±9.36%) were significantly higher than that of control group (38.65%±4.80%) 4 weeks after operation (P < 0.05). The BMD of SIS sponge group [(887.09±61.02) mg/cm³], positive control group [(952.05±132.78) mg/cm³] and control group [(879.29±74.27) mg/cm³] showed no statistical difference 4 weeks after operation (P>0.05). The BV/TV of positive control group (60.57%± 6.56%) was significantly higher than that of SIS sponge group (47.89%±3.59%) and control group (42.99%±2.54%) 12 weeks after operation (P < 0.05). BMD of SIS sponge group [(1047±89.95) mg/cm³] and positive control group [(1101.37±98.85) mg/cm³] were significantly higher than that of control group [(890.36±79.79) mg/cm³] 12 weeks after operation (P < 0.05).

CONCLUSION

The SIS sponge has satisfying physicochemical properties and biocompatibility. The SIS sponge significantly increased bone volume fraction in the early stage of bone formation (4 weeks) and bone mineralized density in the late stage of bone formation (12 weeks).

The identification of articular cartilage and growth plate extracellular matrix-specific proteins supportive of either osteogenesis or stable chondrogenesis of stem cells

Tissue-specific extracellular matrix (ECM) proteins can play a key role in regulating the fate of stem cells and can potentially be utilized for therapeutic applications. Realising this potential requires further characterization of the diversity of biomolecules present in tissue-specific ECMs and an evaluation of their role as regulatory cues for regenerative medicine applications. The goal of this study was to identify specific soluble factors within the ECM of articular cartilage (AC) and growth plate (GP) that may impart chondro-inductivity or osteo-inductivity respectively. To this end, the significantly different proteins between both matrisomes were searched against the STRING database platform, from which C-type lectin domain family-11 member-A (CLEC11A) and S100 calcium-binding protein-A10 (S100A10) were identified as potential candidates for supporting osteogenesis, and Gremlin-1 (GREM1) and TGF-β induced gene human clone-3 (βIGH3) were identified as potential candidates for supporting stable chondrogenesis. Stimulation of chondrogenically-primed bone marrow-derived stem cells (BMSCs) with the AC-specific proteins GREM1 and βIGH3 had no noticeable effect on the deposition of collagen-II, a marker of chondrogenesis, but appeared to suppress the production of the hypertrophic marker collagen-X, particularly for higher concentrations of GREM1. Stimulation with GREM1 was also found to suppress the direct osteoblastic differentiation of BMSCs. In contrast, stimulation with the GP-specific factors CLEC11A and S100A10 was found to enhance osteogenesis of BMSCs, increasing the levels of mineralization, particularly for higher concentration of CLEC11A. Together these results demonstrate that AC- and GP-specific proteins may play a key role in developing novel strategies for engineering phenotypically stable articular cartilage or enhancing the regeneration of critically-sized bone defects.

The Bone Extracellular Matrix in Bone Formation and Regeneration

Bone regeneration repairs bone tissue lost due to trauma, fractures, and tumors, or absent due to congenital disorders. The extracellular matrix (ECM) is an intricate dynamic bio-environment with precisely regulated mechanical and biochemical properties. In bone, ECMs are involved in regulating cell adhesion, proliferation, and responses to growth factors, differentiation, and ultimately, the functional characteristics of the mature bone. Bone ECM can induce the production of new bone by osteoblast-lineage cells, such as MSCs, osteoblasts, and osteocytes and the absorption of bone by osteoclasts. With the rapid development of bone regenerative medicine, the osteoinductive, osteoconductive, and osteogenic potential of ECM-based scaffolds has attracted increasing attention. ECM-based scaffolds for bone tissue engineering can be divided into two types, that is, ECM-modified biomaterial scaffold and decellularized ECM scaffold. Tissue engineering strategies that utilize the functional ECM are superior at guiding the formation of specific tissues at the implantation site. In this review, we provide an overview of the function of various types of bone ECMs in bone tissue and their regulation roles in the behaviors of osteoblast-lineage cells and osteoclasts. We also summarize the application of bone ECM in bone repair and regeneration. A better understanding of the role of bone ECM in guiding cellular behavior and tissue function is essential for its future applications in bone repair and regenerative medicine.

Comparative Study on Acellular Dermal Graft Versus Propylene Mesh Both Either Loaded or Unloaded with BM-MSCs in Healing of Skull Bone Defect in Rats: Histological and Immunohistochemical Study

Bone defect occurs as a consequence of many conditions. Diseased bones don’t heal properly and defects in face area need proper bone reconstruction to avoid psychological and social problems. Tissue engineering is an emerging new modality of treatment. We thought to study different methods to fill skull bone defect in rats in order to find the most safe and effective method. So, this study was designed to evaluate the efficacy of acellular dermal graft (ADM) versus propylene mesh both either loaded or unloaded with bone marrow derived mesenchymal stem cells (BM-MSCs) in healing of skull bone defect of a 5 mm diameter. The study included 36 adult male Wistar albino rats that were divided into three groups according to the way of filling skull bone defect. Group I: Ia (sham control), Ib (negative control). Group II: IIa (unseeded propylene), IIb (seeded propylene) and Group III: IIIa (unseeded ADM), IIIb (seeded ADM). The trephine operation was done on the left parietal bone. Specimens were collected four weeks postoperative and processed for H&E, osteopontin immunohistochemistry and scanning electron microscope. Morphometric and statistical analysis were also performed. After studying the results of the experiment, we found that propylene mesh and ADM were suitable scaffolds that could support new bone formation in clavarial bone defect. Healing of skull bone defect was better in rats that received seeded scaffolds more than rats with unseeded scaffolds. The seeded ADM showed significant increase in bone forming activity as confirmed by histomorphometric and statistical results.

Comparative analysis of bone regeneration behavior using recombinant human BMP-2 versus plasmid DNA of BMP-2

Bone regeneration and the osteoinductive capacity of implants are challenging issues in clinical medicine. Currently, recombinant growth factors and nonviral gene transfer are the most frequently investigated methods for bone growth enhancement, although the more favorable method remains unclear. There is a lack of knowledge in literature about the in vivo comparison of these methods for bone regeneration. BMP-2, which is the most commonly used growth factor for osteogenesis, was applied at its most efficient dose as a recombinant growth factor (rhBMP-2) and as a growth-factor-encoding copolymer protected gene vector (pBMP-2) in a critical size bone defect (CSD) model to determine the most suitable method for bone regeneration. CSDs were induced bilaterally in 32 Sprague-Dawley rats. RhBMP-2 (62.5 μg) or pBMP-2 (2.5 μg) was embedded in poly(d,l-)lactide-coated titanium discs. Survival times were set at 14, 28, 56, and 112 days. After euthanasia, samples were analyzed via micro-computed tomography, polychrome sequential fluorescent labeling, and immunohistochemistry. Whereas defects in both groups were bridged with new bone after 56 days, rhBMP-2 initially induced ectopic new bone formation that was later remodeled in an unorganized hypodense manner. In contrast, pBMP-2 led to slower but steady bone regeneration with physiological tissue morphology, as confirmed by high osteoblast activity shown by osteocalcin staining. CD68 and TRAP staining verified high osteoclast activity for the rhBMP-2 group. pBMP-2 successfully induced locally controlled physiological bone regeneration, whereas rhBMP-2 triggered rapid and ectopic but insufficient bone formation. Thus, nonviral gene transfer appears to be more favorable for clinical applications. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 107A: 163-173, 2019.

Effect of Hydroxyapatite Formation on Titanium Surface with Bone Morphogenetic Protein-2 Loading through Electrochemical Deposition on MG-63 Cells

Calcium phosphate ceramics used in dentistry and orthopedics are some of the most valuable biomaterials, owing to their excellent osteoconduction, osteoinduction, and osseointegration. Osteoconduction and osteoinduction are critical targets for bone regeneration, and osseointegration is essential for any dental implantations. In this study, a hydroxyapatite (HAp) hybrid coating layer with the sequential release of bone morphogenetic protein 2 (BMP-2) was deposited onto an etched titanium substrate by electrochemical deposition. The resulting release of BMP-2 from Ti⁻HAp was assessed by immersing samples in a simulated buffer fluid solution. Through coculture, human osteosarcoma cell proliferation and alkaline phosphatase activity were assessed. The characteristics and effect on cell proliferation of the hybrid coatings were investigated for their functionality through X-ray diffraction (XRD) and cell proliferation assays. Findings revealed that -0.8 V vs. Ag/AgCl (3 M KCl) exhibited the optimal HAp properties and a successfully coated HAp layer. XRD confirmed the crystallinity of the deposited HAp on the titanium surface. Ti-0.8 V Ti⁻HAp co-coating BMP sample exhibited the highest cell proliferation efficiency and was more favorable for cell growth. A successful biocompatible hybrid coating with optimized redox voltage enhanced the osseointegration process. The findings suggest that this technique could have promising clinical applications to enhance the healing times and success rates of dental implantation.

Effects of tissue processing on bioactivity of cartilage matrix-based hydrogels encapsulating osteoconductive particles

In the treatment of severe traumatic brain injury (TBI), decompressive craniectomy is commonly used to remove a large portion of calvarial bone to allow unimpeded brain swelling. Hydrogels have the potential to revolutionize TBI treatment by permitting a single-surgical intervention, remaining pliable during brain swelling, and tuned to regenerate bone after swelling has subsided. With this motivation, our goal is to present a pliable material capable of regenerating calvarial bone across a critical size defect. We therefore proposed the use of a methacrylated solubilized decellularized cartilage (MeSDCC) hydrogel encapsulating synthetic osteogenic particles of hydroxyapatite nanofibers, bioglass microparticles, or added rat bone marrow-derived mesenchymal stem cells (rMSCs) for bone regeneration in critical-size rat calvarial defects. Fibrin hydrogels were employed as a control material for the study. MeSDCC hydrogels exhibited sufficient rheological performance for material placement before crosslinking ([Formula: see text] > 500 Pa), and sufficient compressive moduli post-crosslinking (E > 150 kPa). In vitro experiments suggested increased calcium deposition for cells seeded on the MeSDCC material; however, in vivo bone regeneration was minimal in both MeSDCC and fibrin groups, even with colloidal materials or added rMSCs. Minimal bone regeneration in the MeSDCC test groups may potentially be attributed to cartilage solubilization after decellularization, in which material signals may have degraded from enzymatic treatment. Looking to the future, an improvement in the bioactivity of the material will be crucial to the success of bone regeneration strategies for TBI treatment.

3D printed microchannel networks to direct vascularisation during endochondral bone repair

Bone tissue engineering strategies that recapitulate the developmental process of endochondral ossification offer a promising route to bone repair. Clinical translation of such endochondral tissue engineering strategies will require overcoming a number of challenges, including the engineering of large and often anatomically complex cartilage grafts, as well as the persistence of core regions of avascular cartilage following their implantation into large bone defects. Here 3D printing technology is utilized to develop a versatile and scalable approach to guide vascularisation during endochondral bone repair. First, a sacrificial pluronic ink was used to 3D print interconnected microchannel networks in a mesenchymal stem cell (MSC) laden gelatin-methacryloyl (GelMA) hydrogel. These constructs (with and without microchannels) were next chondrogenically primed in vitro and then implanted into critically sized femoral bone defects in rats. The solid and microchanneled cartilage templates enhanced bone repair compared to untreated controls, with the solid cartilage templates (without microchannels) supporting the highest levels of total bone formation. However, the inclusion of 3D printed microchannels was found to promote osteoclast/immune cell invasion, hydrogel degradation, and vascularisation following implantation. In addition, the endochondral bone tissue engineering strategy was found to support comparable levels of bone healing to BMP-2 delivery, whilst promoting lower levels of heterotopic bone formation, with the microchanneled templates supporting the lowest levels of heterotopic bone formation. Taken together, these results demonstrate that 3D printed hypertrophic cartilage grafts represent a promising approach for the repair of complex bone fractures, particularly for larger defects where vascularisation will be a key challenge.

Cyclic Tensile Strain Can Play a Role in Directing both Intramembranous and Endochondral Ossification of Mesenchymal Stem Cells

Successfully regenerating damaged or diseased bone and other joint tissues will require a detailed understanding of how joint specific environmental cues regulate the fate of progenitor cells that are recruited or delivered to the site of injury. The goal of this study was to explore the role of cyclic tensile strain (CTS) in regulating the initiation of mesenchymal stem cell/multipotent stromal cell (MSC) differentiation, and specifically their progression along the endochondral pathway. To this end, we first explored the influence of CTS on the differentiation of MSCs in the absence of any specific growth factor, and secondly, we examined the influence of the long-term application of this mechanical stimulus on markers of endochondral ossification in MSCs maintained in chondrogenic culture conditions. A custom bioreactor was developed to apply uniaxial tensile deformation to bone marrow-derived MSCs encapsulated within physiological relevant 3D fibrin hydrogels. Mechanical loading, applied in the absence of soluble differentiation factors, was found to enhance the expression of both tenogenic (COL1A1) and osteogenic markers (BMP2, RUNX2, and ALPL), while suppressing markers of adipogenesis. No evidence of chondrogenesis was observed, suggesting that CTS can play a role in initiating direct intramembranous ossification. During long-term culture in the presence of a chondrogenic growth factor, CTS was shown to induce MSC re-organization and alignment, increase proteoglycan and collagen production, and to enhance the expression of markers associated with endochondral ossification (BMP2, RUNX2, ALPL, OPN, and COL10A1) in a strain magnitude-dependent manner. Taken together, these findings indicate that tensile loading may play a key role in promoting both intramembranous and endochondral ossification of MSCs in a context-dependent manner. In both cases, this loading-induced promotion of osteogenesis was correlated with an increase in the expression of the osteogenic growth factor BMP2. The results of this study demonstrate the potent role that extrinsic mechanical loading plays in guiding stem cell fate, which must be carefully considered when designing cell and tissue-engineering therapies if they are to realize their clinical potential.