The effect of mesenchymal stromal cell sheets on the inflammatory stage of flexor tendon healing

Evaluation of Patellar Tendon Structural Changes following Biological Treatments: Secondary Analysis of Double-Blinded Clinical Trial of Bone Marrow Mesenchymal Stromal Cells and Leukocyte-Poor Platelet-Rich Plasma

Objective quantification of tendon structural changes through imaging is only achieved by evaluating tendon structure using ultrasound tissue characterization (UTC) technology. This study compares the effects of bone marrow mesenchymal stromal cells (BM-MSC) and leukocyte-poor platelet-rich plasma (Lp-PRP) on tendon structure and clinical outcomes in male patients with patellar tendinopathy measured with UTC at 3, 6, and 12 months after treatment. This is a double-blinded clinical trial with a randomized active control study with 20 male patients diagnosed with patellar tendinopathy who underwent BM-MSC and Lp-PRP. Bilateral ultrasound tissue characterization scans of the patellar tendon were carried out after 3, 6, and 12 months, as well as tests for strength and pain. UTC patellar tendon was analyzed at the insertion, proximal, and mid-tendon. BM-MSC showed a greater capacity to promote further positive changes than Lp-PRP. Lp-PRP presented higher disorganized echo-type II in the mid-tendon (p = 0.04; ES = 1.06) and III (p = 0.02; ES = -1.47) after 3 months in the Lp-PRP group. Similar results were seen after 6 and 12 months. Pain and strength data show improvement in the treated tendon. BM-MSC treatment demonstrates a superior capacity to promote tendon regeneration and organization, restore strength, and reduce pain compared to Lp-PRP, after 3, 6, and 12 months in male patients with patellar tendinopathy.

Histological and biochemical changes in a rat rotator cuff tear model with or without the subacromial bursa

The subacromial bursa (SAB) plays an important role in the tendon healing process. Based on previous reports, co-culture of the rotator cuff (RC) and SAB have been shown to increase the tendon-related gene expressions, inflammatory cytokines, and tensile strength. However, the nature of the specific biochemical alterations during the inflammatory and repair phases of tendon healing with or without the SAB remain unknown. Using a full-thickness RC tear rat model, we determined how the presence or absence of the SAB alters the histological characteristics and gene expressions. After 3 and 6 weeks, tissues were collected for histological and real-time quantitative polymerase chain reaction (RT-qPCR) evaluations. Results showed greater cell density at 3 weeks, neovascularization and tendon thickening at 6 weeks with SAB preservation. Immunostaining revealed significant increases in type 3 collagen (COL3) expression at 6 weeks with SAB preservation. The RT-qPCR results showed that SAB preservation induced significant increases in the expression of scleraxis, matrix metalloproteinase-13 (MMP-13), interleukin-1β (IL-1β), and inducible nitric oxide synthase (iNOS) at 3 weeks and significant increases in COL3, IL-10, and arginase-1 (Arg-1) at 6 weeks. An RC tear undergoes more appropriate inflammatory and repair phases during the tendon healing process when the SAB is retained.

Understanding exosomes: Part 2-Emerging leaders in regenerative medicine

Exosomes are the smallest subset of extracellular signaling vesicles secreted by most cells with the ability to communicate with other tissues and cell types over long distances. Their use in regenerative medicine has gained tremendous momentum recently due to their ability to be utilized as therapeutic options for a wide array of diseases/conditions. Over 5000 publications are currently being published yearly on this topic, and this number is only expected to dramatically increase as novel therapeutic strategies continue to be developed. Today exosomes have been applied in numerous contexts including neurodegenerative disorders (Alzheimer's disease, central nervous system, depression, multiple sclerosis, Parkinson's disease, post-traumatic stress disorders, traumatic brain injury, peripheral nerve injury), damaged organs (heart, kidney, liver, stroke, myocardial infarctions, myocardial infarctions, ovaries), degenerative processes (atherosclerosis, diabetes, hematology disorders, musculoskeletal degeneration, osteoradionecrosis, respiratory disease), infectious diseases (COVID-19, hepatitis), regenerative procedures (antiaging, bone regeneration, cartilage/joint regeneration, osteoarthritis, cutaneous wounds, dental regeneration, dermatology/skin regeneration, erectile dysfunction, hair regrowth, intervertebral disc repair, spinal cord injury, vascular regeneration), and cancer therapy (breast, colorectal, gastric cancer and osteosarcomas), immune function (allergy, autoimmune disorders, immune regulation, inflammatory diseases, lupus, rheumatoid arthritis). This scoping review is a first of its kind aimed at summarizing the extensive regenerative potential of exosomes over a broad range of diseases and disorders.

Cell Sheet Technology: An Emerging Approach for Tendon and Ligament Tissue Engineering

Tendon and ligament injuries account for a substantial proportion of disorders in the musculoskeletal system. While non-operative and operative treatment strategies have advanced, the restoration of native tendon and ligament structures after injury is still challenging due to its innate limited regenerative ability. Cell sheet technology is an innovative tool for tissue fabrication and cell transplantation in regenerative medicine. In this review, we first summarize different harvesting procedures and advantages of cell sheet technology, which preserves intact cell-to-cell connections and extracellular matrix. We then describe the recent progress of cell sheet technology from preclinical studies, focusing on the application of stem cell-derived sheets in treating tendon and ligament injuries, as well as highlighting its effects on mitigating inflammation and promoting tendon/graft-bone interface healing. Finally, we discuss several prerequisites for future clinical translation including the selection of appropriate cell source, optimization of preparation process, establishment of suitable animal model, and the fabrication of vascularized complex tissue. We believe this review could potentially provoke new ideas and drive the development of more functional biomimetic tissues using cell sheet technology to meet the needs of clinical patients.

Advances in mesenchymal stem cells therapy for tendinopathies

Tendinopathies are chronic diseases of an unknown etiology and associated with inflammation. Mesenchymal stem cells (MSCs) have emerged as a viable therapeutic option to combat the pathological progression of tendinopathies, not only because of their potential for multidirectional differentiation and self-renewal, but also their excellent immunomodulatory properties. The immunomodulatory effects of MSCs are increasingly being recognized as playing a crucial role in the treatment of tendinopathies, with MSCs being pivotal in regulating the inflammatory microenvironment by modulating the immune response, ultimately contributing to improved tissue repair. This review will discuss the current knowledge regarding the application of MSCs in tendinopathy treatments through the modulation of the immune response.

Treatment of Naturally Occurring Tendon Disease with Allogeneic Multipotent Mesenchymal Stromal Cells: A Randomized, Controlled, Triple-Blinded Pilot Study in Horses

The treatment of tendinopathies with multipotent mesenchymal stromal cells (MSCs) is a promising option in equine and human medicine. However, conclusive clinical evidence is lacking. The purpose of this study was to gain insight into clinical treatment efficacy and to identify suitable outcome measures for larger clinical studies. Fifteen horses with early naturally occurring tendon disease were assigned to intralesional treatment with allogeneic adipose-derived MSCs suspended in serum or with serum alone through block randomization (dosage adapted to lesion size). Clinicians and horse owners remained blinded to the treatment during 12 months (seven horses per group) and 18 months (seven MSC-group and five control-group horses) of follow-up including clinical examinations and diagnostic imaging. Clinical inflammation, lameness, and ultrasonography scores improved more over time in the MSC group. The lameness score difference significantly improved in the MSC group compared with the control group after 6 months. In the MSC group, five out of the seven horses were free of re-injuries and back to training until 12 and 18 months. In the control group, three out of the seven horses were free of re-injuries until 12 months. These results suggest that MSCs are effective for the treatment of early-phase tendon disease and provide a basis for a larger controlled study.

The effects of NF-κB suppression on the early healing response following intrasynovial tendon repair in a canine model

The highly variable clinical outcomes noted after intrasynovial tendon repair have been associated with an early inflammatory response leading to the development of fibrovascular adhesions. Prior efforts to broadly suppress this inflammatory response have been largely unsuccessful. Recent studies have shown that selective inhibition of IkappaB kinase beta (IKK-β), an upstream activator of nuclear factor kappa-light chain enhancer of activated B cells (NF-κB) signaling, mitigates the early inflammatory response and leads to improved tendon healing outcomes. In the current study, we test the hypothesis that oral treatment with the IKK-β inhibitor ACHP (2-amino-6-[2-(cyclopropylmethoxy)-6-hydroxyphenyl]-4-piperidin-4-yl nicotinenitrile an inhibitor) will modulate the postoperative inflammatory response and improve intrasynovial flexor tendon healing. To test this hypothesis, the flexor digitorum profundus tendon of 21 canines was transected and repaired within the intrasynovial region and assessed after 3 and 14 days. Histomorphometry, gene expression analyses, immunohistochemistry, and quantitative polarized light imaging were used to examine ACHP-mediated changes. ACHP led to reduction in phosphorylated p-65, indicating that NF-κB activity was suppressed. ACHP enhanced expression of inflammation-related genes at 3 days and suppressed expression of these genes at 14 days. Histomorphometry revealed enhanced cellular proliferation and neovascularization in ACHP-treated tendons compared with time-matched controls. These findings demonstrate that ACHP effectively suppressed NF-κB signaling and modulated early inflammation, leading to increased cellular proliferation and neovascularization without stimulating the formation of fibrovascular adhesions. Together, these data suggest that ACHP treatment accelerated the inflammatory and proliferative phases of tendon healing following intrasynovial flexor tendon repair. Clinical Significance: Using a clinically relevant large-animal model, this study revealed that targeted inhibition of nuclear factor kappa-light chain enhancer of activated B cells signaling with ACHP provides a new therapeutic strategy for enhancing the repair of sutured intrasynovial tendons.

Silver nanoparticles-decorated extracellular matrix graft: fabrication and tendon reconstruction performance

BACKGROUND

The reconstruction of tendons with large defects requires grafts with high mechanical strength and is often hindered by complications such as infection and adhesion. Hence, grafts combining the advantages of mechanical resilience and antibacterial/antiadhesion activity are highly sought after.

METHODS

The silver nanoparticles (GA-Ag NPs) synthesized from gallic acid and silver nitrate were attached to a decellularized extracellular matrix (Decellularized Tendon crosslinking GA-AgNPs, DT-Ag). We examined the histological structure, mechanical property, morphology, Zeta potential, cytotoxicity, antibacterial properties, antioxidant and anti-inflammatory properties, and ability of the DT-Ag to treat tendon defects in animals.

RESULTS

Approximately 108.57 ± 0.94 μg GA-Ag NPs loaded per 50 mg DT, the cross-linked part of GA-Ag NPs was 65.47 ± 0.57%, which provided DT-Ag with long-lasting antibacterial activity. Meanwhile, GA endowed DT-Ag with good antioxidant and anti-inflammatory activities. Additionally, The DT-Ag facilitated M2 macrophage polarization, and suppressed fibrin deposition by hindering fibroblast adhesion. Mormore, the main advantages of DT-Ag, namely its long-lasting antibacterial activity (tested using Escherichia coli and Staphylococcus aureus as models) and the ability to prevent tissue adhesion were confirmed in vivo.

CONCLUSION

The fabricated multifunctional tendon graft was highly hydrophilic, biocompatible, and mechanically resilient, and concluded to be well suited for dealing with the main complications of surgical tendon reconstruction and has bright application prospects.

Mesenchymal Stem Cells in Soft Tissue Regenerative Medicine: A Comprehensive Review

Soft tissue regeneration holds significant promise for addressing various clinical challenges, ranging from craniofacial and oral tissue defects to blood vessels, muscle, and fibrous tissue regeneration. Mesenchymal stem cells (MSCs) have emerged as a promising tool in regenerative medicine due to their unique characteristics and potential to differentiate into multiple cell lineages. This comprehensive review explores the role of MSCs in different aspects of soft tissue regeneration, including their application in craniofacial and oral soft tissue regeneration, nerve regeneration, blood vessel regeneration, muscle regeneration, and fibrous tissue regeneration. By examining the latest research findings and clinical advancements, this article aims to provide insights into the current state of MSC-based therapies in soft tissue regenerative medicine.

M2 Macrophage Membrane-Mediated Biomimetic-Nanoparticle Carrying COX-siRNA Targeted Delivery for Prevention of Tendon Adhesions by Inhibiting Inflammation

Tendon adhesion is the most common outcome of tendon or tendon-to-bone healing after injury. Our group developed a hydrogel-nanoparticle sustained-release system previously to inhibit cyclooxygenases (COXs) expression and consequently prevent tendon adhesion and achieved satisfactory results. However, effective treatment of multiple tendon adhesions is always a challenge in research on the prevention of tendon adhesion. In the present study, an M2M@PLGA/COX-siRNA delivery system is successfully constructed using the cell membranes of M2 macrophages and poly (lactic-co-glycolic acid) (PLGA) nanoparticles. Targeting properties and therapeutic effects are observed in mice or rat models of flexor digitorum longus (FDL) tendon injury combined with rotator cuff injury. The results showed that the M2M@PLGA/COX-siRNA delivery system has low toxicity and remarkable targeting properties to the injured areas. Treatment with the M2M@PLGA/COX-siRNA delivery system reduced the inflammatory reaction and significantly improved tendon adhesion in both the FDL tendon and rotator cuff tissues. These findings indicate that the M2M@PLGA delivery system can provide an effective biological strategy for preventing multiple tendon adhesions.

Nanoscale vibration could promote tenogenic differentiation of umbilical cord mesenchymal stem cells

Regulation of mesenchymal stem cell (MSC) fate for targeted cell therapy applications has been a subject of interest, particularly for tissues such as tendons that possess a marginal regenerative capacity. Control of MSCs' fate into the tendon-specific lineage has mainly been achieved by implementation of chemical growth factors. Mechanical stimuli or 3-dimensional (D) scaffolds have been used as an additional tool for the differentiation of MSCs into tenocytes, but oftentimes, they require a sophisticated bioreactor or a complex scaffold fabrication technique which reduces the feasibility of the proposed method to be used in practice. Here, we used nanovibration to induce the differentiation of MSCs toward the tenogenic fate solely by the use of nanovibration and without the need for growth factors or complex scaffolds. MSCs were cultured on 2D cell culture dishes that were connected to piezo ceramic arrays to apply nanovibration (30-80 nm and 1 kHz frequency) over 7 and 14 d. We observed that nanovibration resulted in significant overexpression of tendon-related markers in both gene expression and protein expression levels, while there was no significant differentiation into adipose and cartilage lineages. These findings could be of assistance in the mechanoregulation of MSCs for stem cell engineering and regenerative medicine applications.

Extracellular Vesicles From Primed Adipose-Derived Stem Cells Enhance Achilles Tendon Repair by Reducing Inflammation and Promoting Intrinsic Healing

Achilles tendon rupture is a common sports-related injury. Even with advanced clinical treatments, many patients suffer from long-term pain and functional deficits. These unsatisfactory outcomes result primarily from an imbalanced injury response with excessive inflammation and inadequate tendon regeneration. Prior studies showed that extracellular vesicles from inflammation-primed adipose-derived stem cells (iEVs) can attenuate early tendon inflammatory response to injury. It remains to be determined if iEVs can both reduce inflammation and promote regeneration in the later phases of tendon healing and the underlying mechanism. Therefore, this study investigated the mechanistic roles of iEVs in regulating tendon injury response using a mouse Achilles tendon injury and repair model in vivo and iEV-macrophage and iEV-tendon cell coculture models in vitro. Results showed that iEVs promoted tendon anti-inflammatory gene expression and reduced mononuclear cell accumulation to the injury site in the remodeling phase of healing. iEVs also increased collagen deposition in the injury center and promoted tendon structural recovery. Accordingly, mice treated with iEVs showed less peritendinous scar formation, much lower incidence of postoperative tendon gap or rupture, and faster functional recovery compared to untreated mice. Further in vitro studies revealed that iEVs both inhibited macrophage M1 polarization and increased tendon cell proliferation and collagen production. The iEV effects were partially mediated by miR-147-3p, which blocked the toll-like receptor 4/NF-κB signaling pathway that activated the M1 phenotype of macrophages. The combined results demonstrate that iEVs are a promising therapeutic agent that can enhance tendon repair by attenuating inflammation and promoting intrinsic healing.

Multicellular Bioprinting of Biomimetic Inks for Tendon-to-Bone Regeneration

Tendon-to-bone interface has a hierarchical structure and gradient component that are conducive to distributing the stresses to achieve movement. Conventional biomaterials lack the capacity to induce synchronous repair of multiple tissues, resulting in the failure of the interface repair. Biomimetic strategies have attracted enormous attention in the field of complex structure regeneration because they can meet the different physiological requirements of multiple tissues. Herein, a biomimetic ink mimicking tendon/bone tissues is developed by combining tendon/bone-related cells and Mo-containing silicate (MS) bioceramics. Subsequently, biomimetic multicellular scaffolds are fabricated to achieve the simulation of the hierarchical structure and cellular composition of tendon-to-bone interfaces by the spatial distribution of the biomimetic inks via 3D bioprinting, which is of great significance for inducing the regeneration of complex structures in the interface region. In addition, attributed to the desirable ionic microenvironment created by MS bioceramics, the biomimetic scaffolds possess the dual function of inducing tendon/bone-related cells tenogenic and osteogenic differentiation in vitro, and promote the integrated regeneration of tendon-to-bone interfaces in vivo. The study offers a feasible strategy to construct biomimetic multicellular scaffolds with bifunction for inducing multi-lineage tissue regeneration, especially for regenerating soft-to-hard tissue interfaces.

Long term functional outcome evaluation in post flexor digitorum profundus tendon zone I rupture repaired by palmaris longus tendon grafting augmented with human amniotic membranes and adipose derived mesenchymal stem cell: A case report

INTRODUCTION AND IMPORTANCE

The sport climbing has many complex maneuvers of the hand producing many potential injuries in flexor digitorum profundus tendon (FDPT). The late management due to an athlete high demand on competition makes the complication of retracted tendon and adhesion tend to occur. We provide the long terms functional outcome in FDPT zone I rupture repaired by palmaris longus (PL) tendon grafting augmented with human amniotic (hAM) and adipose derived mesenchymal stem cell (ASCs).

CASE PRESENTATION

We present a case of a 31-years old male sport climbing athlete with excruciating pain in the right middle finger due to an injury at distal phalangeal area occurred two months earlier. Intraoperatively, Bruner's incision was performed for exploration. A modified Kessler suture technique with running sutures around the sutured stump was used. We slightly overcorrected tension between PL and FDPT distal stumps. We shielded the distal and proximal sutured areas with hAM augmented with ASCs. The result was remarkable as he could return to competitive sport.

CLINICAL DISCUSSION

Zones I and II have a high adhesion risk due to their complex structures. In the case of the PL tendon graft, the sutured stump lies in these zones which can affect outcomes. An HAM augmented with ASCs has an anti-adhesive property that allows smooth gliding of the tendon FDPT on two sutured stump junctions, as well as stimulating the tendon to produce tenocytes, which accelerates tendon healing.

CONCLUSION

The combination of our technique and regenerative therapy effectively prevents adhesions and modulates tendon healing.

Extracellular Vesicles from Inflammation-Primed Adipose-Derived Stem Cells Enhance Achilles Tendon Repair by Reducing Inflammation and Promoting Intrinsic Healing

Achilles tendon rupture is a common sports-related tendon injury. Even with advanced clinical treatments, many patients suffer from long-term pain and reduced function. These unsatisfactory outcomes result primarily from an imbalanced injury response with excessive inflammation and inadequate regeneration. Prior studies showed that extracellular vesicles from inflammation-primed adipose-derived stem cells (iEVs) can attenuate inflammation in the early phase of tendon healing. However, the effect of iEVs on tendon inflammation and regeneration in the later phases of tendon healing and the underlying mechanism remain to be determined. Accordingly, this study investigated the mechanistic roles of iEVs in regulating tendon response to injury using a mouse Achilles tendon injury and repair model in vivo and iEV-macrophage and iEV-tendon cell co-culture models in vitro. Results showed that iEVs promoted tendon anti-inflammatory gene expression and reduced mononuclear cell infiltration in the remodeling phase of tendon healing. iEVs also increased injury site collagen deposition and promoted tendon structural recovery. As such, mice treated with iEVs showed less peritendinous scar formation, much lower incidence of postoperative tendon gap or rupture, and faster functional recovery compared to untreated mice. Further in vitro study revealed that iEVs both inhibited macrophage inflammatory response and increased tendon cell proliferation and collagen production. The iEV effects were partially mediated by miR-147-3p, which blocks the toll-like receptor 4/NF-κB signaling pathway that activates macrophage M1 polarization. The combined results demonstrated that iEVs are a promising therapeutic agent, which can enhance tendon repair by attenuating inflammation and promoting intrinsic healing.

Significance statement

Using a clinically relevant mouse Achilles tendon injury and repair model, this study revealed that iEVs, a biological product generated from inflammation-primed adipose-derived stem cells, can directly target both macrophages and tendon cells and enhance tendon structural and functional recovery by limiting inflammation and promoting intrinsic healing. Results further identified miR-147-3p as one of the active components of iEVs that modulate macrophage inflammatory response by inhibiting toll-like receptor 4/NF-κB signaling pathway. These promising findings paved the road toward clinical application of iEVs in the treatment of tendon injury and other related disorders.

Metabolic regulation of intrasynovial flexor tendon repair: The effects of dichloroacetate administration on early tendon healing in a canine model

Enriched in glycolytic enzymes, paucicellular and hypovascular intrasynovial flexor tendons fail to mount an effective healing response after injury and repair. In contrast, well-vascularized extrasynovial flexor tendons possess high levels of oxidative phosphorylation (OXPHOS) enzymes and have a markedly improved healing capacity. This study was designed to compare the metabolic profiles of the two types of tendons and to evaluate the impact of metabolic reprogramming on early intrasynovial tendon healing in a clinically relevant canine model. Results showed that healthy intrasynovial tendons expressed higher levels of PDK1 and GAPDH and lower levels of SCX and IGF1 than did extrasynovial tendons. PDK1 encodes a subtype of pyruvate dehydrogenase kinase (PDK) that inhibits OXPHOS. Consistently, ATP production via glycolysis was favored in intrasynovial tendon cells whereas OXPHOS was the preferred pathway in extrasynovial tendon cells. Inhibition of glycolysis in vitro increased SCX expression in intrasynovial tendon cells. Therefore, dichloroacetate (DCA), a PDK1 inhibitor, was used in vivo to shift intrasynovial tendon ATP production from glycolysis to OXPHOS. Oral DCA administration reduced serum lactate concentration and increased acetyl-CoA content in repaired intrasynovial tendons and led to reduced TLR4 and IL1B and increased IGF1, SCX, and TGFB3 expressions in treated intrasynovial tendons compared to controls. Immunohistochemistry staining with anti-Ki67 and anti-CD31 antibodies revealed marked increases in cellularity and neovascularization in treated intrasynovial tendons. Clinical significance: The findings of this experiment indicate that improved gene expression and histological outcomes can be achieved by regulating glucose metabolism in the early stages following intrasynovial tendon repair.

Efficacy of Adipose-Derived Mesenchymal Stem Cells and Stromal Vascular Fraction Alone and Combined to Biomaterials in Tendinopathy or Tendon Injury: Systematic Review of Current Concepts

Background and Objectives: Tendon injury and tendinopathy are among the most frequent musculoskeletal diseases and represent a challenging issue for surgeons as well as a great socio-economic global burden. Despite the current treatments available, either surgical or conservative, the tendon healing process is often suboptimal and impaired. This is due to the inherent scarce ability of tendon tissue to repair and return itself to the original structure. Recently, Adipose-derived mesenchymal stem cells (ADSC) and stromal vascular fraction (SVF) have gained a central interest in the scientific community, demonstrating their effectiveness in treatments of acute and chronic tendon disorders in animals and humans. Either enzymatic or mechanical procedures to obtain ADSC and SVF have been described and used in current clinical practice. However, no unified protocols and processes have been established. Materials and Methods: This systematic review aims at providing a comprehensive update of the literature on the clinical application of ADSC enzymatically or mechanically processed to obtain SVF, alone and in association with biomaterials in the local treatment of tendinopathy and tendon injury in vivo, in animal models and humans. The study was performed according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA). Results: Thirty-two articles met our inclusion criteria, with a total of 18 studies in animals, 10 studies in humans and 4 studies concerning the application of biomaterials in vivo in animals. The review of the literature suggests that ADSC/SVF therapy can represent a promising alternative in tendonregenerative medicine for the enhancement of tendon healing. Conclusions: Nevertheless, further investigations and randomized control trials are needed to improve the knowledge, standardize the procedures and extend the consensus on their use for such applications.

The roles and therapeutic potentialof mesenchymal stem/stromal cells and their extracellular vesicles in tendinopathies

Tendinopathies encompass a highly prevalent, multi-faceted spectrum of disorders, characterized by activity-related pain, compromised function, and propensity for an extended absence from sport and the workplace. The pathophysiology of tendinopathy continues to evolve. For decades, it has been related primarily to repetitive overload trauma but more recently, the onset of tendinopathy has been attributed to the tissue's failed attempt to heal after subclinical inflammatory and immune challenges (failed healing model). Conventional tendinopathy management produces only short-term symptomatic relief and often results in incomplete repair or healing leading to compromised tendon function. For this reason, there has been increased effort to develop therapeutics to overcome the tissue's failed healing response by targeting the cellular metaplasia and pro-inflammatory extra-cellular environment. On this basis, stem cell-based therapies have been proposed as an alternative therapeutic approach designed to modify the course of the various tendon pathologies. Mesenchymal stem/stromal cells (MSCs) are multipotent stem cells often referred to as "medicinal signaling cells" due to their immunomodulatory and anti-inflammatory properties that can produce a pro-regenerative microenvironment in pathological tendons. However, the adoption of MSCs into clinical practice has been limited by FDA regulations and perceived risk of adverse events upon infusion in vivo. The introduction of cell-free approaches, such as the extracellular vesicles of MSCs, has encouraged new perspectives for the treatment of tendinopathies, showing promising short-term results. In this article, we review the most recent advances in MSC-based and MSC-derived therapies for tendinopathies. Preclinical and clinical studies are included with comment on future directions of this rapidly developing therapeutic modality, including the importance of understanding tissue loading and its relationship to any treatment regimen.

Constructing Tissue-Engineered Dressing Membranes with Adipose-Derived Stem Cells and Acellular Dermal Matrix for Diabetic Wound Healing: A Comparative Study of Hypoxia- or Normoxia-Culture Modes

Diabetes foot ulcer (DFU) is a serious complication of diabetes, characterized by impaired vascular function, limited angiogenesis, and chronic inflammation. Direct stem cell injection on treating DFU is far from satisfactory in clinical practice, as this therapy neither protects nor localizes the injected cell suspension at the chronic ulcer site. Meanwhile, most of injected cells gradually perished within several days due to senescence or apoptosis. Acellular dermal matrix (ADM) has the potential to act as excellent cell delivery vehicles, considering it is highly biomimetic to native dermal tissue, has low immunogenicity, and suitable for stem cell attachment and proliferation. Hypoxia culture has significantly enhanced effects on the survival ability of in vitro cultured stem cells, indicating this culture mode is a suitable way for inhibiting the senescence or apoptosis of transplanted cells. In the current study, we, respectively, culture adipose-derived stem cells (ADSCs) on an ADM membrane under a hypoxia or normoxia condition to construct two kinds of tissue-engineered dressing membranes (H-ADSCs/ADM and N-ADSCs/ADM) and then comparatively evaluated their efficacy on DFU healing using a diabetic rat model. In vitro results showed that hypoxia precondition could stimulate the ADSCs secreting VEGF-A, and the culture medium from hypoxia-preconditioned ADSCs could enhance the proliferation, migration, and angiogenesis of HUVECs. In vivo results indicated that compared to the N-ADSCs/ADM membrane, the transplanted cells in the H-ADSCs/ADM membrane can survive longer at the chronic ulcer site, consequently improve angiogenesis, inhibit inflammation, and increase extracellular matrix remodeling, eventually accelerating DFU closure. This study provides an innovative covering graft for the treatment of DFU in the clinic.

A Biomaterial-Based Hedging Immune Strategy for Scarless Tendon Healing

Scarring rather than regeneration, is an inevitable outcome of unbalanced amplifications of inflammation-destructive signals and atresia of the regenerative niche. However, identifying and effectively hedging against the risk of scarring and realizing the conversion of regenerative cues remain difficult. In this work, a hedging immune strategy based microfibrous membrane (Him-MFM), by tethering distearoyl phosphoethanolamine layer-supported copoly(lactic/glycolic acid) electrospun fibers with identified CD11b⁺ /CD68⁺ scarring subpopulation membranes in the immune landscape after tendon injury to counterweigh tissue damage, is reported. Him-MFM, carrying relevant risk receptors is shown to shift high type I biased polarization, alleviate apoptosis and metabolic stress, and mitigate inflammatory tenocyte response. Remarkably, the hedging immune strategy reverses the damaged tendon sheath barrier to the innate IL-33 secretory phenotype by 4.36 times and initiates the mucous-IL-33-Th2 axis, directly supplying a transient but obligate regenerative niche for sheath stem cell proliferation. In murine flexor tendon injury, the wrapping of Him-MFM alleviates pathological responses, protects tenocytes in situ, and restores hierarchically arranged collagen fibers covered with basement membrane, and is structurally and functionally comparable to mature tendons, demonstrating that the hedging immunity is a promising strategy to yield regenerative responses not scarring.

Defining the Profile: Characterizing Cytokines in Tendon Injury to Improve Clinical Therapy

Cytokine manipulation has been widely used to bolster innate healing mechanisms in an array of modern therapeutics. While other anatomical locations have a more definitive analysis of cytokine data, the tendon presents unique challenges to detection that make a complete portrayal of cytokine involvement during injury unattainable thus far. Without this knowledge, the advancement of tendon healing modalities is limited. In this review, we discuss what is known of the cytokine profile within the injured tendinous environment and the unique obstacles facing cytokine detection in the tendon while proposing possible solutions to these challenges. IL-1β, TNF-α, and IL-6 in particular have been identified as key cytokines for initiating tendon healing, but their function and temporal expression are still not well understood. Methods used for cytokine evaluation in the tendon including cell culture, tissue biopsy, and microdialysis have their strengths and limitations, but new methods and approaches are needed to further this research. We conclude that future study design for cytokine detection in the injured tendon should meet set criteria to achieve definitive characterization of cytokine expression to guide future therapeutics.

MSC in Tendon and Joint Disease: The Context-Sensitive Link Between Targets and Therapeutic Mechanisms

Mesenchymal stromal cells (MSC) represent a promising treatment option for tendon disorders and joint diseases, primarily osteoarthritis. Since MSC are highly context-sensitive to their microenvironment, their therapeutic efficacy is influenced by their tissue-specific pathologically altered targets. These include not only cellular components, such as resident cells and invading immunocompetent cells, but also components of the tissue-characteristic extracellular matrix. Although numerous in vitro models have already shown potential MSC-related mechanisms of action in tendon and joint diseases, only a limited number reflect the disease-specific microenvironment and allow conclusions about well-directed MSC-based therapies for injured tendon and joint-associated tissues. In both injured tissue types, inflammatory processes play a pivotal pathophysiological role. In this context, MSC-mediated macrophage modulation seems to be an important mode of action across these tissues. Additional target cells of MSC applied in tendon and joint disorders include tenocytes, synoviocytes as well as other invading and resident immune cells. It remains of critical importance whether the context-sensitive interplay between MSC and tissue- and disease-specific targets results in an overall promotion or inhibition of the desired therapeutic effects. This review presents the authors’ viewpoint on disease-related targets of MSC therapeutically applied in tendon and joint diseases, focusing on the equine patient as valid animal model.

Small Leucine-Rich Proteoglycans in Tendon Wound Healing

Significance: Tendon injury possesses a high morbidity rate and is difficult to achieve a satisfying prognosis with currently available treatment strategies. Current approaches used for tendon healing always lead to the formation of fibrovascular scar tissue, which significantly compromises the biomechanics of the healed tendon. Moreover, the related functional deficiency deteriorates over time with an increased injury recurrence risk. Small leucine-rich proteoglycans (SLRPs) link and interact with collagen fibrils to regulate tendon structure and biomechanics, which can provide a new and promising method in the field of tendon injury management. Recent Advances: The effect of SLRPs on tendon development has been extensively investigated. SLRP deficiency impairs tendon collagen fibril structure and biomechanic properties, while administration of SLRPs generally benefits tendon wound healing and regains better mechanical properties. Critical Issues: Current knowledge on the role of SLRPs in tendon development and regeneration mostly comes from uninjured knockout mice, and mainly focuses on the morphology description of collagen fibril profile and mechanical properties. Little is known about the regulatory mechanism on the molecular level. Future Directions: This article reviews the current knowledge in this highly translational topic and provides an evidence-based conclusion, thereby encouraging in-depth investigations of SLRPs in tendons and the development of SLRP-based treatments for desired tendon healing.

Mesenchymal Stem Cell Sheets for Engineering of the Tendon-Bone Interface

Failure to regenerate the gradient tendon-bone interface of the enthesis results in poor clinical outcomes for surgical repair. The goal of this study was to evaluate the potential of composite cell sheets for engineering of the tendon-bone interface to improve regeneration of the functionally graded tissue. We hypothesize that stacking cell sheets at early stages of differentiation into tenogenic and osteogenic progenitors will create a composite structure with integrated layers. Cell sheets were fabricated on methyl cellulose and poly(N-isopropylacrylamide) thermally reversible polymers with human adipose-derived stem cells and differentiated into progenitors of tendon and bone with chemical induction media. Tenogenic and osteogenic cell sheets were stacked, and the engineered tendon-bone interface (TM-OM) was characterized in vitro in comparison to stacked cell sheet controls cultured in basal growth medium (GM-GM), osteogenic medium (OM-OM), and tenogenic medium (TM-TM). Samples were characterized by histology, quantitative real-time polymerase chain reaction, and immunofluorescent staining for markers of tendon, fibrocartilage, and bone including mineralization, scleraxis, tenomodulin, COL2, COLX, RUNX2, osteonectin, and osterix. After 1 week co-culture in basal growth medium, TM-OM cell sheets formed a tissue construct with integrated layers expressing markers of tendon, mineralized fibrocartilage, and bone with a spatial gradient in RUNX2 expression. Tenogenic cell sheets had increased expression of scleraxis and tenomodulin. Osteogenic cell sheets exhibited mineralization 1 week after stacking and upregulation of osterix and osteonectin. Additionally, in the engineered interface, there was significantly increased gene expression of IHH and COLX, indicative of endochondral ossification. These results highlight the potential for composite cell sheets fabricated with adipose-derived stem cells for engineering of the tendon-bone interface. Impact statement This study presents a method for fabrication of the tendon-bone interface using stacked cell sheets of tenogenic and osteogenic progenitors differentiated from human adipose-derived mesenchymal stem cells, resulting in a composite structure expressing markers of tendon, mineralized fibrocartilage, and bone. This work is an important step toward regeneration of the biological gradient of the enthesis and demonstrates the potential for engineering complex tissue interfaces from a single autologous cell source to facilitate clinical translation.

Differentiation Ability of Tendon-Derived Stem Cells and Histological Characteristics of Rotator Cuff Remnant on the Greater Tuberosity Degenerated With Age and Chronicity

PURPOSE

To explore and measure the presence and activity of tendon-derived stem cells (TDSCs), as well as histological changes of rotator cuff remnant by age and chronicity of the rotator cuff tear (RCT).

METHODS

154 patients with a full-thickness tear of supraspinatus and/or infraspinatus tendon were included. 52 qualified remnants of the greater tuberosity were captured through arthroscopy. TDSCs in the remnants were isolated for proliferation ability, basal gene expression, and trilineage differentiation detection. Histological characteristics were evaluated by observation of staining under a light microscope and transmission electron microscopy (TEM). To observe the effect of age, samples were divided into two groups: young (＜60 years old) and old (≥60 years old). For chronicity comparison, samples were divided into three groups: acute group (＜3 months), intermediate group (3-12 months), and chronic group (≥12 months).

RESULTS

Between age groups, the remnants in older patients were found to have lower TDSC proliferation ability (cell counting kit-8 results, old: .5325 ± .050, young: .6623 ± .196; P = .008) and basal expression of aggrecan (.630 ± .239; P = .002) and TGF-β1 (transforming growth factor-β1, .589 ± .326, P = .008), weaker ability of chondrogenic differentiation. Furthermore, the remnant tendons in chronic group was found to have weaker adipogenic and chondrogenic differentiation ability of TDSCs, lower tendon degenerative score (acute: 3.57 ± 1.902, intermediate: 5.94 ± 2.313, chronic: 6.86 ± 2.193; P = .023), increased type III collagen region ratio in insertion area (acute: 86.10% ± 8.29%, intermediate: 94.06% ± 5.36%, chronic: 98.90% ± .49%; P = .023), and larger fibril diameters.

CONCLUSION

Differentiation ability of TDSCs derived from the rotator cuff remnant was reduced with age and chronicity. Histological degeneration of remnant tendon deteriorated with chronicity. Remnant in the greater tuberosity was still alive, but those in young or acute injury patients were more active after full-thickness RCT.

CLINICAL RELEVANCE

TDSCs exist in rotator cuff remnant on the greater tuberosity and have multilineage differentiation ability. But the remnant degenerated with age and chronicity.

Adipose micro-grafts enhance tendinopathy healing in ovine model: An in vivo experimental perspective study

In Europe, approximatively 100 000 to 500 000 tendon repairs are performed every year. These procedures are associated with a considerable rate of postoperative complications (from 6% to 11%). Autologous micro-grafts (AAMG) and stromal vascular fraction (SVF) have been shown to improve tendon healing in 60% to 70% of treated rodents. The purpose of this study was to evaluate the effects of AAMG in a sheep model with tendinopathy. We used sheep models because, as a large animal, they are more comparable to humans. The hypothesis was that SVF injection would improve tendon healing compared with the control group, reducing inflammatory and matrix degrading, while increasing anti-inflammatory expression and collagen synthesis in the early stage of tendon injury. Sixteen Apennine sheep aged 2 to 5 years underwent 500 UI type I collagenase injection into both common calcaneal tendons (CCT) to induce tendinopathy. After 15 days (T0), one CCT in every ovine underwent randomly to 2.5 mL of AAMG obtained by mechanical disruption and the contralateral CCTs received no treatment. Clinical, ecographic, and sonographic evaluations were performed after 4 weeks (T1) and 8 weeks (T2). Histological, immunohistochemical, real-time polymerase chain reaction (RT-PCR), and biomechanical evaluations were performed at T2. At T2, the treated group showed a final tendon diameter (9.1 ± 1.4 mm) and a hardness expression (62%) that were similar to the original healthy tendon (8.1 ± 1.1 mm; 100%), with a significant recovery compared with the control group (9.5 ± 1.7 mm; 39%). Moreover, histological analysis of the treated group revealed an improvement in the fiber orientation score, fiber edema score, infiltrative-inflammatory process, and necrosis score (4.3 ± 3.3) compared with control group (8.8 ± 2.9). Immunohistochemically, the treated group showed high expression of collagen 1, Factor VIII and significantly low expression of collagen 3. These data were confirmed by RT-PCR analysis. The study findings suggested that AAMGs obtained through mechanical disruption present a safe, efficient, and reliable technique, enhancing tendon healing.

Facilitatory effects of artificial nerve filled with adipose-derived stem cell sheets on peripheral nerve regeneration: An experimental study

BACKGROUND

We evaluated how artificial nerves filled with adipose-derived stem cell (ADSC) sheets could facilitate peripheral nerve regeneration.

METHODS

We prepared ADSC sheets following previously described protocols. We transected the sciatic nerve in 12-week-old Wistar rats, fixed the nerve ends to the artificial conduit, and prepared three groups: (1) conduits alone (control group); (2) conduits filled with ADSCs (ADSCs group), and (3) conduits filled with ADSC sheets (ADSC sheet group). We assessed the subjects 4 and 12 weeks post-transplantation (n = 24). We investigated βIII-tubulin and anti-S100 expression at 4 and 12 weeks post-transplantation, in longitudinal- and cross-sections of the central portion in the regenerated tissues. The vascular endothelial growth factor A (VEGFA) and neuregulin-1 expressions were analyzed using real-time reverse-transcription polymerase chain reaction (real-time RT-PCR). We evaluated the tibialis anterior muscle wet weight (affected/healthy sides, %) and sciatic function index (SFI) 12 weeks post-transplantation.

RESULTS

The ADSC sheet group comprised more S100-positive cells than the other groups. The regenerated axon length in the ADSC sheet group was markedly the longest among the studied groups. The immunostaining revealed a positive area in the regenerated tissue center in all groups, tending to be the largest in the ADSC sheet group. The muscle wet weight indicated that the ADSC sheet group exhibited significantly higher weight than the control. The mean SFI showed that the ADSC sheet group exhibited significantly better results than the control. The VEGFA expression was higher both in the ADSC and the ADSC sheet group than in the control. The neuregulin-1 expression was higher both in the ADSC and the ADSC sheet group than in the control.

CONCLUSIONS

The ADSC sheets could potentially support transplanting an adequate number of ADSCs at the target site. Compared with the conventional method of attaching ADSCs, the use of ADSC sheets promotes accelerated nerve regeneration.

Rho/ROCK Inhibition Promotes TGF-β3-Induced Tenogenic Differentiation in Mesenchymal Stromal Cells

Mesenchymal stromal cells (MSC) represent a promising therapeutic tool for tendon regeneration. Their tenogenic differentiation is crucial for tissue engineering approaches and may support their beneficial effects after cell transplantation in vivo. The transforming growth factor (TGF)-β, signalling via intracellular Smad molecules, is a potent paracrine mediator of tenogenic induction. Moreover, scaffold topography or tendon matrix components induced tenogenesis via activation of the Rho/ROCK cascade, which, however, is also involved in pathological adaptations in extracellular matrix pathologies. The aim of this study was to investigate the interplay of Rho/ROCK and TGF-β3/Smad signalling in tenogenic differentiation in both human and equine MSC. Primary equine and human MSC isolated from adipose tissue were cultured as monolayers or on tendon-derived decellularized scaffolds to evaluate the influence of the ROCK inhibitor Y-27632 on TGF-β3-induced tenogenic differentiation. The MSC were incubated with and without TGF-β3 (10 ng/ml), Y-27632 (10 μM), or both. On day 1 and day 3, the signalling pathway of TGF-β and the actin cytoskeleton were visualized by Smad 2/3 and phalloidin staining, and gene expression of signalling molecules and tendon markers was assessed. ROCK inhibition was confirmed by disruption of the actin cytoskeleton. Activation of Smad 2/3 with nuclear translocation was evident upon TGF-β3 stimulation. Interestingly, this effect was most pronounced with additional ROCK inhibition in both species (p < 0.05 in equine MSC). In line with that, the tendon marker scleraxis showed the strongest upregulation when TGF-β3 and ROCK inhibition were combined (p < 0.05 in human MSC). The regulation pattern of tendon extracellular matrix components and the signalling molecules TGF-β3 and Smad 8 showed differences between human and equine MSC. The obtained results showed that ROCK inhibition promotes the TGF-β3/Smad 2/3 axis, with possible implications for future MSC priming regimes in tendon therapy.

MSC-derived immunomodulatory extracellular matrix functionalized electrospun fibers for mitigating foreign-body reaction and tendon adhesion

Adhesion formation during tendon healing remains a severe problem in clinical practice. Multiple factors contribute to postoperative adhesion formation, and macrophage-driven inflammation is thought to be greatly involved in this process. We hypothesize that reducing macrophage-mediated inflammation in the injured tendon by regulating M1 to M2 macrophage polarization may effectively inhibit adhesion formation. Here, we developed an acellular immunomodulatory biomaterial consisting of an electrospun polycaprolactone/silk fibroin (PCL/SF) composite fibrous scaffold functionalized with mesenchymal stem cell (MSC)-derived extracellular matrix (ECM). To enhance the immunoregulatory potential of MSCs, we performed inflammatory licensing with IFN-γ to obtain immunomodulatory ECM (iECM). Proteomic analyses of MSCs and their secreted ECM components from different culture conditions revealed the MSC-ECM molecular signatures and the potential mechanism of ECM immunoregulation. Then, the immunoregulatory potential of the iECM-modified scaffold was evaluated in vitro and in vivo. Relative to the PCL/SF fibrous scaffold, the iECM-functionalized scaffold facilitated M2 macrophage polarization and inhibited the expression of multiple cytokines (IL-1β, IL-6, CXCL11, IL-10, IL-1R2, and TGF-β1) in vitro, strongly suggesting the immunosuppressive ability of iECM derived from inflammatory licensed MSCs. Consistent with the in vitro findings, the results of rat subcutaneous implantation indicated that a markedly lower foreign-body reaction (FBR) was obtained in the PCL/SF-iECM group than in the other groups, as evidenced by thinner fibrotic capsule formation, less type I collagen production and more M2-type macrophage polarization. In the rat Achilles tendon injury model, the PCL/SF-iECM scaffold greatly mitigated tendon adhesion with clear sheath space formation between the tendon and the scaffold. These data highlight the immunomodulatory potential of iECM-functionalized fibrous scaffolds to attenuate FBR by modulating M2 macrophage polarization, thereby preventing tendon adhesion. STATEMENT OF SIGNIFICANCE: Electrospun PCL/SF fibrous scaffolds functionalized with ECM secreted by MSCs stimulated by inflammatory factor IFN-γ was developed that combined physical barrier and immunomodulatory functions to prevent tendon adhesion formation. PCL/SF micro-nanoscale bimodal fibrous scaffolds prepared by emulsion electrospinning possess high porosity and a large pore size beneficial for nutrient transport to promote intrinsic healing; moreover, surface modification with immunomodulatory ECM (iECM) mitigates the FBR of fibrous scaffolds to prevent tendon adhesion. The iECM-functionalized electrospun scaffolds exhibit powerful immunomodulatory potency in vitro and in vivo. Moreover, the iECM-modified scaffolds, as an anti-adhesion physical barrier with immunomodulatory ability, have an excellent performance in a rat Achilles tendon adhesion model. MSC secretome-based therapeutics, as an acellular regenerative medicine strategy, are expected to be applied to other inflammatory diseases due to its strong immunoregulatory potential.

Update on the Basic Science Concepts and Applications of Adipose-Derived Stem Cells in Hand and Craniofacial Surgery

SUMMARY

Adipose-derived stem cell therapy offers plastic surgeons a novel treatment alternative for conditions with few therapeutic options. Adipose-derived stem cells are a promising treatment because of their broad differentiation potential, capacity for self-renewal, and ease of isolation. Over the past decade, plastic surgeons have attempted to harness adipose-derived stem cells' unique cellular characteristics to improve the survival of traditional fat grafting procedures, a process known as cell-assisted lipotransfer. However, the full implications of cell-assisted lipotransfer in clinical practice remain incompletely understood, stressing the urgent need to assess the scientific evidence supporting adipose-derived stem cell-based interventions. Furthermore, with the strict regulatory climate surrounding tissue explantation therapies, reviewing the safety and efficacy of these treatments will clarify their regulatory viability moving forward. In this report, the authors provide a comprehensive, up-to-date appraisal of best evidence-based practices supporting adipose-derived stem cell-derived therapies, highlighting the known mechanisms behind current clinical applications in tissue engineering and regenerative medicine specific to plastic and reconstructive surgery. The authors outline best practices for the harvest and isolation of adipose-derived stem cells and discuss why procedure standardization will elucidate the scientific bases for their broad use. Finally, the authors discuss challenges posed by U.S. Food and Drug Administration oversight of these cell-based therapies and examine the role of adipose-derived stem cell-based applications in the future of plastic surgery.

The role of inflammation in mesenchymal stromal cell therapy in osteoarthritis, perspectives for post-traumatic osteoarthritis: a review

OA is a complex and highly prevalent degenerative disease affecting the whole joint, in which factors like genetic predisposition, gender, age, obesity and traumas contribute to joint destruction. ∼50-80% of OA patients develop synovitis. OA-associated risk factors contribute to joint instability and the release of cartilage matrix fragments, activating the synovium to release pro-inflammatory factors and catabolic enzymes in turn damaging the cartilage and creating a vicious circle. Currently, no cure is available for OA. Mesenchymal stromal cells (MSCs) have been tested in OA for their chondrogenic and anti-inflammatory properties. Interestingly, MSCs are most effective when administered during synovitis. This review focusses on the interplay between joint inflammation and the immunomodulation by MSCs in OA. We discuss the potential of MSCs to break the vicious circle of inflammation and describe current perspectives and challenges for clinical application of MSCs in treatment and prevention of OA, focussing on preventing post-traumatic OA.

Flexor Tendon Injury and Repair. The Influence of Synovial Environment on the Early Healing Response in a Canine Model

BACKGROUND

Environmental conditions strongly influence the healing capacity of connective tissues. Well-vascularized extrasynovial tendons typically undergo a robust wound-healing process following transection and repair. In contrast, avascular intrasynovial tendons do not mount an effective repair response. The current study tests the hypothesis that flexor tendons, as a function of their synovial environment, exhibit unique inflammatory, angiogenic, and metabolic responses to injury and repair.

METHODS

Flexor tendons present a distinct opportunity to test the study hypothesis, as they have proximal regions that are extrasynovial and distal regions that are intrasynovial. In an internally controlled study design, the second and fifth forepaw flexor tendons were transected and repaired in either the extrasynovial or the intrasynovial anatomical region. Histological, gene expression, and proteomics analyses were performed at 3 and 7 days to define the early biological events that drive synovial environment-dependent healing responses.

RESULTS

Uninjured intrasynovial tendons were avascular, contained high levels of proteoglycans, and expressed inflammatory factors, complement proteins, and glycolytic enzymes. In contrast, extrasynovial tendons were well vascularized, contained low levels of proteoglycans, and were enriched in inflammation inhibitors and oxidative phosphorylation enzymes. The response to injury and repair was markedly different between the 2 tendon regions. Extrasynovial tendons displayed a robust and rapid neovascularization response, increased expression levels of complement proteins, and an acute shift in metabolism to glycolysis, whereas intrasynovial tendons showed minimal vascularity and muted inflammatory and metabolic responses.

CONCLUSIONS

The regional molecular profiles of intact and healing flexor tendons revealed extensive early differences in innate immune response, metabolism, vascularization, and expression of extracellular matrix as a function of the synovial environment. These differences reveal mechanisms through which extrasynovial tendons heal more effectively than do intrasynovial tendons.

CLINICAL RELEVANCE

To improve outcomes after operative repair, future treatment strategies should promote features of extrasynovial healing, such as enhanced vascularization and modulation of the complement system and/or glucose metabolism.

Microdamage in the equine superficial digital flexor tendon

The forelimb superficial digital flexor tendon (SDFT) is an energy-storing tendon that is highly susceptible to injury during activities such as galloping and jumping, such that it is one of the most commonly reported causes of lameness in the performance horse. This review outlines the biomechanical and biothermal effects of strain on the SDFT and how these contribute to the accumulation of microdamage. The effect of age-related alterations on strain response and subsequent injury risk is also considered. Given that tendon is a slowly healing and poorly regenerative tissue, prompt detection of early stages of pathology in vivo and timely adaptations to training protocols are likely to have a greater outcome than advances in treatment. Early screening tools and detection protocols could subsequently be of benefit in identifying subclinical signs of degeneration during the training programme. This provides an opportunity for preventative strategies to be implemented to minimise incidences of SDFT injury and reduce recovery periods in elite performance horses. Therefore, this review will focus on the modalities available to implement early screening and prevention protocols as opposed to methods to diagnose and treat injuries.

Cruciate Ligament Cell Sheets Can Be Rapidly Produced on Thermoresponsive poly(glycidyl ether) Coating and Successfully Used for Colonization of Embroidered Scaffolds

Anterior cruciate ligament (ACL) cell sheets combined with biomechanically competent scaffolds might facilitate ACL tissue engineering. Since thermoresponsive polymers allow a rapid enzyme-free detachment of cell sheets, we evaluated the applicability of a thermoresponsive poly(glycidyl ether) (PGE) coating for cruciate ligamentocyte sheet formation and its influence on ligamentocyte phenotype during sheet-mediated colonization of embroidered scaffolds. Ligamentocytes were seeded on surfaces either coated with PGE or without coating. Detached ligamentocyte sheets were cultured separately or wrapped around an embroidered scaffold made of polylactide acid (PLA) and poly(lactic-co-ε-caprolactone) (P(LA-CL)) threads functionalized by gas-phase fluorination and with collagen foam. Ligamentocyte viability, protein and gene expression were determined in sheets detached from surfaces with or without PGE coating, scaffolds seeded with sheets from PGE-coated plates and the respective monolayers. Stable and vital ligamentocyte sheets could be produced within 24 h with both surfaces, but more rapidly with PGE coating. PGE did not affect ligamentocyte phenotype. Scaffolds could be colonized with sheets associated with high cell survival, stable gene expression of ligament-related type I collagen, decorin, tenascin C and Mohawk after 14 d and extracellular matrix (ECM) deposition. PGE coating facilitates ligamentocyte sheet formation, and sheets colonizing the scaffolds displayed a ligament-related phenotype.

Advanced technology-driven therapeutic interventions for prevention of tendon adhesion: Design, intrinsic and extrinsic factor considerations

Tendon adhesion formation describes the development of fibrotic tissue between the tendon and its surrounding tissues, which commonly occurs as a reaction to injury or surgery. Its impact on function and quality of life varies from negligible to severely disabling, depending on the affected area and extent of adhesion formed. Thus far, treatment options remain limited with prophylactic anti-inflammatory medications and revision surgeries constituting the only tools within the doctors' armamentarium - neither of which provides reliable outcomes. In this review, the authors aim to collate the current understanding of the pathophysiological mechanisms underlying tendon adhesion formation, highlighting the significant role ascribed to the inflammatory cascade in accelerating adhesion formation. The bulk of this article will then be dedicated to critically appraising different therapeutic structures like nanoparticles, hydrogels and fibrous membranes fabricated by various cutting-edge technologies for adhesion formation prophylaxis. Emphasis will be placed on the role of the fibrous membranes, their ability to act as drug delivery vehicles as well as the combination with other therapeutic structures (e.g., hydrogel or nanoparticles) or fabrication technologies (e.g., weaving or braiding). Finally, the authors will provide an opinion as to the future direction of the prevention of tendon adhesion formation in view of scaffold structure and function designs.

Transcriptome profiling of digital flexor tendons after injury in a chicken model

BACKGROUND

Modulation of tendon healing remains a challenge because of our limited understanding of the tendon repair process. Therefore, we performed the present study to provide a global perspective of the gene expression profiles of tendons after injury and identify the molecular signals driving the tendon repair process.

RESULTS

The gene expression profiles of flexor digitorum profundus tendons in a chicken model were assayed on day 3, weeks 1, 2, 4, and 6 after injury using the Affymetrix microarray system. Principal component analysis (PCA) and hierarchical cluster analysis of the differentially expressed genes showed three distinct clusters corresponding to different phases of the tendon healing period. Gene ontology (GO) analysis identified regulation of cell proliferation and cell adhesion as the most enriched biological processes. Kyoto encyclopedia of genes and genomes (KEGG) pathway analysis revealed that the cytokine-cytokine receptor interaction and extracellular matrix (ECM)-receptor interaction pathways were the most impacted. Weighted gene co-expression network analysis (WGCNA) demonstrated four distinct patterns of gene expressions during tendon healing. Cell adhesion and ECM activities were mainly associated with genes with drastic increase in expression 6 weeks after injury. The protein-protein interaction (PPI) networks were constructed to identify the key signaling pathways and hub genes involved.

CONCLUSIONS

The comprehensive analysis of the biological functions and interactions of the genes differentially expressed during tendon healing provides a valuable resource to understand the molecular mechanisms underlying tendon healing and to predict regulatory targets for the genetic engineering of tendon repair. Tendon healing, Adhesion, Gene Ontology, Kyoto Encyclopedia of Genes and Genomes, Weighted Gene Co-expression Network Analysis, Protein-protein Interaction.

Adipose-Derived Stromal Cell-Sheets Sandwiched, Book-Shaped Acellular Dermal Matrix Capable of Sustained Release of Basic Fibroblast Growth Factor Promote Diabetic Wound Healing

The management of diabetic wounds is a therapeutic challenge in clinical settings. Current tissue engineering strategies for diabetic wound healing are insufficient, owing to the lack of an appropriate scaffold that can load a large number of stem cells and induce the interaction of stem cells to form granulation tissue. Herein we fabricated a book-shaped decellularized dermal matrix (BDDM), which shows a high resemblance to native dermal tissue in terms of its histology, microstructure, and ingredients, is non-cytotoxic and low-immunogenic, and allows adipose-derived stromal cell (ASC) attachment and proliferation. Then, a collagen-binding domain (CBD) capable of binding collagen was fused into basic fibroblast growth factor (bFGF) to synthetize a recombinant growth factor (termed as CBD-bFGF). After that, CBD-bFGF was tethered onto the collagen fibers of BDDM to improve its endothelial inducibility. Finally, a functional scaffold (CBD-bFGF/BDDM) was fabricated. In vitro and in vivo experiments demonstrated that CBD-bFGF/BDDM can release tethered bFGF with a sustained release profile, steadily inducing the interaction of stem cells down to endothelial differentiation. ASCs were cultured to form a cell sheet and then sandwiched by CBD-bFGF/BDDM, thus enlarging the number of stem cells loaded into the scaffold. Using a rat model, the ASC sheets sandwiched with CBD-bFGF/BDDM (ASCs/CBD-bFGF/BDDM) were capable of enhancing the formation of granulation tissue, promoting angiogenesis, and facilitating collagen deposition and remodeling. Therefore, the findings of this study demonstrate that ASCs/CBD-bFGF/BDDM could be applicable for diabetic wound healing.

Mesenchymal Stem Cell-Derived Extracellular Vesicles for the Promotion of Tendon Repair - an Update of Literature

Tendon injuries are prevalent in physical activities and sports. Tendon heals slowly after injuries. The results of conservative treatments and surgery are not satisfactory with high re-injury rate and scar tissue formation. The application of mesenchymal stem cells (MSCs) to the injured tendons was reported to promote tendon repair. Recent studies have suggested that MSCs supported tendon repair via the secretion of paracrine factors. Extracellular vesicles (EVs) are a heterogeneous group of cell-derived membranous structures that are produced and secreted by most eukaryotic cells. They carry a plethora of proteins, lipids, microRNA and mRNA which reprogram the recipient cells and are involved in multiple physiological and pathological processes. EVs were shown to promote tissue repair and mediate the healing effects of MSCs. In this review, I aim to review the recent literature on the promotion of tendon repair using EVs-derived from MSCs (MSC-EVs). The mechanisms underlying these actions are also reviewed and future research directions are discussed. Better understanding of the roles of MSC-EVs in tendon repair would offer a new treatment strategy to circumvent this devastating soft tissue disorder. Graphical Abstract.

The role of the macrophage in tendinopathy and tendon healing

The role of the macrophage is an area of emerging interest in tendinopathy and tendon healing. The macrophage has been found to play a key role in regulating the healing process of the healing tendon. The specific function of the macrophage depends on its functional phenotype. While the M1 macrophage phenotype exhibits a phagocytic and proinflammatory function, the M2 macrophage phenotype is associated with the resolution of inflammation and tissue deposition. Several studies have been conducted on animal models looking at enhancing or suppressing macrophage function, targeting specific phenotypes. These studies include the use of exogenous biological and pharmacological substances and more recently the use of transgenic and genetically modified animals. The outcomes of these studies have been promising. In particular, enhancement of M2 macrophage activity in the healing tendon of animal models have shown decreased scar formation, accelerated healing, decreased inflammation and even enhanced biomechanical strength. Currently our understanding of the role of the macrophage in tendinopathy and tendon healing is limited. Furthermore, the roles of therapies targeting macrophages to enhance tendon healing is unclear. Clinical Significance: An increased understanding of the significance of the macrophage and its functional phenotypes in the healing tendon may be the key to enhancing tendon healing. This review will present the current literature on the function of macrophages in tendinopathy and tendon healing and the potential of therapies targeting macrophages to enhance tendon healing.

Adipose-derived stem cells improve tendon repair and prevent ectopic ossification in tendinopathy by inhibiting inflammation and inducing neovascularization in the early stage of tendon healing

INTRODUCTION

Achilles tendinopathy is characterized by scar formation or ectopic ossification, both of which result in pain and worsened physical function in athletes and older people. Although cell therapy using adipose-derived stem cells (ASCs) has been shown to be effective for tendinopathy, the underlying mechanisms by which ASCs result in tendon healing in vivo have not yet been fully clarified.

METHODS

ASCs were obtained from the fat pads of EGFP transgenic mice by collagenase digestion. C57BL/6 mice were used in a collagenase-induced injury model. ASCs were transplanted into injury sites at 1 week after injury. Tendons were harvested at 9 days, 2 weeks, and 4 weeks after transplantation, and analyzed by histological examination and μCT scanning.

RESULTS

Histological analysis and μCT scanning revealed greater recovery of collagen fibers and suppression of ectopic ossification in the ASC-treated group than in the control group at 2 and 4 weeks after injury. Immunohistochemical analysis identified transplanted ASCs in the tendon core close to peritenon and connective tissue at 2 days and 1 week after transplantation, but not at 3 weeks. Furthermore, while the expression levels of IL-1β, GLUT1, and CA9 were significantly reduced in the ASC group compared to the control group at 9 days after injury, those of VEGF and the number of CD31 positive vessels were significantly increased.

CONCLUSION

The efficacy of ASCs for tendon repair and the prevention of ectopic ossification in Achilles tendinopathy were demonstrated. Our data suggest that ASCs can modulate inflammation and induce neovascularization in the early stage of tendon injury.

The role of the immune system in tendon healing: a systematic review

INTRODUCTION

The role of the immune system in tendon healing relies on polymorphonucleocytes, mast cells, macrophages and lymphocytes, the 'immune cells' and their cytokine production. This systematic review reports how the immune system affects tendon healing.

SOURCES OF DATA

We registered our protocol (registration number: CRD42019141838). After searching PubMed, Embase and Cochrane Library databases, we included studies of any level of evidence published in peer-reviewed journals reporting clinical or preclinical results. The PRISMA guidelines were applied, and risk of bias and the methodological quality of the included studies were assessed. We excluded all the articles with high risk of bias and/or low quality after the assessment. We included 62 articles assessed as medium or high quality.

AREAS OF AGREEMENT

Macrophages are major actors in the promotion of proper wound healing as well as the resolution of inflammation in response to pathogenic challenge or tissue damage. The immune cells secrete cytokines involving both pro-inflammatory and anti-inflammatory factors which could affect both healing and macrophage polarization.

AREAS OF CONTROVERSY

The role of lymphocytes, mast cells and polymorphonucleocytes is still inconclusive.

GROWING POINTS

The immune system is a major actor in the complex mechanism behind the healing response occurring in tendons after an injury. A dysregulation of the immune response can ultimately lead to a failed healing response.

AREAS TIMELY FOR DEVELOPING RESEARCH

Further studies are needed to shed light on therapeutic targets to improve tendon healing and in managing new way to balance immune response.

Exosomes Derived from Bone Marrow Stromal Cells (BMSCs) Enhance Tendon-Bone Healing by Regulating Macrophage Polarization

Background

Inflammation after tendon-bone junction injury results in the formation of excessive scar tissue and poor biomechanical properties. Recent research has shown that exosomes derived from bone marrow stromal cells (BMSCs) can modulate inflammation during tissue healing. Thus, our study aimed to enhance tendon-bone healing by use of BMSC-derived exosomes (BMSC-Exos).

Material/Methods

The mouse tendon-bone reconstruction model was established, and the mice were randomly divided into 3 groups: the control group, the hydrogel group, and the hydrogel+exosome group, with 30 mice in each group. At 7 days, 14 days, and 1 month after surgery, tendon-bone junction samples were harvested, and the macrophage polarization and tendon-bone healing were evaluated based on histology, immunofluorescence, and quantitative RT-PCR (qRT-PCR) analysis.

Results

In the early phase, we observed significantly higher numbers of M2 macrophages and more anti-inflammatory and chondrogenic-related factors in the hydrogel+BMSC-Exos group compared with the control group and the hydrogel group. The M1 macrophages and related proinflammatory factors decreased. Cell apoptosis decreased in the hydrogel+BMSC-Exos group, while cell proliferation increased; in particular, the CD146+ stem cells substantially increased. At 1 month after surgery, there was more fibrocartilage in the hydrogel+BMSC-Exos group than in the other groups. Biomechanical testing showed that the maximum force, strength, and elastic modulus were significantly improved in the hydrogel+BMSC-Exos group.

Conclusions

Our study provides evidence that the local administration of BMSC-Exos promotes the formation of fibrocartilage by increasing M2 macrophage polarization in tendon-to-bone healing, leading to improved biomechanical properties. These findings provide a basis for the potential clinical use of BMSC-Exos in tendon-bone repair.

Tendon and Ligament Healing and Current Approaches to Tendon and Ligament Regeneration

Ligament and tendon injuries are common problems in orthopedics. There is a need for treatments that can expedite nonoperative healing or improve the efficacy of surgical repair or reconstruction of ligaments and tendons. Successful biologically-based attempts at repair and reconstruction would require a thorough understanding of normal tendon and ligament healing. The inflammatory, proliferative, and remodeling phases, and the cells involved in tendon and ligament healing will be reviewed. Then, current research efforts focusing on biologically-based treatments of ligament and tendon injuries will be summarized, with a focus on stem cells endogenous to tendons and ligaments. Statement of clinical significance: This paper details mechanisms of ligament and tendon healing, as well as attempts to apply stem cells to ligament and tendon healing. Understanding of these topics could lead to more efficacious therapies to treat ligament and tendon injuries. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:7-12, 2020.

Stem cell-derived extracellular vesicles attenuate the early inflammatory response after tendon injury and repair

Adipose-derived stem cells (ASCs) have the potential to enhance tendon repair via paracrine regulation of the inflammatory response to injury. Extracellular vesicles (EVs), which are secreted by ASCs, have shown promise in mediating this process. This study was designed to evaluate the effect of ASC EVs on early tendon healing using a mouse Achilles tendon injury and repair model. EVs were isolated from the conditioned medium of naïve and interferonγ-primed ASCs and applied to the repair site via a collagen sheet. Tendon healing was assessed in nuclear factor-κB (NF-κB)-luciferase reporter mice up to 7 days after suture repair. As anticipated, repair site NF-κB activity increased greater than twofold following tendon repair. Treatment with EVs from primed but not naïve ASCs effectively suppressed the response. Accordingly, the pro-inflammatory genes Il1b and Ifng were both dramatically increased in repaired tendons, while primed, but not naïve ASC EVs attenuated the response. Compared with control repairs, primed ASC EVs further reduced the rate of post-repair tendon gap formation and rupture and facilitated collagen formation at the injury site. Additional experiments demonstrated that EVs target macrophages and that primed ASC EVs were most effective in blocking macrophage NF-κB activity. Collectively, the findings of this study demonstrate that primed ASC EVs, similar to ASCs, attenuate the early tendon inflammatory response after injury via modulation of the macrophage inflammatory response. Statement of clinical significance: These findings introduce a new cell-free therapy, derived from stem cells, for tendon repair with the potential for improved therapeutic efficacy and safety. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:117-127, 2020.

Pulsed Electromagnetic Field Modulates Tendon Cells Response in IL-1β-Conditioned Environment

Strategies aiming at controlling and modulating inflammatory cues may offer therapeutic solutions for improving tendon regeneration. This study aims to investigate the modulatory effect of pulsed electromagnetic field (PEMF) on the inflammatory profile of human tendon-derived cells (hTDCs) after supplementation with interleukin-1β (IL-1β). IL-1β was used to artificially induce inflammatory cues associated with injured tendon environments. The PEMF effect was investigated varying the frequency (5 or 17 Hz), intensity (1.5, 4, or 5 mT), and duty-cycle (10% or 50%) parameters to which IL-1β-treated hTDCs were exposed to. A PEMF actuation with 4 mT, 5 Hz and a 50% duty cycle decreased the production of IL-6 and tumor necrosis factor-α (TNF-α), as well as the expression of TNFα, IL-6, IL-8, COX-2, MMP-1, MMP-2, and MMP-3, while IL-4, IL-10, and TIMP-1 expression increased. These results suggest that PEMF stimulation can modulate hTDCs response in an inflammatory environment holding therapeutic potential for tendon regenerative strategies. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:160-172, 2020.

The cellular basis of fibrotic tendon healing: challenges and opportunities

Tendon injuries are common and can dramatically impair patient mobility and productivity, resulting in a significant socioeconomic burden and reduced quality of life. Because the tendon healing process results in the formation of a fibrotic scar, injured tendons never regain the mechanical strength of the uninjured tendon, leading to frequent reinjury. Many tendons are also prone to the development of peritendinous adhesions and excess scar formation, which further reduce tendon function and lead to chronic complications. Despite this, there are currently no treatments that adequately improve the tendon healing process due in part to a lack of information regarding the contributions of various cell types to tendon healing and how their activity may be modulated for therapeutic value. In this review, we summarize recent efforts to identify and characterize the distinct cell populations involved at each stage of tendon healing. In addition, we examine the mechanisms through which different cell populations contribute to the fibrotic response to tendon injury, and how these responses can be affected by systemic factors and comorbidities. We then discuss gaps in our current understanding of tendon fibrosis and highlight how new technologies and research areas are shedding light on this clinically important and intractable challenge. A better understanding of the complex cellular environment during tendon healing is crucial to the development of new therapies to prevent fibrosis and promote tissue regeneration.

CTGF induces tenogenic differentiation and proliferation of adipose-derived stromal cells

Intrasynovial tendons are paucicellular and hypovascular, resulting in a poor response to injury. Surgical repair of ruptured or lacerated tendons often lead to complications such as adhesions, repair site gapping, and repair site rupture. Adipose-derived stem cells (ASCs) have shown promise for enhancing tendon repair, as they have the capacity to differentiate into tendon fibroblasts and augment the healing response. Furthermore, connective tissue growth factor (CTGF) has been shown to promote tendon regeneration via the stimulation of endogenous tendon stem cells. Here, we evaluated the potential of CTGF to promote tenogenic differentiation of ASCs in vitro. Gene and protein expression, cell proliferation, and FAK and ERK1/2 signaling were assessed. CTGF increased tenogenic genes in mouse ASCs in a dose- and time-dependent manner. Western blot and immunostaining analyses demonstrated increases in tenogenic protein expression in CTGF-treated ASCs at all timepoints studied. CTGF increased ASC proliferation in a dose-dependent manner. CTGF induced phosphorylation of ERK1/2 within 5 min and FAK within 15 min; both signals persisted for 120 min. Blocking FAK and ERK1/2 pathways by selective inhibitors SCH772984 and PF573228, respectively, attenuated the CTGF-induced tenogenic differentiation and proliferation of ASCs. These results suggest that CTGF induces tenogenic differentiation of ASCs via the FAK and ERK1/2 pathway. Statement of clinical significance: Although prior research has led to advances in tendon operative techniques and rehabilitation methods, clinical outcomes after tendon repair remain variable, with high rates of repair site gapping or rupture. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res.

Integration of mesenchymal stem cell sheet and bFGF-loaded fibrin gel in knitted PLGA scaffolds favorable for tendon repair

Tendon injuries are common and require a long time to heal, and are particularly associated with some adverse problems such as adhesion and rupture. Herein, we aim to develop new bioactive scaffolds endowed with stem cell sheets and growth factors to enable cell migration and proliferation favorable for tendon regeneration in situ. An exogenous basic fibroblast growth factor (bFGF)-loaded fibrin gel was firstly incorporated into the porous network of knitted poly(lactide-co-glycolide) (PLGA) scaffolds and then sheets of mesenchymal stem cells (MSCs) were also integrated into the scaffolds. It was shown that the pores in the knitted PLGA scaffold were readily filled with a complex network of fibrin fiber gel and the fibrin fibers were beneficial for the controlled release of bFGF over a long time period. After transplantation in a critical-size Achilles tendon defect model (7 mm) in the rat right hindlimb, gross observation revealed no immunologic incompatibility or rejection derived from the scaffold systems. It was observed that the MSC sheets contributed directly to tendon regeneration, and exerted an environment-modifying effect on the injuries in situ, consistent with the beneficial effect of bFGF. It was interesting that the knitted PLGA-fibrin gel scaffolds loaded with MSC sheets and bFGF showed the highest expression of tendon-related gene markers and outstanding repair efficacy, including appreciable biomechanical strength and native-like histological microstructures. Therefore, the integration of MSC sheets and bFGF into PLGA/bFGF-fibrin gel scaffolds may stimulate the proliferation and tenogenic differentiation of MSCs in situ and synergistically enhance the injured tendon reconstruction.

Extracellular Vesicle-Educated Macrophages Promote Early Achilles Tendon Healing

Tendon healing follows a complex series of coordinated events, which ultimately produces a mechanically inferior tissue more scar‐like than native tendon. More regenerative healing occurs when anti‐inflammatory M2 macrophages play a more dominant role. Mesenchymal stromal/stem cells (MSCs) are able to polarize macrophages to an M2 immunophenotype via paracrine mechanisms. We previously reported that coculture of CD14+ macrophages (MQs) with MSCs resulted in a unique M2‐like macrophage. More recently, we generated M2‐like macrophages using only extracellular vesicles (EVs) isolated from MSCs creating “EV‐educated macrophages” (also called exosome‐educated macrophages [EEMs]), thereby foregoing direct use of MSCs. For the current study, we hypothesized that cell therapy with EEMs would improve in vivo tendon healing by modulating tissue inflammation and endogenous macrophage immunophenotypes. We evaluated effects of EEMs using a mouse Achilles tendon rupture model and compared results to normal tendon healing (without any biologic intervention), MSCs, MQs, or EVs. We found that exogenous administration of EEMs directly into the wound promoted a healing response that was significantly more functional and more regenerative. Injured tendons treated with exogenous EEMs exhibited (a) improved mechanical properties, (b) reduced inflammation, and (c) earlier angiogenesis. Treatment with MSC‐derived EVs alone were less effective functionally but stimulated a biological response as evidenced by an increased number of endothelial cells and decreased M1/M2 ratio. Because of their regenerative and immunomodulatory effects, EEM treament could provide a novel strategy to promote wound healing in this and various other musculoskeletal injuries or pathologies where inflammation and inadequate healing is problematic. Stem Cells2019;37:652–662

Book-Shaped Acellular Fibrocartilage Scaffold with Cell-loading Capability and Chondrogenic Inducibility for Tissue-Engineered Fibrocartilage and Bone-Tendon Healing

Functional fibrocartilage regeneration is a bottleneck during bone-tendon healing, and the currently available tissue-engineering strategies for fibrocartilage regeneration are insufficient because of a lack of appropriate scaffold that can load large seeding-cells and induce chondrogenesis of stem cells. The acellular fibrocartilage scaffold (AFS) contains active growth factors as well as tissue-specific epitopes for cell-matrix interactions, which make it a potential scaffold for tissue-engineered fibrocartilage. A limitation to this scaffold is that its low porosity inhibits cells loading and infiltration. Here, inspired by book appearance, we sectioned native fibrocartilage tissue (NFT) into book-shape to improve cells loading and infiltration, and then decellularized with four protocols: (1) 2% SDS for 6-h, (2) 2% SDS for 24-h, (3) 4 SDS for 6-h, (4) 4% SDS for 24-h, followed by nuclease digestion. The optimal protocol was screened with respect to microstructures, DNA residence, native ingredients reservation, and chondrogenic inducibility of the AFS. In vitro studies demonstrated that this screened scaffold is noncytotoxicity and low-immunogenicity, allows adipose-derived stromal cells (ASCs) attachment and proliferation, shows superior chondrogenic inducibility, and stimulates collagen or glycosaminoglycans secretion. The underlying mechanism for this chondrogenic inducibility may be related to hedgehog pathway activating. Additionally, a novel pattern for fabricating tissue-engineered fibrocartilage was developed to enlarge seeding-cells loading, namely, cell-sheets sandwiched by book-shaped scaffold. In-vivo studies indicate that this screened scaffold alone could induce endogenous cells to satisfactorily regenerate fibrocartilage at 16-week, as characterized by fibrocartilaginous extracellular matrix (ECM) deposition and good interface integration. Interleaving this book-shaped AFS with autologous ASCs-sheets significantly enhanced its ability to regenerate fibrocartilage. Cell tracking demonstrated that fibrochondrocytes, osteoblasts, and osteocytes in the healing interface at postoperative 8-week partly originated from the sandwiched ASCs-sheets. On that basis, we propose the use of this book-shaped AFS and cell sheet technique for fabricating tissue-engineered fibrocartilage to improve bone-tendon healing.