Identification and Distinction of Tenocytes and Tendon-Derived Stem Cells

Single-nucleus transcriptional and chromatin accessibility analyses of maturing mouse Achilles tendon uncover the molecular landscape of tendon stem/progenitor cells

Tendons and ligaments are crucial connective tissues linking bones and muscles, yet achieving full functional recovery after injury remains challenging. We investigated the characteristics of tendon stem/progenitor cells (TSPCs) by focusing on the declining tendon repair capacity with growth. Using single-cell RNA sequencing on Achilles tendon cells from 2- and 6-week-old mice, we identified Cd55 and Cd248 as novel surface antigen markers for TSPCs. Combining single-nucleus ATAC and RNA sequencing analyses revealed that Cd55 and Cd248 positive fractions in tendon tissue are TSPCs, with this population decreasing at 1 weeks. We also identified candidate upstream transcription factors regulating these fractions. Functional analyses of isolated CD55/CD248 positive cells demonstrated high clonogenic potential and tendon differentiation capacity, forming functional tendon-like tissue in vitro . This study establishes CD55 and CD248 as novel TSPC surface antigens, potentially advancing tendon regenerative medicine and contributing to the development of new treatment strategies for tendon and ligament injuries.

Transcriptome-Optimized Hydrogel Design of a Stem Cell Niche for Enhanced Tendon Regeneration

Bioactive hydrogels have emerged as promising artificial niches for enhancing stem cell-mediated tendon repair. However, a substantial knowledge gap remains regarding the optimal combination of niche features for targeted cellular responses, which often leads to lengthy development cycles and uncontrolled healing outcomes. To address this critical gap, an innovative, data-driven materiomics strategy is developed. This approach is based on in-house RNA-seq data that integrates bioinformatics and mathematical modeling, which is a significant departure from traditional trial-and-error methods. It aims to provide both mechanistic insights and quantitative assessments and predictions of the tenogenic effects of adipose-derived stem cells induced by systematically modulated features of a tendon-mimetic hydrogel (TenoGel). The knowledge generated has enabled a rational approach for TenoGel design, addressing key considerations, such as tendon extracellular matrix concentration, uniaxial tensile loading, and in vitro pre-conditioning duration. Remarkably, our optimized TenoGel demonstrated robust tenogenesis in vitro and facilitated tendon regeneration while preventing undesired ectopic ossification in a rat tendon injury model. These findings shed light on the importance of tailoring hydrogel features for efficient tendon repair. They also highlight the tremendous potential of the innovative materiomics strategy as a powerful predictive and assessment tool in biomaterial development for regenerative medicine.

Skeletal Muscle Satellite Cells Co-Opt the Tenogenic Gene Scleraxis to Instruct Regeneration

Skeletal muscles connect bones and tendons for locomotion and posture. Understanding the regenerative processes of muscle, bone and tendon is of importance to basic research and clinical applications. Despite their interconnections, distinct transcription factors have been reported to orchestrate each tissue's developmental and regenerative processes. Here we show that Scx expression is not detectable in adult muscle stem cells (also known as satellite cells, SCs) during quiescence. Scx expression begins in activated SCs and continues throughout regenerative myogenesis after injury. By SC-specific Scx gene inactivation (ScxcKO), we show that Scx function is required for SC expansion/renewal and robust new myofiber formation after injury. We combined single-cell RNA-sequencing and CUT&RUN to identify direct Scx target genes during muscle regeneration. These target genes help explain the muscle regeneration defects of ScxcKO, and are not overlapping with Scx -target genes identified in tendon development. Together with a recent finding of a subpopulation of Scx -expressing connective tissue fibroblasts with myogenic potential during early embryogenesis, we propose that regenerative and developmental myogenesis co-opt the Scx gene via different mechanisms.

Role of tendon-derived stem cells in tendon and ligament repair: focus on tissue engineer

This review offered a comprehensive analysis of tendon and ligament injuries, emphasizing the crucial role of tendon-derived stem cells (TDSCs) in tissue engineering as a potential solution for these challenging medical conditions. Tendon and ligament injuries, prevalent among athletes, the elderly, and laborers, often result in long-term disability and reduced quality of life due to the poor intrinsic healing capacity of these avascular structures. The formation of biomechanically inferior scar tissue and a high rate of reinjury underscore the need for innovative approaches to enhance and guide the regenerative process. This review delved into the complexities of tendon and ligament structure and function, types of injuries and their impacts, and the limitations of the natural repair process. It particularly focused on the role of TDSCs within the context of tissue engineering. TDSCs, with their ability to differentiate into tenocytes, are explored in various applications, including biocompatible scaffolds for cell tracking, co-culture systems to optimize tendon-bone healing, and graft healing techniques. The review also addressed the challenges of immunoreactivity post-transplantation, the importance of pre-treating TDSCs, and the potential of hydrogels and decellularized matrices in supporting tendon regeneration. It concluded by highlighting the essential roles of mechanical and molecular stimuli in TDSC differentiation and the current challenges in the field, paving the way for future research directions.

Healthy Tendon Stem Cell-Derived Exosomes Promote Tendon-To-Bone Healing of Aged Chronic Rotator Cuff Tears by Breaking the Positive-Feedback Cross-Talk between Senescent Tendon Stem Cells and Macrophages through the Modulation of Macrophage Polarization

The re-tear rate of rotator cuff tears (RCT) after surgical repair is high, especially in aged patients with chronic tears. Senescent tendon stem cells (s-TSCs) generally exist in aged and chronically torn rotator cuff tendons and are closely associated with impaired tendon-to-bone healing results. The present study found a positive feedback cross-talk between s-TSCs and macrophages. The conditioned medium (CM) from s-STCs can promote macrophage polarization mainly toward the M1 phenotype, whose CM reciprocally accelerated further s-TSC senescence. Additional healthy tendon stem-cells derived exosomes (h-TSC-Exos) can break this positive feedback cross-talk by skewing macrophage polarization from the M1 phenotype to the M2 phenotype, attenuating s-TSCs senescence. S-TSC senescence acceleration or attenuation effects induced by M1 or M2 macrophages are associated with the inhibition or activation of the bone morphogenetic protein 4 signaling pathway following RNA sequencing analysis. Using an aged-chronic rotator cuff tear rat model, it is found that h-TSC-Exos can shift the microenvironment in the tendon-to-bone interface from a pro-inflammatory to an anti-inflammatory type at the acute postoperative stage and improve the tendon-to-bone healing results, which are associated with the rejuvenated s-TSCs. Therefore, this study proposed a potential strategy to improve the healing of aged chronic RCT.

The Biological Effect of Platelet-Rich Plasma on Rotator Cuff Tears: A Prospective Randomized In Vivo Study

The positive effect of platelet-rich plasma (PRP) on tendon metabolism has been extensively investigated and proven in vitro. Additionally, in vivo animal studies have correlated the application of PRP with the enhancement of tenocyte anabolic activity in the setting of tendon degeneration. However, less is known about its in vivo effect on human tendon biology. The purpose of the current prospective randomized comparative study was to evaluate the effect of PRP on torn human supraspinatus tendon. Twenty consecutive eligible patients with painful and magnetic resonance imaging (MRI)-confirmed degenerative supraspinatus tendon tears were randomized in a one-to-one ratio into two groups. The patients in the experimental group (n = 10) underwent an ultrasound-guided autologous PRP injection in the subacromial space 6 weeks before the scheduled operation. In the control group (n = 10), no injection was made prior to surgery. Supraspinatus tendon specimens were harvested from the lateral end of the torn tendon during shoulder arthroscopy and were evaluated under optical and electron microscopy. In the control group, a mixed cell population of oval and rounded tenocytes within disorganized collagen and sites of accumulated inflammatory cells was detected. In contrast, the experimental group yielded abundant oval-shaped cells with multiple cytoplasmic processes within mainly parallel collagen fibers and less marked inflammation, simulating the intact tendon structure. These findings indicate that PRP can induce microscopic changes in the ruptured tendon by stimulating the healing process and can facilitate a more effective recovery.

Exosomes derived from mir-337-3p over-expressing tendon stem cells protect against apoptosis of tenocytes via targeting caspase3

BACKGROUND

Tendons are important dense fibrous structures connecting muscle to bone, and tendon stem cells (TDSCs) affect their repair and regeneration. The role of TDSC-derived exosomes (TDSC-Exos) is still being unexplored; therefore, this study aimed to investigate the protective effect of TDSC-Exos on tenocytes.

METHODS

The TDSCs and tenocytes were all derived from Sprague Dawley (SD) rats. The expression of positive and negative markers of TDSCs were detected by flow cytometry, and the multi-differentiation ability was also detected to identify TDSCs. Exos were derived from TDSCs using ultracentrifugation; furthermore, Exos enriched with microRNA(miR)-377-3p were generated from TDSCs stably overexpressing miR-377-3p after transfection, identified with transmission electron microscopy (TEM), western blot and PKH26 staining assay. Moreover, the cell functions of tenocytes were evaluated by MTT, EdU, transwell, and flow cytometry. Dual luciferase reporter and RNA pull-down assays were used to verify the binding sites of miR-337-3p and caspase3 (CASP3) predicted by Targetscan.

RESULTS

Exos (miR-337-3p) were taken up by tenocytes, and promoted the proliferation, migration, and invasion and suppressed the apoptosis of tenocytes in a dose-dependent manner. Bioinformatics analysis showed that CASP3 was a target of miR-377-3p, which was further verified by luciferase and RNA pull-down assays. Moreover, over-expressed CASP3 reversed the effects of Exos (miR-337-3p) on cell functions of tenocytes.

CONCLUSIONS

Our findings suggest that Exos derived from miR-337-3p over-expressing TDSCs could potentially protect against tenocyte apoptosis by regulating CASP3. This novel therapeutic approach holds promise for the treatment of tendon injury, offering a glimmer of hope for improved patient outcomes.

3-Dimensional Bioprinting of a Tendon Stem Cell-Derived Exosomes Loaded Scaffold to Bridge the Unrepairable Massive Rotator Cuff Tear

BACKGROUND

Unrepairable massive rotator cuff tears (UMRCTs) are challenging to surgeons owing to the severely retracted rotator cuff musculotendinous tissues and extreme defects in the rotator cuff tendinous tissues.

PURPOSE

To fabricate a tendon stem cell-derived exosomes loaded scaffold (TSC-Exos-S) and investigate its effects on cellular bioactivity in vitro and repair in a rabbit UMRCT model in vivo.

STUDY DESIGN

Controlled laboratory study.

METHODS

TSC-Exos-S was fabricated by loading TSC-Exos and type 1 collagen (COL-I) into a 3-dimensional bioprinted and polycaprolactone (PCL)-based scaffold. The proliferation, migration, and tenogenic differentiation activities of rabbit bone marrow stem cells (BMSCs) were evaluated in vitro by culturing them in saline, PCL-based scaffold (S), COL-I loaded scaffold (COL-I-S), and TSC-Exos-S. In vivo studies were conducted on a rabbit UMRCT model, where bridging was repaired with S, COL-I-S, TSC-Exos-S, and autologous fascia lata (FL). Histological and biomechanical analyses were performed at 8 and 16 weeks postoperatively.

RESULTS

TSC-Exos-S exhibited reliable mechanical strength and subcutaneous degradation, which did not occur before tissue regeneration. TSC-Exos-S significantly promoted the proliferation, migration, and tenogenic differentiation of rabbit BMSCs in vitro. In vivo studies showed that UMRCT repaired with TSC-Exos-S exhibited significant signs of tendinous tissue regeneration at the bridging site with regard to specific collagen staining. Moreover, no significant differences were observed in the histological and biomechanical properties compared with those repaired with autologous FL.

CONCLUSION

TSC-Exos-S achieved tendinous tissue regeneration in UMRCT by providing mechanical support and promoting the trend toward tenogenic differentiation.

CLINICAL RELEVANCE

The present study proposes a potential strategy for repairing UMRCT with severely retracted musculotendinous tissues and large tendinous tissue defects.

Assembly of Interfacial Polyelectrolyte Complexation Fibers with Mineralization Gradient for Physiologically-Inspired Ligament Regeneration

Current synthetic grafts for ligament rupture repair often fail to integrate well with the surrounding biological tissue, leading to complications such as graft wear, fatigue, and subsequent re-rupture. To address this medical challenge, this study aims at advancing the development of a biological ligament through the integration of physiologically-inspired principles and tissue engineering strategies. In this study, interfacial polyelectrolyte complexation (IPC) spinning technique, along with a custom-designed collection system, to fabricate a hierarchical scaffold mimicking native ligament structure, is utilized. To emulate the bone-ligament interface and alleviate stress concentration, a hydroxyapatite (HAp) mineral gradient is strategically introduced near both ends of the scaffold to enhance interface integration and diminish the risk of avulsion rupture. Biomimetic viscoelasticity is successfully displayed to provide similar mechanical support to native ligamentous tissue under physiological conditions. By introducing the connective tissue growth factor (CTGF) and conducting mesenchymal stem cells transplantation, the regenerative potential of the synthetic ligament is significantly amplified. This pioneering study offers a multifaceted solution combining biomimetic materials, regenerative therapies, and advanced techniques to potentially transform ligament rupture treatment.

IL-1β + TGF-β2 dual-licensed mesenchymal stem cells have reduced major histocompatibility class I expression and positively modulate tenocyte migration, metabolism, and gene expression

OBJECTIVE

The study objectives were to 1) determine the mesenchymal stem cell (MSC) surface expression of major histocompatibility complex (MHC) class I and transcriptome-wide gene expression changes following IL-1β + TGF-β2 dual licensing and 2) evaluate if IL-1β + TGF-β2 dual-licensed MSCs had a greater ability to positively modulate tenocyte function compared to naive MSCs.

SAMPLE

Equine bone marrow-derived MSCs from 6 donors and equine superficial digital flexor tenocytes from 3 donors.

METHODS

Experiments were performed in vitro. Flow cytometry and bulk RNA sequencing were utilized to determine naive and dual-licensed MSC phenotype and transcriptome-wide changes in gene expression. Conditioned media were generated from MSCs and utilized in tenocyte cell culture assays as a method to determine the effect of MSC paracrine factors on tenocyte function.

RESULTS

Dual-licensed MSCs have a reduced expression of MHC class I and exhibit enrichment in functional pathways associated with the extracellular matrix, cell signaling, and tissue development. Additionally, dual-licensed MSC-conditioned media significantly improved in vitro tenocyte migration and metabolism to a greater degree than naive MSC-conditioned media. In tenocytes exposed to IL-1β, dual-licensed conditioned media also positively modulated tenocyte gene expression.

CLINICAL RELEVANCE

Our data indicate that conditioned media containing paracrine factors secreted from dual-licensed MSCs significantly modulates in vitro tenocyte function, which may confer benefits in vivo to healing tendons following injury. Additionally, due to reduced MHC class I expression in dual-licensed MSCs, this technique may also provide an avenue to provide an effective "off-the-shelf" allogenic source of MSCs.

Characterization of TGFβ1-induced tendon-like structure in the scaffold-free three-dimensional tendon cell culture system

The biological mechanisms regulating tenocyte differentiation and morphological maturation have not been well-established, partly due to the lack of reliable in vitro systems that produce highly aligned collagenous tissues. In this study, we developed a scaffold-free, three-dimensional (3D) tendon culture system using mouse tendon cells in a differentially adherent growth channel. Transforming Growth Factor-β (TGFβ) signaling is involved in various biological processes in the tendon, regulating tendon cell fate, recruitment and maintenance of tenocytes, and matrix organization. This known function of TGFβ signaling in tendon prompted us to utilize TGFβ1 to induce tendon-like structures in 3D tendon constructs. TGFβ1 treatment promoted a tendon-like structure in the peripheral layer of the constructs characterized by increased thickness with a gradual decrease in cell density and highly aligned collagen matrix. TGFβ1 also enhanced cell proliferation, matrix production, and morphological maturation of cells in the peripheral layer compared to vehicle treatment. TGFβ1 treatment also induced early tenogenic differentiation and resulted in sufficient mechanical integrity, allowing biomechanical testing. The current study suggests that this scaffold-free 3D tendon cell culture system could be an in vitro platform to investigate underlying biological mechanisms that regulate tenogenic cell differentiation and matrix organization.

FGF-stimulated tendon cells embrace a chondrogenic fate with BMP7 in newt tissue culture

Newts can regenerate functional elbow joints after amputation at the joint level. Previous studies have suggested the potential contribution of cells from residual tendon tissues to joint cartilage regeneration. A serum-free tissue culture system for tendons was established to explore cell dynamics during joint regeneration. Culturing isolated tendons in this system, stimulated by regeneration-related factors, such as fibroblast growth factor (FGF) and platelet-derived growth factor, led to robust cell migration and proliferation. Moreover, cells proliferating in an FGF-rich environment differentiated into Sox9-positive chondrocytes upon BMP7 introduction. These findings suggest that FGF-stimulated cells from tendons may aid in joint cartilage regeneration during functional elbow joint regeneration in newts.

Effects of adipose-derived mesenchymal stem cell conditioned medium on human tenocytes exposed to high glucose

Introduction

Diabetic tendinopathy is a common invalidating and challenging disease that may be treated using stem cells. However, the effects of adipose-derived mesenchymal stem cell conditioned medium (ASC-CM) in diabetic tendinopathy have never been explored.

Objectives

The present study evaluated the effects of ASC-CM on morphology, cell viability, structure, and scratch wound closure of human tenocytes (HTNC) exposed to high glucose (HG).

Design

Experimental study.

Methods

HTNC were exposed to HG (25 mM) for 7, 14 and 21 days with or without ASC-CM for the last 24 h. CM was collected from 4 × 105 ASCs, centrifuged for 10 min at 200 g and sterilized with 0.22 μm syringe filter.

Results

At 7 days, HG-HTNC had decreased cell viability [72 ± 2%, p < 0.01 versus normal glucose (NG)] compared to NG-HTNC (90 ± 5%). A further decrement was detected after 14 and 21 days (60 ± 4% and 60 ± 5%, both, p < 0.01 versus NG and p < 0.01 versus HG7). While NG-HTNC evidenced a normal fibroblast cell-like elongated morphology, HG-HTNC showed increased cell roundness. In contrast, HG-HTNC exposed to ASC-CM showed a significant increase in cell viability, an improved cell morphology and higher scratch wound closure at all HG time points. Moreover, the exposure to ASC-CM significantly increased thrombospondin 1 and transforming growth factor beta 1 (TGF-β1) content in HG-HTNC. The TGF-β1 elevation was paralleled by higher Collagen I and Vascular Endothelial Growth Factor in HG-HTNC.

Conclusion

ASC-CM may restore the natural morphology, cell viability and structure of HTNC, promoting their scratch wound closure through TGF-β1 increase.

Extracorporeal membrane oxygenation and effects on tendon tissue: A vibrational spectroscopy study

Extracorporeal membrane oxygenation (ECMO) is an invasive medical technique used to provide life support in persons with insufficient cardiac and respiratory functionalities, or to preserve, postmortem, and organ function addressing organ/tissue transplant. Although a lot of information is available about organs in their entirety, the safety and effectiveness of allogeneic tissues collected from ECMO donors have not been fully elucidated. In this preliminary study, samples of tibial and peroneal human tendons were analyzed along their length with Raman microspectroscopy and attenuated total reflection-Fourier transform infrared micro-imaging. Both techniques evidenced a different chemical composition in the terminal with respect to the central part of the tendon. Thus, a differentiated analysis was performed depending on the specific position with respect to the bone or the muscle junctions. Spectroscopic analyses showed significant differences in the characteristics of the extracellular matrix between tendons from ECMO and non-ECMO donors, suggesting changes in the amino acid (proline and hydroxyproline) content and protein structure.

Endotenon-Derived Type II Tendon Stem Cells Have Enhanced Proliferative and Tenogenic Potential

Tendon injuries caused by overuse or age-related deterioration are frequent. Incomplete knowledge of somatic tendon cell biology and their progenitors has hindered interventions for the effective repair of injured tendons. Here, we sought to compare and contrast distinct tendon-derived cell populations: type I and II tendon stem cells (TSCs) and tenocytes (TNCs). Porcine type I and II TSCs were isolated via the enzymatic digestion of distinct membranes (paratenon and endotenon, respectively), while tenocytes were isolated through an explant method. Resultant cell populations were characterized by morphology, differentiation, molecular, flow cytometry, and immunofluorescence analysis. Cells were isolated, cultured, and evaluated in two alternate oxygen concentrations (physiological (2%) and air (21%)) to determine the role of oxygen in cell biology determination within this relatively avascular tissue. The different cell populations demonstrated distinct proliferative potential, morphology, and transcript levels (both for tenogenic and stem cell markers). In contrast, all tendon-derived cell populations displayed multipotent differentiation potential and immunophenotypes (positive for CD90 and CD44). Type II TSCs emerged as the most promising tendon-derived cell population for expansion, given their enhanced proliferative potential, multipotency, and maintenance of a tenogenic profile at early and late passage. Moreover, in all cases, physoxia promoted the enhanced proliferation and maintenance of a tenogenic profile. These observations help shed light on the biological mechanisms of tendon cells, with the potential to aid in the development of novel therapeutic approaches for tendon disorders.

Primary cilia in skeletal development and disease

Primary cilia are non-motile, microtubule-based sensory organelle present in most vertebrate cells with a fundamental role in the modulation of organismal development, morphogenesis, and repair. Here we focus on the role of primary cilia in embryonic and postnatal skeletal development. We examine evidence supporting its involvement in physiochemical and developmental signaling that regulates proliferation, patterning, differentiation and homeostasis of osteoblasts, chondrocytes, and their progenitor cells in the skeleton. We discuss how signaling effectors in mechanotransduction and bone development, such as Hedgehog, Wnt, Fibroblast growth factor and second messenger pathways operate at least in part at the primary cilium. The relevance of primary cilia in bone formation and maintenance is underscored by a growing list of rare genetic skeletal ciliopathies. We collate these findings and summarize the current understanding of molecular factors and mechanisms governing primary ciliogenesis and ciliary function in skeletal development and disease.

The Functions and Mechanisms of Tendon Stem/Progenitor Cells in Tendon Healing

Tendon injury is one of the prevalent disorders of the musculoskeletal system in orthopedics and is characterized by pain and limitation of joint function. Due to the difficulty of spontaneous tendon healing, and the scar tissue and low mechanical properties that usually develops after healing. Therefore, the healing of tendon injury remains a clinical challenge. Although there are a multitude of approaches to treating tendon injury, the therapeutic effects have not been satisfactory to date. Recent studies have shown that stem cell therapy has a facilitative effect on tendon healing. In particular, tendon stem/progenitor cells (TSPCs), a type of stem cell from tendon tissue, play an important role not only in tendon development and tendon homeostasis, but also in tendon healing. Compared to other stem cells, TSPCs have the potential to spontaneously differentiate into tenocytes and express higher levels of tendon-related genes. TSPCs promote tendon healing by three mechanisms: modulating the inflammatory response, promoting tenocyte proliferation, and accelerating collagen production and balancing extracellular matrix remodeling. However, current investigations have shown that TSPCs also have a negative effect on tendon healing. For example, misdifferentiation of TSPCs leads to a "failed healing response," which in turn leads to the development of chronic tendon injury (tendinopathy). The focus of this paper is to describe the characteristics of TSPCs and tenocytes, to demonstrate the roles of TSPCs in tendon healing, while discussing the approaches used to culture and differentiate TSPCs. In addition, the limitations of TSPCs in clinical application and their potential therapeutic strategies are elucidated.

Equine induced pluripotent stem cells are responsive to inflammatory cytokines before and after differentiation into musculoskeletal cell types

Persistent inflammation is associated with the poor regeneration of musculoskeletal tissues. Embryonic stem cells (ESCs) have an attenuated response to inflammatory cytokines, but there are mixed reports on the response of induced pluripotent stem cells (iPSCs) to inflammation. Horses provide a relevant large animal model for studying musculoskeletal tissue diseases and the testing of novel therapies. The aim of this study was to determine if equine iPSCs are responsive to the inflammatory cytokines IL-1β, TNFα and IFN-γ in their undifferentiated state, or following differentiation into tendon and cartilage-like cells. We demonstrated that in undifferentiated iPSCs, the cytokines induce NF-κB P65 and STAT1 nuclear translocation which leads to cell death, decreased OCT4 expression and increased expression of inflammatory genes. Following differentiation towards cartilage-like cells exposure to the cytokines resulted in STAT1 nuclear translocation, changes in cartilage gene expression and increased expression of matrix metalloproteinases (MMPs) and inflammatory genes. Exposure of iPSC-derived tendon-like cells to the cytokines resulted nuclear translocation of NF-κB P65 and STAT1, altered tendon gene expression, increased MMP expression and increased expression of inflammatory genes. Equine iPSCs are therefore capable of responding to inflammatory stimulation and this may have relevance for their future clinical application.

Three-Dimensional Cell Culture System for Tendon Tissue Engineering

Tendon, connective tissue between bone and muscle has unique component of the musculoskeletal system. It plays important role for transporting mechanical stress from muscle to bone and enabling locomotive motion of the body. There are some restoration capacities in the tendon tissue, but the injured tendons are not completely regenerated after acute and chronic tendon injury. At this point, the treatment options for tendon injuries are limited and not that successful. Therefore, biomedical engineering approaches are emerged to cope with this issue. Among them, three-dimensional cell culture platforms provided similarity to in vivo conditions and suggested opportunities for new therapeutic approaches for treatment of tendon injuries. In this review, we focus on the characteristics of tendon tissue and tendon pathologies which can be targets for tendon tissue engineering strategies. Then proof-of-concept and pre-clinical studies leveraging advanced 3-dimensional cell culture platforms for tendon tissue regeneration have been discussed.

Applications of functionally-adapted hydrogels in tendon repair

Despite all the efforts made in tissue engineering for tendon repair, the management of tendon injuries still poses a challenge, as current treatments are unable to restore the function of tendons following injuries. Hydrogels, due to their exceptional biocompatibility and plasticity, have been extensively applied and regarded as promising candidate biomaterials in tissue regeneration. Varieties of approaches have designed functionally-adapted hydrogels and combined hydrogels with other factors (e.g., bioactive molecules or drugs) or materials for the enhancement of tendon repair. This review first summarized the current state of knowledge on the mechanisms underlying the process of tendon healing. Afterward, we discussed novel strategies in fabricating hydrogels to overcome the issues frequently encountered during the applications in tendon repair, including poor mechanical properties and undesirable degradation. In addition, we comprehensively summarized the rational design of hydrogels for promoting stem-cell-based tendon tissue engineering via altering biophysical and biochemical factors. Finally, the role of macrophages in tendon repair and how they respond to immunomodulatory hydrogels were highlighted.

Tendon tissue engineering: An overview of biologics to promote tendon healing and repair

Tendons are dense connective tissues with a hierarchical polarized structure that respond to and adapt to the transmission of muscle contraction forces to the skeleton, enabling motion and maintaining posture. Tendon injuries, also known as tendinopathies, are becoming more common as populations age and participation in sports/leisure activities increases. The tendon has a poor ability to self-heal and regenerate given its intrinsic, constrained vascular supply and exposure to frequent, severe loading. There is a lack of understanding of the underlying pathophysiology, and it is not surprising that disorder-targeted medicines have only been partially effective at best. Recent tissue engineering approaches have emerged as a potential tool to drive tendon regeneration and healing. In this review, we investigated the physiochemical factors involved in tendon ontogeny and discussed their potential application in vitro to reproduce functional and self-renewing tendon tissue. We sought to understand whether stem cells are capable of forming tendons, how they can be directed towards the tenogenic lineage, and how their growth is regulated and monitored during the entire differentiation path. Finally, we showed recent developments in tendon tissue engineering, specifically the use of mesenchymal stem cells (MSCs), which can differentiate into tendon cells, as well as the potential role of extracellular vesicles (EVs) in tendon regeneration and their potential for use in accelerating the healing response after injury.

Risk factors associated with tendon adhesions after hand tendon repair

Background

Tendon adhesions after hand tendon repair are one of the most difficult complications of hand surgery and can cause severe disability. This study aimed to assess the risk factors associated with tendon adhesions after hand tendon repair to provide a theoretical foundation for the early prevention of tendon adhesions in patients with tendon injuries. Moreover, this study intends to increase doctors' awareness of the issue and serves as a reference for developing new prevention and treatment strategies.

Methods

We retrospectively analyzed 1,031 hand trauma cases that underwent repair after finger tendon injury in our department between June 2009 and June 2019. Tendon adhesions, tendon injury zones, and other relevant information were collected, summarized, and analyzed. The significance of data was determined using a t-test or Pearson's chi-square test, and odds ratios (OR) were calculated using logistic regression tests to describe factors associated with post-tendon repair adhesions.

Results

A total of 1,031 patients were enrolled in this study. There were 817 males and 214 females with an average age of 34.98 (2-82) years. The injured side included 530 left and 501 right hands. Postoperative finger tendon adhesions occurred in 118 cases (11.45%), including 98 males and 20 females, 57 left and 61 right hands. The risk factors for the total sample in the descending order were degloving injury, no functional exercise, zone II flexor tendon injury, time from injury to surgery >12 h, combined vascular injury, and multiple tendon injuries. The flexor tendon sample shared the same risk factors as the total sample. Risk factors for the extensor tendon sample were degloving injury, no functional exercise.

Conclusions

Clinicians should pay close attention to patients with tendon trauma in hand having the following risk factors: degloving injury, zone II flexor tendon injury, lack of functional exercise, time from injury to surgery >12 h, combined vascular injury, and multiple tendon injuries. Due to the high risk of post-repair adhesions in patients with the conditions mentioned above, individualized treatment measures should be designed for the risk factors, and postoperative functional exercise of the hand is required.

The Influence of Different Modes of Exercise on Healthy and Injured Tendons

Tendons are essential components of the musculoskeletal system that links the skeletal muscle to the skeleton. This dense connective tissue exhibits great plasticity. Therefore, research on the influence of types of exercise, including acute and long-term training, on the structural and mechanical properties of tendons in athletic and sedentary populations is of critical importance in the design of scientific-based exercise plans and effective tendinopathy treatment. Here, we review recent studies on the relationship between exercise and tendon health and tendinopathy repair to provide a general understanding of how exercise may reshape tendons.

Challenges and perspectives of tendon-derived cell therapy for tendinopathy: from bench to bedside

Tendon is composed of dense fibrous connective tissues, connecting muscle at the myotendinous junction (MTJ) to bone at the enthesis and allowing mechanical force to transmit from muscle to bone. Tendon diseases occur at different zones of the tendon, including enthesis, MTJ and midsubstance of the tendon, due to a variety of environmental and genetic factors which consequently result in different frequencies and recovery rates. Self-healing properties of tendons are limited, and cell therapeutic approaches in which injured tendon tissues are renewed by cell replenishment are highly sought after. Homologous use of individual’s tendon-derived cells, predominantly differentiated tenocytes and tendon-derived stem cells, is emerging as a treatment for tendinopathy through achieving minimal cell manipulation for clinical use. This is the first review summarizing the progress of tendon-derived cell therapy in clinical use and its challenges due to the structural complexity of tendons, heterogeneous composition of extracellular cell matrix and cells and unsuitable cell sources. Further to that, novel future perspectives to improve therapeutic effect in tendon-derived cell therapy based on current basic knowledge are discussed.

Artificial Microvesicles: New Perspective on Healing Tendon Wounds

Tendons have a limited capacity to repair both naturally and following clinical interventions. Damaged tissue often presents with structural and functional differences, adversely affecting animal performance, mobility, health, and welfare. Advances in cell therapies have started to overcome some of these issues, however complications such as the formation of ectopic bone remain a complication of this technique. Regenerative medicine is therefore looking toward future therapies such as the introduction of microvesicles (MVs) derived from stem cells (SCs). The aim of the present study was to assess the characteristics of artificially derived MVs, from equine mesenchymal stem cells (MSCs), when delivered to rat tendon cells in vitro and damaged tendons in vivo. The initial stages of extracting MVs from equine MSCs and identifying and characterizing the cultured tendon stem/progenitor cells (TSCs) from rat Achilles tendons were undertaken successfully. The horse MSCs and the rat tendon cells were both capable of differentiating in 3 directions: adipogenic, osteogenic, and chondrogenic pathways. The artificially derived equine MVs successfully fused with the TSC membranes, and no cytotoxic or cytostimulating effects were observed. In addition, co-cultivation of TSCs with MVs led to stimulation of cell proliferation and migration, and cytokine VEGF and fractalkine expression levels were significantly increased. These experiments are the first to show that artificially derived MVs exhibited regeneration-stimulating effects in vitro, and that fusion of cytoplasmic membranes from diploid cell lines originating from different species was possible. The experiment in vivo demonstrated the influence of MVs on synthesis of collagen I and III types in damaged tendons of rats. Explorations in vivo showed accelerated regeneration of injured tendons after introduction of the MVs into damaged areas. The results from the studies performed indicated obvious positive modifying effects following the administration of MVs. This represents the initial successful step required prior to translating this regenerative medicine technique into clinical trials, such as for tendon repair in injured horses.

Electrospun aligned poly(ε-caprolactone) nanofiber yarns guiding 3D organization of tendon stem/progenitor cells in tenogenic differentiation and tendon repair

Hierarchical anisotropy structure directing 3D cellular orientation plays a crucial role in designing tendon tissue engineering scaffolds. Despite recent development of fabrication technologies for controlling cellular organization and design of scaffolds that mimic the anisotropic structure of native tendon tissue, improvement of tenogenic differentiation remains challenging. Herein, we present 3D aligned poly (ε-caprolactone) nanofiber yarns (NFYs) of varying diameter, fabricated using a dry-wet electrospinning approach, that integrate with nano- and micro-scale structure to mimic the hierarchical structure of collagen fascicles and fibers in native tendon tissue. These aligned NFYs exhibited good in vitro biocompatibility, and their ability to induce 3D cellular alignment and elongation of tendon stem/progenitor cells was demonstrated. Significantly, the aligned NFYs with a diameter of 50 μm were able to promote the tenogenic differentiation of tendon stem/progenitor cells due to the integration of aligned nanofibrous structure and suitable yarn diameter. Rat tendon repair results further showed that bundled NFYs encouraged tendon repair in vivo by inducing neo-collagen organization and orientation. These data suggest that electrospun bundled NFYs formed by aligned nanofibers can mimic the aligned hierarchical structure of native tendon tissue, highlighting their potential as a biomimetic multi-scale scaffold for tendon tissue regeneration.

Advanced Nanofiber-Based Scaffolds for Achilles Tendon Regenerative Engineering

The Achilles tendon (AT) is responsible for running, jumping, and standing. The AT injuries are very common in the population. In the adult population (21–60 years), the incidence of AT injuries is approximately 2.35 per 1,000 people. It negatively impacts people’s quality of life and increases the medical burden. Due to its low cellularity and vascular deficiency, AT has a poor healing ability. Therefore, AT injury healing has attracted a lot of attention from researchers. Current AT injury treatment options cannot effectively restore the mechanical structure and function of AT, which promotes the development of AT regenerative tissue engineering. Various nanofiber-based scaffolds are currently being explored due to their structural similarity to natural tendon and their ability to promote tissue regeneration. This review discusses current methods of AT regeneration, recent advances in the fabrication and enhancement of nanofiber-based scaffolds, and the development and use of multiscale nanofiber-based scaffolds for AT regeneration.

A bio-inductive collagen scaffold that supports human primary tendon-derived cell growth for rotator cuff repair

Background

Rotator Cuff (RC) tendon tearing is a common clinical problem and there is a high incidence of revision surgery due to re-tearing. In an effort to improve patient outcome and reduce surgical revision, scaffolds have been widely used for augmentation of RC repairs. However, little is known about how scaffolds support tendon stem cell growth or facilitate tendon regeneration. The purpose of this study is to evaluate the structural and biological properties of a bioactive collagen scaffold (BCS) with the potential to promote tendon repair. Additionally, we conducted a pilot clinical study to assess the safety and feasibility of using the BCS for repair of RC tears.

Methods

A series of physical, ultrastructural, molecular and in vitro tests determined the biocompatibility and teno-inductive properties of this BCS. In addition, a prospective case study of 18 patients with RC tendon tears (>20 ​mm in diameter) was performed in an open-label, single-arm study, involving either mini-open or arthroscopic surgical RC repair with the BCS. Clinical assessment of RC repair status was undertaken by MRI-imaging at baseline, 6 and 12 months and patient evaluated questionnaires were taken at baseline as well as 3, 6 & 12 months.

Results

The BCS consists of highly purified type-I collagen, in bundles of varying diameter, arranged in a higher order tri-laminar structure. BCS have minimal immunogenicity, being cell and essentially DNA-free as well as uniformly negative for the porcine α-Gal protein. BCS seeded with human primary tendon-derived cells and exposed to 6% uniaxial loading conditions in vitro, supported increased levels of growth and proliferation as well as up-regulating expression of tenocyte differentiation marker genes including TNMD, Ten-C, Mohawk and Collagen-1α1. To test the safety and feasibility of using the BCS for augmentation of RC repairs, we followed the IDEAL framework and conducted a first, open-label single arm prospective case series study of 18 patients. One patient was withdrawn from the study at 3 months due to wound infection unrelated to the BCS. The remaining 17 cases showed that the BCS is safe to be implanted. The patients reported encouraging improvements in functional outcomes (ASES, OSS and Constant-Murley scores), as well as quality of life assessments (AQoL) and a reduction in VAS pain scores. MRI assessment at 12 months revealed complete healing in 64.8% patients (11/17), 3 partial thickness re-tears (17.6%) and 3 full thickness re-tears (17.6%).

Conclusion

The BCS is composed of type-I collagen that is free of immunogenic proteins and supports tendon-derived cell growth under mechanical loading in vitro. This pilot study shows that it is safe and feasible to use BCS for RC argumentation and further controlled prospective studies are required to demonstrate its efficacy.

The Translational potential of this article

The results of this study indicate that this bioactive collagen scaffold has unique properties for supporting tendon growth and that it is non-immunogenic. The clinical study further confirms that the scaffold is a promising biological device for augment of human rotator cuff repairs.

Stem cells and regenerative medicine in sport science

The estimated cost of acute injuries in college-level sport in the USA is ∼1.5 billion dollars per year, without taking into account the cost of follow up rehabilitation. In addition to this huge financial burden, without appropriate diagnosis and relevant interventions, sport injuries may be career-ending for some athletes. With a growing number of females participating in contact based and pivoting sports, middle aged individuals returning to sport and natural injuries of ageing all increasing, such costs and negative implications for quality of life will expand. For those injuries, which cannot be predicted and prevented, there is a real need, to optimise repair, recovery and function, post-injury in the sporting and clinical worlds. The 21st century has seen a rapid growth in the arena of regenerative medicine for sporting injuries, in a bid to progress recovery and to facilitate return to sport. Such interventions harness knowledge relating to stem cells as a potential for injury repair. While the field is rapidly growing, consideration beyond the stem cells, to the factors they secrete, should be considered in the development of effective, affordable treatments.

Human Tendon Stem/Progenitor Cell Features and Functionality Are Highly Influenced by in vitro Culture Conditions

Our understanding of tendon biology continues to evolve, thus leading to opportunities for developing novel, evidence-based effective therapies for the treatment of tendon disorders. Implementing the knowledge of tendon stem/progenitor cells (TSPCs) and assessing their potential in enhancing tendon repair could fill an important gap in this regard. We described different molecular and phenotypic profiles of TSPCs modulated by culture density, as well as their multipotency and secretory activities. Moreover, in the same experimental setting, we evaluated for different responses to inflammatory stimuli mediated by TNFα and IFNγ. We also preliminarily investigated their immunomodulatory activity and their role in regulating degradation of substance P. Our findings indicated that TSPCs cultured at low density (LD) exhibited cobblestone morphology and a reduced propensity to differentiate. A distinctive immunophenotypic profile was also observed with high secretory and promising immunomodulatory responses when primed with TNFα and IFNγ. In contrast, TSPCs cultured at high density (HD) showed a more elongated fibroblast-like morphology, a greater adipogenic differentiation potential, and a higher expression of tendon-related genes with respect to LD. Finally, HD TSPCs showed immunomodulatory potential when primed with TNFα and IFNγ, which was slightly lower than that shown by LD. A shift from low to high culture density during TSPC expansion demonstrated intermediate features confirming the cellular adaptability of TSPCs. Taken together, these experiments allowed us to identify relevant differences in TSPCs based on culture conditions. This ability of TSPCs to acquire distinguished morphology, phenotype, gene expression profile, and functional response advances our current understanding of tendons at a cellular level and suggests responsivity to cues in their in situ microenvironment.