Progress with stem cell therapies for tendon tissue regeneration

The current status of various preclinical therapeutic approaches for tendon repair

Tendons are fibroblastic structures that link muscle and bone. There are two kinds of tendon injuries, including acute and chronic. Each form of injury or deterioration can result in significant pain and loss of tendon function. The recovery of tendon damage is a complex and time-consuming recovery process. Depending on the anatomical location of the tendon tissue, the clinical outcomes are not the same. The healing of the wound process is divided into three stages that overlap: inflammation, proliferation, and tissue remodeling. Furthermore, the curing tendon has a high re-tear rate. Faced with the challenges, tendon injury management is still a clinical issue that must be resolved as soon as possible. Several newer directions and breakthroughs in tendon recovery have emerged in recent years. This article describes tendon injury and summarizes recent advances in tendon recovery, along with stem cell therapy, gene therapy, Platelet-rich plasma remedy, growth factors, drug treatment, and tissue engineering. Despite the recent fast-growing research in tendon recovery treatment, still, none of them translated to the clinical setting. This review provides a detailed overview of tendon injuries and potential preclinical approaches for treating tendon injuries.

Oncostatin M promotes osteogenic differentiation of tendon-derived stem cells through the JAK2/STAT3 signalling pathway

PURPOSE

Oncostatin M (OSM) is involved in the regulation of osteogenic differentiation and has a major role in the development of heterotopic ossification. The role of OSM in osteogenic differentiation of tendon-derived stem cells (TDSCs) and its mechanism have not been reported. This study aim to investigate the role of OSM in osteogenic differentiation of TDSCs and study the mechanism.

METHODS

TDSCs were differentiated in osteogenic differentiation medium for 7 days. Recombinant OSM was added to the osteogenic differentiation medium for 7 and 14 days. The effect of Janus kinase 2 (JAK2) inhibitor AZD1480 and signal transducer and activator of transcription 3 (STAT3) inhibitor stattic in the presence of recombinant OSM on osteogenic differentiation of TDSCs was examined after differentiation for 7 and 14 days. Alkaline phosphatase and alizarin red staining were used to assess the effects on early and mid-stage osteogenic differentiation, respectively. Western blotting and qPCR were used to assess the expression of receptor and signalling pathway-related proteins and osteogenic marker genes, respectively.

RESULTS

TDSCs were successfully induced to differentiate into osteoblasts. Recombinant OSM promoted osteogenic differentiation of TDSCs to early and mid-stages. After addition of AZD1480 or stattic, decreased alkaline phosphatase and alizarin red staining were observed in the early and mid-stages of osteogenic differentiation. Additionally, decreased expression of receptor and pathway-related proteins, and osteogenic genes was found by western blotting and qPCR, respectively.

CONCLUSION

OSM promotes osteogenic differentiation of TDSCs and the JAK2/STAT3 signalling pathway plays an important role.

Tendon extracellular-matrix-derived tissue engineering micro-tissue for Achilles tendon injury regeneration in rats

BACKGROUND

Achilles tendon is vital in maintaining the stability and function of ankle joint. It is quite difficult to achieve the structural and functional repair of Achilles tendon in tissue engineering.

METHODS

A tissue-engineered tendon micro-tissue was prepared using rat tail tendon extracellular matrix (TECM) combined with rat adipose stem cells (ADSCs) to repair Achilles tendon injuries. The TECM was prepared by repeated freezing and thawing. The in vitro characteristics of TECM and its effect on ADSCs proliferation were detected. This tissue-engineered tendon micro-tissue for Achilles tendon repair in vivo was evaluated based on general characteristics, gait analysis, ultrasound findings, histological analysis, and biomechanical testing.

RESULTS

The results showed that the TECM scaffold had good biocompatibility for ADSCs. At 2 weeks post-surgery, collagen types I and III and tenomodulin expression were higher, and vascular endothelial growth factor expression was lower in the micro-tissue group than other groups. At 4 and 8 weeks post-surgery, the results of histological analysis and ultrasound findings showed that the repaired tendon tissue was smooth and lustrous, and was arranged regularly and evenly in the micro-tissue group. Gait analysis confirmed that better motor function recovery was noted in micro-tissue group than other groups. In addition, the mechanical properties of the repaired tendon tissue in micro-tissue group were better than other groups.

CONCLUSION

Tissue-engineered tendon micro-tissue fabricated by TECM and ADSCs has good biocompatibility and can promote structural and functional repair of tendon in vivo. This composite biomaterial has broad application prospects in tissue engineering.

Tissue-engineered mesenchymal stem cell constructs alleviate tendinopathy by suppressing vascularization

Tendinopathy leads to low-grade tissue inflammation and chronic damage, which progresses due to pathological imbalance in angiogenesis. Reducing early pathological vascularization may be a new approach in helping to regenerate tendon tissue. Conventional stem cell therapy and tissue engineering scaffolds have not been highly effective at treating tendinopathy. In this study, tissue engineered stem cells (TSCs) generated using human umbilical cord mesenchymal stem cells (hUC-MSCs) were combined with microcarrier scaffolds to limit excessive vascularization in tendinopathy. By preventing VEGF receptor activation through their paracrine function, TSCs reduced in vitro angiogenesis and the proliferation of vascular endothelial cells. TSCs also decreased the inflammatory expression of tenocytes while promoting their anabolic and tenogenic characteristics. Furthermore, local injection of TSCs into rats with collagenase-induced tendinopathy substantially reduced early inflammation and vascularization. Mechanistically, transcriptome sequencing revealed that TSCs could reduce the progression of pathological angiogenesis in tendon tissue, attributed to Rap1-mediated vascular inhibition. TSCs may serve as a novel and practical approach for suppressing tendon vascularization, and provide a promising therapeutic agent for early-stage clinical tendinopathy.

Therapeutic potential of GDF-5 for enhancing tendon regenerative healing

Tendon injury is a common disorder of the musculoskeletal system, with a higher possibility of occurrence in elderly individuals and athletes. After a tendon injury, the tendon suffers from inadequate and slow healing, resulting in the formation of fibrotic scar tissue, ending up with inferior functional properties. Therapeutic strategies involving the application of growth factors have been advocated to promote tendon healing. Growth and differentiation-5 (GDF-5) represents one such factor that has shown promising effect on tendon healing in animal models and in vitro cultures. Although promising, these studies are limited as the molecular mechanisms by which GDF-5 exerts its effect remain incompletely understood. Starting from broadly introducing essential elements of current understanding about GDF-5, the present review aims to define the effect of GDF-5 and its possible mechanisms of action in tendon healing. Nevertheless, we still need more in vivo studies to explore dosage, application time and delivery strategy of GDF-5, so as to pave the way for future clinical translation.

Recent Advances in the Use of Stem Cells in Tissue Engineering and Adjunct Therapies for Tendon Reconstruction and Future Perspectives

Due to their function, tendons are exposed to acute injuries. This type of damage to the musculoskeletal system represents a challenge for clinicians when natural regeneration and treatment methods do not produce the expected results. Currently, treatment is long and associated with long-term complications. In this review, we discuss the use of stem cells in the treatment of tendons, including how to induce appropriate cell differentiation based on gene therapy, growth factors, tissue engineering, proteins involved in regenerative process, drugs and three-dimensional (3D) structures. A multidirectional approach as well as the incorporation of novel components of the therapy will improve the techniques used and benefit patients with tendon injuries in the future.

Early delivery of human umbilical cord mesenchymal stem cells improves healing in a rat model of Achilles tendinopathy

Objective: This study aimed to explore the efficacy and optimal delivery time of human umbilical cord mesenchymal stem cells (hUC-MSCs) in treating collagenase-induced Achilles tendinopathy. Methods: Achilles tendinopathy in rats at early or advanced stages was induced by injecting collagenase I into bilateral Achilles tendons. A total of 28 injured rats were injected with a hUC-MSC solution or normal saline into bilateral tendons twice and sampled after 4 weeks for histological staining, gene expression analysis, transmission electron microscope assay and biomechanical testing analysis. Results: The results revealed better histological performance and a larger collagen fiber diameter in the MSC group. mRNA expression of TNF-α, IL-1β and MMP-3 was lower after MSC transplantation. Early MSC delivery promoted collagen I and TIMP-3 synthesis, and strengthened tendon toughness. Conclusion: hUC-MSCs demonstrated a therapeutic effect in treating collagenase-induced Achilles tendinopathy, particularly in the early stage of tendinopathy.

Tendon stem/progenitor cells are promising reparative cell sources for multiple musculoskeletal injuries of concomitant articular cartilage lesions associated with ligament injuries

BACKGROUND

Trauma-related articular cartilage lesions usually occur in conjunction with ligament injuries. Torn ligaments are frequently reconstructed with tendon autograft and has been proven to achieve satisfactory clinical outcomes. However, treatments for the concomitant articular cartilage lesions are still very insufficient. The current study was aimed to evaluate whether stem cells derived from tendon tissue can be considered as an alternative reparative cell source for cartilage repair.

METHODS

Primary human tendon stem/progenitor cells (hTSPCs) were isolated from 4 male patients (32 ± 8 years) who underwent ACL reconstruction surgery with autologous semitendinosus and gracilis tendons. The excessive tendon tissue after graft preparation was processed for primary cell isolation with an enzyme digestion protocol. Decellularization cartilage matrix (DCM) was used to provide a chondrogenic microenvironment for hTSPCs. Cell viability, cell morphology on the DCM, as well as their chondrogenic differentiation were evaluated.

RESULTS

DAPI staining and DNA quantitative analysis (61.47 μg per mg dry weight before and 2.64 μg/mg after decellularization) showed that most of the cells in the cartilage lacuna were removed after decellularization process. Whilst, the basic structure of the cartilage tissue was preserved and the main ECM components, collagen type II and sGAG were retained after decellularization, which were revealed by DMMB assay and histology. Live/dead staining and proliferative assay demonstrated that DCM supported attachment, survival and proliferation of hTSPCs with an excellent biocompatibility. Furthermore, gene expression analysis indicated that chondrogenic differentiation of hTSPC was induced by the DCM microenvironment, with upregulation of chondrogenesis-related marker genes, COL 2 and SOX9, without the use of exogenous growth factors.

CONCLUSION

DCM supported hTSPCs attachment and proliferation with high biocompatibility. Moreover, TSPCs underwent a distinct chondrogenesis after the induction of a chondrogenic microenvironment provided by DCM. These results indicated that TSPCs are promising reparative cell sources for promoting cartilage repair. Particularly, in the cohort that articular cartilage lesions occur in conjunction with ligament injuries, autologous TSPCs can be isolated from a portion of the tendon autograph harvested for ligaments reconstruction. In future clinical practice, combined ligament reconstruction with TSPCs- based therapy for articular cartilage repair can to be considered to achieve superior repair of these associated injuries, in which autologous TSPCs can be isolated from a portion of the tendon autograph harvested for ligaments reconstruction.

Circ_0005736 promotes tenogenic differentiation of tendon-derived stem cells through the miR-636/MAPK1 axis

BACKGROUND

Tendon-derived stem cells (TDSCs) are one of stem cells characterized by greater clonogenicity, tenogenesis, and proliferation capacity. Circ_0005736 has been shown to be decreased in Rotator cuff tendinopathy. Here, we investigated the function and relationship of circ_0005736 in TDSC tenogenic differentiation.

METHODS

Transforming growth factor β1 (TGF-β1) was used to induce the tenogenic differentiation in TDSC. Cell proliferation, invasion and migration were evaluated by Cell Counting Kit-8, 5-Ethynyl-2'-deoxyuridine, transwell, and wound healing assays, respectively. The detection of the levels of genes and proteins was performed by qRT-PCR and Western blot. The binding between miR-636 and circ_0005736 or MAPK1 (Mitogen-Activated Protein Kinase 1) was verified using dual-luciferase reporter assay and RIP assays.

RESULTS

TGF-β1 induced tenogenic differentiation by enhancing the production of tendon-specific markers and TDSC proliferation, invasion and migration. TGF-β1 treatment promoted circ_0005736 expression, knockdown of circ_0005736 abolished TGF-β1-induced tenogenic differentiation in TDSCs. Mechanistically, circ_0005736 acted as a sponge for miR-636 to up-regulate the expression of MAPK1, which was confirmed to be a target of miR-636 in TDSCs. Further rescue assays showed that inhibition of miR-636 could rescue circ_0005736 knockdown-induced suppression on TGF-β1-caused tenogenic differentiation in TDSCs. Moreover, forced expression of miR-636 abolished TGF-β1-caused tenogenic differentiation in TDSCs, which was rescued by MAPK1 up-regulation.

CONCLUSION

Circ_0005736 enhanced TGF-β1-induced tenogenic differentiation in TDSCs via increasing the production of tendon-specific markers and TDSC proliferation, invasion and migration through miR-636/MAPK1 axis.

Bio-Enhanced Neoligaments Graft Bearing FE002 Primary Progenitor Tenocytes: Allogeneic Tissue Engineering & Surgical Proofs-of-Concept for Hand Ligament Regenerative Medicine

Hand tendon/ligament structural ruptures (tears, lacerations) often require surgical reconstruction and grafting, for the restauration of finger mechanical functions. Clinical-grade human primary progenitor tenocytes (FE002 cryopreserved progenitor cell source) have been previously proposed for diversified therapeutic uses within allogeneic tissue engineering and regenerative medicine applications. The aim of this study was to establish bioengineering and surgical proofs-of-concept for an artificial graft (Neoligaments Infinity-Lock 3 device) bearing cultured and viable FE002 primary progenitor tenocytes. Technical optimization and in vitro validation work showed that the combined preparations could be rapidly obtained (dynamic cell seeding of 10⁵ cells/cm of scaffold, 7 days of co-culture). The studied standardized transplants presented homogeneous cellular colonization in vitro (cellular alignment/coating along the scaffold fibers) and other critical functional attributes (tendon extracellular matrix component such as collagen I and aggrecan synthesis/deposition along the scaffold fibers). Notably, major safety- and functionality-related parameters/attributes of the FE002 cells/finished combination products were compiled and set forth (telomerase activity, adhesion and biological coating potentials). A two-part human cadaveric study enabled to establish clinical protocols for hand ligament cell-assisted surgery (ligamento-suspension plasty after trapeziectomy, thumb metacarpo-phalangeal ulnar collateral ligamentoplasty). Importantly, the aggregated experimental results clearly confirmed that functional and clinically usable allogeneic cell-scaffold combination products could be rapidly and robustly prepared for bio-enhanced hand ligament reconstruction. Major advantages of the considered bioengineered graft were discussed in light of existing clinical protocols based on autologous tenocyte transplantation. Overall, this study established proofs-of-concept for the translational development of a functional tissue engineering protocol in allogeneic musculoskeletal regenerative medicine, in view of a pilot clinical trial.

The tendon unit: biochemical, biomechanical, hormonal influences

The current literature has mainly focused on the biology of tendons and on the characterization of the biological properties of tenocytes and tenoblasts. It is still not understood how these cells can work together in homeostatic equilibrium. We put forward the concept of the "tendon unit" as a morpho-functional unit that can be influenced by a variety of external stimuli such as mechanical stimuli, hormonal influence, or pathological states. We describe how this unit can modify itself to respond to such stimuli. We evidence the capability of the tendon unit of healing itself through the production of collagen following different mechanical stimuli and hypothesize that restoration of the homeostatic balance of the tendon unit should be a therapeutic target.

Stem Cell Applications and Tenogenic Differentiation Strategies for Tendon Repair

Tendons are associated with a high injury risk because of their overuse and age-related tissue degeneration. Thus, tendon injuries pose great clinical and economic challenges to the society. Unfortunately, the natural healing capacity of tendons is far from perfect, and they respond poorly to conventional treatments when injured. Consequently, tendons require a long period of healing and recovery, and the initial strength and function of a repaired tendon cannot be completely restored as it is prone to a high rate of rerupture. Nowadays, the application of various stem cell sources, including mesenchymal stem cells (MSCs) and embryonic stem cells (ESCs), for tendon repair has shown great potential, because these cells can differentiate into a tendon lineage and promote functional tendon repair. However, the mechanism underlying tenogenic differentiation remains unclear. Moreover, no widely adopted protocol has been established for effective and reproducible tenogenic differentiation because of the lack of definitive biomarkers for identifying the tendon differentiation cascades. This work is aimed at reviewing the literature over the past decade and providing an overview of background information on the clinical relevance of tendons and the urgent need to improve tendon repair; the advantages and disadvantages of different stem cell types used for boosting tendon repair; and the unique advantages of reported strategies for tenogenic differentiation, including growth factors, gene modification, biomaterials, and mechanical stimulation.

Recent advances in tendon tissue engineering strategy

Tendon injuries often result in significant pain and disability and impose severe clinical and financial burdens on our society. Despite considerable achievements in the field of regenerative medicine in the past several decades, effective treatments remain a challenge due to the limited natural healing capacity of tendons caused by poor cell density and vascularization. The development of tissue engineering has provided more promising results in regenerating tendon-like tissues with compositional, structural and functional characteristics comparable to those of native tendon tissues. Tissue engineering is the discipline of regenerative medicine that aims to restore the physiological functions of tissues by using a combination of cells and materials, as well as suitable biochemical and physicochemical factors. In this review, following a discussion of tendon structure, injury and healing, we aim to elucidate the current strategies (biomaterials, scaffold fabrication techniques, cells, biological adjuncts, mechanical loading and bioreactors, and the role of macrophage polarization in tendon regeneration), challenges and future directions in the field of tendon tissue engineering.

Tendon Aging

Tendons are mechanosensitive connective tissues responsible for the connection between muscles and bones by transmitting forces that allow the movement of the body, yet, with advancing age, tendons become more prone to degeneration followed by injuries. Tendon diseases are one of the main causes of incapacity worldwide, leading to changes in tendon composition, structure, and biomechanical properties, as well as a decline in regenerative potential. There is still a great lack of knowledge regarding tendon cellular and molecular biology, interplay between biochemistry and biomechanics, and the complex pathomechanisms involved in tendon diseases. Consequently, this reflects a huge need for basic and clinical research to better elucidate the nature of healthy tendon tissue and also tendon aging process and associated diseases. This chapter concisely describes the effects that the aging process has on tendons at the tissue, cellular, and molecular levels and briefly reviews potential biological predictors of tendon aging. Recent research findings that are herein reviewed and discussed might contribute to the development of precision tendon therapies targeting the elderly population.

New frontiers of tendon augmentation technology in tissue engineering and regenerative medicine: a concise literature review

Tissue banking programs fail to meet the demand for human organs and tissues for transplantation into patients with congenital defects, injuries, chronic diseases, and end-stage organ failure. Tendons and ligaments are among the most frequently ruptured and/or worn-out body tissues owing to their frequent use, especially in athletes and the elderly population. Surgical repair has remained the mainstay management approach, regardless of scarring and adhesion formation during healing, which then compromises the gliding motion of the joint and reduces the quality of life for patients. Tissue engineering and regenerative medicine approaches, such as tendon augmentation, are promising as they may provide superior outcomes by inducing host-tissue ingrowth and tendon regeneration during degradation, thereby decreasing failure rates and morbidity. However, to date, tendon tissue engineering and regeneration research has been limited and lacks the much-needed human clinical evidence to translate most laboratory augmentation approaches to therapeutics. This narrative review summarizes the current treatment options for various tendon pathologies, future of tendon augmentation, cell therapy, gene therapy, 3D/4D bioprinting, scaffolding, and cell signals.

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.

Cell therapies for chondral defects of the talus: a systematic review

Background

This systematic review investigated the efficacy and safety of surgical procedures augmented with cell therapies for chondral defects of the talus.

Methods

The present systematic review was conducted according to the 2020 PRISMA guidelines. PubMed, Google scholar, Embase, and Scopus databases were accessed in March 2022. All the clinical trials investigating surgical procedures for talar chondral defects augmented with cell therapies were accessed. The outcomes of interest were to investigate whether surgical procedures augmented with cell therapies promoted improvement in patients reported outcomes measures (PROMs) with a tolerable rate of complications.

Results

Data from 477 procedures were retrieved. At a mean follow-up of 34.8 ± 9.7 months, the Visual Analogic Scale (VAS) improved of 4.4/10 (P = 0.002) and the American Orthopaedic Foot and Ankle Score (AOFAS) of 31.1/100 (P = 0.0001) points. No improvement was found in Tegner score (P = 0.4). Few articles reported data on complications. At last follow-up, the rate of reoperation and failure were 0.06% and 0.03%, respectively. No graft delamination or hypertrophy was observed.

Conclusion

The current evidence suggests that cell therapies may be effective and safe to enhance surgical procedures for chondral defects of the talus. These results should be considered within the limitations of the present study. The current literature should be enriched with randomized controlled clinical trials with larger population size and longer follow-up.

Co-Electrospun Silk Fibroin and Gelatin Methacryloyl Sheet Seeded with Mesenchymal Stem Cells for Tendon Regeneration

Silk fibroin (SF) is a promising biomaterial for tendon repair, but its relatively rigid mechanical properties and low cell affinity have limited its application in regenerative medicine. Meanwhile, gelatin-based polymers have advantages in cell attachment and tissue remodeling but have insufficient mechanical strength to regenerate tough tissue such as tendons. Taking these aspects into account, in this study, gelatin methacryloyl (GelMA) is combined with SF to create a mechanically strong and bioactive nanofibrous scaffold (SG). The mechanical properties of SG nanofibers can be flexibly modulated by varying the ratio of SF and GelMA. Compared to SF nanofibers, mesenchymal stem cells (MSCs) seeded on SG fibers with optimal composition (SG7) exhibit enhanced growth, proliferation, vascular endothelial growth factor production, and tenogenic gene expression behavior. Conditioned media from MSCs cultured on SG7 scaffolds can greatly promote the migration and proliferation of tenocytes. Histological analysis and tenogenesis-related immunofluorescence staining indicate SG7 scaffolds demonstrate enhanced in vivo tendon tissue regeneration compared to other groups. Therefore, rational combinations of SF and GelMA hybrid nanofibers may help to improve therapeutic outcomes and address the challenges of tissue-engineered scaffolds for tendon regeneration.

Slowed-Down Rehabilitation Following Percutaneous Repair of Achilles Tendon Rupture

BACKGROUND

Following percutaneous repair of acute Achilles tendon (AT) ruptures, early postoperative weightbearing is advocated; however, it is debatable how aggressive rehabilitation should be. We compared the clinical and functional outcomes in 2 groups of patients who followed either our "traditional" or a "slowed down" rehabilitation after percutaneous surgical repair.

METHODS

Sixty patients were prospectively recruited to a slowed down (29 patients) or a traditional (31 patients) rehabilitation program. Both groups were allowed immediate weightbearing postoperatively; a removable brace with 5 heel wedges was applied at 2 weeks. In the slowed-down group, 1 wedge was removed after 4 weeks. Gradual removal of the boot took place after 4 wedges were kept for 4 weeks. In the traditional group, 1 wedge was removed every 2 weeks, with removal of the boot after 2 wedges had been kept for 2 weeks. The AT Resting Angle (ATRA) evaluated tendon elongation. Patient reported functional outcomes were assessed using the AT Rupture Score (ATRS). Calf circumference difference and the isometric plantarflexion strength of the gastro-soleus complex were evaluated.

RESULTS

At the 12-month follow-up, both ATRA and ATRS were more favorable in the slowed-down group. The isometric strength and the calf circumference were more similar to the contralateral leg in the slowed-down group than in the traditional one.

CONCLUSION

Following percutaneous repair of acute Achilles tendon patients undergoing slowed down rehabilitation performed better than the traditional one. These conclusions must be considered within the limitations of the present study.

LEVEL OF EVIDENCE

Level II, prospective comparative study.

Tendon Immune Regeneration: Insights on the Synergetic Role of Stem and Immune Cells during Tendon Regeneration

Tendon disorders represent a very common pathology in today's population, and tendinopathies that account 30% of tendon-related injuries, affect yearly millions of people which in turn cause huge socioeconomic and health repercussions worldwide. Inflammation plays a prominent role in the development of tendon pathologies, and advances in understanding the underlying mechanisms during the inflammatory state have provided additional insights into its potential role in tendon disorders. Different cell compartments, in combination with secreted immune modulators, have shown to control and modulate the inflammatory response during tendinopathies. Stromal compartment represented by tenocytes has shown to display an important role in orchestrating the inflammatory response during tendon injuries due to the interplay they exhibit with the immune-sensing and infiltrating compartments, which belong to resident and recruited immune cells. The use of stem cells or their derived secretomes within the regenerative medicine field might represent synergic new therapeutical approaches that can be used to tune the reaction of immune cells within the damaged tissues. To this end, promising opportunities are headed to the stimulation of macrophages polarization towards anti-inflammatory phenotype together with the recruitment of stem cells, that possess immunomodulatory properties, able to infiltrate within the damaged tissues and improve tendinopathies resolution. Indeed, the comprehension of the interactions between tenocytes or stem cells with the immune cells might considerably modulate the immune reaction solving hence the inflammatory response and preventing fibrotic tissue formation. The purpose of this review is to compare the roles of distinct cell compartments during tendon homeostasis and injury. Furthermore, the role of immune cells in this field, as well as their interactions with stem cells and tenocytes during tendon regeneration, will be discussed to gain insights into new ways for dealing with tendinopathies.

Mechanism of Action of Mesenchymal Stem Cells (MSCs): impact of delivery method

INTRODUCTION

Mesenchymal stromal cells (MSCs; AKA mesenchymal stem cells) stimulate healing and reduce inflammation. Promising therapeutic responses are seen in many late-phase clinical trials, but others have not satisfied their primary endpoints, making translation of MSCs into clinical practice difficult. These inconsistencies may be related to the route of MSC delivery, lack of product optimization, or varying background therapies received in clinical trials over time.

AREAS COVERED

Here we discuss the different routes of MSC delivery, highlighting the proposed mechanism(s) of therapeutic action as well as potential safety concerns. PubMed search criteria used: MSC plus: local administration; routes of administration; delivery methods; mechanism of action; therapy in different diseases.

EXPERT OPINION

Direct injection of MSCs using a controlled local delivery approach appears to have benefits in certain disease states, but further studies are required to make definitive conclusions regarding the superiority of one delivery method over another.

Check-rein technique for Achilles tendon elongation following conservative management for acute Achilles tendon ruptures: a two-year prospective clinical study

Background

Following conservative management for acute Achilles tendon (AT) ruptures, the tendon may heal in continuity, and some patients may present with an elongated Achilles tendon–gastrosoleus complex. This study investigated the efficacy and feasibility of a novel minimally invasive technique, which we named “check-rein procedure”, in patients with intact and elongated AT following conservative management for AT ruptures.

Methods

All patients who underwent the check-rein procedure for elongation of the gastrosoleus–AT complex by one experienced surgeon were prospectively enrolled. The AT resting angle (ATRA) and AT rupture score (ATRS) were assessed at baseline and repeated at 2-year follow-up, as were calf circumference and isometric plantarflexion strength of both ankles.

Results

Forty-three patients (43 procedures) were analysed. The mean time elapsed from injury to surgery was 28.7 ± 7.9 weeks. The mean age at surgery was 38.5 ± 5.7 years. At the last follow-up, ATRS, ATRA, isometric strength difference, and calf circumference of the affected side were increased (P < 0.0001). The rate of the return to sport was 98% (42 of 43). No wound complications or rupture were experienced by any patient.

Conclusion

The check-rein technique for AT elongation after conservative management of AT tears is effective and feasible to restore tendon length and calf function. The surgical outcome was influenced by the preoperative performance status, and longer time elapsed from injury to surgery worsens the outcomes.

Tendon-Derived Progenitor Cells With Multilineage Potential Are Present Within Human Patellar Tendon

Background:

Progenitor cells serve as a promising source of regenerative potential in a variety of tissue types yet remain underutilized in tendinopathy. Tendon-derived progenitor cells (TDPCs) have previously been isolated from hamstring tendon but only as part of a concomitant medical procedure. Determining the presence of TDPCs in patellar tendon may facilitate clinical utilization of these cells because of the relative accessibility of this location for tissue harvest.

Purpose:

To characterize TDPCs in human patellar tendon samples.

Study Design:

Descriptive laboratory study.

Methods:

Human patellar tendon samples were obtained during elective knee surgery. TDPCs were isolated and seeded at an optimal low cell density and subcultured to confluence for up to 2 passages. Flow cytometry was used to analyze for the expression of CD90+, CD105+, CD44+, and CD31–, CD34–, and CD45– markers. The multilineage differentiation potential of TDPCs was tested in vitro via adipogenic, osteogenic, and chondrogenic culture with subsequent cytochemical staining for Oil Red O, Alizarin Red, and Alcian Blue, respectively. Enzyme-linked immunosorbent assay was used to quantify the amount of adiponectin, alkaline phosphatase, and SRY-box transcription factor 9 secreted into cell culture supernatant for further confirmation of lineage differentiation. Results were analyzed statistically using the 2-tailed Student t test.

Results:

TDPCs demonstrated near-uniform expression of CD90, CD105, and CD44 with minimal expression of CD34, CD31, and CD45. Adipogenic, osteogenic, and chondrogenic differentiation of TDPCs was confirmed using qualitative analysis. The expression of adiponectin, alkaline phosphatase, and SRY-box transcription factor 9 were significantly increased in differentiated cells versus undifferentiated TDPCs (P < .05).

Conclusion:

TDPCs can be successfully isolated from human patellar tendon samples, and they exhibit characteristics of multipotent progenitor cells.

Clinical Relevance:

These data demonstrate the promise of patellar tendon tissue as a source of progenitor cells for use in biologic therapies for the treatment of tendinopathy.

Small interfering RNAs in tendon homeostasis

BACKGROUND

Tenogenesis and tendon homeostasis are guided by genes encoding for the structural molecules of tendon fibres. Small interfering RNAs (siRNAs), acting on gene regulation, can therefore participate in the process of tendon healing.

SOURCES OF DATA

A systematic search of different databases to October 2020 identified 17 suitable studies.

AREAS OF AGREEMENT

SiRNAs can be useful to study reparative processes of tendons and identify possible therapeutic targets in tendon healing.

AREAS OF CONTROVERSY

Many genes and growth factors involved in the processes of tendinopathy and tendon healing can be regulated by siRNAs. It is however unclear which gene silencing determines the expected effect.

GROWING POINTS

Gene dysregulation of growth factors and tendon structural proteins can be influenced by siRNA.

AREAS TIMELY FOR DEVELOPING RESEARCH

It is not clear whether there is a direct action of the siRNAs that can be used to facilitate the repair processes of tendons.

Tendon-bioinspired wavy nanofibrous scaffolds provide tunable anisotropy and promote tenogenesis for tendon tissue engineering

The development of tendon-biomimetic nanofibrous scaffolds with mesenchymal stem cells may represent a promising strategy to improve the unsatisfactory outcomes of traditional treatments in tendon repair. In the present study, the nanofibrous scaffolds comprised of poly(p-dioxanone) (PPDO) and silk fibroin (SF) composites were fabricated by using electrospinning technique and subsequent thermal ethanol treatment. The PPDO/SF composite scaffolds presented parallel fiber arrangement with crimped features and nonlinear mechanical properties, which mimic the structure-function relationship of native tendon tissue mechanics. We demonstrated that the fiber crimp degree and mechanical properties of as-prepared PPDO/SF wavy nanofibrous scaffolds (WNSs) could be tunable by adjusting the mass ratio of PPDO/SF. The biological tests revealed that the addition of SF obviously promoted the cell adhesion, proliferation, and phenotypic maintenance of human tenocytes on the WNSs. A preliminary study on the subcutaneous implantation showed that the PPDO/SF WNSs notably decreased the inflammatory response compared with pure PPDO WNSs. More importantly, a combination of growth factor induction and mechanical stimulation was found to notably enhance the tenogenic differentiation of human adipose derived mesenchymal stem cells on the PPDO/SF WNSs by upregulating the expressions of tendon-associated protein and gene markers. Overall, this study demonstrated that our PPDO/SF WNSs could provide a beneficial microenvironment for various cell activities, making them an attractive candidate for tendon tissue engineering research.

The Biomechanics of Cartilage-An Overview

Articular cartilage (AC) sheathes joint surfaces and minimizes friction in diarthrosis. The resident cell population, chondrocytes, are surrounded by an extracellular matrix and a multitude of proteins, which bestow their unique characteristics. AC is characterized by a zonal composition (superficial (tangential) zone, middle (transitional) zone, deep zone, calcified zone) with different mechanical properties. An overview is given about different testing (load tests) methods as well as different modeling approaches. The widely accepted biomechanical test methods, e.g., the indentation analysis, are summarized and discussed. A description of the biphasic theory is also shown. This is required to understand how interstitial water contributes toward the viscoelastic behavior of AC. Furthermore, a short introduction to a more complex model is given.

Failed Surgery for Patellar Tendinopathy in Athletes: Midterm Results of Further Surgical Management

Background:

Tendon injuries are commonly seen in sports medicine practice. Many elite players involved in high-impact activities develop patellar tendinopathy (PT) symptoms. Of them, a small percentage will develop refractory PT and need to undergo surgery. In some of these patients, surgery does not resolve these symptoms.

Purpose:

To report the clinical results in a cohort of athletes who underwent further surgery after failure of primary surgery for PT.

Study Design:

Case series; Level of evidence, 4.

Methods:

A total of 22 athletes who had undergone revision surgery for failed surgical management of PT were enrolled in the present study. Symptom severity was assessed through the Victorian Institute of Sport Assessment Scale for Patellar Tendinopathy (VISA-P) upon admission and at the final follow-up. Time to return to training, time to return to competition, and complications were also recorded.

Results:

The mean age of the athletes was 25.4 years, and the mean symptom duration from the index intervention was 15.3 months. At a mean follow-up of 30.0 ± 4.9 months, the VISA-P score improved 27.8 points (P < .0001). The patients returned to training within a mean of 9.2 months. Fifteen patients (68.2%) returned to competition within a mean of 11.6 months. Of these 15 patients, a further 2 had decreased their performance, and 2 more had abandoned sports participation by the final follow-up. The overall rate of complications was 18.2%. One patient (4.5%) had a further revision procedure.

Conclusion:

Revision surgery was feasible and effective in patients in whom PT symptoms persisted after previous surgery for PT, achieving a statistically significant and clinically relevant improvement of the VISA-P score as well as an acceptable rate of return to sport at a follow-up of 30 months.

Amnion-Derived Teno-Inductive Secretomes: A Novel Approach to Foster Tendon Differentiation and Regeneration in an Ovine Model

Regenerative medicine has greatly progressed, but tendon regeneration mechanisms and robust in vitro tendon differentiation protocols remain to be elucidated. Recently, tendon explant co-culture (CO) has been proposed as an in vitro model to recapitulate the microenvironment driving tendon development and regeneration. Here, we explored standardized protocols for production and storage of bioactive tendon-derived secretomes with an evaluation of their teno-inductive effects on ovine amniotic epithelial cells (AECs). Teno-inductive soluble factors were released in culture-conditioned media (CM) only in response to active communication between tendon explants and stem cells (CMCO). Unsuccessful tenogenic differentiation in AECs was noted when exposed to CM collected from tendon explants (CMFT) only, whereas CMCO upregulated SCXB, COL I and TNMD transcripts, in AECs, alongside stimulation of the development of mature 3D tendon-like structures enriched in TNMD and COL I extracellular matrix proteins. Furthermore, although the tenogenic effect on AECs was partially inhibited by freezing CMCO, this effect could be recovered by application of an in vivo-like physiological oxygen (2% O2) environment during AECs tenogenesis. Therefore, CMCO can be considered as a waste tissue product with the potential to be used for the development of regenerative bio-inspired devices to innovate tissue engineering application to tendon differentiation and healing.

Cell-based therapy in soft tissue sports injuries of the knee: a systematic review

INTRODUCTION

An ever-increasing number of clinics are offering purportedly 'regenerative' stem-cell treatments, although cell-based therapies may not primarily act as stem cells and have shown the ability to regenerate end-target tissues in some clinical studies only. We aim to systematically review the evidence for their use in soft-tissue sports injuries of the knee.

AREAS COVERED

A search for articles pertaining to the use of preparations of, or containing, mesenchymal stem cells (MSCs) in human subjects in sports knee injuries yielded 14 relevant results for inclusion after screening: 7 used cultured MSCs, 5 bone marrow concentrate (BMC), and the remaining 2 evaluated stromal vascular fraction (SVF) and tenocyte-like-cells. Most studies were level 3 or lower (n = 9).

EXPERT OPINION

There is insufficient high-quality evidence for the use of cell-based therapies that demonstrates either ligamentous or tendinous healing, meniscal volume restoration, or post-traumatic osteoarthritis amelioration/regression. Methods of cell harvesting, preparation, and application are highly heterogenous. Efforts should be directed toward standardization of protocols and their reporting, starting with more basic scientific investigations of MSCs and their niche, as well as rigorous, large clinical RCTs adhering to the reporting principles set out by recent expert consensus.

Dose-Response Tendon-Specific Markers Induction by Growth Differentiation Factor-5 in Human Bone Marrow and Umbilical Cord Mesenchymal Stem Cells

Mesenchymal stem cells derived from human bone marrow (hBM-MSCs) are utilized in tendon tissue-engineering protocols while extra-embryonic cord-derived, including from Wharton's Jelly (hWJ-MSCs), are emerging as useful alternatives. To explore the tenogenic responsiveness of hBM-MSCs and hWJ-MSCs to human Growth Differentiation Factor 5 (hGDF-5) we supplemented each at doses of 1, 10, and 100 ng/mL of hGDF-5 and determined proliferation, morphology and time-dependent expression of tenogenic markers. We evaluated the expression of collagen types 1 (COL1A1) and 3 (COL3A1), Decorin (DCN), Scleraxis-A (SCX-A), Tenascin-C (TNC) and Tenomodulin (TNMD) noting the earliest and largest increase with 100 ng/mL. With 100 ng/mL, hBM-MSCs showed up-regulation of SCX-A (1.7-fold) at Day 1, TNC (1.3-fold) and TNMD (12-fold) at Day 8. hWJ-MSCs, at the same dose, showed up-regulation of COL1A1 (3-fold), DCN (2.7-fold), SCX-A (3.8-fold) and TNC (2.3-fold) after three days of culture. hWJ-MSCs also showed larger proliferation rate and marked aggregation into a tubular-shaped system at Day 7 (with 100 ng/mL of hGDF-5). Simultaneous to this, we explored the expression of pro-inflammatory (IL-6, TNF, IL-12A, IL-1β) and anti-inflammatory (IL-10, TGF-β1) cytokines across for both cell types. hBM-MSCs exhibited a better balance of pro-inflammatory and anti-inflammatory cytokines up-regulating IL-1β (11-fold) and IL-10 (10-fold) at Day 8; hWJ-MSCs, had a slight expression of IL-12A (1.5-fold), but a greater up-regulation of IL-10 (2.5-fold). Type 1 collagen and tenomodulin proteins, detected by immunofluorescence, confirming the greater protein expression when 100 ng/mL were supplemented. In the same conditions, both cell types showed specific alignment and shape modification with a length/width ratio increase, suggesting their response in activating tenogenic commitment events, and they both potential use in 3D in vitro tissue-engineering protocols.