Tenogenesis of bone marrow-, adipose-, and tendon-derived stem cells in a dynamic bioreactor

CTHRC1 Attenuates Tendinopathy via Enhancing EGFR/MAPK Signaling Pathway

Tendinopathy poses a formidable challenge due to the inherent limitations of tendon regenerative capabilities post-injury. At present, effective curative approaches for tendinopathy are still lacking. Collagen triple helix repeat-containing 1 (CTHRC1) is an extracellular matrix protein with significant roles in both physiological and pathological processes. The present study aims to investigate the function and underlying mechanism of CTHRC1 in tendinopathy. In this study, CTHRC1 is identified as a potential effector in promoting tendon regeneration through multi-proteomic analysis of Achilles tendon tissues in mice. In vitro, CTHRC1 enhances the proliferation, migration, and tenogenic differentiation of tendon stem/progenitor cell (TSPC). In vivo, CTHRC1 deletion impairs tendon healing, while its overexpression reverses the detrimental effects caused by CTHRC1 deficiency. Mechanistically, proteomics on TSPC stimulated with recombinant CTHRC1 reveal that CTHRC1 activates the mitogen-activated protein kinase (MAPK) signaling pathway via binding to epidermal growth factor receptor (EGFR), which in turn promotes the proliferative, migrative, and tenogenic capacities of TSPC to attenuate Achilles tendinopathy. Conversely, inhibiting EGFR reverses the tendon-healing effect of CRHRC1. The study demonstrates that CTHRC1 can promote the proliferative, migrative, and tenogenic capacities of TSPC, ultimately facilitating tendon healing through activating the EGFR/MAPK signaling pathway. CTHRC1 holds promise as a potential intervention for tendinopathy.

Recent trends of stem cell therapies in the management of orthopedic surgical challenges

Emerged health-related problems especially with increasing population and with the wider occurrence of these issues have always put the utmost concern and led medicine to outgrow its usual mode of treatment, to achieve better outcomes. Orthopedic interventions are one of the most concerning hitches, requiring advancement in several issues, that show complications with conventional approaches. Advanced studies have been undertaken to address the issue, among which stem cell therapy emerged as a better area of growth. The capacity of the stem cells to renovate themselves and adapt into different cell types made it possible to implement its use as a regenerative slant. Harvesting the stem cells, particularly mesenchymal stem cells (MSCs) is easier and can be further grown in vitro . In this review, we have discussed orthopedic-related issues including bone defects and fractures, nonunions, ligament and tendon injuries, degenerative changes, and associated conditions, which require further approaches to execute better outcomes, and the advanced strategies that can be tagged along with various ways of application of MSCs. It aims to objectify the idea of stem cells, with a major focus on the application of MSCs from different sources in various orthopedic interventions. It also discusses the limitations, and future scopes for further approaches in the field of regenerative medicine. The involvement of MSCs may transition the procedures in orthopedic interventions from predominantly surgical substitution and reconstruction to bio-regeneration and prevention. Nevertheless, additional improvements and evaluations are required to explore the effectiveness and safety of mesenchymal stem cell treatment in orthopedic regenerative medicine.

Engineering interfacial tissues: The myotendinous junction

The myotendinous junction (MTJ) is the interface connecting skeletal muscle and tendon tissues. This specialized region represents the bridge that facilitates the transmission of contractile forces from muscle to tendon, and ultimately the skeletal system for the creation of movement. MTJs are, therefore, subject to high stress concentrations, rendering them susceptible to severe, life-altering injuries. Despite the scarcity of knowledge obtained from MTJ formation during embryogenesis, several attempts have been made to engineer this complex interfacial tissue. These attempts, however, fail to achieve the level of maturity and mechanical complexity required for in vivo transplantation. This review summarizes the strategies taken to engineer the MTJ, with an emphasis on how transitioning from static to mechanically inducive dynamic cultures may assist in achieving myotendinous maturity.

Stem Cell‐Based Therapies and Tissue Engineering Innovations for Tendinopathy: A Comprehensive Review of Current Strategies and Future Directions

Tendon diseases commonly lead to physical disability, exerting a profound impact on the routine of affected patients. These conditions respond poorly to existing treatments, presenting a substantial challenge for orthopedic scientists. Research into clinical translational therapy has yet to yield highly versatile interventions capable of effectively addressing tendon diseases, including tendinopathy. Stem cell‐based therapies have emerged as a promising avenue for modifying the biological milieu through the secretion of regenerative and immunomodulatory factors. The current review provides an overview of the intricate tendon microenvironment, encompassing various tendon stem progenitor cells within distinct tendon sublocations, gene regulation, and pathways pertinent to tendon development, and the pathology of tendon diseases. Subsequently, the advantages of stem cell‐based therapies are discussed that utilize distinct types of autologous and allogeneic stem cells for tendon regeneration at the translational level. Moreover, this review outlines the challenges, gaps, and future innovations to propose a consolidated stem cell‐based therapy to treat tendinopathy. Finally, regenerative soluble therapies, insoluble bio‐active therapies, along with insoluble bio‐active therapies, and implantable 3D scaffolds for tendon tissue engineering are discussed, thereby presenting a pathway toward enhanced tissue regeneration and engineering.

Biomimetic gradient scaffolds for the tissue engineering and regeneration of rotator cuff enthesis

Rotator cuff tear is one of the most common musculoskeletal disorders, which often results in recurrent shoulder pain and limited movement. Enthesis is a structurally complex and functionally critical interface connecting tendon and bone that plays an essential role in maintaining integrity of the shoulder joint. Despite the availability of advanced surgical procedures for rotator cuff repair, there is a high rate of failure following surgery due to suboptimal enthesis healing and regeneration. Novel strategies based on tissue engineering are gaining popularity in improving tendon-bone interface (TBI) regeneration. Through incorporating physical and biochemical cues into scaffold design which mimics the structure and composition of native enthesis is advantageous to guide specific differentiation of seeding cells and facilitate the formation of functional tissues. In this review, we summarize the current state of research in enthesis tissue engineering highlighting the development and application of biomimetic scaffolds that replicate the gradient TBI. We also discuss the latest techniques for fabricating potential translatable scaffolds such as 3D bioprinting and microfluidic device. While preclinical studies have demonstrated encouraging results of biomimetic gradient scaffolds, the translation of these findings into clinical applications necessitates a comprehensive understanding of their safety and long-term efficacy.

Viable tendon neotissue from adult adipose-derived multipotent stromal cells

Background: Tendon healing is frequently prolonged, unpredictable, and results in poor tissue quality. Neotissue formed by adult multipotent stromal cells has the potential to guide healthy tendon tissue formation. Objectives: The objective of this study was to characterize tendon neotissue generated by equine adult adipose-derived multipotent stromal cells (ASCs) on collagen type I (COLI) templates under 10% strain in a novel bioreactor. The tested hypothesis was that ASCs assume a tendon progenitor cell-like morphology, express tendon-related genes, and produce more organized extracellular matrix (ECM) in tenogenic versus stromal medium with perfusion and centrifugal fluid motion. Methods: Equine ASCs on COLI sponge cylinders were cultured in stromal or tenogenic medium within bioreactors during combined perfusion and centrifugal fluid motion for 7, 14, or 21 days under 10% strain. Viable cell distribution and number, tendon-related gene expression, and micro- and ultra-structure were evaluated with calcein-AM/EthD-1 staining, resazurin reduction, RT-PCR, and light, transmission, and scanning electron microscopy. Fibromodulin was localized with immunohistochemistry. Cell number and gene expression were compared between culture media and among culture periods (p < 0.05). Results: Viable cells were distributed throughout constructs for up to 21 days of culture, and cell numbers were higher in tenogenic medium. Individual cells had a round or rhomboid shape with scant ECM in stromal medium in contrast to clusters of parallel, elongated cells surrounded by highly organized ECM in tenogenic medium after 21 days of culture. Transcription factor, extracellular matrix, and mature tendon gene expression profiles confirmed ASC differentiation to a tendon progenitor-like cell in tenogenic medium. Construct micro- and ultra-structure were consistent with tendon neotissue and fibromodulin was present in the ECM after culture in tenogenic medium. Conclusion: Long-term culture in custom bioreactors with combined perfusion and centrifugal tenogenic medium circulation supports differentiation of equine adult ASCs into tendon progenitor-like cells capable of neotissue formation.

Stem cell-based therapeutic strategies for rotator cuff tendinopathy

Rotator cuff tendinopathy is a common musculoskeletal disorder that imposes significant health and economic burden. Stem cell therapy has brought hope for tendon healing in patients with final stage rotator cuff tendinopathy. Some clinical trials have confirmed the effectiveness of stem cell therapy for rotator cuff tendinopathy, but its application has not been promoted and approved. There are still many issues that should be solved prior to using stem cell therapy in clinical applications. The optimal source and dose of stem cells for rotator cuff tendinopathy should be determined. We also proposed novel prospective approaches that can overcome cell population heterogeneity and standardize patient types for stem cell applications.

The translational potential of this article

This review explores the optimal sources of stem cells for rotator cuff tendinopathy and the principles for selecting stem cell dosages. Key strategies are provided for stem cell population standardization and recipient selection.

Equine tendon mechanical behaviour: Prospects for repair and regeneration applications

Tendons are dense connective tissues that play an important role in the biomechanical function of the musculoskeletal system. The mechanical forces have been implicated in every aspect of tendon biology. Tendon injuries are frequently occurring and their response to treatments is often unsatisfactory. A better understanding of tendon biomechanics and mechanobiology can help develop treatment options to improve clinical outcomes. Recently, tendon tissue engineering has gained more attention as an alternative treatment due to its potential to overcome the limitations of current treatments. This review first provides a summary of tendon mechanical properties, focusing on recent findings of tendon mechanobiological responses. In the next step, we highlight the biomechanical parameters of equine energy-storing and positional tendons. The final section is devoted to how mechanical loading contributes to tenogenic differentiation using bioreactor systems. This study may help develop novel strategies for tendon injury prevention or accelerate and improve tendon healing.

Examining the Effects of In Vitro Co-Culture of Equine Adipose-Derived Mesenchymal Stem Cells With Tendon Proper and Peritenon Cells

Tendinopathies remain the leading contributor to career-ending injuries in horses because of the complexity of tendon repair. As such, cell-based therapies like injections of adipose-derived mesenchymal stem cells (ADMSCs, or MSCs) into injured tendons are becoming increasingly popular though their long-term efficacy on a molecular and wholistic level remains contentious. Thus, we co-cultured equine MSCs with intrinsic (tendon proper) and extrinsic (peritenon) tendon cell populations to examine interactions between these cells. Gene expression for common tenogenic, perivascular, and differentiation markers was quantified at 48 and 120 hours. Additionally, cellular metabolism of proliferation was examined every 24 hours for peritenon and tendon proper cells co-cultured with MSCs. MSCs co-cultured with tendon proper or peritenon cells had altered expression profiles demonstrating trend toward tenogenic phenotype with the exception of decreases in type I collagen (COL1A1). Peritenon cells co-cultured with MSCs had a trending and significant decrease in biglycan (BGN) and CSPG4 at 48 hours and 120 hours but overall significant increases in lysyl oxidase (LOX), mohawk (MKX), and scleraxis (SCX) within 48 hours. Tendon proper cells co-cultured with MSCs also exhibited increases in LOX and SCX at 48 hours. Furthermore, cell proliferation improved overall for tendon proper cells co-cultured with MSCs. The co-culture study results suggest that adipose-derived MSCs contribute beneficially to tenogenic stimulation of peritenon or tendon proper cells.

Writing 3D In Vitro Models of Human Tendon within a Biomimetic Fibrillar Support Platform

Tendinopathies are poorly understood diseases for which treatment remains challenging. Relevant in vitro models to study human tendon physiology and pathophysiology are therefore highly needed. Here we propose the automated 3D writing of tendon microphysiological systems (MPSs) embedded in a biomimetic fibrillar support platform based on cellulose nanocrystals (CNCs) self-assembly. Tendon decellularized extracellular matrix (dECM) was used to formulate bioinks that closely recapitulate the biochemical signature of tendon niche. A monoculture system recreating the cellular patterns and phenotype of the tendon core was first developed and characterized. This system was then incorporated with a vascular compartment to study the crosstalk between the two cell populations. The combined biophysical and biochemical cues of the printed pattern and dECM hydrogel were revealed to be effective in inducing human-adipose-derived stem cells (hASCs) differentiation toward the tenogenic lineage. In the multicellular system, chemotactic effects promoted endothelial cells migration toward the direction of the tendon core compartment, while the established cellular crosstalk boosted hASCs tenogenesis, emulating the tendon development stages. Overall, the proposed concept is a promising strategy for the automated fabrication of humanized organotypic tendon-on-chip models that will be a valuable new tool for the study of tendon physiology and pathogenesis mechanisms and for testing new tendinopathy treatments.

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.

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.

Fibrocartilaginous Tissue: Why Does It Fail to Heal?

Tendons and ligaments are critical components in the function of the musculoskeletal system, as they provide stability and guide motion for the biomechanical transmission of forces into bone. Several common injuries in the foot and ankle require the repair of ruptured or attenuated tendon or ligament to its osseous insertion. Understanding the structure and function of injured ligaments and tendons is complicated by the variability and unpredictable nature of their healing. The healing process at the tendon/ligament to bone interface is challenging and often frustrating to foot and ankle surgeons, as they have a high failure rate necessitating the need for revision.

In Vitro Cellular Strain Models of Tendon Biology and Tenogenic Differentiation

Research has shown that the surrounding biomechanical environment plays a significant role in the development, differentiation, repair, and degradation of tendon, but the interactions between tendon cells and the forces they experience are complex. In vitro mechanical stimulation models attempt to understand the effects of mechanical load on tendon and connective tissue progenitor cells. This article reviews multiple mechanical stimulation models used to study tendon mechanobiology and provides an overview of the current progress in modelling the complex native biomechanical environment of tendon. Though great strides have been made in advancing the understanding of the role of mechanical stimulation in tendon development, damage, and repair, there exists no ideal in vitro model. Further comparative studies and careful consideration of loading parameters, cell populations, and biochemical additives may further offer new insight into an ideal model for the support of tendon regeneration studies.

Adult ovine connective tissue cells resemble mesenchymal stromal cells in their propensity for extensive ex vivo expansion

Purpose/Aim: Expanded, human connective tissue cells can adopt mesenchymal stromal cell (MSC) properties that are favorable for applications in regenerative medicine. Sheep are used as a large animal model for cell therapies, although for preclinical testing it is important to establish whether ovine cells resemble humans in their tendency to adopt MSC properties. The objective of this study was to investigate whether cells from five ovine connective tissues are MSC-like in their propensity for extensive expansion and immunophenotype.Materials and Methods: Monolayer cultures were established with cells from annulus fibrosus, cartilage, meniscus, tendon, and nucleus pulposus. Bone marrow MSCs were evaluated as a control. Cultures were seeded at 500 cells/cm², and subcultured every 5 days up to day 20. Flow cytometry was used to evaluate expression of cluster of differentiation (CD) molecules associated with MSCs (29, 44, 166). Colony formation was evaluated using time-lapse imaging of individual cells.Results: By day 20, cumulative population doublings ranged between 22 (chondrocytes) and 27 (MSCs). All cells uniformly expressed CD44 and 73. Expression of CD166 for MSCs was 98-99%, and ranged between 64 and 97% for the other cell types. Time-lapse imaging demonstrated that 58-94% of the cells colonized as indicated by 3 population doublings within 52 hours.Conclusions: Cells from ovine connective tissues resembled MSCs in their propensity for sustained, colony-forming growth and expression of CD molecules. These data supports the potential for preclinical testing of MSC-like connective tissue cells in sheep.

Effects of hypoxia on Achilles tendon repair using adipose tissue-derived mesenchymal stem cells seeded small intestinal submucosa

BACKGROUND

The study was performed to evaluate the feasibility of utilizing small intestinal submucosa (SIS) scaffolds seeded with adipose-derived mesenchymal stem cells (ADMSCs) for engineered tendon repairing rat Achilles tendon defects and to compare the effects of preconditioning treatments (hypoxic vs. normoxic) on the tendon healing.

METHODS

Fifty SD rats were randomized into five groups. Group A received sham operation (blank control). In other groups, the Achilles tendon was resected and filled with the original tendon (Group B, autograft), cell-free SIS (Group C), or SIS seeded with ADMSCs preconditioned under normoxic conditions (Group D) or hypoxic conditions (Group E). Samples were collected 4 weeks after operation and analyzed by histology, immunohistochemistry, and tensile testing.

RESULTS

Histologically, compared with Groups C and D, Group E showed a significant improvement in extracellular matrix production and a higher compactness of collagen fibers. Group E also exhibited a significantly higher peak tensile load than Groups D and C. Additionally, Group D had a significantly higher peak load than Group C. Immunohistochemically, Group E exhibited a significantly higher percentage of MKX + cells than Group D. The proportion of ADMSCs simultaneously positive for both MKX and CM-Dil observed from Group E was also greater than that in Group D.

CONCLUSIONS

In this animal model, the engineered tendon grafts created by seeding ADMSCs on SIS were superior to cell-free SIS. The hypoxic precondition further improved the expression of tendon-related genes in the seeded cells and increased the rupture load after grafting in the Achilles tendon defects.

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.

Biomaterials strategies to balance inflammation and tenogenesis for tendon repair

Adult tendon tissue demonstrates a limited regenerative capacity, and the natural repair process leaves fibrotic scar tissue with inferior mechanical properties. Surgical treatment is insufficient to provide the mechanical, structural, and biochemical environment necessary to restore functional tissue. While numerous strategies including biodegradable scaffolds, bioactive factor delivery, and cell-based therapies have been investigated, most studies have focused exclusively on either suppressing inflammation or promoting tenogenesis, which includes tenocyte proliferation, ECM production, and tissue formation. New biomaterials-based approaches represent an opportunity to more effectively balance the two processes and improve regenerative outcomes from tendon injuries. Biomaterials applications that have been explored for tendon regeneration include formation of biodegradable scaffolds presenting topographical, mechanical, and/or immunomodulatory cues conducive to tendon repair; delivery of immunomodulatory or tenogenic biomolecules; and delivery of therapeutic cells such as tenocytes and stem cells. In this review, we provide the biological context for the challenges in tendon repair, discuss biomaterials approaches to modulate the immune and regenerative environment during the healing process, and consider the future development of comprehensive biomaterials-based strategies that can better restore the function of injured tendon. STATEMENT OF SIGNIFICANCE: Current strategies for tendon repair focus on suppressing inflammation or enhancing tenogenesis. Evidence indicates that regulated inflammation is beneficial to tendon healing and that excessive tissue remodeling can cause fibrosis. Thus, it is necessary to adopt an approach that balances the benefits of regulated inflammation and tenogenesis. By reviewing potential treatments involving biodegradable scaffolds, biological cues, and therapeutic cells, we contrast how each strategy promotes or suppresses specific repair steps to improve the healing outcome, and highlight the advantages of a comprehensive approach that facilitates the clearance of necrotic tissue and recruitment of cells during the inflammatory stage, followed by ECM synthesis and organization in the proliferative and remodeling stages with the goal of restoring function to the tendon.

Basic Research on Tendon Repair: Strategies, Evaluation, and Development

Tendon is a fibro-elastic structure that links muscle and bone. Tendon injury can be divided into two types, chronic and acute. Each type of injury or degeneration can cause substantial pain and the loss of tendon function. The natural healing process of tendon injury is complex. According to the anatomical position of tendon tissue, the clinical results are different. The wound healing process includes three overlapping stages: wound healing, proliferation and tissue remodeling. Besides, the healing tendon also faces a high re-tear rate. Faced with the above difficulties, management of tendon injuries remains a clinical problem and needs to be solved urgently. In recent years, there are many new directions and advances in tendon healing. This review introduces tendon injury and sums up the development of tendon healing in recent years, including gene therapy, stem cell therapy, Platelet-rich plasma (PRP) therapy, growth factor and drug therapy and tissue engineering. Although most of these therapies have not yet developed to mature clinical application stage, with the repeated verification by researchers and continuous optimization of curative effect, that day will not be too far away.

The Lack of a Representative Tendinopathy Model Hampers Fundamental Mesenchymal Stem Cell Research

Overuse tendon injuries are a major cause of musculoskeletal morbidity in both human and equine athletes, due to the cumulative degenerative damage. These injuries present significant challenges as the healing process often results in the formation of inferior scar tissue. The poor success with conventional therapy supports the need to search for novel treatments to restore functionality and regenerate tissue as close to native tendon as possible. Mesenchymal stem cell (MSC)-based strategies represent promising therapeutic tools for tendon repair in both human and veterinary medicine. The translation of tissue engineering strategies from basic research findings, however, into clinical use has been hampered by the limited understanding of the multifaceted MSC mechanisms of action. In vitro models serve as important biological tools to study cell behavior, bypassing the confounding factors associated with in vivo experiments. Controllable and reproducible in vitro conditions should be provided to study the MSC healing mechanisms in tendon injuries. Unfortunately, no physiologically representative tendinopathy models exist to date. A major shortcoming of most currently available in vitro tendon models is the lack of extracellular tendon matrix and vascular supply. These models often make use of synthetic biomaterials, which do not reflect the natural tendon composition. Alternatively, decellularized tendon has been applied, but it is challenging to obtain reproducible results due to its variable composition, less efficient cell seeding approaches and lack of cell encapsulation and vascularization. The current review will overview pros and cons associated with the use of different biomaterials and technologies enabling scaffold production. In addition, the characteristics of the ideal, state-of-the-art tendinopathy model will be discussed. Briefly, a representative in vitro tendinopathy model should be vascularized and mimic the hierarchical structure of the tendon matrix with elongated cells being organized in a parallel fashion and subjected to uniaxial stretching. Incorporation of mechanical stimulation, preferably uniaxial stretching may be a key element in order to obtain appropriate matrix alignment and create a pathophysiological model. Together, a thorough discussion on the current status and future directions for tendon models will enhance fundamental MSC research, accelerating translation of MSC therapies for tendon injuries from bench to bedside.

Using tools in mechanobiology to repair tendons

Purpose of review

The purpose of this review is to describe the mechanobiological mechanisms of tendon repair as well as outline current and emerging tools in mechanobiology that might be useful for improving tendon healing and regeneration. Over 30 million musculoskeletal injuries are reported in the US per year and nearly 50% involve soft tissue injuries to tendons and ligaments. Yet current therapeutic strategies for treating tendon injuries are not always successful in regenerating and returning function of the healing tendon.

Recent findings

The use of rehabilitative strategies to control the motion and transmission of mechanical loads to repairing tendons following surgical reattachment is beneficial for some, but not all, tendon repairs. Scaffolds that are designed to recapitulate properties of developing tissues show potential to guide the mechanical and biological healing of tendon following rupture. The incorporation of biomaterials to control alignment and reintegration, as well as promote scar-less healing, are also promising. Improving our understanding of damage thresholds for resident cells and how these cells respond to bioelectrical cues may offer promising steps forward in the field of tendon regeneration.

Summary

The field of orthopaedics continues to advance and improve with the development of regenerative approaches for musculoskeletal injuries, especially for tendon, and deeper exploration in this area will lead to improved clinical outcomes.

Engineering multi-tissue units for regenerative Medicine: Bone-tendon-muscle units of the rotator cuff

Our body systems are comprised of numerous multi-tissue units. For the musculoskeletal system, one of the predominant functional units is comprised of bone, tendon/ligament, and muscle tissues working in tandem to facilitate locomotion. To successfully treat musculoskeletal injuries and diseases, critical consideration and thoughtful integration of clinical, biological, and engineering aspects are necessary to achieve translational bench-to-bedside research. In particular, identifying ideal biomaterial design specifications, understanding prior and recent tissue engineering advances, and judicious application of biomaterial and fabrication technologies will be crucial for addressing current clinical challenges in engineering multi-tissue units. Using rotator cuff tears as an example, insights relevant for engineering a bone-tendon-muscle multi-tissue unit are presented. This review highlights the tissue engineering strategies for musculoskeletal repair and regeneration with implications for other bone-tendon-muscle units, their derivatives, and analogous non-musculoskeletal tissue structures.

Scaffold-free cell-based tissue engineering therapies: advances, shortfalls and forecast

Cell-based scaffold-free therapies seek to develop in vitro organotypic three-dimensional (3D) tissue-like surrogates, capitalising upon the inherent capacity of cells to create tissues with efficiency and sophistication that is still unparalleled by human-made devices. Although automation systems have been realised and (some) success stories have been witnessed over the years in clinical and commercial arenas, in vitro organogenesis is far from becoming a standard way of care. This limited technology transfer is largely attributed to scalability-associated costs, considering that the development of a borderline 3D implantable device requires very high number of functional cells and prolonged ex vivo culture periods. Herein, we critically discuss advancements and shortfalls of scaffold-free cell-based tissue engineering strategies, along with pioneering concepts that have the potential to transform regenerative and reparative medicine.

Characterization of Tendon-Derived Stem Cells and Rescue Tendon Injury

The natural healing ability of tendon is limited, and it cannot restore the native structure and function of tendon injuries. Tendon-derived stem cells (TDSCs) are a new type of pluripotent stem cells with multi-directional differentiation potential and are expected to become a promising cell-seed for the treatment of tendon injuries in the future. In this review, we outline the latest advances in the culture and identification of TDSCs. In addition, the influencing factors on the differentiation of TDSCs are discussed. Moreover, we aim to discuss recent studies to enhance TDSCs treatment of injured tendons. Finally, we identify the limitations of the current understanding of TDSCs biology, the main challenges of using their use, and potential therapeutic strategies to inform cell-based tendon repair.

Effect of kartogenin-loaded gelatin methacryloyl hydrogel scaffold with bone marrow stimulation for enthesis healing in rotator cuff repair

BACKGROUND

Strategies involving microfracture, biomaterials, growth factors, and chemical agents have been evaluated for improving enthesis healing. Kartogenin (KGN) promotes selective differentiation of bone marrow mesenchymal stem cells (BMSCs) into chondrocytes. Gelatin methacryloyl (GelMA) is a promising biomaterial for engineering scaffolds and drug carriers. Herein, we investigated KGN-loaded GelMA hydrogel scaffolds with a bone marrow-stimulating technique for the repair of rotator cuff tear.

METHODS

KGN-loaded GelMA hydrogel scaffolds were obtained by ultraviolet GelMA crosslinking and vacuum freeze-drying. Fifty-four New Zealand rabbits were randomly divided into (1) repair only (control), (2) microfracture + repair (BMS), and (3) microfracture + repair augmentation with a KGN-loaded GelMA hydrogel scaffold (combined) groups. Tendons were repaired by transosseous sutures. The structure, degradation, and in vitro KGN release of the scaffolds were characterized. Animals were euthanized 4, 8, and 12 weeks after repair. Enthesis healing was evaluated by macroscopy, microcomputed tomography, histology, and biomechanical tests.

RESULTS

The KGN-loaded GelMA hydrogel scaffolds are porous with a 60.4 ± 28.2-μm average pore size, and they degrade quickly in 2.5 units/mL collagenase solution. Nearly 81% of KGN was released into phosphate-buffered saline within 12 hours, whereas the remaining KGN was released in 7 days. Macroscopically, the repaired tendons were attached to the footprint. No differences were detected postoperatively in microcomputed tomography analysis among groups. Fibrous scar tissue was the main component at the tendon-to-bone interface in the control group. Disorderly arranged cartilage formation was observed at the tendon-to-bone interface in the BMS and combined groups 4 weeks after repair; the combined group exhibited relatively more cartilage. The combined group showed improved cartilage regeneration 8 and 12 weeks after repair. Similar results were found in tendon maturation scores. The ultimate load to failure and stiffness of the repaired tendon increased in all 3 groups. At 4 weeks after repair, the BMS and combined groups exhibited greater ultimate load to failure than the control group, although there was no difference in stiffness among groups. The BMS and combined groups exhibited greater ultimate load to failure and stiffness than the control group, and the combined group exhibited better values than the BMS group at 8 and 12 weeks after repair.

CONCLUSION

Compared with the bone marrow-stimulating technique, the KGN-loaded GelMA hydrogel scaffold with bone marrow stimulation improved enthesis healing by promoting fibrocartilage formation and improving the mechanical properties.

Comparative Analysis of Tenogenic Gene Expression in Tenocyte-Derived Induced Pluripotent Stem Cells and Bone Marrow-Derived Mesenchymal Stem Cells in Response to Biochemical and Biomechanical Stimuli

The tendon is highly prone to injury, overuse, or age-related degeneration in both humans and horses. Natural healing of injured tendon is poor, and cell-based therapeutic treatment is still a significant clinical challenge. In this study, we extensively investigated the expression of tenogenic genes in equine bone marrow mesenchymal stem cells (BMSCs) and tenocyte-derived induced pluripotent stem cells (teno-iPSCs) stimulated by growth factors (TGF-β3 and BMP12) combined with ectopic expression of tenogenic transcription factor MKX or cyclic uniaxial mechanical stretch. Western blotting revealed that TGF-β3 and BMP12 increased the expression of transcription factors SCX and MKX in both cells, but the tenocyte marker tenomodulin (TNMD) was detected only in BMSCs and upregulated by either inducer. On the other hand, quantitative real-time PCR showed that TGF-β3 increased the expression of EGR1, COL1A2, FMOD, and TNC in BMSCs and SCX, COL1A2, DCN, FMOD, and TNC in teno-iPSCs. BMP12 treatment elevated SCX, MKX, DCN, FMOD, and TNC in teno-iPSCs. Overexpression of MKX increased SCX, DCN, FMOD, and TNC in BMSCs and EGR1, COL1A2, DCN, FMOD, and TNC in teno-iPSCs; TGF-β3 further enhanced TNC in BMSCs. Moreover, mechanical stretch increased SCX, EGR1, DCN, ELN, and TNC in BMSCs and SCX, MKX, EGR1, COL1A2, DCN, FMOD, and TNC in teno-iPSCs; TGF-β3 tended to further elevate SCX, ELN, and TNC in BMSCs and SCX, MKX, COL1A2, DCN, and TNC in teno-iPSCs, while BMP12 further uptrended the expression of SCX and DCN in BMSCs and DCN in teno-iPSCs. Additionally, the aforementioned tenogenic inducers also affected the expression of signaling regulators SMAD7, ETV4, and SIRT1 in BMSCs and teno-iPSCs. Taken together, our data demonstrate that, in respect to the tenocyte-lineage-specific gene expression, BMSCs and teno-iPSCs respond differently to the tenogenic stimuli, which may affect the outcome of their application in tendon repair or regeneration.

The Application of Mechanical Stimulations in Tendon Tissue Engineering

Tendon injury is the most common disease in the musculoskeletal system. The current treatment methods have many limitations, such as poor therapeutic effects, functional loss of donor site, and immune rejection. Tendon tissue engineering provides a new treatment strategy for tendon repair and regeneration. In this review, we made a retrospective analysis of applying mechanical stimulation in tendon tissue engineering, and its potential as a direction of development for future clinical treatment strategies. For this purpose, the following topics are discussed; (1) the context of tendon tissue engineering and mechanical stimulation; (2) the applications of various mechanical stimulations in tendon tissue engineering, as well as their inherent mechanisms; (3) the application of magnetic force and the synergy of mechanical and biochemical stimulation. With this, we aim at clarifying some of the main questions that currently exist in the field of tendon tissue engineering and consequently gain new knowledge that may help in the development of future clinical application of tissue engineering in tendon injury.

A universal multi-platform 3D printed bioreactor chamber for tendon tissue engineering

A range of bioreactors use linear actuators to apply tensile forces in vitro, but differences in their culture environments can limit a direct comparison between studies. The widespread availability of 3D printing now provides an opportunity to develop a 'universal' bioreactor chamber that, with minimal exterior editing can be coupled to a wide range of commonly used linear actuator platforms, for example, the EBERS-TC3 and CellScale MCT6, resulting in a greater comparability between results and consistent testing of potential therapeutics. We designed a bioreactor chamber with six independent wells that was 3D printed in polylactic acid using an Ultimaker 2+ and waterproofed using a commercially available coating (XTC-3D), an oxirane resin. The cell culture wells were further coated with Sylgard-184 polydimethylsiloxane (PDMS) to produce a low-adhesion well surface. With appropriate coating and washing steps, all materials were shown to be non-cytotoxic by lactate dehydrogenase assay, and the bioreactor was waterproof, sterilisable and reusable. Tissue-engineered tendons were generated from human mesenchymal stem cells in a fibrin hydrogel and responded to 5% cyclic strain (0.5 Hz, 5 h/day, 21 days) in the bioreactor by increased production of collagen-Iα1 and decreased production of collagen-IIIα1. Calcification of the extracellular matrix was observed in unstretched tendon controls indicating abnormal differentiation, while tendons cultured under cyclic strain did not calcify and exhibited a tenogenic phenotype. The ease of manufacturing this bioreactor chamber enables researchers to quickly and cheaply reproduce this culture environment for use with many existing bioreactor actuator platforms by downloading the editable CAD files from a public database and following the manufacturing steps we describe.

One health in regenerative medicine: report on the second Havemeyer symposium on regenerative medicine in horses

Regenerative medicine is commonly used in human and equine athletes. Potential therapies include culture expanded stem cells, stromal vascular fraction of adipose tissue, platelet-rich plasma, bone marrow concentrate, or autologous conditioned serum. The purpose of this manuscript is to disseminate findings from a workshop on the development of translational regenerative medicine in the equine field. Five themes emerged: stem cell characterization and tenogenic differentiation; interactions between mesenchymal stem cells, other cells and the environment; scaffolds and cell packaging; blood- and bone marrow-based regenerative medicines; clinical use of regenerative therapies. Evidence gained through the use of regenerative medicine applications in the horse should continue to translate to the human patient, bringing novel regenerative therapies to both humans and horses.

Growth and Stem Cell Characteristics of Tendon-Derived Cells with Different Initial Seeding Densities: An In Vitro Study in Mouse Flexor Tendon Cells

Tendon stem/progenitor cells (TSPCs) are considered promising seed cells for tendon regeneration. Previous studies reported that a low seeding density favors TSPC growth, whereas a high seeding density favors tenocyte growth. We aimed to distinguish TSPCs from tenocytes by seeding tendon-derived cells at a density gradient. In this study, tendon-derived cells were isolated from flexor digitorum profundus tendons of mice and seeded at the initial densities of 50, 500, 5,000, and 50,000/cm². We found that distinct cell colonies were formed from cells with initial seeding densities of 50 and 500/cm², but colonies were not discernible for cells seeded at 5,000 and 50,000/cm². There was a positive correlation between cell proliferation rate and seeding density, but a negative correlation between cell senescence and seeding density. The cell proliferation rate decreased gradually during serial passages. All cells exhibited restricted differentiation potentials, and expressed stem cell markers and relatively high levels of tenogenic markers without notable differences among cells seeded at different densities. We concluded that a pure population of TSPCs could not be isolated from mouse digital flexor tendons through culturing cells at a density gradient. Cells seeded at low densities had very limited proliferative ability and did not show more prominent stem cell characteristics when compared with cells seeded at high densities.

Tendon and Cytokine Marker Expression by Human Bone Marrow Mesenchymal Stem Cells in a Hyaluronate/Poly-Lactic-Co-Glycolic Acid (PLGA)/Fibrin Three-Dimensional (3D) Scaffold

We developed a (three-dimensional) 3D scaffold, we named HY-FIB, incorporating a force-transmission band of braided hyaluronate embedded in a cell localizing fibrin hydrogel and poly-lactic-co-glycolic acid (PLGA) nanocarriers as transient components for growth factor controlled delivery. The tenogenic supporting capacity of HY-FIB on human-Bone Marrow Mesenchymal Stem Cells (hBM-MSCs) was explored under static conditions and under bioreactor-induced cyclic strain conditions. HY-FIB elasticity enabled to deliver a mean shear stress of 0.09 Pa for 4 h/day. Tendon and cytokine marker expression by hBM-MSCs were studied. Results: hBM-MSCs embedded in HY-FIB and subjected to mechanical stimulation, resulted in a typical tenogenic phenotype, as indicated by type 1 Collagen fiber immunofluorescence. RT-qPCR showed an increase of type 1 Collagen, scleraxis, and decorin gene expression (3-fold, 1600-fold, and 3-fold, respectively, at day 11) in dynamic conditions. Cells also showed pro-inflammatory (IL-6, TNF, IL-12A, IL-1β) and anti-inflammatory (IL-10, TGF-β1) cytokine gene expressions, with a significant increase of anti-inflammatory cytokines in dynamic conditions (IL-10 and TGF-β1 300-fold and 4-fold, respectively, at day 11). Mechanical signaling, conveyed by HY-FIB to hBM-MSCs, promoted tenogenic gene markers expression and a pro-repair cytokine balance. The results provide strong evidence in support of the HY-FIB system and its interaction with cells and its potential for use as a predictive in vitro model.

Current Status and Future Prospects of Genome-Scale Metabolic Modeling to Optimize the Use of Mesenchymal Stem Cells in Regenerative Medicine

Mesenchymal stem cells are a promising source for externally grown tissue replacements and patient-specific immunomodulatory treatments. This promise has not yet been fulfilled in part due to production scaling issues and the need to maintain the correct phenotype after re-implantation. One aspect of extracorporeal growth that may be manipulated to optimize cell growth and differentiation is metabolism. The metabolism of MSCs changes during and in response to differentiation and immunomodulatory changes. MSC metabolism may be linked to functional differences but how this occurs and influences MSC function remains unclear. Understanding how MSC metabolism relates to cell function is however important as metabolite availability and environmental circumstances in the body may affect the success of implantation. Genome-scale constraint based metabolic modeling can be used as a tool to fill gaps in knowledge of MSC metabolism, acting as a framework to integrate and understand various data types (e.g., genomic, transcriptomic and metabolomic). These approaches have long been used to optimize the growth and productivity of bacterial production systems and are being increasingly used to provide insights into human health research. Production of tissue for implantation using MSCs requires both optimized production of cell mass and the understanding of the patient and phenotype specific metabolic situation. This review considers the current knowledge of MSC metabolism and how it may be optimized along with the current and future uses of genome scale constraint based metabolic modeling to further this aim.

Effects of dynamic radial tensile stress on fibrocartilage differentiation of bone marrow mesenchymal stem cells

BACKGROUND

Uniaxial/biaxial tensile stress has been employed to induce chondrocyte differentiation of mesenchymal stem cells. However, the effects of radial tensile stimuli on differentiation of MSCs into fibrocartilage remain unclear.

RESULTS

It was found that induced bone marrow mesenchymal stem cells (BMSCs) were not only similar to TMJ disc cells in morphology, but also could synthesize type I collagen (Col I), a small amount of type II collagen (Col II) and glycosaminoglycans (GAGs). The synthesis of Col I significantly increased while that of Col II gradually decreased with increasing tensile strength. The ratio of Col I to Col II was 1.8 to 1 and 2 to 1 in the 10% and 15% stretching groups, respectively. The gene expression of Col I and GAGs was significantly upregulated, whereas that of Col II was downregulated. However, the higher tensile stimulation (15%) promoted the synthesis of α-smooth muscle actin (α-SMA). Too much α-SMA is not conducive to constructing engineered tissue.

CONCLUSION

Therefore, the 10% radial tensile stimulus was the optimal strength for inducing the BMSCs to differentiate into fibrochondrocytes of the temporomandibular joint (TMJ) disc. This work provided a novel approach for inducing BMSCs to differentiate into fibrochondrocytes.

From the perspective of embryonic tendon development: various cells applied to tendon tissue engineering

There is a high risk of injury from damage to the force-bearing tissue of the tendon. Due to its poor self-healing ability, clinical interventions for tendon injuries are limited and yield unsatisfying results. Tissue engineering might supply an alternative to this obstacle. As one of the key elements of tissue engineering, various cell sources have been used for tendon engineering, but there is no consensue concerning a single optimal source. In this review, we summarized the development of tendon tissue from the embryonic stage and categorized the used cell sources in tendon engineering. By comparing various cell sources as the candidates for tendon regeneration, each cell type was found to have its advantages and limitations; therefore, it is difficult to define the best cell source for tendon engineering. The microenvironment cells located is also crucial for cell growth and differentiation; so, the optimal cells are unlikely to be the same for each patient. In the future, the clinical application of tendon engineering might be more precise and customized in contrast to the current use of a standardized/generic one-size-fits-all procedure. The best cell source for tendon engineering will require a case-based assessment.

Use of Adipose-Derived Orthobiologics for Musculoskeletal Injuries: A Narrative Review

Musculoskeletal injuries are among the most prevalent, disabling, and costly conditions that Americans face, affecting over half of those over 18 and nearly 75% of those over 65 years old. Current treatments are largely palliative for many of these conditions and unmet needs have warranted the emergence of alternative treatments. Orthobiologics, such as adipose tissue derivatives (ATDs), are of high interest because they can be obtained in the office setting and their cellular components, including adipose stem cells and stromal cells, are thought to be beneficial in the treatment of musculoskeletal injuries. Microfragmented adipose tissue (MFAT) and stromal vascular fraction (SVF) are two ATD injectates that are used in the clinical setting to treat musculoskeletal conditions. Our review aimed to clarify the terminology describing the various ATDs used for orthopedic indications while discussing the promising but low-quality evidence, heterogeneity in MFAT and SVF processing methods, and inconsistencies in reported information such as injectate characterization with cell counts, injection technique, and postprocedural rehabilitation.

Urinary Bladder Matrix Does Not Improve Tenogenesis in an In Vitro Equine Model

Extracellular matrix (ECM) is responsible for tendon strength and elasticity. Healed tendon ECM lacks structural integrity, leading to reinjury. Porcine urinary bladder matrix (UBM) provides a scaffold and source of bioactive proteins to improve tissue healing, but has received limited attention for treating tendon injuries. The objective of this study was to evaluate the ability of UBM to induce matrix organization and tenogenesis using a novel in vitro model. We hypothesized that addition of UBM to tendon ECM hydrogels would improve matrix organization and cell differentiation. Hydrogels seeded with bone marrow cells (n = 6 adult horses) were cast using rat tail tendon ECM ± UBM, fixed under static tension and harvested at 7 and 21 days for construct contraction, cell viability, histology, biochemistry, and gene expression. By day 7, UBM constructs contracted significantly from baseline, whereas control constructs did not. Both control and UBM constructs contracted significantly by day 21. In both groups, cells remained viable over time and changed from round and randomly oriented to elongated along lines of tension with visible compaction of the ECM. There were no differences over time or between treatments for nuclear aspect ratio, DNA, or glycosaminoglycan content. Decorin, matrix metalloproteinase 13, and scleraxis expression increased significantly over time, but not in response to UBM treatment. Mohawk expression was constant over time. Cartilage oligomeric matrix protein expression decreased over time in both groups. Using a novel ECM hydrogel model, substantial matrix organization and cell differentiation occurred; however, the addition of UBM failed to induce greater matrix organization than tendon ECM alone. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:1848-1859, 2019.

Regulation of the tenogenic gene expression in equine tenocyte-derived induced pluripotent stem cells by mechanical loading and Mohawk

Cell-based therapeutic strategies afford major potential advantages in the repair of injured tendons. Generation of induced pluripotent stem cells (iPSCs) expands cell sources for “regenerative” therapy. However, its application in tendon repair is still limited and the effects remain unclear. In this study, equine tenocyte-derived iPSCs (teno-iPSCs) were generated by expressing four Yamanaka factors. Compared to parental tenocytes and bone marrow derived mesenchymal stem cells (BMSCs), the transcriptional activities of lineage-specific genes, including Mkx, Col1A2, Col14, DCN, ELN, FMOD, and TNC, were highly repressed in the resulting teno-iPSCs. Exposure to cyclic uniaxial mechanical loading increased the expression of Scx, Egr1, Col1A2, DCN, and TNC in teno-iPSCs and the expression of Scx, Egr1, DCN, and TNC in BMSCs. Reintroduction of tenogenic transcription factor Mohawk (Mkx) upregulated the expression of DCN in teno-iPSCs and the expression of Scx, Col14, and FMOD in BMSCs. Mechanical loading combined with ectopic expression of equine Mkx further enhanced the expression of Egr1, Col1A2, DCN, and TNC in teno-iPSCs and the expression of Scx, Egr1, and TNC in BMSCs. These data suggest that the repressed lineage-specific genes in the teno-iPSCs can be re-activated by mechanical loading and ectopic expression of Mkx. Our findings offer new insights into the application of iPSCs for basic and clinic research in tendon repair.

Strategies of tenogenic differentiation of equine stem cells for tendon repair: current status and challenges

Tendon injuries, as one of the most common orthopedic disorders, are the major cause of early retirement or wastage among sport horses which mainly affect the superficial digital flexor tendon (SDFT). Tendon repair is a slow process, and tendon tissue is often replaced by scar tissue. The current treatment options are often followed by an incomplete recovery that increases the susceptibility to re-injury. Recently, cell therapy has been used in veterinary medicine to treat tendon injuries, although the risk of ectopic bone formation after cell injection is possible in some cases. In vitro tenogenic induction may overcome the mentioned risk in clinical application. Moreover, a better understanding of treatment strategies for musculoskeletal injuries in horse may have future applications for human and vice versa. This comprehensive review outlines the current strategies of stem cell therapy in equine tendon injury and in vitro tenogenic induction of equine stem cell.

The Influence of Cell Source and Donor Age on the Tenogenic Potential and Chemokine Secretion of Human Mesenchymal Stromal Cells

Background

Cellular therapy is proposed for tendinopathy treatment. Bone marrow- (BM-MSC) and adipose tissue- (ASC) derived mesenchymal stromal cells are candidate populations for such a therapy. The first aim of the study was to compare human BM-MSCs and ASCs for their basal expression of factors associated with tenogenesis as well as chemotaxis. The additional aim was to evaluate if the donor age influences these features.

Methods

Cells were isolated from 24 human donors, 8 for each group: hASC, hBM-MSC Y (age ≤ 45), and hBM-MSC A (age > 45). The microarray analysis was performed on RNA isolated from hASC and hBM-MSC A cells. Based on microarray results, 8 factors were chosen for further evaluation. Two genes were additionally included in the analysis: SCLERAXIS and PPARγ. All these 10 factors were tested for gene expression by the qRT-PCR method, and all except of RUNX2 were additionally evaluated for protein expression or secretion.

Results

Microarray analysis showed over 1,400 genes with a significantly different expression between hASC and hBM-MSC groups. Eight of these genes were selected for further analysis: CXCL6, CXCL12, CXCL16, TGF-β2, SMAD3, COLLAGEN 14A1, MOHAWK, and RUNX2. In the subsequent qRT-PCR analysis, hBM-MSCs showed a significantly higher expression than did hASCs in following genes: CXCL12, CXCL16, TGF-β2, SMAD3, COLLAGEN 14A1, and SCLERAXIS (p < 0.05, regardless of BM donor age). In the case of CXCL12, the difference between hASC and hBM-MSC was significant only for younger BM donors, whereas for COLLAGEN 14A1—only for elder BM donors. PPARγ displayed a higher expression in hASCs compared to hBM-MSCs. In regard to CXCL6, MOHAWK, and RUNX2 gene expression, no statistically significant differences between groups were observed.

Conclusions

In the context of cell-based therapy for tendinopathies, bone marrow appears to be a more attractive source of MSCs than does adipose tissue. The age of cell donors seems to be less important than cell source, although cells from elder donors show slightly higher basal tenogenic potential than do cells from younger donors.

Vascular Endothelial Growth Factor Enhances Proliferation of Human Tenocytes and Promotes Tenogenic Gene Expression

BACKGROUND

In obtaining human tenocytes for tendon tissue engineering, a low proliferation rate and phenotype loss during passaging is a problem. It was the authors' aim to evaluate the influence of vascular endothelial growth factor (VEGF) on human tenocyte growth and gene expression.

METHODS

Human tenocytes were exposed to human VEGF in various concentrations (5, 10, and 20 ng/ml) for 5 days. Cell proliferation was counted and expression of tendon-related genes was analyzed.

RESULTS

Tenocyte count was 1.4 × 10(5)/ml, 2.7 × 10(5)/ml, 2.3 × 10(5)/ml, and 3.7 × 10(5)/ml for 0, 5, 10, and 20 ng/ml VEGF, respectively. Expression of Col1 was up-regulated 6.4 ± 4.2-fold, 60.1 ± 21.6-fold, and 15.8 ± 10.2-fold for 5, 10, and 20 ng/ml VEGF; all differences were significant with p < 0.05. Col3 was down-regulated to 0.2 ± 0.1-fold, 0.3 ± 0.1-fold, and 0.1 ± 0.03-fold for 5, 10, and 20 ng/ml VEGF; all differences were significant. Eln was up-regulated 2.3 ± 1.7-fold, 25.5 ± 10.9-fold, and 16.6 ± 9.0-fold for 5, 10, and 20 ng/ml VEGF; differences were significant for 10 and 20 ng/ml VEGF. TSC was down-regulated to 0.3 ± 0.1-fold and 0.3 ± 0.1-fold for 5 and 20 ng/ml VEGF; differences were significant for 5 and 20 ng/ml. SCX was up-regulated to 31.3 ± 8.5-fold, 49.1 ± 23.4-fold, and 20.9 ± 9.5-fold for 5, 10, and 20 ng/ml VEGF; all changes were significant.

CONCLUSIONS

VEGF enhances proliferation and expression of tendon-related genes in human tenocytes. It could therefore be a useful addition for tenocyte cultivation.

Growth Factor-Mediated Tenogenic Induction of Multipotent Mesenchymal Stromal Cells Is Altered by the Microenvironment of Tendon Matrix

Age-related degenerative changes in tendon tissue represent a common cause for acute tendon pathologies. Although the regenerative potential of multipotent mesenchymal stromal cells (MSC) was reported to restore functionality in injured tendon tissue, cellular mechanisms of action remain partly unclear. Potential tenogenic differentiation of applied MSC is affected by various intrinsic and extrinsic factors. The current study presents an in vitro model to evaluate the combined extrinsic effects of decellularized equine tendon matrix, transforming growth factor beta 3 (TGFβ3) and bone morphogenetic protein 12 (BMP12) on the tenogenic fate of equine adipose tissue-derived MSC. Monolayer MSC cultures supplemented with TGFβ3 and BMP12 as well as MSC cultured on tendon matrix scaffolds preloaded with the growth factors were incubated for 3 and 5 days. Histological evaluation and real time reverse transcription polymerase chain reaction (RT-PCR) revealed that growth factor-mediated tenogenic induction of MSC was modified by the conditions of the surrounding microenvironment. While the gene expression pattern in monolayer cultures supplemented with TGFβ3 or TGFβ3 and BMP12 revealed an upregulation for collagen 1A2, collagen 3A1, tenascin c, scleraxis and mohawk (p < 0.05), the presence of tendon matrix led to an upregulation of decorin and osteopontin as well as to a downregulation of smad8 (p < 0.05). Preloading of scaffolds with either TGFβ3, or with TGFβ3 and BMP12 promoted a tenocyte-like phenotype and improved cell alignment. Furthermore, gene expression in scaffold culture was modulated by TGFβ3 and/or BMP12, with downregulation of collagen 1A2, collagen 3A1, decorin, scleraxis, smad8 and osteopontin, whereas gene expression of tenascin c was increased. This study shows that growth factor-induced tenogenic differentiation of equine MSC is markedly altered by topographical constraints of decellularized tendon tissue in vitro. While TGFβ3 represents an effective mediator for tenogenic induction, the role of BMP12 in tenogenesis may be of modulatory character and needs further evaluation.

Tenogenic Properties of Mesenchymal Progenitor Cells Are Compromised in an Inflammatory Environment

Transplantation of multipotent mesenchymal progenitor cells is a valuable option for treating tendon disease. Tenogenic differentiation leading to cell replacement and subsequent matrix modulation may contribute to the regenerative effects of these cells, but it is unclear whether this occurs in the inflammatory environment of acute tendon disease. Equine adipose-derived stromal cells (ASC) were cultured as monolayers or on decellularized tendon scaffolds in static or dynamic conditions, the latter represented by cyclic stretching. The impact of different inflammatory conditions, as represented by supplementation with interleukin-1β and/or tumor necrosis factor-α or by co-culture with allogeneic peripheral blood leukocytes, on ASC functional properties was investigated. High cytokine concentrations increased ASC proliferation and osteogenic differentiation, but decreased chondrogenic differentiation and ASC viability in scaffold culture, as well as tendon scaffold repopulation, and strongly influenced musculoskeletal gene expression. Effects regarding the latter differed between the monolayer and scaffold cultures. Leukocytes rather decreased ASC proliferation, but had similar effects on viability and musculoskeletal gene expression. This included decreased expression of the tenogenic transcription factor scleraxis by an inflammatory environment throughout culture conditions. The data demonstrate that ASC tenogenic properties are compromised in an inflammatory environment, with relevance to their possible mechanisms of action in acute tendon disease.

Mesenchymal Stromal/Stem Cells in Regenerative Medicine and Tissue Engineering

As a result of over five decades of investigation, mesenchymal stromal/stem cells (MSCs) have emerged as a versatile and frequently utilized cell source in the fields of regenerative medicine and tissue engineering. In this review, we summarize the history of MSC research from the initial discovery of their multipotency to the more recent recognition of their perivascular identity in vivo and their extraordinary capacity for immunomodulation and angiogenic signaling. As well, we discuss long-standing questions regarding their developmental origins and their capacity for differentiation toward a range of cell lineages. We also highlight important considerations and potential risks involved with their isolation, ex vivo expansion, and clinical use. Overall, this review aims to serve as an overview of the breadth of research that has demonstrated the utility of MSCs in a wide range of clinical contexts and continues to unravel the mechanisms by which these cells exert their therapeutic effects.

Intratendinous adipose-derived stromal vascular fraction (SVF) injection provides a safe, efficacious treatment for Achilles tendinopathy: results of a randomized controlled clinical trial at a 6-month follow-up

PURPOSE

Although platelet-rich plasma (PRP) injection has shown controversial results for the treatment of Achilles tendinopathy, it remains the most used biological treatment. Recent findings seem to demonstrate that the stromal vascular fraction (SVF) within adipose tissue may counteract the impaired tendon homeostasis. The aim of this study was to prospectively compare the efficacy of PRP and SVF injection for the treatment of non-insertional Achilles tendinopathy.

METHODS

Fourty-four patients were recruited in the study; 23 of them were assigned to the PRP group whereas 21 to the SVF group, treated unilaterally or bilaterally for a total of 28 tendons per group. All patients (age 18-55 years) were clinically assessed pre-operatively and at 15, 30, 60, 120 and 180 days from treatment, using the VAS pain scale, the VISA-A, the AOFAS Ankle-Hindfoot Score and the SF-36 form. The patients were also evaluated by ultrasound and magnetic resonance before treatment and after 4 (US only) and 6 months.

RESULTS

Both treatments allowed for a significant improvement with respect to baseline. Comparing the two groups, VAS, AOFAS and VISA-A scored significantly better at 15 and 30 days in the SVF in comparison to PRP group (p < 0.05). At the following time points the scores were not significantly different between the two groups. No correlation has been found between clinical and radiological findings.

CONCLUSIONS

Both PRP and SVF were safe, effective treatments for recalcitrant Achilles tendinopathy. The patients treated with SVF obtained faster results, thus suggesting that such a treatment should be taken into consideration for those patients who require an earlier return to daily activities or sport.

LEVEL OF EVIDENCE

Randomized Controlled Clinical Trial, Level 1.

Relevance of bioreactors and whole tissue cultures for the translation of new therapies to humans

The purpose of this review is to provide a brief overview of bioreactor-based culture systems as alternatives to conventional two- and three-dimensional counterparts. The role, challenges, and future aspirations of bioreactors in the musculoskeletal field (e.g., cartilage, intervertebral disc, tendon, and bone) are discussed. Bioreactors, by recapitulating physiological processes, can be used effectively as part of the initial in vitro screening, reducing that way the number of animal required for preclinical assessment, complying with the 3R principles and, in most cases, allowing working with human tissues. The clinical significance of bioreactors is that, by providing more physiologically relevant conditions to customarily used two- and three-dimensional cultures, they hold the potential to provide a testing platform that is more predictable of a whole tissue response, thereby facilitating the screening of treatments before the initiation of clinical trials. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:10-21, 2018.

Serum-free human MSC medium supports consistency in human but not in equine adipose-derived multipotent mesenchymal stromal cell culture

For clinical applications of multipotent mesenchymal stromal cells (MSCs), serum-free culture is preferable to standardize cell products and prevent contamination with pathogens. In contrast to human MSCs, knowledge on serum-free culture of large animal MSCs is limited, despite its relevance for preclinical studies and development of veterinary cellular therapeutics. This study aimed to evaluate the suitability of a commercially available serum-free human MSC medium for culturing equine adipose-derived MSCs in comparison with human adipose MSCs. Enzyme-free isolation by explant technique and expansion of equine and human cells in the serum-free medium were feasible. However, serum-free culture altered the morphology and complicated handling of equine MSCs, with cell aggregation and spontaneous detachment of multilayers, compared to culture in standard medium supplemented with fetal bovine serum. Furthermore, proliferation and the surface immunophenotype of equine cells were more variable compared to the controls and appeared to depend on the lot of the serum-free medium. Particularly the expression of CD90 was different between experimental groups (P < 0.05), with lower percentages of CD90⁺ cells found in equine MSC samples cultured in serum-free medium (5.21-83.40%) compared to standard medium (86.20-99.50%). Additionally, small subpopulations expressing MSC exclusion markers such as CD14 (0.28-11.60%), CD34 (0.00-9.87%), CD45 (0.35-10.50%), or MHCII (0.00-3.67%) were found in equine samples after serum-free culture. In contrast, human samples displayed a more consistent morphology and a consistent CD29⁺ (98.60-99.90%), CD73⁺ (94.60-98.40%), CD90⁺ (99.60-99.90%), and CD105⁺ (97.40-99.80%) immunophenotype after culture in serum-free medium. The obtained data demonstrate that the serum-free medium was suitable for human MSC culture but did not lead to entirely satisfactory results in equine MSCs. This underlines that requirements regarding serum-free culture conditions are species-specific, indicating a need for serum-free media to be optimized for MSCs from relevant animal species. © 2017 International Society for Advancement of Cytometry.

Combination of biochemical and mechanical cues for tendon tissue engineering

Tendinopathies negatively affect the life quality of millions of people in occupational and athletic settings, as well as the general population. Tendon healing is a slow process, often with insufficient results to restore complete endurance and functionality of the tissue. Tissue engineering, using tendon progenitors, artificial matrices and bioreactors for mechanical stimulation, could be an important approach for treating rips, fraying and tissue rupture. In our work, C3H10T1/2 murine fibroblast cell line was exposed to a combination of stimuli: a biochemical stimulus provided by Transforming Growth Factor Beta (TGF-β) and Ascorbic Acid (AA); a three-dimensional environment represented by PEGylated-Fibrinogen (PEG-Fibrinogen) biomimetic matrix; and a mechanical induction exploiting a custom bioreactor applying uniaxial stretching. In vitro analyses by immunofluorescence and mechanical testing revealed that the proposed combined approach favours the organization of a three-dimensional tissue-like structure promoting a remarkable arrangement of the cells and the neo-extracellular matrix, reflecting into enhanced mechanical strength. The proposed method represents a novel approach for tendon tissue engineering, demonstrating how the combined effect of biochemical and mechanical stimuli ameliorates biological and mechanical properties of the artificial tissue compared to those obtained with single inducement.