Engineering Tendon: Scaffolds, Bioreactors, and Models of Regeneration

Mechanical loading on cell-free polymer composite scaffold enhances in situ regeneration of fully functional Achilles tendon in a rabbit model

Traditional tendon engineering using cell-loaded scaffold has limited application potential due to the need of autologous cells. We hypothesize that potent mechanical loading can efficiently induce in situ Achilles tendon regeneration in a rabbit model by using a cell-free porous composite scaffold. In this study, melt-spinning was used to fabricate PGA (polyglycolic acid) and PLA (polylactic acid) filament fibers as well as non-woven PGA fibers. The PLA/PGA (4:2) filament fibers were further braided into a hybrid yarn，which was knitted into a PLA/PGA tubular mesh with potent mechanical property for sustaining natural tendon strain. The results showed that a complete cross-section of Achilles tendon created a model of full mechanical loading on the bridging scaffold, which could efficiently induce in situ tendon regeneration by promoting host cell infiltration, matrix production and tissue remodeling. Histologically, mechanical loading assisted in forming parallel aligned collagen fibers and tenocytes in a fashion similar to those of native tendon. Transmission electron microscope further demonstrated that mechanical strain induced collagen fibril development by increasing fibril diameter and forming bipolar structure, which resulted in enhanced mechanical properties. Interestingly, the synergistic effect between mechanical loading and hyaluronic acid modification was also observed on the induced tenogenic differentiation of infiltrated host fibroblasts. In conclusion, potent mechanical loading is the key inductive microenvironment for in situ tendon regeneration for this polymer-based composite scaffold with proper matrix modification, which may serve as a universal scaffold product for tendon regeneration.

Biomechanical Forces in the Tissue Engineering and Regeneration of Shoulder, Hip, Knee, and Ankle Joints

Tear on the tendon, ligament and articular cartilage of the joints do not heal by itself and new modalities of treatment are required to address the need for full restoration of joint functions. Accompanied by degenerative diseases, the healing of these tissues does not occur naturally and hence requires surgical interventions, but with associated morbidity. Tissue engineering strategies are now focusing on the effective incorporation of biomechanical stimulation by the application of biomechanical forces relevant to the tissue of interest to regenerate and engineer functional tissues. Bioreactors are being continuously developed to accomplish this goal. Although bioreactors have been developed, the advancement in the field of biomaterial, basic science, and cell engineering warrant further refinement for their effective use. In this article we reviewed the application of biomechanical forces in the tissue engineering and regeneration of the joints such as rotator cuff of shoulder, ball and socket joint of the hip, articular cartilage of knee, and the ankle joints.

An overview of the material science and knowledge of nanomedicine, bioscaffolds, and tissue engineering for tendon restoration

Tendon wounds are a worldwide health issue affecting millions of people annually. Due to the characteristics of tendons, their natural restoration is a complicated and lengthy process. With the advancement of bioengineering, biomaterials, and cell biology, a new science, tissue engineering, has developed. In this field, numerous ways have been offered. As increasingly intricate and natural structures resembling tendons are produced, the results are encouraging. This study highlights the nature of the tendon and the standard cures that have thus far been utilized. Then, a comparison is made between the many tendon tissue engineering methodologies proposed to date, concentrating on the ingredients required to gain the structures that enable appropriate tendon renewal: cells, growth factors, scaffolds, and scaffold formation methods. The analysis of all these factors enables a global understanding of the impact of each component employed in tendon restoration, thereby shedding light on potential future approaches involving the creation of novel combinations of materials, cells, designs, and bioactive molecules for the restoration of a functional tendon.

High-Performance Polarization Microscopy Reveals Structural Remodeling in Rat Calcaneal Tendons Cultivated In Vitro

Collagenous tissues exhibit anisotropic optical properties such as birefringence and linear dichroism (LD) as a result of their structurally oriented supraorganization from the nanometer level to the collagen bundle scale. Changes in macromolecular order and in aggregational states can be evaluated in tendon collagen bundles using polarization microscopy. Because there are no reports on the status of the macromolecular organization in tendon explants, the objective of this work was to evaluate the birefringence and LD characteristics of collagen bundles in rat calcaneal tendons cultivated in vitro on substrates that differ in their mechanical stiffness (plastic vs. glass) while accompanying the expected occurrence of cell migration from these structures. Tendon explants from adult male Wistar rats were cultivated for 8 and 12 days on borosilicate glass coverslips (n = 3) and on nonpyrogenic polystyrene plastic dishes (n = 4) and were compared with tendons not cultivated in vitro (n = 3). Birefringence was investigated in unstained tendon sections using high-performance polarization microscopy and image analysis. LD was studied under polarized light in tendon sections stained with the dichroic dyes Ponceau SS and toluidine blue at pH 4.0 to evaluate the orientation of proteins and acid glycosaminoglycans (GAG) macromolecules, respectively. Structural remodeling characterized by the reduction in the macromolecular orientation, aggregation and alignment of collagen bundles, based on decreased average gray values concerned with birefringence intensity, LD and morphological changes, was detected especially in the tendon explants cultivated on the plastic substrate. These changes may have facilitated cell migration from the lateral regions of the explants to the substrates, an event that was observed earlier and more intensely upon tissue cultivation on the plastic substrate. The axial alignment of the migrating cells relative to the explant, which occurred with increased cultivation times, may be due to the mechanosensitive nature of the tenocytes. Collagen fibers possibly played a role as a signal source to cells, a hypothesis that requires further investigation, including studies on the dynamics of cell membrane receptors and cytoskeletal organization, and collagen shearing electrical properties.

Automated Assessment of Cell Infiltration and Removal in Decellularized Scaffolds - Experimental Study in Rabbits

Objective  Semiquantitative and automated measurement of nuclear material removal and cell infiltration in decellularized tendon scaffolds (DTSs). Method  16 pure New Zealand rabbits were used, and the gastrocnemius muscle tendon was collected bilaterally from half of these animals (16 tendons collected); 4 were kept as control and 12 were submitted to the decellularization protocol (DTS). Eight of the DTSs were used as an in vivo implant in the experimental rotator cuff tear (RCT) model, and the rest, as well as the controls, were used in the semiquantitative and automated evaluation of nuclear material removal. The eight additional rabbits were used to make the experimental model of RCT and subsequent evaluation of cellular infiltration after 2 or 8 weeks, within the DTS. Results  The semiquantitative and automated analysis used demonstrated a removal of 79% of nuclear material ( p  < 0.001 and power > 99%) and a decrease of 88% (p < 0.001 and power >99%) in the area occupied by nuclear material after the decellularization protocol. On cell infiltration in DTS, an increase of 256% (p < 0.001 and power >99%) in the number of cells within the DTS was observed in the comparison between 2 and 8 weeks postoperatively. Conclusion  The proposed semiquantitative and automated measurement method was able to objectively measure the removal of nuclear material and cell infiltration in DTS.

Tendon Tissue Repair in Prospective of Drug Delivery, Regenerative Medicines, and Innovative Bioscaffolds

The natural healing capacity of the tendon tissue is limited due to the hypovascular and cellular nature of this tissue. So far, several conventional approaches have been tested for tendon repair to accelerate the healing process, but all these approaches have their own advantages and limitations. Regenerative medicine and tissue engineering are interdisciplinary fields that aspire to develop novel medical devices, innovative bioscaffold, and nanomedicine, by combining different cell sources, biodegradable materials, immune modulators, and nanoparticles for tendon tissue repair. Different studies supported the idea that bioscaffolds can provide an alternative for tendon augmentation with an enormous therapeutic potentiality. However, available data are lacking to allow definitive conclusion on the use of bioscaffolds for tendon regeneration and repairing. In this review, we provide an overview of the current basic understanding and material science in the field of bioscaffolds, nanomedicine, and tissue engineering for tendon repair.

Mesenchymal Stromal Cells Adapt to Chronic Tendon Disease Environment with an Initial Reduction in Matrix Remodeling

Tendon lesions are common sporting injuries in humans and horses alike. The healing process of acute tendon lesions frequently results in fibrosis and chronic disease. In horses, local mesenchymal stromal cell (MSC) injection is an accepted therapeutic strategy with positive influence on acute lesions. Concerning the use of MSCs in chronic tendon disease, data are scarce but suggest less therapeutic benefit. However, it has been shown that MSCs can have a positive effect on fibrotic tissue. Therefore, we aimed to elucidate the interplay of MSCs and healthy or chronically diseased tendon matrix. Equine MSCs were cultured either as cell aggregates or on scaffolds from healthy or diseased equine tendons. Higher expression of tendon-related matrix genes and tissue inhibitors of metalloproteinases (TIMPs) was found in aggregate cultures. However, the tenogenic transcription factor scleraxis was upregulated on healthy and diseased tendon scaffolds. Matrix metalloproteinase (MMPs) expression and activity were highest in healthy scaffold cultures but showed a strong transient decrease in diseased scaffold cultures. The release of glycosaminoglycan and collagen was also higher in scaffold cultures, even more so in those with tendon disease. This study points to an early suppression of MSC matrix remodeling activity by diseased tendon matrix, while tenogenic differentiation remained unaffected.

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.

A brief history of tendon and ligament bioreactors: Impact and future prospects

Bioreactors are powerful tools with the potential to model tissue development and disease in vitro. For nearly four decades, bioreactors have been used to create tendon and ligament tissue-engineered constructs in order to define basic mechanisms of cell function, extracellular matrix deposition, tissue organization, injury, and tissue remodeling. This review provides a historical perspective of tendon and ligament bioreactors and their contributions to this advancing field. First, we demonstrate the need for bioreactors to improve understanding of tendon and ligament function and dysfunction. Next, we detail the history and evolution of bioreactor development and design from simple stretching of explants to fabrication and stimulation of two- and three-dimensional constructs. Then, we demonstrate how research using tendon and ligament bioreactors has led to pivotal basic science and tissue-engineering discoveries. Finally, we provide guidance for new basic, applied, and clinical research utilizing these valuable systems, recognizing that fundamental knowledge of cell-cell and cell-matrix interactions combined with appropriate mechanical and chemical stimulation of constructs could ultimately lead to functional tendon and ligament repairs in the coming decades.

Rebuilding Tendons: A Concise Review on the Potential of Dermal Fibroblasts

Tendons are vital to joint movement by connecting muscles to bones. Along with an increasing incidence of tendon injuries, tendon disorders can burden the quality of life of patients or the career of athletes. Current treatments involve surgical reconstruction and conservative therapy. Especially in the elderly population, tendon recovery requires lengthy periods and it may result in unsatisfactory outcome. Cell-mediated tendon engineering is a rapidly progressing experimental and pre-clinical field, which holds great potential for an alternative approach to established medical treatments. The selection of an appropriate cell source is critical and remains under investigation. Dermal fibroblasts exhibit multiple similarities to tendon cells, suggesting they may be a promising cell source for tendon engineering. Hence, the purpose of this review article was in brief, to compare tendon to dermis tissues, and summarize in vitro studies on tenogenic differentiation of dermal fibroblasts. Furthermore, analysis of an open source Gene Expression Omnibus (GEO) data repository was carried out, revealing great overlap in the molecular profiles of both cell types. Lastly, a summary of in vivo studies employing dermal fibroblasts in tendon repair as well as pilot clinical studies in this area is included. Altogether, dermal fibroblasts hold therapeutic potential and are attractive cells for rebuilding injured tendons.

The clinical strategies for tendon repair with biomaterials: A review on rotator cuff and Achilles tendons

Tendon repair is a complex process due to the low tenocyte density, metabolism, and vascularization. Tears of rotator cuff (RCT) and Achilles tendons ruptures have a major impact on healthcare costs and quality of life of patients. Scaffolds are used to improve the healing rate after surgery and long-term results. A systematic search was carried out to identify the different types of scaffolds used during RCT and Achilles tendon repair surgery in the last 10 years. A higher number of clinical studies were reported on RCT ruptures. Biological scaffolds were used more than synthetic ones, for both rotator cuff and Achilles tendons. Moreover, platelet-rich plasma (PRP)-based scaffolds were the most widely used in RCT. A different type of synthetic scaffold was used in each of the five studies found. Biological scaffolds either provide variable results, in particular PRP-based ones, or poor results, such as bovine equine pericardium. All the synthetic scaffolds demonstrated a significant increase in clinical and functional scores in biomechanics, and a significant decrease in pain and re-tear rate in comparison to conventional surgery. Despite the limited number of studies, further investigation in the clinical use of synthetic scaffolds should be carried out.

Design of a Bioreactor to Assess the Effect of Passive Joint Loading in a Live Chick Embryo In Ovo

There is increasing interest in understanding how mechanical cues (e.g., physical forces due to kicking and other movements) influence the embryological development of tissues and organs. For example, recent studies from our laboratory and others have used the chick embryo model to demonstrate that the compositional and mechanical properties of developing tendons are strongly regulated by embryo movement frequency. However, current research tools for manipulating embryological movements and in ovo (or in utero) mechanical forces are generally limited to chemical treatments that either paralyze or overstimulate muscles without allowing for precise control of physical cues. Thus, in this study, we introduce an instrument that enables application of passive, dynamic ankle flexion at prescribed amplitudes and frequencies in live, developing chick embryos. This device meets the design goals of allowing for precise (<1.5°) control of different waveforms of ankle motion at a physiologically relevant frequency (0.17 Hz) across a range of ankle angles (0-90° plantarflexion) with maintenance of embryo viability comparable to other methods. Impact Statement We describe the design and implementation of a novel bioreactor to precisely control ankle motion in a chick embryo within its physiological environment. The chick embryo has been used for decades to study mechanobiology of musculoskeletal tissue development and regeneration, but approaches have been limited to chemical treatments that either paralyze or overstimulate muscles without allowing for precise control of physical cues. Thus, this novel instrument is a major advancement over current research tools for manipulating chick embryological movements in ovo (or in utero).

Hierarchical electrospun tendon-ligament bioinspired scaffolds induce changes in fibroblasts morphology under static and dynamic conditions

The regeneration of injured tendons and ligaments is challenging because the scaffolds needs proper mechanical properties and a biomimetic morphology. In particular, the morphological arrangement of scaffolds is a key point to drive the cells growth to properly regenerate the collagen extracellular matrix. Electrospinning is a promising technique to produce hierarchically structured nanofibrous scaffolds able to guide cells in the regeneration of the injured tissue. Moreover, the dynamic stretching in bioreactors of electrospun scaffolds had demonstrated to speed up cell shape modifications in vitro. The aim of the present study was to combine different imaging techniques such as high-resolution X-ray tomography (XCT), scanning electron microscopy (SEM), fluorescence microscopy and histology to investigate if hierarchically structured poly (L-lactic acid) and collagen electrospun scaffolds can induce morphological modifications in human fibroblasts, while cultured in static and dynamic conditions. After 7 days of parallel cultures, the results assessed that fibroblasts had proliferated on the external nanofibrous sheath of the static scaffolds, elongating themselves circumferentially. The dynamic cultures revealed a preferential axial orientation of fibroblasts growth on the external sheath. The aligned nanofibre bundles inside the hierarchical scaffolds instead, allowed a physiological distribution of the fibroblasts along the nanofibre direction. Inside the dynamic scaffolds, cells appeared thinner compared with the static counterpart. This study had demonstrated that hierarchically structured electrospun scaffolds can induce different fibroblasts morphological modifications during static and dynamic conditions, modifying their shape in the direction of the applied loads. LAY DESCRIPTION: To enhance the regeneration of injured tendons and ligaments cells need to growth on dedicated structures (scaffolds) with mechanical properties and a fibrous morphology similar to the natural tissue. In particular, the morphological organisation of scaffolds is fundamental in leading cells to colonise them, regenerating the collagen extracellular matrix. Electrospinning is a promising technique to produce fibres with a similar to the human collagen fibres, suitable to design complex scaffolds able to guide cells in the reconstruction of the natural tissue. Moreover, it is well established that the cyclic stretching of these scaffolds inside dedicated systems called bioreactors, can speed up cells growth and their shape modification. The aim of the present study was to investigate how hierarchically structured electrospun scaffolds, made of resorbable material such as poly(L-lactic acid) and collagen, could induce morphological changes in human fibroblasts, while cultured during static and dynamic conditions. These scaffolds were composed by an external electrospun membrane that grouped inside it a ring-shaped bundle, made of axially aligned nanofibres, resembling the morphological arrangement of tendon and ligament tissue. After 7 days of parallel cultures, the scaffolds were investigated using the following imaging techniques: (i) high-resolution X-ray tomography (XCT); (ii) scanning electron microscopy (SEM); (iii) fluorescence microscopy and (iv) histology. The results showed that fibroblasts were able to grow on the external nanofibrous sheath of the static scaffolds, by elongating themselves along their circumference. The dynamic cultures revealed instead a preferential axial orientation of fibroblasts grown on the external sheath. The aligned nanofibre bundles inside the hierarchical scaffolds allowed an axial distribution of the fibroblasts along the nanofibres direction. This study has demonstrated that the electrospun hierarchically structured scaffolds investigated can modify the fibroblasts morphology both in static and dynamic conditions, in relation with the direction of the applied loads.

Independent, Controllable Stretch-Perfusion Bioreactor Chambers to Functionalize Cell-Seeded Decellularized Tendons

Tissue-engineered decellularized matrices can progress clinical replacement of full-thickness ruptures or tendon defects. This study develops and validates a custom-made automated bioreactor, called oscillating stretch-perfusion bioreactor (OSPB), consisting of multiple, independent culture chambers able to combine a bidirectional perfusion with a programmable, uniaxial strain to functionalize cell-seeded decellularized tendons. Decellularized tendon matrices were seeded on their surfaces and within the tendon fibers with mesenchymal stem cells. Then, they were subjected to a bidirectional perfusion and programmed stretching cycles of 15–30–60 min on–off two times per day for 7 days of culture. In vitro analyses showed viable cells, homogenously distributed on the surface of the constructs. More importantly, cell-seeded decellularized tendon grafts undergoing cyclic load in our bioreactor had a superior production and organization of newly formed collagen matrix compared to static cultured constructs. The coherency and local alignment of the new collagen deposition within the inner injected channels quantitatively supported histological findings. The designed OSPB could be considered a unique, cost-effective system able to involve multiple independently controlled chambers in terms of biological and mechanical protocols. This system allows parallel processing of several customized tendon constructs to be used as grafts to enhance the surgical repair of large tendon defects.

Orthopaedic regenerative tissue engineering en route to the holy grail: disequilibrium between the demand and the supply in the operating room

Orthopaedic disorders are very frequent, globally found and often partially unresolved despite the substantial advances in science and medicine. Their surgical intervention is multifarious and the most favourable treatment is chosen by the orthopaedic surgeon on a case-by-case basis depending on a number of factors related with the patient and the lesion. Numerous regenerative tissue engineering strategies have been developed and studied extensively in laboratory through in vitro experiments and preclinical in vivo trials with various established animal models, while a small proportion of them reached the operating room. However, based on the available literature, the current strategies have not yet achieved to fully solve the clinical problems. Thus, the gold standards, if existing, remain unchanged in the clinics, notwithstanding the known limitations and drawbacks. Herein, the involvement of regenerative tissue engineering in the clinical orthopaedics is reviewed. The current challenges are indicated and discussed in order to describe the current disequilibrium between the needs and solutions made available in the operating room. Regenerative tissue engineering is a very dynamic field that has a high growth rate and a great openness and ability to incorporate new technologies with passion to edge towards the Holy Grail that is functional tissue regeneration. Thus, the future of clinical solutions making use of regenerative tissue engineering principles for the management of orthopaedic disorders is firmly supported by the clinical need.

Boosting tendon repair: interplay of cells, growth factors and scaffold-free and gel-based carriers

Background

Tendons are dense connective tissues and critical components for the integrity and function of the musculoskeletal system. Tendons connect bone to muscle and transmit forces on which locomotion entirely depends. Due to trauma, overuse and age-related degeneration, many people suffer from acute or chronic tendon injuries. Owing to their hypovascularity and hypocellularity, tendinopathies remain a substantial challenge for both clinicians and researchers. Surgical treatment includes suture or transplantation of autograft, allograft or xenograft, and these serve as the most common technique for rescuing tendon injuries. However, the therapeutic efficacies are limited by drawbacks including inevitable donor site morbidity, poor graft integration, adhesion formations and high rates of recurrent tearing. This review summarizes the literature of the past 10 y concerning scaffold-free and gel-based approaches for treating tendon injuries, with emphasis on specific advantages of such modes of application, as well as the obtained results regarding in vitro and in vivo tenogenesis.

Results

The search was focused on publications released after 2006 and 83 articles have been analysed. The main results are summarizing and discussing the clear advantages of scaffold-free and hydrogels carriers that can be functionalized with cells alone or in combination with growth factors.

Conclusion

The improved understanding of tissue resident adult stem cells has made a significant progress in recent years as well as strategies to steer their fate toward tendon lineage, with the help of growth factors, have been identified. The field of tendon tissue engineering is exploring diverse models spanning from hard scaffolds to gel-based and scaffold-free approaches seeking easier cell delivery and integration in the site of injury. Still, the field needs to consider a multifactorial approach that is based on the combination and fine-tuning of chemical and biomechanical stimuli. Taken together, tendon tissue engineering has now excellent foundations and enters the period of precision and translation to models with clinical relevance on which better treatment options of tendon injuries can be shaped up.

Exposure to buffer solution alters tendon hydration and mechanics

A buffer solution is often used to maintain tissue hydration during mechanical testing. The most commonly used buffer solution is a physiological concentration of phosphate buffered saline (PBS); however, PBS increases the tissue's water content and decreases its tensile stiffness. In addition, solutes from the buffer can diffuse into the tissue and interact with its structure and mechanics. These bathing solution effects can confound the outcome and interpretation of mechanical tests. Potential bathing solution artifacts, including solute diffusion, and their effect on mechanical properties, are not well understood. The objective of this study was to measure the effects of long-term exposure of rat tail tendon fascicles to several concentrations (0.9-25%) of NaCl, sucrose, polyethylene glycol (PEG), and SPEG (NaCl+PEG) solutions on water content, solute diffusion, and mechanical properties. We found that with an increase in solute concentration the apparent water content decreased for all solution types. Solutes diffused into the tissue for NaCl and sucrose, however, no solute diffusion was observed for PEG or SPEG. The mechanical properties changed for both NaCl solutions, in particular after long-term (8h) incubation the modulus and equilibrium stress decreased compared to short-term (15min) for 25% NaCl, and the cross sectional area increased for 0.9% NaCl. However, the mechanical properties were unchanged for both PEG and SPEG except for minor alterations in stress relaxation parameters. This study shows that NaCl and sucrose buffer solutions are not suitable for long-term mechanical tests. We therefore propose using PEG or SPEG as alternative buffer solutions that after long-term incubation can maintain tissue hydration without solute diffusion and produce a consistent mechanical response.

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.

Different combinations of growth factors for the tenogenic differentiation of bone marrow mesenchymal stem cells in monolayer culture and in fibrin-based three-dimensional constructs

Tendon injuries are severe burdens in clinics. The poor tendon healing is related to an ineffective response of resident cells and inadequate vascularization. Thanks to the high proliferation and multi-lineage differentiation capability, bone marrow-derived mesenchymal stem cells (BMSCs) are a promising cell source to support the tendon repair. To date, the association of various growth factors to induce the in vitro tenogenic differentiation of multipotent progenitor cells is poorly investigated. This study aimed to investigate the tenogenic differentiation of rabbit BMSCs by testing the combination of bone morphogenetic proteins (BMP-12 and 14) with transforming growth factor beta (TGF-β) and vascular endothelial growth factor (VEGF) both in 2D and 3D cultures within fibrin-based constructs. After 7 and 14 days, the tenogenic differentiation was assessed by analyzing cell metabolism and collagen content, the gene expression of tenogenic markers and the histological cell distribution and collagen deposition within 3D constructs. Our results demonstrated that the association of BMP-14 with TGF-β3 and VEGF enhanced the BMSC tenogenic differentiation both in 2D and 3D cultures. This study supports the use of fibrin as hydrogel-based matrix to generate spheroids loaded with tenogenic differentiated BMSCs that could be used to treat tendon lesions in the future.

Automated freeze-thaw cycles for decellularization of tendon tissue - a pilot study

Background

Decellularization of tendon tissue plays a pivotal role in current tissue engineering approaches for in vitro research as well as for translation of graft-based tendon restoration into clinics. Automation of essential decellularization steps like freeze-thawing is crucial for the development of more standardized decellularization protocols and commercial graft production under good manufacturing practice (GMP) conditions in the future.

Methods

In this study, a liquid nitrogen-based controlled rate freezer was utilized for automation of repeated freeze-thawing for decellularization of equine superficial digital flexor tendons. Additional tendon specimens underwent manually performed freeze-thaw cycles based on an established procedure. Tendon decellularization was completed by using non-ionic detergent treatment (Triton X-100). Effectiveness of decellularization was assessed by residual nuclei count and calculation of DNA content. Cytocompatibility was evaluated by culturing allogeneic adipose tissue-derived mesenchymal stromal cells on the tendon scaffolds.

Results

There were no significant differences in decellularization effectiveness between samples decellularized by the automated freeze-thaw procedure and samples that underwent manual freeze-thaw cycles. Further, we inferred no significant differences in the effectiveness of decellularization between two different cooling and heating rates applied in the automated freeze-thaw process. Both the automated protocols and the manually performed protocol resulted in roughly 2% residual nuclei and 13% residual DNA content. Successful cell culture was achieved with samples decellularized by automated freeze-thawing as well as with tendon samples decellularized by manually performed freeze-thaw cycles.

Conclusions

Automated freeze-thaw cycles performed by using a liquid nitrogen-based controlled rate freezer were as effective as previously described manual freeze-thaw procedures for decellularization of equine superficial digital flexor tendons. The automation of this key procedure in decellularization of large tendon samples is an important step towards the processing of large sample quantities under standardized conditions. Furthermore, with a view to the production of commercially available tendon graft-based materials for application in human and veterinary medicine, the automation of key procedural steps is highly required to develop manufacturing processes under GMP conditions.

Osteogenic and tenogenic induction of hBMSCs by an integrated nanofibrous scaffold with chemical and structural mimicry of the bone–ligament connection

A novel electrospinning nanofiber collecting device was designed and utilized to fabricate an integrated PCL nanofibrous scaffold with a “random–aligned–random” structure.

Decellularization of Large Tendon Specimens: Combination of Manually Performed Freeze-Thaw Cycles and Detergent Treatment

Reliable decellularization techniques applicable to tendon tissue play a critical role in the field of current tissue engineering. Particularly, an application as three-dimensional culture model for in vitro research and translational approaches to establish graft-based tendon repair as a routine clinical tool represent two main application fields for decellularized tendon scaffolds. Considering methodological issues of tendon decellularization, one of the major challenges lies in the preservation of the tendon-specific extracellular matrix (ECM) architecture to reflect natural tissue characteristic as best as possible. Concurrently, further requirements for high-quality decellularized biological tendon scaffolds include not only the reduction of resident cells, but also an ensured cytocompatibility.To date, a large number and a wide variety of decellularization protocols for natural tendon tissue have already been investigated and usually, physical as well as chemical and/or enzyme-based treatments are used for the purpose of decellularization. However, to the best of our knowledge, there is a lack of evidence-based protocols for the processing of full-thickness large tendon samples, such as the equine flexor tendons.Therefore, the here presented protocol describes a reliable procedure to decellularize equine superficial digital flexor tendons by using a combined treatment of physical decellularization in the form of repetitive freeze-thaw cycles, and of chemical decellularization with the non-ionic detergent Triton X-100. The decellularization effectiveness evaluated by reduction of cell and DNA content, the influence of decellularization on the morphology of the tendon extracellular matrix (ECM) as well as the cytocompatibility of the decellularized tendon scaffolds obtained have been investigated previously. Based on this previous study, the here present protocol is an effective procedure, particularly applicable for large tendon specimens.