Effect of several growth factors on canine flexor tendon fibroblast proliferation and collagen synthesis in vitro

Current progress in growth factors and extracellular vesicles in tendon healing

Tendon injury healing is a complex process that involves the participation of a significant number of molecules and cells, including growth factors molecules in a key role. Numerous studies have demonstrated the function of growth factors in tendon healing, and the recent emergence of EV has also provided a new visual field for promoting tendon healing. This review examines the tendon structure, growth, and development, as well as the physiological process of its healing after injury. The review assesses the role of six substances in tendon healing: insulin-like growth factor-I (IGF-I), transforming growth factor β (TGFβ), vascular endothelial growth factor (VEGF), platelet-derived growth factor (PDGF), basic fibroblast growth factor (bFGF), and EV. Different growth factors are active at various stages of healing and exhibit separate physiological activities. IGF-1 is expressed immediately after injury and stimulates the mitosis of various cells while suppressing the response to inflammation. VEGF, which is also active immediately after injury, accelerates local metabolism by promoting vascular network formation and positively impacts the activities of other growth factors. However, VEGF's protracted action could be harmful to tendon healing. PDGF, the earliest discovered cytokine to influence tendon healing, has a powerful cell chemotaxis and promotes cell proliferation, but it can equally accelerate the response to inflammation and relieve local adhesions. Also useful for relieving tendon adhesion is TGF- β, which is active almost during the entire phase of tendon healing. As a powerful active substance, in addition to its participation in the field of cardiovascular and cerebrovascular vessels, tumour and chronic wounds, TGF- β reportedly plays a role in promoting cell proliferation, activating growth factors, and inhibiting inflammatory response during tendon healing.

Preclinical tendon and ligament models: Beyond the 3Rs (replacement, reduction, and refinement) to 5W1H (why, who, what, where, when, how)

Several tendon and ligament animal models were presented at the 2022 Orthopaedic Research Society Tendon Section Conference held at the University of Pennsylvania, May 5 to 7, 2022. A key objective of the breakout sessions at this meeting was to develop guidelines for the field, including for preclinical tendon and ligament animal models. This review summarizes the perspectives of experts for eight surgical small and large animal models of rotator cuff tear, flexor tendon transection, anterior cruciate ligament tear, and Achilles tendon injury using the framework: "Why, Who, What, Where, When, and How" (5W1H). A notable conclusion is that the perfect tendon model does not exist; there is no single gold standard animal model that represents the totality of tendon and ligament disease. Each model has advantages and disadvantages and should be carefully considered in light of the specific research question. There are also circumstances when an animal model is not the best approach. The wide variety of tendon and ligament pathologies necessitates choices between small and large animal models, different anatomic sites, and a range of factors associated with each model during the planning phase. Attendees agreed on some guiding principles including: providing clear justification for the model selected, providing animal model details at publication, encouraging sharing of protocols and expertise, improving training of research personnel, and considering greater collaboration with veterinarians. A clear path for translating from animal models to clinical practice was also considered as a critical next step for accelerating progress in the tendon and ligament field.

Basic fibroblast growth factor induces proliferation and collagen production by fibroblasts derived from the bovine corpus luteum†

Cyclic regression of the ovarian corpus luteum, the endocrine gland responsible for progesterone production, involves rapid matrix remodeling. Despite fibroblasts in other systems being known for producing and maintaining extracellular matrix, little is known about fibroblasts in the functional or regressing corpus luteum. Vast transcriptomic changes occur in the regressing corpus luteum, among which are reduced levels of vascular endothelial growth factor A (VEGFA) and increased expression of fibroblast growth factor 2 (FGF2) after 4 and 12 h of induced regression, when progesterone is declining and the microvasculature is destabilizing. We hypothesized that FGF2 activates luteal fibroblasts. Analysis of transcriptomic changes during induced luteal regression revealed elevations in markers of fibroblast activation and fibrosis, including fibroblast activation protein (FAP), serpin family E member 1 (SERPINE1), and secreted phosphoprotein 1 (SPP1). To test our hypothesis, we treated bovine luteal fibroblasts with FGF2 to measure downstream signaling, type 1 collagen production, and proliferation. We observed rapid and robust phosphorylation of various signaling pathways involved in proliferation, such as ERK, AKT, and STAT1. From our longer-term treatments, we determined that FGF2 has a concentration-dependent collagen-inducing effect, and that FGF2 acts as a mitogen for luteal fibroblasts. FGF2-induced proliferation was greatly blunted by inhibition of AKT or STAT1 signaling. Our results suggest that luteal fibroblasts are responsive to factors that are released by the regressing bovine corpus luteum, an insight into the contribution of fibroblasts to the microenvironment in the regressing corpus luteum.

Poly-l-lactide-co-ε-caprolactone (PLCL) and poly-l-lactic acid (PLLA)/gelatin electrospun subacromial spacer improves extracellular matrix (ECM) deposition for the potential treatment of irreparable rotator cuff tears

Biodegradable subacromial spacer implantation has become practicable for the treatment of irreparable rotator cuff tears (IRCT). However, the relative high degradation rate and inferior tissue regeneration properties of current subacromial spacer may lead to failure regards to long-term survival. It is reported that satisfactory clinical results lie in the surrounding extracellular matrix (ECM) deposition after implantation. This study aims to develop a biological subacromial spacer that would enhance tissue regeneration properties and results in better ECM deposition. Physicochemical properties were characterized on both poly-l-lactide-co-ε-caprolactone (PLCL) dip-coating spacer (monolayer spacer, MS) and PLCL dip-coating + Poly-l-Lactic Acid (PLLA)/Gelatin electrospun spacer (Bilayer Spacer, BS). Cytocompatibility, angiogenesis, and collagen inducibility were evaluated with tendon fibroblasts and endothelial cells. Ultrasonography and histomorphology were used to analyze biodegradability and surrounding ECM deposition after the implantation in vivo. BS was successfully fabricated with the dip-coating and electrospinning technique, based on the human humeral head data. In vitro studies demonstrated that BS showed a greater cytocompatibility, and increased secretion of ECM proteins comparing to MS. In vivo studies indicated that BS promoted ECM deposition and angiogenesis in the surrounding tissue. Our research highlights that BS exhibits better ECM deposition and reveals a potential candidate for the treatment of IRCT in future.

PDGF-loaded microneedles promote tendon healing through p38/cyclin D1 pathway mediated angiogenesis

Tendon injury is one of the most serious orthopedic diseases often leading to disability of patients. Major shortages of tendon healing are due to its multiple comorbidities, uncertainty of therapeutic efficacy and insufficient of angiogenesis. With a deeper understanding of angiogenic mechanism of tendon healing, we investigated an innovative microneedle patch loaded with platelet derived growth factor (PDGF) to achieve a constant systemic administration of PDGF to enhance topical tendon healing. Rat achilles tendon injury model was performed as in vivo animal models. Histological staining showed an enhancement of tendon healing quality, especially angiogenesis. Biomechanical studies demonstrated an increase of tendon stiffness, maximum load and maximum stress with treatment of PDGF-loaded microneedles. Furthermore, MAPK/p38/Cyclin D1 pathway and angiogenesis were found to play an important role in tendon healing process by using a biological high throughput RNA-sequence method and bioinformatic analysis. The high throughput RNA-seq tendon healing results were confirmed by histochemical staining and western blot. These results suggest the novel therapeutic potential of PDGF-loaded microneedle patch in tendon surgery.

A Promising Candidate in Tendon Healing Events—PDGF-BB

Tendon injuries are one of the most common musculoskeletal disorders for which patients seek medical aid, reducing not only the quality of life of the patient but also imposing a significant economic burden on society. The administration of growth factors at the wound site is a feasible solution for enhancing tendon healing. Platelet-derived growth factor-BB (PDGF-BB) has a well-defined safety profile compared to other growth factors and has been approved by the Food and Drug Administration (FDA). The purpose of this review is to summarize the role of PDGF-BB in tendon healing through a comprehensive review of the published literature. Experimental studies suggest that PDGF-BB has a positive effect on tendon healing by enhancing inflammatory responses, speeding up angiogenesis, stimulating tendon cell proliferation, increasing collagen synthesis and increasing the biomechanics of the repaired tendon. PDGF-BB is regarded as a promising candidate in tendon healing. However, in order to realize its full potential, we still need to carefully consider and study key issues such as dose and application time in the future, so as to explore further applications of PDGF-BB in the tendon healing process.

Axin2-lineage cells contribute to neonatal tendon regeneration

PURPOSE

Tendon injuries are a challenging clinical problem with few treatment options. Identifying the molecular regulators of tendon is required for the development of new therapies. While the Wnt pathway is critical for the maintenance and differentiation of many tissues, the role of Wnt signaling in tendon cell biology remains largely unexplored.

METHODS

The effects of Wnt activation were tested in vitro using neonatal tendon-derived cells cultured in 2D and 3D conditions. The inducible Axin2CreERT2 was then used to label Axin2+ cells in vivo and cells were traced during neonatal tendon regeneration.

RESULTS

We showed that activation of Wnt signaling results in proliferation of neonatal tendon cells. While tendon marker expression was inhibited by Wnt activation under 2D conditions, Scx expression was not affected under 3D uniaxial tension, suggesting that the microenvironment contextualizes tendon cell response to Wnt signaling. Using an in vivo model of neonatal tendon regeneration, we further showed that Wnt signaling cells comprise a subpopulation of tenocyte and epitenon cells that proliferate after injury and are recruited during regeneration.

DISCUSSION

Collectively, these studies suggest that Wnt signaling may play a role in tendon cell proliferation, differentiation, and regeneration.

The Functions and Mechanisms of Basic Fibroblast Growth Factor in Tendon Repair

Tendon injury is a disorder of the musculoskeletal system caused by overuse or trauma, which is characterized by pain and limitations in joint function. Since tendon healing is slowly and various treatments are generally ineffective, it remains a clinically challenging problem. Recent evidences suggest that basic fibroblast growth factor (bFGF) not only plays an important role in tendon healing, but also shows a positive effect in laboratory experimentations. The purpose of this review is to summarize the effects of bFGF in the tendon healing. Firstly, during the inflammatory phase, bFGF stimulates the proliferation and differentiation of vascular endothelial cells to foster neovascularization. Furthermore, bFGF enhances the production of pro-inflammatory factors during the early phase of tendon healing, thereby accelerating the inflammatory response. Secondly, the cell proliferation phase is accompanied by the synthesis of a large number of extracellular matrix components. bFGF speeds up tendon healing by stimulating fibroblasts to secrete type III collagen. Lastly, the remodeling phase is characterized by the transition from type III collagen to type I collagen, which can be promoted by bFGF. However, excessive injection of bFGF can cause tendon adhesions as well as scar tissue formation. In future studies, we need to explore further applications of bFGF in the tendon healing process.

Slowed-Down Rehabilitation Following Percutaneous Repair of Achilles Tendon Rupture

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSION

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

LEVEL OF EVIDENCE

Level II, prospective comparative study.

Comparison of Tendon Development Versus Tendon Healing and Regeneration

Tendon is a vital connective tissue in human skeletal muscle system, and tendon injury is very common and intractable in clinic. Tendon development and repair are two closely related but still not fully understood processes. Tendon development involves multiple germ layer, as well as the regulation of diversity transcription factors (Scx et al.), proteins (Tnmd et al.) and signaling pathways (TGFβ et al.). The nature process of tendon repair is roughly divided in three stages, which are dominated by various cells and cell factors. This review will describe the whole process of tendon development and compare it with the process of tendon repair, focusing on the understanding and recent advances in the regulation of tendon development and repair. The study and comparison of tendon development and repair process can thus provide references and guidelines for treatment of tendon injuries.

The Role of Vascular Endothelial Growth Factor in Tendon Healing

Angiogenesis is crucial to facilitate tendon healing, such as delivering oxygen and nutrients, removing waste products, and controlling immune responses. Vascular endothelial growth factor (VEGF) is one of the most vital angiogenic factors that regulate blood vessel formation in tendon healing. Recently, biological therapies, including the application of exogenous VEGF, have been attracting increasing attention. However, at present, the effect of the application of exogenous VEGF in tendon healing is controversial, as the role of endogenous VEGF in tendons has also not been fully elucidated. This article will summarize the role of both endogenous and exogenous VEGF in tendon healing and discuss possible reasons for the controversy. The present review shows that tendon repair is facilitated only by proper angiogenesis and VEGF at the early stage, whereas the persistent high VEGF expression and prolonged presence of blood vessels may impair tendon repair at a later stage.

The Regenerative Role of Gelatin in PLLA Electrospun Membranes for the Treatment of Chronic Massive Rotator Cuff Injuries

Failing to regenerate native tendon tissue in chronic massive rotator cuff tears (CMRCTs) results in high retear rates after surgery. Gelatin is a hydrolyzed form of collagen which is bioactive and biocompatible. This study intends to investigate the suitability of integrating gelatin to poly (l-lactic acid) (PLLA) fibrous membranes for promoting the healing of CMRCTs. PLLA/Gelatin electrospun membranes (PGEM) are fabricated using electrospinning technology. The fourier transform infrared, static contact angles are tested sequentially. Cytocompatibility is evaluated with rat tendon fibroblasts and human umbilical endothelial cells (HUEVCs) lines. CMRCTs rat models are established and assigned into three groups (the sham group, the repaired group, and the augmentation group) to perform histomorphological and biomechanical evaluations. Gelatin is successfully integrated into PLLA fibrous membranes by the electrospinning technique. In vitro studies indicate that PGEM shows a great cytocompatibility for rat tendon fibroblasts and HUEVCs. In vivo studies find that applications of PGEM significantly promote well-aligned collagen I fibers formation and enhance biomechanical properties of the repaired tendon in CMRCTs rat models. In summary, gelatin promotes tendon fibroblasts and HUEVCs adhesion, migration, and proliferation on the PLLA fibrous membranes, and PGEM may provide a great prospect for clinical application.

Physiological Parameters to Identify Suitable Blood Donor Cows for Preparation of Platelet Rich Plasma

Simple Summary

Platelet rich plasma is a biological product obtained from blood and used for regenerative treatments of different pathologies. It is characterized by a high concentration of platelets (at least 3 times the physiological level) containing many growth factors with anti-inflammatory, bactericidal and regenerative properties. In human medicine, PRP is used in an autologous way, it means that the blood donor is also the recipient. In veterinary medicine, PRP is used to treat different diseases or lesions and in bovine species to treat mastitis. In this context, the opportunity to have PRP ready to use, stored from donor cows of the same farm where it will be used, would be very useful in treating this pathology immediately when it occurs. For this purpose, the present research aimed to detect parameters useful to identify the most suitable cows to be used as blood donors to obtain the highest yield of PRP (milliliters of PRP obtained with respect to milliliters of initial blood). Our results showed that blood collection from the mammary vein within three months of parturition, from nonpregnant cows at 5 years of age, but not the blood collection season, were associated with a high yield of PRP.

Abstract

Platelet rich plasma (PRP) has been shown to be beneficial in the treatment of bovine mastitis, with an action comparable to that of antibiotics. Autologous treatment is feasible in experimental conditions but is difficult to apply in field conditions, particularly in acute mastitis. The ideal scenario would be to have heterologous PRP stored on every farm so that it is readily available when needed. In this paper, we analysed data collected during bovine mastitis treatment with heterologous PRP produced by casual donor cows on several farms. We tried to identify parameters which might be useful to identify the most suitable cows to be used as blood donors, to obtain the highest yield of PRP. Variables considered for each animal were the age, the parity, the date of the last parturition, the season of blood collection, the site of blood collection (jugular or mammary vein) and the reproductive status e.g., pregnant or not pregnant. There were statistically significant differences for all the variables considered from the 135 blood cows, except for the blood collection season. The highest yield of PRP was associated with nonpregnancy blood collection within three months of parturition, parity 3 or 4, and blood collection from the mammary vein.

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.

Nanoparticle-coated sutures providing sustained growth factor delivery to improve the healing strength of injured tendons

Tendon injuries are common diseases. The healing capacity of tendon is limited due to its special composition of extra-cellular matrix and hypocellularity and hypovascularity. The purpose of this study was to evaluate the effectiveness of nanoparticle-coated sutures carrying growth factors for accelerating tendon repair. A variety of experimental methods had been used to investigate the characteristics and therapeutic effects of the modified sutures. Nanoparticles could adhere uniformly to the surface of the suture through polydopamine. Even sutured in the tendon, most of nanoparticles were still remained on the surface of suture, and the loaded proteins could spread into the tendon tissues. In vivo study, the ultimate strength of repaired tendons treated with bFGF and VEGFA-releasing sutures was significantly greater than the tendons repaired with control sutures at multiple time-points, whether in the chicken model of flexor tendon injury or the rat model of Achilles tendon injury. At week 6, the adhesion score in the bFGF and VEGFA-releasing suture group was significantly lower than those of the control suture group. Tendon gliding excursion was significantly longer in the bFGF and VEGFA-releasing suture group than that in the control bare sutures. Work of digital flexion was significantly decreased in the bFGF and VEGFA-releasing suture group. In a word, we developed a platform for local and continuous delivery of growth factors based on the nanoparticle-coated sutures, which could effectively deliver growth factors to tissues and control the release of growth factors. This growth factors delivery system is an attractive therapeutic tool to repair injured tendons. STATEMENT OF SIGNIFICANCE: Tendon rupture is a common clinical injury, due to the special character of the tendon with mainly extra cellular matrix and hypocellularity and hypovascularity, the healing capacity of the injured tendon is limited. In this study, nanoparticle-coated surgical sutures carrying growth factors were prepared to accelerate tendon repair. After treatment, bFGF and VEGFA loaded nanoparticle-coated sutures can significantly enhance tendon healing, and significantly improve tendon gliding function and effectively inhibit the formation of adhesion. Moreover, these nanoparticle-coated sutures have good biocompatibility and no obvious tissue reaction, which provides more guarantee for further clinical application. This is an attractive and promising approach that uses surgical suture as a growth factor delivery tool to repair tendon injury, which can simplify the treatment. And this kind of bioactive sutures may be applied to other tissue repair, such as muscle, nerve, intestinal canal, blood vessel, skin, and so on.

Cell-Free Book-Shaped Decellularized Tendon Matrix Graft Capable of Controlled Release of BMP-12 to Improve Tendon Healing in a Rat Model

BACKGROUND

Achilles tendon (AT) defects often occur in traumatic and chronic injuries. Currently, no graft can satisfactorily regenerate parallel tendinous tissue at the defect site to completely restore AT function.

PURPOSE

To develop a cell-free functional graft by tethering bone morphogenetic protein 12 (BMP-12) on a book-shaped decellularized tendon matrix (BDTM) and to determine whether this graft is more beneficial for AT defect healing than an autograft.

STUDY DESIGN

Controlled laboratory study.

METHODS

Canine patellar tendon was sectioned into a book shape and decellularized to fabricate a BDTM. The collagen-binding domain (CBD) was fused into the N-terminus of BMP-12 to synthesize a recombinant BMP-12 (CBD-BMP-12), which was tethered to the BDTM to prepare a cell-free functional graft (CBD-BMP-12/BDTM). After its tensile resistance, tenogenic inducibility, and BMP-12 release dynamics were evaluated, the efficacy of the graft for tendon regeneration was determined in a rat model. A total of 140 mature male Sprague-Dawley rats underwent AT tenotomy. The defect was reconstructed with reversed AT (autograft group), native BMP-12 tethered to an intact decellularized tendon matrix (IDTM; NAT-BMP-12/IDTM group), native BMP-12 tethered to a BDTM (NAT-BMP-12/BDTM group), CBD-BMP-12 tethered on an IDTM (CBD-BMP-12/IDTM group), and CBD-BMP-12 tethered on a BDTM (CBD-BMP-12/BDTM group). The rats were sacrificed 4 or 8 weeks after surgery to harvest AT specimens. Six specimens from each group at each time point were used for histological evaluation; the remaining 8 specimens were used for biomechanical testing.

RESULTS

In vitro CBD-BMP-12/BDTM was noncytotoxic, showed high biomimetics with native tendons, was suitable for cell adhesion and growth, and had superior tenogenic inducibility. In vivo the defective AT in the CBD-BMP-12/BDTM group regenerated more naturally than in the other groups, as indicated by more spindle-shaped fibroblasts embedded in a matrix of parallel fibers. The biomechanical properties of the regenerated AT in the CBD-BMP-12/BDTM group also increased more significantly than in the other groups.

CONCLUSION

CBD-BMP-12/BDTM is more beneficial than autograft for healing AT defects in a rat model.

CLINICAL RELEVANCE

The findings of this study demonstrate that CBD-BMP-12/BDTM can serve as a practical graft for reconstructing AT defects.

Growth factor and macromolecular crowding supplementation in human tenocyte culture

Cell-assembled tissue engineering strategies hold great potential in regenerative medicine, as three-dimensional tissue-like modules can be produced, even from a patient's own cells. However, the development of such implantable devices requires prolonged in vitro culture time, which is associated with cell phenotypic drift. Considering that the cells in vivo are subjected to numerous stimuli, multifactorial approaches are continuously gaining pace towards controlling cell fate during in vitro expansion. Herein, we assessed the synergistic effect of simultaneous and serial growth factor supplementation (insulin growth factor-1, platelet-derived growth factor ββ, growth differentiation factor 5 and transforming growth factor β3) to macromolecular crowding (carrageenan) in human tenocyte function; collagen synthesis and deposition; and gene expression. TGFβ3 supplementation (without/with carrageenan) induced the highest (among all groups) DNA content. In all cases, tenocyte proliferation was significantly increased as a function of time in culture, whilst metabolic activity was not affected. Carrageenan supplementation induced significantly higher collagen deposition than groups without carrageenan (without/with any growth factor). Of all the growth factors used, TGFβ3 induced the highest collagen deposition when used together with carrageenan in both simultaneous and serial fashion. At day 13, gene expression analysis revealed that TGFβ3 in serial supplementation to carrageenan upregulated the most and downregulated the least collagen- and tendon- related genes and upregulated the least and downregulated the most osteo-, chondro-, fibrosis- and adipose- related trans-differentiation genes. Collectively, these data clearly advocate the beneficial effects of multifactorial approaches (in this case, growth factor and macromolecular crowding supplementation) in the development of functional cell-assembled tissue surrogates.

Nanofiber Scaffolds by Electrospinning for Rotator Cuff Tissue Engineering

Rotator cuff tears continue to be at risk of retear or failure to heal after surgical repair, despite the use of various surgical techniques, which stimulate development of novel scaffolding strategies. They should be able to address the known causes of failure after the conventional rotator cuff repair: (1) failure to reproduce the normal tendon healing process, (2) resultant failure to reproduce four zones of the enthesis, and (3) failure to attain sufficient mechanical strength after repair. Nanofiber scaffolds are suited for this application because they can be engineered to mimic the ultrastructure and properties of the native rotator cuff tendon. Among various methods for tissue-engineered nanofibers, electrospinning has recently been highlighted in the rotator cuff field. Electrospinning can create fibrous and porous structures that resemble natural tendon's extracellular matrix. Other advantages include the ability to create relatively large surface-to-volume ratios, the ability to control fiber size from the micro to the nano scale, and the flexibility of material choices. In this review, we will discuss the anatomical and mechanical features of the rotator cuff tendon, their potential impacts on improper healing after repair, and the current knowledge of the use of electrospinning for rotator cuff tissue engineering.

Stem Cells for the Regeneration of Tendon and Ligament: A Perspective

Tendons are structures that connect muscles to the bones in our body and transmit the force generated by contraction of the muscles to the bones. Ligaments are structures that connect bones to bones, with histological properties similar to tendons. In tendon and ligament tissue, there are very small amounts of cells similar to mesenchymal stem cells (MSCs) called tendon stem/progenitor cells (TSPCs), or tenogenic stem cells. While the role of specific growth factors and transcription factors is well established in the osteogenic and chondrogenic differentiation of stem cells, a consensus has not been established for tenogenic differentiation. Injuries to tendons and ligaments are very common, but natural healing is very slow and inefficient due to limited vascularization. Currently, there is no adequate method for restoring extensive tendon or ligament defects. Procedures addressing the unmet need for regeneration of these tissues are needed. In this review, the current knowledge, as well as the authors’ ideas and perspective on stem cell and regenerative medicine for tendon and ligament defects are presented.

Hip abductor tendon tears: where are we now?

Hip abductor tendon tear is a difficult problem to manage. The hip abductor mechanism is made up of the gluteus medius and minimus muscles, both of which contribute to stabilising the pelvis through the gait cycle. Tears of these tendons are likely due to iatrogenic injury during arthroplasty and chronic degenerative tendinopathy. Ultrasound and magnetic resonance imaging have provided limited clues regarding the pattern of disease and further work is required to clarify both the macro and microscopic pattern of disease. While surgery has been attempted over the last 2 decades, the outcomes are variable and the lack of high-quality studies have limited the uptake of surgical repair. Hip abductor tendon tears share many features with rotator cuff tears, hence, innovations in surgical techniques, materials and biologics may apply to both pathologies.

Treatment of adhesions after Achilles tendon injury using focused ultrasound with targeted bFGF plasmid-loaded cationic microbubbles

Nonviral targeting technology has become promising as a form of gene therapy for diseases and injuries, such as Achilles tendon injuries. In this study, we used avidin–biotin bridge and positive–negative charge attractions to load the intercellular adhesion molecule-1 (ICAM-1) antibody and the basic fibroblast growth factor (bFGF) plasmid onto the surface of the microbubbles. The saturated loading capacity for 1.1 × 108 microbubble was 6.55 ± 0.53 µg. We established the ICAM-1 antigen microenvironment using tumor necrosis factor-alpha-stimulated human umbilical vein endothelial cells and found the targeting ability of the prepared microbubbles in vitro. In vivo, we also found that the injected targeted bFGF gene microbubbles expressed the bFGF gene better when compared with that of the control group. Furthermore, we evaluated adhesions after Achilles tendon injuries in rabbits using hematoxylin and eosin and immunohistochemical (IHC) staining methods. The collagen fibers were properly arranged in the tendon, and there was greater cellularity inside the tendon sheath and a clearer boundary between the internal and external tendon sheath than that of the control group. IHC staining showed greater ICAM-1 expression inside the tendon sheath when compared with outside the tendon sheath. In conclusion, targeted microbubbles can be a useful carrier of genes to provide gene therapy for the prevention of adhesions after tendon injury.

Impact of PDGF-BB on cellular distribution and extracellular matrix in the healing rabbit Achilles tendon three weeks post-operation

Current methods for tendon rupture repair suffer from two main drawbacks: insufficient strength and adhesion formation, which lead to rerupture and impaired gliding. A novel polymer tube may help to overcome these problems by allowing growth factor delivery to the wound site and adhesion reduction, and by acting as a physical barrier to the surrounding tissue. In this study, we used a bilayered DegraPol® tube to deliver PDGF-BB to the wound site in a full-transection rabbit Achilles tendon model. We then performed histological and immunohistochemical analysis at 3 weeks postoperation. Sustained delivery of PDGF-BB to the healing Achilles tendon led to a significantly more homogenous cell distribution within the healing tissue. Lower cell densities next to the implant material were determined for +PDGF-BB samples compared to -PDGF-BB. PDGF-BB application increased proteoglycan content and reduced alpha-SMA+ areas, clusters of different sizes, mainly vessels. Finally, PDGF-BB reduced collagens I and III in the extracellular matrix. The sustained delivery of PDGF-BB via an electrospun DegraPol® tube accelerated tendon wound healing by causing a more uniform cell distribution with higher proteoglycan content and less fibrotic tissue. Moreover, the application of this growth factor reduced collagen III and alpha-SMA, indicating a faster and less fibrotic tendon healing.

Flexor Tendon: Development, Healing, Adhesion Formation, and Contributing Growth Factors

Management of flexor tendon injuries of the hand remains a major clinical problem. Even with intricate repair, adhesion formation remains a common complication. Significant progress has been made to better understand the mechanisms of healing and adhesion formation. However, there has been slow progress in the clinical prevention and reversal of flexor tendon adhesions. The goal of this article is to discuss recent literature relating to tendon development, tendon healing, and adhesion formation to identify areas in need of further research. Additional research is needed to understand and compare the molecular, cellular, and genetic mechanisms involved in flexor tendon morphogenesis, postoperative healing, and mechanical loading. Such knowledge is critical to determine how to improve repair outcomes and identify new therapeutic strategies to promote tissue regeneration and prevent adhesion formation.

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

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

Stem Cell Therapy for Tendon Regeneration: Current Status and Future Directions

In adults the healing tendon generates fibrovascular scar tissue and recovers never histologically, mechanically, and functionally which leads to chronic and to degenerative diseases. In this review, the processes and mechanisms of tendon development and fetal regeneration in comparison to adult defect repair and degeneration are discussed in relation to regenerative therapeutic options. We focused on the application of stem cells, growth factors, transcription factors, and gene therapy in tendon injury therapies in order to intervene the scarring process and to induce functional regeneration of the lesioned tissue. Outlines for future therapeutic approaches for tendon injuries will be provided.

The cellular basis of fibrotic tendon healing: challenges and opportunities

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

Gene-Loaded Nanoparticle-Coated Sutures Provide Effective Gene Delivery to Enhance Tendon Healing

How to accelerate tendon healing remains a clinical challenge. In this study, a suture carrying nanoparticle/pEGFP-basic fibroblast growth factor (bFGF) and pEGFP-vascular endothelial growth factor A (VEGFA) complexes was developed to transfer the growth factor genes into injured tendon tissues to promote healing. Polydopamine-modified sutures can uniformly and tightly absorb nanoparticle/plasmid complexes. After tendon tissues were sutured, the nanoparticle/plasmid complexes still existed on the suture surface. Further, we found that the nanoparticle/plasmid complexes delivered into tendon tissues could diffuse from sutures to tendon tissues and effectively transfect genes into tendon cells, significantly increasing the expression of growth factors in tendon tissues. Finally, biomechanical tests showed that nanoparticle/pEGFP-bFGF and pEGFP-VEGFA complex-coated sutures could significantly increase the ultimate strengths of repaired tendons, especially at 4 weeks after operation. Two kinds of nanoparticle/plasmid complex-coated sutures significantly increased flexor tendon healing strength by 3.7 times for Ethilon and 5.8 times for PDS II, respectively, compared with the corresponding unmodified sutures. In the flexor tendon injury model, at 6 weeks after surgery, compared with the control suture, the nanoparticle/plasmid complex-coated sutures can significantly increase the gliding excursions of the tendon and inhibit the formation of adhesion. These results indicate that this nanoparticle/plasmid complex-coated suture is a promising tool for the treatment of injured tendons.

Limiting angiogenesis to modulate scar formation

Angiogenesis, the process of new blood vessel formation from existing blood vessels, is a key aspect of virtually every repair process. During wound healing an extensive, but immature and leaky vascular plexus forms which is subsequently reduced by regression of non-functional vessels. More recent studies indicate that uncontrolled vessel growth or impaired vessel regression as a consequence of an excessive inflammatory response can impair wound healing, resulting in scarring and dysfunction. However, in order to elucidate targetable factors to promote functional tissue regeneration we need to understand the molecular and cellular underpinnings of physiological angiogenesis, ranging from induction to resolution of blood vessels. Especially for avascular tissues (e.g. cornea, tendon, ligament, cartilage, etc.), limiting rather than boosting vessel growth during wound repair potentially is beneficial to restore full tissue function and may result in favourable long-term healing outcomes.

Tendon stem progenitor cells: Understanding the biology to inform therapeutic strategies for tendon repair

Tendon and ligament injuries are a leading cause of healthcare visits with significant impact in terms of economic cost and reduced quality of life. To date, reparative strategies remain largely restricted to conservative treatment or surgical repair. However, these therapies fail to restore native tendon structure and function; thus, the tissue may re-rupture or degenerate with time. To improve tendon healing, one promising strategy may be harnessing the innate potential of resident tendon stem/progenitor cells (TSPCs) to guide tenogenic regeneration. In this review, we outline recent advances in the identification and characterization of putative TSPC populations, and discuss biochemical, biomechanical, and biomaterial methods employed for their culture and differentiation. Finally, we identify limitations in our current understanding of TSPC biology, key challenges for their use, and potential therapeutic strategies to inform cell-based tendon repair. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:1270-1280, 2019.

Establishment and characterization of a fibroblast-like cell line from the muscle of turbot (Scophthalmus maximus L.)

A continuous fibroblast-like cell line, TMF (turbot muscle fibroblasts), was established from juvenile turbot Scophthalmus maximus muscle with the method of trypsin digestion. It has been subcultured more than 60 passages for over 150 days. The TMF cells were cultured in L-15 medium supplemented with HEPES, fetal bovine serum (FBS), GlutaMAX, and basic fibroblast growth factor (bFGF). The optimal temperature for TMF culture was 24 °C. TMF cells were predominantly composed of fibroblastic-like cells, and the transcription factor 4 (TCF-4) was highly expressed in TMF cells. Chromosome analysis revealed that it had a diploid chromosome number of 2n = 44. The transfection efficiency achieved 54.95 ± 6.59%, and the cell mortality rate was about 8.70% when transfected with the nucleofection method. Meanwhile, the TMF cells showed a sensitive response to amino acid levels and activation target of rapamycin (TOR) signaling pathway. These results indicate that TMF was a potential tool to explore the signal transduction of teleost in vitro.

Assembly of Bifunctional Aptamer-Fibrinogen Macromer for VEGF Delivery and Skin Wound Healing

Macromolecular assembly has been studied for various applications. However, while macromolecules can recognize one another for assembly, their assembled structures usually lack the function of specific molecular recognition. We hypothesized that bifunctional aptamer-protein macromers would possess dual functions of molecular assembly and recognition. The data show that hybrid aptamer-fibrinogen macromers can assemble to form hydrogels. Moreover, the assembled hydrogels can recognize vascular endothelial growth factor (VEGF) for sustained release. When the VEGF-loaded hydrogels are implanted in vivo, they can promote angiogenesis and skin wound healing. Thus, this work has successfully demonstrated a promising macromolecular system for broad applications such as drug delivery and regenerative medicine.

Identification of topographical architectures supporting the phenotype of rat tenocytes

Tenocytes, the main cell type of the tendon, require mechanical stimuli for their proper function. When the tenocyte environment changes due to tissue damage or by transferring tenocytes from their native environment into cell culture, the signals from the tenocyte niche are lost, leading towards a decline of phenotypic markers. It is known that micro-topographies can influence cell fate by the physical cues they provide. To identify the optimal topography-induced biomechanical niche in vitro, we seeded tenocytes on the TopoChip, a micro-topographical screening platform, and measured expression of the tendon transcription factor Scleraxis. Through machine learning algorithms, we associated elevated Scleraxis levels with topological design parameters. Fabricating micro-topographies with optimal surface characteristics on larger surfaces allowed finding an improved expression of multiple tenogenic markers. However, long-term confluent culture conditions coincided with osteogenic marker expression and the loss of morphological characteristics. In contrast, passaging tenocytes which migrated from the tendon directly on the topography resulted in prolonged elongated morphology and elevated Scleraxis levels. This research provides new insights into how micro-topographies influence tenocyte cell fate, and supports the notion that micro-topographical design can be implemented in a new generation of tissue culture platforms for supporting the phenotype of tenocytes. STATEMENT OF SIGNIFICANCE: The challenge in controlling in vitro cell behavior lies in controlling the complex culture environment. Here, we present for the first time the use of micro-topographies as a biomechanical niche to support the phenotype of tenocytes. For this, we applied the TopoChip platform, a screening tool with 2176 unique micro-topographies for identifying feature characteristics associated with elevated Scleraxis expression, a tendon related marker. Large area fabrication of micro-topographies with favorable characteristics allowed us to find a beneficial influence on other tenogenic markers as well. Furthermore, passaging cells is more beneficial for Scleraxis marker expression and tenocyte morphology compared to confluent conditions. This study presents important insights for the understanding of tenocyte behavior in vitro, a necessary step towards tendon engineering.

Rescue plan for Achilles: Therapeutics steering the fate and functions of stem cells in tendon wound healing

Due to the increasing age of our society and a rise in engagement of young people in extreme and/or competitive sports, both tendinopathies and tendon ruptures present a clinical and financial challenge. Tendon has limited natural healing capacity and often responds poorly to treatments, hence it requires prolonged rehabilitation in most cases. Till today, none of the therapeutic options has provided successful long-term solutions, meaning that repaired tendons do not recover their complete strength and functionality. Our understanding of tendon biology and healing increases only slowly and the development of new treatment options is insufficient. In this review, following discussion on tendon structure, healing and the clinical relevance of tendon injury, we aim to elucidate the role of stem cells in tendon healing and discuss new possibilities to enhance stem cell treatment of injured tendon. To date, studies mainly apply stem cells, often in combination with scaffolds or growth factors, to surgically created tendon defects. Deeper understanding of how stem cells and vasculature in the healing tendon react to growth factors, common drugs used to treat injured tendons and promising cellular boosters could help to develop new and more efficient ways to manage tendon injuries.

Fibrin-based delivery strategies for acute and chronic wound healing

Fibrin, a natural hydrogel, is the end product of the physiological blood coagulation cascade and naturally involved in wound healing. Beyond its role in hemostasis, it acts as a local reservoir for growth factors and as a provisional matrix for invading cells that drive the regenerative process. Its unique intrinsic features do not only promote wound healing directly via modulation of cell behavior but it can also be fine-tuned to evolve into a delivery system for sustained release of therapeutic biomolecules, cells and gene vectors. To further augment tissue regeneration potential, current strategies exploit and modify the chemical and physical characteristics of fibrin to employ combined incorporation of several factors and their timed release. In this work we show advanced therapeutic approaches employing fibrin matrices in wound healing and cover the many possibilities fibrin offers to the field of regenerative medicine.

Concise Review: Stem Cell Fate Guided By Bioactive Molecules for Tendon Regeneration

Tendon disorders, which are commonly presented in the clinical setting, disrupt the patients' normal work and life routines, and they damage the careers of athletes. However, there is still no effective treatment for tendon disorders. In the field of tissue engineering, the potential of the therapeutic application of exogenous stem cells to treat tendon pathology has been demonstrated to be promising. With the development of stem cell biology and chemical biology, strategies that use inductive tenogenic factors to program stem cell fate in situ are the most easily and readily translatable to clinical applications. In this review, we focus on bioactive molecules that can potentially induce tenogenesis in adult stem cells, and we summarize the various differentiation factors found in comparative studies. Moreover, we discuss the molecular regulatory mechanisms of tenogenesis, and we examine the various challenges in developing standardized protocols for achieving efficient and reproducible tenogenesis. Finally, we discuss and predict future directions for tendon regeneration. Stem Cells Translational Medicine 2018;7:404-414.

Flexor Tendon Injuries

Flexor tendon injuries of the hand are uncommon, and they are among the most challenging orthopaedic injuries to manage. Proper management is essential to ensure optimal outcomes. Consistent, successful management of flexor tendon injuries relies on understanding the anatomy, characteristics and repair of tendons in the different zones, potential complications, rehabilitation protocols, recent advances in treatment, and future directions, including tissue engineering and biologic modification of the repair site.

Effects of PDGF-BB delivery from heparinized collagen sutures on the healing of lacerated chicken flexor tendon in vivo

Flexor tendon lacerations are traditionally repaired by using non-absorbable monofilament sutures. Recent investigations have explored to improve the healing process by growth factor delivery from the sutures. However, it is difficult to conjugate growth factors to nylon or other synthetic sutures. This study explores the performance of a novel electrochemically aligned collagen suture in a flexor tendon repair model with and without platelet derived growth factor following complete tendon laceration in vivo. Collagen suture was fabricated via electrochemical alignment process. Heparin was covalently bound to electrochemically aligned collagen sutures (ELAS) to facilitate affinity bound delivery of platelet-derived growth factor-BB (PDGF-BB). Complete laceration of the flexor digitorum profundus in the third digit of the foot was performed in 36 skeletally mature White Leghorn chickens. The left foot was used as the positive control. Animals were randomly divided into three groups: control specimens treated with standard nylon suture (n=12), specimens repaired with heparinated ELAS suture without PDGF-BB (n=12) and specimens repaired with heparinated ELAS suture with affinity bound PDGF-BB (n=12). Specimens were harvested at either 4weeks or 12weeks following tendon repair. Differences between groups were evaluated by the degree of gross tendon excursion, failure load/stress, stiffness/modulus, absorbed energy at failure, elongation/strain at failure. Quantitative histological scoring was performed to assess cellularity and vascularity. Closed flexion angle measurements demonstrated no significant differences in tendon excursion between the study groups at 4 or 12weeks. Biomechanical testing showed that the group treated with PDGF-BB bound heparinated ELAS suture had significantly higher stiffness and failure load (p<0.05) at 12-weeks relative to both heparinated ELAS suture and nylon suture. Similarly, the group treated with PDGF-BB bound suture had significantly higher ultimate tensile strength and Young's modulus (p<0.05) at 12-weeks relative to both ELAS suture and nylon suture. Compared to nylon controls, heparinized ELAS with PDGF-BB improved biomechanics and vascularity during tendon healing by 12-weeks following primary repair. The ability of ELAS to deliver PDGF-BB to the lacerated area of tendon presents investigators with a functional bioinductive platform to improve repair outcomes following flexor tendon repair.

STATEMENT OF SIGNIFICANCE

A high strength aligned collagen suture was fabricated via linear electrocompaction and heparinized for prolonged delivery of PDFG-BB. When it was used to suture a complete lacerated flexor tendon in a chicken model controlled release of the PDGF-BB improved the strength of treated tendon after 12 weeks compared to tendon sutured with commercial nylon suture. Furthermore, Collagen suture with affinity bound PDGF-BB enhanced the vascularization and remodeling of lacerated tendon when it compare to synthetic nylon suture. Overall, electrocompacted collagen sutures holds potential to improve repair outcome in flexor tendon surgeries by improving repair strength and stiffness, vascularity, and remodeling via sustained delivery of the PDGF-BB. The bioinductive collagen suture introduces a platform for sustained delivery of other growth factors for a wide-array of applications.

Lactoferrin and parathyroid hormone are not harmful to primary tenocytes in vitro, but PDGF may be

Introduction

Recently, bone-active factors such as parathyroid hormone and lactoferrin, have been used in pre-clinical models to promote tendon healing. How-ever, there is limited understanding of how these boneactive factors may affect the cells of the ten-don themselves. Here, we present an in vitro study assessing the effects of parathyroid hor-mone and lactoferrin on primary tendon cells (tenocytes), and compare their responses to the tenogenic factors, PDGF, IGF-1 and TGF-β.

Materials and Methods

Tenocyte proliferation and collagen production were assessed by alamarBlue® and Sirius red as-says, respectively. To assess tenocyte trans-differentiation, changes in the expression of genes important in tenocyte, chondrocyte and osteoblast biology were determined using real-time PCR.

Results

Parathyroid hormone and lactoferrin had no effect on tenocyte growth or collagen production, with minimal changes in gene expression and no detrimental effects observed to suggest trans-differentiation away from tendon cell behaviour. Tenogenic factors PDGF, IGF-1 and TGF all increasetenocyte collagen production, however, the gene expression data suggests that PDGF promotes severe de-differentiation of the tenocytes.

Discussion

Our findings suggest that using parathyroid hormone or lactoferrin as a singular factor to promote tendon healing may not be of benefit, but for use in tendon-bone healing there would be no detrimental effect on the tendon itself.

Next Generation Tissue Engineering of Orthopedic Soft Tissue-to-Bone Interfaces

Soft tissue-to-bone interfaces are complex structures that consist of gradients of extracellular matrix materials, cell phenotypes, and biochemical signals. These interfaces, called entheses for ligaments, tendons, and the meniscus, are crucial to joint function, transferring mechanical loads and stabilizing orthopedic joints. When injuries occur to connected soft tissue, the enthesis must be re-established to restore function, but due to structural complexity, repair has proven challenging. Tissue engineering offers a promising solution for regenerating these tissues. This prospective review discusses methodologies for tissue engineering the enthesis, outlined in three key design inputs: materials processing methods, cellular contributions, and biochemical factors.

Optimized Repopulation of Tendon Hydrogel: Synergistic Effects of Growth Factor Combinations and Adipose-Derived Stem Cells

Background: Tendon-derived extracellular matrix (ECM) hydrogel has been shown to augment tendon healing in vivo. We hypothesized that reseeding of the gel with adipose-derived stem cells (ASCs) could further assist repopulation of the gel and that combinations of growth factors (GFs) would improve the survival of these cells after reseeding. Methods: A tendon-specific ECM solution was supplemented with varying concentrations of basic fibroblast growth factor (bFGF), insulin-like growth factor-1 (IGF-1), and platelet-derived growth factor-BB (PDGF-BB). Gels were then seeded with ASCs transfected with a green fluorescent protein/luciferin construct. Cell proliferation was determined using the MTT assay and histology, and GF and ASC augmented gels were injected into the back of Sprague Dawley rats. Bioluminescence of seeded gels was continuously followed after reseeding, and cell counts were performed after the gels were explanted at 14 days. Results: Synergistic effects of the GFs were seen, and an optimal combination was determined to be 10 ng/mL bFGF, 100 ng/mL IGF-1, and 100 ng/mL PDGF-BB (2.8-fold increase; P < .05). In vivo bioluminescence showed an improved initial survival of cells in gels supplemented with the optimal concentration of GF compared with the control group (10.6-fold increase at 8 days; P < .05). Cell counts of explants showed a dramatic endogenous repopulation of gels supplemented by GF + ASCs compared with both gels with GF but no ASCs (7.6-fold increase) and gels with ASCs but no GF (1.6-fold increase). Conclusion: Synergistic effects of GFs can be used to improve cellular proliferation of ASCs seeded to a tendon ECM gel. Reseeding with ASCs stimulates endogenous repopulation of the gel in vivo and may be used to further augment tendon healing.

Therapeutics for tendon regeneration: a multidisciplinary review of tendon research for improved healing

Tendon injuries, known as tendinopathies, are common musculoskeletal injuries that affect a wide range of the population. Canonical tendon healing is characterized by fibrosis, scar formation, and the loss of tissue mechanical and structural properties. Understanding the regenerative tendon environment is an area of increasing interest in the field of musculoskeletal research. Previous studies have focused on utilizing individual elements from the fields of biomechanics, developmental biology, cell and growth factor therapy, and tissue engineering in an attempt to develop regenerative tendon therapeutics. Still, the specific mechanism for regenerative healing remains unknown. In this review, we highlight some of the current approaches of tendon therapeutics and elucidate the differences along the tendon midsubstance and enthesis, exhibiting the necessity of location-specific tendon therapeutics. Furthermore, we emphasize the necessity of further interdisciplinary research in order to reach the desired goal of fully understanding the mechanisms underlying regenerative healing.

Cell and Biologic-Based Treatment of Flexor Tendon Injuries

The two primary factors leading to poor clinical results after intrasynovial tendon repair are adhesion formation within the digital sheath and repair-site elongation and rupture. As the outcomes following modern tendon multi-strand repair and controlled rehabilitation techniques are often unsatisfactory, alternative approaches, such as the application of growth factors and mesenchymal stem cells (MSCs), have become increasingly attractive treatment options. Successful biological therapies require carefully controlled spatiotemporal delivery of cells, growth factors, and biocompatible scaffold matrices in order to simultaneously (1) promote matrix synthesis at the tendon repair site leading to increased biomechanical strength and stiffness and (2) suppress matrix synthesis along the tendon surface and synovial sheath preventing adhesion formation. This review summarizes recent cell and biologic-based experimental treatments for flexor tendon injury, with an emphasis on large animal translational studies.

Heparinized collagen sutures for sustained delivery of PDGF-BB: Delivery profile and effects on tendon-derived cells In-Vitro

Suturing is the standard of repair for lacerated flexor tendons. Past studies focused on delivering growth factors to the repair site by incorporating growth factors to nylon sutures which are commonly used in the repair procedure. However, conjugation of growth factors to nylon or other synthetic sutures is not straightforward. Collagen holds promise as a suture material by way of providing chemical sites for conjugation of growth factors. On the other hand, collagen also needs to be reconstituted as a mechanically robust thread that can be sutured. In this study, we reconstituted collagen solutions as suturable collagen threads by using linear electrochemical compaction. Prolonged release of PDGF-BB (Platelet derived growth factor-BB) was achieved by covalent bonding of heparin to the collagen sutures. Tensile mechanical tests of collagen sutures before and after chemical modification indicated that the strength of sutures following chemical conjugation stages was not compromised. Strength of lacerated tendons sutured with epitendinous collagen sutures (11.2±0.7N) converged to that of the standard nylon suture (14.9±2.9N). Heparin conjugation of collagen sutures didn't affect viability and proliferation of tendon-derived cells and prolonged the PDGF-BB release up to 15days. Proliferation of cells seeded on PDGF-BB incorporated collagen sutures was about 50% greater than those seeded on plain collagen sutures. Collagen that is released to the media by the cells increased by 120% under the effects of PDGF-BB and collagen production by cells was detectable by histology as of day 21. Addition of PDGF-BB to collagen sutures resulted in a moderate decline in the expression of the tendon-associated markers scleraxis, collagen I, tenomodulin, and COMP; however, expression levels were still greater than the cells seeded on collagen gel. The data indicate that the effects of PDGF-BB on tendon-derived cells mainly occur through increased cell proliferation and that longer term studies are needed to confirm whether this proliferation is outweighs the moderate reduction in the expression of tendon-associated genes.

STATEMENT OF SIGNIFICANCE

A mechanically robust pure collagen suture was fabricated via linear electrocompaction and conjugated with heparin for prolonged delivery of PDFG-BB. Sustained delivery of the PDGF-BB improved the proliferation of tendon derived cells substantially at the expense of a moderate downregulation of tenogenic markers. The collagen threads were functionally applicable as epitendinous sutures when applied to chicken flexor tendons in vitro. Overall, electrocompacted collagen sutures holds potential to improve repair outcome in flexor tendon surgeries by improving cellularity and collagen production through delivery of the PDGF-BB. The bioinductive suture concept can be applied to deliver other growth factors for a wide-array of applications.

A study to evaluate the safety of platelet-derived growth factor for treatment of osteochondral defects of the talus

PURPOSE

An arthroscopic procedure for the treatment of osteochondral defects using platelet-derived growth factor (PDGF) carried out in a matrix of tricalcium phosphate was developed. This prospective, case-series-based study was designed to evaluate the safety and clinical utility of this procedure.

METHODS

Patients with an isolated osteochondral defect larger than 5 mm long, 3 mm wide, and 5 mm deep and smaller than 30 mm long, 25 mm wide, or 20 mm deep were considered for enrolment. Only patients with chronic lesions were enroled. Arthroscopic debridement was followed by the placement of recombinant human PDGF in a matrix of tricalcium phosphate. The Ankle Osteoarthritis Scale (AOS), visual analogue scale (VAS) for pain, and SF-36 questionnaires were administered at 0, 2, 6, 12, and 24 weeks. Magnetic resonance imaging (MRI) and computed tomography (CT) scans were taken before and after surgery.

RESULTS

Five patients were ultimately enroled in this proof-of-concept trial. All outcome measures demonstrated marked improvement from baseline to final follow-up: The mean weight bearing VAS pain score improved by 49%, and the mean AOS functional score improved by 28%. Bone healing was seen on CT, and reduction in oedema signal was seen on MRI.

CONCLUSION

This new procedure may offer a promising alternative for the treatment of osteochondral defects. Further high-quality studies are needed to confirm these results and to analyse the long-term effects of the procedure. The clinical relevance of this study is that the procedure may provide a less invasive option with improved bone healing compared to standard techniques .

LEVEL OF EVIDENCE

IV.

Platelet concentrate in bovine reproduction: effects on in vitro embryo production and after intrauterine administration in repeat breeder cows

BACKGROUND

A repeat breeder cow (RBC) can be defined as an animal that after 3 or more inseminations cannot get pregnant because of fertilization failure or early embryonic death. If no cause is identified precisely, inadequate uterine receptivity is responsible for implantation failures. Since a large number of identified molecular mediators, such as cytokines, growth factors and lipids have been postulated to be involved in early feto-maternal interaction, in this study a different approach to the treatment of RBC syndrome has been employed using a platelet concentrate (PC) that contains a significant amount of growth factors accumulated in its α-granules.

METHODS

Three explorative studies were performed. Initially, PC was supplemented in the in vitro embryo culture medium to study its effect on embryo-development. After the pilot study, 4 RBCs were treated with intrauterine administration of PC to evaluate proliferative potential of endometrium by immunohistochemical expression of the antigen Ki-67. Lastly, the effect of intrauterine administration of PC at 48 hrs after artificial insemination in RBCs was evaluated.

RESULTS

The in vitro results show that 5 % of PC and 5 % of fetal calf serum (FCS) increase the rate of blastocysts compared with the control containing 10 % FCS only (43.04 % vs 35.00 % respectively). The immunohistochemical study shows more proliferating nuclei in the treated uterine horn compared to the control one. After intrauterine insemination in RBCs, the percentage of pregnant cows in the control group was 33.33 % compared to 70 % of the treated animals.

CONCLUSION

We suppose that when embryo descends in uterus could find a more appropriate environment for nesting and subsequent pregnancy.