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

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

Bioactive Nanostructured Scaffold-Based Approach for Tendon and Ligament Tissue Engineering

An effective therapeutic strategy to treat tendon or ligament injury continues to be a clinical challenge due to the limited natural healing capacity of these tissues. Furthermore, the repaired tendons or ligaments usually possess inferior mechanical properties and impaired functions. Tissue engineering can restore the physiological functions of tissues using biomaterials, cells, and suitable biochemical signals. It has produced encouraging clinical outcomes, forming tendon or ligament-like tissues with similar compositional, structural, and functional attributes to the native tissues. This paper starts by reviewing tendon/ligament structure and healing mechanisms, followed by describing the bioactive nanostructured scaffolds used in tendon and ligament tissue engineering, with emphasis on electrospun fibrous scaffolds. The natural and synthetic polymers for scaffold preparation, as well as the biological and physical cues offered by incorporating growth factors in the scaffolds or by dynamic cyclic stretching of the scaffolds, are also covered. It is expected to present a comprehensive clinical, biological, and biomaterial insight into advanced tissue engineering-based therapeutics for tendon and ligament repair.

Trends in mechanobiology guided tissue engineering and tools to study cell-substrate interactions: a brief review

Sensing the mechanical properties of the substrates or the matrix by the cells and the tissues, the subsequent downstream responses at the cellular, nuclear and epigenetic levels and the outcomes are beginning to get unraveled more recently. There have been various instances where researchers have established the underlying connection between the cellular mechanosignalling pathways and cellular physiology, cellular differentiation, and also tissue pathology. It has been now accepted that mechanosignalling, alone or in combination with classical pathways, could play a significant role in fate determination, development, and organization of cells and tissues. Furthermore, as mechanobiology is gaining traction, so do the various techniques to ponder and gain insights into the still unraveled pathways. This review would briefly discuss some of the interesting works wherein it has been shown that specific alteration of the mechanical properties of the substrates would lead to fate determination of stem cells into various differentiated cells such as osteoblasts, adipocytes, tenocytes, cardiomyocytes, and neurons, and how these properties are being utilized for the development of organoids. This review would also cover various techniques that have been developed and employed to explore the effects of mechanosignalling, including imaging of mechanosensing proteins, atomic force microscopy (AFM), quartz crystal microbalance with dissipation measurements (QCMD), traction force microscopy (TFM), microdevice arrays, Spatio-temporal image analysis, optical tweezer force measurements, mechanoscanning ion conductance microscopy (mSICM), acoustofluidic interferometric device (AID) and so forth. This review would provide insights to the researchers who work on exploiting various mechanical properties of substrates to control the cellular and tissue functions for tissue engineering and regenerative applications, and also will shed light on the advancements of various techniques that could be utilized to unravel the unknown in the field of cellular mechanobiology.

Stem Cell Applications and Tenogenic Differentiation Strategies for Tendon Repair

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

Synergistic effects of growth factor-based serum-free medium and tendon-like substrate topography on tenogenesis of mesenchymal stem cells

Addressing clinical challenges for tendon injuries requires a deeper understanding of the effects that biological and biophysical cues have on tenogenesis. Although prior studies have identified tenogenic growth factors (GFs) or elucidated the effects of substrate topography on tenocyte behavior, few have characterized their combined effect in the presence of a tendon-like substrate. In this study, we assessed the effect of biological (GFs) and biophysical (substrate topography) cues on tenogenic proliferation and differentiation under defined, serum-free conditions. Specifically, human bone marrow-derived mesenchymal stem cells (hMSCs) were cultured in a serum-free culture medium containing a GF cocktail comprised of fibroblast growth factor-2 (FGF-2), transforming growth factor-beta 3 (TGF-β3), and insulin-like growth factor-1 (IGF-1), either alone or in combination with tendon-like substrate topography produced by replica casting of tendon tissue sections. Our data demonstrated that the use of serum-free GF cocktail medium alone promoted hMSC proliferation, as shown via DNA staining as well as Ki67 protein levels and gene expression. In particular, gene expression of Ki67 was increased by 8.46-fold in all three donors relative to serum-free medium control. Also, serum-free GF cocktail promoted tenogenic differentiation, on the basis of expression of tendon-associated gene and protein markers, scleraxis (SCX), tenascin C (TNC), and collagen type I (COL1A1) including increased normalized collagen production by 1.4-fold in two donors relative to serum-free medium control. Interestingly, hMSCs cultured on a tendon-like substrate exhibited highly oriented cell morphology and extracellular matrix (ECM) alignment reminiscent of tendon. In particular, when this GF cocktail was combined with tendon-like topography, they showed a synergistically increased expression of tendon-related markers and anisotropic organization of ECM proteins with moderate-to-large effect sizes. Together, in addition to showing the utility of a GF cocktail for expansion and differentiation of tenocyte-like cells, our findings clearly demonstrate the synergistic relationship between GF-mediated and substrate topography-related effects on hMSC tenogenic differentiation. This information provides insights into the design of strategies that combine biological and biophysical cues for ex vivo tenocyte production and tendon tissue engineering.

Scaffold-Mediated Immunoengineering as Innovative Strategy for Tendon Regeneration

Tendon injuries are at the frontier of innovative approaches to public health concerns and sectoral policy objectives. Indeed, these injuries remain difficult to manage due to tendon’s poor healing ability ascribable to a hypo-cellularity and low vascularity, leading to the formation of a fibrotic tissue affecting its functionality. Tissue engineering represents a promising solution for the regeneration of damaged tendons with the aim to stimulate tissue regeneration or to produce functional implantable biomaterials. However, any technological advancement must take into consideration the role of the immune system in tissue regeneration and the potential of biomaterial scaffolds to control the immune signaling, creating a pro-regenerative environment. In this context, immunoengineering has emerged as a new discipline, developing innovative strategies for tendon injuries. It aims at designing scaffolds, in combination with engineered bioactive molecules and/or stem cells, able to modulate the interaction between the transplanted biomaterial-scaffold and the host tissue allowing a pro-regenerative immune response, therefore hindering fibrosis occurrence at the injury site and guiding tendon regeneration. Thus, this review is aimed at giving an overview on the role exerted from different tissue engineering actors in leading immunoregeneration by crosstalking with stem and immune cells to generate new paradigms in designing regenerative medicine approaches for tendon injuries.

Fibrous heart valve leaflet substrate with native-mimicked morphology

Tissue-engineered heart valves are a promising alternative solution to prosthetic valves. However, long-term functionalities of tissue-engineered heart valves depend on the ability to mimic the trilayered, oriented structure of native heart valve leaflets. In this study, using electrospinning, we developed trilayered microfibrous leaflet substrates with morphological characteristics similar to native leaflets. The substrates were implanted subcutaneously in rats to study the effect of their trilayered oriented structure on in vivo tissue engineering. The tissue constructs showed a well-defined structure, with a circumferentially oriented layer, a randomly oriented layer and a radially oriented layer. The extracellular matrix, produced during in vivo tissue engineering, consisted of collagen, glycosaminoglycans, and elastin, all major components of native leaflets. Moreover, the anisotropic tensile properties of the constructs were sufficient to bear the valvular physiological load. Finally, the expression of vimentin and α-smooth muscle actin, at the gene and protein level, was detected in the residing cells, revealing their growing state and their transdifferentiation to myofibroblasts. Our data support a critical role for the trilayered structure and anisotropic properties in functional leaflet tissue constructs, and indicate that the leaflet substrates have the potential for the development of valve scaffolds for heart valve replacements.

Combinatorial effect of plasma treatment, fiber alignment and fiber scale of poly (ε-caprolactone)/collagen multiscale fibers in inducing tenogenesis in non-tenogenic media

Tendon being a hypocellular, low vascularized tissue often requires assistance for restoration after complete tear. Tendon tissue engineering aims in the development of suitable scaffold that could support the regeneration of tendon after damage. The success of such scaffolds is dependent on its integration with the native tissue which in turn is influenced by the cell-material interaction. In this work aligned poly(ε-caprolactone)/collagen (PCL/collagen) multiscale fibers were developed and plasma treatment using argon, nitrogen and its combination was accessed for inducing tenogenic differentiation in mesenchymal stem cells. The developed fibers mimicked tendon extracellular matrix (ECM) which upon plasma treatment maintained moderate hydrophilicity. Oxygen and nitrogen containing groups were observed to be incorporated after argon and nitrogen treatment respectively. Statistically significant (p < 0.001) enhancement was observed in average and root mean square (RMS) roughness after plasma treatment with the maximum in argon treated fibers. Vitronectin was competitively (statistically significant, p < 0.05) adsorbed after argon and combination treatment whereas nitrogen treatment led to the competitive adsorption of fibronectin (statistically significant, p < 0.05). Human mesenchymal stem cells (hMSCs) showed enhanced proliferation and attachment on plasma treated fibers. Increased porosity due to the presence of sacrificial collagen nanofibers improved cell infiltration which was further enhanced upon plasma treatment. RhoA activation was observed (statistically significant, p < 0.05) on aligned PCL/collagen multiscale fibers and PCL microfibers, which proved its impact on tenogenic differentiation. Further enhancement in rhoA expression was observed on argon (p < 0.01) and combination plasma (p < 0.05) treated fibers. Tenogenic differentiation of hMSCs was enhanced (statistically significant) on argon plasma treated aligned fibers which was confirmed by the expression of scleraxis, mohawk (early markers) and tenomodulin (late marker) at protein level and mohawk, collagen I, collagen III (early markers), thrombospondin 4 and tenascin C (late markers) at gene level. Thus argon plasma treatment on aligned fibers is an effective method to induce tenogenesis even in non-tenogenic media.

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.

Collagen and chondroitin sulfate functionalized bioinspired fibers for tendon tissue engineering application

Functional tendon tissue engineering depends on harnessing the biochemical and biophysical cues of the native tendon extracellular matrix. In this study, we fabricated highly-aligned poly(L-lactic acid) (PLLA) fibers with surfaces decorated by two of the crucial tendon ECM components, type 1 collagen (COL1) and chondroitin sulfate (CS), through a coaxial stable jet electrospinning approach. Effects of the biomimetic COL1-CS (shell)/PLLA (core) fibers on the tenogenic differentiation of human mesenchymal stem cells (hMSCs) in vitro were investigated. Higher rates of cell spreading and proliferation are observed on the aligned COL1-CS/PLLA fibers compared to that on the plain PLLA fibers. Expression of the tendon-associated genes scleraxis (SCX) and COL1 as well as protein tenomodulin (TNMD) are significantly increased. Introduction of mechanical stimulation gives rise to synergistic effect on tenogenic differentiation of hMSCs. Higher expression of TGF-β2, TGFβR-II, and Smad3 by the cells on the COL1-CS/PLLA fiber substrates are observed, which indicates that COL1-CS/PLLA ultrafine fibers dictate the hMSC tenogenic differentiation through activating the TGF-β signaling pathway. Animal study in rat Achilles tendon repair model corroborated the promoting role of COL1-CS/PLLA in regenerating a tendon-like tissue. Thus, our highly aligned biomimicking fibers may serve as an efficient scaffolding system for functional tendon regeneration.

Trilayered tissue construct mimicking the orientations of three layers of a native heart valve leaflet

A tissue-engineered heart valve can be an alternative to a prosthetic valve in heart valve replacement; however, it is not fully efficient in terms of long-lasting functionality, as leaflets in engineered valves do not possess the trilayered native leaflet structure. Previously, we developed a flat, trilayered, oriented nanofibrous (TN) scaffold mimicking the trilayered structure and orientation of native heart valve leaflets. In vivo tissue engineering-a practical regenerative medicine technology-can be used to develop an autologous heart valve. Thus, in this study, we used our flat, trilayered, oriented nanofibrous scaffolds to develop trilayered tissue structures with native leaflet orientations through in vivo tissue engineering in a rat model. After 2 months of in vivo tissue engineering, infiltrated cells and their deposited collagen fibrils were found aligned in the circumferential and radial layers, and randomly oriented in the random layer of the scaffolds, i.e., trilayered tissue constructs (TTCs) were developed. Tensile properties of the TTCs were higher than that of the control tissue constructs (without any scaffolds) due to influence of fibers of the scaffolds in tissue engineering. Different extracellular matrix proteins-collagen, glycosaminoglycans, and elastin-that exist in native leaflets were observed in the TTCs. Gene expression of the TTCs indicated that the tissue constructs were in growing stage. There was no sign of calcification in the tissue constructs. The TTCs developed with the flat TN scaffolds indicate that an autologous leaflet-shaped, trilayered tissue construct that can function as a native leaflet can be developed.

In Vitro Innovation of Tendon Tissue Engineering Strategies

Tendinopathy is the term used to refer to tendon disorders. Spontaneous adult tendon healing results in scar tissue formation and fibrosis with suboptimal biomechanical properties, often resulting in poor and painful mobility. The biomechanical properties of the tissue are negatively affected. Adult tendons have a limited natural healing capacity, and often respond poorly to current treatments that frequently are focused on exercise, drug delivery, and surgical procedures. Therefore, it is of great importance to identify key molecular and cellular processes involved in the progression of tendinopathies to develop effective therapeutic strategies and drive the tissue toward regeneration. To treat tendon diseases and support tendon regeneration, cell-based therapy as well as tissue engineering approaches are considered options, though none can yet be considered conclusive in their reproduction of a safe and successful long-term solution for full microarchitecture and biomechanical tissue recovery. In vitro differentiation techniques are not yet fully validated. This review aims to compare different available tendon in vitro differentiation strategies to clarify the state of art regarding the differentiation process.

Mechanical Strain-Mediated Tenogenic Differentiation of Mesenchymal Stromal Cells Is Regulated through Epithelial Sodium Channels

It has been suggested that mechanical strain may elicit cell differentiation in adult somatic cells through activation of epithelial sodium channels (ENaC). However, such phenomenon has not been previously demonstrated in mesenchymal stromal cells (MSCs). The present study was thus conducted to investigate the role of ENaC in human bone marrow-derived MSCs (hMSCs) tenogenic differentiation during uniaxial tensile loading. Passaged-2 hMSCs were seeded onto silicone chambers coated with collagen I and subjected to stretching at 1 Hz frequency and 8% strain for 6, 24, 48, and 72 hours. Analyses at these time points included cell morphology and alignment observation, immunocytochemistry and immunofluorescence staining (collagen I, collagen III, fibronectin, and N-cadherin), and gene expression (ENaC subunits, and tenogenic markers). Unstrained cells at similar time points served as the control group. To demonstrate the involvement of ENaC in the differentiation process, an ENaC blocker (benzamil) was used and the results were compared to the noninhibited hMSCs. ENaC subunits' (α, β, γ, and δ) expression was observed in hMSCs, although only α subunit was significantly increased during stretching. An increase in tenogenic genes' (collagen1, collagen3, decorin, tenascin-c, scleraxis, and tenomodulin) and proteins' (collagen I, collagen III, fibronectin, and N-cadherin) expression suggests that hMSCs underwent tenogenic differentiation when subjected to uniaxial loading. Inhibition of ENaC function resulted in decreased expression of these markers, thereby suggesting that ENaC plays a vital role in tenogenic differentiation of hMSCs during mechanical loading.

TGF-β3 Loaded Electrospun Polycaprolacton Fibre Scaffolds for Rotator Cuff Tear Repair: An in Vivo Study in Rats

Biological factors such as TGF-β3 are possible supporters of the healing process in chronic rotator cuff tears. In the present study, electrospun chitosan coated polycaprolacton (CS-g-PCL) fibre scaffolds were loaded with TGF-β3 and their effect on tendon healing was compared biomechanically and histologically to unloaded fibre scaffolds in a chronic tendon defect rat model. The biomechanical analysis revealed that tendon-bone constructs with unloaded scaffolds had significantly lower values for maximum force compared to native tendons. Tendon-bone constructs with TGF-β3-loaded fibre scaffolds showed only slightly lower values. In histological evaluation minor differences could be observed. Both groups showed advanced fibre scaffold degradation driven partly by foreign body giant cell accumulation and high cellular numbers in the reconstructed area. Normal levels of neutrophils indicate that present mast cells mediated rather phagocytosis than inflammation. Fibrosis as sign of foreign body encapsulation and scar formation was only minorly present. In conclusion, TGF-β3-loading of electrospun PCL fibre scaffolds resulted in more robust constructs without causing significant advantages on a cellular level. A deeper investigation with special focus on macrophages and foreign body giant cells interactions is one of the major foci in further investigations.

Biodegradable polymer nanocomposites for ligament/tendon tissue engineering

Ligaments and tendons are fibrous tissues with poor vascularity and limited regeneration capacity. Currently, a ligament/tendon injury often require a surgical procedure using auto- or allografts that present some limitations. These inadequacies combined with the significant economic and health impact have prompted the development of tissue engineering approaches. Several natural and synthetic biodegradable polymers as well as composites, blends and hybrids based on such materials have been used to produce tendon and ligament scaffolds. Given the complex structure of native tissues, the production of fiber-based scaffolds has been the preferred option for tendon/ligament tissue engineering. Electrospinning and several textile methods such as twisting, braiding and knitting have been used to produce these scaffolds. This review focuses on the developments achieved in the preparation of tendon/ligament scaffolds based on different biodegradable polymers. Several examples are overviewed and their processing methodologies, as well as their biological and mechanical performances, are discussed.

In vivo tissue engineering of a trilayered leaflet-shaped tissue construct

Aim: We aimed to develop a leaflet-shaped trilayered tissue construct mimicking the morphology of native heart valve leaflets. Materials & methods: Electrospinning and in vivo tissue engineering methods were employed. Results: We developed leaflet-shaped microfibrous scaffolds, each with circumferentially, randomly and radially oriented three layers mimicking the trilayered, oriented structure of native leaflets. After 3 months in vivo tissue engineering with the scaffolds, the generated leaflet-shaped tissue constructs had a trilayered structure mimicking the orientations of native heart valve leaflets. Presence of collagen, glycosaminoglycans and elastin seen in native leaflets was observed in the engineered tissue constructs. Conclusion: Trilayered, oriented fibrous scaffolds brought the orientations of the infiltrated cells and their produced extracellular matrix proteins into the constructs.

Current Progress in Tendon and Ligament Tissue Engineering

Background

Tendon and ligament injuries accounted for 30% of all musculoskeletal consultations with 4 million new incidences worldwide each year and thus imposed a significant burden to the society and the economy. Damaged tendon and ligament can severely affect the normal body movement and might lead to many complications if not treated promptly and adequately. Current conventional treatment through surgical repair and tissue graft are ineffective with a high rate of recurrence.

Methods

In this review, we first discussed the anatomy, physiology and pathophysiology of tendon and ligament injuries and its current treatment. Secondly, we explored the current role of tendon and ligament tissue engineering, describing its recent advances. After that, we also described stem cell and cell secreted product approaches in tendon and ligament injuries. Lastly, we examined the role of the bioreactor and mechanical loading in in vitro maturation of engineered tendon and ligament.

Results

Tissue engineering offers various alternative ways of treatment from biological tissue constructs to stem cell therapy and cell secreted products. Bioreactor with mechanical stimulation is instrumental in preparing mature engineered tendon and ligament substitutes in vitro.

Conclusions

Tissue engineering showed great promise in replacing the damaged tendon and ligament. However, more study is needed to develop ideal engineered tendon and ligament.

Endothelialization of cardiovascular devices

Blood-contacting surfaces of cardiovascular devices are not biocompatible for creating an endothelial layer on them. Numerous research studies have mainly sought to modify these surfaces through physical, chemical and biological means to ease early endothelial cell (EC) adhesion, migration and proliferation, and eventually to build an endothelial layer on the surfaces. The first priority for surface modification is inhibition of protein adsorption that leads to inhibition of platelet adhesion to the device surfaces, which may favor EC adhesion. Surface modification through surface texturing, if applicable, can bring some hopeful outcomes in this regard. Surface modifications through chemical and/or biological means may play a significant role in easy endothelialization of cardiovascular devices and inhibit smooth muscle cell proliferation. Cellular engineering of cells relevant to endothelialization can boost the positive outcomes obtained through surface engineering. This review briefly summarizes recent developments and research in early endothelialization of cardiovascular devices. STATEMENT OF SIGNIFICANCE: Endothelialization of cardiovascular implants, including heart valves, vascular stents and vascular grafts is crucial to solve many problems in our health care system. Numerous research efforts have been made to improve endothelialization on the surfaces of cardiovascular implants, mainly through surface modifications in three ways - physically, chemically and biologically. This review is intended to highlight comprehensive research studies to date on surface modifications aiming for early endothelialization on the blood-contacting surfaces of cardiovascular implants. It also discusses future perspectives to help guide endothelialization strategies and inspire further innovations.

Evaluation of tenogenic differentiation potential of selected subpopulations of human adipose-derived stem cells

Identification of a suitable cell source and bioactive agents guiding cell differentiation towards tenogenic phenotype represents a prerequisite for advancement of cell-based therapies for tendon repair. Human adipose-derived stem cells (hASCs) are a promising, yet intrinsically heterogenous population with diversified differentiation capacities. In this work, we investigated antigenically-defined subsets of hASCs expressing markers related to tendon phenotype or associated with pluripotency that might be more prone to tenogenic differentiation, when compared to unsorted hASCs. Subpopulations positive for tenomodulin (TNMD+ hASCs) and stage specific early antigen 4 (SSEA-4+ hASCs), as well as unsorted ASCs were cultured up to 21 days in basic medium or media supplemented with TGF-β3 (10 ng/ml), or GDF-5 (50 ng/ml). Cell response was evaluated by analysis of expression of tendon-related markers at gene level and protein level by real time RT-PCR, western blot, and immunocytochemistry. A significant upregulation of scleraxis was observed for both subpopulations and unsorted hASCs in the presence of TGF-β3. More prominent alterations in gene expression profile in response to TGF-β3 were observed for TNMD+ hASCs. Subpopulations evidenced an increased collagen III and TNC deposition in basal medium conditions in comparison with unsorted hASCs. In the particular case of TNMD+ hASCs, GDF-5 seems to influence more the deposition of TNC. Within hASCs populations, discrete subsets could be distinguished offering varied sensitivity to specific biochemical stimulation leading to differential expression of tenogenic components suggesting that cell subsets may have distinctive roles in the complex biological responses leading to tenogenic commitment to be further explored in cell based strategies for tendon tissues.

Optimization of polycaprolactone fibrous scaffold for heart valve tissue engineering

Pore size is generally small in nanofibrous scaffolds prepared by electrospinning polymeric solutions. Increase of scaffold thickness leads to decrease in pore size, causing impediment to cell infiltration into the scaffolds during tissue engineering. In contrast, comparatively larger pore size can be realized in microfibrous scaffolds prepared from polymeric solutions at higher concentrations. Further, microfibrous scaffolds are conducive to infiltration of reparative M2 phenotype macrophages during in vivo/in situ tissue engineering. However, rise of mechanical properties of a fibrous scaffold with the increase of polymer concentration may limit the functionality of a scaffold-based, tissue-engineered heart valve. In this study, we developed microfibrous scaffolds from 14%, 16% and 18% (wt/v) polycaprolactone (PCL) polymer solutions prepared with chloroform solvent. Porcine valvular interstitial cells were cultured in the scaffolds for 14 d to investigate the effect of microfibers prepared with different PCL concentrations on the seeded cells. Further, fresh microfibrous scaffolds were implanted subcutaneously in a rat model for two months to investigate the effect of microfibers on infiltrated cells. Cell proliferation, and its morphologies, gene expression and deposition of different extracellular matrix proteins in the in vitro study were characterized. During the in vivo study, we characterized cell infiltration, and myofibroblast and M1/M2 phenotypes expression of the infiltrated cells. Among different PCL concentrations, microfibrous scaffolds from 14% solution were suitable for heart valve tissue engineering for their sufficient pore size and low but adequate tensile properties, which promoted cell adhesion to and proliferation in the scaffolds, and effective gene expression and extracellular matrix deposition by the cells in vitro. They also encouraged the cells in vivo for their infiltration and effective gene expression, including M2 phenotype expression.

Chitosan hydrogel scaffold reinforced with twisted poly(l lactic acid) aligned microfibrous bundle to mimic tendon extracellular matrix

Regeneration of tendon requires construct that provides necessary structural support closely mimicking the native architecture. To recreate this complex architecture a construct made of heat-treated, twisted poly(L lactic acid) (PLLA) microfibers coated with chitosan gel and surrounded by PLLA micro-fibrous layer was developed. The developed construct characterized using SEM showed the macroporous nature of gel coating around four distinct PLLA twisted fibrous bundle and a thin fiber layer surrounding the construct. FTIR analysis confirmed the presence of PLLA and chitosan construct. Mechanical strength increased with increasing number of strips. Protein adsorption was significantly low on the construct with outer covering that could retard cell adhesion to the outer layer. The developed construct showed good cell attachment and proliferation of tenocytes. These results indicate that the construct would find application for tendon tissue engineering.

An asymmetric chitosan scaffold for tendon tissue engineering: In vitro and in vivo evaluation with rat tendon stem/progenitor cells

The poor healing capacity and typically incomplete regeneration of injured tendons has made tendon repair as a primary clinical concern. Several methods for repairing injured tendons have been developed in the last decade. Tendon regeneration using current tissue engineering techniques requires advanced biomaterials to satisfy both microstructural and mechanical criteria. In this study, a novel chitosan (CS)-based scaffold with asymmetric structure was fabricated using a self-deposition technique. The fabricated scaffolds were assessed with regard to the microstructural and mechanical demands of cell ingrowth and the prevention of peritendinous adhesion. In vitro studies showed that rat tendon stem/progenitor cells (TSPCs) seeded onto the CS scaffold displayed higher levels of tenogenic specific genes expression and protein production. Four and six weeks after the implantation of CS scaffolds on full-site Achilles tendon defects, in vivo tendon repair was evaluated by histology, immunohistochemistry, immunofluorescence, and mechanical measurements. The production of collagen I (COL1) and collagen III (COL3) demonstrated that the CS scaffolds were capable of inducing conspicuous tenogenic differentiation, higher tenomodulin (TNMD) production, and superior phenotypic maturity, compared with the empty defect group. The introduction of TSPCs into the CS scaffold resulted in a synergistic effect on tendon regeneration and yielded better-aligned collagen fibers with elongated, spindle-shaped cells. These findings indicated that the application of TSPC-seeded CS scaffolds would be a feasible approach for tendon repair.

STATEMENT OF SIGNIFICANCE

The poor healing capacity of injured tendons and inevitable peritendinous adhesion has made tendon regeneration a clinical priority. In this study, an asymmetric chitosan scaffold was developed to encapsulate rat tendon stem/progenitor cells (TSPCs), which could induce higher levels of tenogenic specific genes and protein expression. Remarkably, the introduction of TSPCs into the asymmetric chitosan scaffold generated a synergistic effect on in vivo tendon regeneration and lead to better-aligned collagen fibers compared with asymmetric chitosan scaffold alone. This work can provide new guidelines for the structure and property design of cell-seeded scaffolds for tendon regeneration.

Living nanofiber yarn-based woven biotextiles for tendon tissue engineering using cell tri-culture and mechanical stimulation

Non-woven nanofibrous scaffolds have been developed for tendon graft application by using electrospinning strategies. However, electrospun nanofibrous scaffolds face some obstacles and limitations, including suboptimal scaffold structure, weak tensile and suture-retention strengths, and compact structure for cell infiltration. In this work, a novel nanofibrous, woven biotextile, fabricated based on electrospun nanofiber yarns, was implemented as a tissue engineered tendon scaffold. Based on our modified electrospinning setup, polycaprolactone (PCL) nanofiber yarns were fabricated with reproducible quality, and were further processed into plain-weaving fabrics interlaced with polylactic acid (PLA) multifilaments. Nonwoven nanofibrous PCL meshes with random or aligned fiber structures were generated using typical electrospinning as comparative counterparts. The woven fabrics contained 3D aligned microstructures with significantly larger pore size and obviously enhanced tensile mechanical properties than their nonwoven counterparts. The biological results revealed that cell proliferation and infiltration, along with the expression of tendon-specific genes by human adipose derived mesenchymal stem cells (HADMSC) and human tenocytes (HT), were significantly enhanced on the woven fabrics compared with those on randomly-oriented or aligned nanofiber meshes. Co-cultures of HADMSC with HT or human umbilical vein endothelial cells (HUVEC) on woven fabrics significantly upregulated the functional expression of most tenogenic markers. HADMSC/HT/HUVEC tri-culture on woven fabrics showed the highest upregulation of most tendon-associated markers than all the other mono- and co-culture groups. Furthermore, we conditioned the tri-cultured constructs with dynamic conditioning and demonstrated that dynamic stretch promoted total collagen secretion and tenogenic differentiation. Our nanofiber yarn-based biotextiles have significant potential to be used as engineered scaffolds to synergize the multiple cell interaction and mechanical stimulation for promoting tendon regeneration.

STATEMENT OF SIGNIFICANCE

Tendon grafts are essential for the treatment of various tendon-related conditions due to the inherently poor healing capacity of native tendon tissues. In this study, we combined electrospun nanofiber yarns with textile manufacturing strategies to fabricate nanofibrous woven biotextiles with hierarchical features, aligned fibrous topography, and sufficient mechanical properties as tendon tissue engineered scaffolds. Comparing to traditional electrospun random or aligned meshes, our novel nanofibrous woven fabrics possess strong tensile and suture-retention strengths and larger pore size. We also demonstrated that the incorporation of tendon cells and vascular cells promoted the tenogenic differentiation of the engineered tendon constructs, especially under dynamic stretch. This study not only presents a novel tissue engineered tendon scaffold fabrication technique but also provides a useful strategy to promote tendon differentiation and regeneration.

Response of Dermal Fibroblasts to Biochemical and Physical Cues in Aligned Polycaprolactone/Silk Fibroin Nanofiber Scaffolds for Application in Tendon Tissue Engineering

Silk fibroin (SF) and fiber alignment were introduced into polycaprolactone (PCL)-based electrospun nanofibers as chemical and physical cues for tendon tissue engineering applications. The physicochemical properties of random PCL (RP) nanofibers, random PCL/SF (RPSF) nanofibers and aligned PCL/SF (APSF) nanofibers were characterized for fiber orientation and SF blending effects. An in vitro cell culture with rabbit dermal fibroblasts (RDFBs) on nanofibers indicated that SF promotes cell proliferation to a higher extent than fiber alignment. Cells aligned in the direction of fiber axes could be confirmed through scanning electron microscopy (SEM) observation and cytoskeleton staining. The quantitative real-time polymerase chain reaction (qRT-PCR) experiments indicated up-regulated gene expression of tendon marker proteins (type I collagen (Col I), fibronectin and biglycan) on APSF nanofibers and tendon reconstruction was confirmed from Col III gene expression. Animal experiments with Achilles tendon defect repairs in rabbits were carried out with RPSF and APSF scaffolds. The beneficial effects of fiber alignment were verified from histological and immunohistochemical staining, where cell migration and extracellular matrix protein deposition tend to stretch in a parallel direction along the axial direction of APSF nanofibers with enhanced Col I and tenascin C production. Biomechanical testing indicated the tensile stiffness and maximum load of cell-seeded APSF scaffolds were 60.2 and 81.3% of normal tendon values, respectively, which are significantly higher than cell-seeded RPSF or acellular APSF and RPSF scaffolds. These results suggest that APSF nanofiber scaffolds combined with RDFBs have the potential to repair the gap defects of Achilles tendons in vivo and to effectively restore the function and structure of tendons.

Tissue-specific bioactivity of soluble tendon-derived and cartilage-derived extracellular matrices on adult mesenchymal stem cells

BACKGROUND

Biological scaffolds composed of tissue-derived extracellular matrix (ECM) can promote homologous (i.e., tissue-specific) cell differentiation through preservation of biophysical and biochemical motifs found in native tissues. Solubilized ECMs derived from decellularized tendon and cartilage have recently been promoted as tissue-specific biomaterials, but whether tissue-specific bioactivity is preserved following solubilization is unknown. This study explored the tissue-specific bioactivity of soluble decellularized tendon and cartilage ECMs on human bone marrow-derived mesenchymal stem cells (MSCs) presented across different culture microenvironments, including two-dimensional (2D) tissue culture plastic, aligned electrospun nanofibers, cell pellets, and cell-seeded photocrosslinkable hydrogels.

METHODS

Tendon and cartilage ECMs were decellularized using established methods and solubilized either via pepsin digestion or urea extraction. The effect of soluble ECMs on cell proliferation and differentiation was initially explored by supplementing basal medium of human MSCs cultured on 2D tissue culture plastic. In subsequent experiments, MSCs were cultured on aligned electrospun nanofibers, ascell pellets, or encapsulated within photocrosslinkable methacrylated gelatin (GelMA) hydrogels. Urea-extracted tendon and cartilage ECMs were added as supplements.

RESULTS

Pepsin-digested ECMs did not promote homologous differentiation in human MSCs, whether provided as a medium supplement or three-dimensional (3D) hydrogels. In contrast, urea-extracted ECMs tended to promote tissue-specific differentiation of MSCs cultured in 2D and 3D microenvironments. The application of the small molecule TGF-β signaling inhibitor SB-431542 largely negated the tissue-specific gene expression patterns mediated by tendon and cartilage ECMs. This suggests that the action of endogenous TGF-β was required, but was not sufficient, to impart tissue-specific bioactivity of urea-extracted ECMs. When urea-extracted cartilage ECM was incorporated within a photocurable GelMA hydrogel it independently enhanced chondrogenesis in encapsulated MSCs, and showed additive prochondrogenesis upon TGF-β supplementation in the medium.

CONCLUSIONS

Urea-extracted ECM fractions of decellularized tendon and cartilage are soluble supplements capable of enhancing tissue-specific differentiation of adult stem cells.

Braided and Stacked Electrospun Nanofibrous Scaffolds for Tendon and Ligament Tissue Engineering

Tendon and ligament injuries are a persistent orthopedic challenge given their poor innate healing capacity. Nonwoven electrospun nanofibrous scaffolds composed of polyesters have been used to mimic the mechanics and topographical cues of native tendons and ligaments. However, nonwoven nanofibers have several limitations that prevent broader clinical application, including poor cell infiltration, as well as tensile and suture-retention strengths that are inferior to native tissues. In this study, multilayered scaffolds of aligned electrospun nanofibers of two designs-stacked or braided-were fabricated. Mechanical properties, including structural and mechanical properties and suture-retention strength, were determined using acellular scaffolds. Human bone marrow-derived mesenchymal stem cells (MSCs) were seeded on scaffolds for up to 28 days, and assays for tenogenic differentiation, histology, and biochemical composition were performed. Braided scaffolds exhibited improved tensile and suture-retention strengths, but reduced moduli. Both scaffold designs supported expression of tenogenic markers, although the effect was greater on braided scaffolds. Conversely, cell infiltration was superior in stacked constructs, resulting in enhanced cell number, total collagen content, and total sulfated glycosaminoglycan content. However, when normalized against cell number, both designs modulated extracellular matrix protein deposition to a similar degree. Taken together, this study demonstrates that multilayered scaffolds of aligned electrospun nanofibers supported tenogenic differentiation of seeded MSCs, but the macroarchitecture is an important consideration for applications of tendon and ligament tissue engineering.

The Horizon of Materiobiology: A Perspective on Material-Guided Cell Behaviors and Tissue Engineering

Although the biological functions of cell and tissue can be regulated by biochemical factors (e.g., growth factors, hormones), the biophysical effects of materials on the regulation of biological activity are receiving more attention. In this Review, we systematically summarize the recent progress on how biomaterials with controllable properties (e.g., compositional/degradable dynamics, mechanical properties, 2D topography, and 3D geometry) can regulate cell behaviors (e.g., cell adhesion, spreading, proliferation, cell alignment, and the differentiation or self-maintenance of stem cells) and tissue/organ functions. How the biophysical features of materials influence tissue/organ regeneration have been elucidated. Current challenges and a perspective on the development of novel materials that can modulate specific biological functions are discussed. The interdependent relationship between biomaterials and biology leads us to propose the concept of "materiobiology", which is a scientific discipline that studies the biological effects of the properties of biomaterials on biological functions at cell, tissue, organ, and the whole organism levels. This Review highlights that it is more important to develop ECM-mimicking biomaterials having a self-regenerative capacity to stimulate tissue regeneration, instead of attempting to recreate the complexity of living tissues or tissue constructs ex vivo. The principles of materiobiology may benefit the development of novel biomaterials providing combinative bioactive cues to activate the migration of stem cells from endogenous reservoirs (i.e., cell niches), stimulate robust and scalable self-healing mechanisms, and unlock the body's innate powers of regeneration.

3D Culture of Chondrocytes in Gelatin Hydrogels with Different Stiffness

Gelatin hydrogels can mimic the microenvironments of natural tissues and encapsulate cells homogeneously, which makes them attractive for cartilage tissue engineering. Both the mechanical and biochemical properties of hydrogels can affect the phenotype of chondrocytes. However, the influence of each property on chondrocyte phenotype is unclear due to the difficulty in separating the roles of these properties. In this study, we aimed to study the influence of hydrogel stiffness on chondrocyte phenotype while excluding the role of biochemical factors, such as adhesion site density in the hydrogels. By altering the degree of methacryloyl functionalization, gelatin hydrogels with different stiffnesses of 3.8, 17.1, and 29.9 kPa Young's modulus were prepared from the same concentration of gelatin methacryloyl (GelMA) macromers. Bovine articular chondrocytes were encapsulated in the hydrogels and cultured for 14 days. The influence of hydrogel stiffness on the cell behaviors including cell viability, cell morphology, and maintenance of chondrogenic phenotype was evaluated. GelMA hydrogels with high stiffness (29.9 kPa) showed the best results on maintaining chondrogenic phenotype. These results will be useful for the design and preparation of scaffolds for cartilage tissue engineering.

Enhancing the Biomechanical Performance of Anisotropic Nanofibrous Scaffolds in Tendon Tissue Engineering: Reinforcement with Cellulose Nanocrystals

Anisotropically aligned electrospun nanofibrous scaffolds based on natural/synthetic polymer blends have been established as a reasonable compromise between biological and biomechanical performance for tendon tissue engineering (TE) strategies. However, the limited tensile properties of these biomaterials restrict their application in this field due to the load-bearing nature of tendon/ligament tissues. Herein, the use of cellulose nanocrystals (CNCs) as reinforcing nanofillers in aligned electrospun scaffolds based on a natural/synthetic polymer blend matrix, poly-ε-caprolactone/chitosan (PCL/CHT) is reported. The incorporation of small amounts of CNCs (up to 3 wt%) into tendon mimetic nanofiber bundles has a remarkable biomaterial-toughing effect (85% ± 5%, p < 0.0002) and raises the scaffolds mechanical properties to tendon/ligament relevant range (σ = 39.3 ± 1.9 MPa and E = 540.5 ± 83.7 MPa, p < 0.0001). Aligned PCL/CHT/CNC nanocomposite fibrous scaffolds meet not only the mechanical requirements for tendon TE applications but also provide tendon mimetic extracellular matrix (ECM) topographic cues, a key feature for maintaining tendon cell's morphology and behavior. The strategy proposed here may be extended to other anisotropic aligned nanofibrous scaffolds based on natural/synthetic polymer blends and enable the full exploitation of the advantages provided by their tendon mimetic fibrous structures in tendon TE.

PEG-chitosan hydrogel with tunable stiffness for study of drug response of breast cancer cells

Mechanical properties of the extracellular matrix have a profound effect on the behavior of anchorage-dependent cells. However, the mechanisms that define the effects of matrix stiffness on cell behavior remains unclear. Therefore, the development and fabrication of synthetic matrices with well-defined stiffness is invaluable for studying the interactions of cells with their biophysical microenvironment in vitro. We demonstrate a methoxypolyethylene glycol (mPEG)-modified chitosan hydrogel network where hydrogel stiffness can be easily modulated under physiological conditions by adjusting the degree of mPEG grafting onto chitosan (PEGylation). We show that the storage modulus of the hydrogel increases as PEGylation decreases and the gels exhibit instant self-recovery after deformation. Breast cancer cells cultured on the stiffest hydrogels adopt a more malignant phenotype with increased resistance to doxorubicin as compared with cells cultured on tissue culture polystyrene or Matrigel. This work demonstrates the utility of mPEG-modified chitosan hydrogel, with tunable mechanical properties, as an improved replacement of conventional culture system for in vitro characterization of breast cancer cell phenotype and evaluation of cancer therapies.

Regeneration ability of valvular interstitial cells from diseased heart valve leaflets

Valvular interstitial cells from diseased aortic valve leaflets show their ability to regenerate–to proliferate and grow, to express appropriate genes and to deposit suitable proteins–in a non-degenerative nanofibrous substrate.

In Vitro Model of a Fibrosa Layer of a Heart Valve

The fibrosa layer of a cardiac aortic valve is composed mostly of a dense network of type I collagen fibers oriented in circumferential direction. This main layer bears the tensile load and responds to the high stress on a leaflet. The inner fibrosa layer is also the site of pathophysiologic changes that result in valvular dysfunction, including stenosis and regurgitation. In vitro studies of these changes are limited by the absence of a substrate that mimics the circumferentially oriented structure of the fibrosa layer. In heart valve tissue engineering, generation of this layer is challenging. This study aimed to develop an artificial fibrosa layer of a native aortic leaflet. A unique morphologically biomimicked, pliable, but standalone substrate with circumferentially oriented nanofibers was fabricated by electrospinning on a novel collector designed for this study. The substrate had low-bulk tensile stiffness and ultimate strength; thus, cultured valvular interstitial cells (VICs) showed a fibroblast phenotype that is generally observed in a healthy aortic leaflet. Furthermore, gene and protein expression and morphology of VICs in substrates were close to those in the fibrosa layer of a native aortic leaflet. This artificial fibrosa layer can be useful for in vitro studies of valvular dysfunctions.

Progress in cell-based therapies for tendon repair

The last decade has seen significant developments in cell therapies, based on permanently differentiated, reprogrammed or engineered stem cells, for tendon injuries and degenerative conditions. In vitro studies assess the influence of biophysical, biochemical and biological signals on tenogenic phenotype maintenance and/or differentiation towards tenogenic lineage. However, the ideal culture environment has yet to be identified due to the lack of standardised experimental setup and readout system. Bone marrow mesenchymal stem cells and tenocytes/dermal fibroblasts appear to be the cell populations of choice for clinical translation in equine and human patients respectively based on circumstantial, rather than on hard evidence. Collaborative, inter- and multi-disciplinary efforts are expected to provide clinically relevant and commercially viable cell-based therapies for tendon repair and regeneration in the years to come.