Tendon-Derived Extracellular Matrix Enhances Transforming Growth Factor-β3-Induced Tenogenic Differentiation of Human Adipose-Derived Stem Cells

4D bioprinting of programmed dynamic tissues

Setting time as the fourth dimension, 4D printing allows us to construct dynamic structures that can change their shape, property, or functionality over time under stimuli, leading to a wave of innovations in various fields. Recently, 4D printing of smart biomaterials, biological components, and living cells into dynamic living 3D constructs with 4D effects has led to an exciting field of 4D bioprinting. 4D bioprinting has gained increasing attention and is being applied to create programmed and dynamic cell-laden constructs such as bone, cartilage, and vasculature. This review presents an overview on 4D bioprinting for engineering dynamic tissues and organs, followed by a discussion on the approaches, bioprinting technologies, smart biomaterials and smart design, bioink requirements, and applications. While much progress has been achieved, 4D bioprinting as a complex process is facing challenges that need to be addressed by transdisciplinary strategies to unleash the full potential of this advanced biofabrication technology. Finally, we present future perspectives on the rapidly evolving field of 4D bioprinting, in view of its potential, increasingly important roles in the development of advanced dynamic tissues for basic research, pharmaceutics, and regenerative medicine.

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.

Non-collagenous proteins, rather than the collagens, are key biochemical factors that mediate tenogenic bioactivity of tendon extracellular matrix

Despite the growing clinical use of extracellular matrix (ECM)-based biomaterials for tendon repair, undesired healing outcomes or complications have frequently been reported. A major scientific challenge has been the limited understanding of their functional compositions and mechanisms of action due to the complex nature of tendon ECM. Previously, we have reported a soluble ECM fraction from bovine tendons (tECM) by urea extraction, which exhibited strong, pro-tenogenic bioactivity on human adipose-derived stem cells (hASCs). In this study, to advance our previous findings and gain insights into the biochemical nature of its pro-tenogenesis activity, tECM was fractionated using (i) an enzymatic digestion approach (pepsin, hyaluronidase, and chondroitinase) to yield various enzyme-digested tECM fractions; and (ii) a gelation-based approach to yield collagen matrix-enriched (CM) and non-collagenous matrix-enriched (NCM) fractions. Their tenogenic bioactivity on hASCs was assessed. Our results collectively indicated that non-collagenous tECM proteins, rather than collagens, are likely the important biochemical factors responsible for tECM pro-tenogenesis bioactivity. Mechanistically, RNA-seq analysis revealed that tECM and its non-collagenous portion induced similar transcriptional profiles of hASCs, particularly genes associated with cell proliferation, collagen synthesis, and tenogenic differentiation, which were distinct from transcriptome induced by its collagenous portion. From an application perspective, the enhanced solubility of the non-collagenous tECM, compared to tECM, should facilitate its combination with various water-soluble biomaterials for tissue engineering protocols. Our work provides insight into the molecular characterization of native tendon ECM, which will help to effectively translate their functional components into the design of well-defined, ECM biomaterials for tendon regeneration. STATEMENT OF SIGNIFICANCE: Significant progress has been made in extracellular matrix (ECM)-based biomaterials for tendon repair. However, their effectiveness remains debated, with conflicting research and clinical findings. Understanding the functional composition and mechanisms of action of ECM is crucial for developing safe and effective bioengineered scaffolds. Expanding on our previous work with bovine tendon ECM extracts (tECM) exhibiting strong pro-tenogenesis activity, we fractionated tECM to evaluate its bioactive moieties. Our findings indicate that the non-collagenous matrix within tECM, rather than the collagenous portions, plays a major role in the pro-tenogenesis bioactivity on human adipose-derived stem cells. These insights will drive further optimization of ECM-based biomaterials, including our advanced method for preparing highly soluble, non-collagenous matrix-enriched tendon ECM for effective tendon repair.

Decellularized extracellular matrix biomaterials for regenerative therapies: Advances, challenges and clinical prospects

Tissue engineering and regenerative medicine have shown potential in the repair and regeneration of tissues and organs via the use of engineered biomaterials and scaffolds. However, current constructs face limitations in replicating the intricate native microenvironment and achieving optimal regenerative capacity and functional recovery. To address these challenges, the utilization of decellularized tissues and cell-derived extracellular matrix (ECM) has emerged as a promising approach. These biocompatible and bioactive biomaterials can be engineered into porous scaffolds and grafts that mimic the structural and compositional aspects of the native tissue or organ microenvironment, both in vitro and in vivo. Bioactive dECM materials provide a unique tissue-specific microenvironment that can regulate and guide cellular processes, thereby enhancing regenerative therapies. In this review, we explore the emerging frontiers of decellularized tissue-derived and cell-derived biomaterials and bio-inks in the field of tissue engineering and regenerative medicine. We discuss the need for further improvements in decellularization methods and techniques to retain structural, biological, and physicochemical characteristics of the dECM products in a way to mimic native tissues and organs. This article underscores the potential of dECM biomaterials to stimulate in situ tissue repair through chemotactic effects for the development of growth factor and cell-free tissue engineering strategies. The article also identifies the challenges and opportunities in developing sterilization and preservation methods applicable for decellularized biomaterials and grafts and their translation into clinical products.

Tendon-derived matrix crosslinking techniques for electrospun multi-layered scaffolds

Tendon tears are common and healing often occurs incompletely and by fibrosis. Tissue engineering seeks to improve repair, and one approach under investigation uses cell-seeded scaffolds containing biomimetic factors. Retention of biomimetic factors on the scaffolds is likely critical to maximize their benefit, while minimizing the risk of adverse effects, and without losing the beneficial effects of the biomimetic factors. The aim of the current study was to evaluate cross-linking methods to enhance the retention of tendon-derived matrix (TDM) on electrospun poly(ε-caprolactone) (PCL) scaffolds. We tested the effects of ultraviolet (UV) or carbodiimide (EDC:NHS:COOH) crosslinking methods to better retain TDM to the scaffolds and stimulate tendon-like matrix synthesis. Initially, we tested various crosslinking configurations of carbodiimide (2.5:1:1, 5:2:1, and 10:4:1 EDC:NHS:COOH ratios) and UV (30 s 1 J/cm2 , 60 s 1 J/cm2 , and 60 s 4 J/cm2 ) on PCL films compared to un-crosslinked TDM. We found that no crosslinking tested retained more TDM than coating alone (Kruskal-Wallis: p > .05), but that human adipose stem cells (hASCs) spread most on the 60 s 1 J/cm2 UV- and 2.5:1:1 EDC-crosslinked films (Kruskal-Wallis: p < .05). Next, we compared the effects of 60 s 1 J/cm2 UV- and 2.5:1:1 EDC-crosslinked to TDM-coated and untreated PCL scaffolds on hASC-induced tendon-like differentiation. UV-crosslinked scaffolds had greater modulus and stiffness than PCL or TDM scaffolds, and hASCs spread more on UV-crosslinked scaffolds (ANOVA: p < .05). Fourier transform infrared spectra revealed that UV- or EDC-crosslinking TDM did not affect the peaks at wavenumbers characteristic of tendon. Crosslinking TDM to electrospun scaffolds improves tendon-like matrix synthesis, providing a viable strategy for improving retention of TDM on electrospun PCL scaffolds.

A systematic investigation of the effects of TGF-β3 and mechanical stimulation on tenogenic differentiation of mesenchymal stromal cells in a poly(ethylene glycol)/gelatin-based hydrogel

Background

High post-surgical failure rates following tendon injury generate high medical costs and poor patient recovery. Cell-based tendon tissue engineering has the potential to produce fully functional replacement tissue and provide new strategies to restore tendon function and healing. In this endeavour, the application of mesenchymal stromal cells (MSCs) encapsulated in biomaterial scaffolds has shown great promise. However, a consensus on optimal promotion of tenogenic differentiation of MSCs has yet to be reached, although growth factors and mechanical cues are generally acknowledged as important factors.

Methods

In this study, we prepared a hydrogel cell culture system consisting of methacrylated poly(d,l-lactic acid-ethylene glycol-d,l-lactic acid) (P(LA-EG-LA)) and gelatin methacrylate (GelMA) to encapsulate human bone marrow-derived MSCs (hBMSCs). We further systematically investigated the influence of static and intermittent cyclic uniaxial strain mechanical stimulation, in combination with transforming growth factor-β3 (TGF-β3) supplementation, on tenogenic differentiation of hBMSCs.

Results

Increased TGF-β3 concentration upregulated the tenogenic genes Scleraxis (SCX) and collagen type I (COL1A1) but showed no effects on tenascin-c (TNC) and collagen type III (COL3A1) expression. Mechanical stimulation had no observable effect on gene expression, but intermittent cyclic uniaxial strain stimulation improved matrix deposition. Together, these data provide new insights into how TGF-β3 and mechanical stimulation regulate MSC tenogenesis, with TGF-β3 promoting the expression of key tenogenic genes whilst mechanical stimulation aided matrix deposition in the engineered tissue. Furthermore, intermittent cyclic uniaxial strain at 3% elongation and 0.33 ​Hz for 1 ​h/day showed improved matrix effects compared to static strain.

Conclusion

Together, the most promising result for tenogenic differentiation of hBMSCs was identified as treatment with 5 ​ng/ml TGF-β3 under intermittent cyclic uniaxial strain (3% strain; 0.33 ​Hz; 1 ​h/day). This knowledge is of importance for the development of an improved protocol for tenogenic differentiation of MSCs and thereby for tendon tissue engineering.

The translational potential of this article

Tissue-engineered strategies for tendon repair require a consensus on the differentiation of mesenchymal stromal cells to tenocytes, which is currently lacking. This article provides a systematic investigation of two main tenogenic differentiation conditions to further development of a tenogenic differentiation protocol.

Bioactivity of human adult stem cells and functional relevance of stem cell-derived extracellular matrix in chondrogenesis

BACKGROUND

Autologous chondrocyte implantation (ACI) has been used to treat articular cartilage defects for over two decades. Adult stem cells have been proposed as a solution to inadequate donor cell numbers often encountered in ACI. Multipotent stem/progenitor cells isolated from adipose, bone marrow, and cartilage are the most promising cell therapy candidates. However, different essential growth factors are required to induce these tissue-specific stem cells to initiate chondrogenic differentiation and subsequent deposition of extracellular matrix (ECM) to form cartilage-like tissue. Upon transplantation into cartilage defects in vivo, the levels of growth factors in the host tissue are likely to be inadequate to support chondrogenesis of these cells in situ. The contribution of stem/progenitor cells to cartilage repair and the quality of ECM produced by the implanted cells required for cartilage repair remain largely unknown. Here, we evaluated the bioactivity and chondrogenic induction ability of the ECM produced by different adult stem cells.

METHODS

Adult stem/progenitor cells were isolated from human adipose (hADSCs), bone marrow (hBMSCs), and articular cartilage (hCDPCs) and cultured for 14 days in monolayer in mesenchymal stromal cell (MSC)-ECM induction medium to allow matrix deposition and cell sheet formation. The cell sheets were then decellularized, and the protein composition of the decellularized ECM (dECM) was analyzed by BCA assay, SDS-PAGE, and immunoblotting for fibronectin (FN), collagen types I (COL1) and III (COL3). The chondrogenic induction ability of the dECM was examined by seeding undifferentiated hBMSCs onto the respective freeze-dried solid dECM followed by culturing in serum-free medium for 7 days. The expression levels of chondrogenic genes SOX9, COL2, AGN, and CD44 were analyzed by q-PCR.

RESULTS

hADSCs, hBMSCs, and hCDPCs generated different ECM protein profiles and exhibited significantly different chondrogenic effects. hADSCs produced 20-60% more proteins than hBMSCs and hCDPCs and showed a fibrillar-like ECM pattern (FN^high, COL1^high). hCDPCs produced more COL3 and deposited less FN and COL1 than the other cell types. The dECM derived from hBMSCs and hCDPCs induced spontaneous chondrogenic gene expression in hBMSCs.

CONCLUSIONS

These findings provide new insights on application of adult stem cells and stem cell-derived ECM to enhance cartilage regeneration.

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.

Tendon microstructural disruption promotes tendon-derived stem cells to express chondrogenic genes by activating endoplasmic reticulum stress

The erroneous differentiation of tendon-derived stem cells (TDSCs) into adipocytes, chondrocytes, and osteoblasts is believed to play an important role in the development of tendinopathy. However, the regulatory mechanisms of TDSC differentiation remain unclear. The aim of this study is to investigate the contribution and mechanism of the tendon microstructural disruption to the differentiation of TDSCs. Bovine Achilles tendons were sliced. The tendon slices were stretched with different tensile strains to mimic the tendon structure alteration at various scales. The TDSCs were cultured on the tendon slices. The differentiation of TDSCs and endoplasmic reticulum (ER) stress in the TDSCs were investigated with quantitative reverse transcription polymerase chain reaction, immunostaining and western blot. The effect of ER stress inhibition on chondrogenic differentiation of the TDSCs was further investigated. The structural alteration did not affect the viability of TDSCs. However, the structural alteration of tendon slices with 6.4% strain promoted TDSCs to express the chondrogenic genes. ER stress-related markers, ATF-4 and PERK, were also upregulated. With the inhibition of ER stress, the expression of ATF-4 and the chondrogenic gene SOX9 of TDSCs were inhibited. The study indicated that tendon microdamage could induce the chondrogenic differentiation of TDSCs through triggering ER stress to activate ATF-4 and SOX9 subsequently.

Mesenchymal Stem Cell Use in Acute Tendon Injury: In Vitro Tenogenic Potential vs. In Vivo Dose Response

Stem cell therapy for the treatment of tendon injury is an emerging clinical practice in the fields of human and veterinary sports medicine; however, the therapeutic benefit of intralesional transplantation of mesenchymal stem cells in tendonitis cases is not well designed. Questions persist regarding the overall tenogenic potential and efficacy of this treatment alone. In this study, we aimed to isolate a rat mesenchymal stem cell lineage for in vitro and in vivo use, to assess the effects of growth factor exposure in vitro on cell morphology, behavior, and tendon-associated glycoprotein production, and to assess the therapeutic potential of intralesional stem cells, as a function of dose, in vivo. First, rat adipose-derived (rAdMSC) and bone marrow-derived (rBMSC) stem cell lineages were isolated, characterized with flow cytometric analysis, and compared in terms of proliferation (MTS assay) and cellular viability (calcein AM staining). Rat AdMSCs displayed superior proliferation and more homogenous CD 73, CD 44H, and CD 90 expression as compared to rBMSC. Next, the tenogenic differentiation potential of the rAdMSC lineage was tested in vitro through isolated and combined stimulation with reported tenogenic growth factors, transforming growth factor (TGF)-β3 and connective tissue growth factor (CTGF). We found that the most effective tenogenic factor in terms of cellular morphologic change, cell alignment/orientation, sustained cellular viability, and tendon-associated glycoprotein upregulation was TGFβ3, and we confirmed that rAdMSC could be induced toward a tenogenic lineage in vitro. Finally, the therapeutic potential of rAdMSCs as a function of dose was assessed using a rat acute Achilles tendon injury model. Amounts of 5 × 10⁵ (low dose) and 4 × 10⁶ (high dose) were used. Subjectively, on the gross morphology, the rAdMSC-treated tendons exhibited fewer adhesions and less scar tissue than the control tendons; however, regardless of the rAdMSC dose, no significant differences in histological grade or tissue collagen I deposition were noted between the rAdMSC-treated and control tendons. Collectively, rAdMSCs exhibited appropriate stem cell markers and tenogenic potential in vitro, but the clinical efficacy of intralesional implantation of undifferentiated cells in acute tendonitis cases could not be proven. Further investigation into complementary therapeutics or specialized culture conditions prior to implantation are warranted.

Tendon tissue engineering: Current progress towards an optimized tenogenic differentiation protocol for human stem cells

Tendons are integral to our daily lives by allowing movement and locomotion but are frequently injured, leading to patient discomfort and impaired mobility. Current clinical procedures are unable to fully restore the native structure of the tendon, resulting in loss of full functionality, and the weakened tissue following repair often re-ruptures. Tendon tissue engineering, involving the combination of cells with biomaterial scaffolds to form new tendon tissue, holds promise to improve patient outcomes. A key requirement for efficacy in promoting tendon tissue formation is the optimal differentiation of the starting cell populations, most commonly adult tissue-derived mesenchymal stem/stromal cells (MSCs), into tenocytes, the predominant cellular component of tendon tissue. Currently, a lack of consensus on the protocols for effective tenogenic differentiation is hampering progress in tendon tissue engineering. In this review, we discuss the current state of knowledge regarding human stem cell differentiation towards tenocytes and tendon tissue formation. Tendon development and healing mechanisms are described, followed by a comprehensive overview of the current protocols for tenogenic differentiation, including the effects of biochemical and biophysical cues, and their combination, on tenogenesis. Lastly, a synthesis of the key features of these protocols is used to design future approaches. The holistic evaluation of current knowledge should facilitate and expedite the development of efficacious stem cell tenogenic differentiation protocols with future impact in tendon tissue engineering. STATEMENT OF SIGNIFICANCE: The lack of a widely-adopted tenogenic differentiation protocol has been a major hurdle in the tendon tissue engineering field. Building on current knowledge on tendon development and tendon healing, this review surveys peer-reviewed protocols to present a holistic evaluation and propose a pathway to facilitate and expedite the development of a consensus protocol for stem cell tenogenic differentiation and tendon tissue engineering.

Engineering a Mechanoactive Fibrous Substrate with Enhanced Efficiency in Regulating Stem Cell Tenodifferentiation

Electrospun-aligned fibers in ultrathin fineness have previously demonstrated a limited capacity in driving stem cells to differentiate into tendon-like cells. In view of the tendon's mechanoactive nature, endowing such aligned fibrous structure with mechanoactivity to exert in situ mechanical stimulus by itself, namely, without any forces externally applied, is likely to potentiate its efficiency of tenogenic induction. To test this hypothesis, in this study, a shape-memory-capable poly(l-lactide-co-caprolactone) (PLCL) copolymer was electrospun into aligned fibrous form followed by a "stretching-recovery" shape-programming procedure to impart shape memory capability. Thereafter, in the absence of tenogenic supplements, human adipose-derived stem cells (ADSCs) were cultured on the programmed fibrous substrates for a duration of 7 days, and the effects of constrained recovery resultant stress-stiffening on cell morphology, proliferation, and tenogenic differentiation were examined. The results indicate that the in situ enacted mechanical stimulus due to shape memory effect (SME) did not have adverse influence on cell viability and proliferation, but significantly promoted cellular elongation along the direction of fiber alignment. Moreover, it revealed that tendon-specific protein markers such as tenomodulin (TNMD) and tenascin-C (TNC) and gene expression of scleraxis (SCX), TNMD, TNC, and collagen I (COL I) were significantly upregulated on the mechanoactive fibrous substrate with higher recovery stress compared to the counterparts. Mechanistically, the Rho/ROCK signaling pathway was identified to be involved in the substrate self-actuation-induced enhancement in tenodifferentiation. Together, these results suggest that constrained shape recovery stress may be employed as an innovative loading modality to regulate the stem cell tenodifferentiation by presenting the fibrous substrate with an aligned tendon-like topographical cue and an additional mechanoactivity. This newly demonstrated paradigm in modulating stem cell tenodifferentiation may improve the efficacy of tendon tissue engineering strategy for tendon healing and regeneration.

Stem cell sheet interpositioned between the tendon and bone would be better for healing than stem cell sheet overlaid above the tendon-to-bone junction in rotator cuff repair of rats

Background

Although stem cells might enhance natural enthesis healing in surgical rotator cuff repair, not much attention has been given to the delivery and location of delivering stem cells. The purpose of this study to know where to locate those stem cells during repair.

Methods

Animal model of chronic rotator cuff tear was created in 24 rats. Adipose-derived stem cells were engineered as a sheet and transplanted 1) between a torn tendon and humerus (interposition group) or 2) over a repaired tendon-to-bone junction (overlay group) at the time of surgical repair. Tracking of stem cells with overexpression of green fluorescent protein (GFP) were carried out at the time of sacrifice in additional 4 shoulders in each group. Histological and Biomechanical evaluation was performed to compare the differences in tendon-to-bone healing.

Results

Histology showed increased fibrocartilage, a clear boundary at the mineralized fibrocartilage, abundant collagen type III, and higher total scores, especially in the interposition group. GFP-overexpression was observed at the transplanted site at 2 weeks after repair. Although two groups where stem cell sheets applied showed higher load to failure than the repair-only group, the load to failure was not different between the interposition and overlay group.

Conclusion

In the chronic rotator cuff repair model, stem cell sheets enhanced regeneration of the tendon-to-bone junction. This regeneration was effective when the stem cell sheet was interpositioned at the tendon-to-bone interface.

Level of evidence

Basic Science Study; In Vivo Animal Model; Histology and Biomechanics.

Evaluation of the Effects of Transforming Growth Factor-Beta 3 (TGF-β3) Loaded Nanoparticles on Healing in a Rat Achilles Tendon Injury Model

BACKGROUND

Achilles tendon (AT) midsubstance injuries may heal suboptimally, especially in athletes. Transforming growth factor-beta 3 (TGF-β3) shows promise because of its recently discovered tendinogenic effects. Using poly(lactic-co-glycolic acid)-b-poly(ethylene glycol) (PLGA-b-PEG) nanoparticles (NPs) may enhance the results by a sustained-release effect.

HYPOTHESIS

The application of TGF-β3 will enhance AT midsubstance healing, and the NP form will achieve better outcomes.

STUDY DESIGN

Controlled laboratory study.

METHODS

A total of 80 rats underwent unilateral AT transection and were divided into 4 groups: (1) control (C); (2) empty chitosan film (Ch); (3) chitosan film containing free TGF-β3 (ChT); and (4) chitosan film containing TGF-β3-loaded NPs (ChN). The animals were sacrificed at 3 and 6 weeks. Tendons were evaluated for morphology (length and cross-sectional area [CSA]), biomechanics (maximum load, stress, stiffness, and elastic modulus), histology, immunohistochemical quantification (types I and III collagen [COL1 and COL3]), and gene expression (COL1A1, COL3A1, scleraxis, and tenomodulin).

RESULTS

Morphologically, at 3 weeks, ChT (15 ± 2.7 mm) and ChN (15.6 ± 1.6 mm) were shorter than C (17.6 ± 1.8 mm) (P = .019 and = .004, respectively). At 6 weeks, the mean CSA of ChN (10.4 ± 1.9 mm²) was similar to that of intact tendons (6.4 ± 1.1 mm²) (P = .230), while the other groups were larger. Biomechanically, at 3 weeks, ChT (42.8 ± 4.9 N) had a higher maximum load than C (27 ± 9.1 N; P = .004) and Ch (29.2 ± 5.7 N; P = .005). At 6 weeks, ChN (26.9 ± 3.9 MPa) had similar maximum stress when compared with intact tendons (34.1 ± 7.8 MPa) (P = .121); the other groups were significantly lower. Histologically, at 6 weeks, the mean Movin score of ChN (4.5 ± 1.5) was lower than that of ChT (6.3 ± 1.8). Immunohistochemically, ChN had higher COL3 (1.469 ± 0.514) at 3 weeks and lower COL1 (1.129 ± 0.368) at 6 weeks. COL1A1 gene expression was higher in ChT and ChN at 3 weeks, but COL3A1 gene expression was higher in ChN.

CONCLUSION

The application of TGF-β3 had a positive effect on AT midsubstance healing, and the sustained-release NP form improved the outcomes, more specifically accelerating the remodeling process.

CLINICAL RELEVANCE

This study demonstrated the effectiveness of TGF-β3 on tendon healing on a rat model, which is an important step toward clinical use. The novel method of using PLGA-b-PEG NPs as a drug-delivery system with sustained-release properties had promising results.

The role of tendon derived stem/progenitor cells and extracellular matrix components in the bone tendon junction repair

Fibrocartilage enthesis is the junction between bone and tendon with a typical characteristics of fibrocartilage transition zones. The regeneration of this transition zone is the bottleneck for functional restoration of bone tendon junction (BTJ). Biomimetic approaches, especially decellularized extracellular matrix (ECM) materials, are strategies which aim to mimic the components of tissues to the utmost extent, and are becoming popular in BTJ healing because of their ability not only to provide scaffolds to allow cells to attach and migrate, but also to provide a microenvironment to guide stem/progenitor cells lineage-specific differentiation. However, the cellular and molecular mechanisms of those approaches, especially the ECM proteins, remain unclear. For BTJ reconstruction, fibrocartilage regeneration is the key for good integrity of bone and tendon as well as its mechanical recovery, so the components which can guide stem cells to a chondrogenic commitment in biomimetic approaches might well be the key for fibrocartilage regeneration and eventually for the better BTJ healing. In this review, we firstly discuss the importance of cartilage-like formation in the healing process of BTJ. Next, we explore the possibility of tendon-derived stem/progenitor cells as cell sources for BTJ regeneration due to their multi-differentiation potential. Finally, we summarize the role of extracellular matrix components of BTJ in guiding stem cell fate to a chondrogenic commitment, so as to provide cues for understanding the mechanisms of lineage-specific potential of biomimetic approaches as well as to inspire researchers to incorporate unique ECM components that facilitate BTJ repair into design.

Application of bone and cartilage extracellular matrices in articular cartilage regeneration

Articular cartilage has an avascular structure with a poor ability for self-repair; therefore, many challenges arise in cases of trauma or disease. It is of utmost importance to identify the proper biomaterial for tissue repair that has the capability to direct cell recruitment, proliferation, differentiation, and tissue integration by imitating the natural microenvironment of cells and transmitting an orchestra of intracellular signals. Cartilage extracellular matrix (cECM) is a complex nanostructure composed of divergent proteins and glycosaminoglycans (GAGs), which regulate many functions of resident cells. Numerous studies have shown the remarkable capacity of ECM-derived biomaterials for tissue repair and regeneration. Moreover, given the importance of biodegradability, biocompatibility, 3D structure, porosity, and mechanical stability in the design of suitable scaffolds for cartilage tissue engineering, demineralized bone matrix (DBM) appears to be a promising biomaterial for this purpose, as it possesses the aforementioned characteristics inherently. To the best of the authors' knowledge, no comprehensive review study on the use of DBM in cartilage tissue engineering has previously been published. Since so much work is needed to address DBM limitations such as pore size, cell retention, and so on, we decided to draw the attention of researchers in this field by compiling a list of recent publications. This review discusses the implementation of composite scaffolds of natural or synthetic origin functionalized with cECM or DBM in cartilage tissue engineering. Cutting-edge advances and limitations are also discussed in an attempt to provide guidance to researchers and clinicians.

Differentiation of human adipose-derived mesenchymal stem cells toward tenocyte by platelet-derived growth factor-BB and growth differentiation factor-6

Mesenchymal Stem Cells (MSCs) are important in regenerative medicine and tissue engineering and will be a very sensible choice for repair and regeneration of tendon. New biological practices, such as cellular therapy using stem cells, are promising for facilitating or expediting tendon therapy. Before using these cells clinically, it is best to check and confirm the optimal conditions for differentiation of these cells in the laboratory. Hence, in the present study, the impacts of PDGF-BB and GDF-6 supplementation on adipose-derived MSCs (ASCs) culture were studied. The frozen ASC were recovered and expanded in basic culture medium (DMEM with 10%FBS). The cells after passage five (P5) were treated with basic medium containing L-Prolin, Ascorbic Acid and only PDGF-BB or GDF-6 (20 ng/ml) or both of them (mix) as 3 groups for 14 days to investigate efficiency of ASCs differentiation towards tenocytes. The cells culturing in basic medium were used as control group. To validate tenogenic differentiation, H&E and Sirius Red staining were used to assess cell morphology and collagen production, respectively. In addition, mRNA levels of collagen I and III, Scleraxis and Tenomodulin as tenogenic markers were analyzed using qPCR. In all test groups, cells appeared slenderer, elongated cytoplasmic attributes compared to the control cells. The intensity of Sirius Red staining was significantly higher in GDF-6, PDGF-BB alone, than in group without supplements. The optical density was higher in the GDF-6 than PDGF-BB and mix-group. QPCR results showed that Col I and III gene expression was increased in all groups compared to the control. SCX expression was significantly increased only in the PDGF-BB group. TNMD mRNA expression was not significant among groups. In this study, we have corroborated that human ASCs are reactionary to tenogenic induction by GDF-6 and PDGF-BB alone or in combination. These outcomes will help greater insight into GDF-6 and PDGF-BB driven tenogenesis of ASCs and new directions of discovery in the design of ASC-based treatments for tendon healing.

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

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

Tendon-like tether formation for tongue-base advancement in an ovine model using a novel implant device intended for the surgical management of obstructive sleep apnoea

Obstructive sleep apnoea (OSA) is a serious debilitating condition with significant morbidity and mortality affecting almost one billion adults globally. The current gold standard in the non-surgical management of airway collapse is continuous positive airway pressure (CPAP). However, non-compliance leads to a high abandon rate (27-46%). While there are multiple sites of airway obstruction during sleep, the tongue base is recognized as the key player in the pathogenesis of OSA. Poor outcomes of current tongue suspension devices are due to fracture, slippage or migration of devices. Three tongue tethering device groups, namely a polydioxanone/polyurethane combination (PDO + PU) treatment group, a PDO analytical control group, and a polypropylene (PP) descriptive control group, were implanted into 22 sheep (75-85 kg) in a two-phased study. After implant times of 8, 16, and 32 weeks, sheep were serially euthanized to allow for explantation of their tongues and chins. The PDO + PU devices remodeled during the 32-week implant period into a hybrid biological tendon-like tether through the process of gradual degradation of the PDO and collagen deposition as shown by electrophoresis, histology and mechanical testing. The control PDO device degraded completely after 32 weeks and the PP devices remained intact. The hybrid biological tendon-like tether exhibited a break-strength of 60 N, thus exceeding the maximum force to overcome upper airway collapse.

The Application of Decellularized Adipose Tissue Promotes Wound Healing

BACKGROUND

Due to adipose-derived stem cells (ASCs) being easy to obtain, their rapid proliferation rate, and their multidirectional differentiation capabilities, they have been widely used in the field of regenerative medicine. With the progress of decellularized adipose tissue (DAT) and adipose tissue engineering research, the role of DAT in promoting angiogenesis has gradually been emphasized.

METHODS

We examined the biological characteristics and biosafety of DAT and evaluated the stem cell maintenance ability and promotion of growth factor secretion through conducting in vitro and in vivo studies.

RESULTS

The tested ASCs showed high rat:es of proliferation and adhered well to DAT. The expression levels of essential genes for cell stem maintenance, including OCT4, SOX2, and Nanog were low at 2-24 h and much higher at 48 and 96 h. The Adipogenic expression level of markers for ASCs proliferation including PPARγ, C/EPBα, and LPL increased from 2 to 96 h. Co-culture of ASCs and DAT increased the secretion of local growth factors, such as VEGF, PDGF-bb, bFGF, HGF, EGF, and FDGF-bb, and secretion gradually increased from 0 to 48 h. A model of full-thickness skin defects on the back of nude mice was established, and the co-culture of ASCs and DAT showed the best in vivo treatment effect.

CONCLUSION

The application of DAT promotes wound healing, and DAT combined with ASCs may be a promising material in adipose tissue engineering and regenerative medicine.

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.

Concurrent multi-lineage differentiation of mesenchymal stem cells through spatial presentation of growth factors

Severe tendon and ligament injuries are estimated to affect between 300 000 and 400 000 people annually. Surgical repairs of these injuries often have poor long-term clinical outcomes because of resection of the interfacial tissue-the enthesis-and subsequent stress concentration at the attachment site. A healthy enthesis consists of distinct regions of bone, fibrocartilage, and tendon, each with distinct cell types, extracellular matrix components, and architecture, which are important for tissue function. Tissue engineering, which has been proposed as a potential strategy for replacing this tissue, is currently limited by its inability to differentiate multiple lineages of cells from a single stem cell population within a single engineered construct. In this study, we develop a multi-phasic gelatin methacrylate hydrogel construct system for spatial presentation of proteins, which is then validated for multi-lineage differentiation towards the cell types of the bone-tendon enthesis. This study determines growth factor concentrations for differentiation of mesenchymal stem cells towards osteoblasts, chondrocytes/fibrochondrocytes, and tenocytes, which maintain similar differentiation profiles in 3D hydrogel culture as assessed by qPCR and immunofluorescence staining. Finally, it is shown that this method is able to guide heterogeneous and spatially confined changes in mesenchymal stem cell genes and protein expressions with the tendency to result in osteoblast-, fibrochondrocyte-, and tenocyte-like expression profiles. Overall, we demonstrate the utility of the culture technique for engineering other musculoskeletal tissue interfaces and provide a biochemical approach for recapitulating the bone-tendon enthesis in vitro.

Tendon-derived extracellular matrix induces mesenchymal stem cell tenogenesis via an integrin/transforming growth factor-β crosstalk-mediated mechanism

Treatment of tendon injuries is challenging. To develop means to augment tendon regeneration, we have previously prepared a soluble, low immunogenic (DNA-free), tendon extracellular matrix fraction (tECM) by urea extraction of juvenile bovine tendons, which is capable of enhancing transforming growth factor-β (TGF-β) mediated tenogenesis in human adipose-derived stem cells (hASCs). Here, we aimed to elucidate the mechanism of tECM-driven hASC tenogenic differentiation in vitro, focusing on the integrin and TGF-β/SMAD pathways. Our results showed that tECM promoted hASC proliferation and tenogenic differentiation in vitro based on tenogenesis-associated markers. tECM also induced higher expression of several integrin subunits and TGF-β receptors, and nuclear translocation of p-SMAD2 in hASCs. Pharmacological inhibition of integrin-ECM binding, focal adhesion kinase (FAK) signaling, or TGF-β signaling independently led to compromised pro-tenogenic effects of tECM and actin fiber polymerization. Additionally, integrin blockade inhibited tECM-driven TGFBR2 expression, while inhibiting TGF-β signaling decreased tECM-mediated expression of integrin α1, α2, and β1 in hASCs. Together, these findings suggest that the strong pro-tenogenic bioactivity of tECM is regulated via integrin/TGF-β signaling crosstalk. Understanding how integrins interact with signaling by TGF-β and/or other growth factors (GFs) within the tendon ECM microenvironment will provide a rational basis for an ECM-based approach for tendon repair.

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

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

Tissue Engineering for Musculoskeletal Regeneration and Disease Modeling

Musculoskeletal injuries and associated conditions are the leading cause of physical disability worldwide. The concept of tissue engineering has opened up novel approaches to repair musculoskeletal defects in a fast and/or efficient manner. Biomaterials, cells, and signaling molecules constitute the tissue engineering triad. In the past 40 years, significant progress has been made in developing and optimizing all three components, but only a very limited number of technologies have been successfully translated into clinical applications. A major limiting factor of this barrier to translation is the insufficiency of two-dimensional cell cultures and traditional animal models in informing the safety and efficacy of in-human applications. In recent years, microphysiological systems, often referred to as organ or tissue chips, generated according to tissue engineering principles, have been proposed as the next-generation drug testing models. This chapter aims to first review the current tissue engineering-based approaches that are being applied to fabricate and develop the individual critical elements involved in musculoskeletal organ/tissue chips. We next highlight the general strategy of generating musculoskeletal tissue chips and their potential in future regenerative medicine research. Exemplary microphysiological systems mimicking musculoskeletal tissues are described. With sufficient physiological accuracy and relevance, the human cell-derived, three-dimensional, multi-tissue systems have been used to model a number of orthopedic disorders and to test new treatments. We anticipate that the novel emerging tissue chip technology will continually reshape and improve our understanding of human musculoskeletal pathophysiology, ultimately accelerating the development of advanced pharmaceutics and regenerative therapies.

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.

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

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

Bi-directional modulation of cellular interactions in an in vitro co-culture model of tendon-to-bone interface

OBJECTIVES

This work aimed at studying in vitro interactions between human tendon-derived cells (hTDCs) and pre-osteoblasts (pre-OBs) that may trigger a cascade of events involved in enthesis regeneration.

MATERIALS AND METHODS

The effect of 5 osteogenic medium (OM) conditions over the modulation of hTDCs and pre-OBs towards the tenogenic and osteogenic phenotypes, respectively, was studied. Three different medium conditions were chosen for subsequently establishing a direct co-culture system in order to study the expression of bone, tendon and interface-related markers.

RESULTS

A higher matrix mineralization and ALP activity was observed in co-cultures in the presence of OM. Higher transcription levels of bone- (ALPL, RUNX2, SPP1) and interface-related genes (ACAN, COMP) were found in co-cultures. The expression of aggrecan was influenced by the presence of OM and cell-cell interactions occurring in co-culture.

CONCLUSIONS

The present work assessed both the influence of OM on cell phenotype modulation and the importance of co-culture models while promoting cell-cell interactions and the exchange of soluble factors in triggering an interface-like phenotype to potentially modulate enthesis regeneration.

Tenogenically differentiated adipose-derived stem cells are effective in Achilles tendon repair in vivo

The purpose of this study was to characterize rat adipose-derived stem cells, induce adipose-derived stem cell tenogenesis, and analyze adipose-derived stem cell effects on tendon repair in vivo. Adipose-derived stem cells demonstrated an immunomodulatory, pro-angiogenic, and pro-proliferatory profile in vitro. Tenogenesis was induced for 1, 7, 14, and 21 days with 24 combinations of growth differentiation factor-5, 6, and 7 and platelet-derived growth factor–BB. Adipose-derived stem cells expression of scleraxis and collagen type I increased the most after 14 days of induction with growth differentiation factor-6 and platelet-derived growth factor–BB. Achilles excision defects injected with hydrogel alone (Gp2), with undifferentiated (Gp3) adipose-derived stem cells, or tenogenically differentiated (Gp4) adipose-derived stem cells exhibited improved tissue repair compared with untreated tendons (Gp1). Addition of adipose-derived stem cells improved tissue cytoarchitecture and increased expression of collagen type I and III, scleraxis, and tenomodulin. Adipose-derived stem cells significantly improved biomechanical properties (ultimate load and elastic toughness) over time more than hydrogel alone, while tenogenically differentiated adipose-derived stem cells improved the mean histological score and collagen fiber dispersion range closest to normal tendon. In addition, tendon sections treated with GFP-adipose-derived stem cells exhibited green fluorescence and positive GFP immunostaining on microscopy confirming the in vivo survival of adipose-derived stem cells that were injected into tendon defects to support the effects of adipose-derived stem cells on tissue up to 4.5 weeks post injury.

Adipose Stem Cell Translational Applications: From Bench-to-Bedside

During the last five years, there has been a significantly increasing interest in adult adipose stem cells (ASCs) as a suitable tool for translational medicine applications. The abundant and renewable source of ASCs and the relatively simple procedure for cell isolation are only some of the reasons for this success. Here, we document the advances in the biology and in the innovative biotechnological applications of ASCs. We discuss how the multipotential property boosts ASCs toward mesenchymal and non-mesenchymal differentiation cell lineages and how their character is maintained even if they are combined with gene delivery systems and/or biomaterials, both in vitro and in vivo.

Tendon-Derived Stem Cell Differentiation in the Degenerative Tendon Microenvironment

Tendinopathy is prevalent in athletic and many occupational populations; nevertheless, the pathogenesis of tendinopathy remains unclear. Tendon-derived stem cells (TDSCs) were regarded as the key culprit for the development of tendinopathy. However, it is uncertain how TDSCs differentiate into adipocytes, chondrocytes, or osteocytes in the degenerative microenvironment of tendinopathy. So in this study, the regulating effects of the degenerative tendon microenvironment on differentiation of TDSCs were investigated. TDSCs were isolated from rat Achilles tendons and were grown on normal and degenerative (prepared by stress-deprived culture) decellularized tendon slices (DTSs). Immunofluorescence staining, H&E staining, real-time PCR, and Western blot were used to delineate the morphology, proliferation, and differentiation of TDSCs in the degenerative microenvironment. It was found that TDSCs were much more spread on the degenerative DTSs than those on normal DTSs. The tenocyte-related markers, COL1 and TNMD, were highly expressed on normal DTSs than the degenerative DTSs. The expression of chondrogenic and osteogenic markers, COL2, SOX9, Runx2, and ALP, was higher on the degenerative DTSs compared with TDSCs on normal DTSs. Furthermore, phosphorylated FAK and ERK1/2 were reduced on degenerative DTSs. In conclusion, this study found that the degenerative tendon microenvironment induced TDSCs to differentiate into chondrogenic and osteogenic lineages. It could be attributed to the cell morphology changes and reduced FAK and ERK1/2 activation in the degenerative microenvironment of tendinopathy.

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

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

Biologics and stem cell-based therapies for rotator cuff repair

The rotator cuff is composed of several distinct muscles and tendons that function in concert to coordinate shoulder motion. Injuries to these tendons frequently result in permanent dysfunction and persistent pain. Despite considerable advances in operation techniques, surgical repair alone still does not fully restore rotator cuff function. This review focuses on recent research in the use of biologics and stem cell-based therapies to augment repair, highlighting promising avenues for future work and remaining challenges. While a number of animal models are used for rotator cuff studies, the anatomy of the rotator cuff varies dramatically between species. Since the rodent rotator cuff shares the most anatomical features with the human, this review will focus primarily on rodent models to enable consistent interpretation of outcome measures.

Nanoscaled and microscaled parallel topography promotes tenogenic differentiation of ASC and neotendon formation in vitro

Background

Topography at different scales plays an important role in directing mesenchymal stem cell differentiation including adipose-derived stem cells (ASCs) and the differential effect remains to be investigated.

Purpose

This study aimed to investigate the similarity and difference between micro- and nanoscaled aligned topography for inducing tenogenic differentiation of human ASCs (hASCs).

Methods

Parallel microgrooved PDMS membrane and a parallel aligned electrospun nanofibers of gelatin/poly-ε-caprolactone mixture were employed as the models for the study.

Results

Aligned topographies of both microscales and nanoscales could induce an elongated cell shape with parallel alignment, as supported by quantitative cell morphology analysis (cell area, cell body aspect, and cell body major axis angle). qPCR analysis also demonstrated that the aligned topography at both scales could induce the gene expressions of various tenogenic markers at the 7th day of in vitro culture including tenomodulin, collagen I and collagen VI, decorin, tenascin-C and biglycan, but with upregulated expression of scleraxis and tenascin-C only in microscaled topography. Additionally, tenogenic differentiation at the 3rd day was confirmed only at microscale. Furthermore, microscaled topography was confirmed for its tenogenic induction at tissue level as neotendon tissue was formed with the evidence of mature type I collagen fibers only in parallel aligned polyglycolic acid (PGA) microfibers after in vitro culture with mouse ASCs. Instead, only fat tissue was formed in random patterned PGA microfibers.

Conclusion

Both microscaled and nanoscaled aligned topographies could induce tenogenic differentiation of hASCs and micro-scaled topography seemed better able to induce elongated cell shape and stable tenogenic marker expression when compared to nanoscaled topography. The microscaled inductive effect was also confirmed at tissue level by neotendon formation in vitro.

Electrospun Heparin-Loaded Core-Shell Nanofiber Sutures for Achilles Tendon Regeneration In Vivo

Achilles tendon reconstruction surgery is the primary clinical method for repairing acute Achilles tendon ruptures. However, the efficacy of the postoperative healing process and the recovery of physiological function are inadequate. This study examines the healing mechanism of ruptured rat Achilles tendons seamed with heparin-loaded core-shell fiber sutures fabricated via near-field electrospinning. High-heparin-concentration sutures (PPH3.0) perform better than the low-heparin-concentration sutures and commercial sutures (CSs). The PPH3.0 suture recruits fewer inflammatory cells and shows good histocompatibility in peritoneal implantation experiments. Staining of the Achilles tendon rupture repair zone demonstrates that a high heparin concentration in sutures reduces immune-inflammatory responses. Immunohistochemical analysis reveals that the transforming growth factor-β staining scores of the PPH3.0 sutures are not significantly different from those of the corresponding control group but are significantly different from those of the CSs and non-heparin-loaded-suture groups. According to vascular endothelial growth factor (VEGF) analysis, the concentration of VEGF in the group treated with the PPH3.0 suture increases by 37.5% compared with that in its control group. No significant difference in tension strength is observed between the PPH3.0 group and healthy Achilles tendons. These findings illustrate that this novel method effectively treats Achilles tendon rupture and promotes healing and regeneration.

Housekeeping Gene Stability in Human Mesenchymal Stem and Tendon Cells Exposed to Tenogenic Factors

The use of biochemical inducers of mesenchymal stem cell (MSC) differentiation into tenogenic lineage represents an investigated aspect of tendon disorder treatment. Bone morphogenetic protein 12 (BMP-12) is a widely studied factor, representing along with ascorbic acid (AA) and basic fibroblast growth factor (bFGF) one of the most promising stimulus in this context so far. Quantitative gene expression of specific tenogenic marker is commonly used to assess the efficacy of these supplements. Nevertheless, the reliability of these data is strongly associated with the choice of stable housekeeping genes. To date, no published studies have evaluated the stability of housekeeping genes in MSCs during tenogenic induction. Three candidate housekeeping genes (YWHAZ, RPL13A, and GAPDH) in human MSCs from bone marrow (BMSCs), adipose tissue (ASCs), and tendon cells (TCs) supplemented with BMP-12 or AA and bFGF in comparison with control untreated cells for 3 and 10 days were evaluated. GeNorm, NormFinder, and BestKeeper tools and the comparative ΔCt method were used to evaluate housekeeping gene stability and the overall ranking was determined by using by the RefFinder algorithm. In all culture conditions, YWHAZ was the most stable gene and RPL13A was the second choice. YWHAZ and RPL13A were the two most stable genes also for ASCs and BMSCs, regardless of the time point analyzed, and for TCs at 10 days of tenogenic induction. Only for TCs at 3 days of tenogenic induction were GAPDH and YWHAZ the best performers. In conclusion, our findings will be useful for the proper selection of housekeeping genes in studies involving MSCs cultured in the presence of tenogenic factors, to obtain accurate and high-quality data from quantitative gene expression analysis.

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.

Regenerative Engineering of the Rotator Cuff of the Shoulder

Rotator cuff tears often heal poorly, leading to re-tears after repair. This is in part attributed to the low proliferative ability of the resident cells (tendon fibroblasts and tendon-stem cells) upon injury to the rotator cuff tissue and the low vascularity of the tendon insertion. In addition, surgical outcomes of current techniques used in clinical settings are often suboptimal, leading to the formation of neo-tissue with poor biomechanics and structural characteristics, which results in re-tears. This has prompted interest in a new approach, which we term as "Regenerative Engineering", for regenerating rotator cuff tendons. In the Regenerative Engineering paradigm, roles played by stem cells, scaffolds, growth factors/small molecules, the use of local physical forces, and morphogenesis interplayed with clinical surgery techniques may synchronously act, leading to synergistic effects and resulting in successful tissue regeneration. In this regard, various cell sources such as tendon fibroblasts and adult tissue-derived stem cells have been isolated, characterized, and investigated for regenerating rotator cuff tendons. Likewise, numerous scaffolds with varying architecture, geometry, and mechanical characteristics of biologic and synthetic origin have been developed. Furthermore, these scaffolds have been also fabricated with biochemical cues (growth factors and small molecules), facilitating tissue regeneration. In this Review, various strategies to regenerate rotator cuff tendons using stem cells, advanced materials, and factors in the setting of physical forces under the Regenerative Engineering paradigm are described.

Advances of stem cell based-therapeutic approaches for tendon repair

Tendon injuries are significant clinical problems. Current treatments often result in incomplete repair or healing, which may lead to reduced function and rupture. Stem cell-based therapy is a promising intervention for tendon repair. In this article, we attempt to provide a brief overview on the recent progress in the field, current understanding of the underlying mechanisms of the approach, and the potential of stem cell-based therapies beyond cell implantation. We conclude the review by sharing our viewpoints on the challenges, opportunities, and future directions of this approach. The translational potential of this article: This paper reviews recent progress on stem cell-based therapeutic approaches for tendon repair, which highlights its translational potential and challenges.

Current concepts on tenogenic differentiation and clinical applications

Tendon is a tissue that transmits force from muscle to bone. Chronic or acute tendon injuries are very common, and are always accompanied by pain and a limited range of motion in patients. In clinical settings, management of tendon injuries still remains a big challenge. Cell therapies, such as the application of stem cells for tenogenic differentiation, were suggested to be an ideal strategy for clinical translation. However, there is still a lack of specific methods for tenogenic differentiation due to the limited understanding of tendon biology currently. This review focuses on the summary of current published strategies for tenogenic differentiation, such as the application of growth factors, mechanical stimulation, biomaterials, coculture, or induced pluripotent stem cells. Current clinical applications of stem cells for treatment of tendon injuries and their limitations have also been discussed in this review.