The proliferative response of isolated human tendon fibroblasts to cyclic biaxial mechanical strain

Addressing muscle-tendon imbalances in adult male athletes with personalized exercise prescription based on tendon strain

PURPOSE

Imbalances of muscle strength and tendon stiffness can increase the operating strain of tendons and risk of injury. Here, we used a new approach to identify muscle-tendon imbalances and personalize exercise prescription based on tendon strain during maximum voluntary contractions (εmax) to mitigate musculotendinous imbalances in male adult volleyball athletes.

METHODS

Four times over a season, we measured knee extensor strength and patellar tendon mechanical properties using dynamometry and ultrasonography. Tendon micromorphology was evaluated through an ultrasound peak spatial frequency (PSF) analysis. While a control group (n = 12) continued their regular training, an intervention group (n = 10) performed exercises (3 × /week) with personalized loads to elicit tendon strains that promote tendon adaptation (i.e., 4.5-6.5%).

RESULTS

Based on a linear mixed model, εmax increased significantly in the control group over the 9 months of observation (pCon = 0.010), while there was no systematic change in the intervention group (pInt = 0.575). The model residuals of εmax, as a measure of imbalances in muscle-tendon adaptation, demonstrated a significant reduction over time exclusively in the intervention group (pInt = 0.007). While knee extensor muscle strength increased in both groups by ~ 8% (pCon < 0.001, pInt = 0.064), only the intervention group showed a trend toward increased normalized tendon stiffness (pCon = 0.824, pInt = 0.051). PSF values did not change significantly in either group (p > 0.05).

CONCLUSION

These results suggest that personalized exercise prescription can reduce muscle-tendon imbalances in athletes and could provide new opportunities for tendon injury prevention.

Unraveling the link: locomotor activity exerts a dual role in predicting Achilles tendon healing and boosting regeneration in mice

Introduction

Tendon disorders present significant challenges in the realm of musculoskeletal diseases, affecting locomotor activity and causing pain. Current treatments often fall short of achieving complete functional recovery of the tendon. It is crucial to explore, in preclinical research, the pathways governing the loss of tissue homeostasis and its regeneration. In this context, this study aimed to establish a correlation between the unbiased locomotor activity pattern of CRL:CD1 (ICR) mice exposed to uni- or bilateral Achilles tendon (AT) experimental injuries and the key histomorphometric parameters that influence tissue microarchitecture recovery.

Methods

The study involved the phenotyping of spontaneous and voluntary locomotor activity patterns in male mice using digital ventilated cages (DVC®) with access to running wheels either granted or blocked. The mice underwent non-intrusive 24/7 long-term activity monitoring for the entire study period. This period included 7 days of pre-injury habituation followed by 28 days post-injury.

Results and discussion

The results revealed significant variations in activity levels based on the type of tendon injury and access to running wheels. Notably, mice with bilateral lesions and unrestricted wheel access exhibited significantly higher activity after surgery. Extracellular matrix (ECM) remodeling, including COL1 deposition and organization, blood vessel remodeling, and metaplasia, as well as cytological tendon parameters, such as cell alignment and angle deviation were enhanced in surgical (bilateral lesion) and husbandry (free access to wheels) groups. Interestingly, correlation matrix analysis uncovered a strong relationship between locomotion and microarchitecture recovery (cell alignment and angle deviation) during tendon healing. Overall, this study highlights the potential of using mice activity metrics obtained from a home-cage monitoring system to predict tendon microarchitecture recovery at both cellular and ECM levels. This provides a scalable experimental setup to address the challenging topic of tendon regeneration using innovative and animal welfare-compliant strategies.

The cellular responses of corneal fibroblasts to cyclic stretching loads

Mechanical signaling plays a crucial role in maintaining extracellular matrix (ECM) homeostasis in various structures. In this study, we investigated the responses of corneal fibroblasts to cyclic stretching loads using an in vitro cell culture system. Bovine corneal fibroblasts were cultured and subjected to equibiaxial cyclic strain of 15% for 72 h at a frequency of 0.25 Hz, with bovine skin fibroblasts used as a comparison. We explored various cellular behaviors, including morphological changes, cell proliferation, and metabolism in response to mechanical stretching loads. The expression of genes, protein secretion, and enzymatic activity for several major metalloproteinases was also determined through Q-PCR, Western blot, and gel zymography. Additionally, we investigated the involvement of mitogen-activated protein kinases (MAPKs) signaling pathways in the corneal fibroblasts when subjected to mechanical stimuli. Our findings revealed that, compared to skin fibroblasts, corneal fibroblasts were reluctant to morphological changes in response to a prolonged (72 h) and high-amplitude (15% of strain) cyclic stretching load. However, cyclic stretching loads stimulated the upregulation of MMP-2 expression in corneal fibroblasts via the MAPK signaling pathways involving extracellular signal-regulated kinase and p38. Together with a lack of upregulation in type I collagen expression, our results indicate the induction of the ECM degradation process in corneal fibroblasts in response to cyclic stretching. These findings emphasize the mechanoresponsive nature of corneal fibroblasts and shed light on the potential impact of intense mechanical stress on the cornea in both normal and pathological conditions such as keratoconus, providing valuable insights for understanding corneal mechanobiology.

Quantification of patellar tendon strain and opportunities for personalized tendon loading during back squats

Tendon strain during exercise is a critical regulatory factor in tendon adaptive responses and there are indications for an optimal range of strain that promotes tendon adaptation. Back squats are used to improve patellar tendon properties in sport and clinical settings. To date, the operating patellar tendon strain during back squats is unknown and current recommendations for individual exercise loading are based on the one repetition maximum (1RM). Here, we quantified patellar tendon strain during loaded back squats at 40, 60 and 80% of the 1RM and during maximum isometric knee extension contractions (MVC) using ultrasonography. Kinematics, ground reaction forces and muscle electromyographic activity were also recorded. Additionally, maximum tendon strain during the MVC and the percentage of 1RM were used as explanatory variables to estimate the individual patellar tendon strain during the squats. Strain increased with increasing 1RM loading (4.7 to 8.2%), indicating that already medium-loading back squats may provide a sufficient stimulus for tendon adaptation. The individual variability was, however, too high to generalize these findings. Yet, there was a high agreement between the individually estimated and measured patellar tendon strain (R² = 0.858) during back squats. We argue that this approach may provide new opportunities for personalized tendon exercise.

Promotion of Melanoma Cell Proliferation by Cyclic Straining through Regulatory Morphogenesis

The genotype and phenotype of acral melanoma are obviously different from UV-radiation-induced melanoma. Based on the clinical data, mechanical stimulation is believed to be a potential cause of acral melanoma. In this case, it is desirable to clarify the role of mechanical stimulation in the progression of acral melanoma. However, the pathological process of cyclic straining that stimulates acral melanoma is still unclear. In this study, the influence of cyclic straining on melanoma cell proliferation was analyzed by using a specifically designed cell culture system. In the results, cyclic straining could promote melanoma cell proliferation but was inefficient after the disruption of cytoskeleton organization. Therefore, the mechanotransduction mechanism of promoted proliferation was explored. Both myosin and actin polymerization were demonstrated to be related to cyclic straining and further influenced the morphogenesis of melanoma cells. Additionally, the activation of mechanosensing transcription factor YAP was related to regulatory morphogenesis. Furthermore, expression levels of melanoma-involved genes were regulated by cyclic straining and, finally, accelerated DNA synthesis. The results of this study will provide supplementary information for the understanding of acral melanoma.

In Vitro Cellular Strain Models of Tendon Biology and Tenogenic Differentiation

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

Bioinspired Scaffold Designs for Regenerating Musculoskeletal Tissue Interfaces

The musculoskeletal system works at a very advanced level of synchrony, where all the physiological movements of the body are systematically performed through well-organized actions of bone in conjunction with all the other musculoskeletal soft tissues, such as ligaments, tendons, muscles, and cartilage through tissue-tissue interfaces. Interfaces are structurally and compositionally complex, consisting of gradients of extracellular matrix components, cell phenotypes as well as biochemical compositions and are important in mediating load transfer between the distinct orthopedic tissues during body movement. When an injury occurs at interface, it must be re-established to restore its function and stability. Due to the structural and compositional complexity found in interfaces, it is anticipated that they presuppose a concomitant increase in the complexity of the associated regenerative engineering approaches and scaffold designs to achieve successful interface regeneration and seamless integration of the engineered orthopedic tissues. Herein, we discuss the various bioinspired scaffold designs utilized to regenerate orthopedic tissue interfaces. First, we start with discussing the structure-function relationship at the interface. We then discuss the current understanding of the mechanism underlying interface regeneration, followed by discussing the current treatment available in the clinic to treat interface injuries. Lastly, we comprehensively discuss the state-of-the-art scaffold designs utilized to regenerate orthopedic tissue interfaces.

Biomaterials in Tendon and Skeletal Muscle Tissue Engineering: Current Trends and Challenges

Tissue engineering is a promising approach to repair tendon and muscle when natural healing fails. Biohybrid constructs obtained after cells&rsquo; seeding and culture in dedicated scaffolds have indeed been considered as relevant tools for mimicking native tissue, leading to a better integration in vivo. They can also be employed to perform advanced in vitro studies to model the cell differentiation or regeneration processes. In this review, we report and analyze the different solutions proposed in literature, for the reconstruction of tendon, muscle, and the myotendinous junction. They classically rely on the three pillars of tissue engineering, i.e., cells, biomaterials and environment (both chemical and physical stimuli). We have chosen to present biomimetic or bioinspired strategies based on understanding of the native tissue structure/functions/properties of the tissue of interest. For each tissue, we sorted the relevant publications according to an increasing degree of complexity in the materials&rsquo; shape or manufacture. We present their biological and mechanical performances, observed in vitro and in vivo when available. Although there is no consensus for a gold standard technique to reconstruct these musculo-skeletal tissues, the reader can find different ways to progress in the field and to understand the recent history in the choice of materials, from collagen to polymer-based matrices.

In vivo heterotopic culturing of prefabricated tendon grafts with mechanical stimulation in a sheep model

BACKGROUND

The goal of this study is to investigate the biomechanical and histological properties of in vivo heterotopically prefabricated cruciate ligament replacement grafts with and without mechanical stimulation. The clinical goal is to heterotopically prefabricate a bone-tendon-bone graft for anterior cruciate ligament reconstruction, which allows rapid ingrowth and early full weight bearing.

METHODS

In a sheep model, eight quadriceps tendon grafts were harvested and introduced into culture chambers at their proximal and distal ends. In group S, four tendon-chamber constructs were mechanically stimulated by direct attachment to the quadriceps tendon and patella. In group NS, the same constructs were cultured without proximal attachment. All sheep were sacrificed six weeks postoperatively and the constructs were examined biomechanically and histologically. The healthy contralateral ACL and quadriceps tendon were used as controls.

RESULTS

Macroscopically, no obvious ossification could be observed at the ends of the tendon-chamber constructs six weeks postoperatively. Histologically, the tendon tissue from the mechanically stimulated constructs revealed higher counts of cells and capillaries. However, there was less regular cell distribution and collagen fiber orientation compared to the control group. In addition, osteoblasts and osteogenesis were observed in the prefabricated constructs both with and without mechanical stimulation. Biomechanically, there were no significant differences in stiffness, elongation and ultimate failure load between the groups.

CONCLUSION

In vivo heterotopic culture of prefabricated tendon grafts may have the potential to stimulate osteoblasts and induce osteogenesis. Future studies with longer follow-up and modifications of the surgical technique and culture conditions are desirable.

Osteopathic Manipulative Treatment in Surgical Care: Short Review of Research Publications in Osteopathic Journals During the Period 1990 to 2017

Background.

A growing trend in surgical care is the investigation and incorporation of multimodal interventions into standardized programs. Additionally, manual therapies such as osteopathic manipulative treatment (OMT) are being used with patients in surgical care. Yet the scientific dialogue and the use of OMT in surgical care are currently insubstantial.

Objective.

The aim of this study was to present an overview of published research articles within the subject field of OMT in surgical care.

Method.

Summative review of peer-reviewed research articles published in osteopathic journals during the period 1990 to 2017. In total, 10 articles were identified.

Result.

Previous research has been conducted within the areas of abdominal, thoracic, gynecological, and/or orthopedic surgery with measured outcomes such as pain, analgesia consumption, length of hospital stay, and range of motion. Heterogeneity was identified in usage of osteopathic techniques, treatment duration, and occurrence, as well as in the treating osteopath’s experience.

Conclusion.

Despite the small number of research articles within this field, both positive measured effects as well as the absence of such effects were identified. Overall, there was a heterogeneity concerning surgical contexts, diagnoses, signs and symptoms, as well as surgical phases in current interprofessional osteopathic publications. In this era of multimodal surgical care, we argue that there is an urgent need to evaluate OMT in this context of care and with a proper research approach.

The effect of estrogen on tendon and ligament metabolism and function

Tendons and ligaments are crucial structures inside the musculoskeletal system. Still many issues in the treatment of tendon diseases and injuries have yet not been resolved sufficiently. In particular, the role of estrogen-like compound (ELC) in tendon biology has received until now little attention in modern research, despite ELC being a well-studied and important factor in the physiology of other parts of the musculoskeletal system. In this review we attempt to summarize the available information on this topic and to determine many open questions in this field.

Cyclic stretch promotes osteogenesis-related gene expression in osteoblast-like cells through a cofilin-associated mechanism

Osteoblasts have the capacity to perceive and transduce mechanical signals, and thus, regulate the mRNA and protein expression of a variety of genes associated with osteogenesis. Cytoskeletal reconstruction, as one of the earliest perception events for external mechanical stimulation, has previously been demonstrated to be essential for mechanotransduction in bone cells. However, the mechanism by which mechanical signals induce cytoskeletal deformation remains poorly understood. The actin‑binding protein, cofilin, promotes the depolymerization of actin and is understood to be important in the regulation of activities in various cell types, including endothelial, neuronal and muscle cells. However, to the best of our knowledge, the importance of cofilin in osteoblastic mechanotransduction has not been previously investigated. In the present study, osteoblast‑like MG‑63 cells were subjected to physiological cyclic stretch stimulation (12% elongation) for 1, 4, 8, 12 and 24 h, and the expression levels of cofilin and osteogenesis-associated genes were quantified with reverse transcription‑quantitative polymerase chain reaction, immunofluorescence staining and western blotting analyses. Additionally, knockdown of cofilin using RNA interference was conducted, and the mRNA levels of osteogenesis‑associated genes were compared between osteoblast‑like cells in the presence and absence of cofilin gene knockdown. The results of the present study demonstrated that cyclic stretch stimulates the expression of genes associated with osteoblastic activities in MG‑63 cells, including alkaline phosphatase (ALP), osteocalcin (OCN), runt‑related transcription factor 2 (Runx2) and collagen‑1 (COL‑1). Cyclic stretch also regulates the mRNA and protein expression of cofilin in MG‑63 cells. Furthermore, stretch‑induced increases in the levels of osteogenesis-associated genes, including ALP, OCN, Runx2 and COL‑1, were reduced following cofilin gene knockdown. Together, these results demonstrate that cofilin is involved in the regulation of mechanical load‑induced osteogenesis and, to the best of our knowledge, provides the first evidence demonstrating the importance of cofilin in osteoblastic mechanotransduction.

Cyclic mechanical stimulation rescues achilles tendon from degeneration in a bioreactor system

Physiotherapy is one of the effective treatments for tendinopathy, whereby symptoms are relieved by changing the biomechanical environment of the pathological tendon. However, the underlying mechanism remains unclear. In this study, we first established a model of progressive tendinopathy-like degeneration in the rabbit Achilles. Following ex vivo loading deprivation culture in a bioreactor system for 6 and 12 days, tendons exhibited progressive degenerative changes, abnormal collagen type III production, increased cell apoptosis, and weakened mechanical properties. When intervention was applied at day 7 for another 6 days by using cyclic tensile mechanical stimulation (6% strain, 0.25 Hz, 8 h/day) in a bioreactor, the pathological changes and mechanical properties were almost restored to levels seen in healthy tendon. Our results indicated that a proper biomechanical environment was able to rescue early-stage pathological changes by increased collagen type I production, decreased collagen degradation and cell apoptosis. The ex vivo model developed in this study allows systematic study on the effect of mechanical stimulation on tendon biology.

Comparative analysis of gene expression in normal and degenerative human tendon cells: effects of cyclic strain

BACKGROUND

Tendinopathy is a clinical problem for which treatment shows mixed results and treatment options are limited. Gene expression signatures early in the mechanotransduction pathway can accurately predict risk and correlate with different clinical outcomes. Studies aimed at elucidating the molecular mechanisms of tendinopathy have focused on small cohorts of genes that show an incomplete picture of the degeneration process. This study compared the effect of cyclic strain on gene expression in tendon cells from normal tendon and chronically painful areas of tendinopathy in 3 patients.

METHODS

We measured a panel of mechanotransduction genes and cytoskeletal tensional balance with and without cyclic strain, which disrupts connective tissue synthetic-degradative equilibrium. Normal and degenerative tendons were obtained from patients undergoing surgery to treat chronic painful tendinopathy. A cyclic strain model was established to measure cytoskeletal tensional homeostasis.

RESULTS

Prior to cyclic strain, the normal tendon cells exhibited varying patterns of elevated expression of 7 genes compared with degenerative tendon cells. In response to cyclic strain, gene expression of COL1A1, ITGA6, CTNNA1, and CLEC3B was up-regulated in normal tendon cells. Cyclic strain had no effect on degenerative tendon cells. Cyclic strain exacerbated the inhibition of protein synthesis in both cell types, especially in the degenerative tendon cells.

CONCLUSION

Alterations in the pattern of gene expression are suggestive of a dynamic equilibrium between synthesis and degradation, whereby cell adhesion molecules are predominantly up-regulated to facilitate cellular reorientation in response to their altered functional environment.

CLINICAL RELEVANCE

These data might have future applications, including the identification of markers for early diagnosis, targets for drug design, and indicators for treatment responsiveness and prognosis.

Low-frequency pulsed electromagnetic fields significantly improve time of closure and proliferation of human tendon fibroblasts

Background

The promotion of the healing process following musculoskeletal injuries comprises growth factor signalling, migration, proliferation and apoptosis of cells. If these processes could be modulated, the healing of tendon tissue may be markedly enhanced. Here, we report the use of the Somagen™ device, which is certified for medical use according to European laws. It generates low-frequency pulsed electromagnetic fields that trigger effects of a nature that are yet to be determined.

Methods

A 1.5-cm wide, linear scrape was introduced into patellar tendon fibroblast cultures (N = 5 donors). Treatment was carried out every second day. The regimen was applied three times in total with 30 minutes comprising pulsed electromagnetic field packages with two fundamental frequencies (10 minutes of 33 Hz, 20 minutes of 7.8 Hz). Control cells remained untreated. All samples were analyzed for gap closure time, proliferation and apoptosis one week after induction of the scrape wound.

Results

The mean time for bridging the gap in the nontreated cells was 5.05 ± 0.33 days, and in treated cells, it took 3.35 ± 0.38 days (P <0.001). For cell cultures with scrape wounds, a mean value for BrdU incorporation of OD = 0.70 ± 0.16 was found. Whereas low-frequency pulsed electromagnetic fields treated samples showed OD = 1.58 ± 0.24 (P <0.001). However, the percentage of apoptotic cells did not differ between the two groups.

Conclusions

Our data demonstrate that low-frequency pulsed electromagnetic fields emitted by the Somagen™ device influences the in vitro wound healing of patellar tendon fibroblasts and, therefore, possibly increases wound healing potential.

Super-paramagnetic responsive nanofibrous scaffolds under static magnetic field enhance osteogenesis for bone repair in vivo

A novel nanofibrous composite scaffold composed of super-paramagnetic γ-Fe₂O₃ nanoparticles (MNP), hydroxyapatite nanoparticles (nHA) and poly lactide acid (PLA) was prepared using electrospinning technique. The scaffold well responds extern static magnetic field with typical saturation magnetization value of 0.049 emu/g as well as possesses nanofibrous architecture. The scaffolds were implanted in white rabbit model of lumbar transverse defects. Permanent magnets are fixed in the rabbit cages to provide static magnetic field for the rabbits post surgery. Results show that MNP incorporated in the nanofibers endows the scaffolds super-paramagnetic responsive under the applied static magnetic field, which accelerates new bone tissue formation and remodeling in the rabbit defect. The scaffold also exhibits good compatibility of CK, Cr, ALT and ALP within normal limits in the serum within 110 days post implantation. In conclusion, the super-paramagnetic responding scaffold with applying of external magnetic field provides a novel strategy for scaffold-guided bone repair.

Mechanical strain using 2D and 3D bioreactors induces osteogenesis: implications for bone tissue engineering

Fracture healing is a complicated process involving many growth factors, cells, and physical forces. In cases, where natural healing is not able, efforts have to be undertaken to improve healing. For this purpose, tissue engineering may be an option. In order to stimulate cells to form a bone tissue several factors are needed: cells, scaffold, and growth factors. Stem cells derived from bone marrow or adipose tissues are the most useful in this regard. The differentiation of the cells can be accelerated using mechanical stimulation. The first part of this chapter describes the influence of longitudinal strain application. The second part uses a sophisticated approach with stem cells on a newly developed biomaterial (Sponceram) in a rotating bed bioreactor with the administration of bone morphogenetic protein-2. It is shown that such an approach is able to produce bone tissue constructs. This may lead to production of larger constructs that can be used in clinical applications.

Tenocyte activation and regulation of complement factors in response to in vitro cell injury

Inferior tendon healing can lead to scarring and tendinopathy. The role of complement in tendon healing is still unclear. The aim of this study was to understand tenocytes response to mechanical injury and whether complement is regulated by injury. Tenocytes were injured using an optimized automated scratch assay model. Using a self-assembled plotter system, 50 parallel lines of injury were created in a 6 cm diameter tenocyte cell layer. Tenocytes mitotic activity and survival post injury was assessed using FDA/ethidiumbromide assay. Furthermore, this injury model was combined with stimulation of the tenocytes with the complement split fragment C3a. Gene expression of C3aR, C5aR (CD88), CD46, CD55, tumor necrosis factor (TNF)α, interleukin (IL)-1β, matrix metalloproteinase (MMP)-1 was analyzed. Immunolabeling for C5aR and CD55 was performed. An enhanced mitotic activity and some dead cells were detected in the vicinity of the scratches. Gene expression of the C3aR was suppressed after 4 h but induced after 24 h post injury. C5aR was down-regulated at 24 h, CD46 and CD55 were induced at 24 h in response to injury and CD55 was also elevated at 4 h. MMP-1 was upregulated by injury but both proinflammatory cytokines remained mainly unaffected. Combination of injury with C3a stimulation led to an enhanced C3aR, CD55 and TNFα gene expression. According to the gene expression data, the protein expression of C5aR was reduced and that of CD55 induced. In summary, a specific response of complement regulation was found in mechanically injured tenocytes which may be involved in healing responses.

Cyclic strain-induced cytoskeletal rearrangement of human periodontal ligament cells via the Rho signaling pathway

Background

Although mechanical stimulations are known have a significant impact on cytoskeletal rearrangement, little is known regarding the behavioral alteration of human periodontal ligament cells (hPDLCs) under cyclic strain. The aim of this study was to elucidate the role of the Rho signaling pathway on cyclic strain-induced cytoskeletal rearrangement of hPDLCs.

Methods

Healthy hPDLCs obtained from teeth extracted for orthodontic purposes were subjected to cyclic strain with physiological loading (10%) at a frequency of 0.1 Hz for 6 h or 24 h using a FX-5000T system. Changes in cell morphology were examined by phase-contrast microscopy, while F-actin reorganization was observed by phalloidin staining and confocal microscopy. Protein expression was analyzed through western blot analysis.

Results

Significant enhancement of cytoskeletal reorganization was observed following exposure to the cyclic strain. In addition, a significant increase was noted in the expression levels of GTP-Rho, Rho-associated protein kinase (ROCK) and p-cofilin, whereas the expression levels of Rho GDP dissociation inhibitors alpha (Rho-GDIa) were reduced in the hPDLCs, compared with the static control cells. More importantly, the Rock inhibitor Y-27632 suppressed cyclic strain-induced cytoskeletal rearrangement of hPDLCs. Additionally, Y-27632 and overexpression of Rho-GDIa were found to lower p-cofilin protein expressions under cyclic strain, while Rho-GDIa siRNA transfection had the opposite effect on the hPDLCs.

Conclusion

Cyclic strain promotes cytoskeletal rearrangement of hPDLCs by downregulating the expression levels of Rho-GDIa and upregulating the expression levels of GTP-Rho, Rock and p-cofilin. These observations may provide valuable insight into understanding orthodontic tooth movement as well as alveolar bone remodeling.

DIFFERENCES BETWEEN PROTEIN EXPRESSION AND EXTRACELLULAR MATRIX STATE ON UNIAXIAL STRETCHING FOR TENOGENIC DIFFERENTIATION

There have been no previous reports of tendon tissue engineering using mesenchymal stem cells (MSCs) with regard to quantitative evaluation of protein expression levels and observation of derived extracellular matrix (ECM) state. Therefore, we approached tendon tissue engineering from both perspectives. Human bone marrow MSCs (hBMSCs) were subjected to 8% or 10% cyclic stretching at 1 Hz to promote differentiation into tenocytes and ECM production. The type I collagen (Col I) and Tenascin-C (Tnc) protein expression levels were evaluated quantitatively by enzyme-linked immunosorbent assay (ELISA). Confocal fluorescence microscopy was employed to observe the derived ECM state. Col I state derived from 10%-stretched cells as ECM was elongated like actual tendon ECM, although the quantitative protein expression levels were slightly higher in 8%-stretched cells. The results suggested that the optimal uniaxial stretching ratio was different between protein expression levels and derived ECM state. Therefore, it is important to pay attention to both protein expression levels and ECM state in tendon tissue engineering.

Evaluation of stem cell-to-tenocyte differentiation by atomic force microscopy to measure cellular elastic moduli

In the present study, we evaluated whether stem cell-to-tenocyte differentiation could be evaluated via measurement of the mechanical properties of the cell. We used mechanical uniaxial cyclic stretching to induce the differentiation of human bone marrow mesenchymal stem cells into tenocytes. The cells were subjected to cyclic elongation of 10 or 15 % at a cyclic frequency of 1 Hz for 24 or 48 h, and differentiation was assessed by real-time PCR (rtPCR) determination of messenger RNA expression levels for four commonly used markers of stem cell-to-tenocyte differentiation: type I collagen, type III collagen, tenascin-C, and scleraxis. The rtPCR results showed that cells subjected to 10 % cyclic elongation for 24 or 48 h differentiated into tenocytes. Atomic force microscopy (AFM) was then used to measure the force curves around the cell nuclei, and the AFM data were used to calculate the elastic moduli of the cell surfaces. The elastic modulus values of the control (non-stretched) cells differed significantly from those of cells stretched at 10 % for 24 or 48 h (P < 0.01). Confocal fluorescence microscopic observations of actin stress fibers suggested that the change in elastic modulus was ascribable to the development of the cellular cytoskeleton during the differentiation process. Therefore, we conclude that the atomic force microscopic measurement of the elastic modulus of the cell surface can be used to evaluate stem cell-to-tenocyte differentiation.

The role of mechanical loading in tendon development, maintenance, injury, and repair

Tendon injuries often result from excessive or insufficient mechanical loading, impairing the ability of the local tendon cell population to maintain normal tendon function. The resident cell population composing tendon tissue is mechanosensitive, given that the cells are able to alter the extracellular matrix in response to modifications of the local loading environment. Natural tendon healing is insufficient, characterized by improper collagen fibril diameter formation, collagen fibril distribution, and overall fibril misalignment. Current tendon repair rehabilitation protocols focus on implementing early, well-controlled eccentric loading exercises to improve repair outcome. Tissue engineers look toward incorporating mechanical loading regimens to precondition cell populations for the creation of improved biological augmentations for tendon repair.

Augmenting tendon and ligament repair with platelet-rich plasma (PRP)

Tendon and ligament injuries (TLI) commonly occur in athletes and non-athletes alike, and remarkably debilitate patients' athletic and personal abilities. Current clinical treatments, such as reconstruction surgeries, do not adequately heal these injuries and often result in the formation of scar tissue that is prone to re-injury. Platelet-rich plasma (PRP) is a widely used alternative option that is also safe because of its autologous nature. PRP contains a number of growth factors that are responsible for its potential to heal TLIs effectively. In this review, we provide a comprehensive report on PRP. While basic science studies in general indicate the potential of PRP to treat TLIs effectively, a review of existing literature on the clinical use of PRP for the treatment of TLIs indicates a lack of consensus due to varied treatment outcomes. This suggests that current PRP treatment protocols for TLIs may not be optimal, and that not all TLIs may be effectively treated with PRP. Certainly, additional basic science studies are needed to develop optimal treatment protocols and determine those TLI conditions that can be treated effectively.

Detection of slow-cycling and stem/progenitor cells in different regions of rat Achilles tendon: response to treadmill exercise

PURPOSE

Tendons generally exhibit poor healing capacity, probably due to slow cell regeneration potential and low vascularization. The potential to regenerate may partly be due to activation of stem/progenitor cells localized in the tendon or its vicinity. In the present study, we attempted to determine where in the rat Achilles tendon stem/progenitor cells reside and to investigate the effect of exercise on cell proliferation in the in vivo situation.

METHOD

We used bromodeoxyuridine (BrdU) labelling to investigate proliferation and label-retaining cells (i.e. slow-cycling cells) in non-exercised and exercised rats, in combination with immunostaining of the stem cell marker nucleostemin. Rat Achilles tendons were harvested 14, 28, 56 and 105 days after BrdU administration.

RESULTS

We found the proportion of stem/progenitor cells to be twice as high in the distal tendon (DT) compared with the mid/proximal tendon (MPT) and that paratenon/endotenon regions appear to host a pool of existing stem/progenitor cells. Exercise increased the BrdU-stained cell population after 14 days only (DT region p = 0.032, MPT p = 0.065), indicating effect mainly on more differentiated cells, since the nucleostemin-positive cells (i.e. stem/progenitor cells) remained unaffected in the intact Achilles tendon.

CONCLUSION

Stem/progenitor cells exist in several areas of the rat Achilles tendon which implies a possible stem cell regeneration pool of different origins. The distal region has twice the amount of stem/progenitor cells compared with the mid/proximal region, indicating a potentially higher stem cell activity in this tissue. Daily moderate exercise (treadmill running) mainly improves in vivo cell proliferation in rapidly proliferating cells, whereas the stem/progenitor pool remains constant.

Effects of mechanical strain on human mesenchymal stem cells and ligament fibroblasts in a textured poly(L-lactide) scaffold for ligament tissue engineering

The purpose of this study was to prove the effect of cyclic uniaxial intermittent strain on the mRNA expression of ligament-specific marker genes in human mesenchymal stem cells (MSC) and anterior cruciate ligament-derived fibroblasts (ACL-fibroblasts) seeded onto a novel textured poly(L-lactide) scaffold (PLA scaffold). Cell-seeded scaffolds were mechanically stimulated by cyclic uniaxial stretching. The expression of ligament matrix gene markers: collagen types I and III, fibronectin, tenascin C and decorin, as well as the proteolytic enzymes matrix metalloproteinase MMP-1 and MMP-2 and their tissue specific inhibitors TIMP-1 and TIMP-2 was investigated by analysing the mRNA expression using reverse transcriptase polymerase chain reaction and related to the static control. In ACL-fibroblasts seeded on PLA, mechanical load induced up-regulation of collagen types I and III, fibronectin and tenascin C. No effect of mechanical stimulation on the expression of ligament marker genes was found in undifferentiated MSC seeded on PLA. The results indicated that the new textured PLA scaffold could transfer the mechanical load to the ACL-fibroblasts and improved their ligament phenotype. This scaffold might be suitable as a cell-carrying component of ACL prostheses.

Operative versus nonoperative management of acute Achilles tendon ruptures: a quantitative systematic review of randomized controlled trials

BACKGROUND

Despite several randomized controlled trials comparing operative to nonoperative management of Achilles tendon ruptures, the optimal management of this condition remains the subject of significant debate. Rerupture is a known complication, but most level I studies have not shown a significant difference in the incidence of reruptures when comparing operative to nonoperative management.

PURPOSE

The goal of this systematic review was to identify all randomized controlled trials comparing operative and nonoperative management of Achilles tendon ruptures and to meta-analyze the data with reruptures being the primary outcome. Secondary outcomes including strength, time to return to work, and other complications were analyzed as well.

STUDY DESIGN

Meta-analysis.

METHODS

We searched multiple online databases to identify English-language, prospective randomized controlled trials comparing open surgical repair of acute Achilles tendon ruptures to nonoperative management. Rerupture was our primary outcome. Secondary outcomes included strength, time to return to work, deep infections, sural nerve sensory disturbances, noncosmetic scar complaints, and deep venous thrombosis. Coleman methodology scores were calculated for each included study. Data were extracted from all qualifying articles and, when appropriate, pooled and meta-analyzed.

RESULTS

Seven level I trials involving 677 patients met inclusion criteria. Coleman scores were 95, 95, 95, 89, 78, 97, and 92. Open repair was associated with a significantly lower rerupture rate compared with nonoperative treatment (3.6% vs 8.8%; odds ratio, 0.425; 95% confidence interval, 0.222-0.815). The incidence of deep infections was significantly higher for patients treated with surgery (P = .0113). The incidences of noncosmetic scar complaints and sural nerve sensory disturbances were also significantly higher in patients treated with surgery (P < .001 for each). Strength measurements were not standardized and therefore could not be meta-analyzed.

CONCLUSION

Open surgical repair of acute Achilles tendon ruptures significantly reduces the risk of reruptures when compared with nonoperative management. Several other complications, which are clearly avoided with nonoperative treatment, occur with a significantly higher incidence when surgical repair is performed. The available literature makes it difficult to compare the return of strength in the involved lower extremity after operative or nonoperative management. Future studies may focus on testing strength in a more functional and reproducible manner than isokinetic testing.

Effects of perfusion and cyclic compression on in vitro tissue engineered meniscus implants

PURPOSE

The purpose of this study was to investigate the influence of continuous perfusion and mechanical stimulation on bone marrow stromal cells seeded on a collagen meniscus implant.

METHODS

Bone marrow aspirates from 6 donors were amplified in vitro. 10(6) human BMSC were distributed on a collagen meniscus implant. Scaffolds were cultured under static conditions (control) or placed into a bioreactor system where continuous perfusion (10 ml/min) or perfusion and mechanical stimulation (8 h of 10% cyclic compression at 0.5 Hz) were administered daily. After 24 h, 7 and 14 days, cell proliferation, synthesis of procollagen I and III peptide (PIP, PIIIP), histology, and the equilibrium modulus of the constructs were analyzed.

RESULTS

Proliferation demonstrated a significant increase over time in all groups (p < 0.001). PIP synthesis was found to increase from 0.1 ± 0.0 U/ml/g protein after 24 h to 2.0 ± 0.5 (perfusion), 3.8 ± 0.3 (mechanical stimulation), and 1.8 ± 0.2 U/ml/g protein (static control, lower than perfusion and mechanical stimulation, p < 0.05). These differences were also evident after 2 weeks (2.7 ± 0.3, 4.0 ± 0.6, and 1.8 ± 0.2 U/ml/g protein, p < 0.01); PIIIP synthesis was found to increase from 0.1 ± 0.0 U/ml/g protein after 24 h to 2.9 ± 0.7 (perfusion), 3.1 ± 0.9 (mechanical stimulation), and 1.6 ± 0.3 U/ml/g protein (controls) after 1 week and remained significantly elevated under the influence of perfusion and mechanical stimulation (p < 0.01) after 2 weeks. Mechanical stimulation increased the equilibrium modulus more than static culture and perfusion after 2 weeks (24.7 ± 7.6; 12.3 ± 3.7; 15.4 ± 2.6 kPa; p < 0.02).

CONCLUSION

Biomechanical stimulation and perfusion have impact on collagen scaffolds seeded with BMSCs. Cell proliferation can be enhanced using continuous perfusion and differentiation is fostered by mechanical stimulation.

Effect of alpine skiing training on tendon mechanical properties in older men and women

Strain is one of the parameters determining tendon adaptation to mechanical stimuli. The aim of this study was to test whether the patellar tendon strain induced during recreational alpine skiing would affect tendon mechanical properties in older individuals. Twenty-two older males and females (67 ± 2 years) were assigned to a 12-week guided skiing programme (IG) and 20 aged-matched volunteers served as controls (CG). Patellar tendon mechanical properties and cross-sectional area (CSA) were measured before and after training, with combined dynamometry and ultrasonography scanning. None of the variables changed significantly in the CG after training. In the IG, tendon stiffness and Young's modulus were increased (respectively, 14% and 12%, P<0.01), without any significant change in tendon CSA. In addition, changes in tendon stiffness were blunted in women (9%) compared with men (19%). Serum IGF-1 concentration tended to be lower in women (-19%, P=0.07). These results demonstrate that the mechanical stimulus induced by alpine skiing is sufficient to elicit adaptive changes in patellar tendon mechanical and material properties in older subjects. Furthermore, the present sex-specific adaptations are consistent with previous reports of lower collagen metabolic responsiveness in women and may be underpinned by anthropometric and metabolic differences.

Tissue engineering approaches for the construction of a completely autologous tendon substitute

Tissue engineering is a multidisciplinary field that involves the application of the principles and methods of engineering and life sciences towards i) the fundamental understanding of structure-function relationships in normal and pathological mammalian tissues and ii) the development of biological substitutes that restore, maintain or improve tissue function. The goal of tissue engineering is to surpass the limitations of conventional treatments based on organ transplantation and biomaterial implantation. The field of tendon tissue engineering is relatively unexplored due to the difficulty in in vitro preservation of tenocyte phenotype. Only recently has mechanobiology allowed us to gain a better understanding of the fundamental role of in vitro mechanical stimuli in maintaining the phenotype of tendinous tissue. This review analyzes the techniques used so far for in vitro regeneration of tendinous tissue.

Tenogenic differentiation of stem cells for tendon repair-what is the current evidence?

Tendon/ligament injuries are very common in sports and other rigorous activities. Tendons regenerate and repair slowly and inefficiently in vivo after injury. The limited ability of tendon to self-repair and the general inefficiencies of current treatment regimes have hastened the motivation to develop tissue-engineering strategies for tissue repair. Of particular interest in recent years has been the use of adult mesenchymal stem cells (MSCs) to regenerate functional tendons and ligaments. Different sources of MSCs have been studied for their effects on tendon repair. However, ectopic bone and tumour formation has been reported in some special circumstances after transplantation of MSCs. The induction of MSCs to differentiate into tendon-forming cells in vitro prior to transplantation is a possible approach to avoid ectopic bone and tumour formation while promoting tendon repair. While there are reports about the factors that might promote tenogenic differentiation, the study of tenogenic differentiation is hampered by the lack of definitive biomarkers for tendons. This review aims to summarize the cell sources currently used for tendon repair as well as their advantages and limitations. Factors affecting tenogenic differentiation were summarized. Molecular markers currently used for assessing tenogenic differentiation or neotendon formation are summarized and their advantages and limitations are commented upon. Finally, further directions for promoting and assessing tenogenic differentiation of stem cells for tendon repair are discussed.

Increased cellular proliferation in rat skeletal muscle and tendon in response to exercise: use of FLT and PET/CT

PURPOSE

The purpose of this study is to investigate exercise-induced cellular proliferation in rat skeletal muscle/tendon with the use of 3'-[F-18]fluoro-3'deoxythymidine (FLT) and to quantitatively study concomitant changes in the proliferation-associated factor, Ki67.

PROCEDURES

Wistar rats (n = 13) performed 3 days of treadmill running. Cellular proliferation was investigated 3 days before and 48 h after the running exercise with the use of FLT and positron emission tomography/computed tomography (PET/CT). Results were compared to a sedentary control group (n = 10). Image-derived standardized uptake values were calculated for Achilles tendons and calf muscles and compared to gene expression and immunohistochemical evaluations of Ki67.

RESULTS

Treadmill running induced increased uptake of FLT uptake in calf muscles (30%; p < 0.001) and in Achilles tendon (21%, p < 0.001). The image-derived results were supported by a correlation in calf muscle to Ki67 (protein and mRNA level), while this coherence was not found in tendon.

CONCLUSION

FLT-PET seems to be a promising tool for imaging of exercise-induced cellular proliferation in musculo-tendinous tissue.

A strain-cue hypothesis for biological network formation

The direction of migration of a cell invading a host population is assumed to be controlled by the magnitude of the strains in the host medium (cells plus extracellular matrix) that arise as the host medium deforms to accommodate the invader. The single assumption that invaders are cued by strains external to themselves is sufficient to generate network structures. The strain induced by a line of invaders is greatest at the extremity of the line and thus the strain field breaks symmetry, stabilizing branch formation. The strain cue also triggers sprouting from existing branches, with no further model assumption. Network characteristics depend primarily on the ratio of the rate of advance of the invaders to the rate of relaxation of the host cells after their initial deformation. Intra-cell mechanisms that govern these two rates control network morphology. The strain field that cues an individual invader is a collective response of the combined cell populations, involving the nearest 100 cells, to order of magnitude, to any invader. The mechanism does not rely on the pre-existence of the entire host medium prior to invasion; the host cells need only maintain a layer several cells thick around each invader. Consistent with recent experiments, networks result only from a strain cue that is based on strain magnitudes. Spatial strain gradients do not break symmetry and therefore cannot stabilize branch formation. The theory recreates most of the geometrical features of the nervous network in the mouse gut when the most influential adjustable parameter takes a value consistent with one inferred from human and mouse amelogenesis. Because of similarity in the guiding local strain fields, strain cues could also be a participating factor in the formation of vascular networks.

Effect of cyclic three-dimensional strain on cell proliferation and collagen synthesis of fibroblast-seeded chitosan-hyaluronan hybrid polymer fiber

BACKGROUND

Tissue engineering techniques using biodegradable three-dimensional (3D) scaffolds with cultured cells offer more potential alternatives for the treatment of severe ligament and tendon injuries. In tissue engineering, one of the crucial roles of 3D scaffolds is to provide a temporary template with the biomechanical characteristics of the native extracellular matrix (ECM) until the regenerated tissue matures. The purpose of the present study was to assess the effect of various cyclic mechanical stresses on cell proliferation and ECM production in a 3D scaffold made from chitosan and hyaluronan for ligament and tendon tissue engineering.

METHODS

Three-dimensional scaffolds seeded with rabbit patella tendon fibroblasts were attached to a bioreactor under various conditions: static group, no strain; stretch group, tensile strain; rotational group, rotational strain; combined group, rotational and tensile strain. In the Static group, 3 weeks of stationary culture was performed. In the remaining three groups, a loading regimen of 0.5 Hz for 18 h and then 6 h rest was carried out for 2 weeks after 1 week of static culture. The DNA content was determined to quantify cell proliferation. Real-time reverse transcription polymerase chain reaction analysis was performed to assess the mRNA levels of the ECM products.

RESULTS

DNA content of the combined group was significantly higher than that of the static and stretch groups, and that of the rotational group was significant higher than that of the static and stretch groups at 21 days after cultivation. The mRNA level of types I and III collagen and fibromodulin in the combined group was significantly higher than that in the other three groups. The amount of collagen synthesis in the combined group was higher than that in the static group, but the difference was not significant.

CONCLUSIONS

Multidimensional cyclic mechanical strain to mimic the physiological condition in vivo has the potential to improve or accelerate tissue regeneration in ligament and tendon tissue engineering using 3D scaffolds in vitro.

Combined effects of scaffold stiffening and mechanical preconditioning cycles on construct biomechanics, gene expression, and tendon repair biomechanics

Our group has previously reported that in vitro mechanical stimulation of tissue-engineered tendon constructs significantly increases both construct stiffness and the biomechanical properties of the repair tissue after surgery. When optimized using response surface methodology, our results indicate that a mechanical stimulus with three components (2.4% strain, 3000 cycles/day, and one cycle repetition) produced the highest in vitro linear stiffness. Such positive correlations between construct and repair stiffness after surgery suggest that enhancing structural stiffness before surgery could not only accelerate repair stiffness but also prevent premature failures in culture due to poor mechanical integrity. In this study, we examined the combined effects of scaffold crosslinking and subsequent mechanical stimulation on construct mechanics and biology. Autologous tissue-engineered constructs were created by seeding mesenchymal stem cells (MSCs) from 15 New Zealand white rabbits on type I collagen sponges that had undergone additional dehydrothermal crosslinking (termed ADHT in this manuscript). Both constructs from each rabbit were mechanically stimulated for 8h/day for 12 consecutive days with half receiving 100 cycles/day and the other half receiving 3000 cycles/day. These paired MSC-collagen autologous constructs were then implanted in bilateral full-thickness, full-length defects in the central third of rabbit patellar tendons. Increasing the number of in vitro cycles/day delivered to the ADHT constructs in culture produced no differences in stiffness or gene expression and no changes in biomechanical properties or histology 12 weeks after surgery. Compared to MSC-based repairs from a previous study that received no additional treatment in culture, ADHT crosslinking of the scaffolds actually lowered the 12-week repair stiffness. Thus, while ADHT crosslinking may initially stiffen a construct in culture, this specific treatment also appears to mask any benefits of stimulation among repairs postsurgery. Our findings emphasize the importance of properly preconditioning a scaffold to better control/modulate MSC differentiation in vitro and to further enhance repair outcome in vivo.

Tensile stimulation of murine stem cell-collagen sponge constructs increases collagen type I gene expression and linear stiffness

The objectives of this study were to determine how tensile stimulation delivered up to 14 days in culture influenced type I collagen gene expression in stem cells cultured in collagen sponges, and to establish if gene expression, measured using a fluorescence method, correlates with an established method, real-time quantitative reverse transcriptase polymerase chain reaction (qRT-PCR). Using a novel model system, mesenchymal stem cells were harvested from six double transgenic mice in which the type I and type II collagen promoters were linked to green fluorescent protein-topaz and enhanced cyan fluorescent protein, respectively. Tissue-engineered constructs were created by seeding 0.5 x 10(6) mesenchymal stem cells onto type I collagen sponge scaffolds in a silicone dish. Constructs were then transferred to a custom pneumatic mechanical stimulation system housed in a standard incubator and stimulated for 5 h=day in tension for either 7 or 14 days using a repeated profile (2.4% peak strain for 20 s at 1 Hz followed by a rest period at 0% strain for 100 s). Control specimens were exposed to identical culture conditions but without mechanical stimulation. At three time points (0, 7, and 14 days), constructs were then prepared for evaluation of gene expression using fluorescence analysis and qRT-PCR, and the remaining constructs were failed in tension. Both analytical methods showed that constructs stimulated for 7 and 14 days showed significantly higher collagen type I gene expression than nonstimulated controls at the same time interval. Gene expression measured using qRT-PCR and fluorescence analysis was positively correlated (r = 0.9). Linear stiffness of stimulated constructs was significantly higher at both 7 and 14 days than that of nonstimulated controls at the same time intervals. Linear stiffness of the stimulated constructs at day 14 was significantly different from that of day 7. Future studies will vary the mechanical signal to optimize type I collagen gene expression to improve construct biomechanics and in vivo tendon repair.

Stem cells in drug discovery, tissue engineering, and regenerative medicine: emerging opportunities and challenges

Stem cells, irrespective of their origin, have emerged as valuable reagents or tools in human health in the past 2 decades. Initially, a research tool to study fundamental aspects of developmental biology is now the central focus of generating transgenic animals, drug discovery, and regenerative medicine to address degenerative diseases of multiple organ systems. This is because stem cells are pluripotent or multipotent cells that can recapitulate developmental paths to repair damaged tissues. However, it is becoming clear that stem cell therapy alone may not be adequate to reverse tissue and organ damage in degenerative diseases. Existing small-molecule drugs and biologicals may be needed as "molecular adjuvants" or enhancers of stem cells administered in therapy or adult stem cells in the diseased tissues. Hence, a combination of stem cell-based, high-throughput screening and 3D tissue engineering approaches is necessary to advance the next wave of tools in preclinical drug discovery. In this review, the authors have attempted to provide a basic account of various stem cells types, as well as their biology and signaling, in the context of research in regenerative medicine. An attempt is made to link stem cells as reagents, pharmacology, and tissue engineering as converging fields of research for the next decade.

Bone marrow stromal cells in a liquid fibrin matrix improve the healing process of patellar tendon window defects

Following injury, ligaments and tendons do not regain their normal biological and biomechanical status. This study analyzed whether an injection of human bone marrow stromal cells (BMSC) or human fibroblast in a liquid fibrin matrix influences the histological results, ultrastructural morphology, mRNA expression of essential extracellular matrix proteins, and material properties of the healing tissue. Standardized full-thickness, full-length defects of the central portion of patellar tendons were created in 96 immunodeficient rats, and filled with human BMSC in a fibrin matrix (BMSC group), human fibroblasts in a fibrin matrix (fibroblast group), or fibrin matrix only (matrix group), or left untreated (defect group). Histological sections revealed more mature tissue formation with more regular patterns of cell distribution in the BMSC group, without signs of ectopic tissue formation into bone or cartilage. Mean collagen fibril diameter and relative area covered by collagen fibrils were significantly higher at 10 and 20 days postoperatively in the BMSC group compared to the defect and matrix groups, and comparable to normal tendon tissue. Further, collagen I mRNA expression, collagen I/collagen III mRNA ratio, and Young's modulus were significantly increased at 20 days postoperatively in comparison to the defect and matrix groups. In the fibroblast group, only mean collagen fibril diameter was significantly higher compared to the defect group, whereas the other biological and biomechanical parameters were not significantly improved. This study reveals that an injection of BMSC in a liquid fibrin matrix stimulates histological, ultrastructural, molecular biologic, and biomechanical parameters of patellar tendon healing, whereas injection of fibroblasts in fibrin matrix had only minor effects on the stimulation of tendon healing.

Tissue engineering of osteochondral constructs in vitro using bioreactors

Articular cartilage is a relatively simple tissue, but has a limited capacity of restoration. Tissue engineering is a promising field that seeks to accomplish the in vitro generation of complex, functional, 3-dimensional tissues. Various cell types and scaffolds have been tested for these purposes. The results of tissue engineered cartilage and bone are as yet inferior to native tissue. Strain and perfusion have been shown to stimulate cell proliferation and differentiation of various cell phenotypes. The perfect protocol to produce articular cartilage has not been defined yet. Bioreactors could provide the environment to engineer osteochondral constructs in vitro and to provide a stress protocol. The bioreactor has to provide an economically viable approach to automated manufacture of functional grafts under clinical aspects. Composite engineered tissues, like an engineered joint, represent a future goal. Cross-disciplinary approaches are necessary in order to succeed in engineering osteochondral grafts that provide adequate primary biomechanical stability and incorporate rapidly in vivo with histological appearance close to healthy osteochondral tissue. This review surveys current clinical and experimental concepts and discusses challenges and future expectations in this advancing field of regenerative medicine focusing human osteochondral constructs in bioreactors.