An experimental model for studying the biomechanics of embryonic tendon: Evidence that the development of mechanical properties depends on the actinomyosin machinery

Engineering interfacial tissues: The myotendinous junction

The myotendinous junction (MTJ) is the interface connecting skeletal muscle and tendon tissues. This specialized region represents the bridge that facilitates the transmission of contractile forces from muscle to tendon, and ultimately the skeletal system for the creation of movement. MTJs are, therefore, subject to high stress concentrations, rendering them susceptible to severe, life-altering injuries. Despite the scarcity of knowledge obtained from MTJ formation during embryogenesis, several attempts have been made to engineer this complex interfacial tissue. These attempts, however, fail to achieve the level of maturity and mechanical complexity required for in vivo transplantation. This review summarizes the strategies taken to engineer the MTJ, with an emphasis on how transitioning from static to mechanically inducive dynamic cultures may assist in achieving myotendinous maturity.

Enthesis maturation in engineered ligaments is differentially driven by loads that mimic slow growth elongation and rapid cyclic muscle movement

Entheses are complex attachments that translate load between elastic-ligaments and stiff-bone via organizational and compositional gradients. Neither natural healing, repair, nor engineered replacements restore these gradients, contributing to high re-tear rates. Previously, we developed a culture system which guides ligament fibroblasts in high-density collagen gels to develop early postnatal-like entheses, however further maturation is needed. Mechanical cues, including slow growth elongation and cyclic muscle activity, are critical to enthesis development in vivo but these cues have not been widely explored in engineered entheses and their individual contribution to maturation is largely unknown. Our objective here was to investigate how slow stretch, mimicking ACL growth rates, and intermittent cyclic loading, mimicking muscle activity, individually drive enthesis maturation in our system so to shed light on the cues governing enthesis development, while further developing our tissue engineered replacements. Interestingly, we found these loads differentially drive organizational maturation, with slow stretch driving improvements in the interface/enthesis region, and cyclic load improving the ligament region. However, despite differentially affecting organization, both loads produced improvements to interface mechanics and zonal composition. This study provides insight into how mechanical cues differentially affect enthesis development, while producing some of the most organized engineered enthesis to date. STATEMENT OF SIGNIFICANCE: Entheses attach ligaments to bone and are critical to load transfer; however, entheses do not regenerate with repair or replacement, contributing to high re-tear rates. Mechanical cues are critical to enthesis development in vivo but their individual contribution to maturation is largely unknown and they have not been widely explored in engineered replacements. Here, using a novel culture system, we provide new insight into how slow stretch, mimicking ACL growth rates, and intermittent cyclic loading, mimicking muscle activity, differentially affect enthesis maturation in engineered ligament-to-bone tissues, ultimately producing some of the most organized entheses to date. This system is a promising platform to explore cues regulating enthesis formation so to produce functional engineered replacements and better drive regeneration following repair.

Tendon cell and tissue culture: Perspectives and recommendations

The wide variety of cell and tissue culture systems used to study and engineer tendons can make it difficult to choose the best approach and "optimal" culture conditions to test a given hypothesis. Therefore, a breakout session was organized at the 2022 ORS Tendon Section Meeting that focused on establishing a set of guidelines for conducting cell and tissue culture studies of tendon. This paper summarizes the outcomes of that discussion and presents recommendations for future studies. In the case of studying tendon cell behavior, cell and tissue culture systems are reductionist models in which the culture conditions should be strictly defined to approximate the in vivo condition as closely as possible. In contrast, for tissue engineering tendon replacements, the culture conditions do not need to replicate native tendon, but the outcome measures for success should be narrowly defined for the specific clinical application. Common recommendations for both applications are that researchers should perform a baseline phenotypic characterization of the cells that are ultimately used for experimentation. For models of tendon cell behavior, culture conditions should be well justified by existing literature and meticulously reported, tissue explant viability should be assessed, and comparisons to in vivo conditions should be made to determine baseline physiological relevance. For tissue engineering applications, the functional/structural/compositional outcome targets should be defined by the specific tendons they seek to replace, with key biologic and material properties prioritized for construct assessment. Lastly, when engineering tendon replacements, researchers should utilize clinically approved cGMP materials to facilitate clinical translation.

Collagen-Based Biomimetic Systems to Study the Biophysical Tumour Microenvironment

The extracellular matrix (ECM) is a pericellular network of proteins and other molecules that provides mechanical support to organs and tissues. ECM biophysical properties such as topography, elasticity and porosity strongly influence cell proliferation, differentiation and migration. The cell's perception of the biophysical microenvironment (mechanosensing) leads to altered gene expression or contractility status (mechanotransduction). Mechanosensing and mechanotransduction have profound implications in both tissue homeostasis and cancer. Many solid tumours are surrounded by a dense and aberrant ECM that disturbs normal cell functions and makes certain areas of the tumour inaccessible to therapeutic drugs. Understanding the cell-ECM interplay may therefore lead to novel and more effective therapies. Controllable and reproducible cell culturing systems mimicking the ECM enable detailed investigation of mechanosensing and mechanotransduction pathways. Here, we discuss ECM biomimetic systems. Mainly focusing on collagen, we compare and contrast structural and molecular complexity as well as biophysical properties of simple 2D substrates, 3D fibrillar collagen gels, cell-derived matrices and complex decellularized organs. Finally, we emphasize how the integration of advanced methodologies and computational methods with collagen-based biomimetics will improve the design of novel therapies aimed at targeting the biophysical and mechanical features of the tumour ECM to increase therapy efficacy.

Quantifying Cell-Derived Changes in Collagen Synthesis, Alignment, and Mechanics in a 3D Connective Tissue Model

Dysregulation of extracellular matrix (ECM) synthesis, organization, and mechanics are hallmark features of diseases like fibrosis and cancer. However, most in vitro models fail to recapitulate the three-dimensional (3D) multi-scale hierarchical architecture of collagen-rich tissues and as a result, are unable to mirror native or disease phenotypes. Herein, using primary human fibroblasts seeded into custom fabricated 3D non-adhesive agarose molds, a novel strategy is proposed to direct the morphogenesis of engineered 3D ring-shaped tissue constructs with tensile and histological properties that recapitulate key features of fibrous connective tissue. To characterize the shift from monodispersed cells to a highly-aligned, collagen-rich matrix, a multi-modal approach integrating histology, multiphoton second-harmonic generation, and electron microscopy is employed. Structural changes in collagen synthesis and alignment are then mapped to functional differences in tissue mechanics and total collagen content. Due to the absence of an exogenously added scaffolding material, this model enables the direct quantification of cell-derived changes in 3D matrix synthesis, alignment, and mechanics in response to the addition or removal of relevant biomolecular perturbations. To illustrate this, the effects of nutrient composition, fetal bovine serum, rho-kinase inhibitor, and pro- and anti-fibrotic compounds on ECM synthesis, 3D collagen architecture, and mechanophenotype are quantified.

Development of Fluorescently Labeled, Functional Type I Collagen Molecules

While de novo collagen fibril formation is well-studied, there are few investigations into the growth and remodeling of extant fibrils, where molecular collagen incorporation into and erosion from the fibril surface must delicately balance during fibril growth and remodeling. Observing molecule/fibril interactions is difficult, requiring the tracking of molecular dynamics while, at the same time, minimizing the effect of the observation on fibril structure and assembly. To address the observation-interference problem, exogenous collagen molecules are tagged with small fluorophores and the fibrillogenesis kinetics of labeled collagen molecules as well as the structure and network morphology of assembled fibrils are examined. While excessive labeling significantly disturbs fibrillogenesis kinetics and network morphology of assembled fibrils, adding less than ≈1.2 labels per collagen molecule preserves these characteristics. Applications of the functional, labeled collagen probe are demonstrated in both cellular and acellular systems. The functional, labeled collagen associates strongly with native fibrils and when added to an in vitro model of corneal stromal development at low concentration, the labeled collagen is incorporated into a fine extracellular matrix (ECM) network associated with the cells within 24 h.

Maintenance of Ligament Homeostasis of Spheroid-Colonized Embroidered and Functionalized Scaffolds after 3D Stretch

Anterior cruciate ligament (ACL) ruptures are usually treated with autograft implantation to prevent knee instability. Tissue engineered ACL reconstruction is becoming promising to circumvent autograft limitations. The aim was to evaluate the influence of cyclic stretch on lapine (L) ACL fibroblasts on embroidered scaffolds with respect to adhesion, DNA and sulphated glycosaminoglycan (sGAG) contents, gene expression of ligament-associated extracellular matrix genes, such as type I collagen, decorin, tenascin C, tenomodulin, gap junctional connexin 43 and the transcription factor Mohawk. Control scaffolds and those functionalized by gas phase fluorination and cross-linked collagen foam were either pre-cultured with a suspension or with spheroids of LACL cells before being subjected to cyclic stretch (4%, 0.11 Hz, 3 days). Stretch increased significantly the scaffold area colonized with cells but impaired sGAGs and decorin gene expression (functionalized scaffolds seeded with cell suspension). Stretching increased tenascin C, connexin 43 and Mohawk but decreased decorin gene expression (control scaffolds seeded with cell suspension). Pre-cultivation of functionalized scaffolds with spheroids might be the more suitable method for maintaining ligamentogenesis in 3D scaffolds compared to using a cell suspension due to a significantly higher sGAG content in response to stretching and type I collagen gene expression in functionalized scaffolds.

Mechanobiology in Tendon, Ligament, and Skeletal Muscle Tissue Engineering

Tendon, ligament, and skeletal muscle are highly organized tissues that largely rely on a hierarchical collagenous matrix to withstand high tensile loads experienced in activities of daily life. This critical biomechanical role predisposes these tissues to injury, and current treatments fail to recapitulate the biomechanical function of native tissue. This has prompted researchers to pursue engineering functional tissue replacements, or dysfunction/disease/development models, by emulating in vivo stimuli within in vitro tissue engineering platforms-specifically mechanical stimulation, as well as active contraction in skeletal muscle. Mechanical loading is critical for matrix production and organization in the development, maturation, and maintenance of native tendon, ligament, and skeletal muscle, as well as their interfaces. Tissue engineers seek to harness these mechanobiological benefits using bioreactors to apply both static and dynamic mechanical stimulation to tissue constructs, and induce active contraction in engineered skeletal muscle. The vast majority of engineering approaches in these tissues are scaffold-based, providing interim structure and support to engineered constructs, and sufficient integrity to withstand mechanical loading. Alternatively, some recent studies have employed developmentally inspired scaffold-free techniques, relying on cellular self-assembly and matrix production to form tissue constructs. Whether utilizing a scaffold or not, incorporation of mechanobiological stimuli has been shown to improve the composition, structure, and biomechanical function of engineered tendon, ligament, and skeletal muscle. Together, these findings highlight the importance of mechanobiology and suggest how it can be leveraged to engineer these tissues and their interfaces, and to create functional multitissue constructs.

Neonatal Spinal Cord Transection Decreases Hindlimb Weight-Bearing and Affects Formation of Achilles and Tail Tendons

Mechanical loading may be required for proper tendon formation. However, it is not well understood how tendon formation is impacted by the development of weight-bearing locomotor activity in the neonate. This study assessed tendon mechanical properties, and concomitant changes in weight-bearing locomotion, in neonatal rats subjected to a low thoracic spinal cord transection or a sham surgery at postnatal day (P)1. On P10, spontaneous locomotion was evaluated in spinal cord transected and sham controls to determine impacts on weight-bearing hindlimb movement. The mechanical properties of P10 Achilles tendons (ATs), as representative energy-storing, weight-bearing tendons, and tail tendons (TTs), as representative positional, non-weight-bearing tendons were evaluated. Non- and partial weight-bearing hindlimb activity decreased in spinal cord transected rats compared to sham controls. No spinal cord transected rats showed full weight-bearing locomotion. ATs from spinal cord transected rats had increased elastic modulus, while cross-sectional area trended lower compared to sham rats. TTs from spinal cord transected rats had higher stiffness and cross-sectional area. Collagen structure of ATs and TTs did not appear impacted by surgery condition, and no significant differences were detected in the collagen crimp pattern. Our findings suggest that mechanical loading from weight-bearing locomotor activity during development regulates neonatal AT lateral expansion and maintains tendon compliance, and that TTs may be differentially regulated. The onset and gradual increase of weight-bearing movement in the neonate may provide the mechanical loading needed to direct functional postnatal tendon formation.

Influence of Bias in the Assessment of Patellar Tendon Stiffness: A Systematic Review and Meta-analysis

The aim of the present study was to provide a systematic review and meta-analysis of the main biases in practical applications of the different types of elastography in assessment of the stiffness of healthy patellar tendons. A literature search on four databases (Scopus, Web of Science, Pubmed and SportDiscus) was conducted on March 25, 2020. After analysis of 1,052 resultant articles, studies were included if they met two inclusion criteria: (1) studies were observational or randomized controlled trials; (2) studies included outcomes of patellar tendon stiffness. Twenty-one studies met the inclusion criteria and were included in this meta-analysis. Four meta-analyses were performed with respect to assessment procedure and participant characteristics. This meta-analysis found a low effect of examined portion of patellar tendon (standardized mean difference [SMD] = 0.27, 95% confidence interval [CI]: 0.01-0.54, n = 379, Z = 2.01, p = 0.04) and a high effect of knee flexion angle during assessment (SMD = -2.12; 95% CI: -2.67 to -1.58, n = 97, Z = 7.68, p < 0.01) in stiffness outcomes. The risk of bias was generally low, but the heterogenicity of the results downgraded the level of evidence. There is evidence in the elastography assessment of patellar tendon suggesting that the knee position and patellar tendon portion evaluated influence stiffness outcomes for patellar tendon in healthy volunteers.

Effects of genipin crosslinking on mechanical cell-matrix interaction in 3D engineered tendon constructs

It is well known that cells can generate endogenous forces onto the extracellular matrix, but to what extent the mechanical properties of the matrix influences these endogenous cellular forces remains unclear. We therefore sought to quantify the influence of matrix rigidity on cell-matrix interactions by inducing cross-links using increasing concentrations of genipin (0.01-1 mM) or by blocking cross-link formation using beta-aminopropionitrile (BAPN) in engineered human tendon tissue constructs. The cell-matrix mechanics of the tendon constructs were evaluated as cell-generated tissue re-tensioning and stress-relaxation responses using a novel custom-made force monitor, which can apply and detect tensional forces in real-time in addition to mechanical failure testing. Genipin treatment had no influence on the biochemical profile (hydroxyproline, glycosaminoglycan and DNA content) of the constructs and cell viability was comparable between genipin-treated and control constructs, except at the highest genipin concentration. Endogenous re-tension after unloading was significantly decreased with increasing genipin concentrations compared to controls. Mechanical failure testing of tendon constructs showed increased (56%) peak stress at the highest genipin concentration but decreased (72%) with BAPN treatment when compared to controls. Tendon construct stiffness increased with high genipin concentrations (0.1 and 1 mM) and decreased by 70% in BAPN-treated constructs, relative to the controls. These data demonstrate that human tendon fibroblasts regulate their force exertion inversely proportional to increased cross-link capacity but did so independently of matrix stiffness. Overall, these findings support the notion of an interaction between cell force generation and cross-linking, and thus a role for this interplay in mechanical homeostasis of the tissue.

The effects of shoe upper construction on mechanical ankle joint work during lateral shuffle movements

Lateral shuffles are common movements in sports and are facilitated by the hip, knee, and ankle joints. Shoe uppers can change ankle kinetics during walking and running. However, it is not known how shoe upper modifications affect ankle kinetics during shuffling. The purpose of this study was to investigate the effects of shoe upper construction on mechanical ankle joint work during shuffling. It was hypothesized that a shoe with a reinforced upper will result in decreased negative ankle joint work. Twenty participants performed Maximal (MLST) and Submaximal Lateral Shuffle Tests (90% of MLST) in footwear with a minimal (MU) and reinforced upper (RU). Ground reaction forces and ankle kinematics were collected to compute ankle joint work. Performing lateral shuffles in the RU condition resulted in significantly reduced positive (MU: 0.62 ± 0.16 J/kg, RU: 0.55 ± 0.16 J/kg; p = 0.001, d = 0.44) and negative (MU: -0.60 ± 0.20 J/kg, RU: -0.53 ± 0.19 J/kg; p = 0.004, d = 0.41) ankle work. A decrease in positive and negative work could be a performance benefit, enabling the athlete to perform the same movement with a lower energy cost. More extreme upper interventions may yield even larger performance benefits.

Collagen fibril diameter distribution affects permeability of ligament tissue: A computational study on healthy and injured tissues

Background and objective In avascular or hypovascular tissues, elements required for maintaining tissue functions are recruited through diffusion, which is highly related with the permeability of the extracellular matrix in health and injury. Here, we investigate the effect of collagen fibril diameter distribution of bovine Anterior Cruciate Ligament (ACL) tissue on the hydraulic permeability of the matrix. Based on the fact that the diameter distribution is significantly different between healthy and injured ACL tissues, our study aims to investigate the effect of such variability on the hydraulic permeability. Methods Simulations are carried out in 3D geometries reconstructed from actual collagen filament/fibril diameter distributions obtained from healthy and injured tissue samples (n=3). The fluid flow through the fibrous tissue is modeled based on Eringen's theory of micropolar fluid flow to determine the effects of vortex viscosity (m) and spin gradient viscosity (N) on hydraulic permeability. Results Computational results indicate that the hydraulic permeability of models which are replicates of healthy ACL tissues is higher than that of the injured, indicating that the filament size distribution might play an important role on fluid and nutrient transport through ligament tissues. Conclusions These findings underscore the need for increased attention on replicating the diameter distribution of healthy collagens in tissue engineering scaffolds and allowing adequate supply of elements through permeation during ACL reconstruction procedures.

A universal multi-platform 3D printed bioreactor chamber for tendon tissue engineering

A range of bioreactors use linear actuators to apply tensile forces in vitro, but differences in their culture environments can limit a direct comparison between studies. The widespread availability of 3D printing now provides an opportunity to develop a 'universal' bioreactor chamber that, with minimal exterior editing can be coupled to a wide range of commonly used linear actuator platforms, for example, the EBERS-TC3 and CellScale MCT6, resulting in a greater comparability between results and consistent testing of potential therapeutics. We designed a bioreactor chamber with six independent wells that was 3D printed in polylactic acid using an Ultimaker 2+ and waterproofed using a commercially available coating (XTC-3D), an oxirane resin. The cell culture wells were further coated with Sylgard-184 polydimethylsiloxane (PDMS) to produce a low-adhesion well surface. With appropriate coating and washing steps, all materials were shown to be non-cytotoxic by lactate dehydrogenase assay, and the bioreactor was waterproof, sterilisable and reusable. Tissue-engineered tendons were generated from human mesenchymal stem cells in a fibrin hydrogel and responded to 5% cyclic strain (0.5 Hz, 5 h/day, 21 days) in the bioreactor by increased production of collagen-Iα1 and decreased production of collagen-IIIα1. Calcification of the extracellular matrix was observed in unstretched tendon controls indicating abnormal differentiation, while tendons cultured under cyclic strain did not calcify and exhibited a tenogenic phenotype. The ease of manufacturing this bioreactor chamber enables researchers to quickly and cheaply reproduce this culture environment for use with many existing bioreactor actuator platforms by downloading the editable CAD files from a public database and following the manufacturing steps we describe.

Early Growth Response Genes Increases Rapidly After Mechanical Overloading and Unloading in Tendon Constructs

Tendon cells exist in a dense extracellular matrix and mechanical loading is important for the strength development of this matrix. We therefore use a three-dimensional (3D) culture system for tendon formation in vitro. The objectives of this study were to elucidate the temporal expression of tendon-related genes during the formation of artificial tendons in vitro and to investigate if early growth response-1 (EGR1), EGR2, FOS, and cyclooxygenase-1 and -2 (PTGS1 and PTGS2) are sensitive to mechanical loading. First, we studied messenger RNA (mRNA) levels of several tendon-related genes during formation of tendon constructs. Second, we studied the mRNA levels of, for example, EGR1 and EGR2 after different degrees of loading; dynamic physiologic-range loading (2.5% strain), dynamic overloading (approximately 10% strain), or tension release. The gene expression for tendon-related genes (i.e., EGR2, MKX, TNMD, COL3A1) increased with time after seeding into this 3D model. EGR1, EGR2, FOS, PTGS1, and PTGS2 did not respond to physiologic-range loading. But overloading (and tension release) lead to elevated levels of EGR1 and EGR2 (p ≤ 0.006). FOS and PTGS2 were increased after overloading (both p < 0.007) but not after tension release (p = 0.06 and 0.08). In conclusion, the expression of tendon-related genes increases during the formation of artificial tendons in vitro, including EGR2. Furthermore, the gene expression of EGR1 and EGR2 in human tendon cells appear to be sensitive to overloading and unloading but did not respond to the single episode of physiologic-range loading. These findings could be helpful for the understanding of tendon tensional homeostasis. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:173-181, 2020.

Models of tendon development and injury

Tendons link muscle to bone and transfer forces necessary for normal movement. Tendon injuries can be debilitating and their intrinsic healing potential is limited. These challenges have motivated the development of model systems to study the factors that regulate tendon formation and tendon injury. Recent advances in understanding of embryonic and postnatal tendon formation have inspired approaches that aimed to mimic key aspects of tendon development. Model systems have also been developed to explore factors that regulate tendon injury and healing. We highlight current model systems that explore developmentally inspired cellular, mechanical, and biochemical factors in tendon formation and tenogenic stem cell differentiation. Next, we discuss in vivo, in vitro, ex vivo, and computational models of tendon injury that examine how mechanical loading and biochemical factors contribute to tendon pathologies and healing. These tendon development and injury models show promise for identifying the factors guiding tendon formation and tendon pathologies, and will ultimately improve regenerative tissue engineering strategies and clinical outcomes.

A novel mechanism for the protection of embryonic stem cell derived tenocytes from inflammatory cytokine interleukin 1 beta

Interleukin 1β (IL-1β) is upregulated following tendon injury. Here we demonstrate that in adult and fetal tenocytes IL-1β increases the expression of matrix metalloproteinases, tenascin-C and Sox9 and decreases the expression of scleraxis and cartilage oligomeric matrix protein. When cultured in 3-dimensional collagen gels adult and fetal tenocytes exposed to IL-1β have reduced contraction ability and generate tendon-like constructs with a lower storage modulus. In contrast, equine embryonic stem cell (ESC) derived tenocytes exposed to IL-1β exhibit no changes in gene expression and generate identical tendon-like constructs. We propose that ESC-derived tenocytes do not respond to IL-1β due to their low expression of interleukin 1 (IL-1) receptor 1 and high expression of the decoy receptor IL-1 receptor 2 and IL-1 receptor antagonist protein (IL1Ra). This may make ESC-derived tenocytes an advantageous source of cells for tissue regeneration and allow the development of novel pharmaceutical interventions to protect endogenous cells from inflammation.

A review of methods to measure tendon dimensions

Tendons are soft tissues of the musculoskeletal system that are designed to facilitate joint movement. Tendons exhibit a wide range of mechanical properties matched to their functions and, as a result, have been of interest to researchers for many decades. Dimensions are an important aspect of tendon properties.

Change in the dimensions of tissues is often seen as a sign of injury and degeneration, as it may suggest inflammation or general disorder of the tissue. Dimensions are also important for determining the mechanical properties and behaviours of materials, particularly the stress, strain, and elastic modulus. This makes the dimensions significant in the context of a mechanical study of degenerated tendons. Additionally, tendon dimensions are useful in planning harvesting for tendon transfer and joint reconstruction purposes.

Historically, many methods have been used in an attempt to accurately measure the dimensions of soft tissue, since improper measurement can lead to large errors in the calculated properties. These methods can be categorised as destructive (by approximation), contact, and non-contact and can be considered in terms of in vivo and ex vivo.

Synchronized mechanical oscillations at the cell-matrix interface in the formation of tensile tissue

The formation of uniaxial fibrous tissues with defined viscoelastic properties implies the existence of an orchestrated mechanical interaction between the cytoskeleton and the extracellular matrix. This study addresses the nature of this interaction. The hypothesis is that this mechanical interplay underpins the mechanical development of the tissue. In embryonic tendon tissue, an early event in the development of a mechanically robust tissue is the interaction of the pointed tips of extracellular collagen fibrils with the fibroblast plasma membrane to form stable interface structures (fibripositors). Here, we used a fibroblast-generated tissue that is structurally and mechanically matched to embryonic tendon to demonstrate homeostasis of cell-derived and external strain-derived tension over repeated cycles of strain and relaxation. A cell-derived oscillatory tension component is evident in this matrix construct. This oscillatory tension involves synchronization of individual cell forces across the construct and is induced in each strain cycle by transient relaxation and transient tensioning of the tissue. The cell-derived tension along with the oscillatory component is absent in the presence of blebbistatin, which disrupts actinomyosin force generation of the cell. The time period of this oscillation (60-90 s) is well-defined in each tissue sample and matches a primary viscoelastic relaxation time. We hypothesize that this mechanical oscillation of fibroblasts with plasma membrane anchored collagen fibrils is a key factor in mechanical sensing and feedback regulation in the formation of tensile tissues.

Cellular homeostatic tension and force transmission measured in human engineered tendon

Tendons transmit contractile muscular force to bone to produce movement, and it is believed cells can generate endogenous forces on the extracellular matrix to maintain tissue homeostasis. However, little is known about the direct mechanical measurement of cell-matrix interaction in cell-generated human tendon constructs. In this study we examined if cell-generated force could be detected and quantified in engineered human tendon constructs, and if glycosaminoglycans (GAGs) contribute to tendon force transmission. Following de-tensioning of the tendon constructs it was possible to quantify an endogenous re-tensioning. Further, it was demonstrated that the endogenous re-tensioning response was markedly blunted after interference with the cytoskeleton (inhibiting non-muscle myosin-dependent cell contraction by blebbistatin), which confirmed that re-tensioning was cell generated. When the constructs were elongated and held at a constant length a stress relaxation response was quantified, and removing 27% of the GAG content of tendon did not alter the relaxation behavior, which indicates that GAGs do not play a meaningful role in force transmission within this system.

Optimizing a 3D model system for molecular manipulation of tenogenesis

PURPOSE

Tendon injuries are clinically challenging due to poor healing. A better understanding of the molecular events that regulate tendon differentiation would improve current strategies for repair. The mouse model system has been instrumental to tendon studies and several key molecules were initially established in mouse. However, the study of gene function has been limited by the absence of a standard in vitro tendon system for efficiently testing multiple mutations, physical manipulations, and mis-expression. The purpose of this study is therefore to establish such a system.

METHODS

We adapted an existing design for generating three-dimensional (3D) tendon constructs for use with mouse progenitor cells harboring the ScxGFP tendon reporter and the Rosa26-TdTomato Cre reporter. Using these cells, we optimized the parameters for construct formation, inducing tenogenesis via transforming growth factor-β2 (TGFβ2), and genetic recombination via an adenovirus encoding Cre recombinase. Finally, for proof of concept, we used Smad4 floxed cells and tested the robustness of the system for gene knockdown.

RESULTS

We found that TGFβ2 treatment induced a tenogenic phenotype depending on the timing of initiation. Addition of TGFβ2 after 3D "tensioning" enhanced tendon differentiation. Interestingly, while TGFβ2-induced proliferation depended on Smad4, tenogenic parameters such as ScxGFP expression and fibril diameter were independent of Smad4.

CONCLUSIONS

Our results demonstrate the feasibility of this optimized system for harnessing the power of mouse genetics for in vitro applications.

Cellular Changes of Stem Cells in 3-Dimensional Culture

PURPOSE

During various operations and procedures, such as distraction osteogenesis and orthodontics, skeletal tissues use mechanotransduction. Mechanotransduction is important for maintaining bone health and converting mechanical forces into biochemical signals. We hypothesized that cells put under mechanical stress would adapt and change morphologically and respond with a decrease in cellular proliferation to accommodate the stress differences. These differences will be measured at the molecular and genetic level. We also wanted to test the practicality of an in vitro 3-dimensional gel model system.

MATERIALS AND METHODS

We implemented a 3-dimensional cell culture model. The sample was composed of isolated mouse mesenchymal prefibroblast bone marrow cells from the femurs and tibias of 6- to 8-week-old wild-type C57BL6 mice. The cells were seeded on fibronectin-coated hydrogels along with fibrin and nodulin growth factors. The variables tested were a no-force model (control) and a force model. The force model required two 0.1-mm suture pins put through one 0.25-cm length of cell-gel matrix. After the experiments were run to completion, the samples were fixed with 4% paraformaldehyde and embedded in paraffin. Serial sections were cut at a thickness of 5 μm along the long axis for the force construct and encompassing the entire circular area of the control construct. Descriptive and bivariate statistics were computed, and the P value was set at 5%.

RESULTS

There was a statistically significant difference between the 2 models. The force model had longer and straighter primary cilia, less apoptosis, and an increase in cell proliferation. In addition, the shape of the cells was markedly different after the experiment.

CONCLUSIONS

The results of the study suggest cells put under tensile stress have the ability to mechanically sense the environment to provide improved adaptation. Our work also confirms the usefulness of the in vitro 3-dimensional gel model system to mimic in vivo applications.

Fell Muir Lecture: Collagen fibril formation in vitro and in vivo

It is a great honour to be awarded the Fell Muir Prize for 2016 by the British Society of Matrix Biology. As recipient of the prize, I am taking the opportunity to write a minireview on collagen fibrillogenesis, which has been the focus of my research for 33 years. This is the process by which triple helical collagen molecules assemble into centimetre-long fibrils in the extracellular matrix of animals. The fibrils appeared a billion years ago at the dawn of multicellular animal life as the primary scaffold for tissue morphogenesis. The fibrils occur in exquisite three-dimensional architectures that match the physical demands of tissues, for example orthogonal lattices in cornea, basket weaves in skin and blood vessels, and parallel bundles in tendon, ligament and nerves. The question of how collagen fibrils are formed was posed at the end of the nineteenth century. Since then, we have learned about the structure of DNA and the peptide bond, understood how plants capture the sun's energy, cloned animals, discovered antibiotics and found ways of editing our genome in the pursuit of new cures for diseases. However, how cells generate tissues from collagen fibrils remains one of the big unsolved mysteries in biology. In this review, I will give a personal account of the topic and highlight some of the approaches that my research group are taking to find new insights.

A systems biology approach to defining regulatory mechanisms for cartilage and tendon cell phenotypes

Phenotypic plasticity of adult somatic cells has provided emerging avenues for the development of regenerative therapeutics. In musculoskeletal biology the mechanistic regulatory networks of genes governing the phenotypic plasticity of cartilage and tendon cells has not been considered systematically. Additionally, a lack of strategies to effectively reproduce in vitro functional models of cartilage and tendon is retarding progress in this field. De- and redifferentiation represent phenotypic transitions that may contribute to loss of function in ageing musculoskeletal tissues. Applying a systems biology network analysis approach to global gene expression profiles derived from common in vitro culture systems (monolayer and three-dimensional cultures) this study demonstrates common regulatory mechanisms governing de- and redifferentiation transitions in cartilage and tendon cells. Furthermore, evidence of convergence of gene expression profiles during monolayer expansion of cartilage and tendon cells, and the expression of key developmental markers, challenges the physiological relevance of this culture system. The study also suggests that oxidative stress and PI3K signalling pathways are key modulators of in vitro phenotypes for cells of musculoskeletal origin.

Matrix metalloproteinase 14 is required for fibrous tissue expansion

Type I collagen-containing fibrils are major structural components of the extracellular matrix of vertebrate tissues, especially tendon, but how they are formed is not fully understood. MMP14 is a potent pericellular collagenase that can cleave type I collagen in vitro. In this study, we show that tendon development is arrested in Scleraxis-Cre::Mmp14 lox/lox mice that are unable to release collagen fibrils from plasma membrane fibripositors. In contrast to its role in collagen turnover in adult tissue, MMP14 promotes embryonic tissue formation by releasing collagen fibrils from the cell surface. Notably, the tendons grow to normal size and collagen fibril release from fibripositors occurs in Col-r/r mice that have a mutated collagen-I that is uncleavable by MMPs. Furthermore, fibronectin (not collagen-I) accumulates in the tendons of Mmp14-null mice. We propose a model for cell-regulated collagen fibril assembly during tendon development in which MMP14 cleaves a molecular bridge tethering collagen fibrils to the plasma membrane of fibripositors.

DOI:http://dx.doi.org/10.7554/eLife.09345.001

A scaffold of proteins called the extracellular matrix surrounds each of the cells that make up our organs and tissues. This matrix, which contains fibres made of proteins called collagens, provides the physical support needed to hold organs and tissues together. This support is especially important in the tendons—a tough tissue that connects the muscle to bone—and other ‘connective’ tissues.

An enzyme called MMP14 is able to cut through chains of collagen proteins. It belongs to a family of proteins that are involved in breaking down the extracellular matrix to enable cells to divide and for other important processes in cells. Some cancer cells exploit MMP14 to enable them to leave their tissue of origin and spread around the body. Therefore, when researchers bred mutant mice that lacked MMP14, they expected to see excessive growth of collagen fibres in the connective tissues of the mice. However, these mice actually have extremely thin, fragile connective tissue and die soon after birth.

Earlier in 2015, a group of researchers demonstrated that the first stage of tendon development in mice involves the formation of collagen fibres, which are attached to structures that project from tendon cells called fibripositors. Then, soon after the mice are born, the fibripositors disappear and the collagen fibres are released into the extracellular matrix where they grow longer and become thicker. Now, Taylor, Yeung, Kalson et al.—including some of the researchers from the earlier work—have used electron microscopy to investigate how a lack of MMP14 leads to fragile tendons in young mice.

The experiments show that MMP14 plays a crucial role in the first stage of tendon development by detaching the collagen fibres from the fibripositors. MMP14 also promotes the formation of new collagen fibres; the tendons of mutant mice that lack MMP14 have fewer collagen fibres than normal mice. Further experiments revealed that the release of collagen fibres from fibripositors does not require MMP14 to cleave the chains of collagen proteins themselves. Instead, it appears that MMP14 cleaves another protein that is associated with the fibres, called fibronectin.

Taylor, Yeung, Kalson et al.'s findings show that MMP14 plays an important role in the development of tendons by releasing collagen fibres from fibripositors and promoting the formation of new fibres. The next challenge is to find out how MMP14 regulates the number of collagen fibres in mature tendons and other tissues, and how defects in this enzyme can lead to cancer and other diseases.

DOI:http://dx.doi.org/10.7554/eLife.09345.002

pGlcNAc Nanofiber Treatment of Cutaneous Wounds Stimulate Increased Tensile Strength and Reduced Scarring via Activation of Akt1

Treatment of cutaneous wounds with poly-N-acetyl-glucosamine containing nanofibers (pGlcNAc), a novel polysaccharide material derived from a marine diatom, results in increased wound closure, antibacterial activities and innate immune responses. We have shown that Akt1 plays a central role in the regulation of these activities. Here, we show that pGlcNAc treatment of cutaneous wounds results in a smaller scar that has increased tensile strength and elasticity. pGlcNAc treated wounds exhibit decreased collagen content, increased collagen organization and decreased myofibroblast content. A fibrin gel assay was used to assess the regulation of fibroblast alignment in vitro. In this assay, fibrin lattice is formed with two pins that provide focal points upon which the gel can exert force as the cells align from pole to pole. pGlcNAc stimulation of embedded fibroblasts results in cellular alignment as compared to untreated controls, by a process that is Akt1 dependent. We show that Akt1 is required in vivo for the pGlcNAc-induced increased tensile strength and elasticity. Taken together, our findings suggest that pGlcNAc nanofibers stimulate an Akt1 dependent pathway that results in the proper alignment of fibroblasts, decreased scarring, and increased tensile strength during cutaneous wound healing.

Insulin-like growth factor I enhances collagen synthesis in engineered human tendon tissue

OBJECTIVE

Isolated human tendon cells form 3D tendon constructs that demonstrate collagen fibrillogenesis and feature structural similarities to tendon when cultured under tensile load. The exact role of circulating growth factors for collagen formation in tendon is sparsely examined. We investigated the influence of insulin-like growth factor I (IGF-I) on tendon construct formation in 3D cell culture.

DESIGN

Tendon constructs were grown in 0.5 or 10% FBS with or without IGF-I (250 mg/ml) supplementation. Collagen content (fluorometric), mRNA levels (PCR) and fibril diameter (transmission electron microscopy) were determined at 7, 10, 14, 21 and 28 days.

RESULTS

IGF-I revealed a stimulating effect on fibril diameter (up to day 21), mRNA for collagen (to day 28), tenomodulin (to day 28) and scleraxis (at days 10 and 14), and on overall collagen content. 10% FBS diminished the development of fibril diameter (day 14), collagen content (at days 21 and 28) and mRNA expression for collagen, tenomodulin and scleraxis.

CONCLUSION

IGF-I supplementation promotes early onset of tensile load induced collagen formation and tendon structural arrangement, whereas the FBS concentration routinely used in cultures diminishes collagen expression, collagen content and fibril formation.

The effects of scaffold architecture and fibrin gel addition on tendon cell phenotype

Development of tissue engineering scaffolds relies on careful selection of pore architecture and chemistry of the cellular environment. Repair of skeletal soft tissue, such as tendon, is particularly challenging, since these tissues have a relatively poor healing response. When removed from their native environment, tendon cells (tenocytes) lose their characteristic morphology and the expression of phenotypic markers. To stimulate tendon cells to recreate a healthy extracellular matrix, both architectural cues and fibrin gels have been used in the past, however, their relative effects have not been studied systematically. Within this study, a combination of collagen scaffold architecture, axial and isotropic, and fibrin gel addition was assessed, using ovine tendon-derived cells to determine the optimal strategy for controlling the proliferation and protein expression. Scaffold architecture and fibrin gel addition influenced tendon cell behavior independently in vitro. Addition of fibrin gel within a scaffold doubled cell number and increased matrix production for all architectures studied. However, scaffold architecture dictated the type of matrix produced by cells, regardless of fibrin addition. Axial scaffolds, mimicking native tendon, promoted a mature matrix, with increased tenomodulin, a marker for mature tendon cells, and decreased scleraxis, an early transcription factor for connective tissue. This study demonstrated that both architectural cues and fibrin gel addition alter cell behavior and that the combination of these signals could improve clinical performance of current tissue engineering constructs.

Fibrin gels exhibit improved biological, structural, and mechanical properties compared with collagen gels in cell-based tendon tissue-engineered constructs

The prevalence of tendon and ligament injuries and inadequacies of current treatments is driving the need for alternative strategies such as tissue engineering. Fibrin and collagen biopolymers have been popular materials for creating tissue-engineered constructs (TECs), as they exhibit advantages of biocompatibility and flexibility in construct design. Unfortunately, a few studies have directly compared these materials for tendon and ligament applications. Therefore, this study aims at determining how collagen versus fibrin hydrogels affect the biological, structural, and mechanical properties of TECs during formation in vitro. Our findings show that tendon and ligament progenitor cells seeded in fibrin constructs exhibit improved tenogenic gene expression patterns compared with their collagen-based counterparts for approximately 14 days in culture. Fibrin-based constructs also exhibit improved cell-derived collagen alignment, increased linear modulus (2.2-fold greater) compared with collagen-based constructs. Cyclic tensile loading, which promotes the maturation of tendon constructs in a previous work, exhibits a material-dependent effect in this study. Fibrin constructs show trending reductions in mechanical, biological, and structural properties, whereas collagen constructs only show improved tenogenic expression in the presence of mechanical stimulation. These findings highlight that components of the mechanical stimulus (e.g., strain amplitude or time of initiation) need to be tailored to the material and cell type. Given the improvements in tenogenic expression, extracellular matrix organization, and material properties during static culture, in vitro findings presented here suggest that fibrin-based constructs may be a more suitable alternative to collagen-based constructs for tissue-engineered tendon/ligament repair.

Arhgap28 is a RhoGAP that inactivates RhoA and downregulates stress fibers

The small GTPase RhoA is a major regulator of actin reorganization during the formation of stress fibers; thus identifying molecules that regulate Rho activity is necessary for a complete understanding of the mechanisms that determine cell contractility. Here, we have identified Arhgap28 as a Rho GTPase activating protein (RhoGAP) that switches RhoA to its inactive form. We generated an Arhgap28-LacZ reporter mouse that revealed gene expression in soft tissues at E12.5, pre-bone structures of the limb at E15.5, and prominent expression restricted mostly to ribs and limb long bones at E18.5 days of development. Expression of recombinant Arhgap28-V5 in human osteosarcoma SaOS-2 cells caused a reduction in the basal level of RhoA activation and disruption of actin stress fibers. Extracellular matrix assembly studies using a 3-dimensional cell culture system showed that Arhgap28 was upregulated during Rho-dependent assembly of the ECM. Taken together, these observations led to the hypothesis that an Arhgap28 knockout mouse model would show a connective tissue phenotype, perhaps affecting bone. Arhgap28-null mice were viable and appeared normal, suggesting that there could be compensation from other RhoGAPs. Indeed, we showed that expression of Arhgap6 (a closely related RhoGAP) was upregulated in Arhgap28-null bone tissue. An upregulation in RhoA expression was also detected suggesting that Arhgap28 may be able to additionally regulate Rho signaling at a transcriptional level. Microarray analyses revealed that Col2a1, Col9a1, Matn3, and Comp that encode extracellular matrix proteins were downregulated in Arhgap28-null bone. Although mutations in these genes cause bone dysplasias no bone phenotype was detected in the Arhgap-28 null mice. Together, these data suggest that the regulation of Rho by RhoGAPs, including Arhgap28, during the assembly and development of mechanically strong tissues is complex and may involve multiple RhoGAPs.

Strategies for directing the structure and function of three-dimensional collagen biomaterials across length scales

Collagen type I is a widely used natural biomaterial that has found utility in a variety of biological and medical applications. Its well-characterized structure and role as an extracellular matrix protein make it a highly relevant material for controlling cell function and mimicking tissue properties. Collagen type I is abundant in a number of tissues, and can be isolated as a purified protein. This review focuses on hydrogel biomaterials made by reconstituting collagen type I from a solubilized form, with an emphasis on in vitro studies in which collagen structure can be controlled. The hierarchical structure of collagen from the nanoscale to the macroscale is described, with an emphasis on how structure is related to function across scales. Methods of reconstituting collagen into hydrogel materials are presented, including molding of macroscopic constructs, creation of microscale modules and electrospinning of nanoscale fibers. The modification of collagen biomaterials to achieve the desired structures and functions is also addressed, with particular emphasis on mechanical control of collagen structure, creation of collagen composite materials and crosslinking of collagenous matrices. Biomaterials scientists have made remarkable progress in rationally designing collagen-based biomaterials and in applying them both to the study of biology and for therapeutic benefit. This broad review illustrates recent examples of techniques used to control collagen structure and thereby to direct its biological and mechanical functions.

Nonmuscle myosin II powered transport of newly formed collagen fibrils at the plasma membrane

Collagen fibrils can exceed thousands of microns in length and are therefore the longest, largest, and most size-pleomorphic protein polymers in vertebrates; thus, knowing how cells transport collagen fibrils is essential for a more complete understanding of protein transport and its role in tissue morphogenesis. Here, we identified newly formed collagen fibrils being transported at the surface of embryonic tendon cells in vivo by using serial block face-scanning electron microscopy of the cell-matrix interface. Newly formed fibrils ranged in length from ~1 to ~30 µm. The shortest (1-10 µm) occurred in intracellular fibricarriers; the longest (~30 µm) occurred in plasma membrane fibripositors. Fibrils and fibripositors were reduced in numbers when collagen secretion was blocked. ImmunoEM showed the absence of lysosomal-associated membrane protein 2 on fibricarriers and fibripositors and there was no effect of leupeptin on fibricarrier or fibripositor number and size, suggesting that fibricarriers and fibripositors are not part of a fibril degradation pathway. Blebbistatin decreased fibricarrier number and increased fibripositor length; thus, nonmuscle myosin II (NMII) powers the transport of these compartments. Inhibition of dynamin-dependent endocytosis with dynasore blocked fibricarrier formation and caused accumulation of fibrils in fibripositors. Data from fluid-phase HRP electron tomography showed that fibricarriers could originate at the plasma membrane. We propose that NMII-powered transport of newly formed collagen fibrils at the plasma membrane is fundamental to the development of collagen fibril-rich tissues. A NMII-dependent cell-force model is presented as the basis for the creation and dynamics of fibripositor structures.

The effects of age and the expression of SPARC on extracellular matrix production by cardiac fibroblasts in 3-D cultures

Fibrillar collagen is the primary component of the cardiac interstitial extracellular matrix. This extracellular matrix undergoes dramatic changes from birth to adulthood and then into advanced age. As evidence, fibrillar collagen content was compared in sections from neonates, adult, and old hearts and was found to increase at each respective age. Cardiac fibroblasts are the principle cell type that produce and control fibrillar collagen content. To determine whether fibroblast production, processing, and deposition of collagen differed with age, primary cardiac fibroblasts from neonate, adult, and old mice were isolated and cultured in 3-dimensional (3D) fibrin gels. Fibroblasts from each age aligned in fibrin gels along points of tension and deposited extracellular matrix. By confocal microscopy, wild-type neonate fibroblasts appeared to deposit less collagen into fibrillar structures than fibroblasts from adults. However, by immunoblot analysis, differences in procollagen production and processing of collagen I were not detected in neonate versus adult fibroblasts. In contrast, fibroblasts from old mice demonstrated increased efficiency of procollagen processing coupled with decreased production of total collagen. SPARC is a collagen-binding protein previously shown to affect cardiac collagen deposition. Accordingly, in the absence of SPARC, less collagen appeared to be associated with fibroblasts of each age grown in fibrin gels. In addition, the increased efficiency of procollagen alpha 1(I) processing in old wild-type fibroblasts was not detected in old SPARC-null fibroblasts. Increased levels of fibronectin were detected in wild-type neonate fibroblasts over that of adult and old fibroblasts but not in SPARC-null neonate fibroblasts versus older ages. Immunostaining of SPARC overlapped with that of collagen I but not to that of fibronectin in 3D cultures. Hence, whereas increases in procollagen processing, influenced by SPARC expression, plausibly contribute to increased collagen deposition in old hearts, other cellular mechanisms likely affect differential collagen deposition by neonate fibroblasts.

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.

Mechanical factors in embryonic tendon development: potential cues for stem cell tenogenesis

Tendons are connective tissues required for motion and are frequently injured. Poor healing and inadequate return to normal tissue structure and mechanical function make tendon a prime candidate for tissue engineering; however functional tendons have yet to be engineered. The physical environment, from substrate stiffness to dynamic mechanical loading, may regulate tenogenic stem cell differentiation. Tissue stiffness and loading parameters derived from embryonic development may enhance tenogenic stem cell differentiation and tendon tissue formation. We highlight the current understanding of the mechanical environment experienced by embryonic tendons and how progenitor cells may sense and respond to physical inputs. We further discuss how mechanical factors have only recently been used to induce tenogenic fate in stem cells.

Proteomics-based identification of novel proteins in temporal tendons of patients with masticatory muscle tendon--aponeurosis hyperplasia

Masticatory muscle tendon-aponeurosis hyperplasia (MMTAH) is a new disease associated with limited mouth opening that is often misdiagnosed as a temporomandibular disorder; subsequently, patients are mistakenly treated with irreversible operations. Due to the poor presentation and characterization of symptoms, the underlying pathological conditions remain unclear. We have previously conducted a proteomic analysis of tendons derived from one MMTAH subject and one facial deformity subject using two-dimensional fluorescence difference gel electrophoresis and liquid chromatography coupled with tandem mass spectrometry. However, the results were obtained for only one subject. The aim of the present study was to confirm the expression of specific molecules in tendon tissues from multiple subjects with MMTAH by applying two-dimensional polyacrylamide gel electrophoresis with matrix-assisted laser desorption ionization time-of-flight mass spectrometry. Of the 19 proteins identified in tendons from both MMTAH and facial deformity patients, fibrinogen fragment D and beta-crystallin A4 were up-regulated, whereas myosin light chain 4 was down-regulated in MMTAH. We also found fibrinogen to be expressed robustly in tendon tissues of MMTAH patients. Our data provide the possibility that the distinctive expression of these novel proteins is associated with the pathology of MMTAH.

Enhancing the mechanical properties of engineered tissue through matrix remodeling via the signaling phospholipid lysophosphatidic acid

Knee meniscus fibrocartilage is frequently injured, resulting in approximately 1 million procedures annually in the US and Europe. Its near-avascularity contributes heavily to its inability to heal, and places it as a prime candidate for replacement through regenerative medicine. Here, we describe a novel approach to increase extracellular matrix organization, rather than content, in order to augment the mechanical properties of engineered tissue. To synthesize fibrocartilage, we employ a self-assembling process, which is free of exogenous scaffolds and relies on cell-to-cell interactions to form all-biologic constructs. When treated with the signaling phospholipid lysophosphatidic acid (LPA), tissue constructs displayed increased tensile properties and collagen organization, while total collagen content remained unchanged. LPA-treated constructs exhibited greater DNA content, indicative that the molecule exerted a signaling effect. Furthermore, LPA-treated cells displayed significant cytoskeletal reorganization. We conclude that LPA induced cytoskeletal reorganization and cell-matrix traction, which resulted in matrix reorganization and increased tensile properties. This study emphasizes the potential of non-traditional stimuli, such as signaling phospholipids, for use in tissue development studies. The extension of these results to other collagen-rich tissues represents a promising avenue for future exploration.

Re-establishment of cytoskeletal tensional homeostasis in lax tendons occurs through an actin-mediated cellular contraction of the extracellular matrix

Cytoskeletal tensional homeostasis is known to be an important factor in controlling catabolic gene expression in tendon cells. Loss of cell tension in lax rat tail tendon fascicles (RTTfs) has been associated with an upregulation of MMP-13 gene expression and protein synthesis. To determine the role of the actin cytoskeleton in re-establishing tensional homeostasis in lax tendons, RTTfs were allowed to freely contract in vitro for 8 days. The cultured RTTfs contracted rapidly, reaching 50% of their initial length by 3 days. This contraction was associated with the presence of α-smooth muscle actin positive cells within the tendon. Disruption of the actin network by cytochalasian D caused an immediate and significant elongation of the contracted RTTfs. Subsequent removal of the cytochalasian D re-initiated the contraction process. When lax RTTfs were allowed to contract between fixed clamps in culture and become taut, they demonstrated a marked decrease in MMP-13 staining intensity when compared to freely contracting RTTfs. The ability of native tendon cells to contract lax tendons and re-establish their homeostatic "set point" with respect to collagenase production may be an important mechanism in the recovery of tendons elongated by injury, surgical positioning, or cyclic, viscoelastic creep secondary to repetitive exercise.

Making parallel lines meet: transferring information from microtubules to extracellular matrix

The extracellular matrix is constructed beyond the plasma membrane, challenging mechanisms for its control by the cell. In plants, the cell wall is highly ordered, with cellulose microfibrils aligned coherently over a scale spanning hundreds of cells. To a considerable extent, deploying aligned microfibrils determines mechanical properties of the cell wall, including strength and compliance. Cellulose microfibrils have long been seen to be aligned in parallel with an array of microtubules in the cell cortex. How do these cortical microtubules affect the cellulose synthase complex? This question has stood for as many years as the parallelism between the elements has been observed, but now an answer is emerging. Here, we review recent work establishing that the link between microtubules and microfibrils is mediated by a protein named cellulose synthase-interacting protein 1 (CSI1). The protein binds both microtubules and components of the cellulose synthase complex. In the absence of CSI1, microfibrils are synthesized but their alignment becomes uncoupled from the microtubules, an effect that is phenocopied in the wild type by depolymerizing the microtubules. The characterization of CSI1 significantly enhances knowledge of how cellulose is aligned, a process that serves as a paradigmatic example of how cells dictate the construction of their extracellular environment.

Slow stretching that mimics embryonic growth rate stimulates structural and mechanical development of tendon-like tissue in vitro

A distinctive feature of embryonic tendon development is the steady increase in collagen fibril diameter and associated improvement of tissue mechanical properties. A potential mechanical stimulus for these changes is slow stretching of the tendon during limb growth. Testing this hypothesis in vivo is complicated by the presence of other developmental processes including muscle development and innervation. Here we used a cell culture tendon-like construct to determine if slow stretch can explain the increases in fibril diameter and mechanical properties that are observed in vivo. Non-stretched constructs had an ultrastructural appearance and mechanical properties similar to those of early embryonic tendon. However, slowly stretching during 4 days in culture increased collagen fibril diameter, fibril packing volume, and mechanical stiffness, and thereby mimicked embryonic development. 3D EM showed cells with improved longitudinal alignment and elongated nuclei, which raises the hypothesis that nuclear deformation could be a novel mechanism during tendon development. Developmental Dynamics 240:2520–2528, 2011. © 2011 Wiley Periodicals, Inc.

Tenocyte contraction induces crimp formation in tendon-like tissue

Tendons are composed of longitudinally aligned collagen fibrils arranged in bundles with an undulating pattern, called crimp. The crimp structure is established during embryonic development and plays a vital role in the mechanical behaviour of tendon, acting as a shock-absorber during loading. However, the mechanism of crimp formation is unknown, partly because of the difficulties of studying tendon development in vivo. Here, we used a 3D cell culture system in which embryonic tendon fibroblasts synthesise a tendon-like construct comprised of collagen fibrils arranged in parallel bundles. Investigations using polarised light microscopy, scanning electron microscopy and fluorescence microscopy showed that tendon constructs contained a regular pattern of wavy collagen fibrils. Tensile testing indicated that this superstructure was a form of embryonic crimp producing a characteristic toe region in the stress-strain curves. Furthermore, contraction of tendon fibroblasts was the critical factor in the buckling of collagen fibrils during the formation of the crimp structure. Using these biological data, a finite element model was built that mimics the contraction of the tendon fibroblasts and monitors the response of the Extracellular matrix. The results show that the contraction of the fibroblasts is a sufficient mechanical impulse to build a planar wavy pattern. Furthermore, the value of crimp wavelength was determined by the mechanical properties of the collagen fibrils and inter-fibrillar matrix. Increasing fibril stiffness combined with constant matrix stiffness led to an increase in crimp wavelength. The data suggest a novel mechanism of crimp formation, and the finite element model indicates the minimum requirements to generate a crimp structure in embryonic tendon.

Development of the zebrafish myoseptum with emphasis on the myotendinous junction

Zebrafish myosepta connect two adjacent muscle cells and transmit muscular forces to axial structures during swimming via the myotendinous junction (MTJ). The MTJ establishes transmembrane linkages system consisting of extracellular matrix molecules (ECM) surrounding the basement membrane, cytoskeletal elements anchored to sarcolema, and all intermediate proteins that link ECM to actin filaments. Using a series of zebrafish specimens aged between 24 h post-fertilization and 2 years old, the present paper describes at the transmission electron microscope level the development of extracellular and intracellular elements of the MTJ. The transverse myoseptum development starts during the segmentation period by deposition of sparse and loosely organized collagen fibrils. During the hatching period, a link between actin filaments and sarcolemma is established. The basal lamina underlining sarcolemma is well differentiated. Later, collagen fibrils display an orthogonal orientation and fibroblast-like cells invade the myoseptal stroma. A dense network of collagen fibrils is progressively formed that both anchor myoseptal fibroblasts and sarcolemmal basement membrane. The differentiation of a functional MTJ is achieved when sarcolemma interacts with both cytoskeletal filaments and extracellular components. This solid structural link between contractile apparatus and ECM leads to sarcolemma deformations resulting in the formation of regular invaginations, and allows force transmission during muscle contraction. This paper presents the first ultrastructural atlas of the zebrafish MTJ development, which represents an useful tool to analyse the mechanisms of the myotendinous system formation and their disruption in muscle disorders.

The knee meniscus: structure-function, pathophysiology, current repair techniques, and prospects for regeneration

Extensive scientific investigations in recent decades have established the anatomical, biomechanical, and functional importance that the meniscus holds within the knee joint. As a vital part of the joint, it acts to prevent the deterioration and degeneration of articular cartilage, and the onset and development of osteoarthritis. For this reason, research into meniscus repair has been the recipient of particular interest from the orthopedic and bioengineering communities. Current repair techniques are only effective in treating lesions located in the peripheral vascularized region of the meniscus. Healing lesions found in the inner avascular region, which functions under a highly demanding mechanical environment, is considered to be a significant challenge. An adequate treatment approach has yet to be established, though many attempts have been undertaken. The current primary method for treatment is partial meniscectomy, which commonly results in the progressive development of osteoarthritis. This drawback has shifted research interest toward the fields of biomaterials and bioengineering, where it is hoped that meniscal deterioration can be tackled with the help of tissue engineering. So far, different approaches and strategies have contributed to the in vitro generation of meniscus constructs, which are capable of restoring meniscal lesions to some extent, both functionally as well as anatomically. The selection of the appropriate cell source (autologous, allogeneic, or xenogeneic cells, or stem cells) is undoubtedly regarded as key to successful meniscal tissue engineering. Furthermore, a large variation of scaffolds for tissue engineering have been proposed and produced in experimental and clinical studies, although a few problems with these (e.g., byproducts of degradation, stress shielding) have shifted research interest toward new strategies (e.g., scaffoldless approaches, self-assembly). A large number of different chemical (e.g., TGF-β1, C-ABC) and mechanical stimuli (e.g., direct compression, hydrostatic pressure) have also been investigated, both in terms of encouraging functional tissue formation, as well as in differentiating stem cells. Even though the problems accompanying meniscus tissue engineering research are considerable, we are undoubtedly in the dawn of a new era, whereby recent advances in biology, engineering, and medicine are leading to the successful treatment of meniscal lesions.