Characteristics of Sonography in a Rat Achilles Tendinopathy Model: Possible Non-invasive Predictors of Biomechanics

Novel In Vitro Platform for Studying the Cell Response to Healthy and Diseased Tendon Matrices

Current in vitro models poorly represent the healthy or diseased tendon microenvironment, limiting the translation of the findings to clinics. The present work aims to establish a physiologically relevant in vitro tendon platform that mimics biophysical aspects of a healthy and tendinopathic tendon matrix using a decellularized bovine tendon and to characterize tendon cells cultured using this platform. Bovine tendons were subjected to various decellularization techniques, with the efficacy of decellularization determined histologically. The biomechanical and architectural properties of the decellularized tendons were characterized using an atomic force microscope. Tendinopathy-mimicking matrices were prepared by treating the decellularized tendons with collagenase for 3 h or collagenase-chondroitinase (CC) for 1 h. The tendon tissue collected from healthy and tendinopathic patients was characterized using an atomic force microscope and compared to that of decellularized matrices. Healthy human tendon-derived cells (hTDCs) from the hamstring tendon were cultured on the decellularized matrices for 24 or 48 h, with cell morphology characterized using f-actin staining and gene expression characterized using real-time PCR. Tendon matrices prepared by freeze-thawing and 48 h nuclease treatment were fully decellularized, and the aligned structure and tendon stiffness (1.46 MPa) were maintained. Collagenase treatment prepared matrices with a disorganized architecture and reduced stiffness (0.75 MPa), mimicking chronic tendinopathy. Treatment with CC prepared matrices with a disorganized architecture without altering stiffness, mimicking early tendinopathy (1.52 MPa). hTDCs on a healthy tendon matrix were elongated, and the scleraxis (SCX) expression was maintained. On tendinopathic matrices, hTDCs had altered morphological characteristics and lower SCX expression. The expression of genes related to actin polymerization, matrix degradation and remodeling, and immune cell invasion were higher in hTDCs on tendinopathic matrices. Overall, the present study developed a physiological in vitro system to mimic healthy tendons and early and late tendinopathy, and it can be used to better understand tendon cell characteristics in healthy and diseased states.

A Novel Tendon Injury Model, Induced by Collagenase Administration Combined with a Thermo-Responsive Hydrogel in Rats, Reproduces the Pathogenesis of Human Degenerative Tendinopathy

Patellar tendinopathy is a common clinical problem, but its underlying pathophysiology remains poorly understood, primarily due to the absence of a representative experimental model. The most widely used method to generate such a model is collagenase injection, although this method possesses limitations. We developed an optimized rat model of patellar tendinopathy via the ultrasound-guided injection of collagenase mixed with a thermo-responsive Pluronic hydrogel into the patellar tendon of sixty male Wistar rats. All analyses were carried out at 3, 7, 14, 30, and 60 days post-injury. We confirmed that our rat model reproduced the pathophysiology observed in human patients through analyses of ultrasonography, histology, immunofluorescence, and biomechanical parameters. Tendons that were injured by the injection of the collagenase-Pluronic mixture exhibited a significant increase in the cross-sectional area (p < 0.01), a high degree of tissue disorganization and hypercellularity, significantly strong neovascularization (p < 0.01), important changes in the levels of types I and III collagen expression, and the organization and presence of intra-tendinous calcifications. Decreases in the maximum rupture force and stiffness were also observed. These results demonstrate that our model replicates the key features observed in human patellar tendinopathy. Collagenase is evenly distributed, as the Pluronic hydrogel prevents its leakage and thus, damage to surrounding tissues. Therefore, this model is valuable for testing new treatments for patellar tendinopathy.

Repurposing of Angiotensin-converting-enzyme Inhibitor on Prevention of Post-surgical Tendon Adhesion

BACKGROUND

Formation of adhesion bands is a frequent clinical complication after tendon injury or surgery with limited treatment options. This study investigates the repurposing of Angiotensin-Converting-Enzyme Inhibitor (ACEI) in attenuating post-operative tendon-sheath adhesion bands in an Achilles tendon rat model.

METHODS

Structural, mechanical, histological, and biochemical characteristics of the Achilles tendons were compared in the presence and absence of oral ACEI (enalapril) using the Achilles tendon adhesion (TA) model in rats. Inflammation and total fibrosis of tendon tissues were compared between groups using molecular investigations along with macroscopic and histological scoring methods.

RESULTS

ACEI significantly alleviated the severity, length, and density of Achilles TAs. Moreover, histopathological changes, recruitment of inflammatory cells, and inflammation were significantly decreased in post-operative tissue samples as quantified with the Moran scoring model. We showed that ACEI treatment elicits a potent anti-fibrotic effect on tendon tissue samples, as illustrated by decreasing the severity and extent of the formed fibrotic tissue and collagen accumulation at the site of surgery when scored either by Tang or Ishiyama grading systems. The H&E staining showed no histopathological changes or damage to the principal organs.

CONCLUSION

Our results showed that ACEI is a safe and effective therapeutic candidate with potent immunomodulatory and anti-fibrotic features to alleviate surgery-induced development of fibrotic adhesive tissue. However, its efficacy needs to be further validated in clinical studies.

The LINC complex regulates Achilles tendon elastic modulus, Achilles and tail tendon collagen crimp, and Achilles and tail tendon lateral expansion during early postnatal development

There is limited understanding of how mechanical signals regulate tendon development. The nucleus has emerged as a major regulator of cellular mechanosensation, via the linker of nucleoskeleton and cytoskeleton (LINC) protein complex. Specific roles of LINC in tenogenesis have not been explored. In this study, we investigate how LINC regulates tendon development by disabling LINC-mediated mechanosensing via dominant negative (dn) expression of the Klarsicht, ANC-1, and Syne Homology (KASH) domain, which is necessary for LINC to function. We hypothesized that LINC regulates mechanotransduction in developing tendon, and that disabling LINC would impact tendon mechanical properties and structure in a mouse model of dnKASH. We used Achilles (AT) and tail (TT) tendons as representative energy-storing and limb-positioning tendons, respectively. Mechanical testing at postnatal day 10 showed that disabling the LINC complex via dnKASH significantly impacted tendon mechanical properties and cross-sectional area, and that effects differed between ATs and TTs. Collagen crimp distance was also impacted in dnKASH tendons, and was significantly decreased in ATs, and increased in TTs. Overall, we show that disruption to the LINC complex specifically impacts tendon mechanics and collagen crimp structure, with unique responses between an energy-storing and limb-positioning tendon. This suggests that nuclear mechanotransduction through LINC plays a role in regulating tendon formation during neonatal development.

Pro-inflammatory activity of long noncoding RNA FOXD2-AS1 in Achilles tendinopathy

Achilles tendinopathy is a prevalent clinical problem that plagues athletes and general populations. Achilles tendon healing is a complex process, and so far, there is no successful long-term solution to Achilles tendinopathy in the field of microsurgery due to its poor natural regeneration ability. Limitations in understanding the pathogenesis of Achilles tendon development and Achilles tendon injury hinder clinical treatment developments. There is an increasing demand for innovative conservative treatments that can improve Achilles tendon injury. In this study, a Sprague-Dawley rat model of Achilles tendinopathy was established. Lentiviral vectors that interfere with the expression of FOXD2-AS1, miR-21-3p, or PTEN were injected every 3 days. Rats were euthanized after 3 weeks, and the effect of FOXD2-AS1, miR-21-3p, or PTEN on Achilles tendon healing was analyzed by histological observation, biomechanical test, and examinations of inflammatory factors and tendon markers. As measured, downregulating FOXD2-AS1 or upregulating miR-21-3p improved histological structure, suppressed inflammation, promoted the expression of tendon markers, and optimized the biomechanical properties of Achilles tendon. Upregulating PTEN was capable of reversing the promoting effect of inhibition of FOXD2-AS1 on Achilles tendon healing. As concluded, deficiency of FOXD2-AS1 accelerates the healing of Achilles tendon injury and improves tendon degeneration by regulating the miR-21-3p/PTEN axis and promoting the activation of the PI3K/AKT signaling pathway.

Scavenging of reactive oxygen species can adjust the differentiation of tendon stem cells and progenitor cells and prevent ectopic calcification in tendinopathy

Tendinopathy is a common disorder that leads to pain and impaired quality of life. Recent studies revealed that osteogenic differentiation of tendon stem/progenitor cells (TSPCs) played an important role in the pathogenesis of tendon calcification and tendinopathy. In this study, we found that the growth hormone-releasing hormone agonist (GA) can prevent matrix degradation and osteogenic differentiation in TSPCs. As oxidative stress is a key factor in the osteogenic differentiation of TSPCs, we used bovine serum albumin/heparin nanoparticles (BHNPs), which have biocompatibility and drug loading capacity, to scavenge reactive oxygen species (ROS) and achieve sustained release of GA at the site of inflammation. The newly developed BHNPs@GA had a synergetic effect on reducing ROS production in TSPCs. In addition, BHNPs@GA effectively inhibited tendon calcification and promoted collagen formation in a rat model of tendinopathy. Focusing on the ROS underlying the differentiation and dedifferentiation of TSPCs, this work demonstrated that sustained release of GA targeting ROS and ectopic ossification is a practical therapeutic strategy for treating tendinopathy. STATEMENT OF SIGNIFICANCE: Osteogenic differentiation of tendon stem/progenitor cells (TSPCs) plays an important role in the pathogenesis of ectopic calcification in tendinopathy. In this study, we found that growth hormone-releasing hormone agonist (GA) can reduce reactive oxygen species (ROS) production and adjust TSPCs differentiation. Bovine serum albumin/heparin nanoparticles (BHNPs) were developed to encapsulate GA and achieve sustained release of GA at the site of inflammation. The developed compound, BHNPs@GA, with a synergistic effect of inhibiting ROS and thus, can effectively adjust TSPCs differentiation, inhibit tendon calcification, and promote collagen formation in tendinopathy. This study highlighted the role of ROS underlying the differentiation and dedifferentiation of TSPCs in tendinopathy, and findings may help to identify new therapeutic targets and develop novel strategy for treating tendinopathy.

Amelioration of experimental tendinopathy by lentiviral CD44 gene therapy targeting senescence-associated secretory phenotypes

CD44 exerts anti-senescence effects in many disease models. We examined senescence in tendinopathy and the effect of CD44 on senescence-associated secretory phenotypes (SASPs). Senescent markers were determined in human tendinopathic long head of bicep (LHB) and normal hamstring tendons. CD44 gene transfer in rat tendinopathic tenocytes stimulated with interleukin (IL)-1β and a rat Achilles tendinopathy model were performed using lentiviral vectors. Expression levels of p53, p21, and p16 and senescence-associated β-galactosidase (SA-β-gal) activity were positively correlated with the severity of human tendinopathy and were higher in rat and human tendinopathic tenocytes than in normal controls. CD44 overexpressed tenocyte transfectants exhibited reduced levels of IL-6, matrix metalloproteinases (MMPs), cyclooxygenase (COX)-2, p53, p21, p16, SA-β-gal, and phospho-nuclear factor (NF)-κB, whereas their collagen type I alpha 1 (COL1A1) and tenomodulin (tnmd) levels were increased when compared with control transfectants under IL-1β-stimulated conditions. In the animal model, CD44 overexpression lowered the ultrasound and histology scores and expression levels of the senescent and SASP markers COX-2 and phospho-NF-κB. Bromodeoxyuridine (BrdU)- and tnmd-positive cell numbers were increased in the LVCD44-transduced tendinopathic tendons. Senescence is positively correlated with tendinopathic severity, and CD44 overexpression may protect the tendinopathic tendons from SASPs via anti-inflammation and maintenance of extracellular matrix homeostasis.

Cross-Linked Hyaluronate and Corticosteroid Combination Ameliorate the Rat Experimental Tendinopathy through Anti-Senescent and -Apoptotic Effects

The combination of cross-linked hyaluronate (cHA) and corticosteroid showed more rapid pain or functional improvement in knee osteoarthritis and adhesive capsulitis. However, rare evidence of this combination in treating tendinopathy has been reported. We hypothesized that the specific formulations of cHA and dexamethasone (DEX) conferred amelioration of tendinopathy via anti-apoptosis and anti-senescence. In this controlled laboratory study, primary tenocytes from the human tendinopathic long head of biceps were treated with three cHA formulations (cHA:linealized HA = 80:20, 50:50, and 20:80) + DEX with or without IL-1β stimulation. Cell viability, inflammatory cytokines, tendon-related proliferation markers, matrix metalloproteinases (MMPs), senescent markers, and apoptosis were examined. The in vivo therapeutic effects of the selected cHA + DEX combinations were evaluated in a collagenase-induced rat patellar tendinopathy model. The expression levels of inflammatory mediators, including IL-1β, IL-6, COX-2, MMP-1, and MMP-3 were significantly reduced in all cHA + DEX-treated tenocytes (p < 0.05, all). The cHA (50:50) + DEX and cHA (20:80) + DEX combinations protected tenocytes from cytotoxicity, senescence, and apoptosis induced by DEX in either IL-1β stimulation or none. Furthermore, the two combinations significantly improved the rat experimental tendinopathy by reducing ultrasound feature scores and histological scores as well as the levels of apoptosis, senescence, and senescence-associated secretory phenotypes (p < 0.05, all). We identified two specific cHA formulations (cHA (50:50) and cHA (20:80)) + DEX that could ameliorate tendinopathy through anti-senescence and -apoptosis without cytotoxicity. This study provides a possible approach to treating tendinopathy using the combination of two well-known agents.

Evaluation of cytokines in protective effect of docosahexaenoic acid in experimental achilles tendinopathy rat model induced with type-1 collagenase

BACKGROUND

We aimed to investigate the effectiveness of docosahexaenoic acid (DHA) as a treatment for Achilles tendinopathy (AT) induced with type-I collagenase in rats and compare it with collagen.

METHODS

The AT model was induced with type I collagenase, and animals were randomly assigned to groups. Group 1:AT, Group 2: Collagen (7.2 mg/kg/day), Group 3:DHA (300 mg/kg/day), and Group 4:DHA (100 mg/kg/day). Right tendons of Group1 were used as a healthy control (HC). Oral treatments were applied for eight weeks. Serum tumor necrosis factor-alpha(TNF-α), matrix metalloproteinase-13 (MMP-13), and interleukin-1 beta(IL-1β) concentrations were determined by ELISA. Tendon samples were taken for histopathological evaluation and examined immunohistochemically with antibodies specific for Col1A1, TNF-α, MMP-13, IL-1β, and nitric oxide synthase-2(NOS-2). The ultimate tensile force (UTF) yield force(YF) and stiffness were measured by biomechanical assessments.

RESULTS

UTF,YF and stiffness values were increased in all treatment groups compared to the AT control, a significant increase was found in Group 2 (p < 0.05). There was severe degeneration of tendon cells in the AT control. The tendon cells in samples from Groups 2-3 were less degraded, and this was statistically significant (p < 0.05). TNF-α, MMP-13, IL-1β, and NOS-2 expressions were significantly higher in the AT control compared to the HC. In all treatment groups, their concentrations were lower than in the AT control. Serum TNF-α, MMP-13, and IL-1β levels were lower in all treatment groups (Especially in Group3 (p < 0.001)) compared to Group1.

CONCLUSION

The efficacy of high-dose DHA as a treatment for AT was investigated from biochemical, histopathological, and biomechanical perspectives. The results showed that DHA could be an alternative treatment compound to collagen.

Tendinopathy and tendon material response to load: What we can learn from small animal studies

Tendinopathy is a debilitating disease that causes as much as 30% of all musculoskeletal consultations. Existing treatments for tendinopathy have variable efficacy, possibly due to incomplete characterization of the underlying pathophysiology. Mechanical load can have both beneficial and detrimental effects on tendon, as the overall tendon response depends on the degree, frequency, timing, and magnitude of the load. The clinical continuum model of tendinopathy offers insight into the late stages of tendinopathy, but it does not capture the subclinical tendinopathic changes that begin before pain or loss of function. Small animal models that use high tendon loading to mimic human tendinopathy may be able to fill this knowledge gap. The goal of this review is to summarize the insights from in-vivo animal studies of mechanically-induced tendinopathy and higher loading regimens into the mechanical, microstructural, and biological features that help characterize the continuum between normal tendon and tendinopathy. STATEMENT OF SIGNIFICANCE: This review summarizes the insights gained from in-vivo animal studies of mechanically-induced tendinopathy by evaluating the effect high loading regimens have on the mechanical, structural, and biological features of tendinopathy. A better understanding of the interplay between these realms could lead to improved patient management, especially in the presence of painful tendon.

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.

Changes in Gene Expression Associated with Collagen Regeneration and Remodeling of Extracellular Matrix after Percutaneous Electrolysis on Collagenase-Induced Achilles Tendinopathy in an Experimental Animal Model: A Pilot Study

Percutaneous electrolysis is an emerging intervention proposed for the management of tendinopathies. Tendon pathology is characterized by a significant cell response to injury and gene expression. No study investigating changes in expression of those genes associated with collagen regeneration and remodeling of extracellular matrix has been conducted. The aim of this pilot study was to investigate gene expression changes after the application of percutaneous electrolysis on experimentally induced Achilles tendinopathy with collagenase injection in an animal model. Fifteen Sprague Dawley male rats were randomly divided into three different groups (no treatment vs. percutaneous electrolysis vs. needling). Achilles tendinopathy was experimentally induced with a single bolus of collagenase injection. Interventions consisted of 3 sessions (one per week) of percutaneous electrolysis or just needling. The rats were euthanized, and molecular expression of genes involved in tendon repair and remodeling, e.g., Cox2, Mmp2, Mmp9, Col1a1, Col3a1, Vegf and Scx, was examined at 28 days after injury. Histological tissue changes were determined with hematoxylin–eosin and safranin O analyses. The images of hematoxylin–eosin and Safranin O tissue images revealed that collagenase injection induced histological changes compatible with a tendinopathy. No further histological changes were observed after the application of percutaneous electrolysis or needling. A significant increase in molecular expression of Cox2, Mmp9 and Vegf genes was observed in Achilles tendons treated with percutaneous electrolysis to a greater extent than after just needling. The expression of Mmp2, Col1a1, Col3a1, or Scx genes also increased, but did not reach statistical significance. This animal study demonstrated that percutaneous electrolysis applied on an experimentally induced Achilles tendinopathy model could increase the expression of some genes associated with collagen regeneration and remodeling of extracellular matrix. The observed gene overexpression was higher with percutaneous electrolysis than with just needling.

Systematic Review of Photobiomodulation Therapy (PBMT) on the Experimental Calcaneal Tendon Injury in Rats

This systematic review analyzed the light parameters and the effects of photobiomodulation therapy (PBMT) through low-level laser therapy (LLLT) and/or LED (light-emitting diode) on tendon repair of rats submitted to calcaneal injury. This study was conducted in accordance with the guidelines of the Preferred Reporting Items for Meta-Analysis, and PubMed and MEDLINE databases were accessed to search eligible studies published in English. The search terms were as follows: "Achilles tendon" or "Calcaneal tendon" or "tendon injuries" or "soft tissue injuries" and "tendinopathy" or "tendinitis" and "low-level light therapy" or "low-level laser therapy" or "low intensity power therapy" or "light-emitting diode" or photobiomodulation." The SYRCLE (SYstematic Review Center for Laboratory animal Experimentation) risks of bias was used to assess the risk of bias for selected studies. A total of 225 studies were found based on the descriptors used, and only 33 studies were eligible. Light parameters identified per point of irradiation were approximately 60 mW (continuous mode at infrared spectra), 2 W cm^-2 , 2 J and 45 J cm^-2 . Light parameters at red spectra, continuous versus pulsed mode, and PBMT combined or compared with other therapies such as ultrasound, and studies using unhealthy rats (ovariectomized and/or diabetic models) were also identified and grouped according to these similarities. The main effects found were decreased inflammatory markers and signs of inflammatory process. PBMT (laser/LED) has positive effects in reducing the inflammatory and time for tissue repair in animal models of tendon injury and/or tendinitis using parameters identified.

Treatment with Human Amniotic Suspension Allograft Improves Tendon Healing in a Rat Model of Collagenase-Induced Tendinopathy

Treatment of tendon injuries is challenging, with neither conservative nor surgical approaches providing full recovery. Placental-derived tissues represent a promising tool for the treatment of tendon injuries. In this study, human amniotic suspension allograft (ASA) was investigated in a pre-clinical model of Achilles tendinopathy. Collagenase type I was injected in the right hind limb of Sprague Dawley rats to induce disease. Contralateral tendons were either left untreated or injected with saline as controls. Seven days following induction, tendons were injected with saline, ASA, or left untreated. Rats were sacrificed 14 and 28 days post-treatment. Histological and biomechanical analysis of tendons was completed. Fourteen days after ASA injection, improved fiber alignment and reduced cell density demonstrated improvement in degenerated tendons. Twenty-eight days post-treatment, tendons in all treatment groups showed fewer signs of degeneration, which is consistent with normal tendon healing. No statistically significant differences in histological or biomechanical analyses were observed between treatment groups at 28 days independent of the treatment they received. In this study, ASA treatment was safe, well-tolerated, and resulted in a widespread improvement of the tissue. The results of this study provide preliminary insights regarding the potential use of ASA for the treatment of Achilles tendinopathy.

Non-Invasive Ultrasound Quantification of Scar Tissue Volume Identifies Early Functional Changes During Tendon Healing

Tendon injuries are very common and disrupt the transmission of forces from muscle to bone, leading to impaired function and quality of life. Successful restoration of tendon function after injury is a challenging clinical problem due to the pathological, scar-mediated manner in which the tendons heal. Currently, there are no standard treatments to modulate scar tissue formation and improve tendon healing. A major limitation to the identification of therapeutic candidates has been the reliance on terminal endpoint metrics of healing in pre-clinical studies, which require a large number of animals and result in destruction of the tissue. To address this limitation, we have identified quantification of scar tissue volume (STV) from ultrasound (US) imaging as a longitudinal, non-invasive metric of tendon healing. STV was strongly correlated with established endpoint metrics of gliding function including gliding resistance and metatarsophalangeal (MTP) flexion angle. However, no associations were observed between STV and structural or material properties. To define the sensitivity of STV to identify differences between functionally discrete tendon healing phenotypes, we utilized S100a4 haploinsufficient mice (S100a4^GFP/+ ), which heal with improved gliding function relative to wild-type (WT) littermates. A significant decrease in STV was observed in S100a4^GFP/+ repairs, relative to WT at day 14. Taken together, these data suggest US quantification of STV as a means to facilitate the rapid screening of biological and pharmacological interventions to improve tendon healing, and identify promising therapeutic targets, in an efficient, cost-effective manner. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:2476-2485, 2019.

Onset of neonatal locomotor behavior and the mechanical development of Achilles and tail tendons

Tendon tissue engineering approaches are challenged by a limited understanding of the role mechanical loading plays in normal tendon development. We propose that the increased loading that developing postnatal tendons experience with the onset of locomotor behavior impacts tendon formation. The objective of this study was to assess the onset of spontaneous weight-bearing locomotion in postnatal day (P) 1, 5, and 10 rats, and characterize the relationship between locomotion and the mechanical development of weight-bearing and non-weight-bearing tendons. Movement was video recorded and scored to determine non-weight-bearing, partial weight-bearing, and full weight-bearing locomotor behavior at P1, P5, and P10. Achilles tendons, as weight-bearing tendons, and tail tendons, as non-weight-bearing tendons, were mechanically evaluated. We observed a significant increase in locomotor behavior in P10 rats, compared to P1 and P5. We also found corresponding significant differences in the maximum force, stiffness, displacement at maximum force, and cross-sectional area in Achilles tendons, as a function of postnatal age. However, the maximum stress, strain at maximum stress, and elastic modulus remained constant. Tail tendons of P10 rats had significantly higher maximum force, maximum stress, elastic modulus, and stiffness compared to P5. Our results suggest that the onset of locomotor behavior may be providing the mechanical cues regulating postnatal tendon growth, and their mechanical development may proceed differently in weight-bearing and non-weight-bearing tendons. Further analysis of how this loading affects developing tendons in vivo may inform future engineering approaches aiming to apply such mechanical cues to regulate engineered tendon formation in vitro.

Estrogen and mechanical loading-related regulation of estrogen receptor-β and apoptosis in tendinopathy

Female-dominant tendinopathies are musculoskeletal disorders caused by repetitive hand posture and motion; they are considered overuse syndromes. Both external mechanical stress and changes in hormone levels might affect disease progression. We have previously reported that estrogen receptor-β (ER)-β expression was associated with the pathogenesis of de Quervain's disease. To study the underlying mechanisms, a cyclic stretching culture system was applied to tendon tissue from ovariectomized (OVX) rats. Furthermore, a collagenase I-induced rat tendinopathy model was established to examine the association of ER-β with disease progression. Our results showed that ER-β expression and the number of apoptotic cells were higher and associated with disease severity in rats with tendinopathy. Mechanical stress altered the morphology of primary tenocytes and collagen fiber alignment in tendons, and up-regulated the expression of matrix metalloproteinase-9, ER-β, and interleukin-1β, as well as induced apoptosis in tenocytes and tendon tissue from OVX rats. This is the first report on the effects of ER-β and mechanical stress in tendinopathy. We hope these findings contribute to new pharmacological therapies targeting ER-β signaling pathways to treat tendon-related diseases.