The Impact of Timing of Delayed Autologous Breast Reconstruction following Postmastectomy Radiation Therapy on Postoperative Morbidity.

BACKGROUND: The ideal time to perform reconstruction after the completion of postmastectomy radiation therapy (PMRT) in patients with locally advanced breast cancer is currently unknown. We evaluate the association between the timing of delayed autologous breast reconstruction following PMRT and postoperative complications. METHODS: Patients who underwent mastectomy, PMRT, and then delayed autologous breast reconstruction from 2009 to 2016 were identified from the Truven Health MarketScan Research Databases. Timing of reconstruction following PMRT was grouped 0-3, 3-6, 6-12, 12-24, and after 24 months. Multivariable models were used to assess associations between timing of reconstruction following PMRT and key measures of morbidity. RESULTS: A total of 1,039 patients met inclusion criteria. The rate of any complications for the analytic cohort was 39.4%, including 13.3% of patients who experienced wound complications and 11.3% of patients requiring additional flaps. Unadjusted rates of complications increased from 23.4% between 0 and 3 months to 49.4% between 3 and 6 months and decreased thereafter. Need for additional flaps was highest within 3 to 6 months (14.0%). Multivariate analysis revealed higher rates of any complications when reconstruction was performed between 3 and 6 months (odds ratio [OR]: 3.04, p < 0.001), 6 and 12 months (OR: 2.66, p < 0.001), or 12 and 24 months (OR: 2.13, p = 0.001) after PMRT. No difference in complications were noted in reconstructions performed after 24 months compared with those performed before 3 months (p > 0.05). However, rates of wound complications were least likely in reconstructions after 24 months (OR: 0.34, p = 0.035). CONCLUSION: These findings suggest plastic surgeons may consider performing autologous breast reconstruction early for select patients, before 3 months following PMRT without increasing postoperative morbidity.

Local shifts in inflammatory and resolving lipid mediators in response to tendon overuse.

Tendon inflammation has been implicated in both adaptive connective tissue remodeling and overuse-induced tendinopathy. Lipid mediators control both the initiation and resolution of inflammation, but their roles within tendon are largely unknown. Here, we profiled local shifts in intratendinous lipid mediators via liquid chromatography-tandem mass spectrometry in response to synergist ablation-induced plantaris tendon overuse. Sixty-four individual lipid mediators were detected in homogenates of plantaris tendons from ambulatory control rats. This included many bioactive metabolites of the cyclooxygenase (COX), lipoxygenase (LOX), and epoxygenase (CYP) pathways. Synergist ablation induced a robust inflammatory response at day 3 post-surgery characterized by epitenon infiltration of polymorphonuclear leukocytes and monocytes/macrophages (MΦ), heightened expression of inflammation-related genes, and increased intratendinous concentrations of the pro-inflammatory eicosanoids thromboxane B2 and prostaglandin E2 . By day 7, MΦ became the predominant myeloid cell type in tendon and there were further delayed increases in other COX metabolites including prostaglandins D2 , F2α , and I2 . Specialized pro-resolving mediators including protectin D1, resolvin D2 and D6, as well as related pathway markers of D-resolvins (17-hydroxy-docosahexaenoic acid), E-resolvins (18-hydroxy-eicosapentaenoic acid), and lipoxins (15-hydroxy-eicosatetraenoic acid) were also increased locally in response to tendon overuse, as were anti-inflammatory fatty acid epoxides of the CYP pathway (eg, epoxy-eicosatrienoic acids). Nevertheless, intratendinous prostaglandins remained markedly increased even following 28 days of tendon overuse together with a lingering MΦ presence. These data reveal a delayed and prolonged local inflammatory response to tendon overuse characterized by an overwhelming predominance of pro-inflammatory eicosanoids and a relative lack of specialized pro-resolving lipid mediators.

Insulin-like growth factor 1 signaling in tenocytes is required for adult tendon growth.

Tenocytes serve to synthesize and maintain collagen fibrils and other extracellular matrix proteins in tendon. Despite the high prevalence of tendon injury, the underlying biologic mechanisms of postnatal tendon growth and repair are not well understood. IGF1 plays an important role in the growth and remodeling of numerous tissues but less is known about IGF1 in tendon. We hypothesized that IGF1 signaling is required for proper tendon growth in response to mechanical loading through regulation of collagen synthesis and cell proliferation. To test this hypothesis, we conditionally deleted the IGF1 receptor (IGF1R) in scleraxis (Scx)-expressing tenocytes using a tamoxifen-inducible Cre-recombinase system and caused tendon growth in adult mice via mechanical overload of the plantaris tendon. Compared with control Scx-expressing IGF1R-positive (Scx:IGF1R+) mice, in which IGF1R is present in tenocytes, mice that lacked IGF1R in their tenocytes [Scx-expressing IGF1R-negative (Scx:IGF1RΔ) mice] demonstrated reduced cell proliferation and smaller tendons in response to mechanical loading. Additionally, we identified that both the PI3K/protein kinase B and ERK pathways are activated downstream of IGF1 and interact in a coordinated manner to regulate cell proliferation and protein synthesis. These studies indicate that IGF1 signaling is required for proper postnatal tendon growth and support the potential use of IGF1 in the treatment of tendon disorders.-Disser, N. P., Sugg, K. B., Talarek, J. R., Sarver, D. C., Rourke, B. J., Mendias, C. L. Insulin-like growth factor 1 signaling in tenocytes is required for adult tendon growth.

Postnatal tendon growth and remodeling require platelet-derived growth factor receptor signaling.

Platelet-derived growth factor receptor (PDGFR) signaling plays an important role in the fundamental biological activities of many cells that compose musculoskeletal tissues. However, little is known about the role of PDGFR signaling during tendon growth and remodeling in adult animals. Using the hindlimb synergist ablation model of tendon growth, our objectives were to determine the role of PDGFR signaling in the adaptation of tendons subjected to a mechanical growth stimulus, as well as to investigate the biological mechanisms behind this response. We demonstrate that both PDGFRs, PDGFRα and PDGFRß, are expressed in tendon fibroblasts and that the inhibition of PDGFR signaling suppresses the normal growth of tendon tissue in response to mechanical growth cues due to defects in fibroblast proliferation and migration. We also identify membrane type-1 matrix metalloproteinase (MT1-MMP) as an essential proteinase for the migration of tendon fibroblasts through their extracellular matrix. Furthermore, we report that MT1-MMP translation is regulated by phosphoinositide 3-kinase/Akt signaling, while ERK1/2 controls posttranslational trafficking of MT1-MMP to the plasma membrane of tendon fibroblasts. Taken together, these findings demonstrate that PDGFR signaling is necessary for postnatal tendon growth and remodeling and that MT1-MMP is a critical mediator of tendon fibroblast migration and a potential target for the treatment of tendon injuries and diseases.

Anterior cruciate ligament tear induces a sustained loss of muscle fiber force production.

INTRODUCTION: Patients with anterior cruciate ligament (ACL) tears have persistent quadriceps strength deficits that are thought to be due to altered neurophysiological function. Our goal was to determine the changes in muscle fiber contractility independent of the ability of motor neurons to activate fibers. METHODS: We obtained quadriceps biopsies of patients undergoing ACL reconstruction, and additional biopsies 1, 2, and 6 months after surgery. Muscles fiber contractility was assessed in vitro, along with whole muscle strength testing. RESULTS: Compared with controls, patients had a 30% reduction in normalized muscle fiber force at the time of surgery. One month later, the force deficit was 41%, and at 6 months the deficit was 23%. Whole muscle strength testing demonstrated similar trends. DISCUSSION: While neurophysiological dysfunction contributes to whole muscle weakness, there is also a reduction in the force generating capacity of individual muscle cells independent of alpha motor neuron activation. Muscle Nerve, 2018.

Stromal vascular stem cell treatment decreases muscle fibrosis following chronic rotator cuff tear.

PURPOSE: Rotator cuff injuries are associated with atrophy and fat infiltration into the muscle, commonly referred to as "fatty degeneration." As the poor function of chronically torn muscles may limit recovery after surgical repair, there is considerable interest in finding therapies to enhance muscle regeneration. Stromal vascular fraction stem cells (SVFCs) can improve muscle regeneration in other chronic injury states, and our objective was to evaluate the ability of SVFCs to reduce fibrosis and fat accumulation, and enhance muscle fibre specific force production after chronic rotator cuff tear. METHODS: Chronic supraspinatus tears were induced in adult immunodeficient rats, and repaired one month following tear. Rats received vehicle control, or injections of 3 × 10(5) or 3 × 10(6) human SVFCs into supraspinatus muscles. RESULTS: Two weeks following repair, we detected donor human DNA and protein in SVFC treated muscles. There was a 40 % reduction in fibrosis in the treated groups compared to controls (p = 0.03 for 3 × 10(5), p = 0.04 for 3 × 10(6)), and no differences between groups for lipid content or force production were observed. CONCLUSIONS: As there has been much interest in the use of stem cell-based therapies in musculoskeletal regenerative medicine, the reduction in fibrosis and trend towards an improvement in single fiber contractility suggest that SVFCs may be beneficial to enhance the treatment and recovery of patients with chronic rotator cuff tears.

TGF-ß superfamily signaling in muscle and tendon adaptation to resistance exercise.

Numerous studies in muscle and tendon have identified a central role of the transforming growth factor-ß (TGF-ß) superfamily of cytokines in the regulation of extracellular matrix growth and remodeling, protein degradation, and cell proliferation and differentiation. We provide a novel framework for TGF-ß and myostatin signaling in controlling the coordinated adaptation of both skeletal muscle and tendon tissue to resistance training.

Contralateral Autologous Augmentation in DIEP Flap Reconstruction: Employing CT Angiography & Volumetric Analysis for Preoperative Planning.

SUMMARY: Autologous reconstruction accounts for approximately 20% of all breast reconstruction. In cases of unilateral reconstruction, contralateral breast augmentation using autologous tissue can be performed to improve symmetry and is a viable option for patients interested in having more volume relative to their current size without the use of implants. CT scans have been used for preoperative planning for autologous reconstruction to evaluate available perforators. In this study, we report our experience using CT angiography for preoperative volumetric assessment for autologous contralateral breast augmentation in the setting of unilateral autologous breast reconstruction. Twelve patients underwent autologous augmentation during the study period. The average reconstruction flap weight was 561.2±253.6 grams, while the average augmentation flap weight was 218.0±133.7 grams. No patients experienced flap loss and we demonstrate that the predicted volumes for the augmented and reconstructed breasts were comparable to the actual respective flap volumes. Additionally, post-operative patient-reported outcome measures demonstrate high levels of satisfaction across multiple BREAST-Q subscales. This study demonstrates the utility of using CT angiography to estimate reconstructive volumes to help preoperative planning and achieve predictable postoperative breast volumes. It also supports that contralateral autologous augmentation is a good option for patients who would like to avoid implants and are interested in a small to moderate increase in size.

Outcomes of tethered cord repair with a layered soft tissue closure.

BACKGROUND: Previous studies have demonstrated the benefit of releasing symptomatic tethered cords; however, complications such as seroma, cerebrospinal fluid leak, and infection continue to plague these patients. We propose that composite tissue closure of tethered cord repairs yields superior outcomes and that a collaborative effort between neurosurgery and plastic surgery may result in enhanced structural and functional results. METHODS: This is a retrospective study comprised of consecutive patients with tethered cord syndrome by 2 neurosurgeons and 2 plastic surgeons between 1994 and 2008 at a single institution. All consecutive patients who underwent tethered cord release by neurosurgery and subsequent composite tissue closure with fascial and musculofascial flaps by plastic surgery were included. Data were collected by retrospective chart review and analyzed using parametric methods. RESULTS: A total of 86 consecutive patients were included in this study, with follow-up ranged from 12 to 144 months (average follow-up, 29 months). There were no statistical differences in follow-up time, comorbidities, or surgeon when comparing hospital readmission or reoperation. There was no statistical difference in complications when comparing the different flap closures. We had a 1.2% infection rate, a 4.7% readmission rate, and a 3.5% reoperation rate. CONCLUSION: We believe that local soft tissue rearrangement improves the closure by providing an additional layer of vascularized tissue between the skin and the spinal cord. We believe our series represents a significant sample size compared with those previously reported for an experience that achieves multilayered soft tissue closure after tethered cord repair. Our results support the idea that neurosurgeons should consider consultation of plastic surgeons when treating patients with tethered cord syndrome surgically.

Dermal Sensory Regenerative Peripheral Nerve Interface for Reestablishing Sensory Nerve Feedback in Peripheral Afferents in the Rat.

BACKGROUND: Without meaningful, intuitive sensory feedback, even the most advanced myoelectric devices require significant cognitive demand to control. The dermal sensory regenerative peripheral nerve interface (DS-RPNI) is a biological interface designed to establish high-fidelity sensory feedback from prosthetic limbs. METHODS: DS-RPNIs were constructed in rats by securing fascicles of residual sensory peripheral nerves into autologous dermal grafts, with the objectives of confirming regeneration of sensory afferents within DS-RPNIs and establishing the reliability of afferent neural response generation with either mechanical or electrical stimulation. RESULTS: Two months after implantation, DS-RPNIs were healthy and displayed well-vascularized dermis with organized axonal collaterals throughout and no evidence of neuroma. Electrophysiologic signals were recorded proximal from DS-RPNI's sural nerve in response to both mechanical and electrical stimuli and compared with (1) full-thickness skin, (2) deepithelialized skin, and (3) transected sural nerves without DS-RPNI. Mechanical indentation of DS-RPNIs evoked compound sensory nerve action potentials (CSNAPs) that were like those evoked during indentation of full-thickness skin. CSNAP firing rates and waveform amplitudes increased in a graded fashion with increased mechanical indentation. Electrical stimuli delivered to DS-RPNIs reliably elicited CSNAPs at low current thresholds, and CSNAPs gradually increased in amplitude with increasing stimulation current. CONCLUSIONS: These findings suggest that afferent nerve fibers successfully reinnervate DS-RPNIs, and that graded stimuli applied to DS-RPNIs produce proximal sensory afferent responses similar to those evoked from normal skin. This confirmation of graded afferent signal transduction through DS-RPNI neural interfaces validate DS-RPNI's potential role of facilitating sensation in human-machine interfacing. CLINICAL RELEVANCE STATEMENT: The DS-RPNI is a novel biotic-abiotic neural interface that allows for transduction of sensory stimuli into neural signals. It is expected to advance the restoration of natural sensation and development of sensorimotor control in prosthetics.

Adaptive and innate immune cell responses in tendons and lymph nodes after tendon injury and repair.

Tendon injuries are a common clinical condition with limited treatment options. The cellular components of the innate immune system, such as neutrophils and macrophages, have been studied in tendon injuries. However, the adaptive immune system, comprising specialized lymphocytes, plays an important role in orchestrating the healing of numerous tissues, but less is known about these cells in tendon healing. To gain a greater understanding of the biological processes that regulate tendon healing, we determined how the cellular components of the adaptive and innate immune system respond to a tendon injury using two-month-old male mice. We observed that lymphatic vasculature is present in the epitenon and superficial regions of Achilles tendons, and that the lymphatics drain into the popliteal lymph node. We then created an acute Achilles tenotomy followed by repair, and collected tendons and popliteal lymph nodes 1, 2, and 4 wk after injury. Tendon injury resulted in a robust adaptive immune cell response that followed an initial innate immune cell response in tendons and lymph nodes. Monocytes, neutrophils, and macrophages initially accumulated at 1 wk after injury in tendons, while dendritic cells and CD4+ T cells peaked at 2 wk after injury. B cells and CD8+ T cells progressively increased over time. In parallel, immune cells of the popliteal lymph node demonstrated a similarly coordinated response to the injury. These results suggest that there is an adaptive immune response to tendon injury, and adaptive immune cells may play a role in regulating tendon healing.NEW & NOTEWORTHY While the innate immune system, consisting of macrophages and related hematopoietic cells, has been studied in tendon injury, less is known about the adaptive immune system. Using a mouse model of Achilles tendon tenotomy and repair, we observed an adaptive immune cell response, consisting of CD4+ and CD8+ T cells, and B cells, which occur through 4 wk after tendon injury. This response appeared to be coordinated by the draining popliteal lymph node.

Resolvin D1 supports skeletal myofiber regeneration via actions on myeloid and muscle stem cells.

Specialized proresolving mediators (SPMs) actively limit inflammation and expedite its resolution by modulating leukocyte recruitment and function. Here we profiled intramuscular lipid mediators via liquid chromatography-tandem mass spectrometry-based metabolipidomics following myofiber injury and investigated the potential role of SPMs in skeletal muscle inflammation and repair. Both proinflammatory eicosanoids and SPMs increased following myofiber damage induced by either intramuscular injection of barium chloride or synergist ablation-induced functional muscle overload. Daily systemic administration of the SPM resolvin D1 (RvD1) as an immunoresolvent limited the degree and duration of inflammation, enhanced regenerating myofiber growth, and improved recovery of muscle strength. RvD1 suppressed inflammatory cytokine expression, enhanced polymorphonuclear cell clearance, modulated the local muscle stem cell response, and polarized intramuscular macrophages to a more proregenerative subset. RvD1 had minimal direct impact on in vitro myogenesis but directly suppressed myokine production and stimulated macrophage phagocytosis, showing that SPMs can modulate both infiltrating myeloid and resident muscle cell populations. These data reveal the efficacy of immunoresolvents as a novel alternative to classical antiinflammatory interventions in the management of muscle injuries to modulate inflammation while stimulating tissue repair.

Prostaglandin D2 signaling is not involved in the recovery of rat hind limb tendons from injury.

Injured tendons heal through the formation of a fibrovascular scar that has inferior mechanical properties compared to native tendon tissue. Reducing inflammation that occurs as a result of the injury could limit scar formation and improve functional recovery of tendons. Prostaglandin D2 (PGD2 ) plays an important role in promoting inflammation in some injury responses and chronic disease processes, and the inhibition of PGD2 has improved healing and reduced disease burden in animal models and early clinical trials. Based on these findings, we sought to determine the role of PGD2 signaling in the healing of injured tendon tissue. We tested the hypothesis that a potent and specific inhibitor of hematopoietic PGD synthase (HPGDS), GSK2894631A, would improve the recovery of tendons of adult male rats following an acute tenotomy and repair. To test this hypothesis, we performed a full-thickness plantaris tendon tenotomy followed by immediate repair and treated rats twice daily with either 0, 2, or 6 mg/kg of GSK2894631A. Tendons were collected either 7 or 21 days after surgical repair, and mechanical properties of tendons were assessed along with RNA sequencing and histology. While there were some differences in gene expression across groups, the targeted inhibition of HPGDS did not impact the functional repair of tendons after injury, as HPGDS expression was surprisingly low in injured tendons. These results indicate that PGD2 signaling does not appear to be important in modulating the repair of injured tendon tissue.

Factors Influencing American Plastic Surgery Residents Toward an Academic Career.

BACKGROUND: Plastic surgery residency program directors have an interest in recruiting applicants who show an interest in an academic practice. Medical school achievements (ie, United States Medical Licensing Examination® scores, publications, and Alpha Omega Alpha status) are metrics assessed to grade applicants but may not correlate with ultimately choosing an academic career. OBJECTIVE: This study was designed to investigate factors influencing residents' choices for or against academic careers. METHODS: A 25-item online questionnaire was designed to measure baseline interest in academic plastic surgery and factors that influence decisions to continue on or abandon that career path. This questionnaire was disseminated to the integrated/combined plastic surgery residents during the 2013 to 2014 academic year. RESULTS: One hundred twenty-five respondents indicated that they were currently interested in pursuing academic practice (n = 78) or had lost interest in academic practice (n = 47). Among all respondents, 92.8% (n = 116) stated they were interested in academic careers at the time of residency application, but one-third (n = 41) subsequently lost interest. Those residents who retained interest in academic careers indicated resident/medical student educational opportunities (57%) and complexity of patients (52%) as reasons. Those who lost interest cited a lack of autonomy (43%), publishing requirements (32%), and income discrepancy (26%) as reasons. DISCUSSION: Many residents report losing interest in academics during residency. Traditional metrics valued in the recruitment process may not serve as positive predictors of an academic career path. CONCLUSION: Reasons why residents lose interest are not easily correctable, but mentorship, adequate career counseling, and research opportunities during training remain factors that can be addressed across all residency programs.

HISTORIQUE: Les directeurs des programmes de résidence en chirurgie plastique ont intérêt à recruter des candidats qui souhaitent mener une carrière en milieu universitaire. Les réalisations en faculté de médecine (indices de l'USMLE, publications et statut à l'AOA) sont des mesures utilisées pour classer les candidats, mais elles ne sont peut-être pas corrélées avec le choix d'une carrière en milieu universitaire. OBJECTIF: La présente étude visait à évaluer les facteurs qui influent sur les choix des résidents à opter ou non pour une carrière en milieu universitaire. MÉTHODOLOGIE: Les chercheurs ont conçu un questionnaire en ligne de 25 questions pour mesurer l'intérêt de départ envers une carrière en chirurgie plastique en milieu universitaire et les facteurs qui influent sur la décision de poursuivre en ce sens ou non. Ils l'ont distribué aux résidents en chirurgie plastique intégrés ou combinés pendant l'année scolaire 2013-2014. RÉSULTATS: Au total, 125 répondants ont affirmé s'intéresser à une carrière en milieu universitaire (n = 78) ou avoir perdu leur intérêt envers la pratique universitaire (n = 47). Dans l'ensemble, 92.8 % d'entre eux (n = 116) ont affirmé qu'ils envisageaient de mener une carrière en milieu universitaire au moment de leur demande de résidence, mais le tiers (n = 41) a ensuite perdu cet intérêt. Parmi leurs raisons, les résidents qui continuaient de vouloir mener une carrière en milieu universitaire ont cité les possibilités d'enseignement aux résidents et aux étudiants en médecine (57 %) et la complexité des cas (52 %). Ceux qui avaient perdu l'intérêt ont invoqué le manque d'autonomie (43 %), les exigences en matière de publication (32 %) et l'écart du revenu (26 %). EXPOSÉ: De nombreux résidents ont déclaré perdre leur intérêt envers une carrière en milieu universitaire pendant leur résidence. Les mesures habituelles utilisées pendant le processus de recrutement ne sont peut-être pas des prédicteurs positifs d'un cheminement de carrière universitaire. CONCLUSION: Les raisons pour lesquelles les résidents perdent l'intérêt envers la carrière en milieu universitaire ne sont pas faciles à corriger, mais le mentorat, une bonne orientation de carrière et des occasions de recherche pendant la formation font partie des facteurs qui peuvent être abordés dans tous les programmes de résidence.

Pathogenesis and Management of Tendinopathies in Sports Medicine.

OBJECTIVE: Tendinopathy is a major clinical problem in sports medicine and is often difficult to treat. Traditional therapeutic approaches have focused on reducing inflammation, yet research suggests that little to no inflammation is present in the tendons that fail to heal. The purpose of this review was to evaluate the effectiveness of the available treatment options for tendinopathy and to inform best clinical practices. DESIGN: A narrative review. METHODS: A comprehensive search of electronic databases (PubMed, Google Scholar and Web of Science) was conducted to identify relevant studies through June 2016. Studies were deemed relevant if they were published in English and contained original research on the management of tendinopathy in humans. RESULTS: Studies varied in methodological quality and were often limited by small sample size and lack of sufficient control groups. Critical evaluation of the literature suggests that physical therapy with or without eccentric exercise should be considered a first-line treatment. Corticosteroids and nonsteroidal anti-inflammatory drugs provide short-term symptomatic relief, but long-term efficacy has not been demonstrated. Inconsistent results do not support the routine use of prolotherapy, platelet-rich plasma injections and topical nitric oxide patches. Operative intervention should be reserved until conservative measures fail or an obvious operative lesion is present. CONCLUSIONS: While numerous therapeutic modalities exist for tendinopathy in the athlete, the ideal treatment protocol has not been clearly defined. The development of new targeted therapies for tendinopathy is likely to follow a greater understanding of the cellular and molecular mechanisms that underlie its pathogenesis.

Active shortening protects against stretch-induced force deficits in human skeletal muscle.

Skeletal muscle contraction results from molecular interactions of myosin "crossbridges" with adjacent actin filament binding sites. The binding of myosin to actin can be "weak" or "strong," and only strong binding states contribute to force production. During active shortening, the number of strongly bound crossbridges declines with increasing shortening velocity. Forcibly stretching a muscle that is actively shortening at high velocity results in no apparent negative consequences, whereas stretch of an isometrically (fixed-length) contracting muscle causes ultrastructural damage and a decline in force-generating capability. Our working hypothesis is that stretch-induced damage is uniquely attributable to the population of crossbridges that are strongly bound. We tested the hypothesis that stretch-induced force deficits decline as the prevailing shortening velocity is increased. Experiments were performed on permeabilized segments of individual skeletal muscle fibers obtained from human subjects. Fibers were maximally activated and allowed either to generate maximum isometric force (Fo), or to shorten at velocities that resulted in force maintenance of ≈50% Fo or ≈2% Fo For each test condition, a rapid stretch equivalent to 0.1 × optimal fiber length was applied. Relative to prestretch Fo, force deficits resulting from stretches applied during force maintenance of 100, ≈50, and ≈2% Fo were 23.2 ± 8.6, 7.8 ± 4.2, and 0.3 ± 3.3%, respectively (means ± SD, n = 20). We conclude that stretch-induced damage declines with increasing shortening velocity, consistent with the working hypothesis that the fraction of strongly bound crossbridges is a causative factor in the susceptibility of skeletal muscle to stretch-induced damage.NEW & NOTEWORTHY Force deficits caused by stretch of contracting muscle are most severe when the stretch is applied during an isometric contraction, but prevented if the muscle is shortening at high velocity when the stretch occurs. This study indicates that velocity-controlled modulation of the number of strongly bound crossbridges is the basis for the observed relationship between stretch-induced muscle damage and prevailing shortening velocity.

Inhibition of platelet-derived growth factor signaling prevents muscle fiber growth during skeletal muscle hypertrophy.

The platelet-derived growth factor receptors alpha and beta (PDGFRα and PDGFRß) mark fibroadipogenic progenitor cells/fibroblasts and pericytes in skeletal muscle, respectively. While the role that these cells play in muscle growth and development has been evaluated, it was not known whether the PDGF receptors activate signaling pathways that control transcriptional and functional changes during skeletal muscle hypertrophy. To evaluate this, we inhibited PDGFR signaling in mice subjected to a synergist ablation muscle growth procedure, and performed analyses 3 and 10 days after induction of hypertrophy. The results from this study indicate that PDGF signaling is required for fiber hypertrophy, extracellular matrix production, and angiogenesis that occur during muscle growth.

Changes in muscle fiber contractility and extracellular matrix production during skeletal muscle hypertrophy.

Skeletal muscle can adapt to increased mechanical loads by undergoing hypertrophy. Transient reductions in whole muscle force production have been reported during the onset of hypertrophy, but contractile changes in individual muscle fibers have not been previously studied. Additionally, the extracellular matrix (ECM) stores and transmits forces from muscle fibers to tendons and bones, and determining how the ECM changes during hypertrophy is important in understanding the adaptation of muscle tissue to mechanical loading. Using the synergist ablation model, we sought to measure changes in muscle fiber contractility, collagen content, and cross-linking, and in the expression of several genes and activation of signaling proteins that regulate critical components of myogenesis and ECM synthesis and remodeling during muscle hypertrophy. Tissues were harvested 3, 7, and 28 days after induction of hypertrophy, and nonoverloaded rats served as controls. Muscle fiber specific force (sFo), which is the maximum isometric force normalized to cross-sectional area, was reduced 3 and 7 days after the onset of mechanical overload, but returned to control levels by 28 days. Collagen abundance displayed a similar pattern of change. Nearly a quarter of the transcriptome changed over the course of overload, as well as the activation of signaling pathways related to hypertrophy and atrophy. Overall, this study provides insight into fundamental mechanisms of muscle and ECM growth, and indicates that although muscle fibers appear to have completed remodeling and regeneration 1 mo after synergist ablation, the ECM continues to be actively remodeling at this time point.NEW & NOTEWORTHY This study utilized a rat synergist ablation model to integrate changes in single muscle fiber contractility, extracellular matrix composition, activation of important signaling pathways in muscle adaption, and corresponding changes in the muscle transcriptome to provide novel insight into the basic biological mechanisms of muscle fiber hypertrophy.

Sex differences in tendon structure and function.

Tendons play a critical role in the transmission of forces between muscles and bones, and chronic tendon injuries and diseases are among the leading causes of musculoskeletal disability. Little is known about sex-based differences in tendon structure and function. Our objective was to evaluate the mechanical properties, biochemical composition, transcriptome, and cellular activity of plantarflexor tendons from 4 month old male and female C57BL/6 mice using in vitro biomechanics, mass spectrometry-based proteomics, genome-wide expression profiling, and cell culture techniques. While the Achilles tendons of male mice were approximately 6% larger than female mice (p < 0.05), the cell density of female mice was around 19% greater than males (p < 0.05). No significant differences in mechanical properties (p > 0.05) of plantaris tendons were observed. Mass spectrometry proteomics analysis revealed no significant difference between sexes in the abundance of major extracellular matrix (ECM) proteins such as collagen types I (p = 0.30) and III (p = 0.68), but female mice had approximately twofold elevations (p < 0.05) in less abundant ECM proteins such as fibronectin, periostin, and tenascin C. The transcriptome of male and female tendons differed by only 1%. In vitro, neither the sex of the serum that fibroblasts were cultured in, nor the sex of the ECM in which they were embedded, had profound effects on the expression of collagen and cell proliferation genes. Our results indicate that while male mice expectedly had larger tendons, male and female tendons have very similar mechanical properties and biochemical composition, with small increases in some ECM proteins and proteoglycans evident in female tendons. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:2117-2126, 2017.