Distinct myofibre domains of the human myotendinous junction revealed by single-nucleus RNA sequencing.

The myotendinous junction (MTJ) is a specialized domain of the multinucleated myofibre that is faced with the challenge of maintaining robust cell-matrix contact with the tendon under high mechanical stress and strain. Here, we profiled 24,124 nuclei in semitendinosus muscle-tendon samples from three healthy males by using single-nucleus RNA sequencing (snRNA-seq), alongside spatial transcriptomics, to gain insight into the genes characterizing this specialization in humans. We identified a cluster of MTJ myonuclei represented by 47 enriched transcripts, of which the presence of ABI3BP, ABLIM1, ADAMTSL1, BICD1, CPM, FHOD3, FRAS1 and FREM2 was confirmed at the MTJ at the protein level in immunofluorescence assays. Four distinct subclusters of MTJ myonuclei were apparent, comprising two COL22A1-expressing subclusters and two subclusters lacking COL22A1 expression but with differing fibre type profiles characterized by expression of either MYH7 or MYH1 and/or MYH2. Our findings reveal distinct myonuclei profiles of the human MTJ, which represents a weak link in the musculoskeletal system that is selectively affected in pathological conditions ranging from muscle strains to muscular dystrophies.

Exercise entrainment of musculoskeletal connective tissue clocks.

The musculoskeletal system, crucial for movement and support, relies on the delicate balance of connective tissue homeostasis. Maintaining this equilibrium is essential for tissue health and function. There has been increasing evidence in the past decade that shows the circadian clock as a master regulator of extracellular matrix (ECM) homeostasis in several connective tissue clocks. Very recently, exercise has emerged as a significant entrainment factor for cartilage and intervertebral disk circadian rhythms. Understanding the implications of exercise on connective tissue peripheral clocks holds promise for enhancing tissue health and disease prevention. Exercise-induced factors such as heat, glucocorticoid release, mechanical loading, and inter-tissue cross talk may play pivotal roles in entraining the circadian rhythm of connective tissues. This mini review underscores the importance of elucidating the mechanisms through which exercise influences circadian rhythms in connective tissues to optimize ECM homeostasis. Leveraging exercise as a modulator of circadian rhythms in connective tissues may offer novel therapeutic approaches to physical training for preventing musculoskeletal disorders and enhancing recovery.

Mechanical properties, collagen and glycosaminoglycan content of equine superficial digital flexor tendons are not affected by training.

Physical activity can activate extracellular matrix (ECM) protein synthesis and influence the size and mechanical properties of tendon. In this study, we aimed to investigate whether different training histories of horses would influence the synthesis of collagen and other matrix proteins and alter the mechanical properties of tendon. Samples from superficial digital flexor tendon (SDFT) from horses that were either (a) currently race trained (n = 5), (b) previously race trained (n = 5) or (c) untrained (n = 4) were analysed for matrix protein abundance (mass spectrometry), collagen and glycosaminoglycan (GAG) content, ECM gene expression and mechanical properties. It was found that ECM synthesis by tendon fibroblasts in vitro varied depending upon the previous training history. In contrast, fascicle morphology, collagen and GAG content, mechanical properties and ECM gene expression of the tendon did not reveal any significant differences between groups. In conclusion, although we could not identify any direct impact of the physical training history on the mechanical properties or major ECM components of the tendon, it is evident that horse tendon cells are responsive to loading in vivo, and the training background may lead to a modification in the composition of newly synthesised matrix.

Insulin-like growth factor-1 infusion in preterm piglets does not affect growth parameters of skeletal muscle or tendon tissue.

Prematurity has physical consequences, such as lower birth weight, decreased muscle mass and increased risk of adult-onset metabolic disease. Insulin-like growth factor 1 (IGF-1) has therapeutic potential to improve the growth and quality of muscle and tendon in premature births, and thus attenuate some of these sequalae. We investigated the effect of IGF-1 on extensor carpi radialis muscle and biceps brachii tendon of preterm piglets. The preterm group consisted of 19-day-old preterm (10 days early) piglets, treated with either IGF-1 or vehicle. Term controls consisted of groups of 9-day-old piglets (D9) and 19-day-old piglets (D19). Muscle samples were analysed by immunofluorescence to determine the cross-sectional area (CSA) of muscle fibres, fibre type composition, satellite cell content and central nuclei-containing fibres in the muscle. Tendon samples were analysed for CSA, collagen content and maturation, and vascularization. Gene expression of the tendon was measured by RT-qPCR. Across all endpoints, we found no significant effect of IGF-1 treatment on preterm piglets. Preterm piglets had smaller muscle fibre CSA compared to D9 and D19 control group. Satellite cell content was similar across all groups. For tendon, we found an effect of age on tendon CSA, and mRNA levels of COL1A1, tenomodulin and scleraxis. Immunoreactivity for elastin and CD31, and several markers of tendon maturation, were increased in D9 compared to the preterm piglets. Collagen content was similar across groups. IGF-1 treatment of preterm-born piglets does not influence the growth and maturation of skeletal muscle and tendon.

Clinical and Imaging Outcomes Over 12 Weeks in Elite Athletes With Early-Stage Tendinopathy.

Knowledge of how to treat chronic tendinopathy has advanced in recent years, but the treatment of early tendinopathy is not well understood. The main purpose of this prospective observational study was to investigate if changes occur in clinical and imaging outcomes over 12 weeks in elite athletes with recent debut of tendinopathy. Sixty-five elite adult athletes (24 ± 5 years) with early Achilles or patellar tendinopathy (symptoms < 3 months) were examined at baseline and after 12 weeks. Patients were divided into groups based on the duration of symptoms at the time of inclusion: (T1): 0-1 month, (T2): 1-2 months, or (T3): 2-3 months. Pain-guided activity modification was the only intervention. We assessed the following clinical outcomes: Questionnaires (Victorian Institute of Sports Assessment (VISA)) and pain scores (0-10 numeric rating scale, NRS), structural outcomes from ultrasonography: Thickness, echogenicity, and Doppler flow, and from magnetic resonance imaging: Cross-sectional area (CSA), thickness and length. Tendinopathic Achilles and patellar tendons displayed no significant differences on imaging tendon structural outcomes between T1 (n = 19), T2 (n = 23), and T3 (n = 20) at baseline or after 12 weeks, with one exception: Patellar tendons in T1 were thicker than T2 and T3 at baseline. Although athletes improved clinically on VISA and most NRS scores after 12 weeks, affected tendons with greater thickness, CSA and Doppler flow than contralateral tendons at baseline remained unchanged after 12 weeks. In conclusion, these data suggest that early tendinopathy in elite athletes can improve clinically after 12 weeks while morphology remains unchanged.

Disruption of day-to-night changes in circadian gene expression with chronic tendinopathy.

Overuse injury in tendon tissue (tendinopathy) is a frequent and costly musculoskeletal disorder and represents a major clinical problem with unsolved pathogenesis. Studies in mice have demonstrated that circadian clock-controlled genes are vital for protein homeostasis and important in the development of tendinopathy. We performed RNA sequencing, collagen content and ultrastructural analyses on human tendon biopsies obtained 12 h apart in healthy individuals to establish whether human tendon is a peripheral clock tissue and we performed RNA sequencing on patients with chronic tendinopathy to examine the expression of circadian clock genes in tendinopathic tissues. We found time-dependent expression of 280 RNAs including 11 conserved circadian clock genes in healthy tendons and markedly fewer (23) differential RNAs with chronic tendinopathy. Further, the expression of COL1A1 and COL1A2 was reduced at night but was not circadian rhythmic in synchronised human tenocyte cultures. In conclusion, day-to-night changes in gene expression in healthy human patellar tendons indicate a conserved circadian clock as well as the existence of a night reduction in collagen I expression. KEY POINTS: Tendinopathy is a major clinical problem with unsolved pathogenesis. Previous work in mice has shown that a robust circadian rhythm is required for collagen homeostasis in tendons. The use of circadian medicine in the diagnosis and treatment of tendinopathy has been stifled by the lack of studies on human tissue. Here, we establish that the expression of circadian clock genes in human tendons is time dependent, and now we have data to corroborate that circadian output is reduced in diseased tendon tissues. We consider our findings to be of significance in advancing the use of the tendon circadian clock as a therapeutic target or preclinical biomarker for tendinopathy.

Is it all in the baseline? Trajectories of chair stand performance over 4 years and their association with grey matter structure in older adults.

Understanding individual variability in response to physical activity is key to developing more effective and personalised interventions for healthy ageing. Here, we aimed to unpack individual differences by using longitudinal data from a randomised-controlled trial of a 12-month muscle strengthening intervention in older adults. Physical function of the lower extremities was collected from 247 participants (66.3 ± 2.5 years) at four time-points. At baseline and at year 4, participants underwent 3 T MRI brain scans. K-means longitudinal clustering was used to identify patterns of change in chair stand performance over 4 years, and voxel-based morphometry was applied to map structural grey matter volume at baseline and year 4. Results identified three groups showing trajectories of poor (33.6%), mid (40.1%), and high (26.3%) performance. Baseline physical function, sex, and depressive symptoms significantly differed between trajectory groups. High performers showed greater grey matter volume in the motor cerebellum compared to the poor performers. After accounting for baseline chair stand performance, participants were re-assigned to one of four trajectory-based groups: moderate improvers (38.9%), maintainers (38.5%), improvers (13%), and decliners (9.7%). Clusters of significant grey matter differences were observed between improvers and decliners in the right supplementary motor area. Trajectory-based group assignments were unrelated to the intervention arms of the study. In conclusion, patterns of change in chair stand performance were associated with greater grey matter volumes in cerebellar and cortical motor regions. Our findings emphasise that how you start matters, as baseline chair stand performance was associated with cerebellar volume 4 years later.

Differences in the cross-sectional area along the ankle tendons with both age and sex.

Increasing age appears to influence several morphologic changes in major tendons. However, the effects of aging on the cross-sectional area (CSA) of different ankle tendons are much less understood. Furthermore, potential differences in specific tendon regions along the length of the tendons have not been investigated in detail. Sixty healthy adult participants categorized by age as young (n = 20; mean ± SD age = 22.5 ± 4.5 years), middle-age (n = 20; age = 40.6 ± 8. 0 years), or old (n = 20; age = 69.9 ± 9.1 years), from both sexes, were included. The tendon CSA of tibialis anterior (TA), tibialis posterior (TP), fibularis (FT), and Achilles (AT) was measured from T1-weighted 1.5 T MR images in incremental intervals of 10% along its length (from proximal insertion) and compared between different age groups and sexes. The mean CSA of the AT was greater in the middle-age group than both young and old participants (p < 0.01) and large effect sizes were observed for these differences (Cohen's d > 1). Furthermore, there was a significant difference in CSA in all three groups along the length of the different tendons. Region-specific differences between groups were observed in the distal portion (90% and 100% of the length), in which the FT presented greater CSA comparing middle-age to young and old (p < 0.05). In conclusion, (1) great magnitude of morpho-structural differences was discovered in the AT; (2) there are region-specific differences in the CSA of ankle tendons within the three groups and between them; and (3) there were no differences in tendon CSA between sexes.

Tendon properties in a mouse model of severe osteogenesis imperfecta.

PURPOSE/AIM OF THE STUDY: Osteogenesis imperfecta is a heritable bone disorder that is usually caused by mutations in collagen type I encoding genes. The impact of such mutations on tendons, a structure with high collagen type I content, remains largely unexplored. We hypothesized that tendon properties are abnormal in the context of a mutation affecting collagen type I. The main purpose of the study was to assess the anatomical, mechanical, and material tendon properties of Col1a1Jrt/+ mice, a model of severe dominant OI. MATERIALS AND METHODS: The Flexor Digitorum Longus (FDL) tendon of Col1a1Jrt/+ mice and wild-type littermates (WT) was assessed with in vitro mechanical testing. RESULTS: The results showed that width and thickness of FDL tendons were about 40% larger in WT (p < 0.01) than in Col1a1Jrt/+ mice, whereas the cross-sectional area was 138% larger (p < 0.001). The stiffness, peak- and yield-force were between 160% and 194% higher in WT vs. Col1a1Jrt/+ mice. The material properties did not show significant differences between mouse strains with differences <15% between WT and Col1a1Jrt/+ (p > 0.05). Analysis of the Achilles tendon collagen showed no difference between mice strains for the content but collagen solubility in acetic acid was 66% higher in WT than in Col1a1Jrt/+ (p < 0.001). CONCLUSIONS: This study shows that the FDL tendon of Col1a1Jrt/+ mice has reduced mechanical properties but apparently normal material properties. It remains unclear whether the tendon phenotype of Col1a1Jrt/+ mice is secondary to muscle weakness or a direct effect of the Col1a1 mutation or a combination of both.

Human derived tendon cells contribute to myotube formation in vitro.

Skeletal muscle possesses remarkable adaptability to mechanical loading and regenerative potential following muscle injury primarily due to satellite cell activity. Although the roles of several types of interstitial cells in skeletal muscle have been documented, the signaling interplay between the skeletal muscle and the adjacent tendon tissue has not been elucidated. Here, we tested whether human tendon derived cells (tenocytes) could induce human myogenic cells (myoblasts) proliferation and differentiation in vitro using co-culture experiments that allowed us to investigate the effect of tenocytes secretion upon myogenic progression. This was done in vitro by introducing insert wells with either myoblasts, tenocytes, or no cells (control) into a myoblast containing well (co-culture). Immunofluorescence analysis revealed a higher fusion index (≥5 nuclei within one Desmin + myotube) and a higher myotube diameter in co-cultures with tenocytes compared to myoblasts condition. Correspondingly, MHC-IIX gene expression was up-regulated when co-cultured with tenocytes. However, the proliferation of myoblasts (either Ki67 or BrdU + cells) was not enhanced under the presence of tenocytes. These findings show that tenocytes influence myotube formation upon human primary cells in vitro and contribute to understanding the role of tendon derived cells in skeletal muscle during development and regeneration.

Larger interface area at the human myotendinous junction in type 1 compared with type 2 muscle fibers.

The myotendinous junction (MTJ) is structurally specialized to transmit force. The highly folded muscle membrane at the MTJ increases the contact area between muscle and tendon and potentially the load tolerance of the MTJ. Muscles with a high content of type II fibers are more often subject to strain injury compared with muscles with type I fibers. It is hypothesized that this is explained by a smaller interface area of MTJ in type II compared with type I muscle fibers. The aim was to investigate by confocal microscopy whether there is difference in the surface area at the MTJ between type I and II muscle fibers. Individual muscle fibers with an intact MTJ were isolated by microscopic dissection in samples from human semitendinosus, and they were labeled with antibodies against collagen XXII (indicating MTJ) and type I myosin (MHCI). Using a spinning disc confocal microscope, the MTJ from each fiber was scanned and subsequently reconstructed to a 3D-model. The interface area between muscle and tendon was calculated in type I and II fibers from these reconstructions. The MTJ was analyzed in 314 muscle fibers. Type I muscle fibers had a 22% larger MTJ interface area compared with type II fibers (p < 0.05), also when the area was normalized to fiber diameter. By the new method, it was possible to analyze the structure of the MTJ from a large number of human muscle fibers. The finding that the interface area between muscle and tendon is higher in type I compared with type II fibers suggests that type II fibers are less resistant to strain and therefore more susceptible to injury.

Chronic changes in muscle architecture and aponeurosis structure following calf muscle strain injuries.

BACKGROUND: Muscle strain injuries in the human calf muscles are frequent sports injuries with high recurrence. Potential structural and functional changes in the medial head of the musculus gastrocnemius (GM) and the associated aponeurosis are not well documented. PURPOSE: To test whether a GM muscle strain injury affects muscle fascicle length, pennation angle, and the morphology of the deep aponeurosis at rest and during muscle contraction long time after the injury. Additionally, electromyography (EMG) of the GM and the soleus muscle during a unilateral heel rise was measured in the injured and uninjured calf. METHODS: GM fascicle length, pennation angle, and aponeurosis thickness was analyzed on dynamic ultrasonography (US) recordings in 10 participants with a chronic calf strain. In addition, US images taken across the distal portion and mid-belly of the GM were analyzed at three different ankle positions. EMG recordings were obtained during a unilateral heel rise. RESULTS: The pennation angle of the injured distal GM was significantly larger compared to the uninjured GM in the contracted, but not the relaxed state. Pennation angle increased more in the injured compared to the uninjured GM during contraction. Fascicle length was shorter in the most distal portion of the injured GM. Fascicles at the distal portion of the injured GM showed a pronounced curvilinear shape as the muscle contracted and the aponeurosis was enlarged in the injured compared to the uninjured GM. The ratio between GM and soleus EMG activity showed a significantly higher relative soleus activity in the injured compared to the healthy calf. CONCLUSION: The greater change in pennation angle and curvilinear fascicle shape during contraction suggest that a long-term consequence after a muscle strain injury is that some muscle fibers at the distal GM are not actively engaged. The significantly enlarged aponeurosis indicates a substantial and long-lasting connective tissue involvement following strain injuries.

Human skeletal muscle acetylcholine receptor gene expression in elderly males performing heavy resistance exercise.

Muscle fiber denervation is a major contributor to the decline in muscle mass and function during aging. Heavy resistance exercise is an effective tool for increasing muscle mass and strength, but whether it can rescue denervated muscle fibers remains unclear. Therefore, the purpose of this study was to investigate the potential of heavy resistance exercise to modify indices of denervation in healthy elderly individuals. Thirty-eight healthy elderly men (72 ± 5 yr) underwent 16 wk of heavy resistance exercise, whereas 20 healthy elderly men (72 ± 6 yr) served as nonexercising sedentary controls. Muscle biopsies were obtained pre and post training, and midway at 8 wk. Biopsies were analyzed by immunofluorescence for the prevalence of myofibers expressing embryonic myosin [embryonic myosin heavy chain (MyHCe)], neonatal myosin [neonatal myosin heavy chain (MyHCn)], nestin, and neural cell adhesion molecule (NCAM), and by RT-qPCR for gene expression levels of acetylcholine receptor (AChR) subunits, MyHCn, MyHCe, p16, and Ki67. In addition to increases in strength and type II fiber hypertrophy, heavy resistance exercise training led to a decrease in AChR α1 and ε subunit messenger RNA (mRNA; at 8 wk). Changes in gene expression levels of the α1 and ε AChR subunits with 8 wk of heavy resistance exercise supports the role of this type of exercise in targeting stability of the neuromuscular junction. The number of fibers positive for NCAM, nestin, and MyHCn was not affected, suggesting that a longer timeframe is needed for adaptations to manifest at the protein level.

Preserved stem cell content and innervation profile of elderly human skeletal muscle with lifelong recreational exercise.

Muscle fibre denervation and declining numbers of muscle stem (satellite) cells are defining characteristics of ageing skeletal muscle. The aim of this study was to investigate the potential for lifelong recreational exercise to offset muscle fibre denervation and compromised satellite cell content and function, both at rest and under challenged conditions. Sixteen elderly lifelong recreational exercisers (LLEX) were studied alongside groups of age-matched sedentary (SED) and young subjects. Lean body mass and maximal voluntary contraction were assessed, and a strength training bout was performed. From muscle biopsies, tissue and primary myogenic cell cultures were analysed by immunofluorescence and RT-qPCR to assess myofibre denervation and satellite cell quantity and function. LLEX demonstrated superior muscle function under challenged conditions. When compared with SED, the muscle of LLEX was found to contain a greater content of satellite cells associated with type II myofibres specifically, along with higher mRNA levels of the beta and gamma acetylcholine receptors (AChR). No difference was observed between LLEX and SED for the proportion of denervated fibres or satellite cell function, as assessed in vitro by myogenic cell differentiation and fusion index assays. When compared with inactive counterparts, the skeletal muscle of lifelong exercisers is characterised by greater fatigue resistance under challenged conditions in vivo, together with a more youthful tissue satellite cell and AChR profile. Our data suggest a little recreational level exercise goes a long way in protecting against the emergence of classic phenotypic traits associated with the aged muscle. KEY POINTS: The detrimental effects of ageing can be partially offset by lifelong self-organized recreational exercise, as evidence by preserved type II myofibre-associated satellite cells, a beneficial muscle innervation status and greater fatigue resistance under challenged conditions. Satellite cell function (in vitro), muscle fibre size and muscle fibre denervation determined by immunofluorescence were not affected by recreational exercise. Individuals that are recreationally active are far more abundant than master athletes, which sharply increases the translational perspective of the present study. Future studies should further investigate recreational activity in relation to muscle health, while also including female participants.

Nestin and osteocrin mRNA increases in human semitendinosus myotendinous junction 7 days after a single bout of eccentric exercise.

The myotendinous junction (MTJ), a specialized interface for force transmission between muscle and tendon, has a unique transcriptional activity and is highly susceptible to muscle strain injury. Eccentric exercise training is known to reduce this risk of injury, but knowledge of the influence of exercise on the MTJ at the molecular and cellular levels is limited. In this study, 30 subjects were randomized to a single bout of eccentric exercise 1 week prior to tissue sampling (exercised) or no exercise (control). Samples were collected from the semitendinosus as part of reconstruction of the anterior cruciate ligament and divided into fractions containing muscle, MTJ and tendon, respectively. The concentrations of macrophages and satellite cells were counted, and the expression of genes previously known to be active at the MTJ were analyzed by real-time-quantitative PCR. An effect of the single bout of exercise was found on the expression of nestin (NES) and osteocrin (OSTN) mRNA in the MTJ and tendon fractions. Genes earlier identified at the MTJ (COL22A1, POSTN, ADAMTS8, MNS1, NCAM1) were confirmed to be expressed at a significantly higher level in the MTJ compared to muscle and tendon but were unaffected by exercise. In the exercise group a higher concentration of macrophages, but not of satellite cells, was seen in muscle close to the MTJ. The expression of NES and OSTN was higher in human semitendinosus MTJ 1 week after a single session of heavy eccentric exercise. Based on these results, NES and OSTN could have a part in explaining how the MTJ adapts to eccentric exercise.

Effects of age on muscle power, postural control and functional capacity after short-term immobilization and retraining.

OBJECTIVES: This study investigated the effect of lower limb immobilization and retraining on postural control and muscle power in healthy old and young men. METHODS: Twenty men, nine old (OM:67.3±4.4 years) and eleven young (YM:24.4±1.6 years) underwent 2 weeks of unilateral whole-leg casting, followed by 4 weeks of retraining. Measures included center of pressure (CoP) sway length and area during single- and double-leg stance, maximal leg extensor muscle power, habitual and maximal 10-m gait speed, sit-to-stand performance, and 2-min step test. RESULTS: After immobilization, leg extension muscle power decreased by 15% in OM (from 2.68±0.60 to 2.29±0.63 W/kg, p<0.05) and 17% in YM (4.37±0.76 to 3.63±0.69 W/kg, p<0.05). Double-leg CoP sway area increased by 45% in OM (218±82 to 317±145 mm2; p<0.05), with no change in YM (p=0.43). Physical function did not change after immobilization but sit-to-stand performance (+20%, p<0.05) and 2-min step test (+28%, p<0.05) increased in OM following retraining. In both groups, all parameters returned to baseline levels after retraining. CONCLUSION: Two weeks of lower limb immobilization led to decreases in maximal muscle power in both young and old, whereas postural control was impaired selectively in old men. All parameters were restored in both groups after 4 weeks of resistance-based retraining.

RNA sequencing and immunofluorescence of the myotendinous junction of mature horses and humans.

The myotendinous junction (MTJ) is a specialized interface for transmitting high forces between the muscle and tendon and yet the MTJ is a common site of strain injury with a high recurrence rate. The aim of this study was to identify previously unknown MTJ components in mature animals and humans. Samples were obtained from the superficial digital flexor (SDF) muscle-tendon interface of 20 horses, and the tissue was separated through a sequential cryosectioning approach into muscle, MTJ (muscle tissue enriched in myofiber tips attached to the tendon), and tendon fractions. RT-PCR was performed for genes known to be expressed in the three tissue fractions and t-distributed stochastic neighbor embedding (t-SNE) plots were used to select the muscle, MTJ, and tendon samples from five horses for RNA sequencing. The expression of previously known and unknown genes identified through RNA sequencing was studied by immunofluorescence on human hamstring MTJ tissue. The main finding was that RNA sequencing identified the expression of a panel of 61 genes enriched at the MTJ. Of these, 48 genes were novel for the MTJ and 13 genes had been reported to be associated with the MTJ in earlier studies. The expression of known [COL22A1 (collagen XXII), NCAM (neural cell adhesion molecule), POSTN (periostin), NES (nestin), OSTN (musclin/osteocrin)] and previously undescribed [MNS1 (meiosis-specific nuclear structural protein 1), and LCT (lactase)] MTJ genes was confirmed at the protein level by immunofluorescence on tissue sections of human MTJ. In conclusion, in muscle-tendon interface tissue enriched with myofiber tips, we identified the expression of previously unknown MTJ genes representing diverse biological processes, which may be important in the maintenance of the specialized MTJ.

Right-left asymmetry in corticospinal tract microstructure and dexterity are uncoupled in late adulthood.

Ageing leads to a decline in white matter microstructure and dexterous function of the hand. In adolescents, it has previously been shown that the degree of right-left asymmetry in the corticospinal tract (CST) is linearly related with right-left asymmetry in dexterity. Here, we tested whether this association is also expressed in older adults. Participants completed a simple circle drawing task with their right and left hand as a measure of dexterity and underwent whole-brain diffusion weighted imaging at 3 Tesla (n = 199; aged 60-72 years). Fractional anisotropy and mean diffusivity of right and left CST were extracted from a manually defined region-of-interest. Linear regression analyses were computed to replicate the analyses in adolescents. Frequentist analyses were complemented with a Bayesian analytical framework. Outcome measures were compared with those previously reported in adolescents (aged 11-16 years). Asymmetries in white matter microstructure of the CST were evident and comparable to the degree of lateralisation observed in adolescence. Similarly, asymmetries in dexterity were evident, but to a lesser degree than in adolescents. Unlike in adolescents, we found no evidence of a linear relationship between asymmetries in CST microstructure and dexterity. Complementary Bayesian regression analysis provided moderate evidence in favour of the null hypothesis, pointing towards a lack of association between the structural and functional measures of right-left asymmetry. Our findings are compatible with the notion that, by late adulthood, a diverging impact of age on white matter structure and dexterous hand function dilutes the structure-function relationship between CST microstructure and manual proficiency that has been reported in adolescents.

Collagens in primary frozen shoulder: expression of collagen mRNA isoforms in the different phases of the disease.

OBJECTIVES: Primary frozen shoulder (pFS) has three phases that differ in clinical presentation. It is characterized by contracture of the joint capsule. We hypothesized that there is a general upregulation of collagens in pFS, and that this is highest in the first phase of the disease. The aims of this study were to investigate the expression of various collagens and degradation of collagens in patients with primary pFS and relate this to the three phases of the condition. METHODS: From twenty-six patients with pFS and eight control patients with subacromial impingement, biopsies were obtained during shoulder arthroscopy from the middle glenohumeral ligament and the anterior capsule, and mRNA levels for collagens, MMP-2 and -14 and TGF-ß1, - ß2 and -ß3 in the tissue were analysed using real-time PCR. RESULTS: Genes for collagens type I, III, IV, V, VI and XIV, were activated in pFS, and the total mRNA for all collagens was increased (P < 0.05). This upregulation was independent of disease phases in pFS. In addition, MMP-2, MMP-14, TGF-ß1 and TGF-ß3 were upregulated in all phases of the disease. CONCLUSION: There is a general upregulation and an increased degradation of collagens in pFS in all three phases of the disease. This indicates a constantly increased turnover of the fibrotic tissue in the capsule from pFS. The difference in clinical presentation of pFS observed in the three phases of the disease is not primarily a result of variations in collagen production.

Vicariance followed by secondary gene flow in a young gazelle species complex.

Grant's gazelles have recently been proposed to be a species complex comprising three highly divergent mtDNA lineages (Nanger granti, N. notata and N. petersii). The three lineages have nonoverlapping distributions in East Africa, but without any obvious geographical divisions, making them an interesting model for studying the early-stage evolutionary dynamics of allopatric speciation in detail. Here, we use genomic data obtained by restriction site-associated (RAD) sequencing of 106 gazelle individuals to shed light on the evolutionary processes underlying Grant's gazelle divergence, to characterize their genetic structure and to assess the presence of gene flow between the main lineages in the species complex. We date the species divergence to 134,000 years ago, which is recent in evolutionary terms. We find population subdivision within N. granti, which coincides with the previously suggested two subspecies, N. g. granti and N. g. robertsii. Moreover, these two lineages seem to have hybridized in Masai Mara. Perhaps more surprisingly given their extreme genetic differentiation, N. granti and N. petersii also show signs of prolonged admixture in Mkomazi, which we identified as a hybrid population most likely founded by allopatric lineages coming into secondary contact. Despite the admixed composition of this population, elevated X chromosomal differentiation suggests that selection may be shaping the outcome of hybridization in this population. Our results therefore provide detailed insights into the processes of allopatric speciation and secondary contact in a recently radiated species complex.