Coordinated development of the limb musculoskeletal system: Tendon and muscle patterning and integration with the skeleton

Dynamic interactions between cartilaginous and tendinous/ligamentous primordia during musculoskeletal integration

Proper connections between cartilaginous and muscular primordia through tendinous/ligamentous primordia are essential for musculoskeletal integration. Herein, we report a novel double-reporter mouse model for investigating this process via fluorescently visualising scleraxis (Scx) and SRY-box containing gene 9 (Sox9) expression. We generated ScxTomato transgenic mice and crossed them with Sox9EGFP knock-in mice to obtain ScxTomato;Sox9EGFP mice. Deep imaging of optically cleared double-reporter embryos at E13.5 and E16.5 revealed previously unknown differences in the dynamic interactions between cartilaginous and tendinous/ligamentous primordia in control and Scx-deficient mice. Tendon/ligament maturation was evaluated through simultaneous detection of fluorescence and visualisation of collagen fibre formation using second harmonic generation imaging. Lack of deltoid tuberosity in Scx-deficient mice caused misdirected muscle attachment with morphological changes. Loss of Scx also dysregulated progenitor cell fate determination in the chondrotendinous junction, resulting in the formation of a rounded enthesis rather than the protruding enthesis observed in the control. Hence, our double-reporter mouse system, in combination with loss- or gain-of-function approaches, is a unique and powerful tool that could be used to gain a comprehensive understanding of musculoskeletal integration.

Nutrition and Physical Activity in Musculoskeletal Health

A balanced diet and regular physical activity are essential for maintaining musculoskeletal health. Key nutrients such as calcium, vitamin D, and protein are especially important for preventing falls and fractures. While the benefits of these nutrients are well-established, other dietary components have not been studied as extensively. For instance, vegetables, which are rich in nutrients vital for muscle and bone health, play a crucial role in preventing falls and fractures. Over recent decades, a great emphasis has been given to the combinations of nutrients and foods in dietary patterns that may have synergistic or antagonistic effects. Despite the challenges in researching the impact of nutrition and physical activity on musculoskeletal health due to the extensive heterogeneity of the results, healthcare professionals should continue to promote healthy eating and regular physical activity, and these principles should be emphasized in public health initiatives. Ultimately, a sufficient and balanced diet, abundant in plant-based foods and low in processed or discretionary foods, along with consistent physical activity, remains the most effective strategy for the prevention of musculoskeletal issues. This article aims to review the updated literature of recent years on the links between nutrition and physical activity with bone and skeletal muscle health.

Preparation of black phosphorus@sodium alginate microspheres with bone matrix vesicle structure via electrospraying for bone regeneration

Bone matrix vesicles are commonly acknowledged as the primary site of biomineralization in human skeletal tissue. Black phosphorus has exhibited favorable properties across various chemical and physical domains. In this investigation, a novel composite microsphere was synthesized through the amalgamation of sodium alginate (ALG) with black phosphorus nanosheets (BP) utilizing the electrospray (ES) technique. These microspheres were tailored to mimic the regulatory function of matrix vesicles (MV) upon exposure to a biomimetic mineralization fluid (SBF) during the biomineralization process. Results revealed that black phosphorus nanosheets facilitated the generation of hydroxyapatite (HA) on the microsphere surface. Live-dead assays and cell proliferation experiments showcased a cell survival rate exceeding 85 %. Moreover, wound healing assessments unveiled that M-ALG-BP microspheres exhibited superior migration capacity, with a migration rate surpassing 50 %. Furthermore, after 7 days of osteogenic induction, M-ALG-BP microspheres notably stimulated osteoblast differentiation. Particularly noteworthy, M-ALG-BP microspheres significantly enhanced osteogenic differentiation of osteoblasts and induced collagen production in vitro. Additionally, experiments involving microsphere implantation into mouse skeletal muscle demonstrated the potential for ectopic mineralization by ALG-BP microspheres. This investigation underscores the outstanding mineralization properties of ALG-BP microspheres and their promising clinical prospects in bone tissue engineering.

Extracellular matrix protein composition dynamically changes during murine forelimb development

The extracellular matrix (ECM) is an integral part of multicellular organisms, connecting different cell layers and tissue types. During morphogenesis and growth, tissues undergo substantial reorganization. While it is intuitive that the ECM remodels in concert, little is known regarding how matrix composition and organization change during development. Here, we quantified ECM protein dynamics in the murine forelimb during appendicular musculoskeletal morphogenesis (embryonic days 11.5-14.5) using tissue fractionation, bioorthogonal non-canonical amino acid tagging, and mass spectrometry. Our analyses indicated that ECM protein (matrisome) composition in the embryonic forelimb changed as a function of development and growth, was distinct from other developing organs (brain), and was altered in a model of disease (osteogenesis imperfecta murine). Additionally, the tissue distribution for select matrisome was assessed via immunohistochemistry in the wild-type embryonic and postnatal musculoskeletal system. This resource will guide future research investigating the role of the matrisome during complex tissue development.

Variations in the Extensor Pollicis Brevis-Extensor Pollicis Longus Tendon Complex

Despite several reports on the running of the extensor pollicis brevis (EPB) tendons, the classification of tendon insertions remains ununified due to differences in reports. This diversity in tendon patterning is attributed to the process of tendon development. In this study, we assessed the running of the EPB tendons of 44 cadaver hands fixed in ethanol/formalin in detail and examined the existing classification method. The specimens were obtained from 15 women and seven men, with an average age of 86 years. Consistent with previous reports, we observed a wide diversity in the running of the EPB tendons. Further, we found that EPB tendon insertions showed diverse variations in the proportion and running of fibers, making it difficult to classify them into independent patterns. It is speculated that the EPB tendon develops through a different process than that of the muscle body of the EPB and that the entire muscle-tendon module of the EPB is evolving. The diversity of the EPB tendons observed in this study may reflect the ongoing process of evolution. In clinical practice, a wide variation in the running of the EPB tendons should be considered.

Cardiac toxicity of phenanthrene depends on developmental stage in Atlantic cod (Gadus morhua)

Complex mixtures like crude oil, and single components such as Phenanthrene (Phe), induce cardiotoxicity by interfering with excitation-contraction coupling. However, recent work has demonstrated that the timing of pollutant exposure during embryogenesis greatly impacts the degree of cardiac dysfunction caused. Here, we aimed to clarify the temporal dependence of Phe toxicity and the downstream effects of cardiac dysfunction using Atlantic cod (Gadus morhua). Phe (nominal concentration, 1.12 μmol/L), or the L-type‑calcium channel blocker Nicardipine (Nic) (nominal concentration, 2 and 4 μmol/L), were individually applied to cod embryos either during cardiogenesis (early) or after the onset of cardiac function (late). Phe toxicity was highly dependent on the timing of exposure. Exposure after the onset of cardiac function (i.e. late) caused more severe cardiac and extracardiac abnormalities at 3 days post hatching (dph) than early exposure. Late Phe exposure resulted in a smaller ventricle, eliminated ventricular contraction, and reduced atrial contraction. In contrast, early Phe exposure did not have an effect on cardiac development and function. This temporal difference was not as evident in the Nic treatment. Early Nic exposure created similar morphological phenotypes to the late Phe exposure. The two treatments (early Nic and late Phe) also shared a cardiofunctional phenotype, comprised of eliminated ventricular, and reduced atrial, contraction. These data suggest that extracardiac abnormalities, such as the craniofacial deformities seen after late embryonic exposure to cardiotoxic oil components and mixtures, are mostly downstream effects of cardiac dysfunction.

Bifurcated distal biceps brachii tendon coexisting with separated bicipital aponeurosis: a complex variational case report

Variations appearing in biceps brachii muscle are common with accessory head, different origins, variant insertion, and different pattern of nerve innervation. However, variations appearing in both origin and insertion, and with other anomalous morphology at the same time are seldom. Here we report a complex variational case on the right arm of a 91-year-old Japanese female cadaver. The complex variations included (1) the biceps brachii muscle bifurcated at its distal ending; (2) the long head had its own tendon, which divided into two parts, i.e., a lateral part fused into the fascia between the brachioradialis and extensor carpi brevis, and a medial part attached to the radius about one centimeter ahead of the radial tuberosity; (3) the short head had an accessory origin from the shoulder capsule; (4) the bicipital aponeurosis was of two parts with an anterior superior layer formed by the long head and a posterior deep one formed by the short head; (5) the musculocutaneous nerve was especially underdeveloped that only innervated the coracobrachialis; (6) the existence of communicating branch between the musculocutaneous and median nerves, and the median nerve issued muscular branches to the biceps brachii and brachialis muscles, and (7) the brachioradial muscle had two accessory muscular bundles that originated from the fascia of the brachial muscle (proximal one) and from the bicipital aponeurosis (distal one).

A single-cell census of mouse limb development identifies complex spatiotemporal dynamics of skeleton formation

Limb development has long served as a model system for coordinated spatial patterning of progenitor cells. Here, we identify a population of naive limb progenitors and show that they differentiate progressively to form the skeleton in a complex, non-consecutive, three-dimensional pattern. Single-cell RNA sequencing of the developing mouse forelimb identified three progenitor states: naive, proximal, and autopodial, as well as Msx1 as a marker for the naive progenitors. In vivo lineage tracing confirmed this role and localized the naive progenitors to the outer margin of the limb, along the anterior-posterior axis. Sequential pulse-chase experiments showed that the progressive transition of Msx1⁺ naive progenitors into proximal and autopodial progenitors coincides with their differentiation to Sox9⁺ chondroprogenitors, which occurs along all the forming skeletal segments. Indeed, tracking the spatiotemporal sequence of differentiation showed that the skeleton forms progressively in a complex pattern. These findings suggest an alternative model for limb skeleton development.

The emergence of the stem cell niche

Stem cell niches are composed of dynamic microenvironments that support stem cells over a lifetime. The emerging niche is distinct from the adult because its main role is to support the progenitors that build organ systems in development. Emerging niches mature through distinct stages to form the adult niche and enable proper stem cell support. As a model of emerging niches, this review highlights how differences in the skeletal muscle microenvironment influence emerging versus satellite cell (SC) niche formation in skeletal muscle, which is among the most regenerative tissue systems. We contrast how stem cell niches regulate intrinsic properties between progenitor and stem cells throughout development to adulthood. We describe new applications for generating emerging niches from human pluripotent stem cells (hPSCs) using developmental principles and highlight potential applications for regeneration and therapeutics.

Three-dimensional visualization and quantitative analysis of embryonic and fetal thigh muscles using magnetic resonance and phase-contrast X-ray imaging

The musculoskeletal system around the human hip joint has acquired a suitable structure for erect bipedal walking. However, little is known about the process of separation and maturation of individual muscles during the prenatal period, when muscle composition is acquired. Understanding the maturation process of the normal musculoskeletal system contributes to elucidating the acquisition of bipedal walking in humans and to predicting normal growth and detecting congenital muscle disorders and anomalies. In this study, we clarify the process of thigh muscle maturation from the embryonic stage to the mid-fetal stage using serial sections, phase-contrast X-ray computed tomography, and magnetic resonance imaging. We also provide a 4D atlas of human thigh muscles between 8 and 23 weeks of gestation. As a result, we first show that muscle separation in the lower thigh tends to progress from the superficial to the deep layers and that all musculoskeletal components are formed by Carnegie Stage 22. Next, we show that femur and muscle volume grow in correlation with crown-rump length. Finally, we show that the anterior, abductor, and posterior muscle groups in the thigh contain a high percentage of monoarticular muscle volume by the end of the embryonic period. This ratio approaches that of adult muscle composition during normal early fetal development and is typical of bipedal walking. This study of fetal muscle composition suggests that preparation for postnatal walking may begin in early fetal period.

Osteosarcopenia in Patients with Chronic Obstructive Pulmonary Diseases: Which Pathophysiologic Implications for Rehabilitation?

Chronic obstructive pulmonary disease (COPD) is a burdensome condition affecting a growing number of people worldwide, frequently related to major comorbidities and functional impairment. In these patients, several factors might have a role in promoting both bone and muscle loss, including systemic inflammation, corticosteroid therapies, sedentary behaviours, deconditioning, malnutrition, smoking habits, and alcohol consumption. On the other hand, bone and muscle tissues share several linkages from functional, embryological, and biochemical points of view. Osteosarcopenia has been recently defined by the coexistence of osteoporosis and sarcopenia, but the precise mechanisms underpinning osteosarcopenia in patients with COPD are still unknown. In this scenario, a deeper understanding of the molecular basis of osteosarcopenia might guide clinicians in a personalized approach integrating skeletal muscle health with the pulmonary rehabilitation framework in COPD. Taken together, our results summarized the currently available evidence about the multilevel interactions between osteosarcopenia and COPD to pave the way for a comprehensive approach targeting the most common risk factors of these pathological conditions. Further studies are needed to clarify the role of modern clinical strategies and telemedicine solutions to optimize healthcare delivery in patients with COPD, including osteopenia, osteoporosis, and sarcopenia screening in these subjects.

TWIST1, a gene associated with Saethre-Chotzen syndrome, regulates extraocular muscle organization in mouse

Heterozygous loss of function mutations in TWIST1 cause Saethre-Chotzen syndrome, which is characterized by craniosynostosis, facial asymmetry, ptosis, strabismus, and distinctive ear appearance. Individuals with syndromic craniosynostosis have high rates of strabismus and ptosis, but the underlying pathology is unknown. Some individuals with syndromic craniosynostosis have been noted to have absence of individual extraocular muscles or abnormal insertions of the extraocular muscles on the globe. Using conditional knock-out alleles for Twist1 in cranial mesenchyme, we test the hypothesis that Twist1 is required for extraocular muscle organization and position, attachment to the globe, and/or innervation by the cranial nerves. We examined the extraocular muscles in conditional Twist1 knock-out animals using Twist2-cre and Pdgfrb-cre drivers. Both are expressed in cranial mesoderm and neural crest. Conditional inactivation of Twist1 using these drivers leads to disorganized extraocular muscles that cannot be reliably identified as specific muscles. Tendons do not form normally at the insertion and origin of these dysplastic muscles. Knock-out of Twist1 expression in tendon precursors, using scleraxis-cre, however, does not alter EOM organization. Furthermore, developing motor neurons, which do not express Twist1, display abnormal axonal trajectories in the orbit in the presence of dysplastic extraocular muscles. Strabismus in individuals with TWIST1 mutations may therefore be caused by abnormalities in extraocular muscle development and secondary abnormalities in innervation and tendon formation.

Ultrasound evaluation of fetal bone development in the collared (Pecari tajacu) and white-lipped peccary (Tayassu pecari)

The study of fetal development allows for evaluating the different strategies adopted by mammal species to maximize neonatal survival. Autonomous locomotion is fundamental for newborns to perform foraging activities and increases newborn survival from predation. In this study, we assess the gestational bone development of 53 collared (CP, Pecari tajacu) and 61 white-lipped (WLP, Tayassu pecari) peccaries, collected through the collaboration of subsistence hunters in the Amazon. The bone mineralization and biometry of the axial and appendicular skeleton were assessed by ultrasound examinations, and the timing of the main bone developmental events was calculated in relation to the total dorsal length (TDL) and the percentage of the total gestational period (GP). The first US signs of mineralization of the axial skeleton in CP and WLP were observed in fetuses with 3.4 cm (42 gestation days, 30% GPCP ) and 5.1 cm (51 gestation days, 32% GPWLP ). The early development of the appendicular skeleton was observed by the synchronic appearance of the mineralized scapula, humerus, radius, ulna, ilium, ischium, femur, tibia, and fibula at 36% GPCP (50 gestation days), and 35% GPWLP (56 gestation days). The pubis was mineralized in fetuses at 55% GPCP (75 gestation days) and 59% GPWLP (94 gestation days). The mineralization was observed in all autopod bones at 79% GPCP (109 gestation days) and 67% GPWLP (106 gestation days). All primary ossification centers in long bones of thoracic and pelvic limbs were mineralized in advanced fetuses (GPCP and GPWLP ≥75%). The mineralized patella was not observed in advanced fetuses in either species. Secondary ossification centers first appeared at the distal epiphysis of the femur in the CP (99 gestation days, 72% GPCP ) and the distal epiphysis of the radius, femur, and tibia in the WLP (106 gestation days, 67% GPWLP ). Advanced fetuses of CP and WLP presented 60% (15/25) and 68% (17/25) of the total secondary ossification centers observed present in the adult domestic pig, while newborns from the domestic pig presented 52% (13/25). The early intrauterine development of the skeletal system in both peccary species suggests a precocial development strategy, which likely correlates with neonatal ability to escape predators and reduces the dependence on parental care.

In situ integrated microrobots driven by artificial muscles built from biomolecular motors

Microrobots have been developed for applications in the submillimeter domain such as the manipulation of micro-objects and microsurgery. Rapid progress has been achieved in developing miniaturized components for microrobotic systems, resulting in a variety of functional microactuators and soft components for creating untethered microrobots. Nevertheless, the integration of microcomponents, especially the assembly of actuators and mechanical components, is still time-consuming and has inherent restrictions, thus limiting efficient fabrications of microrobots and their potential applications. Here, we propose a method for fabricating microrobots in situ inspired by the construction of microsystems in living organisms. In a microfluidic chip, hydrogel mechanical components and artificial muscle actuators are successively photopatterned from hydrogel prepolymer and biomolecular motors, respectively, and integrated in situ into functional microrobots. The proposed method allows the fast fabrication of microrobots through simple operations and affordable materials while providing versatile functions through the precise spatiotemporal control of in situ integration and reconfiguration of artificial muscles. To validate the method, we fabricated microrobots to elicit different motions and on-chip robots with unique characteristics for microfluidic applications. This study may establish a new paradigm for microrobot integration and lead to the production of unique biohybrid microrobots with various advantages.

Heterogeneous Skeletal Muscle Cell and Nucleus Populations Identified by Single-Cell and Single-Nucleus Resolution Transcriptome Assays

Single-cell RNA-seq (scRNA-seq) has revolutionized modern genomics, but the large size of myotubes and myofibers has restricted use of scRNA-seq in skeletal muscle. For the study of muscle, single-nucleus RNA-seq (snRNA-seq) has emerged not only as an alternative to scRNA-seq, but as a novel method providing valuable insights into multinucleated cells such as myofibers. Nuclei within myofibers specialize at junctions with other cell types such as motor neurons. Nuclear heterogeneity plays important roles in certain diseases such as muscular dystrophies. We survey current methods of high-throughput single cell and subcellular resolution transcriptomics, including single-cell and single-nucleus RNA-seq and spatial transcriptomics, applied to satellite cells, myoblasts, myotubes and myofibers. We summarize the major myonuclei subtypes identified in homeostatic and regenerating tissue including those specific to fiber type or at junctions with other cell types. Disease-specific nucleus populations were found in two muscular dystrophies, FSHD and Duchenne muscular dystrophy, demonstrating the importance of performing transcriptome studies at the single nucleus level in muscle.

Principles for the design of multicellular engineered living systems

Remarkable progress in bioengineering over the past two decades has enabled the formulation of fundamental design principles for a variety of medical and non-medical applications. These advancements have laid the foundation for building multicellular engineered living systems (M-CELS) from biological parts, forming functional modules integrated into living machines. These cognizant design principles for living systems encompass novel genetic circuit manipulation, self-assembly, cell-cell/matrix communication, and artificial tissues/organs enabled through systems biology, bioinformatics, computational biology, genetic engineering, and microfluidics. Here, we introduce design principles and a blueprint for forward production of robust and standardized M-CELS, which may undergo variable reiterations through the classic design-build-test-debug cycle. This Review provides practical and theoretical frameworks to forward-design, control, and optimize novel M-CELS. Potential applications include biopharmaceuticals, bioreactor factories, biofuels, environmental bioremediation, cellular computing, biohybrid digital technology, and experimental investigations into mechanisms of multicellular organisms normally hidden inside the "black box" of living cells.

Comparative development of limb musculature in phylogenetically and ecologically divergent lizards

BACKGROUND

Squamate reptiles (lizards, snakes, and amphisbaenians) exhibit incredible diversity in their locomotion, behavior, morphology, and ecological breadth. Although they often are used as models of locomotor diversity, surprisingly little attention has been given to muscle development in squamate reptiles. In fact, the most detailed examination was conducted almost 80 years ago and solely focused on the proximal limb regions. Herein, we present forelimb and hindlimb muscle morphogenesis data for three lizard species with different locomotion and feeding strategies: the desert grassland whiptail lizard, the central bearded dragon, and the veiled chameleon. This study fills critical gaps in our understanding of muscle morphogenesis in squamate reptiles and presents a comparative and temporospatial analysis of muscle development.

RESULTS

Our results reveal a conserved pattern of early muscle development among lizards with different adult morphologies and ecologies. The variations that exist are concentrated in distal regions, particularly the specialized autopodia of chameleons, where differentiation of muscles associated with the digits is delayed.

CONCLUSIONS

The chameleon autopod provides an example of major evolutionary modifications to the skeleton with only minor disruption of the conserved order and pattern of limb muscle development. This robustness of muscle patterning facilitates the evolution of extreme yet functional phenotypes.

Transcriptomic and Genetic Analyses Identify the Krüppel-Like Factor Dar1 as a New Regulator of Tube-Shaped Long Tendon Development

To ensure locomotion and body stability, the active role of muscle contractions relies on a stereotyped muscle pattern set in place during development. This muscle patterning requires a precise assembly of the muscle fibers with the skeleton via a specialized connective tissue, the tendon. Like in vertebrate limbs, Drosophila leg muscles make connections with specific long tendons that extend through different segments. During the leg disc development, cell precursors of long tendons rearrange and collectively migrate to form a tube-shaped structure. A specific developmental program underlies this unique feature of tendon-like cells in the Drosophila model. We provide for the first time a transcriptomic profile of leg tendon precursors through fluorescence-based cell sorting. From promising candidates, we identified the Krüppel-like factor Dar1 as a critical actor of leg tendon development. Specifically expressed in the leg tendon precursors, loss of dar1 disrupts actin-rich filopodia formation and tendon elongation. Our findings show that Dar1 acts downstream of Stripe and is required to set up the correct number of tendon progenitors.

Engineering Musculoskeletal Grafts for Multi-Tissue Unit Repair: Lessons From Developmental Biology and Wound Healing

In the musculoskeletal system, bone, tendon, and skeletal muscle integrate and act coordinately as a single multi-tissue unit to facilitate body movement. The development, integration, and maturation of these essential components and their response to injury are vital for conferring efficient locomotion. The highly integrated nature of these components is evident under disease conditions, where rotator cuff tears at the bone-tendon interface have been reported to be associated with distal pathological alterations such as skeletal muscle degeneration and bone loss. To successfully treat musculoskeletal injuries and diseases, it is important to gain deep understanding of the development, integration and maturation of these musculoskeletal tissues along with their interfaces as well as the impact of inflammation on musculoskeletal healing and graft integration. This review highlights the current knowledge of developmental biology and wound healing in the bone-tendon-muscle multi-tissue unit and perspectives of what can be learnt from these biological and pathological processes within the context of musculoskeletal tissue engineering and regenerative medicine. Integrating these knowledge and perspectives can serve as guiding principles to inform the development and engineering of musculoskeletal grafts and other tissue engineering strategies to address challenging musculoskeletal injuries and diseases.

Engineering multi-tissue units for regenerative Medicine: Bone-tendon-muscle units of the rotator cuff

Our body systems are comprised of numerous multi-tissue units. For the musculoskeletal system, one of the predominant functional units is comprised of bone, tendon/ligament, and muscle tissues working in tandem to facilitate locomotion. To successfully treat musculoskeletal injuries and diseases, critical consideration and thoughtful integration of clinical, biological, and engineering aspects are necessary to achieve translational bench-to-bedside research. In particular, identifying ideal biomaterial design specifications, understanding prior and recent tissue engineering advances, and judicious application of biomaterial and fabrication technologies will be crucial for addressing current clinical challenges in engineering multi-tissue units. Using rotator cuff tears as an example, insights relevant for engineering a bone-tendon-muscle multi-tissue unit are presented. This review highlights the tissue engineering strategies for musculoskeletal repair and regeneration with implications for other bone-tendon-muscle units, their derivatives, and analogous non-musculoskeletal tissue structures.

Loss of Smad4 in the scleraxis cell lineage results in postnatal joint contracture

Growth of the musculoskeletal system requires precise coordination between bone, muscle, and tendon during development. Insufficient elongation of the muscle-tendon unit relative to bone growth results in joint contracture, a condition characterized by reduction or complete loss of joint range of motion. Here we establish a novel murine model of joint contracture by targeting Smad4 for deletion in the tendon cell lineage using Scleraxis-Cre (ScxCre). Smad4^ScxCre mutants develop a joint contracture shortly after birth. The contracture is stochastic in direction and increases in severity with age. Smad4^ScxCre mutant tendons exhibited a stable reduction in cellularity and a progressive reduction in extracellular matrix volume. Collagen fibril diameters were reduced in the Smad4^ScxCre mutants, suggesting a role for Smad4 signaling in the regulation of matrix accumulation. Although ScxCre also has sporadic activity in both cartilage and muscle, we demonstrate an essential role for Smad4 loss in tendons for the development of joint contractures. Disrupting the canonical TGFβ-pathway in Smad2;3^ScxCre mutants did not result in joint contractures. Conversely, disrupting the BMP pathway by targeting BMP receptors (Alk3^ScxCre/Alk6^null) recapitulated many features of the Smad4^ScxCre contracture phenotype, suggesting that joint contracture in Smad4^ScxCre mutants is caused by disruption of BMP signaling. Overall, these results establish a model of murine postnatal joint contracture and a role for BMP signaling in tendon elongation and extracellular matrix accumulation.

Local retinoic acid signaling directs emergence of the extraocular muscle functional unit

Coordinated development of muscles, tendons, and their attachment sites ensures emergence of functional musculoskeletal units that are adapted to diverse anatomical demands among different species. How these different tissues are patterned and functionally assembled during embryogenesis is poorly understood. Here, we investigated the morphogenesis of extraocular muscles (EOMs), an evolutionary conserved cranial muscle group that is crucial for the coordinated movement of the eyeballs and for visual acuity. By means of lineage analysis, we redefined the cellular origins of periocular connective tissues interacting with the EOMs, which do not arise exclusively from neural crest mesenchyme as previously thought. Using 3D imaging approaches, we established an integrative blueprint for the EOM functional unit. By doing so, we identified a developmental time window in which individual EOMs emerge from a unique muscle anlage and establish insertions in the sclera, which sets these muscles apart from classical muscle-to-bone type of insertions. Further, we demonstrate that the eyeballs are a source of diffusible all-trans retinoic acid (ATRA) that allow their targeting by the EOMs in a temporal and dose-dependent manner. Using genetically modified mice and inhibitor treatments, we find that endogenous local variations in the concentration of retinoids contribute to the establishment of tendon condensations and attachment sites that precede the initiation of muscle patterning. Collectively, our results highlight how global and site-specific programs are deployed for the assembly of muscle functional units with precise definition of muscle shapes and topographical wiring of their tendon attachments.

The TMJ Disc Is a Common Ancestral Feature in All Mammals, as Evidenced by the Presence of a Rudimentary Disc During Monotreme Development

The novel mammalian jaw joint, known in humans as the temporomandibular joint or TMJ, is cushioned by a fibrocartilage disc. This disc is secondarily absent in therian mammals that have lost their dentition, such as giant anteaters and some baleen whales. The disc is also absent in all monotremes. However, it is not known if the absence in monotremes is secondary to the loss of dentition, or if it is an ancestral absence. We use museum held platypus and echidna histological sections to demonstrate that the developing monotreme jaw joint forms a disc primordium that fails to mature and become separated from the mandibular condyle. We then show that monotreme developmental anatomy is similar to that observed in transgenic mouse mutants with reduced cranial musculature. We therefore suggest that the absence of the disc on monotremes is a consequence of the changes in jaw musculature associated with the loss of adult teeth. Taken together, these data indicate that the ancestors of extant monotremes likely had a jaw joint disc, and that the disc evolved in the last common ancestor of all mammals.

MET promotes the proliferation and differentiation of myoblasts

The receptor tyrosine kinase MET plays a vital role in skeletal muscle development and in postnatal muscle regeneration. However, the effect of MET on myogenesis of myoblasts has not yet been fully understood. This study aimed to investigate the effects of MET on myogenesis in vivo and in vitro. Decreased myonuclei and down-regulated expression of myogenesis-related markers were observed in Met p.Y1232C mutant heterozygous mice. To explore the effects of MET on myoblast proliferation and differentiation, Met was overexpressed or interfered in C2C12 myoblast cells through the lentiviral transfection. The Met overexpression cells exhibited promotion in myoblast proliferation, while the Met deficiency cells showed impediment in proliferation. Moreover, myoblast differentiation was enhanced by the stable Met overexpression, but was impaired by Met deficiency. Furthermore, this study demonstrated that SU11274, an inhibitor of MET kinase activity, suppressed myoblast differentiation, suggesting that MET regulated the expression of myogenic regulatory factors (MRFs) and of desmin through the classical tyrosine kinase pathway. On the basis of the above findings, our work confirmed that MET promoted the proliferation and differentiation of myoblasts, deepening our understanding of the molecular mechanisms underlying muscle development.

Models of tendon development and injury

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

Requirement for scleraxis in the recruitment of mesenchymal progenitors during embryonic tendon elongation

The transcription factor scleraxis (Scx) is required for tendon development; however, the function of Scx is not fully understood. Although Scx is expressed by all tendon progenitors and cells, only long tendons are disrupted in the Scx ^-/- mutant; short tendons appear normal and the ability of muscle to attach to skeleton is not affected. We recently demonstrated that long tendons are formed in two stages: first, by muscle anchoring to skeleton via a short tendon anlage; and second, by rapid elongation of the tendon in parallel with skeletal growth. Through lineage tracing, we extend these observations to all long tendons and show that tendon elongation is fueled by recruitment of new mesenchymal progenitors. Conditional loss of Scx in mesenchymal progenitors did not affect the first stage of anchoring; however, new cells were not recruited during elongation and long tendon formation was impaired. Interestingly, for tenocyte recruitment, Scx expression was required only in the recruited cells and not in the recruiting tendon. The phenotype of Scx mutants can thus be understood as a failure of tendon cell recruitment during tendon elongation.

The zebrafish HGF receptor met controls migration of myogenic progenitor cells in appendicular development

The hepatocyte growth factor receptor C-met plays an important role in cellular migration, which is crucial for many developmental processes as well as for cancer cell metastasis. C-met has been linked to the development of mammalian appendicular muscle, which are derived from migrating muscle progenitor cells (MMPs) from within the somite. Mammalian limbs are homologous to the teleost pectoral and pelvic fins. In this study we used Crispr/Cas9 to mutate the zebrafish met gene and found that the MMP derived musculature of the paired appendages was severely affected. The mutation resulted in a reduced muscle fibre number, in particular in the pectoral abductor, and in a disturbed pectoral fin function. Other MMP derived muscles, such as the sternohyoid muscle and posterior hypaxial muscle were also affected in met mutants. This indicates that the role of met in MMP function and appendicular myogenesis is conserved within vertebrates.

Odd-skipped and Stripe act downstream of Notch to promote the morphogenesis of long appendicular tendons in Drosophila

Multiple tissue interactions take place during the development of the limb musculoskeletal system. While appendicular myogenesis has been extensively studied, development of connective tissue associated with muscles has received less attention. In the developing Drosophila leg, tendon-like connective tissue arises from clusters of epithelial cells that invaginate into the leg cavity and then elongate to form internal tube-shape structures along which muscle precursors are distributed. Here we show that stripe-positive appendicular precursors of tendon-like connective tissue are set up among intersegmental leg joint cells expressing odd-skipped genes, and that Notch signaling is necessary and locally sufficient to trigger stripe expression. This study also finds that odd-skipped genes and stripe are both required downstream of Notch to promote morphogenesis of tube-shaped internal tendons of the leg.

Summary: In this paper, we show that Notch promotes the tendon development by inducing Stripe expression in leg discs and that both Stripe and Odd-skipped are required to form tube-like tendons.

Mechanobiology of limb musculoskeletal development

While there has been considerable progress in identifying molecular regulators of musculoskeletal development, the role of physical forces in regulating induction, differentiation, and patterning events is less well understood. Here, we highlight recent findings in this area, focusing primarily on model systems that test the mechanical regulation of skeletal and tendon development in the limb. We also discuss a few of the key signaling pathways and mechanisms that have been implicated in mechanotransduction and highlight current gaps in knowledge and opportunities for further research in the field.