Response of rabbit skeletal muscle to tibial lengthening

Fibroblast fusion to the muscle fiber regulates myotendinous junction formation

Vertebrate muscles and tendons are derived from distinct embryonic origins yet they must interact in order to facilitate muscle contraction and body movements. How robust muscle tendon junctions (MTJs) form to be able to withstand contraction forces is still not understood. Using techniques at a single cell resolution we reexamine the classical view of distinct identities for the tissues composing the musculoskeletal system. We identify fibroblasts that have switched on a myogenic program and demonstrate these dual identity cells fuse into the developing muscle fibers along the MTJs facilitating the introduction of fibroblast-specific transcripts into the elongating myofibers. We suggest this mechanism resulting in a hybrid muscle fiber, primarily along the fiber tips, enables a smooth transition from muscle fiber characteristics towards tendon features essential for forming robust MTJs. We propose that dual characteristics of junctional cells could be a common mechanism for generating stable interactions between tissues throughout the musculoskeletal system.

Bone Formation and Adaptive Morphology of the Anterior Tibial Muscle in 3-mm Daily Lengthening Using High-Fractional Automated Distraction and Osteosynthesis with the Ilizarov Apparatus Combined with Intramedullary Hydroxyapatite-Coated Wire

Purpose

We studied osteogenesis and morphofunctional features of the anterior tibial muscle using 3-mm high-frequency automated lengthening with the Ilizarov apparatus alone and in combination with intramedullary nailing.

Material and Methods

Tibia was lengthened with a round-the-clock automated distractor at a 3-mm daily rate for 10 days in 16 mongrel dogs. In group 1 (n = 8), a 1.8-mm intramedullary titanium wire coated with hydroxyapatite was introduced into the tibial canal followed by Ilizarov frame mounting and transverse osteotomy of the diaphysis. Distraction mode was 0.025 mm x 120 increments a day. In group 2 (n = 8), distraction mode was the same but nailing was not used. Bone formation and the anterior tibial muscle were studied at two time points: (1) upon distraction completion; (2) three months after the apparatus removal. Bone formation was studied radiographically. Muscle preparations were examined histologically and stereomicroscopically.

Results

There was a threefold reduction in the distraction time in both groups. Consolidation took 13.83±4.02 days in group 1 and 33.7±2.4 days in group 2. Muscle macropreparations of the experimental limb in group 1 at study time points did not show significant differences from intact tissues. Muscle histostructure in both groups was characterized by activation of angiogenesis and myohistogenesis, but the volumetric density of microvessels in the lengthening phase was three times higher in group 1.

Conclusion

Combined technology significantly reduces the total lengthening procedure and does not compromise limb functions. Intramedullary HA-coated wires promote faster bone formation. The muscle was able to exhibit structural adaptation and plasticity of a restitution type.

Shortening and Angulation Strategies to Address Composite Bone and Soft Tissue Defects

Gustilo-Anderson IIIB tibial fractures and infected tibial nonunions represent particular challenges for the orthopaedic trauma surgeon. Debridement of dysvascular bone and soft tissues can create composite bone and soft tissue defects. Restoring the soft tissue envelope is a critical step in preventing or treating ongoing infection and in restoring local blood supply. Shortening and angulation techniques, including the gradual expansion muscle flap, rely on distraction histogenesis to address composite bone and soft tissue loss. These strategies can be used to treat large soft tissue defects when flap coverage or free tissue transfer is not available.

Muscle metabolism during tibial lengthening with regular and high distraction rates

INTRODUCTION

Muscle regeneration is promoted when the Ilizarov method is used for limb lengthening and deformity correction, but the regenerative ability of muscles decreases when achieving large amounts of elongation. Much research has been dedicated to studying the capabilities of muscles under lengthening, but no reports are available that investigate the muscle metabolism. We supposed that energy turnover would be activated in skeletal muscles under lengthening as a response to distraction, and the activity of the energy turnover would grow in proportion to the increase in the distraction rate or amount.

MATERIALS AND METHODS

We compared the metabolism of canine anterior tibial muscles (ATMs) by regular and 3-mm high-frequency bone distraction in 30 dogs to obtain 14.5 ± 0.8% lengthening from the initial tibial length. Group 1 (n = 12) had manual lengthening with a rate of 1 mm per day. Three millimeters per day was produced with 120 increments in automated mode in group 2 (n = 12). An intact group (n = 6) served as controls. ATMs were harvested at the end of distraction, after 30 days of fixation, and 30 days after frame removal. We assessed the activity of lactate dehydrogenase, creatine phosphokinase, glucoso-6-phosphate dehydrogenase, and catalase and calculated the concentration of malone dialdehyde, sarcoplasmic and contractile proteins in the ATM extract.

RESULTS

Energy turnover reactions were activated in the ATM as a response to distraction forces, but the activity of the energy turnover did not grow proportionally to the increased distraction rate. Levels of sarcoplasmic and contractile proteins in the ATM decreased insignificantly in both groups.

CONCLUSIONS

High-frequency 3-mm daily lengthening results in compensatory energy turnover changes in the muscle, sufficient for prevention of catabolic processes.

The gradual expansion muscle flap

High-energy open fractures of the tibia have traditionally been fraught with challenges to include bone comminution or loss, soft tissue loss, nonunion, and infection. A number of techniques have been implemented to treat the severe soft tissue loss typically involving the anteromedial surface of the tibia to include wet-to-dry dressings or Papineau techniques, negative pressure wound therapy, acellular dermal matrices, and rotational or free tissue transfer with Masquelet technique, primary shortening, and distraction osteogenesis to address bone loss. We present a novel technique and subsequent case series that obviates the need of free tissue transfer while treating high-energy type IIIB open tibia fractures by performing an acute shortening and angulation of the tibia and rotational muscle flap coverage and split-thickness skin grafting of the soft tissue defect. Distraction histiogenesis with circular external fixation is then used to correct the residual osseous deformity while stretching the rotational muscle flap.

The harmonies played by TGF-β in stem cell biology

To rejuvenate tissues and/or repair wounds, stem cells must receive extrinsic signals from their surrounding environment and integrate them with their intrinsic abilities to self-renew and differentiate to make tissues. Increasing evidence suggests that the superfamily of transforming growth factor-βs (TGF-βs) constitute integral components in the intercellular crosstalk between stem cells and their microenvironment. In this review, we summarize recent advances in our understanding of TGF-β superfamily functions in embryonic and adult stem cells. We discuss how these pathways help to define the physiological environment where stem cells reside, and how perturbations in the signaling circuitry contribute to cancers.

Mechanical strain applied to human fibroblasts differentially regulates skeletal myoblast differentiation

Cyclic short-duration stretches (CSDS) such as those resulting from repetitive motion strain increase the risk of musculoskeletal injury. Myofascial release is a common technique used by clinicians that applies an acyclic long-duration stretch (ALDS) to muscle fascia to repair injury. When subjected to mechanical strain, fibroblasts within muscle fascia secrete IL-6, which has been shown to induce myoblast differentiation, essential for muscle repair. We hypothesize that fibroblasts subjected to ALDS following CSDS induce myoblast differentiation through IL-6. Fibroblast conditioned media and fibroblast-myoblast cocultures were used to test fibroblasts' ability to induce myoblast differentiation. The coculture system applies strain to fibroblasts only but still allows for diffusion of potential differentiation mediators to unstrained myoblasts on coverslips. To determine the role of IL-6, we utilized myoblast unicultures ± IL-6 (0-100 ng/ml) and cocultures ± α-IL-6 (0-200 μg/ml). Untreated uniculture myoblasts served as a negative control. After 96 h, coverslips (n = 6-21) were microscopically analyzed and quantified by blinded observer for differentiation endpoints: myotubes per square millimeter (>3 nuclei/cell), nuclei/myotube, and fusion efficiency (%nuclei within myotubes). The presence of fibroblasts and fibroblast conditioned media significantly enhanced myotube number (P < 0.05). However, in coculture, CSDS applied to fibroblasts did not reproduce this effect. ALDS following CSDS increased myotube number by 78% and fusion efficiency by 96% vs. CSDS alone (P < 0.05). Fibroblasts in coculture increase IL-6 secretion; however, IL-6 secretion did not correlate with enhanced differentiation among strain groups. Exogenous IL-6 in myoblast uniculture failed to induce differentiation. However, α-IL-6 attenuated differentiation in all coculture groups (P < 0.05). Fibroblasts secrete soluble mediators that have profound effects on several measures of myoblast differentiation. Specific biophysical strain patterns modify these outcomes, and suggest that myofascial release after repetitive strain increases myoblast differentiation and thus may improve muscle repair in vivo. Neutralization of IL-6 in coculture significantly reduced differentiation, suggesting fibroblast-IL-6 is necessary but not sufficient in this process.

BMPs are mediators in tissue crosstalk of the regenerating musculoskeletal system

The musculoskeletal system is a tight network of many tissues. Coordinated interplay at a biochemical level between tissues is essential for development and repair. Traumatic injury usually affects several tissues and represents a large challenge in clinical settings. The current demand for potent growth factors in such applications thus accompanies the keen interest in molecular mechanisms and orchestration of tissue formation. Of special interest are multitasking growth factors that act as signals in a variety of cell types, both in a paracrine and in an autocrine manner, thereby inducing cell differentiation and coordinating not only tissue assembly at specific sites but also maturation and homeostasis. We concentrate here on bone morphogenetic proteins (BMPs), which are important crosstalk mediators known for their irreplaceable roles in vertebrate development. The molecular crosstalk during embryonic musculoskeletal tissue formation is recapitulated in adult repair. BMPs act at different levels from the initiation to maturation of newly formed tissue. Interestingly, this is influenced by the spatiotemporal expression of different BMPs, their receptors and co-factors at the site of repair. Thus, the regenerative potential of BMPs needs to be evaluated in the context of highly connected tissues such as muscle and bone and might indeed be different in more poorly connected tissues such as cartilage. This highlights the need for an understanding of BMP signaling across tissues in order to eventually improve BMP regenerative potential in clinical applications. In this review, the distinct members of the BMP family and their individual contribution to musculoskeletal tissue repair are summarized by focusing on their paracrine and autocrine functions.

Bmp signaling at the tips of skeletal muscles regulates the number of fetal muscle progenitors and satellite cells during development

Muscle progenitors, labeled by the transcription factor Pax7, are responsible for muscle growth during development. The signals that regulate the muscle progenitor number during myogenesis are unknown. We show, through in vivo analysis, that Bmp signaling is involved in regulating fetal skeletal muscle growth. Ectopic activation of Bmp signaling in chick limbs increases the number of fetal muscle progenitors and fibers, while blocking Bmp signaling reduces their numbers, ultimately leading to small muscles. The Bmp effect that we observed during fetal myogenesis is diametrically opposed to that previously observed during embryonic myogenesis and that deduced from in vitro work. We also show that Bmp signaling regulates the number of satellite cells during development. Finally, we demonstrate that Bmp signaling is active in a subpopulation of fetal progenitors and satellite cells at the extremities of muscles. Overall, our results show that Bmp signaling plays differential roles in embryonic and fetal myogenesis.

Dynamics of muscle fibre growth during postnatal mouse development

Background

Postnatal growth in mouse is rapid, with total skeletal muscle mass increasing several-fold in the first few weeks. Muscle growth can be achieved by either an increase in muscle fibre number or an increase in the size of individual myofibres, or a combination of both. Where myofibre hypertrophy during growth requires the addition of new myonuclei, these are supplied by muscle satellite cells, the resident stem cells of skeletal muscle.

Results

Here, we report on the dynamics of postnatal myofibre growth in the mouse extensor digitorum longus (EDL) muscle, which is essentially composed of fast type II fibres in adult. We found that there was no net gain in myofibre number in the EDL between P7 and P56 (adulthood). However, myofibre cross-sectional area increased by 7.6-fold, and length by 1.9-fold between these ages, resulting in an increase in total myofibre volume of 14.1-fold: showing the extent of myofibre hypertrophy during the postnatal period. To determine how the number of myonuclei changes during this period of intense muscle fibre hypertrophy, we used two complementary mouse models: 3F-nlacZ-E mice express nlacZ only in myonuclei, while Myf5^nlacZ/+ mice have β-galactosidase activity in satellite cells. There was a ~5-fold increase in myonuclear number per myofibre between P3 and P21. Thus myofibre hypertrophy is initially accompanied by a significant addition of myonuclei. Despite this, the estimated myonuclear domain still doubled between P7 and P21 to 9.2 × 10³ μm³. There was no further addition of myonuclei from P21, but myofibre volume continued to increase, resulting in an estimated ~3-fold expansion of the myonuclear domain to 26.5 × 10³ μm³ by P56. We also used our two mouse models to determine the number of satellite cells per myofibre during postnatal growth. Satellite cell number in EDL was initially ~14 satellite cells per myofibre at P7, but then fell to reach the adult level of ~5 by P21.

Conclusions

Postnatal fast muscle fibre type growth is divided into distinct phases in mouse EDL: myofibre hypertrophy is initially supported by a rapid increase in the number of myonuclei, but nuclear addition stops around P21. Since the significant myofibre hypertrophy from P21 to adulthood occurs without the net addition of new myonuclei, a considerable expansion of the myonuclear domain results. Satellite cell numbers are initially stable, but then decrease to reach the adult level by P21. Thus the adult number of both myonuclei and satellite cells is already established by three weeks of postnatal growth in mouse.

The role of variable muscle adaptation to limb lengthening in the development of joint contractures: an experimental study in the goat

Muscle stiffness and joint contractures are currently regarded as the most common complications of limb lengthening. To better understand the mechanisms of joint contractures, architectural changes of all involved muscles were analyzed in 9 goats after 20% tibial lengthening with standard distraction protocol.All 13 muscles of the goat's tibia were found to be organized into an anterior compartment with 2 longitudinal and 4 pennate muscles and a posterior compartment with 1 longitudinal and 6 pennate muscles. Longitudinal muscles showed better compliance to distraction than pinnate muscles. Although muscle-to-bone lengthening ratio ranged widely (0-1.2), most of the muscles and especially those located in the posterior compartment showed much less lengthening than the bone. Muscular portions of the muscles lengthened more substantially (average, 17%) than their associated tendons (average, 7%). Muscle fiber length changes varied greatly between muscles (range, 0%-88%). Normalization of muscle fiber length revealed considerable elongation of anterior muscles fibers (25%) that was associated with an addition of new sarcomeres in series. Fiber length increase of all posterior muscles but one occurred by stretching of existing sarcomeres, with little addition or even dissolution of sarcomeres in series. This correlated with muscle mass changes showing significant muscle atrophy in the posterior compartment and better mass preservation in the anterior compartment.The study revealed striking difference in response to limb lengthening between individual muscles and muscles from antagonistic compartments in particular. Poor sarcomerogenesis in the posterior muscles leading to their insufficient length increase seems to play major role in the development of joint contractures.

Effects of hyperbaric oxygen therapy on rabbit skeletal muscle during extremity lengthening

INTRODUCTION

Extremity lengthening through distraction osteogenesis is limited by the surrounding skeletal muscle and neurovascular structures rather than the bone itself. The purpose of this study is to evaluate the effects of hyperbaric oxygen therapy on skeletal muscle during distraction osteogenesis.

MATERIALS AND METHODS

Twenty New Zealand white rabbits were randomly divided into two groups. Right tibia of all rabbits was distracted at a rate of 0.125 mm per 6 h (0.5 mm/day) for 10 days with circular external fixator. Experimental group rabbits (N=10) underwent 2.5 ATA hyperbaric oxygen therapy for 2 h everyday for 20 days, control group rabbits (N=10) did not receive any corresponding treatment. Skeletal muscle perfusion was evaluated with scintigraphy before and after the distraction period. Serum CPK, LDH and AST levels were measured before and after the distraction period. All animals were killed on the 27th day. The right tibias of all animals were removed and tibialis posterior muscle was harvested for histopathologic and histomorphometric assessment with light and electron microscopy.

RESULTS

Skeletal muscle perfusion was decreased in the control group in comparison with pre-distraction level (P=0.008). However, no significant decrease was observed in the experimental group (P=0.678). There were no statistical differences in serum CPK, LDH and AST levels between groups (P=0.340, P=0.077, P=0.796). The mean area of the muscle fibers was measured as 398.66+/-9.16 micro2 in the experimental group and 349.44+/-5.76 micro2 in the control group (P=0.000) with light microscopy. Mild fibrosis was observed in connective tissue component of muscle tissue in control group. An average of 26 myofibrils (20-32) was counted in a 16-cm2 unit area in experimental group and 50 myofibrils (35-65) in the control group with electron microscopy. Enlargement in the sarcoplasmic reticulum, degenerative changes in nuclear cytoplasm and increase in myofibril diameter were observed in the control group, which was not observed in the experimental group

CONCLUSION

Results of this study suggest that HBO treatment alleviates the detrimental effects of distraction on skeletal muscles and preserves its ultrastructure.

Structural changes in the lengthened rabbit muscle

This study evaluated the histological changes in muscle tissue after limb lengthening in skeletally mature and immature rabbits and assessed the most vulnerable level of striated muscle. Twenty-three male domestic white rabbits, divided into six groups, were operated on and different lengthening protocols were used in the mature and immature rabbits. The histopathological changes were analysed by a semi-quantitative method according to the scoring system of Lee et al. (Acta Orthop Scand 64(6):688-692, 1993). After the evaluation of the five main degenerative parameters (muscle atrophy, muscle nuclei internalisation, degeneration of the muscle fibre, perimysial and endomysial fibrosis, haematomas), it is evident that the adults lengthened at a rate of 1.6 mm/day showed more degenerative changes than those lengthened at 0.8 mm/day. The adult 1.6 mm/day lengthened group presented significantly higher damage in the muscle and lower regenerative signs compared with the young 1.6 mm/day lengthened group, according to the summarised degenerative scores.

Pax7 reveals a greater frequency and concentration of satellite cells at the ends of growing skeletal muscle fibers

The main sites of longitudinal growth in skeletal muscle are the ends of the fibers. This study tests the hypothesis that satellite cells (SCs) are at a greater frequency (#SC nuclei/all nuclei within basal laminae) and concentration (closer together) within growing fiber ends of posthatch chicken pectoralis. SCs were localized by their Pax7 expression, and fiber ends were identified by their retention of neonatal myosin heavy chains and small cross-sectional profiles. Whereas SC frequency decreased from about 20% at 9 days posthatch to <5% at 115 days, fiber ends retained a frequency of approximately 16%. Calculated mean area of sarcolemma per SC revealed higher concentrations of SCs at fiber ends. There was also a strong inverse correlation between SC frequency and fiber profile cross-sectional size throughout development. This study suggests that SCs at fiber ends play a key role in the longitudinal growth of muscle fibers, and that fiber profile size may impact SC distribution.