Transected myofibres may remain permanently divided in two parts

ALM Therapy Promotes Functional and Histologic Regeneration of Traumatized Peripheral Skeletal Muscle

Skeletal muscle trauma is a common injury with a range of severity. Adenosine, lidocaine and Mg²⁺ (ALM) is a protective solution and improves tissue perfusion and coagulopathy. Male Wistar rats were anesthetized and subjected to standardized skeletal muscle trauma of the left soleus muscle with the protection of the neurovascular structures. Seventy animals were randomly assigned to saline control or ALM. Immediately after trauma, a bolus of ALM solution was applied intravenously, followed by a one-hour infusion. After 1, 4, 7, 14 and 42 days, the biomechanical regenerative capacity was examined using incomplete tetanic force and tetany, and immunohistochemistry was used to examine for proliferation and apoptosis characteristics. Biomechanical force development showed a significant increase following ALM therapy for incomplete tetanic force and tetany on days 4 and 7. In addition, the histological evaluation showed a significant increase in proliferative BrdU-positive cells with ALM therapy on days 1 and 14. Ki67 histology also detected significantly more proliferative cells on days 1, 4, 7, 14 and 42 in ALM-treated animals. Furthermore, a simultaneous decrease in the number of apoptotic cells was observed using the TUNEL method. ALM solution showed significant superiority in biomechanical force development and also a significant positive effect on cell proliferation in traumatized skeletal muscle tissue and reduced apoptosis.

Discovery of Muscle-Tendon Progenitor Subpopulation in Human Myotendinous Junction at Single-Cell Resolution

The myotendinous junction (MTJ) is a complex and special anatomical area that connects muscles and tendons, and it is also the key to repairing tendons. Nevertheless, the anatomical structure and connection structure of MTJ, the cluster and distribution of cells, and which cells are involved in repairing the tissue are still unclear. Here, we analyzed the cell subtype distribution and function of human MTJ at single-cell level. We identified four main subtypes, including stem cell, muscle, tendon, and muscle-tendon progenitor cells (MTP). The MTP subpopulation, which remains the characteristics of stem cells and also expresses muscle and tendon marker genes simultaneously, may have the potential for bidirectional differentiation. We also found the muscle-tendon progenitor cells were distributed in the shape of a transparent goblet; muscle cells first connect to the MTP and then to the tendon. And after being transplanted in the MTJ injury model, MTP exhibited strong regenerative capability. Finally, we also demonstrated the importance of mTOR signaling for MTP maintenance by in vitro addition of rapamycin and in vivo validation using mTOR-ko mice. Our research conducted a comprehensive analysis of the heterogeneity of myotendinous junction, discovered a special cluster called MTP, provided new insights into the biological significance of myotendinous junction, and laid the foundation for future research on myotendinous junction regeneration and restoration.

Muscle Precursor Cells Enhance Functional Muscle Recovery and Show Synergistic Effects With Postinjury Treadmill Exercise in a Muscle Injury Model in Rats

BACKGROUND

Skeletal muscle injuries represent a major concern in sports medicine. Cell therapy has emerged as a promising therapeutic strategy for muscle injuries, although the preclinical data are still inconclusive and the potential clinical use of cell therapy has not yet been established.

PURPOSE

To evaluate the effects of muscle precursor cells (MPCs) on muscle healing in a small animal model.

STUDY DESIGN

Controlled laboratory study.

METHODS

A total of 27 rats were used in the study. MPCs were isolated from rat (n = 3) medial gastrocnemius muscles and expanded in primary culture. Skeletal muscle injury was induced in 24 rats, and the animals were assigned to 3 groups. At 36 hours after injury, animals received treatment based on a single ultrasound-guided MPC (10⁵ cells) injection (Cells group) or MPC injection in combination with 2 weeks of daily exercise training (Cells+Exercise group). Animals receiving intramuscular vehicle injection were used as controls (Vehicle group). Muscle force was determined 2 weeks after muscle injury, and muscles were collected for histological and immunofluorescence evaluation.

RESULTS

Red fluorescence-labeled MPCs were successfully transplanted in the site of the injury by ultrasound-guided injection and were localized in the injured area after 2 weeks. Transplanted MPCs participated in the formation of regenerating muscle fibers as corroborated by the co-localization of red fluorescence with developmental myosin heavy chain (dMHC)-positive myofibers by immunofluorescence analysis. A strong beneficial effect on muscle force recovery was detected in the Cells and Cells+Exercise groups (102.6% ± 4.0% and 101.5% ± 8.5% of maximum tetanus force of the injured vs healthy contralateral muscle, respectively) compared with the Vehicle group (78.2% ± 5.1%). Both Cells and Cells+Exercise treatments stimulated the growth of newly formed regenerating muscles fibers, as determined by the increase in myofiber cross-sectional area (612.3 ± 21.4 µm² and 686.0 ± 11.6 µm², respectively) compared with the Vehicle group (247.5 ± 10.7 µm²), which was accompanied by a significant reduction of intramuscular fibrosis in Cells and Cells+Exercise treated animals (24.2% ± 1.3% and 26.0% ± 1.9% of collagen type I deposition, respectively) with respect to control animals (40.9% ± 4.1% in the Vehicle group). MPC treatment induced a robust acceleration of the muscle healing process as demonstrated by the decreased number of dMHC-positive regenerating myofibers (enhanced replacement of developmental myosin isoform by mature myosin isoforms) (4.3% ± 2.6% and 4.1% ± 1.5% in the Cells and Cells+Exercise groups, respectively) compared with the Vehicle group (14.8% ± 13.9%).

CONCLUSION

Single intramuscular administration of MPCs improved histological outcome and force recovery of the injured skeletal muscle in a rat injury model that imitates sports-related muscle injuries. Cell therapy showed a synergistic effect when combined with an early active rehabilitation protocol in rats, which suggests that a combination of treatments can generate novel therapeutic strategies for the treatment of human skeletal muscle injuries.

CLINICAL RELEVANCE

Our study demonstrates the strong beneficial effect of MPC transplant and the synergistic effect when the cell therapy is combined with an early active rehabilitation protocol for muscle recovery in rats; this finding opens new avenues for the development of effective therapeutic strategies for muscle healing and clinical trials in athletes undergoing MPC transplant and rehabilitation protocols.

Local low-intensity vibration improves healing of muscle injury in mice

Recovery from traumatic muscle injuries is typically prolonged and incomplete. Our previous study demonstrated that whole‐body low‐intensity vibration (LIV) enhances healing in a mouse laceration model. We sought to determine whether locally applied LIV (a) improves muscle repair following injury in mice and (b) is directly transduced by cultured muscle cells, via increased IGF‐1 activity. C57BL/6J mice were subjected to laceration of the gastrocnemius muscle and were treated with LIV applied directly to the lower leg for 30 min/day or non‐LIV sham treatment (controls) for 7 or 14 days. LIV was also applied to differentiating myotubes in culture for 30 min/day for 3 or 6 days. Compared with control mice, LIV increased myofiber cross‐sectional area, diameter, and percent area of peripherally nucleated fibers, and decreased percent damaged area after 14 days of treatment. In cultured myotubes, LIV increased fusion and diameter compared with controls after 6 days of treatment. These LIV‐induced effects were associated with increased total Akt on day 7 in injured muscle and on day 3 in myotubes, whereas phosphorylated‐to‐total Akt ratio increased on day 14 in injured muscle and on day 6 in myotubes but were not associated with increased IGF‐1 levels at any time point. These changes were also associated with LIV‐induced suppression of FOXO1 and Atrogin‐1 gene expression at day 7 in injured muscle. These findings demonstrate that muscle cells can directly transduce LIV signals into increased growth and differentiation, and this effect is associated with increased Akt signaling.

Low-Intensity Vibration Improves Muscle Healing in a Mouse Model of Laceration Injury

Recovery from traumatic muscle injuries is typically prolonged and incomplete, leading to impaired muscle and joint function. We sought to determine whether mechanical stimulation via whole-body low-intensity vibration (LIV) could (1) improve muscle regeneration and (2) reduce muscle fibrosis following traumatic injury. C57BL/6J mice were subjected to a laceration of the gastrocnemius muscle and were treated with LIV (0.2 g at 90 Hz or 0.4 g at 45 Hz for 30 min/day) or non-LIV sham treatment (controls) for seven or 14 days. Muscle regeneration and fibrosis were assessed in hematoxylin and eosin or Masson's trichrome stained muscle cryosections, respectively. Compared to non-LIV control mice, the myofiber cross-sectional area was larger in mice treated with each LIV protocol after 14 days of treatment. Minimum fiber diameter was also larger in mice treated with LIV of 90 Hz/0.2 g after 14 days of treatment. There was also a trend toward a reduction in collagen deposition after 14 days of treatment with 45 Hz/0.4 g (p = 0.059). These findings suggest that LIV may improve muscle healing by enhancing myofiber growth and reducing fibrosis. The LIV-induced improvements in muscle healing suggest that LIV may represent a novel therapeutic approach for improving the healing of traumatic muscle injuries.

Regeneration techniques for bone-to-tendon and muscle-to-tendon interfaces reconstruction

INTRODUCTION

The complex structure of the bone-tendon and muscle-tendon junctions makes their reproduction for tissue engineering applications very difficult. Relatively few studies have investigated the characteristics of these regions from a tissue engineering view point.

SOURCES OF DATA

PubMed, Thomson Reuters, Scopus and Google Scholar databases were searched using various combinations of the keywords 'Tendon', 'Myotendinous junction', 'Osteotendinous junction', 'Tissue engineering' and 'Scaffold'.

AREAS OF AGREEMENT

The available studies can be divided according to whether the objective is to build an entire composite tissue unit or to assist the recreation of interfaces, such as improving integration of autografts with the surrounding bone or with the muscle. The most used techniques are based on the electrospinning and the self-reorganized constructs process, which were applied to both bone-to-tendon junction (BTJ) and muscle-to-tendon junction (MTJ) regeneration. The use of nanofibers that mimic the hierarchical structure of the extracellular matrix (ECM), eventually functionalized by encapsulation of bioactive components, allowed cell attachment and differentiation.

AREAS OF CONTROVERSY

There have been no translational investigations.

GROWING POINTS

There is a need to devise suitable techniques that allow suitable tissue engineering of BTJ and MTJ.

AREAS TIMELY FOR DEVELOPING RESEARCH

Appropriately planned studies are needed to translate tissue engineering from a scientific challenge to a clinically applicable technique.

Interaction between HSP 70 and iNOS in skeletal muscle injury and repair

Muscle injuries are frequently occurred in various sports. The biological process and mechanism of muscle repair after injury are well known through the many studies. This study aimed at presenting heat shock protein and nitric oxide synthase are to respond to muscle damage and repair. This section discusses the results obtained through many articles. Heat shock proteins (HSPs) are considered to play an essential role in protecting cells from damage, preparing them to survive on new environmental challenges. In addition, exercise-induced changes such as heat shock, oxidative, metabolic, muscular, and cytokine stress seem to be responsible for the HSP response to exercise. Also, inducible nitric oxide synthase (iNOS) generates nitric oxide (NO) for prolonged period and causes pathophysiological effects. Furthermore, iNOS is involved in processes such as cell injury, wound repair, embryogenesis, tissue differentiation, and suppression of tumorigenesis. In conclusion, the inhibition of HSP 70 on caspase-3 and apoptosis is associated with its inhibition on iNOS that leads to less NO production.

Activity attenuates skeletal muscle fiber damage after ischemia and reperfusion

INTRODUCTION

In this investigation we aimed to determine whether: (1) physical activity protects rat skeletal muscle from ischemia/reperfusion (I/R) injury; and (2) continued activity after I/R improves the rate of healing.

METHODS

Rats were divided into sedentary or active (voluntary wheel running) groups. Active rats ran for 4 weeks before I/R or 4 weeks before plus 4 weeks after I/R.

RESULTS

Activity before I/R resulted in 73.2% less muscle damage (Evans blue dye inclusion). Sedentary and active rats had a similar decline in neural-evoked (∼ 99%) and directly stimulated (∼ 70%) in vivo muscle torque, and a similar reduction in junctophilin 1. Active rats produced 19% and 15% greater neural-evoked torque compared with sedentary rats at 14 and 28 days postinjury, respectively, although the rate of recovery appeared similar.

CONCLUSIONS

Activity protects against long-term muscle damage, but not short-term neural injury or excitation-contraction uncoupling. Continued activity neither accelerates nor hinders the rate of functional recovery.

Transfection efficiency along the regenerating soleus muscle of the rat

We investigated the efficiency of a single plasmid transfection along the longitudinal axis of the regenerating soleus of young rats. This also reflected transfection efficiency along the fibers because the soleus is a nearly fusiform muscle in young animals. The complete regeneration was induced by notexin and the transfection was made by intramuscular injection of enhanced green fluorescent protein- or Discosoma red-coding plasmids after 4 days. One week after transfection the number of transfected fibers was higher at the place of injection (i.e., in the muscle belly) and lower or absent at the ends of the muscle. The inspection of longitudinal sections and neuromuscular endplates indicated that one of the reasons of uneven transfection might be the shortness of transfected myotubes and the other reason might be the limit of diffusion of transgenic proteins from the expressing nuclei. As a result, the efficiency of transfection in the whole regenerating muscle was much lower than it could be estimated from the most successfully transfected part.

Muscle injury: review of experimental models

Skeletal muscle is the most abundant tissue in the human body. Its main characteristic is the capacity to regenerate after injury independent of the cause of injury through a process called inflammatory response. Mechanical injuries are the most common type of the skeletal muscle injuries and are classified into one of three areas strain, contusion, and laceration. First, this review aims to describe and compare the main experimental methods that replicate the mechanical muscle injuries. There are several ways to replicate each kind of mechanical injury; there are, however, specific characteristics that must be taken into account when choosing the most appropriate model for the experiment. Finally, this review discusses the context of mechanical injury considering types, variability of methods, and the ability to reproduce injury models.

Intra-Arterial MSC Transplantation Restores Functional Capacity After Skeletal Muscle Trauma

Introduction:

Skeletal muscle trauma leads to severe functional deficits, which cannot be addressed by current treatment options. Our group could show the efficacy of local transplantation of mesenchymal stroma cells (MSCs) for the treatment of injured muscles. While local application of MSCs has proven to be effective, we hypothesized that a selective intra-arterial transplantation would lead to a better distribution of the cells and so improved physiological recovery of muscle function.

Materials and Methodology:

18 female Sprague Dawley rats received an open crush trauma of the left soleus muscle. Autologous MSC were transduced using dsCOP-GFP and 2.5 x 10⁶cells were transplanted into the femoral artery of 9 animals one week after trauma. Control animals (n=9) received a saline injection. Cell tracking, analysis of tissue fibrosis and muscle force measurements were performed after 3 weeks.

Results:

Systemic MSC-therapy improved the muscle force significantly compared to control (fast twitch: 82.4%, tetany: 61.6%, p = 0.02). The histological analysis showed no differences in the quantity of fibrotic tissue. Histological examination revealed no cells in the traumatized muscle tissue 21 days after transplantation.

Conclusions:

The present study demonstrated an effect of systemically administered MSCs in the treatment of skeletal muscle injuries. For possible future therapeutic approaches a systemic application of MSCs seems to present an alternative to a local administration. Such systemic treatment would be preferable since it allows functional improvement and possible cellular concentration at injury sites that are not easily accessible

[Muscle injuries: diagnostics and treatments]

Muscle injuries are common in sports. They are usually caused by either acute (mostly eccentric mechanisms) or chronic overloading with a lack of muscle coordination. They present in clinical practice as bruises and muscle sprains. Due to the rigours of a modern society and the high economic cost of time off work, an effective treatment needs to be employed. The key to an optimised therapy rests in the appropriate timing between immobilisation and mobilisation. The interval to muscle repair might be shortened by certain adjuvant therapies. In doing so, it is important that no physiological phases of wound healing are overlooked. Muscle healing can be accelerated by externally induced higher metabolic turnover. Surgical therapy is sometimes necessary in selected cases and in serious injuries.

Long-term adaptation of human microneurovascular muscle flaps to the paralyzed face: An immunohistochemical study

The purpose of this study was to characterize microneurovascular (MNV) muscle transplants immunohistochemically up to 10 years after transfer. Histological data was related to long-term functional outcome. The study comprised 17 patients with a mean age of 41 years suffering from complete unilateral long-lasting facial paralysis. A two-stage procedure was performed between 1986 and 2001. The gracilis, latissimus dorsi, and serratus muscles were used in four, eight, and five patients, respectively. Eighteen biopsy samples were taken from MNV muscle grafts during secondary refinement procedures. In one patient, the tissue samples were collected at two different time points. Immunohistochemistry testing revealed muscle fiber type distribution (anti-myosin fast), proliferating satellite cells (Ki-67), and reinnervation (S-100). Muscle atrophy was assessed histomorphometrically. In a recent study, patient characteristics and the function of the flap were evaluated. Histological data were compared with clinical data and long-term functional outcomes of the patients. In biopsy samples taken 1-10 (mean 31 months) years after MNV muscle transfer, the mean muscle fiber diameter was 38 (range 14-70) microm, indicating a 40% decrease compared with control values. Muscle atrophy was not type-specific and the mean percentage of type II fibers was not altered. Individual variation was, however, considerable. Proliferative activity of satellite cells was seen in 60% of the samples but it tended to decline with an increase in follow-up time. All samples showed intramuscular reinnervation. In statistical analysis severe atrophy correlated with prolonged intraoperative ischemia (P=0.04). The good long-term functional outcome correlated with dominance of fast fibers in muscle grafts (P=0.03). Atrophy tended to be more pronounced in the serratus than in the other muscles (ns). In summary, despite dense muscle reinnervation, morphology of the muscle is not fully restored after muscle transfer. Ischemia time affects muscle morphology. Adaptation of the graft to fast-twitch muscle activity favors better mimic function. The proliferative activity of satellite cells declines with prolonged follow-up time.

The cut intramuscular nerve affects the recovery in the lacerated skeletal muscle

The recovery of lacerated skeletal muscles are said to be slow and incomplete. Often the intramuscular (IM-) nerve is concomitantly cut, but never repaired. We questioned whether the IM-nerve should also be reanastamosed before repairing the skeletal muscle. Before answering this, it was necessary to know if the cut IM nerve would have an effect on the recovery of the segment of muscle distal to the level of the laceration. This study investigates the recovery of lacerated muscles after repair, and compares a complete muscle laceration where the main IM-nerve was concomitantly cut and an incomplete muscle laceration where the IM-nerve was preserved intact. The medial gastrocnemius (MG) of the adult male New Zealand White rabbit was used, with the contralateral muscle as a sham control. The laceration was at the proximal quarter of the muscle, distal to the entry point of the nerve branch from the tibial nerve into the muscle belly. Twenty-eight weeks post-repair, the lacerated MG with the IM-nerve intact showed improved muscle wet weight, near normal morphology and contractile properties, and return of muscle fiber type mix and size. The repaired lacerated MG with their IM-nerve concomitantly cut demonstrated loss of muscle wet weight, obvious fibrosis, mononuclear proliferation with fatty infiltration, increase in type-1 fibers and muscle fiber atrophy in the distal portion. We postulate that it might be important to repair the intramuscular nerve branch by microanastomosis when repairing a vital skeletal muscle that is lacerated.

Muscle injuries: biology and treatment

Muscle injuries are one of the most common traumas occurring in sports. Despite their clinical importance, few clinical studies exist on the treatment of these traumas. Thus, the current treatment principles of muscle injuries have either been derived from experimental studies or been tested only empirically. Although nonoperative treatment results in good functional outcomes in the majority of athletes with muscle injuries, the consequences of failed treatment can be very dramatic, possibly postponing an athlete's return to sports for weeks or even months. Moreover, the recognition of some basic principles of skeletal muscle regeneration and healing processes can considerably help in both avoiding the imminent dangers and accelerating the return to competition. Accordingly, in this review, the authors have summarized the prevailing understanding on the biology of muscle regeneration. Furthermore, they have reviewed the existing data on the different treatment modalities (such as medication, therapeutic ultrasound, physical therapy) thought to influence the healing of injured skeletal muscle. In the end, they extend these findings to clinical practice in an attempt to propose an evidence-based approach for the diagnosis and optimal treatment of skeletal muscle injuries.

Restoration of myofiber continuity after transection injury in the rat soleus

In a shearing type of muscle injury, scar formation prevents restoration of myofiber continuity and the transected myofibers may become permanently divided into two separate myofibers. We have analysed whether the injured myofiber stumps can fuse and continuity of the transected fibers be re-established, if the stumps are surgically closely apposed immediately after injury. 55 rat soleus muscles were transected, after which the epimysium was carefully sutured and the leg was immobilised for seven days. The animals were sacrificed at 2, 5, 7, 10, 14 and 25 days after surgery. All muscles were analysed by light and electron microscopy as well as by immunohistochemistry. Mechanical strength was also measured at day 10 and 25. We observed that suturing reduced the extent of the intervening scar and accelerated healing. More importantly our results indicate that fusion of the stumps and thus restoration of myofiber continuity, is possible after myofiber transection injury.

Long-term morphometric and immunohistochemical findings in human free microvascular muscle flaps

Reinnervation, muscle regeneration, density of microvessels, and muscle-type specific atrophy were studied 3-4 years after surgery in surgically nonreinnervated free microvascular muscle flaps to 13 patients transplanted to the upper or lower extremities. Routine histology and immunohistochemistry for PGP 9.5 and S-100 (neuronal markers), Ki-67 (cell proliferation), myosin (muscle fiber types), and CD-31 (endothelium) were carried out, and results were analyzed morphometrically. Three to 4 years after surgery, severe atrophy of predominantly slow-type fibers was seen in 9 cases. In 4 cases, muscle-fiber diameter and fiber-type distribution were close to normal. Long intraoperative muscle ischemia and postoperative immobilization were associated with poor muscle bulk in flaps. The density of microvessels in flaps did not differ from control muscles. PGP 9.5 and S-100 immunopositive nerve fibers were detected in 7 patients. Reinnervation was associated with good muscle bulk. In 4 patients, activation of satellite cells was evident. The results suggest that in some cases, spontaneous reinnervation may occur in free muscle flaps, and that several years after microvascular free flap transfer, the muscle still attempts to regenerate.

Satellite cell proliferation, reinnervation, and revascularization in human free microvascular muscle flaps

BACKGROUND

Satellite cell proliferation, reinnervation, and revascularization were studied in human nonreinnervated free microvascular muscle flaps to characterize mechanisms of muscle regeneration after flap surgery.

MATERIALS AND METHODS

Patient biopsies (n = 19) were taken at operation and five timepoints up to 9 months after operation, and corresponding clinical data were obtained. Immunohistochemistry for Ki-67 was used to detect proliferating satellite cells, CD-31 to identify endothelial cells, and S-100 and PGP 9.5 proteins to detect reinnervation.

RESULTS

Two weeks after operation, the expression of PGP 9.5 and S-100 had virtually disappeared in all larger nerve fibers and half of smaller nerve fibers. By 6 months, however, a strong expression of PGP 9.5 and S-100 had reappeared in larger nerve fibers in three of four flaps, suggesting that reinnervation had taken place. The number of mitotic satellite cells already peaked at 2 weeks, indicating onset of muscle regeneration. The number of intramuscular capillaries first increased but later decreased to lower than original level. Flaps with more muscle volume showed more reinnervation and satellite cell mitotic activity. In cases of a delay occurring in reconstructive surgery, a low level of reinnervation was seen.

CONCLUSION

Three patients of four showed spontaneous muscle reinnervation in microvascular free flaps with satellite cell activation followed by restored morphology. Late reconstruction and obesity lead to poor reinnervation, placing emphasis on timing of surgery and patient selection.