Nascent muscle fiber appearance in overloaded chicken slow-tonic muscle

Assessment of black soldier fly (Hermetia illucens) larvae meal as a potential substitute for soybean meal on growth performance and flesh quality of grass carp Ctenopharyngodon idellus

A 90-day feeding trial was conducted to assess the effects of black soldier fly larvae meal (BSFLM) as a replacement for soybean meal (SM) on growth performance and flesh quality of grass carp. A total of 420 grass carp (299.93 ± 0.85 g) were randomly divided into 7 groups (triplicate) and fed 7 diets with SM substitution of 0% (SM, control), 15% (BSFLM15), 30% (BSFLM30), 45% (BSFLM45), 60% (BSFLM60), 75% (BSFLM75) and 100% (BSFLM100) by BSFLM. The growth performance of grass carp in the BSFLM75 and BSFLM100 groups were significantly lower compared to other groups (P < 0.05). The mid-gut villus height was the lowest in the BSFLM100 group (P < 0.05). Muscle nutritional value was improved due to increased DHA (docosahexaenoic acid), EPA (eicosapentaenoic acid), total HUFA (highly unsaturated fatty acids) and glycine levels, and reached the optimum in the BSFLM100 group (P < 0.05). According to the results of principal component analysis and weight analysis of muscle texture and body color, all the BSFLM diets except BSFLM15 could improve muscle texture and body color and reached the optimum level in the BSFLM100 group. Muscle drip loss and hypoxanthine content were the lowest and muscle antioxidant capacity was the highest in the BSFLM75 group, and water- and salt-soluble protein contents reached the optimum level in the BSFLM60 group (P < 0.05). Dietary BSFLM significantly reduced muscle fiber area and diameter, and increased muscle fiber density and the proportion of small fiber (diameter <20 μm) (P < 0.05). Additionally, sarcomere lengths in the BSFLM75 and BSFLM100 groups were significantly higher than that in the SM group (P < 0.05). The mRNA relative expression levels of MyoD, Myf5, MyHC and FGF6b were remarkably up-regulated at an appropriate dietary BSFLM level (P < 0.05). In conclusion, BSFLM could replace up to 60% SM without an adverse effect on growth performance and improve the flesh quality of grass carp. The optimum levels of dietary BSFLM were 71.0 and 69.1 g/kg diet based on the final body weight and feed conversion ratio. The flesh quality was optimal when dietary SM was completely replaced with BSFLM (227 g/kg diet).

Flexibility of equine bioenergetics and muscle plasticity in response to different types of training: An integrative approach, questioning existing paradigms

Equine bioenergetics have predominantly been studied focusing on glycogen and fatty acids. Combining omics with conventional techniques allows for an integrative approach to broadly explore and identify important biomolecules. Friesian horses were aquatrained (n = 5) or dry treadmill trained (n = 7) (8 weeks) and monitored for: evolution of muscle diameter in response to aquatraining and dry treadmill training, fiber type composition and fiber cross-sectional area of the M. pectoralis, M. vastus lateralis and M. semitendinosus and untargeted metabolomics of the M. pectoralis and M. vastus lateralis in response to dry treadmill training. Aquatraining was superior to dry treadmill training to increase muscle diameter in the hindquarters, with maximum effect after 4 weeks. After dry treadmill training, the M. pectoralis showed increased muscle diameter, more type I fibers, decreased fiber mean cross sectional area, and an upregulated oxidative metabolic profile: increased β-oxidation (key metabolites: decreased long chain fatty acids and increased long chain acylcarnitines), TCA activity (intermediates including succinyl-carnitine and 2-methylcitrate), amino acid metabolism (glutamine, aromatic amino acids, serine, urea cycle metabolites such as proline, arginine and ornithine) and xenobiotic metabolism (especially p-cresol glucuronide). The M. vastus lateralis expanded its fast twitch profile, with decreased muscle diameter, type I fibers and an upregulation of glycolytic and pentose phosphate pathway activity, and increased branched-chain and aromatic amino acid metabolism (cis-urocanate, carnosine, homocarnosine, tyrosine, tryptophan, p-cresol-glucuronide, serine, methionine, cysteine, proline and ornithine). Trained Friesians showed increased collagen and elastin turn-over. Results show that branched-chain amino acids, aromatic amino acids and microbiome-derived xenobiotics need further study in horses. They feed the TCA cycle at steps further downstream from acetyl CoA and most likely, they are oxidized in type IIA fibers, the predominant fiber type of the horse. These study results underline the importance of reviewing existing paradigms on equine bioenergetics.

Muscle Fiber Splitting Is a Physiological Response to Extreme Loading in Animals

Skeletal muscle fiber branching and splitting typically is associated with damage and regeneration and is considered pathological when observed during loading-induced hypertrophy. We hypothesize that fiber splitting is a nonpathological component of extreme loading and hypertrophy, which is primarily supported by evidence in animals, and propose that the mechanisms and consequences of fiber splitting deserve further exploration.

Mononuclear cells from dedifferentiation of mouse myotubes display remarkable regenerative capability

Certain lower organisms achieve organ regeneration by reverting differentiated cells into tissue-specific progenitors that re-enter embryonic programs. During muscle regeneration in the urodele amphibian, postmitotic multinucleated skeletal myofibers transform into mononucleated proliferating cells upon injury, and a transcription factor-msx1 plays a role in their reprograming. Whether this powerful regeneration strategy can be leveraged in mammals remains unknown, as it has not been demonstrated that the dedifferentiated progenitor cells arising from muscle cells overexpressing Msx1 are lineage-specific and possess the same potent regenerative capability as their amphibian counterparts. Here, we show that ectopic expression of Msx1 reprograms postmitotic, multinucleated, primary mouse myotubes to become proliferating mononuclear cells. These dedifferentiated cells reactivate genes expressed by embryonic muscle progenitor cells and generate only muscle tissue in vivo both in an ectopic location and inside existing muscle. More importantly, distinct from adult muscle satellite cells, these cells appear both to fuse with existing fibers and to regenerate myofibers in a robust and time-dependent manner. Upon transplantation into a degenerating muscle, these dedifferentiated cells generated a large number of myofibers that increased over time and replenished almost half of the cross-sectional area of the muscle in only 12 weeks. Our study demonstrates that mammals can harness a muscle regeneration strategy used by lower organisms when the same molecular pathway is activated.

Physiological factors influencing the growth of skeletal muscle

The growth of muscle can be regulated by developmental changes or by alterations in hormone levels or in the rate or amount of work demanded. The mechanisms and structures involved in growth processes can be studied by controlling these factors. The models used are chicken anterior latissimus dorsi (ALD) muscle under the influence of overloading and rabbit tibialis anterior (TA) muscle under the influence of chronic nerve stimulation. Both models involve changes in the isoform of myosin that is expressed. Methods of study include quantitative ultrastructural analysis, immunofluorescence and in situ mRNA hybridization. In overloaded chick ALD fibres polysomes are nonuniformly distributed between the myofibrils and in a peripheral annulus even though subcellular concentrations of the new isoform are not found. In normal rabbit muscle the highest concentration of myosin mRNA detected by in situ hybridization is found in the subsarcolemmal zone. In stimulated TA polysomes are found between myofibrils. It appears that the myosin mRNA accumulates at specific cell locations before translation; then diffusion of isomyosin and rapid exchange into myofibrils follows. Therefore, regulation of growth may be possible at the transcriptional, translational and assembly stages.

Distinct roles of neurofilament and tubulin gene expression in axonal growth

Tubulin and the neurofilament (NF) proteins, which are major constituents of the axonal cytoskeleton, play distinct roles in longitudinal and radial growth of axons. The predominantly longitudinal growth of axons in developing neurons correlates with relatively high levels of expression for a particular tubulin gene, encoding the class II beta tubulin isotype, whereas levels of gene expression for two other beta tubulin isotypes (classes I and IV) are comparable in developing and maturing neurons. Gene expression for the 68 kDa NF protein (NF68) is low during longitudinal growth. Conversely, relatively high levels of gene expression for NF68 and low levels for class II beta tubulin in maturing neurons correlate with the radial growth of axons. The developmental pattern of gene expression is recapitulated during axonal regeneration. Expression of the class II beta tubulin isotype correlates with the outgrowth (elongation) of regenerating sprouts.

Satellite cell activation in stretched skeletal muscle and the role of nitric oxide and hepatocyte growth factor

In the present study, we examined the roles of hepatocyte growth factor (HGF) and nitric oxide (NO) in the activation of satellite cells in passively stretched rat skeletal muscle. A hindlimb suspension model was developed in which the vastus, adductor, and gracilis muscles were subjected to stretch for 1 h. Satellite cells were activated by stretch determined on the basis of 5-bromo-2′-deoxyuridine (BrdU) incorporation in vivo. Extracts from stretched muscles stimulated BrdU incorporation in freshly isolated control rat satellite cells in a concentration-dependent manner. Extracts from stretched muscles contained the active form of HGF, and the satellite cell-activating activity could be neutralized by incubation with anti-HGF antibody. The involvement of NO was investigated by administering nitro-l-arginine methyl ester (l-NAME) or the inactive enantiomer NG-nitro-d-arginine methyl ester HCl (d-NAME) before stretch treatment. In vivo activation of satellite cells in stretched muscle was not inhibited by d-NAME but was inhibited by l-NAME. The activity of stretched muscle extract was abolished by l-NAME treatment but could be restored by the addition of HGF, indicating that the extract was not inhibitory. Finally, NO synthase activity in stretched and unstretched muscles was assayed in muscle extracts immediately after 2-h stretch treatment and was found to be elevated in stretched muscle but not in stretched muscle from l-NAME-treated rats. The results of these experiments demonstrate that stretching muscle liberates HGF in a NO-dependent manner, which can activate satellite cells.

Low-pH preparation of skeletal muscle satellite cells can be used to study activation in vitro

When skeletal muscle is stretched or injured, satellite cells are activated to enter the cell cycle, and this process could be mediated by hepatocyte growth factor (HGF) and nitric oxide (NO) as revealed by primary culture technique. In this system, which was originally developed by Allen et al. [Allen, R. E., Temm-Grove, C. J., Sheehan, S. M., & Rice, G. (1997). Skeletal muscle satellite cell cultures. Methods Cell Biol., 52, 155-176], however, some populations of satellite cells would receive activation signals during the cell isolation procedure; the high baseline level of activation diminishes the magnitude of the observed effect of HGF and NO. In this study, we modified the cell isolation procedure by lowering pH of muscle and isolation media from 7.2 (original) to 6.5. This modification was designed to block the activation signal generation, based on our previous observations that the satellite cell activation in response to mechanical stimulation only occurred between pH 7.1 and 7.5. Satellite cells prepared at low-pH showed a low baseline level of activation in bromodeoxyuridine incorporation and MyoD expression assays on control cultures, and demonstrated a large activation response to mechanical stretch, exogenous HGF and NO donor. Cell yield and myogenic purity were not affected by the modifications. The low-pH procedure could provide an improved satellite cell model for in vitro activation experiments.

The effect of resection on satellite cell activity in rabbit extraocular muscle

PURPOSE

A common treatment for motility disorders of the extraocular muscles (EOMs) is a resection procedure in which there is surgical shortening of the muscle. This procedure results in rotation of the globe toward the resected muscle, increased resting tension across the agonist-antagonist pair, and stretching of the elastic components of the muscles. In the rabbit, due to orbital constraints and limited rotation, resection results in more significant stretch of the surgically treated muscle than the antagonist. This surgical preparation allows for the examination of the effects of surgical shortening of one rectus muscle and passive stretch of its ipsilateral antagonist.

METHODS

The insertional 6 mm of the superior rectus muscles of adult rabbits were resected and reattached to the original insertion site. After 7 and 14 days, the animals were injected intraperitoneally with bromodeoxyuridine (BrdU) every 2 hours for 12 hours, followed by a 24-hour BrdU-free period. All superior and inferior rectus muscles from both globes were examined for BrdU incorporation, MyoD expression, neonatal and developmental myosin heavy chain (MyHC) isoform expression, and myofiber cross-sectional area alterations.

RESULTS

In the resected muscle and in the passively stretched antagonist muscle, there was a dramatic increase in the number of myofibers positive for neonatal MyHC and in the number of BrdU- and MyoD-positive satellite cells. The addition of BrdU-positive myonuclei increased from 1 per 1000 myofibers in cross sections of control muscles to 2 to 3 per 100 myofibers in the resected muscles. Single myofiber reconstructions showed that multiple BrdU-positive myonuclei were added to individual myofibers. Addition of new myonuclei occurred in random locations along the myofiber length of single fibers. There was no correlation between myofibers with BrdU-positive myonuclei and neonatal MyHC isoform expression.

CONCLUSIONS

Both active and passive stretch of the rectus muscles produced by strabismus surgery dramatically upregulated the processes of satellite cell activation, integration of new myonuclei into existing myofibers, and concomitant upregulation of immature myosin heavy chain isoforms. Understanding the effects of strabismus surgery on muscle cell biological reactions and myofiber remodeling may suggest new approaches for improving surgical outcomes.

Muscle hypertrophy models: applications for research on aging

Muscle hypertrophy is an adaptive response to overload that requires increasing gene transcription and synthesis of muscle-specific proteins resulting in increased protein accumulation. Progressive resistance training (P(RT)) is thought to be among the best means for achieving hypertrophy in humans. However, hypertrophy and functional adaptations to P(RT) in the muscles of humans are often difficult to evaluate because adaptations can take weeks, months, or even years before they become evident, and there is a large variability in response to P(RT) among humans. In contrast, various animal models have been developed which quickly result in extensive muscle hypertrophy. Several such models allow precise control of the loading parameters and records of muscle activation and performance throughout overload. Scientists using animal models of muscle hypertrophy should be familiar with the advantages and disadvantages of each and thereby choose the model that best addresses their research question. The purposes of this paper are to review animal models currently being used in basic research laboratories, discuss the hypertrophic and functional outcomes as well as applications of these models to aging, and highlight a few mechanisms involved in regulating hypertrophy as a result of applying these animal models to questions in research on aging.

Transcriptional profiling of tissue plasticity: role of shifts in gene expression and technical limitations

Reprogramming of gene expression has been recognized as a main instructive modality for the adjustments of tissues to various kinds of stress. The recent application of gene expression profiling has provided a powerful tool to elucidate the molecular pathways underlying such tissue remodeling. However, the biological interpretations of expression profiling results critically depend on normalization of transcript signals to mRNA standards before statistical evaluation. A hypothesis is proposed whereby the “fluctuating nature” of gene expression represents an inherent limitation of the test system used to quantify RNA levels. Misinterpretation of gene expression data occurs when RNA quantities are normalized to a subset of mRNAs that are subject to strong regulation. The contention of contradictory biological outcomes using different RNA-normalization schemes is demonstrated in two models of skeletal muscle plasticity with data from custom-designed microarrays and biochemical and ultrastructural evidence for correspondingly altered RNA content and nucleolar activity. The prevalence of these biological constraints is underlined by a literature survey in different models of tissue plasticity with emphasis on the unique malleability of skeletal muscle. Finally, recommendations on the optimal experimental layout are given to control biological and technical variability in microarray and RT-PCR studies. It is proposed to approach normalization of transcript signals by measuring total RNA and DNA content per sample weight and by correcting for concurrently estimated endogenous standards such as major ribosomal RNAs and spiked RNA and DNA species. This allows for later conversion to diverse tissue-relevant references and should improve the physiological interpretations of phenotypic plasticity.

Revisional Operations Improve Results of Neurovascular Free Muscle Transfer for Treatment of Facial Paralysis

BACKGROUND

Neurovascular free muscle transfer is currently the mainstay for smile reconstruction. However, problems such as excessive muscle bulk and dislocation of the transferred muscle attachment have been described. Furthermore, dynamic movements of the transferred muscle are sometimes too strong or too weak, resulting in facial asymmetry. In these cases, secondary revisional operations for the transferred muscle are required after neurovascular free muscle transfer. This report describes revisional operative procedures in detail and examines the extent of improvement of the smile by comparing preoperative and postoperative results.

METHODS

Of 468 patients in whom neurovascular free muscle transfer was performed between 1977 and 2000, a total of 183 received revisional operations for the transferred muscle. Operations included revision of muscle attachment in 129 patients, debulking of the cheek in 114 patients, and fascia graft in 21 patients.

RESULTS

Evaluation with the grading scale was performed in 117 of the 183 patients. Grading improved in 59 patients and worsened in seven patients. The remaining 51 patients displayed no change in grading. Differences between preoperative and post-operative grading were compared statistically, and revisional operations improved the grading score.

CONCLUSIONS

Revisional operations are effective and important as secondary operations after neurovascular free muscle transfer. However, care must be taken not to damage the neurovascular pedicles.

Nandrolone decanoate and load increase remodeling and strength in human supraspinatus bioartificial tendons

BACKGROUND

To date, no studies document the effect of anabolic steroids on rotator cuff tendons.

STUDY DESIGN

Controlled laboratory study.

HYPOTHESIS

Anabolic steroids enhance remodeling and improve the biomechanical properties of bioartificially engineered human supraspinatus tendons.

METHODS

Bioartificial tendons were treated with either nandrolone decanoate (nonload, steroid, n = 18), loading (load, nonsteroid, n = 18), or both (load, steroid, n = 18). A control group received no treatment (nonload, nonsteroid [NLNS], n = 18). Bioartificial tendons' remodeling was assessed by daily scanning, cytoskeletal organization by staining, matrix metalloproteinase-3 levels by ELISA assay, and biomechanical properties by load-to-failure testing.

RESULTS

The load, steroid group showed the greatest remodeling and the best organized actin cytoskeleton. Matrix metallo-proteinase-3 levels in the load, steroid group were greater than those of the nonload, nonsteroid group (P <.05). Ultimate stress and ultimate strain in the load, steroid group were greater than those of the nonload, nonsteroid and nonload, steroid groups (P <.05). The strain energy density in the load, steroid group was greater when compared to other groups (P <.05).

CONCLUSIONS

Nandrolone decanoate and load acted synergistically to increase matrix remodeling and biomechanical properties of bioartificial tendons.

CLINICAL RELEVANCE

Data suggest anabolic steroids may enhance production of bioartificial tendons and rotator cuff tendon healing in vitro. More research is necessary before such clinical use is recommended.

Ectopic expression of an embryonic skeletal myosin heavy chain in human fetal and Syrian hamster hearts

The mammalian heart is known to contain only two isoformic myosin heavy chain (MHC) genes, alpha and beta. A previously uncharacterized MHC gene was isolated in Syrian hamster hearts (McCully et al., JMol Biol 1991). We identified the novel MHC gene as a hamster embryonic skeletal MHC gene based on the developmental stage- and tissue-specific expression pattern: the restricted expression ofmRNA to striated muscles was highest in embryonic skeletal muscle and was developmentally down-regulated. We confirmed that the embryonic skeletal MHC gene exhibited higher expression in cardiomyopathic than in normal hamster hearts, and was up-regulated during the development of cardiomyopathy. The sporadic expression was highly localized in the endocardium. The present study identified that a very small number of undifferentiated myogenic cells existed in adult hamster endocardium. Moreover, using RT-PCR, a homologue of embryonic skeletal MHC mRNA was also expressed in human embryonic, but not adult ventricles. Our data provide a new insight into the regulatory mechanisms of MHCs in the cardiomyopathic hamster heart.

Mechanical stretch induces activation of skeletal muscle satellite cells in vitro

Cultured quiescent satellite cells were subjected to mechanical stretch in a FlexerCell System. In response to stretch, satellite cells entered the cell cycle earlier than if they were under control conditions. Only a brief period of stretch, as short as 2 h, was necessary to stimulate activation. Additionally, conditioned medium from stretched cells could activate unstretched satellite cells. The presence of HGF on c-met-positive myogenic cells was detected by immunofluorescence at 12 h in culture, and immunoblots demonstrated that HGF was released by stretched satellite cells into medium. Also, stretch activation could be abolished by the addition of anti-HGF antibodies to stretched cultures, and activity in conditioned medium from stretched cells could be neutralized by anti-HGF antibodies. In addition, stretch appeared to cause release of preexisting HGF from the extracellular matrix. These experiments suggest that HGF may be involved in linking mechanical perturbation of muscle to satellite cell activation.

Experimental induction of ring fibers in regenerating skeletal muscle

Light microscopy and electron microscopy were used to study the formation of ring fibers induced experimentally in regenerating muscle subjected to tenotomy-induced tension deficiency. Anterior tibial rat muscles were injured by intramuscular injection of mepivacain, tenotomized at varying stages of the regenerative process, and analyzed 30 days after sectioning the tendon. The combination of regeneration and tenotomy led to the appearance of ring fibers at different developmental stages. Ring fibers were not observed in regenerating control muscles and were scarce in tenotomized controls. Our results showed that the regenerative phase in which tension deficiency was established had a significant influence on the number of developing ring fibers; the number increased when tenotomy was performed during subsarcolemmic myofibrillogenesis in regenerating fibers. As a consequence, one might hypothesize that tension deficiency during muscle fiber repair plays a critical role in ring fiber formation.

Selected Contribution: Skeletal muscle focal adhesion kinase, paxillin, and serum response factor are loading dependent

This investigation examined the effect of mechanical loading state on focal adhesion kinase (FAK), paxillin, and serum response factor (SRF) in rat skeletal muscle. We found that FAK concentration and tyrosine phosphorylation, paxillin concentration, and SRF concentration are all lower in the lesser load-bearing fast-twitch plantaris and gastrocnemius muscles compared with the greater load-bearing slow-twitch soleus muscle. Of these three muscles, 7 days of mechanical unloading via tail suspension elicited a decrease in FAK tyrosine phosphorylation only in the soleus muscle and decreases in FAK and paxillin concentrations only in the plantaris and gastrocnemius muscles. Unloading decreased SRF concentration in all three muscles. Mechanical overloading (via bilateral gastrocnemius ablation) for 1 or 8 days increased FAK and paxillin concentrations in the soleus and plantaris muscles. Additionally, whereas FAK tyrosine phosphorylation and SRF concentration were increased by < or =1 day of overloading in the soleus muscle, these increases did not occur until somewhere between 1 and 8 days of overloading in the plantaris muscle. These data indicate that, in the skeletal muscles of rats, the focal adhesion complex proteins FAK and paxillin and the transcription factor SRF are generally modulated in association with the mechanical loading state of the muscle. However, the somewhat different patterns of adaptation of these proteins to altered loading in slow- vs. fast-twitch skeletal muscles indicate that the mechanisms and time course of adaptation may partly depend on the prior loading state of the muscle.

Rapid and reciprocal regulation of tenascin-C and tenascin-Y expression by loading of skeletal muscle

Tenascin-C and tenascin-Y are two structurally related extracellular matrix glycoproteins that in many tissues show a complementary expression pattern. Tenascin-C and the fibril-associated minor collagen XII are expressed in tissues bearing high tensile stress and are located in normal skeletal muscle, predominantly at the myotendinous junction that links muscle fibers to tendon. In contrast, tenascin-Y is strongly expressed in the endomysium surrounding single myofibers, and in the perimysial sheath around fiber bundles. We previously showed that tenascin-C and collagen XII expression in primary fibroblasts is regulated by changes in tensile stress. Here we have tested the hypothesis that the expression of tenascin-C, tenascin-Y and collagen XII in skeletal muscle connective tissue is differentially modulated by mechanical stress in vivo. Chicken anterior latissimus dorsi muscle (ALD) was mechanically stressed by applying a load to the left wing. Within 36 hours of loading, expression of tenascin-C protein was ectopically induced in the endomysium along the surface of single muscle fibers throughout the ALD, whereas tenascin-Y protein expression was barely affected. Expression of tenascin-C protein stayed elevated after 7 days of loading whereas tenascin-Y protein was reduced. Northern blot analysis revealed that tenascin-C mRNA was induced in ALD within 4 hours of loading while tenascin-Y mRNA was reduced within the same period. In situ hybridization indicated that tenascin-C mRNA induction after 4 hours of loading was uniform throughout the ALD muscle in endomysial fibroblasts. In contrast, the level of tenascin-Y mRNA expression in endomysium appeared reduced within 4 hours of loading. Tenascin-C mRNA and protein induction after 4–10 hours of loading did not correlate with signs of macrophage infiltration. Tenascin-C protein decreased again with removal of the load and nearly disappeared after 5 days. Furthermore, loading was also found to induce expression of collagen XII mRNA and protein, but to a markedly lower level, with slower kinetics and only partial reversibility. The results suggest that mechanical loading directly and reciprocally controls the expression of extracellular matrix proteins of the tenascin family in skeletal muscle.

Regulation of bovine satellite cell proliferation and differentiation by insulin and triiodothyronine

Satellite cells activity contributes to postnatal muscle growth. Herein, we have studied the respective influence of insulin and triiodothyronine (T3) on the proliferation and differentiation of primary bovine satellite cells isolated from Semitendinosus muscle of Montbéliard steers. Under basal conditions, satellite cells proliferated until the fifth day of culture, began to fuse into myotubes and expressed differentiation markers such as connectin, myogenin, and myosin heavy chain (MHC) isoforms. Insulin behaved as an effective mitogen. Moreover, it promoted extensive myotube formation and enhanced differentiation as shown by an increase in the accumulation of differentiation markers. Maximal differentiation occurred with insulin physiological range concentrations. A delay in the stimulation of differentiation was registered with a high dose that promoted maximal proliferation. Conversely, T3 decreased cell proliferation in a dose-dependent manner. In addition, fusion and biochemical differentiation (accumulation of connectin, MyoD1, myogenin, and myosin heavy chain isoforms) were also enhanced. Bovine satellite cells seemed to respond differentially to insulin and T3 for proliferation. Interestingly, both hormones displayed a myogenic influence. Our observations suggest that both hormones could influence bovine satellite cells in vivo and contribute to the regulation of postnatal muscle growth.

Effects of anabolic steroids on the muscle cells of strength-trained athletes

PURPOSE

Athletes who use anabolic steroids get larger and stronger muscles. How this is reflected at the level of the muscle fibers has not yet been established and was the topic of this investigation.

METHODS

Muscle biopsies were obtained from the trapezius muscles of high-level power lifters who have reported the use of anabolic steroids in high doses for several years and from high-level power lifters who have never used these drugs. Enzyme-immunohistochemical investigation was performed to assess muscle fiber types, fiber area, myonuclear number, frequency of satellite cells, and fibers expressing developmental protein isoforms.

RESULTS

The overall muscle fiber composition was the same in both groups. The mean area for each fiber type in the reported steroid users was larger than that in the nonsteroid users (P < 0.05). The number of myonuclei and the proportion of central nuclei were also significantly higher in the reported steroid users (P < 0.05). Likewise, the frequency of fibers expressing developmental protein isoforms was significantly higher in the reported steroid users group (P < 0.05).

CONCLUSION

Intake of anabolic steroids and strength-training induce an increase in muscle size by both hypertrophy and the formation of new muscle fibers. We propose that activation of satellite cells is a key process and is enhanced by the steroid use. The incorporation of the satellite cells into preexisting fibers to maintain a constant nuclear to cytoplasmic ratio seems to be a fundamental mechanism for muscle fiber growth. Although all the subjects in this study have the same level of performance, the possibility of genetic differences between the two groups cannot be completely excluded.

Slow tonic muscle fibers in the thyroarytenoid muscles of human vocal folds; a possible specialization for speech

Most of the sounds of human speech are produced by vibration of the vocal folds, yet the biomechanics and control of these vibrations are poorly understood. In this study the muscle within the vocal fold, the thyroarytenoid muscle (TA), was examined for the presence and distribution of slow tonic muscle fibers (STF), a rare muscle fiber type with unique contraction properties. Nine human TAs were frozen and serially sectioned in the frontal plane. The presence and distribution pattern of STF in each TA were examined by immunofluorescence microscopy using the monoclonal antibodies (mAb) ALD-19 and ALD-58 which react with the slow tonic myosin heavy chain (MyHC) isoform. In addition, TA muscle samples from adjacent frozen sections were also examined for slow tonic MyHC isoform by electrophoretic immunoblotting. STF were detected in all nine TAs and the presence of slow tonic MyHC isoform was confirmed in the immunoblots. The STF were distributed predominantly in the medial aspect of the TA, a distinct muscle compartment called the vocalis which is the vibrating part of the vocal fold. STF do not contract with a twitch like most muscle fibers, instead, their contractions are prolonged, stable, precisely controlled, and fatigue resistant. The human voice is characterized by a stable sound with a wide frequency spectrum that can be precisely modulated and the STF may contribute to this ability. At present, the evidence suggests that STF are not presented in the vocal folds of other mammals (including other primates), therefore STF may be a unique human specialization for speech.

Focal adhesion proteins FAK and paxillin increase in hypertrophied skeletal muscle

Components of signaling pathways for mechanotransduction during load-induced enlargement of skeletal muscle have not been completely defined. We hypothesized that loading of skeletal muscle would result in an adaptive increase in the expression of two focal adhesion complex (FAC)-related proteins, focal adhesion kinase (FAK) and paxillin, as well as increased FAK activity. FAK protein was immunolocalized to the sarcolemmal region of rooster anterior latissimus dorsi (ALD) myofibers in the middle of the ALD muscle. FAK (77 and 81%) and paxillin (206 and 202%) protein concentrations per unit of total protein in Western blots increased significantly after 1.5 and 7 days, but not after 13 days, of stretch-induced hypertrophy-hyperplasia of the ALD muscle. FAK autokinase activity in immunoprecipitates was increased after 1.5, 7, and 13 days in stretched ALD muscles. To determine whether increased FAK and paxillin protein concentrations are associated with hypertrophy and/or new fiber formation, two additional experiments were performed. First, during formation of primary chicken myotubes (a model of new fiber formation), FAK protein concentration (63%), FAK activity (157%), and paxillin protein concentration (97%) increased compared with myoblasts. Second, FAK (112% and 611%) and paxillin (87% and 431%) protein concentrations per unit of total protein in the soleus muscle increased at 1 and 8 days after surgical ablation of the synergistic gastrocnemius muscle (a model of hypertrophy without hyperplasia). Thus increases in components of the FAC occur in hypertrophying muscle of animals and in newly formed muscle fibers in culture. Furthermore, increased FAK activity suggests a possible convergence of signaling at the FAC in load-induced growth of skeletal muscle.

Prolonged passive stretch of rat soleus muscle provokes an increase in the mRNA levels of the muscle regulatory factors distributed along the entire length of the fibers

The mRNA levels of the adult and the neonatal sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPases (SERCA1a and SERCA1b, respectively) and those of the muscle regulatory factors (MRFs: myoD, myf-5, myogenin, MRF4) have been assessed by RT PCR in rat soleus muscles immobilized for 3 days in an extended position (passive stretch). The transcript level of the fast type SERCA1a Ca(2+)-transport ATPase decreased to half of its normal value, whereas that of neonatal SERCA1b isoform increased 5-fold above control in stretched muscles. Immunostaining of muscle cross sections showed that the fraction of fibers expressing the SERCA1a protein was decreased evenly along the length of the stretched muscles indicating that a transformation occurred of fast fibers to slow ones. The mRNA levels of MRFs were elevated 3- to 6-fold above the normal level and were distributed evenly along the length of the stretched muscles. However in the controls these transcripts were more abundant at both ends of the muscle. The stretch increased the level of myoD and immunocytochemistry showed the expression of myoD protein in a number of nuclei of the stretched muscles whereas it was practically undetectable by this method in the control muscles. Western blotting did not indicate a significant stretch-induced increase in the level of the myogenin protein, in spite of the fact that immunocytochemistry tended to show more myogenin-positive nuclei in stretched muscles as compared to the controls. These data indicate that after 3 days of passive stretch the central and the terminal parts of the soleus muscle adapt similarly by increasing the levels of the MRFs, by decreasing the overall levels of the fast SERCA1-type of ATPase and by partially re-establishing a neonatal mode of alternative SERCA1 transcript splicing resulting in an increased SERCA1b/1a ratio.

Functional properties of myosin isoforms in avian muscle

Sarcomeric myosin is the major skeletal muscle protein and is encoded by a large and complex multigene family whose members are differentially expressed in developing and adult muscle cells. The structure and function of sarcomeric myosins have been extensively analyzed and many myosin genes have now been cloned and sequenced. This manuscript reviews the broad spectrum of myosin research with emphasis on studies in avian systems and discusses how advances in myosin isoform analysis have contributed to muscle and meat science.

Tensile force in limb lengthening: histogenesis or only mechanical elongation

Although bone response can be evaluated by radiography, there have been no reports in human confirming formation of new soft tissue in limb lengthening. This study evaluated the tensile force between pin clamps in 14 lower limb lengthenings. Legs were lengthened 0.5 mm every 12 hours and the tensile was measured continuously. The tensile force increased simultaneously with each lengthening and decreased gradually. However, the reduction rate of tensile force during the nighttime (120+/-22%) was significantly higher than that during the daytime (72+/-10%). This differed from the stress relaxation phenomenon shown by viscoelastic material and suggested the presence of other phenomena such as histogenesis.

Expression of fibroblast growth factor family during postnatal skeletal muscle hypertrophy

The potential role of the fibroblast growth factor (FGF) family during stretch-induced postnatal skeletal muscle hypertrophy was analyzed by using an avian wing-weighting model. After 2 or 11 days of weighted stretch, anterior latissimus dorsi (ALD) muscles were, on average, 34 (P < 0.01) and 85% (P < 0.01) larger, respectively, than unweighted ALD control muscles. By using quantitative RT-PCR, FGF-1 mRNA expression was found to be significantly decreased in ALD muscles stretched for 2 or 11 days. In contrast, FGF-4 and FGF-10 mRNA expression was significantly increased 2 days after initiation of stretch. FGF-2, FGF-10, fibroblast growth factor receptor 1, and FREK mRNA expression was significantly increased at 11 days poststretch. Increases in FGF-2 and FGF-4 protein could be detected throughout the myofiber periphery after 11 days of stretch. On a cellular level, FGF-2 and FGF-4 proteins were differentially localized. This differential expression pattern and protein localization of the FGF family in response to stretch-induced hypertrophy suggest distinct roles for individual FGFs during the postnatal hypertrophy process.

Myotrophic effects of muscle extracts obtained at different intervals after denervation

A study was made of the myotrophic effects of denervated muscle extracts on normal Wistar rat soleus muscle. Extracts obtained 1 h, 2, 4 and 7 days after sectioning of the sciatic nerve were administered intraperitoneally over five consecutive days. Soleus muscles were routinely processed for morphological and morphometrical analysis using light microscopic techniques. Quantitative differences were observed in the effects of different extracts on total muscle area, fibre cross-sectional area, mean minimum diameter and number of fibres/ area. The greatest myotrophic response was elicited by extracts obtained at 2 and 4 days; differences with respect to controls and extracts obtained at 1 day were significant (P < 0.05) for all parameters studied. Statistically significant differences (P < 0.05) were also recorded for fibre cross-sectional area and mean minimum diameter between the 2- and 4-day groups and the 7-day group. It may thus be concluded that the time elapsing between denervation and extract obtention influences the effect of the extract on normal rat muscle.

Comparative analysis of satellite cell properties in heavy- and lightweight strains of turkey

The growth of muscle during postnatal development results partly from the proliferation of satellite cells and their fusion with muscle fibres. We analysed the properties of satellite cells in a heavyweight (HW) turkey strain characterized by high body weight and a fast growth rate, and in a lightweight farm strain (LW) characterized by low body weight and a slow growth rate. Satellite cell activation was then examined in stretched-overloaded anterior latissimus dorsi (ALD) muscle by weighting one wing in young turkeys from both strains. As early as day 1 of stretching for HW and day 2 for LW, small embryonic-like fibres expressing ventricular cardiac myosin heavy chain (MHC) isoform were observed. Following four days of stretching, the number of nascent fibres had increased in both strains but was significantly greater in HW than LW ALD muscle. The proliferation and differentiation capacities of satellite cells from HW and LW strains were investigated in culture. As judged by in vitro measurements of 3H-thymidine incorporation and DNA content, satellite cells of HW turkey exhibited a greater proliferative capability than those of LW turkey. No differences in the temporal appearance of muscle markers (desmin, MHC isoforms) were noted in vitro between the two strains. These data confirm our in vivo observations indicating that selection based on growth rate does not modify muscle fibre maturation. Our in vivo and in vitro observations suggest that variations in the postnatal muscle growth pattern between HW and LW strains may be related to a difference in the capacity of their satellite cells to proliferate.

Morphological and biochemical evidence of muscle hyperplasia following weight-lifting exercise in rats

We used a rat model of weight lifting to examine the serial biochemical and morphological changes following muscle fiber hyperplasia during 14 days of exercise. [3H]thymidine and [14C]leucine labeling were used to determine the serial changes in cellular mitotic activity and the level of amino acid uptake and myosin synthesis. Morphological changes were assessed with light and transmission electron microscopy, whereas proliferation of cells was evaluated immunohistochemically with 5-bromo-2'-deoxyuridine (BrdU). The intensity of the exercise and degree of muscle damage were monitored by serum creatine kinase (CK) activity. Damaged fibers were sparsely distributed, and a significant CK leakage was observed 30-60 min after exercise. Anti-BrdU-positive cells were observed in damaged fibers and at the periphery of undamaged fibers. Changes typical of muscle regeneration were observed; however, the formation of new fibers in the interstitial space was also evident. The mitotic activity also changed and reflected the appearance of anti-BrdU-positive cells and activated satellite cells. Amino acid uptake increased during the first week of exercise, probably reflecting muscle hypertrophy and synthesis of other noncontractile related proteins. The uptake also increased during the second week, probably due to hyperplasia, a finding also supported by electron microscopy. Our results suggest that one bout of weight-lifting exercise in untrained rats induced muscle hyperplasia following regeneration. The process of muscle hyperplasia was activated by muscle fiber damage in our model.

Effect of radiation on satellite cell activity and protein expression in overloaded mammalian skeletal muscle

BACKGROUND

To gain insight into the role of satellite cells in skeletal muscle hypertrophy, the effect of radiation on small fiber formation, embryonic myosin heavy chain (embryonic MHC) production, and insulin-like growth factor I (IGF-I) production in overloaded adult rat soleus muscle was examined.

METHODS

Adult rat soleus muscle was overloaded by ablation of the synergistic gastrocnemius, plantaris, and flexor digitorum profundus muscles of the right hindlimb. Half of the rats were subjected to gamma irradiation of the right hindlimb prior to ablation in an attempt to prevent satellite cell proliferation.

RESULTS

Wet weight of the non-irradiated overloaded soleus muscle increased almost 40% compared to contralateral control muscle following 4 weeks of overload. Small fibers, which were rare in control muscle, accounted for 6.76 +/- 5.08% to 12.74 +/- 7.76% of the total fiber number of the non-irradiated soleus following 1 to 4 weeks of overload. Although usually absent in control muscle, IGF-I or embryonic MHC was immunolocalized in a small percentage (< 11%) of the mature fibers in the non-irradiated overloaded soleus. Irradiation prevented compensatory hypertrophy and nearly abolished small fiber formation in the overloaded soleus. However, irradiation did not diminish the percentage of mature fibers producing immunocytochemically detectable levels of embryonic MHC or IGF-I.

CONCLUSIONS

Irradiation may prevent hypertrophy by impairing activation, proliferation, and/or differentiation of satellite cells. Small fibers in overloaded muscle appear to be new fibers arising from satellite cells. IGF-I may have a role in muscle hypertrophy involving satellite cell activation, or perhaps a more direct role that requires additional factors.

Mechanical overload and skeletal muscle fiber hyperplasia: a meta-analysis

With use of the meta-analytic approach, the purpose of this study was to examine the effects of mechanical overload on skeletal muscle fiber number in animals. A total of 17 studies yielding 37 data points and 360 subjects met the initial criteria: 1) "basic" research studies published in journals, 2) animals (no humans) as subjects, 3) control group included, 4) some type of mechanical overload (stretch, exercise, or compensatory hypertrophy) used to induce changes in muscle fiber number, and 5) sufficient data to accurately calculate percent changes in muscle fiber number. Across all designs and categories, statistically significant increases were found for muscle fiber number [15.00 +/- 19.60% (SD), 95% confidence interval = 8.65-21.53], muscle fiber area (31.60 +/- 44.30%, 95% confidence interval = 16.83-46.37), and muscle mass (90.50 +/- 86.50%, 95% confidence interval = 61.59-119.34). When partitioned according to the fiber-counting technique, larger increases in muscle fiber number were found by using the histological vs. nitric acid digestion method (histological = 20.70%, nitric acid digestion = 11.10%; P = 0.14). Increases in fiber number partitioned according to species were greatest among those groups that used an avian vs. mammalian model (avian = 20.95%, mammalian = 7.97%; P = 0.07). Stretch overload yielded larger increases in muscle fiber number than did exercise and compensatory hypertrophy (stretch = 20.95%, exercise = 11.59%, compensatory hypertrophy = 5.44%; P = 0.06). No significant differences between changes in fiber number were found when data were partitioned according to type of control (intra-animal = 15.20%, between animal = 13.90%; P = 0.82) or fiber arrangement of muscle (parallel = 15.80%, pennate = 11.60%; P = 0.61). The results of this study suggest that in several animal species certain forms of mechanical overload increase muscle fiber number.

Inherited muscular disorder in mutant Japanese quail (Coturnix coturnix japonica): an immunohistochemical study

Cryostat sections of myofibres from the Musculus pectoralis thoracicus of a newly established mutant strain (LWC) of Japanese quail with a myotonic dystrophy-like myopathy were labelled with antibody against myosin heavy chain (MHC) isoforms and neural cell adhesion molecule (N-CAM). The characteristic lesions found in sections of muscle of LWC quail stained with haematoxylin and eosin were type 2B fibre atrophy, sarcoplasmic masses, and ring fibres. Immunohistochemical examination failed to distinguish type 2A and 2B fibres in the LWC quail. Antibody to adult fast MHC, which reacted only with type 2A fibres in normal quail, reacted in LWC quail with type 2B fibres, and to a limited degree with type 2A fibres. Sarcoplasmic masses reacted with both fast and slow MHC antibodies. Some masses also reacted with NCAM antibody, but apparently independently of similar reactions in fibres. These findings suggest that the changes observed in the myofibres of the LWC quail were not neurogenic but represented defects in both the plasma membrane and intermediate filaments.

Adaptations of ligamentum teres in ischemic necrosis of human femoral head

Little is known about the biomechanical properties of human ligamentum teres. To more fully understand the ligamentum teres, its dimensions and mechanical properties were measured in 22 cases of acute fracture of the femoral neck and 21 cases of ischemic necrosis of the femoral head. The specimens first were preconditioned and then loaded to failure with a testing machine at a fast strain rate of 100% s(-1). The ischemic necrosis group had a significantly larger volume (3.09 +/- 1.81 ml versus 1.30 +/- 0.62 ml) and cross section area (65.3 +/- 59.1 mm2 versus 30.6 +/- 27.2 mm2) than did the acute fracture group. The former also had a significantly greater ultimate load (234 +/- 168 N versus 130 +/- 111 N) and strain energy to failure (1.22 +/- 1.04 J versus 0.41 +/- 0.39 J), but a significantly smaller linear modulus (4.72 +/- 3.31 MPa versus 8.69 +/- 7.97 MPa) than did the latter. Histologic studies showed differences in the amount of organized collagen and components of subsynovial tissue between the 2 groups. Mechanical and morphologic adaptations of the ligamentum teres in a group of ischemic femoral heads are described, and a possible biomechanical role is suggested for the ligamentum teres in the hip joint in conjunction with the ischemic necrosis of the femoral head.

Stretch overload-induced satellite cell activation in slow tonic muscle from adult and aged Japanese quail

Stretch overload-induced activation of satellite cells in the anterior latissimus dorsi (ALD) muscle was examined in full-grown adult (12 wk old) and aged (90 wk old) Japanese quail. 5'-Bromo-2'-deoxyuridine (BrdU) constant-release pellets (0.22 mg BrdU.g body wt-1.day-1) were implanted subcutaneously before weighting the left wing of each bird. Nuclei that incorporated BrdU were localized by immunohistochemistry after 1 or 2 wk of stretch overload. Total fiber number was quantified by counting all fibers in a histological cross section from the midbelly of the ALD. Aging reduced increases in ALD mass and fiber number during 2 wk of stretch overload. Fiber proliferation in the ALD of aged birds also demonstrated an altered time course. The percentage of BrdU-positive nuclei associated with muscle fibers and the percentage of fibers associated with BrdU-positive nuclei did not differ between age groups. The altered time course of new fiber formation in the ALD of aged birds during 2 wk of stretch overload does not appear to be related to the capability to activate satellite cells.

Myosin heavy chain expression within the tapered ends of skeletal muscle fibers

BACKGROUND

The pectoralis muscle of the chicken contains fast-twitch glycolytic fibers, which during development undergo a transformation in their myosin heavy chain (MyHC) content from embryonic to a neonatal to an adult isoform (Bandman et al., 1990). Little, however, is known of MyHC expression within the ends of these or other muscle fibers. Here we test the hypothesis that the tapered ends of mature skeletal muscle fibers contain a less mature MyHC isoform than that typically found throughout their lengths.

METHODS

We apply an ammoniacal silver histological stain for endomysium and monoclonal antibodies against neonatal and adult MyHCs of chicken pectoralis to transverse serial sections of pectoralis from five mature chickens. The "lesser fiber diameters" of populations of fibers from each bird are also measured.

RESULTS

Most (approximately 81.8%) of the small (< 12 microns) and none of the larger (> 20 microns) diameter fibers contain the neonatal MyHC. Following these smaller fibers through serial sections, we show that they are the tapered ends of the larger fibers. Whereas neonatal MyHC is restricted to the tapered fiber ends, adult MyHC is present throughout the entire lengths of all fibers. We also demonstrate acetylcholinesterase (AChE) activity at some of these fiber ends.

CONCLUSIONS

We postulate that longitudinal growth of myofibrils in adult muscle is characterized by the sequential expression of MyHC isoforms similar to that observed in rapidly growing muscle and that the presence of the neurotransmitter hydrolase AChE at the tapered fiber ends may be related to the retention of neonatal MyHC.

MRF4, Myf-5, and myogenin mRNAs in the adaptive responses of mature rat muscle

We studied the possible role of specific muscle regulatory factors (MRF) in the adaptive response to changes in contractile activity in mature skeletal muscle. The tibialis anterior muscle of anesthetized female rats was subjected to low-frequency stimulation, static stretch, or a combination of both. Message levels of MRF were observed after 2 h of activity, and the subsequent 20-h recovery period by slot blot and in situ hybridizations for MRF4, Myf-5, and myogenin. A combination of stimulation and stretch for 2 h increased MRF4 (11.6 +/- 5.3-fold) and Myf-5 (6.6 +/- 1.4-fold). In situ hybridization showed abundance in some regions of the muscle with positive staining near peripheral nuclei of both large and small fibers. Message levels remained high for 30 min and declined to near control levels by 20 h of recovery. Myogenin mRNA levels were unaffected by any manipulations. Neither stretch alone nor 10 Hz of electrical stimulation alone induced a significant increase in MRF. We conclude that myonuclei, and possibly activated myoblasts, increase expression of Myf-5 and MRF4 after a combination of both stimulation and stretch for 2 h.

Adaptation in myosin expression of avian skeletal muscle after weighting and unweighting

Stretch-induced hypertrophy of the quail anterior latissimus dorsi is associated with decreased slow myosin 1 and increased slow myosin 2 expression and a small increase in expression of fast myosins. Because reduced neural activity has also been shown to accelerate expression from slow myosin 1 to slow myosin 2, we tested whether the increased expression of slow myosin 2 would be maintained when stretch was removed during a time when muscle activity would not be expected to increase. Quail anterior latissimus dorsi muscles were examined after 0-30 days of stretch overload and after 30 or 60 days of unweighting following 30 days of stretch. As expected, slow myosin 2 expression increased and slow myosin 1 expression decreased after 14-30 days of stretch. Novel findings were that slow myosin 1 and slow myosin 2 returned to control levels after unweighting. Furthermore, the expression of developmental and fast myosin heavy chains were evident by day 7, and maintained throughout wing unweighting. These data are consistent with the hypothesis that alterations in fast and developmental myosin expression result from formation and subsequent maintenance of new fibres during hypertrophy and regression. The relative amount and expression of myosin appears dependent upon mechanical stretch in the anterior latissimus dorsi muscle.

New fiber formation in rat soleus muscle following administration of denervated muscle extract

A study was made of Wistar rat soleus muscle following intraperitoneal administration of denervated muscle extract over 1 and 2 days. Light microscopy revealed the appearance on fiber surfaces of basophilic satellite structures whose histochemical behaviour differed from that of the parent fiber. Small fibers showing regenerative characteristics were also detected, mainly in the extrafascicular spaces. At ultrastructural examination, activated satellite cells were visible, and there was evidence of splitting in subsarcolemmal regions of apparently hypertrophic muscle fibers. Interstitial cells were occasionally observed, containing structures like myofilaments. The hypothesis is advanced that denervated muscle extract contains substances able to stimulate new fiber formation in adult skeletal muscle.

The pathology of the lower leg muscles in pure forefoot pes cavus

Enlargement of the peroneus longus muscle is a common occurrence in patients with forefoot pes cavus, and may contribute to the cavus deformity. The present study compares the morphology of up to five lower leg muscles from 17 patients with forefoot pes cavus with those of normal muscles. Eight cases had an identifiable neurogenic cause for the cavus. In four cases of hereditary motor-sensory neuropathy, the tibialis anterior showed more severe damage than the peroneus longus. In two cases of cerebral palsy, fibre atrophy and increased oxidative enzyme activity were observed. In nine clinically idiopathic cases, the histological appearances ranged from normal to generalised fibre atrophy or hypertrophy in individual muscles. There was a trend for the mean fibre area to be greater in peroneus longus than in tibialis anterior in six of the idiopathic group of patients. The muscle cross-sectional area on magnetic resonance imaging was correlated closely with the mean fibre area measured on tissue sections. In idiopathic forefoot pes cavus, fibre hypertrophy in peroneus longus (relative to tibialis anterior) may contribute to the cavus deformity. Muscle fibre hyperplasia may contribute to the peroneal muscle enlargement in Friedreich's ataxia. In none of the cases was peroneus longus enlargement due to fat or fibrous tissue replacement.

Skeletal muscle satellite cells

Evidence now suggests that satellite cells constitute a class of myogenic cells that differ distinctly from other embryonic myoblasts. Satellite cells arise from somites and first appear as a distinct myoblast type well before birth. Satellite cells from different muscles cannot be functionally distinguished from one another and are able to provide nuclei to all fibers without regard to phenotype. Thus, it is difficult to ascribe any significant function to establishing or stabilizing fiber type, even during regeneration. Within a muscle, satellite cells exhibit marked heterogeneity with respect to their proliferative behavior. The satellite cell population on a fiber can be partitioned into those that function as stem cells and those which are readily available for fusion. Recent studies have shown that the cells are not simply spindle shaped, but are very diverse in their morphology and have multiple branches emanating from the poles of the cells. This finding is consistent with other studies indicating that the cells have the capacity for extensive migration within, and perhaps between, muscles. Complexity of cell shape usually reflects increased cytoplasmic volume and organelles including a well developed Golgi, and is usually associated with growing postnatal muscle or muscles undergoing some form of induced adaptive change or repair. The appearance of activated satellite cells suggests some function of the cells in the adaptive process through elaboration and secretion of a product. Significant advances have been made in determining the potential secretion products that satellite cells make. The manner in which satellite cell proliferative and fusion behavior is controlled has also been studied. There seems to be little doubt that cellcell coupling is not how satellite cells and myofibers communicate. Rather satellite cell regulation is through a number of potential growth factors that arise from a number of sources. Critical to the understanding of this form of control is to determine which of the many growth factors that can alter satellite cell behavior in vitro are at work in vivo. Little work has been done to determine what controls are at work after a regeneration response has been initiated. It seems likely that, after injury, growth factors are liberated through proteolytic activity and initiate an activation process whereby cells enter into a proliferative phase. After myofibers are formed, it also seems likely that satellite cell behavior is regulated through diffusible factors arising from the fibers rather than continuous control by circulating factors.(ABSTRACT TRUNCATED AT 400 WORDS)

Role of satellite cells in altering myosin expression during avian skeletal muscle hypertrophy

This study examined whether satellite cells express an embryonic isoform of myosin upon fusion with hypertrophying muscle fibers. Anterior latissimus dorsi (ALD) muscle hypertrophy was induced in adult chickens by weighting one wing. One and 7 days of wing-weighting produced significant increases in ALD muscle wet weight and in the number of mature fibers expressing ventricular-like embryonic (V-EMB) myosin. V-EMB myosin expression could be an event during regeneration of fibers injured by overload or part of the hypertrophy process itself. Although there was an increase in both the number of damaged fibers and the number of mature fibers expressing embryonic myosin after wing-weighting, results from this study suggest that these two events were not necessarily related. The apparent health of fibers expressing V-EMB myosin and the lack of correlation between the numbers of damaged and V-EMB myosin positive fibers (r = 0.20) suggest that embryonic myosin expression in mature fibers was likely a feature of the hypertrophy process itself. The appearance of V-EMB myosin in mature fibers 1 day after wing-weighting suggests that the change in myosin expression did not involve satellite cells since 24 hr is too short a time to permit more than limited satellite cell fusion. The relationship between satellite cells and embryonic myosin expression was examined more closely by labeling dividing satellite cells and their progeny with 5-bromo-2-deoxyuridine, and then colocalizing labeled myofiber nuclei and embryonic myosin in consecutive transverse sections of hypertrophied ALD muscle. One week of wing-weighting resulted in marked increases in myofiber nuclear labeling index and myofiber nuclear density compared to contralateral control. V-EMB myosin was not expressed uniformly throughout individual fibers, but rather in discrete regions of varying length. Many V-EMB myosin positive regions had a higher labeled nuclear density than V-EMB myosin negative regions indicating that V-EMB myosin expression was associated with an accumulation of satellite cell progeny in a restricted area. However, it was also clear that satellite cell progeny were not the sole source of V-EMB myosin since labeled nuclei were completely absent from 41% of the V-EMB positive regions. Furthermore, the presence of new nuclei did not result in obligatory expression of embryonic myosin because many V-EMB negative regions had a high labeled nuclear density. Thus, recently incorporated nuclei arising by satellite cell division are implicated as one, but not the sole source of embryonic myosin in hypertrophying muscle.(ABSTRACT TRUNCATED AT 400 WORDS)

Stretch induces non-uniform isomyosin expression in the quail anterior latissimus dorsi muscle

Stretch-overload has been shown to elevate muscle mass in the avian anterior latissimus dorsi (ALD) by increasing both fiber size and fiber number; however, it is not known if these adaptations result in uniform regulation of myosin isoform expression along the length of the fibers in this slow tonic muscle. When a weight was added to the left wing of 20 adult quail for 30 days, ALD mass was increased by 161%. ALDs were divided into proximal, middle, and distal portions. Tissue cross-sections from each muscle portion were reacted against monoclonal antibodies for either fast (FM) or slow myosin (SM), or processed for identification of native myosin isoforms. The number of fibers expressing FM increased by 190% in the distal region after stretch; however, most of these were small fibers. Electrophoretic analyses of stretched muscles demonstrated an upregulation of SM2 in all regions of the ALD. SM1 was only down-regulated in the proximal region of the ALD. FM isoforms had greater increases in the proximal region than other regions of the overloaded ALD. These data indicate that stretch-induced hypertrophy induces a non-uniform increase in fast myosin isoforms and SM1 along the length of the fibers of the ALD.

Plasticity of human satellite cells

Satellite cells were isolated from human quadriceps and masseter muscles and the phenotype of these cells examined in vitro. The expression of the different isoforms of the myosin heavy chains (embryonic, fetal, fast and slow) and light chain isoforms was used to assay myotube diversification. We found that fused cultures of human satellite cells express adult fast and slow MHCs in addition to the embryonic and fetal isoforms. Only the four fast light chains (MLC1emb, MLC1F, MLC2F and MLC3F) were synthesized. No slow MLCs were ever detected in these cultures. In order to determine if the human satellite cells were committed to distinct fast and slow myogenic lineages, a clonal analysis was carried out on both cell populations. All myogenic clones expressed fast and slow MHCs, suggesting that there is no evidence for different fast and slow satellite cell lineages in human skeletal muscle.

A myogenic regulatory gene, qmf1, is expressed by adult myonuclei after injury

Myogenic regulatory factors (MRFs) induce differentiation in developing muscle. We examined the role of MRFs in the repair of adult muscle using a model of stretch-induced injury in 5-wk-old chickens. The anterior latissimus dorsi muscle was stretched by loading the wing with 10% of body weight, while the contralateral muscle served as a control. At various intervals (0.5-72 h), chickens were killed by CO2 asphyxiation and the muscles were frozen. Slot hybridizations showed that the onset of high qmf1 expression occurred as early as 0.5 h, which was before regenerative processes involving satellite cell proliferation were observed. Maximal qmf1 expression varied among animals from 3 to 16 h and returned to control levels by 72 h. Within a muscle, in situ hybridization showed that maximal qmf1 expression varied spatially with > 60% of the nuclei within active fascicles being positive. We interpret this high percentage to mean that many of the nuclei of preexisting muscle fibers must be expressing qmf1. The expression of the protooncogene c-myc (presumably by proliferating cells such as satellite cells, fibroblasts, and capillary epithelial cells) and the MRF qmf1 (by myoblasts and adult muscle nuclei) are among the early molecular responses of injured muscle. We conclude that myogenic regulatory factors are not permanently repressed after embryonic development and that derepression plays a role in the repair of terminally differentiated myofibers.