Muscle Fiber Satellite Cell Research Articles

Muscle fibre satellite cells are located at a greater distance from capillaries in patients with COPD compared with healthy controls.

COPD is a disease characterised by skeletal muscle dysfunction. A spatial relationship exists between satellite cells and muscle fibre capillaries, which has been suggested to be of major importance for satellite cell function. In the present study we compared the spatial relationship between satellite cells and capillaries in patients with COPD and age-matched healthy older adults. Muscle biopsies were obtained from the vastus lateralis of n=18 patients with COPD (8 female, 10 male; age 66±5 years, mild-to-severe airflow obstruction) and n=18 age-, sex- and body mass index-matched healthy control adults (8 female, 10 male; age 68±5 years). Immunohistochemistry was used to assess type I/II muscle fibre size, distribution, myonuclear content, satellite cell number and fibre capillarisation. In addition, type I/II muscle fibre satellite cell distance to its nearest capillary was assessed. The percentage of type II muscle fibres was significantly greater in patients with COPD (62±10%) compared with controls (50±12%, p<0.05). Muscle fibre capillarisation was significantly lower in patients with COPD compared with controls (p<0.05). While satellite cell content was not different between groups, type I and type II satellite cell distance to its nearest capillary was significantly greater in patients with COPD (type I: 21.3±4.8 µm; type II: 26.7±9.3 µm) compared with controls (type I: 16.1±3.5 µm; type II: 22.7±5.8 µm; p<0.05). Satellite cells are located at a greater distance from their nearest capillary in patients with COPD compared with age-matched controls. This increased distance could play a role in impaired satellite cell function in patients with COPD.

Effects of combined exercise training in older adults: a potential relationship between muscle fibre satellite cell function and capillarization.

Effects of combined exercise training in older adults: a potential relationship between muscle fibre satellite cell function and capillarization.

Prolonged exercise training improves the acute type II muscle fibre satellite cell response in healthy older men.

Skeletal muscle stem cells, termed satellite cells, play a crucial role in repair and remodelling of muscle in response to exercise An age-related decline in satellite cell number and/or function has been hypothesized to be a key factor in the development of sarcopenia and/or the blunted muscle fibre adaptive response to prolonged exercise training in older persons We report that performing prolonged exercise training improves the acute type II muscle fibre satellite cell response following a single bout of resistance exercise in older men. The observed improvement in muscle satellite function is associated with an increase in muscle fibre capillarization following exercise training suggesting a possible functional link between capillarization and satellite cell function. Age-related type II muscle fibre atrophy is accompanied by a fibre type-specific decline in satellite cell number and function. Exercise training restores satellite cell quantity in older adults; however, whether it can restore the impaired satellite cell response to exercise in older adults remains unknown. Therefore we assessed the acute satellite cell response to a single exercise session before and after prolonged exercise training in older men. Fourteen older men (74±8years) participated in a 12-week exercise training programme (resistance exercise performed twice per week, high intensity interval training once per week). Before and after training, percutaneous biopsies from the vastus lateralis muscle were taken prior to and following 24 and 48h of post-exercise recovery. Muscle fibre characteristics were evaluated by immunohistochemistry and mRNA expression by RT-PCR. Whereas no changes were observed in type II muscle fibres, type I muscle fibre satellite cell content increased significantly at 24 and 48h after a single bout of resistance exercise before the exercise training programme (P<0.01). Following the exercise training programme, both type I and type II muscle fibre satellite cell content increased significantly at 24 and 48h after a single bout of resistance exercise (P<0.05). The greater acute increase in type II muscle fibre satellite cell content at 24 h post-exercise recovery after training was correlated with an increase in type II muscle fibre capillarization (r=0.671, P=0.012). We show that the acute muscle satellite cell response following exercise can be improved by prolonged exercise training in older men.

Ingestion of a Multi-Ingredient Supplement Does Not Alter Exercise-Induced Satellite Cell Responses in Older Men

Nutritional supplementation can have beneficial effects on body composition, strength, and function in older adults. However, whether the response of satellite cells can be altered by nutritional supplementation in older adults remains unknown. We assessed whether a multi-ingredient protein-based supplement taken over a prolonged period of time could alter the muscle satellite cell response after exercise in older men. Twenty-seven older men [mean ± SD age: 73±1 y; mean ± SD body mass index (kg/m2): 28±1] participated in a randomized double-blind experiment. Participants were randomly divided into an experimental (EXP) group (n=13) who consumed a multi-ingredient protein-based supplement [30 g whey protein, 2.5 g creatine, 500 IU vitamin D, 400 mg Ca, and 1500 mg n-3 (ω-3) polyunsaturated fatty acids] 2 times/d for 7 wk or a control (CON; 22 g maltodextrin) group (n=14). After 7 wk of supplementation, all participants performed a single resistance exercise session, and muscle biopsy samples were taken from the vastus lateralis before and 24 and 48 h after exercise. Immunohistochemistry was used to assess the change in type I and II muscle fiber satellite cell content and activation status of the cells. In addition, mRNA expression of the myogenic regulatory factors was determined by using reverse transcriptase-polymerase chain reaction. In response to the single bout of exercise, type I muscle fiber satellite cell content was significantly increased at 24 h (0.132±0.015 and 0.131±0.011 satellite cells/fiber in CON and EXP groups, respectively) and 48 h (0.126±0.010 and 0.120±0.012 satellite cells/fiber in CON and EXP groups, respectively) compared with pre-exercise (0.092±0.007 and 0.118±0.017 satellite cells/fiber in CON and EXP groups, respectively) muscle biopsy samples (P<0.01), with no difference between the 2 groups. In both groups, we observed no significant changes in type II muscle fiber satellite cell content after exercise. Ingesting a multi-ingredient protein-based supplement for 7 wk did not alter the type I or II muscle fiber satellite cell response during postexercise recovery in older men. This trial was registered at www.clinicaltrials.gov as NCT02281331.

Extensive Type II Muscle Fiber Atrophy in Elderly Female Hip Fracture Patients.

Sarcopenia, or the loss of muscle mass and strength, is known to increase the risk for falls and (hip) fractures in older people. The objective of this study was to assess the skeletal muscle fiber characteristics in elderly female hip fracture patients. Percutaneous needle biopsies were collected from the vastus lateralis muscle in 15 healthy young women (20 ± 0.4 years), 15 healthy elderly women (79 ± 1.7 years), and 15 elderly women with a fall-related hip fracture (82 ± 1.5 years). Immunohistochemical analyses were performed to assess Type I and Type II muscle fiber size, and myonuclear and satellite cell content. Type II muscle fiber size was significantly different between all groups (p < .05), with smaller Type II muscle fibers in the hip fracture patients (2,609 ± 185 µm2) compared with healthy elderly group (3,723 ± 322 µm2) and the largest Type II muscle fibers in the healthy young group (4,755 ± 335 µm2). Furthermore, Type I muscle fiber size was significantly lower in the hip fracture patients (4,684 ± 211 µm2) compared with the healthy elderly group (5,842 ± 316 µm2, p = .02). The number of myonuclei per Type II muscle fiber was significantly lower in the healthy elderly and hip fracture group compared with the healthy young group (p = .011 and p = .002, respectively). Muscle fiber satellite cell content did not differ between groups. Elderly female hip fracture patients show extensive Type II muscle fiber atrophy when compared with healthy young or age-matched healthy elderly controls. Type II muscle fiber atrophy is an important hallmark of sarcopenia and may predispose to falls and (hip) fractures in the older population.

Satellite Cell Reponse to Concurrent Resistance Exercise and High Intensity Interval Training in Overweight/Obese Individuals

It is recommended that individuals perform a combination of resistance and endurance exercise to improve cardio-metabolic health. Lack of time is often cited as a reason for being unable to do so. Therefore, combining both in one session may be beneficial. However, work continues to elucidate whether an interference in adaptive outcomes occur when resistance and endurance exercise are performed concurrently. PURPOSE: To determine whether concurrent resistance exercise (RE) and high intensity interval training (HIIT) acutely alters the satellite cell density and activity following exercise compared to single-mode resistance exercise. METHOD: In a balanced crossover design, 8 sedentary overweight/obese individuals (3 male, 5 females) performed either RE only (8 x 8 leg extensions at 70% 1 RM), or RE followed by HIIT (RE + HIIT; 10 x 1 min at 90% HRmax on a cycle ergometer). Muscle biopsies were obtained before exercise and at 96 h after the RE component. Fiber type-specific satellite cell density (Pax7+ cells) and activity (MyoD+ cells) were analysed by immunofluorescence microscopy. Two-way repeated-measure ANOVA was used for statistical analysis. RESULTS: The number of mixed muscle fiber satellite cells (Pax7+ cells) increased by 27% at 96 h compared to baseline (P<0.05) with no difference between exercise trials. There was a 60% increase in muscle fiber type-I-specific satellite cell number at 96 h compared to baseline (P<0.05), but no significant difference in type II muscle fibers. In mixed muscle fibers, the increase (P<0.05) in MyoD+ cells at 96 h compared to baseline for both exercise trials was greatest in RE (98% vs. 48% in RE + HIIT). In both exercise trials, there was an increase in MyoD+ cells per type I muscle fibers (120%, P<0.05), but no significant difference in type II muscle fibers. CONCLUSION: Combining the additional HIIT session to RE does not interfere with the increase in satellite cell density when compared to RE only. However following the concurrent exercise, in this study, there are fewer active satellite cells. This may attenuate the number of myogenic precursors cells, a key requirement for hypertrophy. ACKNOWLEDGEMENTS: This work was in part supported by Technogym, The Wellness Company and the National Institute for Health Research (NIHR) Diet, Lifestyle & Physical Activity Biomedical Research Unit.

Satellite cells in human skeletal muscle; from birth to old age

Changes in satellite cell content play a key role in regulating skeletal muscle growth and atrophy. Yet, there is little information on changes in satellite cell content from birth to old age in humans. The present study defines muscle fiber type-specific satellite cell content in human skeletal muscle tissue over the entire lifespan. Muscle biopsies were collected in 165 subjects, from different muscles of children undergoing surgery (<18 years; n = 13) and from the vastus lateralis muscle of young adult (18–49 years; n = 50), older (50–69 years; n = 53), and senescent subjects (70–86 years; n = 49). In a subgroup of 51 aged subjects (71 ± 6 years), additional biopsies were collected after 12 weeks of supervised resistance-type exercise training. Immunohistochemistry was applied to assess skeletal muscle fiber type-specific composition, size, and satellite cell content. From birth to adulthood, muscle fiber size increased tremendously with no major changes in muscle fiber satellite cell content, and no differences between type I and II muscle fibers. In contrast to type I muscle fibers, type II muscle fiber size was substantially smaller with increasing age in adults (r = −0.56; P < 0.001). This was accompanied by an age-related reduction in type II muscle fiber satellite cell content (r = −0.57; P < 0.001). Twelve weeks of resistance-type exercise training significantly increased type II muscle fiber size and satellite cell content. We conclude that type II muscle fiber atrophy with aging is accompanied by a specific decline in type II muscle fiber satellite cell content. Resistance-type exercise training represents an effective strategy to increase satellite cell content and reverse type II muscle fiber atrophy.

Influx of Ca2+ via Cav1.3 calcium channels in satellite cells of muscle fibers in rats

Voltage-dependent L-type Cav1.3 channels have been detected in satellite cells localized to muscle fibers. It was established that the action of carbachol, which activates nicotinic acetylcholine receptors and causes cell membrane to depolarize, resulted in the activation of these channels. In addition, verapamil and amlodipine, selective L-type calcium channel blockers, suppressed extracellular calcium influx into the cytoplasm. It was noted that in a calcium-free medium, carbachol had no influence on the concentration of calcium in the cytoplasm of satellite cells, whereas adrenaline induced calcium efflux from intracellular stores. In addition, calcium influx into the cytoplasm was not suppressed by verapamil and amlodipine under the action of adrenaline and noradrenalin in a medium with calcium, and an ICI- 118551 blocker of β2-adrenoreceptros significantly decreased the increase in the concentration of calcium in the cytoplasm.

Eccentric Exercise Increases Satellite Cell Content in Type II Muscle Fibers

Satellite cells (SCs) are of key importance in skeletal muscle tissue growth, repair, and regeneration. A single bout of high-force eccentric exercise has been demonstrated to increase mixed muscle SC content after 1-7 d of postexercise recovery. However, little is known about fiber type-specific changes in SC content and their activation status within 24 h of postexercise recovery. Nine recreationally active young men (23 ± 1 yr) performed 300 eccentric actions of the knee extensors on an isokinetic dynamometer. Skeletal muscle biopsies from the vastus lateralis were collected preexercise and 24 h postexercise. Muscle fiber type-specific SC content and the number of activated SCs were determined by immunohistochemical analyses. There was no difference between Type I and Type II muscle fiber SC content before exercise. SC content significantly increased 24 h postexercise in Type II muscle fibers (from 0.085 ± 0.012 to 0.133 ± 0.016 SCs per fiber, respectively; P < 0.05), whereas there was no change in Type I fibers. In accordance, activation status increased from preexercise to 24 h postexercise as demonstrated by the increase in the number of DLK1+ SCs in Type II muscle fibers (from 0.027 ± 0.008 to 0.070 ± 0.017 SCs per muscle fiber P < 0.05). Although no significant changes were observed in the number of Ki-67+ SCs, we did observe an increase in the number of proliferating cell nuclear antigen-positive SCs after 24 h of postexercise recovery. A single bout of high-force eccentric exercise increases muscle fiber SC content and activation status in Type II but not Type I muscle fibers.

A single bout of exercise activates skeletal muscle satellite cells during subsequent overnight recovery

Skeletal muscle satellite cell (SC) content has been reported to increase following a single bout of exercise. Data on muscle fibre type-specific SC content and/or SC activation status are presently lacking. The objective of the study was to determine the impact of a single bout of exercise on muscle fibre type-specific SC content and activation status following subsequent overnight recovery. Eight healthy men (age, 20 ± 1 years) performed a single bout of combined endurance- and resistance-type exercise. Muscle biopsies were collected before and immediately after exercise, and following 9 h of postexercise, overnight recovery. Muscle fibre type-specific SC and myonuclear content and SC activation status were determined by immunohistochemical analyses. Satellite cell activation status was assessed by immunohistochemical staining for both Delta-like homologue 1 (DLK1) and Ki-67. Muscle fibre size and fibre area per nucleus were greater in type II compared with type I muscle fibres (P < 0.05). At baseline, no differences were observed in the percentage of SCs staining positive for DLK1 and/or Ki67 between fibre types. No significant changes were observed in SC content following 9 h of postexercise, overnight recovery; however, the percentage of DLK1-positive SCs increased significantly during overnight recovery, from 22 ± 5 to 41 ± 5% and from 24 ± 6 to 51 ± 9% in the type I and II muscle fibres, respectively. No changes were observed in the percentage of Ki-67-positive SCs. A single bout of exercise activates both type I and II skeletal muscle fibre SCs within a single night of postexercise recovery, preceding the subsequent increase in SC content.

Concurrent aerobic exercise interferes with the satellite cell response to acute resistance exercise

The addition of aerobic exercise (AE) to a resistance exercise (RE) program (concurrent exercise, CE) can interfere with maximum muscle fiber growth achieved with RE. Further, CE appears to markedly affect the growth of myosin heavy chain (MHC) I, but not MHC IIa fibers. The mechanism responsible for this "interference" is unclear. Satellite cell (SC) responsiveness to exercise appears to influence muscle adaptation but has not yet been examined following acute concurrent exercise. Thus, we assessed the fiber-type-specific SC response to RE, AE, and CE exercise. Eight college-aged males completed the following two exercise trials: the RE trial, which consisted of unilateral leg extensions and presses (4 sets ≥ 10 repetitions: 75% 1 repetition maximum, RM); and the AE/CE trial, which included an identical RE protocol with the opposite leg, immediately followed by subjects cycling for 90 min (60% W(max)). Muscle biopsies were obtained from the vastus lateralis before and 4 days after each session. Samples were cross-sectioned, stained with antibodies against NCAM, Ki-67, and MHC I, counterstained with DAPI, and analyzed for SC density (SC per fiber), SC activation, and fiber type. SC density increased to a greater extent following RE (38 ± 10%), compared with CE (-6 ± 8%). Similarly, MHC I muscle fiber SC density displayed a greater increase following RE (46 ± 14%), compared with AE (-7 ± 17%) and CE (-8 ± 8%). Our data indicate that the SC response to RE is blunted when immediately followed by AE, at least in MHC I muscle fibers, and possibly MHC II fibers. This suggests that the physiological environment evoked by AE might attenuate the eventual addition of myonuclei important for maximum muscle fiber growth and consequent force-producing capacity.

Down-regulation of MyoD by Calpain 3 Promotes Generation of Reserve Cells in C2C12 Myoblasts

Calpain 3 is a calcium-dependent cysteine protease that is primarily expressed in skeletal muscle and is implicated in limb girdle muscular dystrophy type 2A. To date, its best characterized function is located within the sarcomere, but this protease is found in other cellular compartments, which suggests that it exerts multiple roles. Here, we present evidence that calpain 3 is involved in the myogenic differentiation process. In the course of in vitro culture of myoblasts to fully differentiated myotubes, a population of quiescent undifferentiated "reserve cells" are maintained. These reserve cells are closely related to satellite cells responsible for adult muscle regeneration. In the present work, we observe that reserve cells express higher levels of endogenous Capn3 mRNA than proliferating myoblasts. We show that calpain 3 participates in the establishment of the pool of reserve cells by decreasing the transcriptional activity of the key myogenic regulator MyoD via proteolysis independently of the ubiquitin-proteasome degradation pathway. Our results identify calpain 3 as a potential new player in the muscular regeneration process by promoting renewal of the satellite cell compartment.

Skeletal Muscle Hypertrophy Following Resistance Training Is Accompanied by a Fiber Type-Specific Increase in Satellite Cell Content in Elderly Men

We determined muscle fiber type-specific hypertrophy and changes in satellite cell (SC) content following a 12-week resistance training program in 13 healthy, elderly men (72 +/- 2 years). Leg strength and body composition (dual-energy X-ray absorptiometry and computed tomography) were assessed, and muscle biopsy samples were collected. Leg strength increased 25%-30% after training (p < .001). Leg lean mass and quadriceps cross-sectional area increased 6%-9% (p < .001). At baseline, mean fiber area and SC content were smaller in the Type II versus Type I muscle fibers (p < .01). Following training, Type II muscle fiber area increased from 5,438 +/- 319 to 6,982 +/- 503 microm(2) (p < .01). Type II muscle fiber SC content increased from 0.048 +/- 0.003 to 0.084 +/- 0.008 SCs per fiber (p < .001). No changes were observed in the Type I muscle fibers. In older adults, skeletal muscle tissue is still capable of inducing SC proliferation and differentiation, resulting in Type II muscle fiber hypertrophy.

Changes in satellite cell mitotic activity during acute period of unilateral diaphragm denervation.

The acute period of unilateral diaphragm denervation (DNV) is associated with increases in cell mitotic activity, protein synthesis, and muscle fiber hypertrophy. Our purpose was to determine whether acute unilateral diaphragm DNV is associated with changes in muscle isometric contractile properties, cross-sectional area of different muscle fiber types, mitotic activity of muscle fiber satellite cells, and muscle fiber ultrastructural properties indicative of injury. Adult male Fischer 344 rats underwent a right phrenicotomy, and DNV and intact (INT) hemidiaphragms were studied 72 h later. DNV hemidiaphragm displayed a significant decline in maximal isometric force (8.7 vs. 24.3 N/cm2) and a prolonged time to peak twitch force (47.8 vs. 37.5 ms) and time to half relaxation (72.3 vs. 44.3 ms) compared with INT contralateral hemidiaphragm (P < 0.05). DNV resulted in a significant increase in cross-sectional area of types I (33%), IIa (35%), and IIb (28%) fibers relative to INT hemidiaphragm (P < 0.05). Satellite cell mitotic activity (assessed by incorporation of bromodeoxyuridine) was approximately 5.5 times greater in DNV than in INT muscle (DNV 25.0 +/- 3.8, INT 4.5 +/- 1.4 labeled satellite cell nuclei/1,000 nuclei; P < 0.05). Ultrastructural examination of electron micrographs revealed alterations in Z-line and sarcomeric structure indicative of muscle injury. Cellular infiltration and segmental necrosis were also noted in some fibers. We conclude that acute unilateral diaphragm DNV results in muscle fiber injury that induces satellite cell activation. We also speculate that the specific force decrement associated with DNV is at least partially the result of muscle injury.

Muscle Tissue Reactions to Implantation of Bone Matrix Gelatin

Histologic changes of muscle tissue in the early stage of heterotopic osteogenesis induced by syngeneic insoluble bone matrix gelatin (BMG) with bone morphogenetic protein in rats was observed by light and electron microscopy. BMG induced cartilage in muscle tissue by Day 7 after its implantation, woven bone by Day 10, and lamellar bone with bone marrow by Day 14. The new findings in this work include (1) the disappearance of the basement membrane of muscle fibers; (2) the activation of the satellite cells of muscle fibers; (3) the appearance of fibroblastlike cells that closely resembled activated satellite cells among the degenerated muscle fibers or on the surface of the BMG; and (4) the change of fibroblastlike cells to chondroblasts or osteoblasts. These findings suggest that intramuscular implantation of BMG caused the conspicuous disappearance of the basement membrane of the muscle fiber and may play a part in osteogenesis induced by BMG.

Satellite cell and growth factor involvement in skeletal muscle growth

WHITE, T. P. and K. A. ESSER. Satellite cell and growth factor involvement in skeletal muscle growth. Med. Sci. Sports Exerc., Vol. 21, No. 5 (Supplement), pp. S158–S163, 1989. The activity of the satellite cell, discovered by Alexander Mauro, is of fundamental importance in postnatal skeletal muscle development, muscle adaptation to certain activity stimuli, and to muscle fiber regeneration following injury and transplantation operations. There are numerous mitogens and growth factors that influence satellite cell proliferation and differentiation in vitro and likely in vivo. The best understood purified growth factors are fibroblast growth factor (FGF), the insulin-like growth factors (IGF-I and -II), and transforming growth factor-β (TGF-β). Soluble extracts from injured muscle and chronically stretched muscle are also known to be mitogenic and are yet to be purified. Skeletal muscle development, hypertrophy, and regeneration can be viewed as points on a continuum with respect to the regulatory mechanisms of myogenic cell growth. The occurrence of fiber hyperplasia differs amongst some models of activity-induced growth and may reflect differences in the magnitude of the stimulus relative to the capacity of fibers to adapt. The relationships between the mechanical and environmental events coincident with an activity or injury stimulus and the role of specific muscle fiber satellite cell populations and growth factors are fertile areas for investigation. Insights from these experiments will yield a comprehensive understanding of the muscle growth process at the molecular, cellular, and tissue levels, and have implications for development and aging, health, disease, and adaptation.

SATELLITE CELL AND GROWTH FACTOR INVOLVEMENT IN SKELETAL MUSCLE GROWTH.

The activity of the satellite cell, discovered by Alexander Mauro, is of fundamental importance in postnatal skeletal muscle development, muscle adaptation to certain activity stimuli, and to muscle fiber regeneration following injury and transplantation operations. There are numerous mitogens and growth factors that influence satellite cell proliferation and differentiation in vitro and likely in vivo. The best understood purified growth factors are fibroblast growth factor (FGF), the insulin-like growth factors (IGF-I and -II), and transforming growth factor-beta (TGF-beta). Soluble extracts from injured muscle and chronically stretched muscle are also known to be mitogenic and are yet to be purified. Skeletal muscle development, hypertrophy, and regeneration can be viewed as points on a continuum with respect to the regulatory mechanisms of myogenic cell growth. The occurrence of fiber hyperplasia differs amongst some models of activity-induced growth and may reflect differences in the magnitude of the stimulus relative to the capacity of fibers to adapt. The relationships between the mechanical and environmental events coincident with an activity or injury stimulus and the role of specific muscle fiber satellite cell populations and growth factors are fertile areas for investigation. Insights from these experiments will yield a comprehensive understanding of the muscle growth process at the molecular, cellular, and tissue levels, and have implications for development and aging, health, disease, and adaptation.

Two unusual satellite cell-intrafusal muscle fiber relationships.

We studied the location and ultrastructure features of rat teres minor intrafusal muscle fiber satellite cells. They were abundant in myotube and polar regions, particularly near sensory nerve terminals and terminations of the spindle capsule and intrafusal muscle fibers. One satellite cell was wedged between two “paired” nuclear bag fibers in the myotube region. No basement membrane surrounded this cell. A polar satellite cell was encased by the basement membrane for only part its length. This cell contained numerous myofilaments. It terminated in the intercellular space surrounded by its own basement membrane adjacent to the host nuclear bag fiber. Recent satellite cell investigations are summarized and compared with the results of this study. We conclude there may be more than one cell type in the satellite cell position and intrafusal and extrafusal satellite cells may function in different capacities.