Size and myonuclear domains in Rhesus soleus muscle fibers: short-term spaceflight

The cross-sectional area (CSA), myonuclear number per mm of fiber length, and myonuclear domain (cytoplasmic volume/myonucleus) of mechanically isolated single fibers from biopsies of the soleus muscle of 5 vivarium control, 3 flight simulation and 2 flight (BION 11) Rhesus monkeys (Macaca [correction of Macacca] mulatta) were determined using confocal microscopy before and after a 14-day experimental period. Simulation monkeys were confined in chairs placed in capsules identical to those used during the flight. Fibers were classified as type I, type II or hybrid (containing both types I and II) based on myosin heavy chain (MHC) gel electrophoresis. A majority of the fibers sampled contained only type I MHC, i.e. 89, 62 and 68% for the control, simulation and flight groups, respectively. Most of the remaining fibers were hybrids, i.e. 8, 36 and 32% for the same groups. There were no significant pre-post differences in the fiber type composition for any of the experimental groups. There also were no significant pre-post differences in fiber CSA, myonuclear number or myonuclear domain. There was, however, a tendency for the fibers in the post-flight biopsies to have a smaller mean CSA and myonuclear domain (approximately 10%, p=0.07) than the fibers in the pre-flight biopsy. The combined mean cytoplasmic volume/myonucleus for all muscle fiber phenotypes in the Rhesus soleus muscle was approximately 25,000 micrometers3 and there were no differences in pre-post samples for the control and simulated groups. The cytoplasmic domains tended to be lower (p=0.08) after than before flight. No phenotype differences in cytoplasmic domains were observed. These data suggest that after a relatively short period of actual spaceflight, modest fiber atrophy occurs in the soleus muscle fibers without a concomitant change in myonuclear number.     

Cytoplasm-to-myonucleus ratios and succinate dehydrogenase activities in adult rat slow and fast muscle fibers

The relationship between myonuclear number, cellular size, succinate dehydrogenase activity, and myosin type was examined in single fiber segments (n = 54; 9 ± 3 mm long) mechanically dissected from soleus and plantaris muscles of adult rats. One end of each fiber segment was stained for DNA before quantitative photometric analysis of succinate dehydrogenase activity; the other end was double immunolabelled with fast and slow myosin heavy chain monoclonal antibodies. Mean ± S.D. cytoplasmic volume/myonucleus ratio was higher in fast and slow plantaris fibers (112 ± 69 vs. 34 ± 21 x 10 ³µm ³) than fast and slow soleus fibers (40 ± 20 vs. 30 ± 14 x 10 ³µm ³), respectively. Slow fibers always had small volumes/myonucleus, regardless of fiber diameter, succinate dehydrogenase activity, or muscle of origin. In contrast, smaller diameter (<70 µm) fast soleus and plantaris fibers with high succinate dehydrogenase activity appeared to have low volumes/myonucleus while larger diameter (>70 µm) fast fibers with low succinate dehydrogenase activity always had large volume/myonucleus. Slow soleus fibers had significantly greater numbers of myonuclei/mm than did either fast soleus or fast plantaris fibers (116 ± 51 vs. 55 ± 22 and 44 ± 23), respectively. These data suggest that the myonuclear domain is more limited in slow than fast fibers and in the fibers with a high, compared to a low, oxidative metabolic capability.     

Tumor necrosis factor-α in macrophages of heart,liver, kidney,and in the pituitary gland

The relationship between myonuclear number, cellular size, succinate dehydrogenase activity, and myosin type was examined in single fiber segments (n=54; 9±3 mm long) mechanically dissected from soleus and plantaris muscles of adult rats. One end of each fiber segment was stained for DNA before quantitative photometric analysis of succinate dehydrogenase activity; the other end was double immunolabelled with fast and slow myosin heavy chain monoclonal antibodies. Mean±S.D. cytoplasmic volume/myonucleus ratio was higher in fast and slow plantaris fibers (112±69 vs. 34±21x10³ m³) than fast and slow soleus fibers (40±20 vs. 30±14x10³ m³), respectively. Slow fibers always had small volumes/myonucleus, regardless of fiber diameter, succinate dehydrogenase activity, or muscle of origin. In contrast, smaller diameter (<70 m) fast soleus and plantaris fibers with high succinate dehydrogenase activity appeared to have low volumes/myonucleus while larger diameter (>70 m) fast fibers with low succinate dehydrogenase activity always had large volume/myonucleus. Slow soleus fibers had significantly greater numbers of myonuclei/mm than did either fast soleus or fast plantaris fibers (116±51 vs. 55±22 and 44±23), respectively. These data suggest that the myonuclear domain is more limited in slow than fast fibers and in the fibers with a high, compared to a low, oxidative metabolic capability.     

Influence of corticosteroids on myonuclear domain size in the rat diaphragm muscle.

Skeletal muscle fibers are multinucleated. Each myonucleus regulates gene products and protein expression in only a restricted portion of the muscle fiber, the myonuclear domain (MND). In the rat diaphragm muscle (DIAm), corticosteroid (CoS) treatment causes atrophy of fibers containing myosin heavy chain (MHC): MHC2X and/or MHC2B. We hypothesized that DIAm fiber MND size is maintained during CoS-induced atrophy. Adult male rats received methylprednisolone for 11 days at 1 (CoS-Low, n = 8) or 8 mg x kg(-1) x day(-1) (CoS-High, n = 8). Age-matched (CTL-AgeM, n = 8), sham-operated (SHAM-AgeM, n = 8), and weight-matched (CTL-WtM, n = 8) animals served as controls. In single DIAm fibers, cross-sectional area (CSA), MND size, and MHC expression were determined. Fiber CSA and MND size were similar in CTL-AgeM and SHAM-AgeM groups. Only fibers containing MHCslow or MHC2A displayed smaller CSA in CTL-WtM than in CTL-AgeM and SHAM-AgeM groups, and MND size was reduced in all fibers. Thus fibers containing MHCslow and MHC2A maintain the number of myonuclei, whereas MHC2X or MHC2B fibers show loss of myonuclei during normal muscle growth. Both CoS groups displayed smaller CSA and MND size than CTL-AgeM and SHAM-AgeM groups. However, compared with CTL-WtM DIAm fibers, only fibers containing MHC2X or MHC2B displayed reduced CSA and MND size after CoS treatment. Thus little, if any, loss of myonuclei was associated with CoS-induced atrophy of MHC2X or MHC2B DIAm fibers. In summary, MND size does not appear to be regulated during CoS-induced DIAm atrophy.     

Developmental effects on myonuclear domain size of rat diaphragm fibers.

During early postnatal development in rat diaphragm muscle (Diam), significant fiber growth and transitions in myosin heavy chain (MHC) isoform expression occur. Similar to other skeletal muscles, Diam fibers are multinucleated, and each myonucleus regulates the gene products within a finite volume: the myonuclear domain (MND). We hypothesized that postnatal changes in fiber cross-sectional area (CSA) are associated with increased number of myonuclei so that the MND size is maintained. The Diam was removed at postnatal days 14 (P-14) and 28 (P-28). MHC isoform expression was determined by SDS-PAGE. Fiber CSA, myonuclear number, and MND size were measured using confocal microscopy. By P-14, significant coexpression of MHC isoforms was present with no fiber displaying singular expression of MHCNeo. By P-28, singular expression was predominant. MND size was not different across fiber types at P-14. Significant fiber growth was evident by P-28 at all fiber types (fiber CSA increased by 61, 93, and 147% at fibers expressing MHCSlow, MHC2A, and MHC2X, respectively). The number of myonuclei per unit of fiber length was similar across fibers at P-14, but it was greater at fibers expressing MHC2X at P-28. The total number of myonuclei per fiber also increased between P-14 and P-28 at all fiber types. Accordingly, MND size increased significantly by P-28 at all fiber types, and it became larger at fibers expressing MHC2X compared with fibers expressing MHCSlow or MHC2A. These results suggest that MND size is not maintained during the considerable fiber growth associated with postnatal development of the Diam.     

Effects of inactivity on fiber size and myonuclear number in rat soleus muscle.

The effects of short-term (4 days) and long-term (60 days) neuromuscular inactivity on myonuclear number, size, and myosin heavy chain (MHC) composition of isolated rat soleus fibers were determined using confocal microscopy and gel electrophoresis. Inactivity was produced via spinal cord isolation (SI), i.e., complete spinal cord transections at a midthoracic and a high sacral level and bilateral deafferentation between the transection sites. Compared with control, there was an increase in the percentage of fibers containing the faster MHC isoforms after 60, but not 4, days of SI. The mean sizes of type I and type I+IIa fibers were 41 and 27% and 66 and 56% smaller after 4 and 60 days of SI, respectively. Thus atrophy occurred earlier than the shift in myosin heavy chain (MHC) profile. The number of myonuclei was approximately 30% higher in type I than type I+IIa fibers in control soleus, but after 60 days of SI these values were similar. The number of myonuclei per millimeter in type I fibers was significantly lower than control after 60 days of SI, whereas there was no change in type I+IIa fibers. Thus myonuclei were eliminated from fibers containing only type I MHC. Because the magnitude of the loss of myonuclei was less than the level of atrophy, the myonuclear domains of both type I and type I+IIa fibers were significantly lower than control. Thus chronic (60 days) inactivity results in smaller, faster fibers that contain a higher than normal amount of DNA per unit of cytoplasm. The absence of activation of muscle fibers that are normally the most active (pure type I fibers) resulted in most, but not all, fibers expressing some fast MHC isoforms. The results also indicate that a loss of myonuclei is not a prerequisite for sustained muscle fiber atrophy.     

Skeletal muscle morphology in power-lifters with and without anabolic steroids

The morphological appearance of the vastus lateralis (VL) muscle from high-level power-lifters on long-term anabolic steroid supplementation (PAS) and power-lifters never taking anabolic steroids (P) was compared. The effects of long- and short-term supplementation were compared. Enzyme-immunohistochemical investigations were performed to assess muscle fiber type composition, fiber area, number of myonuclei per fiber, internal myonuclei, myonuclear domains and proportion of satellite cells. The PAS group had larger type I, IIA, IIAB and IIC fiber areas (p<0.05). The number of myonuclei/fiber and the proportion of central nuclei were significantly higher in the PAS group (p<0.05). Similar results were seen in the trapezius muscle (T) but additionally, in T the proportion of fibers expressing developmental myosin isoforms was higher in the PAS group compared to the P group. Further, in VL, the PAS group had significantly larger nuclear domains in fibers containing ≥5 myonuclei. The results of AS on VL morphology in this study were similar to previously reported short-term effects of AS on VL. The initial effects from AS appear to be maintained for several years.     

Role(s) of nucleoli and phosphorylation of ribosomal protein S6 and/or HSP27 in the regulation of muscle mass

Effects of 14 days of hindlimb unloading or synergist ablation-related overloading with or without deafferentation on the fiber cross-sectional area, myonuclear number, size, and domain, the number of nucleoli in a single myonucleus, and the levels in the phosphorylation of the ribosomal protein S6 (S6) and 27-kDa heat shock protein (HSP27) were studied in rat soleus. Hypertrophy of fibers (+24%), associated with increased nucleolar number (from 1–2 to 3–5) within a myonucleus and myonuclear domain (+27%) compared with the preexperimental level, was induced by synergist ablation. Such phenomena were associated with increased levels of phosphorylated S6 (+84%) and HSP27 (+28%). Fiber atrophy (–52%), associated with decreased number (–31%) and domain size (–28%) of myonuclei and phosphorylation of S6 (–98%) and HSP27 (–63%), and with increased myonuclear size (+19%) and ubiquitination of myosin heavy chain (+33%, P > 0.05), was observed after unloading, which inhibited the mechanical load. Responses to deafferentation, which inhibited electromyogram level (–47%), were basically similar to those caused by hindlimb unloading, although the magnitudes were minor. The deafferentation-related responses were prevented and nucleolar number was even increased (+18%) by addition of synergist ablation, even though the integrated electromyogram level was still 30% less than controls. It is suggested that the load-dependent maintenance or upregulation of the nucleolar number and/or phosphorylation of S6 and HSP27 plays the important role(s) in the regulation of muscle mass. It was also indicated that such regulation was not necessarily associated with the neural activity. rat soleus muscle; functional overload; deafferentation; 27-kDa heat shock protein; ubiquitination of myosin heavy chain     

Is the myonuclear domain size fixed?

It has been suggested that the number of myonuclei in a muscle fibre changes in proportion to the change in fibre size, resulting in a constant myonuclear domain size, defined as the cytoplasmic volume per myonucleus. The myonuclear domain size varies, however, between fibre types and is inversely related with the oxidative capacity of a fibre. Overall, the observations of an increase in myonuclear domain size during both maturational growth and overload-induced hypertrophy, and the decrease in myonuclear domain size during disuse- and ageing-associated muscle atrophy suggest that the concept of a constant myonuclear domain size needs to be treated cautiously. It also suggests that only when the myonuclear domain size exceeds a certain threshold during growth or overload-induced hypertrophy acquisition of new myonuclei is required for further fibre hypertrophy.     

Modulation of myonuclear number in functionally overloaded and exercised rat plantaris fibers.

The effects of 10 wk of functional overload (FO), with and without daily treadmill endurance training, on the cross-sectional area, myonuclear number, and myonuclear domain size of mechanically isolated single fiber segments of the adult rat plantaris were determined. The fibers were typed on the basis of high-resolution gel electrophoresis for separation of specific myosin heavy chain (MHC) isoforms and grouped as type I(+) (containing some type I MHC with or without any combination of fast MHCs), type IIa(+) (containing some type IIa with or without some type IIx and/or IIb but no type I MHC), and type IIx/b (containing only type IIx and/or IIb MHCs). Type I(+) fibers had a higher myonuclear number than did both fast types of fibers in the control and FO, but not in the FO and treadmill trained, rats. All fiber types in both FO groups had a significantly larger (36-90%) cross-sectional area and a significantly higher (61-109%) myonuclear number than did control. The average myonuclear domain size of each fiber type was similar among the three groups, except for a smaller domain size in the type IIx/b fibers of the FO compared with control. In general, these data indicate that during hypertrophy the number of myonuclei increase proportionally to the increase in fiber volume. The maintenance of myonuclear domain size near control values suggests that regulatory mechanisms exist that ensure a tight coupling between the quantity of genetic machinery and the protein requirements of a fiber.     

Is functional hypertrophy and specific force coupled with the addition of myonuclei at the single muscle fiber level?

Muscle force is typically proportional to muscle size, resulting in constant force normalized to muscle fiber cross-sectional area (specific force). Mice overexpressing insulin-like growth factor-1 (IGF-1) exhibit a proportional gain in muscle force and size, but not the myostatin-deficient mice. In an attempt to explore the role of the cytoplasmic volume supported by individual myonuclei [myonuclear domain (MND) size] on functional capacity of skeletal muscle, we have investigated specific force in relation to MND and the content of the molecular motor protein, myosin, at the single muscle fiber level from myostatin-knockout (Mstn(-/-)) and IGF-1-overexpressing (mIgf1(+/+)) mice. We hypothesize that the addition of extra myonuclei is a prerequisite for maintenance of specific force during muscle hypertrophy. A novel algorithm was used to measure individual MNDs in 3 dimensions along the length of single muscle fibers from the fast-twitch extensor digitorum longus and the slow-twitch soleus muscle. A significant effect of the size of individual MNDs in hypertrophic muscle fibers on both specific force and myosin content was observed. This effect was muscle cell type specific and suggested there is a critical volume individual myonuclei can support efficiently. The large MNDs found in fast muscles of Mstn(-/-) mice were correlated with the decrement in specific force and myosin content in Mstn(-/-) muscles. Thus, myostatin inhibition may not be able to maintain the appropriate MND for optimal function.     

Cytomorphometry of skeletal muscle: The influence of age and testosterone on the rat M. levator ani

Summary The levator ani muscle of the rat was examined by correlated light and electron microscopic morphometry. Corrections were made for shrinkage, compression, and differences in stretching. Age, castration, and subsequent testosterone treatment do not affect the fiber number, the filament lattice, and the size of the filaments and myonuclei. The fibers in intact growing males increase in width and length. The number of myonuclei rises, although relatively slower than the amount of contractile material.Castration, performed at six weeks, partially suppresses fiber growth. The increase of mean fiber width is more strongly inhibited than that of fiber length. Myonuclear multiplication is almost completely arrested in castrates, and the amount of contractile material per myonucleus is lower than in intact males of equal age.Testosterone, administered at about two months following orchidectomy, highly accelerates the transversal fiber growth, but fiber length is not significantly influenced. Between the fourth and seventh day of treatment a marked increase in myonuclear number occurs.Analysis of the frequency distribution of the individual fiber widths, which is logarithmic-normal in intact males, revealed that the hormonal influence on the net result of protein anabolism and catabolism markedly differs in the various fibers of a single muscle.With the technical assistance of Tineke J. Hoogenboezem.     

Growth hormone/IGF-I and/or resistive exercise maintains myonuclear number in hindlimb unweighted muscles

Allen, David L., Jon K. Linderman, Roland R. Roy, Richard E. Grindeland, Venkat Mukku, and V. Reggie Edgerton. Growth hormone/IGF-I and/or resistive exercise maintains myonuclearnumber in hindlimb unweighted muscles. J. Appl.Physiol. 83(5): 1857-1861, 1997.In the presentstudy of rats, we examined the role, during 2 wk ofhindlimb suspension, of growth hormone/insulin-like growth factor I(GH/IGF-I) administration and/or brief bouts of resistance exercise in ameliorating the loss of myonuclei in fibers of the soleusmuscle that express type I myosin heavy chain. Hindlimb suspensionresulted in a significant decrease in mean soleus wet weight that wasattenuated either by exercise alone or by exercise plus GH/IGF-Itreatment but was not attenuated by hormonal treatment alone. Both meanmyonuclear number and mean fiber cross-sectional area (CSA) of fibersexpressing type I myosin heavy chain decreased after 2 wk of suspensioncompared with control (134 vs. 162 myonuclei/mm and 917 vs. 2,076 µm², respectively). NeitherGH/IGF-I treatment nor exercise alone affected myonuclear number orfiber CSA, but the combination of exercise and growth-factor treatmentattenuated the decrease in both variables. A significant correlationwas found between mean myonuclear number and mean CSA across allgroups. Thus GH/IGF-I administration and brief bouts of muscle loadinghad an interactive effect in attenuating the loss of myonuclei inducedby chronic unloading.

     

Myonuclear domain and myosin phenotype in human soleus after bed rest with or without loading.

After 2 or 4 mo of bed rest (6 degrees head-down tilt) and 1 mo of ambulation, there was a tendency toward a higher percentage of fibers expressing fast myosin heavy chain (MHC) isoforms and a de novo appearance of fibers coexpressing type I+IIa+IIx and IIa+IIx MHC in human soleus fibers. After 2 and 4 mo of bed rest, the mean size of type I fibers decreased by 12 (P > 0.05) and 39%, respectively. Because myonuclear number/mm of fiber length was unchanged, myonuclear domain was smaller after bed rest than before. The mean size and myonuclear domain of type I fibers were largest after 1 mo of recovery. The effects of wearing an antigravity device (Penguin suit), which had a modest but continuous resistance at the knee and ankle (Penguin-1) or knee resistance without loading on the ankle (Penguin-2), for 10 consecutive h/day were determined during 2 mo of bed rest. Mean fiber sizes in Penguin-1, but not Penguin-2, group were maintained at or above pre-bed-rest levels, whereas neither group showed phenotype changes. Myonuclear domain in type I fibers was larger in Penguin-1 and smaller in Penguin-2 group post- compared with pre-bed rest, indicating that a single daily 10-h bout of modest muscle loading can prevent bed-rest-induced soleus fiber atrophy but has minimal effect on myosin phenotype. The specific adaptive cellular strategies involved may be a function of the duration and magnitude of the adaptive stimulus as well as the immediate activity history of the fiber before the newly changed functional demands.     

Effects of aging and gender on the spatial organization of nuclei in single human skeletal muscle cells

The skeletal muscle fibre is a syncitium where each myonucleus regulates the gene products in a finite volume of the cytoplasm, i.e., the myonuclear domain (MND). We analysed aging‐ and gender‐related effects on myonuclei organization and the MND size in single muscle fibres from six young (21–31 years) and nine old men (72–96 years), and from six young (24–32 years) and nine old women (65–96 years), using a novel image analysis algorithm applied to confocal images. Muscle fibres were classified according to myosin heavy chain (MyHC) isoform expression. Our image analysis algorithm was effective in determining the spatial organization of myonuclei and the distribution of individual MNDs along the single fibre segments. Significant linear relations were observed between MND size and fibre size, irrespective age, gender and MyHC isoform expression. The spatial organization of individual myonuclei, calculated as the distribution of nearest neighbour distances in 3D, and MND size were affected in old age, but changes were dependent on MyHC isoform expression. In type I muscle fibres, average NN‐values were lower and showed an increased variability in old age, reflecting an aggregation of myonuclei in old age. Average MND size did not change in old age, but there was an increased MND size variability. In type IIa fibres, average NN‐values and MND sizes were lower in old age, reflecting the smaller size of these muscle fibres in old age. It is suggested that these changes have a significant impact on protein synthesis and degradation during the aging process.     

Role of gravity in mammalian development: effects of hypergravity and/or microgravity on the development of skeletal muscles.

Effects of hindlimb suspension or exposure to 2-G between postnatal day 4 and month 3 and of 3-month recovery at 1-G environment on the characteristics of rat hindlimb muscles were studied. Pronounced growth inhibition was induced by unloading, but not by 2-G loading. It is suggested that the development and/or differentiation of soleus muscle fibers are closely associated with gravitational loading. The data indicated that gravitational unloading during postnatal development inhibits the myonuclear accretion in accordance with subnormal numbers of both mitotic active and quiescent satellite cells. Even though the fiber formation and longitudinal fiber growth were not influenced, cross-sectional growth of muscle fibers was also inhibited in association with lesser myonuclear domain and DNA content per unit volume of myonucleus. Unloading-related inhibition was generally normalized following the recovery.     

Exercise-enhanced satellite cell proliferation and new myonuclear accretion in rat skeletal muscle.

The effects of increased functional loading on early cellular regenerative events after exercise-induced injury in adult skeletal muscle were examined with the use of in vivo labeling of replicating myofiber nuclei and immunocyto- and histochemical techniques. Satellite cell proliferation in the soleus (Sol) of nonexercised rats (0.4 +/- 0.2% of fibers) was unchanged after an initial bout of declined treadmill exercise but was elevated after two (1.0 +/- 0.2%, P < or = 0.01), but not four or seven, daily bouts of the same task. Myonuclei produced over the 7-day period comprised 0.9-1.9% of myonuclei in isolated fibers of Sol, tibialis anterior, and vastus intermedius of nonexercised rats. The accretion of new myonuclei was enhanced (P < or = 0.05) in Sol and vastus intermedius by the initial exercise followed by normal activity (to 3.1-3.4% of myonuclei) and more so by continued daily exercise (4.2-5.3%). Observed coincident with a lower incidence of histological fiber injury and unchanged fiber diameter and myonuclei per millimeter, the greater new myonuclear accretion induced by continued muscle loading may contribute to an enhanced fiber repair and regeneration after exercise-induced injury.     

The nuclear-cytoplasmic relationship in 'mosaic' skeletal muscle fibers from mouse chimaeras

In mouse chimaeras, individual skeletal muscle fibers typically contain populations of myonuclei derived from both cell lines. This 'mosaic' circumstance has provided an opportunity to investigate directly whether the mammalian myofiber syncytium is functionally subdivided into territories, each preferentially influenced by products encoded by the local myonucleus, or whether the multiple nuclei direct the synthesis of products that achieve a uniform distribution throughout the fiber. Chimaeras were produced in which one cell line was derived from an embryo homozygous for gpi-1a, whereas the other was homozygous for the gpi-1b; each allele specifies electrophoretically distinguishable isozymes of the cytosolic enzyme glucosephosphate isomerase (GPI-1). Microtechniques capable of measuring the proportion of each isozyme expressed within small samples of individual muscle fibers have been established, permitting the comparison of the relative quantitative distributions of the GPI-1 isozyme types along the length of individual chimaera fibers. From individual mosaic fibers, all samples yielded identical isozyme profiles, demonstrating that GPI-1 is not sequestered adjacent to the nucleus directing its synthesis; rather, it achieves a homogeneous distribution throughout the mosaic syncytium. The GPI-1 gene locus encodes only the GPI-1 monomer, whereas the functional enzyme detected in our analysis is a dimer that results from the aggregation of monomers in the cytoplasm. The quantitative distribution of dimer types within each mosaic fiber was consistent with random aggregation amongst all monomers represented in the final isozyme pattern, a result requiring that monomers or earlier precursors were mixed in the myofiber cytoplasm prior to assembly of the enzymatically active dimer. Thus, both the final distribution of enzyme dimers within fibers and the patterns of monomer aggregation suggest that there are no subdivisions related to the spatial separation of the genotypically distinct myonuclei within mosaic muscle fibers.     

Essential role of satellite cells in the growth of rat soleus muscle fibers

Effects of gravitational loading or unloading on the growth-associated increase in the cross-sectional area and length of fibers, as well as the total fiber number, in soleus muscle were studied in rats. Furthermore, the roles of satellite cells and myonuclei in growth of these properties were also investigated. The hindlimb unloading by tail suspension was performed in newborn rats from postnatal day 4 to month 3 with or without 3-mo reloading. The morphological properties were measured in whole muscle and/or single fibers sampled from tendon to tendon. Growth-associated increases of soleus weight and fiber cross-sectional area in the unloaded group were approximately 68% and 69% less than the age-matched controls. However, the increases of number and length of fibers were not influenced by unloading. Growth-related increases of the number of quiescent satellite cells and myonuclei were inhibited by unloading. And the growth-related decrease of mitotically active satellite cells, seen even in controls (20%, P > 0.05), was also stimulated (80%). The increase of myonuclei during 3-mo unloading was only 40 times vs. 92 times in controls. Inhibited increase of myonuclear number was not related to apoptosis. The size of myonuclear domain in the unloaded group was less and that of single nuclei, which was decreased by growth, was larger than controls. However, all of these parameters, inhibited by unloading, were increased toward the control levels generally by reloading. It is suggested that the satellite cell-related stimulation in response to gravitational loading plays an essential role in the cross-sectional growth of soleus muscle fibers.