Increase In Fiber Cross-sectional Area Research Articles

Muscle Stem Cell Depletion Blunts Skeletal Muscle Adaptation to Weighted Wheel Running

Satellite cells, the resident muscle stem cell, are not required for short term skeletal muscle hypertrophy in mice when subjected to mechanical overload via synergist ablation, however, long term growth is blunted. This study aimed to further investigate the effect of satellite cell depletion on muscle adaptation using Progressive Weighted Wheel Running (PoWeR). We hypothesized that satellite cell depletion would reduce the adaptive response of skeletal muscle to the training stimulus. The Pax7‐DTA mouse model was used to deplete satellite cell content in adult skeletal muscle. PoWeR training involved adding weight to the running wheel of the mice incrementally over the 8‐week training period. Hindlimb muscles were excised at the 8‐week time point, and the plantaris was immunohistochemically analyzed. PoWeR induced an oxidative shift in fiber type distribution and increased muscle fiber cross sectional area (CSA). However, this increase in fiber CSA was blunted in mice without satellite cells. To investigate the blunted growth in satellite cell‐depleted mice, the plantaris was analyzed for extracellular matrix content and capillarization. We found that neither extracellular matrix buildup nor capillary density differed between satellite cell replete and depleted groups following PoWeR. These findings suggest that loss of satellite cells reduces the capacity for hypertrophy in response to PoWeR, but this was not due to excessive extracellular matrix accumulation. Further, satellite cells were not required for a robust fiber type shift in response to PoWeR training.Support or Funding InformationThis work was funded by the American Physiological Society’s UGSRF program and a grant from the National Institute of Health to Dr. Charlotte Peterson and Dr. John McCarthy (AR060701).

MiR-29c improves skeletal muscle mass and function throughout myocyte proliferation and differentiation and by repressing atrophy-related genes.

AimTo identify microRNAs (miRs) involved in the regulation of skeletal muscle mass. For that purpose, we have initially utilized an in silico analysis, resulting in the identification of miR‐29c as a positive regulator of muscle mass.MethodsmiR‐29c was electrotransferred to the tibialis anterior to address its morphometric and functional properties and to determine the level of satellite cell proliferation and differentiation. qPCR was used to investigate the effect of miR‐29c overexpression on trophicity‐related genes. C2C12 cells were used to determine the impact of miR‐29c on myogenesis and a luciferase reporter assay was used to evaluate the ability of miR‐29c to bind to the MuRF1 3′UTR.ResultsThe overexpression of miR‐29c in the tibialis anterior increased muscle mass by 40%, with a corresponding increase in fibre cross‐sectional area and force and a 30% increase in length. In addition, satellite cell proliferation and differentiation were increased. In C2C12 cells, miR‐29c oligonucleotides caused increased levels of differentiation, as evidenced by an increase in eMHC immunostaining and the myotube fusion index. Accordingly, the mRNA levels of myogenic markers were also increased. Mechanistically, the overexpression of miR‐29c inhibited the expression of the muscle atrophic factors MuRF1, Atrogin‐1 and HDAC4. For the key atrogene MuRF1, we found that miR‐29c can bind to its 3′UTR to mediate repression.ConclusionsThe results herein suggest that miR‐29c can improve skeletal muscle size and function by stimulating satellite cell proliferation and repressing atrophy‐related genes. Taken together, our results indicate that miR‐29c might be useful as a future therapeutic device in diseases involving decreased skeletal muscle mass.

Deletion of p16 expressing senescent cells improves muscle mass and age‐dependent outcomes in multiple organ systems

BackgroundCellular senescence is defined as permanent cell cycle arrest due to upregulation of cyclin dependent kinases such as p16. Senescent cells accumulate with age and can contribute to aging related pathologies. Recently, deletion of p16 expressing senescent cells in vivo has been shown to increase healthspan and lifespan, and ameliorate some tissue specific pathologies in mice. However, it is unknown whether cellular senescence plays a role in age associated declines in skeletal muscle function. We hypothesized that deletion of p16 expressing senescent cells will preserve age‐induced declines in muscle function and mass.MethodsTo eliminate senescent cells in vivo, we utilized p16‐3MR transgenic mice in which administration of the drug ganciclovir will induce cell death in p16 expressing cells. Mice were treated with 25 mg/kg ganciclovir or saline (control) intraperitoneally for 5 days at monthly intervals from 20–26 months of age. Animals were sacrificed at 26 months of age, muscles were weighed, and gastrocnemius muscles were tested for force production in vivo, while extensor digitorum longus and soleus muscles were tested in vitro. Muscle fiber cross sectional areas and fiber type were determined by immunohistochemistry. Additional organs were snap frozen for downstream protein, cytokine, and RNA analyses. mRNA expression was measured using qPCR.ResultsMice that were treated with ganciclovir lived longer and had a lower incidence of cancer than control mice. Muscle masses of the soleus, tibialis anterior and gastrocnemius were greater in ganciclovir‐treated mice compared with controls. The higher muscle mass in treated mice was not associated with any shift in fiber type distribution, increase in fiber cross‐sectional area, or any expansion of the volume of extracellular matrix, suggesting the higher muscle mass was due to preservation of fiber number. There was no effect of p16 deletion on muscle force production. However, upon examining muscle, spinal cord, serum and lung tissue, p16 deletion impacted age‐dependent changes in gene expression and cytokines.ConclusionsDifferential effects of senescent cell deletion on skeletal muscle mass and function suggests the role of senescent cells in skeletal muscle aging may be context specific. Further studies are needed to more thoroughly determine tissue‐specific benefits, but our work suggests that deletion of senescent cells even late in life may provide protection against global aging processes.Support or Funding InformationAG051442K12GM111725This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Muscle Fiber Hypertrophy and Myonuclei Addition: A Systematic Review and Meta-analysis.

The myonuclear domain theory postulates that myonuclei are added to muscle fibers when increases in fiber cross-sectional area (i.e., hypertrophy) are ≥26%. However, recent studies have reported increased myonuclear content with lower levels (e.g., 12%) of muscle fiber hypertrophy. This study aimed to determine whether a muscle fiber hypertrophy "threshold" is required to drive the addition of new myonuclei to existing muscle fibers. Studies of resistance training endurance training with or without nutrient (i.e., protein) supplementation and steroid administration with measures of muscle fiber hypertrophy and myonuclei number as primary or secondary outcomes were considered. Twenty-seven studies incorporating 62 treatment groups and 903 subjects fulfilled the inclusion criteria and were included in the analyses. Muscle fiber hypertrophy of ≤10% induces increases in myonuclear content, although a significantly higher number of myonuclei are observed when muscle hypertrophy is ~22%. Additional analyses showed that age, sex, and muscle fiber type do not influence muscle fiber hypertrophy or myonuclei addition. Although a more consistent myonuclei addition occurs when muscle fiber hypertrophy is >22%, our results challenge the concept of a muscle hypertrophy threshold as significant myonuclei addition occurs with lower muscle hypertrophy (i.e., <10%).

Swimming-induced exercise promotes hypertrophy and vascularization of fast skeletal muscle fibres and activation of myogenic and angiogenic transcriptional programs in adult zebrafish.

BackgroundThe adult skeletal muscle is a plastic tissue with a remarkable ability to adapt to different levels of activity by altering its excitability, its contractile and metabolic phenotype and its mass. We previously reported on the potential of adult zebrafish as a tractable experimental model for exercise physiology, established its optimal swimming speed and showed that swimming-induced contractile activity potentiated somatic growth. Given that the underlying exercise-induced transcriptional mechanisms regulating muscle mass in vertebrates are not fully understood, here we investigated the cellular and molecular adaptive mechanisms taking place in fast skeletal muscle of adult zebrafish in response to swimming.ResultsFish were trained at low swimming speed (0.1 m/s; non-exercised) or at their optimal swimming speed (0.4 m/s; exercised). A significant increase in fibre cross-sectional area (1.290 ± 88 vs. 1.665 ± 106 μm2) and vascularization (298 ± 23 vs. 458 ± 38 capillaries/mm2) was found in exercised over non-exercised fish. Gene expression profiling by microarray analysis evidenced the activation of a series of complex transcriptional networks of extracellular and intracellular signaling molecules and pathways involved in the regulation of muscle mass (e.g. IGF-1/PI3K/mTOR, BMP, MSTN), myogenesis and satellite cell activation (e.g. PAX3, FGF, Notch, Wnt, MEF2, Hh, EphrinB2) and angiogenesis (e.g. VEGF, HIF, Notch, EphrinB2, KLF2), some of which had not been previously associated with exercise-induced contractile activity.ConclusionsThe results from the present study show that exercise-induced contractile activity in adult zebrafish promotes a coordinated adaptive response in fast muscle that leads to increased muscle mass by hypertrophy and increased vascularization by angiogenesis. We propose that these phenotypic adaptations are the result of extensive transcriptional changes induced by exercise. Analysis of the transcriptional networks that are activated in response to exercise in the adult zebrafish fast muscle resulted in the identification of key signaling pathways and factors for the regulation of skeletal muscle mass, myogenesis and angiogenesis that have been remarkably conserved during evolution from fish to mammals. These results further support the validity of the adult zebrafish as an exercise model to decipher the complex molecular and cellular mechanisms governing skeletal muscle mass and function in vertebrates.Electronic supplementary materialThe online version of this article (doi:10.1186/1471-2164-15-1136) contains supplementary material, which is available to authorized users.

Human slow troponin T (TNNT1) pre-mRNA alternative splicing is an indicator of skeletal muscle response to resistance exercise in older adults.

Slow skeletal muscle troponin T (TNNT1) pre-messenger RNA alternative splicing (AS) provides transcript diversity and increases the variety of proteins the gene encodes. Here, we identified three major TNNT1 splicing patterns (AS1-3), quantified their expression in the vastus lateralis muscle of older adults, and demonstrated that resistance training modifies their relative abundance; specifically, upregulating AS1 and downregulating AS2 and AS3. In addition, abundance of TNNT1 AS2 correlated negatively with single muscle fiber-specific force after resistance training, while abundance of AS1 correlated negatively with V max. We propose that TNNT1 AS1, AS2 and the AS1/AS2 ratio are potential quantitative biomarkers of skeletal muscle adaptation to resistance training in older adults, and that their profile reflects enhanced single fiber muscle force in the absence of significant increases in fiber cross-sectional area.

A cellular memory mechanism aids overload hypertrophy in muscle long after an episodic exposure to anabolic steroids

Previous strength training with or without the use of anabolic steroids facilitates subsequent re-acquisition of muscle mass even after long intervening periods of inactivity. Based on in vivo and ex vivo microscopy we here propose a cellular memory mechanism residing in the muscle cells. Female mice were treated with testosterone propionate for 14 days, inducing a 66% increase in the number of myonuclei and a 77% increase in fibre cross-sectional area. Three weeks after removing the drug, fibre size was decreased to the same level as in sham treated animals, but the number of nuclei remained elevated for at least 3 months (>10% of the mouse lifespan). At this time, when the myonuclei-rich muscles were exposed to overload-exercise for 6 days, the fibre cross-sectional area increased by 31% while control muscles did not grow significantly. We suggest that the lasting, elevated number of myonuclei constitutes a cellular memory facilitating subsequent muscle overload hypertrophy. Our findings might have consequences for the exclusion time of doping offenders. Since the ability to generate new myonuclei is impaired in the elderly our data also invites speculation that it might be beneficial to perform strength training when young in order to benefit in senescence.

Predicted high-performing piglets exhibit more and larger skeletal muscle fibers1

Postnatal (muscle) growth potential in pigs depends on the total number and hypertrophy of myofibers in skeletal muscle tissue. In a previous study an algorithm was developed to predict piglet BW at the end of the nursery period (10 wk of age) on the basis of BW at birth, at weaning, and at 6 wk of age. The objective of this study was to determine whether the differences in growth performance between poor (PP) and high (HP) performing piglets could be the result of different skeletal muscle properties. Therefore, from a total of 368 piglets (offspring from Hypor sows bred to TOPIGS sires) 2 groups with a divergent growth performance were selected at 6 wk of age: HP (n = 20, predicted BW at 10 wk of age 26.8-30.9 kg) and PP (n = 20, predicted BW at 10 wk of age 16.0-22.9 kg). Piglets were euthanized at 10 wk of age, and samples of the semitendinosus muscle (STN) were collected for histochemistry and gene expression analysis using quantitative PCR (qPCR). At 10 wk of age, realized BW did not differ from predicted BW in either group (P > 0.880). The HP piglets exhibited greater ADG and ADFI from 6 to 10 wk and greater BW at birth and 6 and 10 wk of age (P ≤ 0.002) compared with the PP piglets, whereas G:F ratio was similar (P = 0.417). Superior growth performance of HP piglets was associated with a 1.27-fold higher IGF1 plasma concentration at 10 wk compared with the PP piglets (P = 0.044). The greater weight and muscle cross-sectional area of STN in HP piglets was due to a 1.20-fold increase in total muscle fiber number (TFN; P = 0.009) and 1.34-fold increase in fiber cross-sectional area (FCSA; P = 0.004) compared with the PP piglets. The number of myonuclei per red and intermediate fiber was greater in HP piglets (P ≤ 0.097), but the nucleus-to-cytoplasm ratio was unaffected by the performance group (P = 0.861). The mRNA expression of proliferating cell nuclear antigen (PCNA), paired box 7 (PAX7), myogenic factor 5 (MYF5), and myogenic differentiation factor (MYOD) did not differ between groups (P ≥ 0.327). However, IGF2-specific mRNA expression was numerically higher in the HP piglets (P = 0.101). The greater myofiber number, the higher degree of myofiber hypertrophy, and the increased muscular mRNA expression of IGF2 indicate that HP piglets exhibit a greater capacity for lean accretion and may grow faster until market weight. In summary, pigs that were selected for predicted high BW at 10 wk of age using a complex selection model had a superior muscularity in terms of greater TFN and FCSA, which may be of advantage for lean mass accretion in later life and for meat quality.

Effects of a high fat diet and Metformin on skeletal muscle membrane proteins and fiber size in young mice

A prolonged high fat diet (HFD) is known to impair insulin signaling in skeletal muscle. Specifically, HFD reduces caveolin‐3, Akt activation, and GLUT‐4 translocation to the membrane. Type II diabetes results in the loss of sarcolemmal proteins of the dystrophin‐glycoprotein complex (DGC), particularly dystrophin and nNOS. However, the effect of HFD on DGC proteins is unknown. We hypothesized that HFD would cause translocation of DGC proteins away from the membrane. We further postulated that Metformin, an oral anti‐diabetic drug, would alleviate translocation of DGC proteins. To test our hypotheses, 8 week old C57Bl‐6 mice were fed a high fat diet for 14 weeks (n=8). After 9 weeks, mice were also given either PBS (n=4) or Metformin (n=4) for the remaining 5 weeks. Soleus muscle fiber cross sectional area (CSA) and DGC protein expression were quantified using immunohistochemistry assays. Surprisingly, we found a marked increase in fiber CSA that was normalized by Metformin. HFD resulted in a small decrease of dystrophin membrane expression with no change in nNOS, while the Metformin group expressed normal levels. These findings indicate that the DGC may undergo only minor functional changes in young mice fed a short term HFD, which are similar findings seen with muscle fiber hypertrophy. We postulate that intact DGC proteins may help preserve insulin signaling during short term HFD. Our future research will therefore focus on understanding how insulin signaling (e.g., IRS‐1, mTOR) might be preserved, and the point at which intact muscle insulin pathways in young mice become perturbed with prolonged HFD. This work was supported in part by ADA grant 1–10‐JF‐54 and AHA 12BGIA9050003 (to C.W).

Reduced Ribosomal Protein S6 Phosphorylation After Progressive Resistance Exercise in Growing Adolescent Rats

The purpose of this study was to investigate moderate intensity progressive resistance exercise (PRE) in growing adolescent rats and its effect on muscle hypertrophy (defined as an increase in fiber cross-sectional area [CSA]). We hypothesized that in adolescent animals moderate intensity PRE would increase (a) fiber CSA; (b) myosin heavy chain (MyHC) content; and (c) expression and phosphorylation of cell signaling molecules involved in translational regulation, compared with that in age-matched sedentary (SED) controls. In the PRE group, 3-week-old male rats were trained to climb a vertical ladder as a mode of PRE training such that by 10 weeks all animals in the PRE group had progressed to carry an additional 80% of their body weight per climb. In agreement with our hypotheses, we observed that 10 weeks of moderate PRE in adolescent animals was sufficient to increase the CSA of muscle fibers and increase MyHC content. The average muscle fiber CSA increased by >10%, and the total MyHC content increased by 35% (p < 0.05) in the PRE group compared with that in the SED animals. Concurrently, we investigated sustained changes in the expression and phosphorylation of key signaling molecules that are previously identified regulators of hypertrophy in adult animal models. Contrary to our hypotheses, expression and phosphorylation of the translational regulators mammalian target of rapamycin and Akt were not increased in the PRE group. In addition, we observed that the ratio of phosphorylated-to-unphosphorylated ribosomal protein S6 (rpS6) was reduced over sixfold in PRE animals (p < 0.05) and that total rpS6 protein levels were unchanged between PRE and SED animals (p > 0.05). We conclude that moderate intensity PRE is sufficient to induce muscle hypertrophy in adolescent animals, whereas the signaling mechanisms associated with muscle hypertrophy may differ between growing adolescents and adults.

Local Injections of Adipose-Derived Mesenchymal Stem Cells Modulate Inflammation and Increase Angiogenesis Ameliorating the Dystrophic Phenotype in Dystrophin-Deficient Skeletal Muscle

The effects of adipose-derived mesenchymal stem cells (ADMSC) transplantation on degeneration, regeneration and skeletal muscle function were investigated in dystrophin-deficient mice (24-week-old). ADMSC transplantation improved muscle strength and, resistance to fatigue. An increase in fiber cross-sectional area and in the number of fibers with centralized nuclei and augment of myogenin content were observed. In ADMSC-treated muscles a decrease in muscle content of TNF-α, IL-6 and oxidative stress measured by Amplex(®) reagent were observed. The level of TGF-β1 was lowered whereas that of VEGF, IL-10 and IL-4 were increased by ADMSC treatment. An increase in markers of macrophage M1 (CD11 and F4-80) and a decrease in T lymphocyte marker (CD3) and arginase-1 were also observed in ADMSCs-treated dystrophic muscle. No change was observed in iNOS expression. Increased phosphorylation of Akt, p70S6k and 4E-BP1 was found in dystrophic muscles treated with ADMSC. These results suggest that ADMSC transplantation modulates inflammation and improves muscle tissue regeneration, ameliorating the dystrophic phenotype in dystrophin-deficient mice.

The time course of myonuclear accretion during hypertrophy in young adult and older rat plantaris muscle

To investigate whether accretion of myonuclei precedes or follows the increase in fibre cross-sectional area and whether this time course is affected by age, left plantaris muscle of 5- and 25-month-old male Wistar rats was overloaded by denervation of its synergists for 1, 2 or 4 weeks. Contralateral plantaris muscle served as control. Myonuclei were counted in haematoxylin-stained cross-sections. While hypertrophy (33% in young adult) became significant after 2 weeks overload (p<0.01), the myonuclear number was increased only at 4 weeks of overload (p<0.001). The time course and magnitude of hypertrophy were similar in young adult and older rats. Older muscles contained 26% more myonuclei per mm fibre length (p=0.001) and had a 10-fold larger proportion of central myonuclei (p<0.001) than young adult muscles. In conclusion, our data indicate that muscle fibre hypertrophy precedes the acquisition of new myonuclei and that the ability to develop hypertrophy is not attenuated or delayed in 25-month-old rat muscle.

Both type 1 and type 2a muscle fibers can respond to enzyme therapy in Pompe disease

Muscle weakness is the main symptom of Pompe disease, a lysosomal storage disorder for which major clinical benefits of enzyme replacement therapy (ERT) have been documented recently. Restoration of skeletal muscle function is a challenging goal. Type 2 muscle fibers of mice with Pompe disease have proven resistant to therapy. To investigate the response in humans, we studied muscle biopsies of a severely affected infant before and after 17 months of therapy. Type 1 and 2a fibers were marked with antibodies, and lysosome-associated membrane protein-1 (Lamp1) was used as the lysosomal membrane marker. Quantitative measurements showed a 2.5-3-fold increase of fiber cross-sectional area of both fiber types during therapy and normalization of the Lamp1 signal in approximately 95% of type 1 and approximately 75% of type 2a fibers. The response of both type 1 and 2a muscle fibers in the patient studied herein corroborates the beneficial effects of enzyme therapy seen in patients with Pompe disease.

Capillarization and vascular endothelial growth factor expression in hypertrophying anterior latissimus dorsi muscle of the Japanese quail.

Hypertrophy may increase the diffusion distances from capillaries to the interior of the muscle fibers. We hypothesized that capillary proliferation occurs during hypertrophy, which is accompanied by an up-regulation of vascular endothelial growth factor (VEGF). Hypertrophy of the left anterior latissimus dorsi muscle of Japanese quail (2-3 months old) was induced by 1-4 week stretch-overload. Capillarization was analyzed in cross-sections stained for ATPase. VEGF expression was determined with RT-PCR. Initially, hypertrophy was not accompanied by increases in fiber cross-sectional area (FCSA), but after 1 week the average FCSA did increase. The capillary to fiber ratio was decreased after 1 week, but returned to control values in subsequent weeks. This indicates that capillary proliferation occurred, because this model is characterized by extensive fiber proliferation. The absence of any significant change in VEGF mRNA levels indicates that increased levels of VEGF mRNA are not crucial for capillary proliferation during muscle hypertrophy.

β2-Agonist fenoterol has greater effects on contractile function of rat skeletal muscles than clenbuterol

Potential treatments for skeletal muscle wasting and weakness ideally possess both anabolic and ergogenic properties. Although the beta(2)-adrenoceptor agonist clenbuterol has well-characterized effects on skeletal muscle, less is known about the therapeutic potential of the related beta(2)-adrenoceptor agonist fenoterol. We administered an equimolar dose of either clenbuterol or fenoterol to rats for 4 wk to compare their effects on skeletal muscle and tested the hypothesis that fenoterol would produce more powerful anabolic and ergogenic effects. Clenbuterol treatment increased fiber cross-sectional area (CSA) by 6% and maximal isometric force (P(o)) by 20% in extensor digitorum longus (EDL) muscles, whereas fiber CSA in soleus muscles decreased by 3% and P(o) was unchanged, compared with untreated controls. In the EDL muscles, fenoterol treatment increased fiber CSA by 20% and increased P(o) by 12% above values achieved after clenbuterol treatment. Soleus muscles of fenoterol-treated rats exhibited a 13% increase in fiber CSA and a 17% increase in P(o) above that of clenbuterol-treated rats. These data indicate that fenoterol has greater effects on the functional properties of rat skeletal muscles than clenbuterol.

Regulation of fiber size, oxidative potential, and capillarization in human muscle by resistance exercise.

To examine the hypothesis that increases in fiber cross-sectional area mediated by high-resistance training (HRT) would result in a decrease in fiber capillarization and oxidative potential, regardless of fiber type, we studied six untrained males (maximum oxygen consumption, 45.6 +/- 2.3 ml. kg-1. min-1; mean +/- SE) participating in a 12-wk program designed to produce a progressive hypertrophy of the quadriceps muscle. The training sessions, which were conducted 3 times/wk, consisted of three sets of three exercises, each performed for 6-8 repetitions maximum (RM). Measurements of fiber-type distribution obtained from tissue extracted from the vastus lateralis at 0, 4, 7, and 12 wk indicated reductions (P < 0.05) in type IIB fibers (15.1 +/- 2.1% vs. 7.2 +/- 1.3%) by 4 wk in the absence of changes in the other fiber types (types I, IIA, and IIAB). Training culminated in a 17% increase (P < 0.05) in cross-sectional area by 12 wk with initial increases observed at 4 wk. The increase was independent of fiber type-specific changes. The number of capillaries in contact with each fiber type increased by 12 wk, whereas capillary contacts-to-fiber area ratios remained unchanged. In a defined cross-sectional field, HRT also increased the capillaries per fiber at 12 wk. Training failed to alter cellular oxidative potential, as measured by succinic dehydrogenase (SDH) activity, regardless of fiber type and training duration. It is concluded that modest hypertrophy induced by HRT does not compromise cellular tissue capillarization and oxidative potential regardless of fiber type.

Oxygen availability and temperature affect embryonic muscle development in Atlantic salmon ( Salmo salar L.)

In Atlantic salmon, muscle fibre hypertrophy (increase in fibre cross-sectional area) and muscle fibre hyperplasia (increase in muscle fibre number) are differentially affected by the incubation temperature during embryonic development. This affects muscle cellularity (number and mean cross-sectional area of the fibres) at hatching and can affect posthatch growth. When the egg capsule, an oxygen barrier, is removed temperature has a different effect on muscle cellularity. Oxygen levels may thus play a role in the muscle development of late embryos. The effect of different oxygen levels (50%, 100% and 150% air saturation level) and temperatures (5°C and 10°C) during late embryonic development on the muscle cellularity at a developmental stage just before hatching was therefore investigated. Fibre numbers at 100% air saturation were 17% lower at 10°C in the presence of the egg capsule (chorionated) whereas no difference between temperature regimes was found in its absence (dechorionated). Under low oxygen conditions a reduction in fibre numbers (–12%) was found at 5°C in chorionated embryos, whereas no effect was found in the dechorionated groups. Muscle fibre cross-sectional area was affected also by oxygen levels with lowered O 2 leading to a reduction. An increase in temperature also led to a reduction in fibre cross-sectional area. Furthermore, nuclear numbers in the presumptive white muscle were strongly affected. Removal of the egg capsule caused an increase in nuclear numbers at both temperatures (5°C: +40%; 10°C: +70%). The oxygen level only affected nuclear numbers at 10°C: an increase of 46% was found at the increased O 2 level in chorionated embryos, whereas a reduction by 26% was observed at the low O 2 level in the dechorionated group. O 2 levels therefore clearly affected muscle cellularity in a temperature-dependent way. Overall the results suggest that O 2 availability is a significant contributing factor to temperature effects during late muscle development in Atlantic salmon embryos.

The Ontogeny of Inter- and Intrasexual Vocal Muscle Dimorphisms in a Sound-Producing Fish

This study documents the development of inter- and intrasexual dimorphisms in the vocal ('sonic') muscles of the swimbladder in the plainfin midshipman fish, Porichthys notatus. Midshipman have two male reproductive morphs, Types I and II. Only Type I males build nests and generate mate calls to attract females; Type II males sneak or satellite spawn. Vocal muscles in the mate-calling Type I males were 25-fold larger in absolute size (sixfold larger when scaled to body size) compared with females or non-calling Type II males. Dimorphisms in muscle mass were correlated with dimorphisms in fiber number and diameter. Only nascent Type I males experienced a pre-maturational, fourfold increase of fiber number followed by a rapid, fivefold increase in fiber cross-sectional area at sexual maturity. Also specific to Type I males was a striking change in cell structure: the ratio of sarcoplasm to myofibril area increased fivefold. By contrast, Type II males and females matured without showing any of these changes in sonic muscle--the trajectories which described juvenile growth continued to hold through maturity and adulthood. The results indicate distinct, non-sequential, ontogenetic trajectories for Type I and Type II males. The origin of the Type II male morphotype is described as paedomorphosis by progenesis: size distributions of free-living animals suggest that Type II males can mature precocially as one-year-old fish, whereas Type I males defer reproduction until they are at least two years of age.

Volume density and distribution of mitochondria in myocardial growth and hypertrophy

The volume density and distribution of mitochondria relative to capillaries have been measured using the technique of point counting in concentric rings, on normal and hypertrophied guinea pig myocardium. Right ventricular hypertrophy was produced by hypobaric hypoxia. In control guinea pigs right ventricular weight increased linearly with body weight. In control and hypoxic guinea pigs fiber cross sectional area (FCSA) and ventricular weight were related by a single straight line. Capillary density decreased hyperbolically with FCSA in both control and hypoxic animals. Capillary density was significantly higher ( P < 0.001) in the right ventricles of the hypoxic animals than in controls when the fibers were small but this difference disappeared as the fibers hypertrophied beyond FCSA values of 260 μm 2. Volume density of mitochondria reached a peak (29–35%) close to the capillary but then decreased significantly in the spaces farthest from the capillary (mean 25.2%). There was no significant difference in peak mitochondrial volume density in controls versus hypoxic animals. There was thus an increase in the total volume of mitochondria in these myofibers that was in direct proportion to the increases in FCSA and heart volume. The highest volume density of mitochondria was found at a distance that is approximately 15% of the total distance over which O 2 must diffuse in myocardial tissue and the peak volume density of mitochondria was thus a function of capillary density. The speculation is presented that the distribution of mitochondria may be related both to oxygen supply to the mitochondria and to the movement of high energy phosphate compounds out of mitochondria for use by myofibrils.

Volume density and distribution of mitochondria in myocardial growth and hypertrophy

The volume density and distribution of mitochondria relative to capillaries have been measured using the technique of point counting in concentric rings, on normal and hypertrophied guinea pig myocardium. Right ventricular hypertrophy was produced by hypobaric hypoxia. In control guinea pigs right ventricular weight increased linearly with body weight. In control and hypoxic guinea pigs fiber cross sectional area (FCSA) and ventricular weight were related by a single straight line. Capillary density decreased hyperbolically with FCSA in both control and hypoxic animals. Capillary density was significantly higher ( P < 0.001) in the right ventricles of the hypoxic animals than in controls when the fibers were small but this difference disappeared as the fibers hypertrophied beyond FCSA values of 260 μm 2. Volume density of mitochondria reached a peak (29–35%) close to the capillary but then decreased significantly in the spaces farthest from the capillary (mean 25.2%). There was no significant difference in peak mitochondrial volume density in controls versus hypoxic animals. There was thus an increase in the total volume of mitochondria in these myofibers that was in direct proportion to the increases in FCSA and heart volume. The highest volume density of mitochondria was found at a distance that is approximately 15% of the total distance over which O 2 must diffuse in myocardial tissue and the peak volume density of mitochondria was thus a function of capillary density. The speculation is presented that the distribution of mitochondria may be related both to oxygen supply to the mitochondria and to the movement of high energy phosphate compounds out of mitochondria for use by myofibrils.