Figure 1 - available via license: Creative Commons Attribution 4.0 International
Content may be subject to copyright.
![(A) Illustration of skeletal muscle structure copied with permission under the Creative Commons Attribution 4.0 International license and adapted for this review, available online: https://openstax.org/books/anatomy-and-physiology/pages/10-2-skeletal-muscle (accessed on 6 January 2020) [28]. (B) Cross-section of a mouse plantaris muscle that was subjected to](publication/342856222/figure/fig1/AS:911929656750081@1594432381323/A-Illustration-of-skeletal-muscle-structure-copied-with-permission-under-the-Creative.png)
(A) Illustration of skeletal muscle structure copied with permission under the Creative Commons Attribution 4.0 International license and adapted for this review, available online: https://openstax.org/books/anatomy-and-physiology/pages/10-2-skeletal-muscle (accessed on 6 January 2020) [28]. (B) Cross-section of a mouse plantaris muscle that was subjected to
Source publication
The maintenance of skeletal muscle mass plays a critical role in health and quality of life. One of the most potent regulators of skeletal muscle mass is mechanical loading, and numerous studies have led to a reasonably clear understanding of the macroscopic and microscopic changes that occur when the mechanical environment is altered. For instance...
Contexts in source publication
Context 1
... the macroscopic level, it can be noted that skeletal muscles are connected to bones via tendinous attachments and enact their contractile function by providing movement and articulation of the skeletal system. Moreover, as illustrated in Figure 1, skeletal muscles are surrounded by an outer layer of connective tissue called the epimysium, and underneath the epimysium are bundles of myofibers (i.e., fascicles) that are surrounded by another layer of connective tissue called the perimysium [24]. In most skeletal muscles, the fascicles, and their associated myofibers, are not directly aligned with the longitudinal axis of the muscle, but instead are offset at an angle called the pennation angle ( Figure 2) [27]. ...
Context 2
... Higher magnification of the boxed region in D reveals the presence of the thick and thin myofilaments. At the microscopic level, a cross-section of skeletal muscle will reveal the presence of individual myofibers ( Figure 1A,B). The myofibers are multinucleated cells that are encased by a layer of connective tissue called the endomysium, and they are surrounded by interstitial cells such as fibroblasts, immune cells, pericytes and fibro-adipogenic progenitors ( Figure 1A-C) [29,30]. ...
Context 3
... the microscopic level, a cross-section of skeletal muscle will reveal the presence of individual myofibers ( Figure 1A,B). The myofibers are multinucleated cells that are encased by a layer of connective tissue called the endomysium, and they are surrounded by interstitial cells such as fibroblasts, immune cells, pericytes and fibro-adipogenic progenitors ( Figure 1A-C) [29,30]. Furthermore, another important class of cells, called satellite cells, resides between the endomysium and the plasma membrane of the myofibers (i.e., the sarcolemma) [31]. ...
Context 4
... gelatinous sarcoplasm contains the primary ultrastructural elements of the myofiber, and as illustrated in Figure 1, an examination at the ultrastructural level reveals that ≈80% of the sarcoplasm is filled with an in-parallel array of rod-like structures called myofibrils [32][33][34][35][36]. The myofibrils are composed of a long in-series array of force-generating elements called sarcomeres and are surrounded by a mitochondrial reticulum and a membranous structure called the sarcoplasmic reticulum [37,38]. ...
Context 5
... in this study, rat diaphragm muscles were incubated with 3 H-leucine to label newly synthesized proteins, and then electron microscope autoradiography was used to identify the location of the newly synthesized proteins [192]. As shown in Figure 10A, the location of the newly synthesized proteins was indicated by the presence of relatively large (≈300 nm) electron dense grains. The quantitative results from this study are shown in Figure 10B with the bars indicating how frequently the center of the grains appeared at various distances from the periphery of the myofibril, and the green highlighted curve illustrating the theoretical distribution of the grains that would be expected if the myofibrils were labeled exclusively at the periphery. ...
Context 6
... shown in Figure 10A, the location of the newly synthesized proteins was indicated by the presence of relatively large (≈300 nm) electron dense grains. The quantitative results from this study are shown in Figure 10B with the bars indicating how frequently the center of the grains appeared at various distances from the periphery of the myofibril, and the green highlighted curve illustrating the theoretical distribution of the grains that would be expected if the myofibrils were labeled exclusively at the periphery. At first glance, the close match between the theoretical and observed values appears to provide compelling support for the conclusion that new myofilaments are added to the periphery of the myofibrils [192]. ...
Context 7
... first glance, the close match between the theoretical and observed values appears to provide compelling support for the conclusion that new myofilaments are added to the periphery of the myofibrils [192]. However, this evidence becomes less persuasive when one considers that ribosomes are typically localized in the intermyofibrillar space and many of these ribosomes appear in polysomal configurations which is indicative of active protein synthesis ( Figure 10C) [193,194]. This point leads us to question how well the data from Morkin (1970) would fit with a different hypothesis. ...
Context 8
... this case, the hypothesis was that the ribosomes in the intermyofibrillar space are actively engaged in the synthesis of new proteins. In Figure 10D-E, we have illustrated how well the data from Morkin (1970) fit with the theoretical distribution of the grains that would be expected if the myofibrils were labeled exclusively at the periphery, and compared that with the theoretical distribution of the grains that would be expected if newly synthesized proteins were located exclusively within the intermyofibrillar space. The key point from this illustration is that the data appears to be consistent with both theoretical distributions, and this is because the resolution provided by autoradiography simply does not allow for a clear distinction between the two possibilities. ...
Context 9
... limitations of the resolution that can be obtained with electron microscope autoradiography have been thoroughly described by Caro (1962) and Salpeter et al. (1969) [195,198]. Importantly, both of these studies demonstrate that under typical conditions, 50% of the grains will develop within ≈130 nm of the source and 95% of the grains will develop within ≈300 nm [195,198] (Figure 10F). This level of resolution would be outstanding if the goal was to identify the location of newly synthesized proteins within a myofiber (typical diameter of 25,000 nm), but it is far from ideal when the goal is to identify the location of newly synthesized proteins within a myofibril (typical diameter 850 nm). ...
Context 10
... instance, it is now possible to identify the location of newly synthesized proteins with immunological and click-chemistry-based technologies [199,200]. This is noteworthy because, as illustrated in Figure 10F, a typical immunoelectron microscopy-based approach will result in 100% of the signal appearing within 20 nm of the source, and the use of more advanced approaches (e.g., 1 nm gold conjugated Fab antibody fragments, or click-chemistry-based linkers) can allow for a resolution of less than 7 nm [197,[201][202][203]. Thus, although we still do not know whether mechanical load-induced hypertrophy of myofibers is driven by myofibril hypertrophy, or where new myofilaments get added during the process of myofibril hypertrophy, the technologies that are need to answer these fundamental questions are now within our reach. ...
Context 11
... on our collective view of the literature, we propose that both processes are involved, and can be explained by a model that we have defined as the "myofibril expansion cycle". Specifically, as illustrated in Figure 11, the myofibril expansion cycle begins with the deposition of new myofilaments around the periphery of the pre-existing myofibrils, and results in myofibril hypertrophy. Once the myofibrils reach a critical size, they split and subsequently form two smaller daughter myofibrils. ...
Citations
... The observation that specific types of exercise training can lead to increases in both MT and FL aligns with our mediation model. Furthermore, recent insights from a study by Hornberger et al. 47 offer a mechanistic explanation for the observed sex differences in MT and PA. According to their findings, mechanical loading induces changes in fascicle length and diameter, leading to alterations in whole-muscle CSA. ...
Muscle morphological architecture, a crucial determinant of muscle function, has fascinated researchers since the Renaissance. Imaging techniques enable the assessment of parameters such as muscle thickness (MT), pennation angle (PA), and fascicle length (FL), which may vary with growth, sex, and physical activity. Despite known interrelationships, robust mathematical models like causal mediation analysis have not been extensively applied to large population samples. We recruited 109 males and females, measuring knee flexor and extensor, and plantar flexor MT, PA, and FL using real-time ultrasound imaging at rest. A mixed-effects model explored sex, leg (dominant vs. non-dominant), and muscle region differences. Males exhibited greater MT in all muscles (0.1 to 2.1 cm, p < 0.01), with no sex differences in FL. Dominant legs showed greater rectus femoris (RF) MT (0.1 cm, p = 0.01) and PA (1.5°, p = 0.01), while vastus lateralis (VL) had greater FL (1.2 cm, p < 0.001) and PA (0.6°, p = 0.02). Regional differences were observed in VL, RF, and biceps femoris long head (BFlh). Causal mediation analyses highlighted MT’s influence on PA, mediated by FL. Moderated mediation occurred in BFlh, with FL differences. Gastrocnemius medialis and lateralis exhibited FL-mediated MT and PA relationships. This study unveils the intricate interplay of MT, FL, and PA in muscle architecture.
... Its clinical significance has even been formally challenged by Lieber and collaborators in a recent publication (Lieber 2022). The intricacy of all the interrelated architectural variables, thus, calls for cautious interpretations, as even decisive postulates seem controversial (Jorgenson et al. 2020;Lieber 2022;Sayed et al. 2023). An endless number of geometrical scenarios, relying on advanced models (Maxwell et al. 1974), have indeed been proposed in the literature (Jorgenson et al. 2020;Sayed et al. 2023). ...
... The intricacy of all the interrelated architectural variables, thus, calls for cautious interpretations, as even decisive postulates seem controversial (Jorgenson et al. 2020;Lieber 2022;Sayed et al. 2023). An endless number of geometrical scenarios, relying on advanced models (Maxwell et al. 1974), have indeed been proposed in the literature (Jorgenson et al. 2020;Sayed et al. 2023). To conclude on these architectural issues with a few perspectives, we might refer to the speculation of Kawakami and collaborators stating that architectural components, such as FL, might limit muscular hypertrophy (Kawakami et al. 2006). ...
Purpose
While muscle mass and skeletal muscle fibers phenotype have been shown atypical in constitutional thinness (CT), force production capacities and its architectural determinants have never been explored. The present study compared muscle functionality and architecture between participants with CT and their normal-weight (NW) counterparts.
Methods
Anthropometry, body composition (Dual-X-ray Absorptiometry), physical activity/sedentary behavior (ActiGraph wGT3X-BT), ultrasound recording of the Vastus Lateralis (2D-ultrasound system), and functional capacities at maximal isometric and isokinetic voluntary contractions (MVCISO and MVCCON) during knee extension (isokinetic dynamometer chair Biodex) have been measured in 18 women with CT (body mass index < 17.5 kg/m²) and 17 NW women.
Results
A lower fat-free mass (ES: −1.94, 95%CI: −2.76 to −1.11, p < 0.001), a higher sedentary time, and a trend for a lower time spent at low-intensity physical activity, were observed in CT vs NW participants. While absolute MVCISO, MVCCON, rate of torque development (RTD), and torque work were all markedly lower in CT, these differences disappeared when normalized to body or muscle mass. Muscle thickness and fascicle length were found lower in CT (ES: −1.29, 95%CI: −2.03 to −0.52, p < 0.001; and ES: −0.87, 95%CI: −1.58 to −0.15, p = 0.02, respectively), while pennation angle was found similar.
Conclusion
Despite lower absolute strength capacities observed in CT, present findings support the hypothesis of physiological adaptations to the low body and muscle mass than to some intrinsic contractile impairments. These results call for further studies exploring hypertrophy-targeted strategies in the management of CT.
... In this regard, the literature on fascicle angle parallels the literature on fascicle length: while LML-RT may enhance adaptations, data are inconsistent. Since increases in fascicle angle have been hypothesized to represent increases in radial hypertrophy (Jorgenson et al., 2020), these results suggest that LML-RT may lead to both greater increases in longitudinal hypertrophy as well as radial hypertrophy. ...
... One possible hypothesis for the reduced macroscopic muscle size in CP without a clear reduction in Fiber-CSA, could be that children with CP have fewer muscle fibers compared to TD children. A review on the mechanisms of muscle hypertrophy after training in healthy adults suggested that the increase in Belly-CSA in response to training results from fascicle growth due to fiber hypertrophy and/or due to an increase in the number of fibers per cross section, next to longitudinal fascicle growth (Jorgenson et al., 2020). Previous literature is characterised by inconsistent findings on the alterations of fascicle length in children with CP. ...
... In general, it is important to acknowledge that only few studies have investigated the microscopic muscle size properties and related structures (such as the satellite cells and components of the extracellular matrix) of the MG in young children with CP. Moreover, it has been suggested that muscle growth is characterized by an uneven growth distribution (Jorgenson et al., 2020), highlighting the importance of the biopsy site. ...
Children with spastic cerebral palsy (CP) are characterized by altered muscle growth, secondary to the pathological neural input to the muscular system, caused by the primary brain lesion. As a result, their medial gastrocnemius is commonly affected and is characterized by altered macro and microscopic muscular alterations. At the macroscopic level, the muscle volume (MV), anatomical cross-sectional area of the muscle belly (Belly-CSA), muscle belly length (ML), and the intrinsic muscle quality are reduced. At the microscopic level, the cross-sectional area of the fiber (Fiber-CSA) is characterized by an increased within-patient variability (coefficient of variation (CV)), the fiber type proportion is altered and capillarization is reduced. However, the relations between the muscular alterations at the macro- and microscopic level are not yet known. Therefore, this cross-sectional study integrated macro- and microscopic parameters of the medial gastrocnemius in one cohort of young ambulant children with CP and age-matched TD children, and explored how deficits in macroscopic muscle size are associated with alterations at the microscopic level. A group of 46 children with CP (median age 5.4 (3.3) years) and a control group of 34 TD children (median age 6.3 (3.4) years), who had data on microscopic muscular properties (defined through the histological analyses of muscle biopsies), as well as macroscopic muscle properties (defined by 3D freehand ultrasound) were included. We defined Pearson or Spearman correlations, depending on the data distribution. The macroscopic muscle size parameters (MV, Belly-CSA, ML) showed significant moderate correlations (0.504-0.592) with the microscopic average Fiber-CSA in TD and CP. To eliminate the common effect of anthropometric growth at the macro- as well as microscopic level, the data were expressed as deficits (i.e. z-scores from normative centile curves or means) or were normalized to body size parameters. A significant but low correlation was found between the z-scores of MV with the z-scores of the Fiber-CSA (r=0.420, p=0.006). The normalized muscle parameters also showed only low correlations between the macro- and microscopic muscle size parameters, namely between Belly-CSA and Fiber-CSA, both in the TD (r=0.408, p=0.023) and the CP (ρ=0.329, p=0.041) group. Explorations between macroscopic muscle parameters and other microscopic muscle parameters (capillary density, capillary to fiber ratio, and fiber type proportion) revealed no or only low correlations. These results highlighted the complexity of the interacting network of intrinsic muscle structures, with mainly low associations between the macro- and microstructural level, and it remains unclear how alterations in microscopic muscle structures contribute to the macroscopic muscle size deficits in children with CP.
... As muscle mass can also be influenced by edema, we measured type-specific cross-sectional fiber size 14 ( MOV, with no genotype effect. While increased fiber size did contribute to MOV-induced muscle growth, the contribution was mild compared to the increased fiber numbers present in cross-sections, which could technically be due to either muscle fiber hyperplasia or increased muscle fiber length, such that more fibers cross the muscle mid belly [30,31]. In either case, MOV-induced increases in cross sectional fiber number is likely to rely on MuSC fusion to support muscle growth, in line with a strong increase in the number of centrally nucleated fibers after 14 and 28 days MOV (Fig. S1A). ...
The oncogenic transcription factor Myc stimulates many growth processes including cell cycle progression and ribosome biogenesis. Myc expression is low in adult skeletal muscle, but is upregulated upon growth stimuli. Furthermore, muscle fiber Myc overexpression recapitulates many aspects of growth-related gene expression, suggesting Myc may mediate pro-growth responses to anabolic stimuli, such as exercise. Here, we tested this hypothesis by examining mouse models in which Myc was specifically eliminated or overexpressed in skeletal muscle fibers or muscle stem cells (MuSC). While muscle fiber Myc expression increased during muscle growth and Myc expression in MuSCs was required for successful muscle regeneration, muscle fiber Myc expression was dispensable for post-natal, mechanical overload or PKB/Akt-induced muscle growth in mice. Similarly, constitutive Myc expression did not promote skeletal muscle hypertrophy, but instead impaired muscle fiber structure and function within days. These data question the role of Myc in skeletal muscle growth.
... Skeletal muscle mass can be regulated by a variety of different stimuli with one of the most widely recognized being mechanical signals (Bodine, 2013;Adams and Bamman, 2012;Goldberg et al., 1975;Roberts et al., 2023). For instance, a plethora of studies have shown that an increase in mechanical loading, such as that which occurs during resistance exercise (RE), can induce radial growth of the muscle fibers (Conceição et al., 2018;Ema et al., 2016;Williams and Goldspink, 1971;Haun et al., 2019;Jorgenson et al., 2020). Surprisingly, however, the ultrastructural adaptations that drive this response have not been well defined (Haun et al., 2019;Jorgenson et al., 2020;Roberts et al., 2020). ...
... For instance, a plethora of studies have shown that an increase in mechanical loading, such as that which occurs during resistance exercise (RE), can induce radial growth of the muscle fibers (Conceição et al., 2018;Ema et al., 2016;Williams and Goldspink, 1971;Haun et al., 2019;Jorgenson et al., 2020). Surprisingly, however, the ultrastructural adaptations that drive this response have not been well defined (Haun et al., 2019;Jorgenson et al., 2020;Roberts et al., 2020). Indeed, a number of seemingly simple and fundamental important questions have not been answered. ...
... myofibril hypertrophy) and/or the number of myofibrils (i.e. myofibrillogenesis) has not been resolved (Jorgenson et al., 2020;Roberts et al., 2020;Wang et al., 1993;Ashmore and Summers, 1981). ...
An increase in mechanical loading, such as that which occurs during resistance exercise, induces radial growth of muscle fibers (i.e. an increase in cross-sectional area). Muscle fibers are largely composed of myofibrils, but whether radial growth is mediated by an increase in the size of the myofibrils (i.e. myofibril hypertrophy) and/or the number of myofibrils (i.e. myofibrillogenesis) is not known. Electron microscopy (EM) can provide images with the level of resolution that is needed to address this question, but the acquisition and subsequent analysis of EM images is a time- and cost-intensive process. To overcome this, we developed a novel method for visualizing myofibrils with a standard fluorescence microscope (fluorescence imaging of myofibrils with image deconvolution [FIM-ID]). Images from FIM-ID have a high degree of resolution and contrast, and these properties enabled us to develop pipelines for automated measurements of myofibril size and number. After extensively validating the automated measurements, we used both mouse and human models of increased mechanical loading to discover that the radial growth of muscle fibers is largely mediated by myofibrillogenesis. Collectively, the outcomes of this study offer insight into a fundamentally important topic in the field of muscle growth and provide future investigators with a time- and cost-effective means to study it.
... It is possible however that the increased cytoplamic γ-actin in D2-mdx+OVL mice contributed to the improved fragility since transgenic overexpression of cytoplamic γ-actin reduces fragility in mdx mice [4]. Finally, and in a manner always linked to hypertrophy, it is possible that the reduction in fragility in response to OVL is due to the increase in the ratio of fiber length to muscle length [35]. This increase would have the effect of reducing the percentage of lengthening at the fiber level, since we achieved a stretch of 10% of the length of the muscle. ...
Mechanical overloading (OVL) resulting from the ablation of muscle agonists, a supra-physiological model of resistance training, reduces skeletal muscle fragility, i.e. the immediate maximal force drop following lengthening contractions, and increases maximal force production, in mdx mice, a murine model of Duchene muscular dystrophy (DMD). Here, we further analyzed these beneficial effects of OVL by determining whether they were blocked by cyclosporin, an inhibitor of the calcineurin pathway, and whether there were also observed in the D2-mdx mice, a more severe murine DMD model. We found that cyclosporin did not block the beneficial effect of 1-month OVL on plantaris muscle fragility in mdx mice, nor did it limit the increases in maximal force and muscle weight (an index of hypertrophy). Fragility and maximal force were also ameliorated by OVL in the plantaris muscle of D2- mdx mice. In addition, OVL increased the expression of utrophin, cytoplamic γ-actin, MyoD, and p-Akt in the D2- mdx mice, proteins playing an important role in fragility, maximal force gain and muscle growth. In conclusion, OVL reduced fragility and increased maximal force in the more frequently used mild mdx model but also in D2- mdx mice, a severe model of DMD, closer to human physiopathology. Moreover, these beneficial effects of OVL did not seem to be related to the activation of the calcineurin pathway. Thus, this preclinical study suggests that resistance training could have a potential benefit in the improvement of the quality of life of DMD patients.
... There has been a renewed interest in examining myofibrillar adaptations to resistance training (Jorgenson et al., 2020;Roberts et al., 2020;Roberts et al., 2023). MacDougall et al. (MacDougall et al., 1982) used electron microscopy investigation to show that longer-term resistance training disproportionately increases sarcoplasmic spacing. ...
... It has been suggested that skeletal muscle hypertrophy induced by mechanical overload can be attributed mainly to the proportional increase in contractile and non-contractile components of myofibers (Jorgenson et al., 2020;Roberts et al., 2023). This phenomenon has been termed conventional hypertrophy (Roberts et al., 2020). ...
Blood flow restriction applied during low-load resistance training (LL-BFR) induces a similar increase in the cross-sectional area of muscle fibers (fCSA) compared to traditional high-load resistance training (HL-RT). However, it is unclear whether LL-BFR leads to differential changes in myofibrillar spacing in muscle fibers and/or extracellular area compared to HL-RT. Therefore, this study aimed to investigate whether the hypertrophy of type I and II fibers induced by LL-BFR or HL-RT is accompanied by differential changes in myofibrillar and non-myofibrillar areas. In addition, we examined if extracellular spacing was differentially affected between these two training protocols. Twenty recreationally active participants were assigned to LL-BFR or HL-RT groups and underwent a 6-week training program. Muscle biopsies were taken before and after the training period. The fCSA of type I and II fibers, the area occupied by myofibrillar and non-myofibrillar components, and extracellular spacing were analyzed using immunohistochemistry techniques. Despite the significant increase in type II and mean (type I + II) fCSA (p < 0.05), there were no significant changes in the proportionality of the myofibrillar and non-myofibrillar areas [∼86% and ∼14%, respectively (p > 0.05)], indicating that initial adaptations to LL-BFR are primarily characterized by conventional hypertrophy rather than disproportionate non-myofibrillar expansion. Additionally, extracellular spacing was not significantly altered between protocols. In summary, our study reveals that LL-BFR, like HL-RT, induces skeletal muscle hypertrophy with proportional changes in the areas occupied by myofibrillar, non-myofibrillar, and extracellular components.
... Skeletal muscle mass can be regulated by a variety of different stimuli with one of the most widely recognized being mechanical signals (Bodine, 2013;Adams and Bamman, 2012;Goldberg et al., 1975;Roberts et al., 2023). For instance, a plethora of studies have shown that an increase in mechanical loading, such as that which occurs during resistance exercise (RE), can induce radial growth of the muscle fibers (Conceição et al., 2018;Ema et al., 2016;Williams and Goldspink, 1971;Haun et al., 2019;Jorgenson et al., 2020). Surprisingly, however, the ultrastructural adaptations that drive this response have not been well defined (Haun et al., 2019;Jorgenson et al., 2020;Roberts et al., 2020). ...
... For instance, a plethora of studies have shown that an increase in mechanical loading, such as that which occurs during resistance exercise (RE), can induce radial growth of the muscle fibers (Conceição et al., 2018;Ema et al., 2016;Williams and Goldspink, 1971;Haun et al., 2019;Jorgenson et al., 2020). Surprisingly, however, the ultrastructural adaptations that drive this response have not been well defined (Haun et al., 2019;Jorgenson et al., 2020;Roberts et al., 2020). Indeed, a number of seemingly simple and fundamental important questions have not been answered. ...
... myofibril hypertrophy) and/or the number of myofibrils (i.e. myofibrillogenesis) has not been resolved (Jorgenson et al., 2020;Roberts et al., 2020;Wang et al., 1993;Ashmore and Summers, 1981). ...
An increase in mechanical loading, such as that which occurs during resistance exercise, induces radial growth of muscle fibers (i.e., an increase in cross-sectional area). Muscle fibers are largely composed of myofibrils, but whether radial growth is mediated by an increase in the size of the myofibrils (i.e., myofibril hypertrophy) and/or the number of myofibrils (i.e., myofibrillogenesis) is not known. Electron microscopy (EM) can provide images with the level of resolution that is needed to address this question, but the acquisition and subsequent analysis of EM images is a time- and cost-intensive process. To overcome this, we developed a novel method for visualizing myofibrils with a standard fluorescence microscope (FIM-ID). Images from FIM-ID have a high degree of resolution and contrast, and these properties enabled us to develop pipelines for automated measurements of myofibril size and number. After extensively validating the automated measurements, we used both mouse and human models of increased mechanical loading to discover that the radial growth of muscle fibers is largely mediated by myofibrillogenesis. Collectively, the outcomes of this study offer insight into a fundamentally important topic in the field of muscle growth and provide future investigators with a time- and cost-effective means to study it.
... Skeletal muscle mass can be regulated by a variety of different stimuli with one of the most widely recognized being mechanical signals (Bodine, 2013;Adams and Bamman, 2012;Goldberg et al., 1975;Roberts et al., 2023). For instance, a plethora of studies have shown that an increase in mechanical loading, such as that which occurs during resistance exercise (RE), can induce radial growth of the muscle fibers (Conceição et al., 2018;Ema et al., 2016;Williams and Goldspink, 1971;Haun et al., 2019;Jorgenson et al., 2020). Surprisingly, however, the ultrastructural adaptations that drive this response have not been well defined (Haun et al., 2019;Jorgenson et al., 2020;Roberts et al., 2020). ...
... For instance, a plethora of studies have shown that an increase in mechanical loading, such as that which occurs during resistance exercise (RE), can induce radial growth of the muscle fibers (Conceição et al., 2018;Ema et al., 2016;Williams and Goldspink, 1971;Haun et al., 2019;Jorgenson et al., 2020). Surprisingly, however, the ultrastructural adaptations that drive this response have not been well defined (Haun et al., 2019;Jorgenson et al., 2020;Roberts et al., 2020). Indeed, a number of seemingly simple and fundamental important questions have not been answered. ...
... myofibril hypertrophy) and/or the number of myofibrils (i.e. myofibrillogenesis) has not been resolved (Jorgenson et al., 2020;Roberts et al., 2020;Wang et al., 1993;Ashmore and Summers, 1981). ...
An increase in mechanical loading, such as that which occurs during resistance exercise, induces radial growth of muscle fibers (i.e., an increase in cross-sectional area). Muscle fibers are largely composed of myofibrils, but whether radial growth is mediated by an increase in the size of the myofibrils (i.e., myofibril hypertrophy) and/or the number of myofibrils (i.e., myofibrillogenesis) is not known. Electron microscopy (EM) can provide images with the level of resolution that is needed to address this question, but the acquisition and subsequent analysis of EM images is a time- and cost-intensive process. To overcome this, we developed a novel method for visualizing myofibrils with a standard fluorescence microscope (FIM-ID). Images from FIM-ID have a high degree of resolution and contrast, and these properties enabled us to develop pipelines for automated measurements of myofibril size and number. After extensively validating the automated measurements, we used both mouse and human models of increased mechanical loading to discover that the radial growth of muscle fibers is largely mediated by myofibrillogenesis. Collectively, the outcomes of this study offer insight into a foundationally important topic in the field of muscle growth and provide future investigators with a time- and cost-effective means to study it.
