key: cord-0683294-j356z6cc
authors: Anderson, Judy E.
title: Key concepts in muscle regeneration: muscle “cellular ecology” integrates a gestalt of cellular cross-talk, motility, and activity to remodel structure and restore function
date: 2021-12-20
journal: Eur J Appl Physiol
DOI: 10.1007/s00421-021-04865-4
sha: ae84b1885c042205617627df0fc80daed0ac754c
doc_id: 683294
cord_uid: j356z6cc

This review identifies some key concepts of muscle regeneration, viewed from perspectives of classical and modern research. Early insights noted the pattern and sequence of regeneration across species was similar, regardless of the type of injury, and differed from epimorphic limb regeneration. While potential benefits of exercise for tissue repair was debated, regeneration was not presumed to deliver functional restoration, especially after ischemia–reperfusion injury; muscle could develop fibrosis and ectopic bone and fat. Standard protocols and tools were identified as necessary for tracking injury and outcomes. Current concepts vastly extend early insights. Myogenic regeneration occurs within the environment of muscle tissue. Intercellular cross-talk generates an interactive system of cellular networks that with the extracellular matrix and local, regional, and systemic influences, forms the larger gestalt of the satellite cell niche. Regenerative potential and adaptive plasticity are overlain by epigenetically regionalized responsiveness and contributions by myogenic, endothelial, and fibroadipogenic progenitors and inflammatory and metabolic processes. Muscle architecture is a living portrait of functional regulatory hierarchies, while cellular dynamics, physical activity, and muscle–tendon–bone biomechanics arbitrate regeneration. The scope of ongoing research—from molecules and exosomes to morphology and physiology—reveals compelling new concepts in muscle regeneration that will guide future discoveries for use in application to fitness, rehabilitation, and disease prevention and treatment.

The basis of any scientific exploration lies in the richness of observation and experimentation conveyed by researchers, including their respectful discussion of previous and concurrent work. Generations of emerging scientists, finding a research niche, contributing new concepts, and augmenting earlier ideas as their experience and resources grow, further decorate Communicated by Michael Lindinger.

the process of providing this review. The field of research on muscle regeneration is remarkably broad in its diversity of approaches and prospects, though nowhere near as remarkable as the myriad processes that are integrated to restore function and enable adaptation. So, a toast to skeletal muscle and all its fascinatingly interactive components, and to the ingeniously creative use of tools and concepts to push the frontiers of understanding muscle regeneration and how it can shape life.

Early observations, including potential influences of weather (barometric pressure, temperature) on strength (manual grip strength) by Fischer (1947) , and the sociopolitical context of rehabilitation after World Wars I and II described by Boynton (1947) , illustrate the broad scope of many early ideas about recovery and tissue repair. Few reports are specific to what we now understand as muscle-tissue regeneration, although the perspectives illustrate the way concepts can shape understanding and either extend or squelch curiositydriven research.

In 1900, for instance, a pathologist (and assistant medical officer for the Royal Exchange Assurance Company) noted that exercise should not be abused (i.e., over-indulged), or undertaken over the level of normal daily activity, as it led to many negative consequences. Lister (1900) cautioned against "violent efforts" on summer holidays and offered now-quaint commentary on the consequence of over-exertion in the non-athletic. He also described that incidence of such injury is likely to increase with the "growing demands made on energies, knowledge, and intelligence in the working months by the increasingly-rapid inter-communications and severe competition of advancing civilization." The comment that over-exertion was much less frequent among women as they did household chores gives the paper a historical context.

In 1922, Rowlands (1922) suggested for the first time, that activity might accelerate wound healing. While not specific to muscle regeneration per se, Rowlands questioned the prescription of prolonged bedrest after surgery-typical accompaniment to hospitalization after many procedures well into the 1980s. Rowlands was "not aware of any delay in wound healing due to getting up early and taking gentle exercise…[since] gentle movements of wounded soft parts do not hinder but help the process of healing." In concluding his conference paper, Rowlands called for a more thorough assessment of the influence of activity in post-operative recovery, to prevent adhesions and muscle shortening that appear after prolonged inactivity. He politely recommended that "taking gradually increasing exercise, just short of fatigue" would be useful and provide interesting "mental diversion" during recovery. , Newman (1949 recognized "the importance of an accurate evaluation of a disability", since "muscle strength, to a great extent, governs the amount of function of a part." While this might seem obvious now, many early measurement devices were not standardized and their use was not subject to guidelines for positioning a patient or calibration that could be reproduced over time. Use of the tools was not standardized across investigators or clinical staff, to assure consistent application, and thus jeopardized assessment of function. In 1949, Woodard (1949) noted that muscle injury in athletes differed from that incurred by non-athletes, and thus required particular therapy including gentle muscle activity rather than rest, for optimal repair. Brewerton and Darcus (1956) prefaced a 1956 review paper on methods of increasing muscle strength by stating that there little new information on muscle function had accrued since 1900. He cited Siebert (writing in German in Z. Klin. Med., 1928) for recognizing that an increase in muscle size is not achieved by increasing the number of muscle fibers. (This is the current understanding of growth by fiber hypertrophy rather than a hyperplasia of fibers that occurs in indeterminate growers such as sturgeon (Hiebert and Anderson 2020) . Darcus noted that "one of the few well-established facts is that the performance of muscles can be improved by systematic voluntary exercise." He then discussed the lack of consensus on how that could happen or the changes in muscle structure and function that happen during strengthening, despite "the determination of the workers in this field" in developing training routines and equipment. He also noted that researchers were still looking for evidence that the muscle capillary network increases with training and adaptation.

Studying the neuromuscular system, Darcus distinguished changes to muscle (from force generation) from those to the nervous system (related to making best use of available force generation). By 1923, it was already established that reinnervating axons could regenerate through nerve stumps from early parabiotic-pair experiments (Morpugo 1923) . However, Darcus was not sure that the normal process of muscle hypertrophy was different from that in muscle atrophied by denervation or inactivity. Further, it was not established how much fiber hypertrophy actually contributed to increases in human muscle strength, since findings from animal experimentation were not directly applicable to rehabilitation in people. Darcus recognized that training-induced increases in strength implied the existence of important neuromuscular adaptations in skill, recruitment of motor neurons and motor units, and sensory (afferent) inflow to the central nervous system from muscle spindle contractions and from other tissues. However, he did not know whether the exercise by patients recovering from injury should be allowed to induce muscle fatigue, and cautioned against inducing "subjectively unpleasant" sensations that produced residual stiffness or soreness. In short, Darcus considered neuromuscular adaptations to be more important in the outcome of training than structural and biochemical changes in muscle. Neuromuscular adaptation, later termed muscle plasticity, was reviewed more recently (Pette 2001) .

By 1949, Denny-Brown had published some of his classical work on electromyography and motor unit recruitment (Denny-Brown 1949) ; others had reported the importance of afferent information from muscle, tendons, and muscle spindles in regulating contractile force. Darcus noted that neuromuscular impulses facilitated synaptic transmission at nerve-muscle junctions (NMJs) and all along the reflex arc, and with sufficient repetition, might reduce resistance to signal transmission from stimulus to contraction.

Writing in the context of a raging polio epidemic, Darcus and his contemporaries would have had many daily reminders that compensatory movements during rehabilitation were critical to restoring overall function. Relevant to the current review, Darcus cited a 1943 paper by Griffiths (1943) that "the ultimate recovery of function may be delayed by inhibiting the normal habit" of a muscle. This further emphasized that exercise or at least muscle activity is important to functional recovery. Griffiths distinguished passive movement and anti-gravity assistance from active movement during recovery to restore muscle power after injury or surgery, and noted effective venous return was dependent on muscle contraction, without discussing myogenic repair itself.

The following year, Eccles (1944) concluded from animal physiology experiments that disuse atrophy is best countered by exercise in which muscle shortens during contraction; he noted that strong isometric contractions can maintain muscle condition reasonably well. Eccles also determined that the effectiveness of muscle stimulation to maintain muscle mass depended on more than changes in muscle length since the outcome of stimulation differed between extensor and flexor muscles. A 1957 address to the British Medical Society (Anonymous 1957) referenced Darcus' work on neuromuscular adaptation, and noted a lack of consensus on how best to tailor resistance-exercise protocols to specifically build endurance, speed, strength, or coordination.

The cultivation of chopped muscle in a medium of embryo extract, Tyrode's solution, and plasma, using a "lying drop" technique was described by Popogeff and Murray in 1956 (1956) . Use of that culturing method quickly revealed that precursor cell dormancy (muscle satellite cells) was longer in normal than in dystrophic human muscle, and demonstrated myotube growth or hypertrophy in culture (Geiger and Garvin 1957) . The advent of tissue-culture methodologies opened the door to many research questions.

As early as 1947, Diaz-Guerrero and Thomson reported (Diaz-Guerrero et al. 1947b ) that the restoration of function after denervating gastrocnemius muscle (using a crush injury of the tibial nerve) was slower in hypothyroid than hyperthyroid rats. As well, they reported that denervation atrophy occurred more quickly in hyper-than hypothyroid animals. These findings were based on initial studies of muscle strength without denervation. The force of muscle contraction produced after indirect stimulation (i.e., stimulation to an innervating nerve) was lower, and fatigue greater in hyperthyroid than normal animals; hypothyroid animals showed no change in strength and had only a slightly increased fatigue of muscles tested with the nerve-stimulation protocol (Diaz-Guerrero et al. 1947a) . In that exercise and activity provide voluntary (nerve-dependent) generation of muscle contractile force, these early studies established the important impact of the metabolic state on nerve-muscle interactions, atrophy, and neuromuscular repair.

Studitsky conducted early experiments focussed on muscle regeneration from 1952 to 1964, He compared regeneration in young and old animals (often rats), and transplanted crushed or minced muscle into an emptied muscle bed. He also documented the progressive return of function during muscle recovery from damage. Unfortunately, the intriguing titles in Russian are not accompanied by translations (Carlson 1968) . His book chapter in English in 1963 (Studitsky 1963) describes the fascinating process of myogenesis in cultures of muscle explants. Fortunately, Carlson visited and worked in Studitsky's lab, and was able to bring to broad attention to those experiments in later reports in English. By 1968, Carlson confirmed that use of minced-muscle in modeling cellular processes provides major opportunities for observing the chronology of a synchronized process of muscle regeneration (Carlson 1968) .

Notably, by 1945, the surgical literature had identified the features of muscle regeneration induced by loss of vascular supply, including the "myotube stage" discussed by Le Gros Clark and Blomfield (1945) and took note of previous studies of contractures after traumatic wartime injuries or vascular occlusion (Brooks 1922 ). Brooks confirmed findings of an earlier paper by Rowlands (1905) that identified the serious sequelae of muscle damage from ischemia-reperfusion injury. Church cited even earlier work by Waldeyer (1865) and Volkman (1893) that described regeneration processes that involved "continuous sprouting from viable fibres… and discontinuous budding from surviving fibres of single cells… that ultimately give rise to new fibers" (Church et al. 1966) . These were early observations that identified the role of activated satellite cells in myogenic repair, although the cells were not named as such, or identified in their quiescent state, for years to come.

Carlson's impressive monograph in 1973, The Regeneration of Minced Muscles (Carlson 1972) , detailed the histological progression of muscle tissue repair. In the epilogue, Carlson notes seven "potentially… most profitable directions for future research," including how damaged muscle survives, the source of muscle progenitor cells, muscle interactions (with vessels, nerves, and endocrine signals), the functional outcome of muscle regenerates, and the possibility that transplanting muscle minces or cells could be used to treat muscle damage or disease. Carlson identified problems in repairing muscle damage that involved excessive deposition of connective tissue and skeletal nodules; those sequelae were often related to poor vascularization at a particular stage of myogenic regeneration.

Our current knowledge of the muscle regeneration process was founded on the rich descriptions of histological features of normal or pathological muscle repair under various conditions reported by many investigators, typically using mammalian models (e.g., Le Gros Clark and Blomfield 1945; Murray and Kodicek 1949) . The often-voluminous descriptive text in these reports would never be accepted for publication in current journals, it clearly provides important details. Early histologists thoroughly explored the chemistry of tissue staining and learned about muscle metabolism. This was followed within a few decades by enzyme histochemistry (Snow 1973) and an early method of staining muscle satellite cells in a particular state (Ontell 1974) . Sometimes descriptions were "persuaded" by presumptions about the process under investigation. For example, a description of fiber fragmentation (Murray and Kodicek 1949) refers in error, to two structures shown in figures that are actually muscle spindle fibers and "cuffs" of myoblasts and myotubes within endomysial sheaths of fiber remnants. Since such observations, confounded by interpretation or assumption, can linger in the literature and our brains, their influence on understanding may persist longer than new evidence. There is real merit to digging back with a skeptical lens, through classical reports to explore the basis of our current etymologies and insights. Often, though, early descriptions are valuable, and serve as a caution when we consider an observation is "the first report" on a topic.

Histology studies were quickly augmented by major advances in development of electron microscopy techniques in the 1950s. Techniques for ultrathin sections and electrondense stains led to Mauro's report (1961) of a presumptive muscle precursor cell, termed the satellite cell due to the position of those cells on muscle fibers. The same cells were the subject of speculation by Church et al. (1966) , Shafiq et al. (1967) , and Carlson (1968) who suggested they may become activated into myoblast precursors for muscle formation. By 1966 (Church et al. 1966) , satellite cells were known to survive muscle damage and serve as reserve cells that transform into myoblasts that proliferate and form new fibers.

The 1969 monograph on Regeneration of Striated muscle, and Myogenesis (Mauro et al. 1970) illustrates the intense discussions during an international conference at the Institute for Muscle Disease in New York on the time-course of regeneration in amphibians and rodent muscle. Attendees at the conference also discussed the myoblast cell cycle, myoblast fusion into myotubes in vivo and in vitro, and the ultrastructural morphology of the satellite cell. In 1968, Carlson showed that muscle fibers regenerate with guidance from the internal structure of connective tissue in a muscle, such that new fibers are aligned to the direction of tension in the muscle regenerated from minced fragments, and that early muscle regeneration occurs independent of nerve supply (Carlson 1968 ).

In 1975, Bischoff (1975) reported use of a method of culturing single muscle fibers to examine the population of inactive satellite cells in residence on isolated fibers. In addition, he used a direct method, autoradiography to track the entry of satellite cells into the cell cycle, by their uptake of tritiated-thymidine into new strands of DNA during mitosis. Although immune detection methods are much faster, autoradiography is still a valuable technique, especially in combination with other methods of tracking gene expression and the histology of regenerative processes (Anderson et al. , 1998a McIntosh et al. 1998) . By the 1980s, research had identified differences between muscle development in the embryo and muscle regeneration in adults (e.g., Hansen-Smith and Carlson 1979; Carlson and Faulkner 1983) and many details of the time-course of muscle regeneration (e.g., Aloisi 1970; Reznik 1970; Shafiq 1970; Hall-Craggs 1980; Maxwell et al. 1984; Schultz and Jaryszak 1985; Ontell 1986; Grounds 1987; Grounds and McGeachie 1987; McGeachie and Grounds 1987; Brooks and Faulkner 1988; Carlson and Faulkner 1989) . In 1968, McComas and Thomas (1968) reported electrophysiology studies that implicated the existence of slow and fast muscles in humans. In 1976, Khan confirmed the likelihood that slow-twitch muscle had the same histochemical pH-dependent staining pattern as type-I oxidative fibers (Khan 1976b ) and had identified fast-oxidative-glycolytic fibers (type-IIA) (Khan 1976a) . By the end of the 1980s, there was a burgeoning literature describing fiber-typing studies that compared different muscles in a huge range of species, and during development, adaptation, aging, regeneration, reinnervation, metabolic shifts, and disease (McComas and Thomas 1968; Tomonaga 1977; Haggmark et al. 1981; Haggmark et al. 1986; Pette 1986, 1987a, b; Anderson et al. 1988; Coulton et al. 1988; Jakobsson et al. 1988; Termin et al. 1989; Dusterhoft et al. 1990; Klitgaard et al. 1990; LaFramboise et al. 1990; Parry and Wilkinson 1990; Goldspink, 1991; Coggan et al. 1992; Welle et al. 1993 ).

Myogenic regeneration begins with damage to fibers that degenerate and are removed by phagocytosis. Damage also activates satellite cells which proliferate as spindle-shaped myoblasts that cycle, fuse and form new syncytial fibers, initially with central nuclei. Myotubes subsequently elongate toward attachment sites at tendons and on bone. Bone also responds to the presence or loss of pull from muscle (Anderson et al. 1993) . Tissue repair in a muscle belly can be highly successful, comprising restoration of many types of tissue: myogenic, vascular (initially capillaries), nervous (initially Wallerian degeneration of axons and then elongation of growth cones through any surviving nerve stumps, or toward NMJs on remnant tubes of endomysial basal lamina tubes (previously surrounding fibers), and connective tissues (extracellular matrix (ECM) including endomysium, perimysium, epimysium, and tendon connections to fibers). Regenerated muscle fibers are typically aligned to the original orientation of fibers.

Given the architecture of skeletal muscle tissue and the wide range of injuries and diseases that provoke muscle to regenerate, it is important to remember that a reproducible approach to sampling the regeneration time-course requires attention to the location and type of injury in the tissue and the time elapsed since injury. Regeneration is typically localized around an area of injury, with some extension into the surrounding undamaged tissue due to the migration of myoblasts along fiber and endomysial remnants in the ECM (McIntosh et al. 1994) . Injury can be very focal, more broadly segmental across a short portion of the fiber length as in muscular dystrophy, or widespread as in avascular necrosis modelled by denervation-devascularization injury (Anderson 1991; Mechalchuk and Bressler 1992; Lefaucheur and Sébille 1995) . Thus, the extent and location of initial tissue damage, whether it is synchronous or sporadic and recurrent, and its homogeneity in a muscle belly should determine the timing and location of any approach to investigating the time-course or outcome of regeneration. It is useful to remember that delayed regeneration is not necessarily defective.

i. Myogenic regeneration follows a similar pattern and sequence, albeit with different timing, across species and many types of damage; the pattern differs from the epimorphic regeneration of limbs in urodele amphibians. ii. Muscle tissue regeneration can be very successful but does not always restore function. iii. Large areas of damage often leave a deep fibrotic scar. iv. Cells in skeletal muscle can form bone, adipose, and fibrous connective tissues. v. Regeneration from ischemia-reperfusion injury has a less functional outcome than after anoxia. vi. Assessing muscle function to track regeneration requires standardized protocols and tools.

Myogenic regeneration is the subject of many excellent reports and reviews (Charge and Rudnicki 2004; Ciciliot and Schiaffino 2010; Carosio et al. 2011; Tidball 2011; Musaro 2014; Dumont et al. 2015a; Domingues-Faria et al. 2016; Qaisar et al. 2016; Joanisse et al. 2017; Le Moal et al. 2017; Zammit 2017; Cornelison 2018; Li et al. 2018; Wosczyna and Rando 2018; Chen and Shan 2019; Chazaud 2020; Forcina et al. 2020) , and there are many more related to the benefit of activity or exercise, the impact of aging, differential injury-dependent regenerative responses, roles of micro-RNAs in regulating the stem cell niche, and the responsiveness of satellite cells. Although selected topics in the overall process will be highlighted, this review is primarily focussed on a high-level synthesis rather than a comprehensive overview of the wealth of important or essential factors involved in orchestrating effective myogenic regeneration and muscle tissue repair.

A diverse range of injuries leads to muscle regeneration, and some damage to fibers is needed to prompt the onset of the muscle regeneration cascade; this is a truly beautiful example of tissue homeostasis in action (Musaro 2014; Musarò 2020) . The impact of trauma changes the environment of all types of proximate and surrounding cells and the acellular matrix, and cells respond in marvelously coordinated fashion toward resolving an injury and restoring muscle function. However, the extent to which muscle-tissue damage is followed by regeneration and functionality is highly dependent on many variables. There are considerations about the initial insult, such as whether it is an acute or chronic injury or a single or repetitive injury; the volume of tissue injury and its distribution (widespread or focal) are also important. The age of the individual or animal are notable features as well, since age affects the number of satellite cells, the state of satellite cells (activation, senescence, stemness and selfrenewal capacity, and exhaustion), and their responsiveness to muscle injury. Age also affects the composition and character of the ECM, the vascular supply, innervation, insertion points on bone, and any accrual of prior injury, scarring or disease conditions. The nature and configuration of the ECM (fibrotic or not) are additional determinants to the ultimate recovery of function, as is the possible involvement of denervation that may accompany the injury to fibers. The timing by which inflammation is resolved has a major impact on functional recovery, as is the timing of vascular regeneration to ensure blood supply for debris removal and support regenerating tissue. Adiposity within muscle fascicles and the level of physical activity prior to an injury also play into the timing and extent of recovery, along with stresses of metabolic disease, transient systemic conditions, and/or the secondary impact of genetic, biochemical, developmental or epigenetic modifications to cells or molecules that mediate the outcome of regeneration, as reviewed elsewhere (Urso 2013; Mok et al. 2017; Robinson and Dilworth 2018; Mitchell et al. 2019; Long et al. 2021; Machado et al. 2021; Vechetti et al. 2021; Wen et al. 2021) .

The discovery of muscle regulatory factor genes (MRFs) was a remarkable turning point in our understanding of the embryological development of muscle (Esteves de Lima and Relaix 2021); the four MRFs are re-expressed during the post-natal myogenic process of muscle regeneration. Expression of the first two MRF genes, MyoD and Myf5 by satellite cells increases after satellite cell activation. MyoD and Myf5 regulate the commitment of daughter myoblasts into the muscle lineage and myoblast proliferation, and induce expression of myogenin. Myogenin and MRF4 are expressed soon after under appropriate conditions, and induce differentiation Knappe et al. 2015; Zammit 2017) . Pax7 and Pax3 function in advance of MRFs in postnatal myogenesis (Buckingham and Rigby 2014; Buckingham and Relaix 2015) .

Initial damage induces degeneration of fibers and at the same time, activates muscle satellite cells, the stem cells that possess the all-important dual capability of self-renewing the stem cell compartment and producing committed myogenic cells. Satellite cells activation and cell cycle entry are regulated through Notch Wnt-signaling pathways (Conboy and Rando 2002; Conboy et al. 2003; Dhawan and Rando 2005; Luo et al. 2005; Le Grand and Rudnicki 2007; Srivastava et al. 2010; von Maltzahn et al. 2012; Subramaniam et al. 2013) . It is well established that muscle satellite cells are a heterogeneous population (Tierney and Sacco 2016) that is regulated to maintain quiescence (Subramaniam et al. 2013; Arora et al. 2017; van Velthoven et al. 2017; Purohit and Dhawan 2019; Puri et al. 2021) . Satellite cells are rapidly activated in response to damage, mechanical stretching and nitric oxide release, exercise and physical activity (Anderson 2000; Anderson and Pilipowicz 2002; Wozniak et al. 2003; Anderson 2007, 2009; Wang et al. 2009 ). Such regulation is affected by aging (Brack et al. 2005 Collins et al. 2007; Joanisse et al. 2017; Snijders and Parise 2017; Hwang and Brack 2018; Tierney et al. 2018 Tierney et al. , 2019 Kimmel et al. 2020) .

Recent study of the capacity for stem-cell renewal-by way of CD34 expression modulated through insulin-like growth factor-1 (IGF1)-mediated Akt activation that reduces CD34 expression by inhibiting FoxO-introduced even further complexity in satellite cell heterogeneity. Experiments identified a state of "primed stemness" (cells with myogenic commitment) and more prevalent "genuine stemness" in muscle of younger animals (García-Prat et al. 2020), expanding on earlier studies of CD34-mediated satellite cell motility (Alfaro et al. 2011) .

Although the proportion of the satellite-cell population with genuine-stem capability declines with age (Day et al. 2010; García-Prat et al. 2020 ) and satellite cells become increasingly refractory to stretch-induced activation with age (Leiter and Anderson 2010) , those cells retain the capability of responding to damage and producing effective repair in old animals (Smythe et al. 2008; Shavlakadze et al. 2010; Lee et al. 2013; Domingues-Faria et al. 2016; Snijders and Parise 2017; Franco et al. 2019; García-Prat et al. 2020; Kimmel et al. 2020; Yamakawa et al. 2020; Delsmann et al. 2021 ). Thus, satellite cells clearly respond to mechanical stimulation through exercise (Wozniak et al. 2003; Gomes et al. 2004; Tidball 2005; Tatsumi and Allen 2008; Wozniak and Anderson 2009; Leiter and Anderson 2010; Tatsumi 2010; Hara et al. 2012; Zhang and Anderson 2014; Li et al. 2018; Eliazer et al. 2019) ; that and the regenerative response to injury are muscle-specific and age-dependent (Leiter and Anderson 2010; Gigliotti et al. 2015 Gigliotti et al. , 2016 .

The timing of satellite cell activation is important in muscle tissue regeneration. Premature activation before injury, established using treatment with a nitric oxide donor, can accelerate myogenesis following a myotoxic or crush injury, whereas the restoration of NMJs on newly formed myotubes is disrupted (Daneshvar et al. 2020) . This reminds us that the desired outcome of regeneration, strength and voluntary control, results from coordinated nerve-muscle interactions, and that myogenesis is only one, albeit an important aspect of muscle-tissue regeneration. Interestingly, experiments with nitric oxide-donor treatments in mice also improve regeneration and reduce exercise-induced damage in dystrophic mice (Archer et al. 2006; Wang et al. 2009; Mizunoya et al. 2011) , attenuate disuse atrophy after hindlimb suspension (Anderson et al. 2017b), enable exercise-induced hypertrophy in atrophic muscle of old mice (Leiter et al. 2012) . While many mouse studies highlight the exciting potential for treating human Duchenne dystrophy via nitric oxide donors (Sciorati et al. 2006 (Sciorati et al. , 2010 (Sciorati et al. , 2011 (Sciorati et al. , 2013 Brunelli et al. 2007; De Palma and Clementi 2012) , simply increasing its bioavailability by inhibiting phosphodiesterase can be detrimental (Timpani et al. 2020) . Further investigations of applications to safely deliver nitric oxide to replace the normal functional release during muscle activity and mechanical stretch Anderson 2007, 2009 ) in a dosing regimen that avoids excessive oxidative stress could promote muscle growth and regeneration, and benefit people with dystrophy, disuse atrophy, and sarcopenia (Alrushaid et al. 2018 ). Satellite cells are essential for myogenesis, hypertrophy during maturation, and regeneration. By contrast, muscle hypertrophy in adults can occur without satellite cell contributions to myofibers. However, that hypertrophy ultimately restricts growth, vascular arborization, strength, and remodelling of collagen, which affect longer term plasticity and adaptability (McCarthy et al. 2011; Jackson et al. 2012; Fry et al. 2014 Fry et al. , 2015 Englund et al. 2020 Englund et al. , 2021 . The elegant experimentation demonstrated that the satellite cell population affects expression by myonuclei inside fibers, possibly through their production of extracellular vesicles. To quote a perspective on the paper, "in essence, satellite cells are more than just myonuclei in waiting, capable of affecting muscle health in a fusion-independent fashion"(Hawke 2020).

Migratory movements by myoblasts and others cells are implicated as determinants of the success of muscle regeneration (Siegel et al. 2009 Alfaro et al. 2011; Stark et al. 2011; Kowalski et al. 2017 ). Myoblast movement is itself, mediated from the supporting substrate containing fibronectin, which regulates Wnt7a signaling and myoblast proliferation (Bentzinger et al. 2013 (Bentzinger et al. , 2014 . The research platform of microfluidics technology with applications to tissue engineering, is a powerful tool in studying migration.

Microfluidics experiments provide major advances over earlier approaches to the real-time visualization of cellular physiology under exquisitely controlled conditions (Roveimiab et al. 2019 ) and disease modeling . For example, our recent microfluidics study explored the mechanisms by which the niche substrate has significant impact on responses by myoblasts (Roveimiab et al. 2020) . Myoblast interactions with the substrate composition (collagens, fibronectin, laminin, etc.) as well as the extent and nature of connective tissue elements in the substrate, affect myoblast migration by the complex process of haptotaxis. Very recent experiments with inhibitors (Roveimiab, Lin and Anderson, unpublished) suggest a hierarchy of disruptions to different aspects of adhesion, speed and direction of movement, and cell-cell fusion result from interference in the fibronectin-integrin signaling pathway, affect the alignment of one myoblast and its juxtaposition toward the nucleus of a differentiated myotube. The movements of individual cells play a significant role in the outcome of myogenesis during muscle-tissue regeneration.

In addition, soluble elements in the niche also provide input to guide myoblast movements including the speed and direction of migration along isolated fibers (Siegel et al. 2009 or within a microfluidic device (Roveimiab et al. 2020) . In particular, hepatocyte growth factor (HGF), also called scatter factor, binds to the "motogenic" c-met receptor expressed by both activated and quiescent satellite cells (Hartmann et al. 1992; Sonnenberg et al. 1993; Cornelison and Wold 1997; Dietrich et al. 1999; Webster and Fan 2013) . HGF delivers a potent signal for positive chemotaxis and mobilizes populations of myoblasts (Roveimiab et al. 2020) . Traction enabled by a haptotaxis substrate interacts with the configuration of the chemokine signal provided by the concentration or a directional gradient of HGF (Roveimiab et al. 2020) . Even more complex is the signaling among different cell types including leukocytes (Panci and Chazaud 2021) and other inflammatory cells (Sakaguchi et al. 2014; Chen et al. 2015; Ferreira et al. 2015; Saini et al. 2016) in the environment of muscle precursors that are mobilized by injury.

Recently Hox10 expression and epigenetic methylation events at the Hox-A locus were found to mediate the topographical distribution of differential responsiveness by adult muscle stem cells (Yoshioka et al. 2021 ). This paper recalled earlier reports on cranial muscles that present continuous "remodeling" through satellite cell turnover and fusion into undamaged muscle fibers above levels seen in limb muscle fibers (e.g., Wirtschafter 2002, 2003; McLoon et al. 2004) . In these cases, there was higher Hox-A hypermethylation and Hox-A and Hox-C cluster gene expression, both robustly sustained after transplantation and regeneration by engrafted satellite cells. Loss of Hoxa10 expression by postnatal muscle reduced the capacity for regeneration, indicating that satellite cells display a fascinating "positional memory" based on developmental topography (including distinction between limb muscles and those innervated by cranial nerves). This memory-of-source feature of regeneration did not affect Pax7 or MyoD expression, the size of the satellite cell population or its ability to become activated. However, with Hoxa10 inactivation, satellite cells resident on fibers cultured ex vivo, were not as good at migration-an essential process in myogenic repair; they also had impaired proliferation without displaying apoptosis. Rather, satellite cell-specific disruption of Hoxa10 affected genes important in chromosomal segregation and spindle formation during mitosis, and in limb muscle, induced changes that led to the formation of micronuclei and chromosomal bridges without affecting myogenic differentiation or self-renewal capacity. Human satellite cells in culture mirrored the position-dependent distinctions in cell-cycling, gene expression, and the impact of HOXA10 knockdown by short interference RNA (siRNA) found in the transgenic mouse experiments. This report opens a new avenue toward understanding regional and muscle-specific differences in regeneration capacity and plasticity of limb muscles toward adaptation.

Two ideas: that the potential of muscle for regeneration and plasticity is regionalized, and that stem-cell function is influenced by epigenetic acetylation of Pax7 (Sincennes et al. 2021 ), together bring new implications to considering epigenetic modifications in responses to exercise, training, aging, hormones. Such ideas also raise to mind the regional patterning of genetic neuromuscular diseases such as the family of muscular dystrophies involving disruptions of the dystrophin-associated cytoskeleton (Sunada and Campbell 1995; Cohn and Campbell 2000; Durbeej and Campbell 2002) . Epigenetic modifications to post-mitotic muscle fibers, recently identified to involve long non-coding RNA molecules, also affect adaptive responses to stress (El Said et al. 2021) . The full-length influences of RNA sequences will likely be revealed to have further impact on skeletal muscle adaptation and repair.

Inflammatory cells are resident (as macrophages) in muscle tissue and attracted to the site of injury. They promote sarcolemmal repair, fiber growth, and regeneration (Cheung and Tidball 2003; Tidball and Wehling-Henricks 2007) in addition to removing damaged tissue following muscle injury. Macrophages resident within muscle tissue are highly plastic in establishing the stem cell niche and interact with satellite and other cells (Ratnayake et al. 2021 ). Experiments using a beautifully designed system to follow individual macrophages in real time in living zebrafish (Montandon et al. 2021 ) revealed that there are early interactions of muscle-resident macrophages with satellite cells through a macrophage-secreted compound, nicotinamide phosphoribosyl transferase (NAMPT). NAMPT in turn, acts via the CCR5 chemokine receptor and establishes a short-lived niche that activates satellite cells; the same signaling contributes to regulate the pace and timing of myogenic regeneration (Ratnayake et al. 2021) .

Muscle stem cell interactions with pro-inflammatory macrophages are thus very broad, and as macrophages rapidly shift to become anti-inflammatory, clues for both myogenic and non-myogenic processes are compiled over time through interactions of immune cells including macrophages with muscle stem cells (Chazaud 2020) . Recent literature highlights the notion that macrophage activity determines whether repair is regenerative or fibrotic (Moyer and Wagner 2011; Muñoz-Cánoves and Serrano 2015; Juban et al. 2018; Yang and Hu 2018) through considerable cross-talk with muscle cells (Mann et al. 2011; Muñoz-Cánoves et al. 2013; Yang and Hu 2018) . Thus, there is a balance between inflammation and myogenic repair in regenerating muscle: the inflammatory response to damage will ideally maximize the prospects for an effective regenerative response while avoiding a large catabolic response (Urso 2013) .

The timing of macrophage transition from pro-to antiinflammatory-required to trigger the resolution of inflammation during muscle regeneration-is critical, since early and late transition are both detrimental for recovery (Chazaud 2020). Signals that resolve inflammation begin when prostaglandins and other proinflammatory signals are inactivated or blocked; inflammatory cells are then removed from the damaged tissue by reverse migration, drainage and cell apoptosis, and apoptotic neutrophils (to minimize secondary necrosis) are cleared from the tissue. The transition to an anti-inflammatory state is facilitated by macrophage plasticity (Sugimoto et al. 2019) .

Notably, muscle regeneration is impaired in autoimmune myopathies that result from inflammatory cell activation by antibodies against signal-recognition protein or HMG-CoA reductase; the impairment, a defect in myoblast fusion and fiber formation, was associated with deficient levels of IL-4 and IL-13 (Arouche-Delaperche et al. 2017). Thus, inflammation, its resolution and its etiology including direct muscle infection by viruses such as Zika, arboviruses, and SARS-CoV-2 (COVID-19) (Disser et al. 2020; Filippone et al. 2020; Legros et al. 2020; Paliwal et al. 2020) , are important during regeneration.

Fibro-adipogenic precursors (FAPs) in muscle are derived from neural crest; since the first report in 2010, FAPs are now understood to play key roles in configuring lineage determination and the outcome of proliferation by mesenchymal cells-both essential in regeneration of muscle (Joe et al. 2010; Uezumi et al. 2010; Theret et al. 2021 ) and other tissues Lemos et al. 2012; Paylor et al. 2014; Giuliani et al. 2021) . FAPs are non-myogenic and give rise to fibroblasts and adipocytes, depending on signals such as from the innate immune system that regulates eosinophil and anti-inflammatory macrophage activity soon after tissue damage. Interleukin signaling, specifically IL-4 from eosinophils, promotes FAP proliferation and restricts the progression of FAPs to adipocytes. Notably, FAPs also mediate the removal of damaged, necrotic tissues after injury or in culture, and stimulate myogenic differentiation and MRF-gene expression by muscle precursors (Heredia et al. 2013) .

Recently, a subset of FAPs that express Gli1 and Hh (hedgehog) proliferates rapidly after muscle injury and was identified as being responsible for restricting the development of adipose tissue during muscle regeneration. The role of Gli1-positive FAPs was emphasized by genetic ablation of Gli1-expressing cells, which impaired muscle regeneration through a transient expansion that produces paracrine signaling factors such as IL-6 and IGF-1 that act on satellite cells (Yao et al. 2021) . That fibrosis-another essential aspect of muscle regeneration-reduces satellite cell proliferation and delays fiber growth during the regenerative response when FAP proliferation is inhibited (Fiore et al. 2016) . Essentially, although the influences of FAPs during muscle repair may be short-lived, they make major contributions to the overall "tuning" of different molecular players regulating myogenic and other stem cells (Biferali et al. 2019; Forcina et al. 2020) . Interactions of FAPs with fibroblast growth factor 2 (FGF2) through via miRNA-29a signaling promotes formation of adipose tissue in muscle during aging (Mathes et al. 2021) .

Fascinatingly, fibroblasts were recently shown to fuse to skeletal muscle, a feature that enables the transfer of fibroblast-specific mRNAs that facilitate the transition of the fiber toward its tendinous attachment sites (Yaseen et al. 2021) . That report solved a puzzle from earlier electron microscopy observations of muscle regeneration (Anderson 1991) .

Very recently, direct isolation of FAPs, muscle stem cells and macrophages from human skeletal muscle biopsies, using a combination of fluorescence-activated cell sorting and mRNA sequencing at the single-cell level, was found to preserve the phenotypic behavior and expression of those cells (Jensen et al. 2021 ). The transcriptomics approach should prove exceedingly useful in translating discoveries of FAP contributions to muscle regeneration, and age-related changes in muscle tissue.

The ECM plays a major role in muscle health, development, disease, and regeneration (Goetsch et al. 2003; Csapo et al. 2020; Forcina et al. 2020) . Dynamic ECM interactions with every type of cell in the tissue (Cisternas et al. 2014; Dunn et al. 2018 Marcinczyk et al. 2019; Patel et al. 2019) occur through enzymatic degradation and remodelling by proteinases, including matrix metalloproteinases (Chen and Li 2009; Lu et al. 2011) . The ECM also serves as a reservoir for growth factors, including HGF and IGF-1, with which matrix metalloproteinases can interact to promote regeneration (Kok and Barton 2021) . ECM actually promotes muscle regeneration (Kuraitis et al. 2012) , in part by haptotaxis-induced myoblast migration (Roveimiab et al. 2020) . And, ECM proteins mediate adhesion or attachment of muscle stem cells, meaning the ECM mediates satellite cell responses to mechanical activity and tissue perturbation (Li et al. 2018; Moyle et al. 2020) .

The ECM forms a complex three-dimensional meshwork of proteins that surrounds myogenic and vascular cells, fibroblasts, and nerves within muscle tissue; that meshwork is synthesized and then modified by epigenetic and posttranslational modifications with important implications in disease pathophysiology (Theocharis et al. 2016 ) and stem cell function (Sincennes et al. 2021) . Proteins such as proteoglycan, integrin, collagen, fibronectin, hyaluronan, elastin, tenascin, and others, form a vast reference "library" that participates in any aspect of cell physiology, including regeneration. Realignment, damage, and remodelling of the ECM during regeneration also affect function and the outcome of regeneration, partly through cell attachments to the ECM through growth factor and other ligands it displays for binding with receptor-expressing cells. Such binding thus enables the internal actin cytoskeleton of those cells to be polarized according to their residency and migration within the interstitial space (Li and Gundersen 2008) .

Our understanding of the signaling interplay between myogenic cells (and other cells) with components of the ECM and basement membrane adherent to cells (Pozzi et al. 2017 ) through receptors, proteolysis, and unmasking of cryptic sites in ECM proteins (Clause and Barker 2013; Brown et al. 2015; Barker and Engler 2017; Yeh et al. 2021 ) has shown major recent advances. These new ideas have brought biomechanics, tissue engineering, and nanoscale three-dimensional scaffold production into the realm of therapeutics for muscle regeneration (Turner and Badylak 2012; Choi et al. 2018; Marcinczyk et al. 2019; Patel et al. 2019; Baiguera et al. 2020; Gilbert-Honick and Grayson 2020; Mihaly et al. 2021) , promising prospects for implantable volumes of muscle tissue that will promote the ingrowth of vessels and nerves (Laumonier and Menetrey 2016; Gilbert-Honick and Grayson 2020) . Interstitial fibroblasts between muscle fibers, produce many components of the extracellular matrix (ECM) and wrap around collagen cables (Gillies and Lieber 2011; Gillies et al. 2017) .

Excessive deposits of connective tissue are detrimental to muscle function in muscular dystrophy, as suggested over 40 years ago (Duance et al. 1980) ; that impact is now well established in physiological experiments, computational modeling (Mann et al. 2011; Martin et al. 2016; Virgilio et al. 2021) , and the clinical literature (Best et al. 2013; Lieber and Ward 2013; Teixeira and Duarte 2016) . For example, the negative functional impact of extensive fibrosis and collagen deposition in skeletal and cardiac muscle of dystrophic mdx mice was demonstrated by functional gains after an anti-fibrosis treatment that reduced expression of collagens I and III. Treatment attenuated dystrophic damage due to the tissue stiffness that affects myofibers (Huebner et al. 2008) , the satellite cell niche (Moyle et al. 2020) , satellite cell proliferation, and myogenesis (Teixeira and Duarte 2016).

The supply and configuration of blood vessels around, along, and sometimes within muscle fibers is critical to regeneration and maintenance of muscle. Plasma carries potent systemic factors, and proteins including hormones, and cytokines (some anabolic and/or catabolic) (Conboy et al. 2005; Cornish et al. 2020a ) while perfusion removes waste products and conveys red blood cells. Vascular architecture itself regenerates differently among various models of injury by freezing or toxicity (Hardy et al. 2016) . Muscle satellite cells in particular, survive the loss of vascular supply and local ischemia, and will actively recruit endothelial cells to their niche during muscle regeneration (Collins and Kardon 2018) . Notably, the impact of many circulating factors including metabolic regulators and signaling ligands conveyed via vascular supply, will depend on revascularization in regeneration.

Clearly there is cross-talk-a "social network"-among cells and between cells and the abiotic environment of degenerating and regenerating muscle tissue (Wosczyna and Rando 2018; Biferali et al. 2019; Scognamiglio et al. 2020) . Drawing a parallel between populations of organisms and populations of cells in muscle tissue [similar to cancerous cells (Somarelli 2021 )], such a network could be considered a "cellular ecosystem," since both populations are influenced by their environment (as in ecology, the interactions of organisms to each other and their surroundings), during adaptation and regeneration. There is very keen interest in what drives the outcome, which cell or combination of cells coordinates the many possible responses by other cells, and a strong push to identify the signaling networks that integrate cellular, tissue, and systemic processes. For example, research on the dystrophin cytoskeleton connected to ideas of mechanically induced satellite cell activation by transients of nitric oxide (NO) gas (Wozniak et al. 2003 ). Other research on activity-induced angiogenesis (McAllister et al. 2008; Alfaro et al. 2011 ) and changes in NO concentration during muscle disuse, aging or dystrophin deficiency (Anderson 2000; Anderson and Pilipowicz 2002; Tatsumi et al. 2002; Anderson and Wozniak 2004; Leiter et al. 2012; Janke et al. 2013; Allen et al. 2016; Anderson et al. 2017b; Rogers et al. 2017 ) are related. The important structural role of dystrophin in the dystroglycan complex, and its anchorage of neuronal NO synthase are considered to "orchestrate" the epigenetic profile of muscle cells and affect adipogenesis through miRNA regulatory pathways during differentiation and regeneration (Marrone and Shcherbata 2011) .

Redox control mechanisms also contribute to the process and outcome of muscle regeneration (Le Moal et al. 2017) through NO mediation of initial inflammation, myogenic repair, and the FAP-mediated processes of fibrosis and adipogenesis (Filippin et al. 2009 (Filippin et al. , 2011a . Potential therapeutics to manipulate NO concentration or the NO-cGMP signaling pathway that could promote muscle regeneration or growth, attenuate muscular dystrophy, or prevent muscle-disuse atrophy were broadly explored (Archer et al. 2006; Pisconti et al. 2006; Tatsumi et al. 2006 Tatsumi et al. , 2009b Betters et al. 2008a Anderson et al. 2017b) . Mechanically mediated cellular interactions and the potency of NO signaling to satellite cells and surrounding blood vessels must be placed in context with the overarching genetic regulation of satellite cell activation and quiescence by Wnts, Notch, quiescencespecific gene-signaling networks, and epigenetics.

Satellite cells recruit endothelial cells and also promote angiogenesis by secreting vascular endothelial growth factor (VEGF); in turn, endothelial cells promote satellite cell expansion (Latroche et al. 2015) . Satellite cells also pattern the architecture of nearby capillaries through VEGF-A signaling, and endothelial cells produce the Notch ligand Dll4 that helps sustain the stemness of satellite cells close to vessels by maintaining their quiescence (Verma et al. 2018 ). The proximate, even intimate, juxtaposition of two cell types, in this case endothelial and muscle satellite cells, has potent meaning for interpretation of regenerative signaling, just as the proximity of satellite cells on the fiber sarcolemma has for mechanical signal transduction in satellite cell activation (Anderson 2000; Wozniak et al. 2003; Wozniak and Anderson 2009 ) and satellite cell influences on myonuclei and fibroblasts (Englund et al. 2021) . Satellite cell interactions with macrophages include receiving macrophage-derived HGF which mediates secretion of semaphorin3A (Sema3A) by myoblasts (Yamada et al. 2010; Do et al. 2011; Sato et al. 2013; Sakaguchi et al. 2014) . In turn, this influences many cells involved in muscle regeneration (Anderson et al. 2017a) , and the development of slow fibers through neuropilinA-plex-inA3, myogenin, and Mef2D signaling to slow myosin expression (Tatsumi et al. 2017) . Importantly, Sema3A is implicated in synchronizing motor neurite elongation during the re-establishment of NMJs (Tatsumi et al. 2009a; Sato et al. 2013) .

It is interesting to consider that while FAPs contribute to regulating satellite cell activity, satellite cells also configure the responsiveness of fibroblasts in the niche around fibers during hypertrophy (Fry et al. 2014) . This cross-talk among cells during muscle regeneration thus has a systemic context (Pillon et 

The fascinating intricacies of nerve-muscle interaction, 

Physical activity and exercise induce muscle plasticity and adaptive responses in the absence of injury. During activity, fibers release myokines (cytokines) that exert autocrine, paracrine and systemic effects on muscle fibers (anabolic and catabolic) and satellite cells (Bugera et al. 2018; Cornish et al. 2018 Cornish et al. , 2020a . Those actions are integrated into the network of signals released from inflammatory and other cells Pedersen 2005, 2020; Febbraio 2008, 2012) including bone and adipose tissues (Kirk et al. 2020) . Regeneration of muscle after injury is significantly enhanced by exercise especially in synergy with injection of muscle stem cells (Contreras-Muñoz et al. 2021) . Physical activity also modulates the immune response, releasing growth factors that promote angiogenesis (Gregory et al. 1995; Brutsaert et al. 2002; Faria et al. 2008; Aurora et al. 2014) , and muscle contractile activity induced by electrical stimulation promotes nerve repair by upregulating neurotrophins and brain-derived neurotrophic factor (Gordon and English 2016) .

Interestingly, optimal fiber hypertrophy requires satellite cell proliferation and accrued contributions of their daughter cells toward fusion, fiber hypertrophy is possible through myonuclear domain expansion without satellite cell contributions, and satellite cells are not essential for fiber-type adaptations to life-long physical activity (Egan and Zierath 2013; Qaisar et al. 2016; Englund et al. 2020) . One form of severe muscle atrophy in humans is associated with admission to an intensive care unit (ICU) and is exceedingly resistant to treatment; it is attributed to the critical illness behind admission. A recent review distinguished ICU-acquired weakness by the predominant loss of myosin that affects both type-I (slow-twitch) and type-II (fast-twitch) fibers, and suggested resistance exercise during restricted blood flow (BFR) as a potential treatment (Lad et al. 2020 ). Metformin, a drug compound used to increase glucose transport into muscle and reduce blood glucose, reduces hypertrophy and transcriptional responses of skeletal muscle to resistance exercise training (Walton et al. 2019; Kulkarni et al. 2020) . Interestingly, recent findings on a trial of BFR resistanceexercise training suggested that changes in systemic concentrations of a subset of myokines show differential responses based on age and resistance training, in association with increases in muscle strength and quality (Cordingly, Anderson, and Cornish, submitted) .

A very recent paper demonstrated that exercise training in adult mice has differential impact on DNA methylation in muscle and non-muscle interstitial cells; detraining and also retraining demonstrated further differential responses, implicating "epigenetic memory" of prior adaptation that may help account for well-established differential response to exercise among muscles (Lavin et al. 2021; Sharples 2021; Wen et al. 2021) . Of note, that even non-muscle cells respond to exercise training highlights the system-wide impact of physical activity.

Systemic and muscle-tissue metabolism, including variations due to nutritional status, metabolic diseases (e.g., cachexia), and non-neuromuscular conditions such as diabetes and non-alcoholic fatty liver, affect the outcome of muscle regeneration-so many features of muscle growth, power, strength, endurance, and plasticity are reliant on anabolic processes. Anything that impacts protein synthesis, turnover, and remodelling-including responses to systemic stress-will reduce the capacity for timely regeneration (i.e., the speed of tissue repair) if not its effectiveness. The issue of protein turnover is especially important in interpreting results, given that use of transgenic technologies to conditionally alter the expression of particular genes in a cell or tissue of interest at a particular age, are typically validated by testing the level of mRNA or protein expression from tissue, without identifying the half-life of the targeted protein or synthetic rate after tamoxifen administration ends.

A recently reported atlas of single cells and nuclei in damaged muscle and liver revealed a common stress-response profile or signature is shared by many types of cells in both tissues, and other tissues as found in published datasets. In both liver and muscle stem cells, quiescent prior to tissue damage, ERK1/2 expression was essential prior to the onset of Notch-regulated myogenesis (Machado et al. 2021) . With parallel stress-response expression patterns in muscle and liver, it is not surprising to find that muscle is receptive to systemic physiological stressors (or that cell isolation itself is a stressor). The observation that metabolism itself makes system-wide interconnections across many types of cells including muscle stem cells. Each cell type has its own internal metabolic character, responsiveness and regulatory patterns mediated in its own distinctive niche; that idea forms a window into understanding tissue physiology Purohit and Dhawan 2019) .

A recent exciting report demonstrated systemic signaling through release of exosomal vesicles from myoblasts. Those vesicles are now known to mediate muscle growth Vechetti et al. 2021) in the absence of satellite cell cycling. The notion of non-cellular, circulating and/or local influences on growth, cell-cell signaling, and likely regeneration in muscle, extend findings of systemic influences on regenerative capacity between old and younger animals in earlier parabiosis experiments (Conboy et al. 2005; ).

Muscle regeneration is a "team effort"; an individual cell lineage acting on its own, cannot establish a fully functional muscle. The effective outcome of muscle regeneration therefore, depends on all the cellular players, the timing of damage and inflammation, muscle-specific architecture, and the physical use and activity of a muscle. The exquisite genetic, biochemical, physiological, and epigenetic regulation of the timing of satellite cell activation and changes in satellite cell transitions to and from quiescence with aging (Addicks et al. 2019; Kimmel et al. 2020; Sincennes et al. 2021 ) have major implications for muscle regeneration capacity. As well, the distribution of fibrosis, vascular compromise and oxygenation, inflammation, and muscle anatomy before injury will affect the whole gamut of myogenic processes that ensue, from a particular mode of injury in each muscle to the outcome of satellite cell cycling and myotube maturation (Csete et al. 2001) . Again, fiber type plays a role, since shear forces on the sarcolemma are higher in rapidly contracting type-II fibers and make them more susceptible to exercise-related damage. Different species and taxa also present fascinating distinctions across many features of muscle development, satellite cell distribution and regulation, muscle use, and evolutionary modifications based on life-history that may affect regeneration capacity, as recently explored in a variety of fish species (Zhang and Anderson 2014; Knappe et al. 2015; Gurevich et al. 2016; Ratnayake and Currie 2017; Anderson et al. 2019; Pourghadamyari et al. 2019; Tingle et al. 2019; Christian and Benian 2020; Hiebert and Anderson 2020; Montandon et al. 2021) . Although autophagy may decrease with aging, inducing an increase in autophagy can rescue the effectiveness of regeneration by muscles in old animals (Park et al. 2019; .

The balance between stem capability and myogenic commitment by proliferating myoblasts is highly dependent on the cytoskeleton of the underlying muscle fiber. The dystrophin-associated protein complex (DAPC) distributes neuronal NO synthase (NOS-Iµ) within the dividing cell; that distribution, in turn, shapes the polarity of asymmetric division by muscle stem cells and normally ensures stem cell renewal (Dumont et al. 2015a, b; Chang et al. 2018; Feige et al. 2018; Addicks et al. 2019) . Loss of dystrophin from the cytoskeletal complex has serious impact in dystrophic muscle, and leads to exhaustion of the stem cells (and self-renewal capacity) during the ongoing regenerative response to dystrophic damage. The mechanistic finetuning that retains or restores and sustains the viability of stem cell pool ) is an essential concept for undertaking future research; that viability encapsulates what otherwise may appear as a stochastic or randomized process that determines the fate of daughter cells-that was the basis of a fascinating discussion at conferences in the late 1990s, and the puzzle took decades of focussed research to resolve. The process by which epidermal growth factor rescues the function of dystrophic satellite cells and builds the stem-cell potential of their progeny ) is only one of many details that now guides the search for therapies to promote regeneration.

The regulation of regenerative-muscle-cell lineages follows the pattern of networks mediated by muscle regulatory genes during development (Hernández-Hernández et al. 2017) . The discovery of MyoD and other MRF genes in the mid-1980s (Zammit 2017) and the impact of their deficient expression (Megeney et al. 1996; Anderson et al. 1998b; McIntosh et al. 1998; Huijbregts et al. 2001 ) have since revealed many key aspects of muscle regeneration (Lassar 2017) . The identification of non-coding micro RNAs that help regulate development (Mok et al. 2017 ) and regeneration (Drummond et al. 2008; Safdar et al. 2009; Aoi et al. 2010; Guller and Russell 2010; Nielsen et al. 2010; Wessner et al. 2010; Cheung et al. 2012; Sharma et al. 2014; Diniz and Wang 2016) adds additional layers of complexity to muscle responses to injury and disease. Further, the notion that exosomal vesicles convey important intercellular signals vociferously demands new approaches for testing hypotheses, including those used to characterize exocrine regulation (Mitchell et al. 2019) .

A systems-biology approach incorporates the complexities of stem cell function (Wosczyna and Rando 2018) and myogenic regeneration that proceeds in synchrony with inflammation and its resolution (Csete and Doyle 2002; Vodovotz et al. 2008) , exosome signaling, re-innervation processes (Daneshvar et al. 2020), vascular perfusion, metabolism, and exercise (Henriksen et al. 2012; Bugera et al. 2018; Cornish et al. 2020a) . Future research will reveal key intersections of the different processes that regulate muscle regeneration.

Notably, the mechanisms and actions of at least 25 gene products considered essential for myoblast fusion into the multinucleated syncytium known as a muscle fiber are not known (Deng et al. 2017 ). There is so much more to learn about myoblast interaction with, and migration through the ECM, and many physical properties of the cellular and acellular components of the niche around myoblasts as they fuse (Roveimiab et al. 2020) . Figure 1 provides an overview of local influences, muscle-specific environment, regional adaptive plasticity, and systemic influences on muscle regeneration from a generic injury.

Current key concepts of muscle regeneration 1. Myogenic regeneration occurs within the cellular ecosystem of muscle tissue. The ecosystem includes signals and responses by satellite cells, inflammatory and vascular cells, fibro-adipogenic precursors and derivatives, the ECM, and nervous tissue. Cells and their interactions within muscle tissue during myogenesis are further embedded in systemic physiology, physical activity, and aging. Evaluating the integrative physiology of this "ecosystem" with an open-ended systems-biology approach will become an increasingly essential, counterpart to the exciting and more focussed research that targets the single-cell behavior or molecular interactions. 2. Regenerative potential and the adaptive plasticity of skeletal muscle are overlain by regionalized epigenetic modifications implicated in responses to exercise, aging, hormones, circulating factors, and disease. The extent and timing of regeneration are muscle-specific and interrelated with the vascular architecture and the use of that muscle. 3. Cells "talk" and socialize amongst themselves and with the ECM while they respond to systemic influences. Precisely how this happens is exquisitely complex, as tantalizingly illustrated by ongoing discovery of processes influenced via nanotubes, exosomal vesicles, gaseous transmission, and systemic factors. New hypotheses now must account for the hierarchy by which responses of one cell induce and direct subsequent responses by other cells, including fibers. This systemic, intercellular crosstalk during muscle regeneration forms the gestalt at the satellite cell niche, the focal point of muscle regeneration. However, we should not forget that satellite cells also influence the postmitotic nuclei inside fibers, and can optimize long-term adaptation and muscle responses to exercise. 4. Cells move; all types of cells in muscle, will move, and their migration processes are essential to regeneration. Migration behavior is highly dynamic, both biomechanically and in time. Myoblasts build and change their own operational niche and leave exosome and nano-scale "clues" to other cells along their trajectory, with major implications to fiber formation and health, as well as regeneration. Anticipating cell movement necessitates innovative experiments to visualize and track functional molecular cascades -these methods are real-time "lenses" to help interpret observations by accounting for motility. 5. Physical activity and concomitant signaling are highimpact arbiters of the outcome of regeneration. As in muscle hypertrophy, such signals to and from satellite (and other) cells in muscle, are mediated by integrated influences of metabolism, myokines, angiogenesis, innervation, and the configuration and composition of the ECM. 6. The intricate architecture of skeletal muscle, from the level of gross morphology in classical anatomy to its structure at ultrastructural, spectroscopic, and nanoscale resolutions, displays the functional hierarchy of a highly evolved muscle physiology. The complexity of muscle regeneration in three dimensions over time cannot be fully modeled by deconstructing the tissue processes in culture.

Astute experimental design is critical to the clarity of research findings. The variables we measure, the research tools we use, the independent animal controls or human participants, and the specificity of our hypotheses-the questions we ask-are all part of anticipating research outcomes. The tools for tracking the outcome of muscle regeneration have evolved tremendously since the early development of standardized instrumentation. Even a new, simple way to isolating primary satellite cells using ice-cold incubation, can advance research on regeneration (Benedetti et al. 2021) . Recent discovery of MYC-dependent satellite cell function, revealed through CRISPR/Cas9 editing of the MyoD locus in young growing muscle, opened new potential for understanding how genome topology mediates activation of satellite cells (He et al. 2021) . Exciting new avenues for conditional, cell-specific genetic manipulation will provide highly refined, wonderful insights into processes that change with gain-or loss-of-function by particular cells at particular junctures in regeneration, disease, and activity [e.g., ].

Multivariate readouts of muscle and single-cell transcriptomics, physiology and metabolism; systemic metabolism and biochemical interactions; and muscle structure coupled with functional genomics, cell-lineage tracing, and singlecell real-time tracking are all powerful tools, often accessible through collaboration. In effect, data readouts from studies on single fibers, dispersed cell cultures and co-cultures, and in vivo muscle tissue, coupled with diagnostics and treatment outcomes in a clinical setting can now be re-integrated by systems approaches (Owens et al. 2015) . My lab ventured a short distance in that direction in our investigations of potential denervation and preserved satellite-cell responsiveness in muscle after rotator-cuff injury (Gigliotti et al. 2015 (Gigliotti et al. , 2016 (Gigliotti et al. , 2017 , aiming toward eventual clinical trials to promote regeneration (Gigliotti et al. 2016) .

There are still major gaps in our knowledge of muscle regeneration related to the impact and type of physical activity, with all the local and systemic mediators released by exercise (Bugera et al. 2018; Cornish et al. 2018 Cornish et al. , 2020a . Physical activity likely influences the resolution of inflammation, where the timing is critical; myotube alignment during regeneration; and the role of the ECM and ECM-bound ligands in guiding cells during movement. Myoblast migration (Roveimiab et al. 2020 ) and the mechanisms of axonal targeting to neuromuscular junctions through synchronized myogenesis and nerve-muscle connectivity during regeneration (Daneshvar et al. 2020) can be closely scrutinized by microfluidics applications, now used for high-throughput screening for mutations (Markin et al. 2021 ) and biological assays (Grant et al. 2018) .

Newly formed fibers will continue to grow by hypertrophy and nuclear accretion as part of adaptations to activity or compensation for disease, but the epigenetic "finesse" on the outcome of regeneration (Giordani and Puri 2013) remains to be mapped. As well, the mitotic clock that shortens telomere length with repetitive cell cycling of precursor cells during regeneration from disease (Decary et al. 2000; Renault et al. 2000; Cooper et al. 2003 ) plays a role that is still contentious. We have yet to understand observations that in athletes, minimum terminal restriction fragment length is reduced only in proportion to the degree of stress during distance running and power lifting (Mouly et al. 2005; Kadi and Ponsot 2010) .

Overall, the philosophy of research on muscle regeneration, if it could be characterized as a philosophy, is ultimately directed toward improving the outcome, however, remotely feasible. Indeed, functional recovery is the primary hallmark of muscle regeneration (Forcina et al. 2020) . In considering how key concepts in this field of muscle regeneration have changed in the past 100 years (Table 1 ) and how methodologies and technologies have advanced, we find that even classical ideas, misinterpretations and assumptions (revealed by our current knowledge), and speculation and discussion by experts, all contribute to re-imagining the horizons of possibility.

The stepwise pattern of tissue changes during muscle regeneration is well established, involving inflammatory cells (M1 and M2), vasculature, satellite cells, fibers, FAPs, extracellular matrix, nerves, and myelinating and terminal Schwann cells. Each of type of cell both produces and Fig. 1 Skeletal muscle regeneration in context of local, regional, and systemic influences. A Under normal conditions (top right), muscle fibers with adjacent muscle satellite cells are innervated at neuromuscular junctions (NMJs) and situated in an active environment of multiplex cross-talk with components of the extracellular matrix (ECM), fibro-adipogenic precursors (FAPs), resident macrophages, fibroblasts, terminal Schwann cells, and endothelial cells lining blood vessels. The tissue complex is under tension from muscle insertions. B Soon after a disruptive injury (pointed wedge at the right), cells and fiber fragments (bottom right) are dispersed in the ECM, inflammatory (M1) macrophages infiltrate, satellite cells are activated, and many types of cells proliferate. Axons from spinal cord motor neurons (centre) begin to undergo Wallerian degeneration. Cellular cross-talk is fully engaged at cellular and molecular levels. Musclespecific ecology related to 3-dimensional position and architecture (formed through development), the vascular and nerve supplies, and influences of genetics and epigenetics, engages signaling pathways through microRNAs, muscle regulatory genes, growth factors, exosomal vesicles, and their interplay with tension, inflammatory processes, ECM composition and fibrosis, with impact on cell cycling, migration, and differentiation behavior. C Regional adaptive plasticity shapes the regenerating muscle tissue (left) through a balance of tension, activity, and exercise with metabolic regulatory feedback loops (through endocrine, exocrine, and myokine pathways), age, prior injury, and extant disease. These influences act on myoblasts, fibroblasts, FAPs, endothelial and anti-inflammatory (M2) macrophages, as neurites begin to reconnect with elongating myotubes at nascent NMJs. Systemic influences of other tissues on the regenerating muscle, including genetics, metabolism, nutrition, disease and age, all contribute to the maturation of muscle 3-dimensional structure, stiffness, function, and adaptive responsiveness. Legend to symbols appears in the box (bottom left) receives multiple signals within their occupied niche at any given time. While the niche locations of satellite cells and terminal Schwann are particularly identifiable, every cell occupies and tailors its niche-a niche that presents signaling ligands and receptors, and stores signaling molecules produced elsewhere. And every cell responds or very likely will respond to metabolic and mechanical or activity-based signals, from local to systemic.

All those signals change in a time-dependent manner during regeneration; each type of cell and structure "recognizes" the signals and compiles or integrates them in a way that directs a particular response or action. That integration parallels the way in which the electrical output of a neuron is compiled from excitatory and inhibitory input at dendritic spines, soma, and axon in a complex summative and time-dependent mechanism. In muscle, systemic and localized signaling combine to impact the manner of such responses. That understanding enhances our ability to dissect and describe the behaviour of a given cell population in a region of regenerating muscle tissue.

Notably, the population-level behaviour of any type of cell will be displayed as an average response that accounts for the level of population heterogeneity. Single-cell approaches are astute in displaying responses based on gene and protein expression, morphological interactions, and extant physiology at a single time point; vital and video-capture imaging help capture those responses. We use many models of muscle regeneration, imposing particular injuries or metabolic and disease conditions to test new ideas, and have found great success in revealing how those cells interact. Over the decades of inquiry, those many models of muscle damage were explored with increasing ability to resolve patterns. Technologies have developed through application of highly specific molecular, cellular, and imaging tools that can observe and manipulate individual cells or a cell population. And objective observations and data can be analyzed with powerful computational and statistical approaches of genomics, genetics, computational biology, and time-lapse imaging. Ongoing research will probe ever-further into the mechanisms regulating cell behaviour and interactions.

While science rapidly embraces new methods, classical methods are still valuable, as is a review of historical literature. Understanding the trends in thinking can gives insights that refine an experiment or modify an approach. Reading old monographs is also fascinating as it helps us feel the energy of previous scientists as they reached toward their own technological limits to pose questions and open doors, ever hopeful they too, might advance the concepts of muscle regeneration.

The goal of this review was not to characterize the burgeoning literature on molecular genetics, epigenetics, and cell signaling by all types of cells involved in muscle regeneration. Nor was it to review the exquisite combinations of modern technology such as CRISPR in intraosseous chimeric cell therapy to promote systemic muscle regeneration and prevent disease progression, now supporting huge strides in systemic treatment of diseases such as Duchenne muscular dystrophy (Siemionow et al. 2019 (Siemionow et al. , 2021 . Rather, the review aimed to identify distinctions between current concepts in muscle regeneration and earlier historical ideas, and trace the general timeline of research advances (Table 2 ). Our conceptualization of a process shapes the way we approach its exploration, just as the model of injury we select for an experiment will shape the nature of findings and variables that are available in studying the outcome of a perturbation or potential therapy we might superimpose (Hardy et al. 2016; Tatsumi et al. 2017) .

It is the accessible scope of activities or responses made by a single cell, a cell population, or a mixed collection of all the cells within a tissue, that provides the basis of our clinical and research practices. Concepts-how we think Cells are dynamic in time and through biomechanical influences on molecular signaling pathways often Less successful after ischemia-reperfusion injury than after anoxia Physical activity arbitrates the outcome, mediated by metabolism, myokines, angiogenesis, innervation, and the extracellular matrix Tracking functional repair requires standardized protocols and tools 3-Dimensional skeletal muscle architecture over a broad range of resolutions, is an important readout of a highly evolved hierarchy of function about each level of organization-form the foundational map of our understanding. Alas, our concepts also form the basis of our assumptions, since we use that map to speculate on the unknown, choose a direction, and anticipate the next horizon. It's exciting to learn new concepts in our own and other fields, since those ideas bring, even force, a paradigm shift in perspectives and redirect our attention. Our focus shifts to the next question, whether new or refreshed from classical literature by modern technologies, insights, and skepticism. The impact of intricate molecular-genetic signaling networks-a critical foundation in understanding cell behaviour in muscle regeneration and therapeutics-must be interpreted in context of the bigger picture of structure-function relationships and systemic influences (Swaggart and McNally 2014) . Ongoing discoveries highlight that every part of the muscle structure is important in some fashion. That notion provides important clues to us as researchers, about the functional context and functionality of muscle during regeneration. The beautiful three-dimensional architecture of skeletal muscle has evolved to optimize the functions of all cells in normal muscle; recognizing how that architecture integrates with physiology is critical to advancing our understanding of muscle regeneration. Deciphering the interplay of layered mechanisms of damage in traumatic muscle injury, for example, by high-energy ballistic damage from firearms (Moriscot et al. 2021) , will advance our thinking about rehabilitation approaches that promote regeneration and function.

Research on muscle regeneration intersects at many levels, with ideas gleaned from other research domains. Even apparently simple concepts can spark creativity and drive new hypotheses directed to pose the why? and how? and what happens then? questions at the core of research. Skeletal muscle, the engine of our voluntary activity, is critical to health, both physical and mental; one can only hope that the remarkable capability of muscle tissue to regenerate through myriad dynamic inter-cellular interactions and signaling processes, continues to capture the imagination of future generations of researchers. Key concepts in skeletal muscle regeneration will evolve as does our understanding of the processes influencing cellular and molecular interactions. (2002) 

MLL1 is required for PAX7 expression and satellite cell self-renewal in mice

Nitric oxide synthase inhibition impairs muscle regrowth following immobilization

CD34 promotes satellite cell motility and entry into proliferation to facilitate efficient skeletal muscle regeneration

Absence of dystrophin disrupts skeletal muscle signaling: roles of Ca2+, reactive oxygen species, and nitric oxide in the development of muscular dystrophy

Regneration of striated muscle, and myogenesis

Pharmaceutical characterization of MyoNovin, a novel skeletal muscle regenerator: in silico, in vitro and in vivo studies

Dystrophic changes in mdx muscle regenerating from denervation and devascularization

A role for nitric oxide in muscle repair: nitric oxide-mediated activation of muscle satellite cells

Activation of muscle satellite cells in single-fiber cultures

Satellite cell activation on fibers: modeling events in vivo-an invited review

Functional regeneration in the hindlimb skeletal muscle of the mdx mouse

Recovery from disuse osteopenia coincident to restoration of muscle strength in mdx mice

Deflazacort but not prednisone improves both muscle repair and fiber growth in diaphragm and limb muscle in vivo in the mdx dystrophic mouse

Dystrophy and myogenesis in mdx diaphragm muscle

Levels of MyoD protein expression following injury of mdx and normal limb muscle are modified by thyroid hormone

The role of semaphorin3A in myogenic regeneration and the formation of functional neuromuscular junctions on new fibres

Nitric oxide treatment attenuates muscle atrophy during hind limb suspension in mice

Novel "omega muscle units" in superficial body-wall myotomes during metamorphosis in the northern brook lamprey (Ichthyomyzon fossor)

Anonymous (1957) Increasing muscle strength

The microRNA miR-696 regulates PGC-1{alpha} in mouse skeletal muscle in response to physical activity

Persistent and improved functional gain in mdx dystrophic mice after treatment with L-arginine and deflazacort

Skeletal muscle regulates metabolism via interorgan crosstalk: roles in health and disease

Mimicking muscle stem cell quiescence in culture: methods for synchronization in reversible arrest

Pathogenic role of antisignal recognition protein and anti-3-Hydroxy-3-methylglutaryl-CoA reductase antibodies in necrotizing myopathies: Myofiber atrophy and impairment of muscle regeneration in necrotizing autoimmune myopathies

Physical rehabilitation improves muscle function following volumetric muscle loss injury

3D printing decellularized extracellular matrix to design biomimetic scaffolds for skeletal muscle tissue engineering

Schwann cells in neuromuscular junction formation and maintenance

The provisional matrix: setting the stage for tissue repair outcomes

Skeletal muscle wasting and renewal: a pivotal role of myokine IL-6

A novel approach for the isolation and longterm expansion of pure satellite cells based on ice-cold treatment

Wnt7a stimulates myogenic stem cell motility and engraftment resulting in improved muscle strength

Fibronectin regulates Wnt7a signaling and satellite cell expansion

Building muscle: molecular regulation of myogenesis

Stem cells, angiogenesis and muscle healing: a potential role in massage therapies?

Supplemental nitric oxide augments satellite cell activity on cultured myofibers from aged mice

Nitric oxide reverses prednisolone-induced inactivation of muscle satellite cells

Fibro-Adipogenic Progenitors Cross-Talk in Skeletal Muscle: The Social Network

Regeneration of single skeletal muscle fibers in vitro

Mechanisms modulating skeletal muscle phenotype

Nitric oxide regulates multiple functions and fate of adult progenitor and stem cells

Trends in training in physical medicine

Evidence that satellite cell decrement contributes to preferential decline in nuclear number from large fibres during murine age-related muscle atrophy

Increased Wnt signaling during aging alters muscle stem cell fate and increases fibrosis

Intrinsic changes and extrinsic influences of myogenic stem cell function during aging

Discussion on an evaluation of the methods of increasing muscle strength

Pathological changes in muscle as a result of disturbances of circulation. An experimental study of Volkmann's ischemic paralysis

Contractile properties of skeletal muscles from young, adult and aged mice

Integrin α3β1 binding to fibronectin is dependent on the ninth type III repeat

Nitric oxide release combined with nonsteroidal antiinflammatory activity prevents muscular dystrophy pathology and enhances stem cell therapy

Regional differences in expression of VEGF mRNA in rat gastrocnemius following 1 hr exercise or electrical stimulation

PAX3 and PAX7 as upstream regulators of myogenesis

Gene regulatory networks and transcriptional mechanisms that control myogenesis

The systemic myokine response of decorin, interleukin-6 (IL-6) and interleukin-15 (IL-15) to an acute bout of blood flow restricted exercise

Regeneration of the completely excised gastrocnemius muscle in the frog and rat from minced muscle fragments

The regeneration of minced muscles

The regeneration of skeletal muscle fibers following injury: a review

Muscle transplantation between young and old rats: age of host determines recovery

Impact of ageing on muscle cell regeneration

The dystrophin glycoprotein complex regulates the epigenetic activation of muscle stem cell commitment

Cellular and molecular regulation of muscle regeneration

Inflammation and skeletal muscle regeneration: leave it to the macrophages!

Single-cell migration chip for chemotaxis-based microfluidic selection of heterogeneous cell populations

Role of matrix metalloproteinases in skeletal muscle: migration, differentiation, regeneration and fibrosis

The role of satellite and other functional cell types in muscle repair and regeneration

YY1 regulates skeletal muscle regeneration through controlling metabolic reprogramming of satellite cells

Maintenance of muscle stem-cell quiescence by microRNA-489

Administration of the non-steroidal antiinflammatory drug ibuprofen increases macrophage concentrations but reduces necrosis during modified muscle use

Musclederived extracellular matrix on sinusoidal wavy surfaces synergistically promotes myogenic differentiation and maturation

Animal models of sarcopenia

Satellite cells and skeletal muscle regeneration

Regeneration of mammalian skeletal muscle. Basic mechanisms and clinical implications

Wnt signaling in skeletal muscle dynamics: myogenesis, neuromuscular synapse and fibrosis

Extracellular matrix signaling in morphogenesis and repair

Skeletal muscle adaptations to endurance training in 60-to 70-yr-old men and women

Molecular basis of muscular dystrophies

Won't you be my neighbor? Muscle stem cells recruit endothelial cells to their niche

A population of myogenic stem cells that survives skeletal muscle aging

Notch-mediated restoration of regenerative potential to aged muscle

Regulation of the muscle fiber microenvironment by activated satellite cells during hypertrophy

Inducible depletion of satellite cells in adult, sedentary mice impairs muscle regenerative capacity without affecting sarcopenia

FoxO maintains a genuine muscle stem-cell quiescent state until geriatric age

Pattern of regeneration of muscle from progressive muscular dystrophy patients cultivated in vitro as compared to normal human skeletal muscle

Altered satellite cell responsiveness and denervation implicated in progression of rotator-cuff injury

Atrophy, inducible satellite cell activation, and possible denervation of supraspinatus muscle in injured human rotator-cuff muscle

Anderson JE (2017) Fibrosis, low vascularity, and fewer slow fibers after rotator-cuff injury

Vascularized and innervated skeletal muscle tissue engineering

High resolution three-dimensional reconstruction of fibrotic skeletal muscle extracellular matrix

Structure and function of the skeletal muscle extracellular matrix

Epigenetic control of skeletal muscle regeneration: Integrating genetic determinants and environmental changes

Signaling pathways regulating the fate of fibro/adipogenic progenitors (FAPs) in skeletal muscle regeneration and disease

Exercise-related changes in protein turnover in mammalian striated muscle

Effect of one stretch a week applied to the immobilized soleus muscle on rat muscle fiber morphology

2020) peripheral nerve regeneration and muscle reinnervation

The use of the rat as a model for studying peripheral nerve regeneration and sprouting after complete and partial nerve injuries

Strategies to promote peripheral nerve regeneration: electrical stimulation and/or exercise

Bone muscle crosstalk targets muscle regeneration pathway regulated by core circadian transcriptional repressors DEC1 and DEC2

Satellite and stem cells in muscle growth and repair

High-throughput enzyme kinetics with 3D microfluidics and imaging SAMDI mass spectrometry

The effect of exercise on the presence of leukocytes, erythrocytes and collagen fibers in skeletal muscle after contusion

Principles of occupational therapy in the treatment of the injured

The efficiency of intramuscular anastomoses, with observations on the regeneration of devascularized muscle

Phagocytosis of necrotic muscle in muscle isografts is influenced by the strain, age, and sex of host mice

A model of myogenesis in vivo, derived from detailed autoradiographic studies of regenerating skeletal muscle, challenges the concept of quantal mitosis

MicroRNAs in skeletal muscle: their role and regulation in development, disease and function

Asymmetric division of clonal muscle stem cells coordinates muscle regeneration in vivo

Muscle fiber type changes in human skeletal muscle after injuries and immobilization

Fiber type area and metabolic potential of the thigh muscle in man after knee surgery and immobilization

Early ultrastructural changes in skeletal muscle exposed to the local anaesthetic bupivacaine (Marcaine)

Cellular responses to free grafting of the extensor digitorum longus muscle of the rat

Calcium influx through a possible coupling of cation channels impacts skeletal muscle satellite cell activation in response to mechanical stretch

Comparative study of injury models for studying muscle regeneration in mice

A functional domain in the heavy chain of scatter factor/hepatocyte growth factor binds the c-Met receptor and induces cell dissociation but not mitogenesis

Expanding roles for muscle satellite cells in exerciseinduced hypertrophy

CRISPR/Cas9/AAV9-mediated in vivo editing identifies MYC regulation of 3D genome in skeletal muscle stem cell

Myokines in myogenesis and health

Type 2 innate signals stimulate fibro/adipogenic progenitors to facilitate muscle Regeneration

The myogenic regulatory factors, determinants of muscle development, cell identity and regeneration

NO more muscle fatigue

Satellite cell division and fiber hypertrophy alternate with new fiber formation during indeterminate muscle growth in juvenile lake sturgeon (Acipenser fulvescens)

Functional resolution of fibrosis in mdx mouse dystrophic heart and skeletal muscle by halofuginone

The absence of MyoD in regenerating skeletal muscle affects the expression pattern of basement membrane, interstitial matrix and integrin molecules that is consistent with delayed myotube formation

Muscle stem cells and aging

Satellite cell depletion does not inhibit adult skeletal muscle regrowth following unloading-induced atrophy

Use of motor units in relation to muscle fiber type and size in man

A new look at cytoskeletal NOS-1 and ƒ-dystroglycan changes in developing muscle and brain in control and mdx dystrophic mice

Jessen N (2021) Isolation and characterization of muscle stem cells, fibro-adipogenic progenitors, and macrophages from human skeletal muscle biopsies

Skeletal muscle regeneration, repair and remodelling in aging: the importance of muscle stem cells and vascularization

Muscle injury activates resident fibro/ adipogenic progenitors that facilitate myogenesis

TGF-B1 secretion by pro-inflammatory macrophages and controls fibrosis in Duchenne muscular dystrophy

The biology of satellite cells and telomeres in human skeletal muscle: effects of aging and physical activity

Histochemical and ultrastructural characteristics of a new muscle fibre type in avian striated muscle

Histochemical characteristics of vertebrate striated muscle: a review

Aging induces aberrant state transition kinetics in murine muscle stem cells

Muscle, bone, and fat crosstalk: the biological role of myokines, osteokines, and adipokines

) Function, morphology and protein expression of ageing skeletal muscle: a cross-sectional study of elderly men with different training backgrounds

A population of Pax7-expressing muscle progenitor cells show differential responses to muscle injury dependent on developmental stage and injury extent

Actions and interactions of IGF-I and MMPs during muscle regeneration

Stem cells migration during skeletal muscle regeneration -the role of Sdf-1/Cxcr4 and Sdf-1/Cxcr7 axis

Metformin alters skeletal muscle transcriptome adaptations to resistance training in older adults

Injected matrix stimulates myogenesis and regeneration of mouse skeletal muscle after ischaemic injury

Myosin isoforms in neonatal rat extensor digitorum longus, diaphragm, and soleus muscles

Intensive care unit-acquired weakness: not just another muscle atrophying condition

Finding MyoD and lessons learned along the way

Skeletal muscle microvasculature: a highly dynamic lifeline

Muscle injuries and strategies for improving their repair

Muscle transcriptional networks linked to resistance exercise training hypertrophic response heterogeneity

Aged skeletal muscle retains the ability to fully regenerate functional architecture

The cellular events of injured muscle regeneration depend on the nature of the injury

Differentiation-dependent susceptibility of human muscle cells to Zika virus infection

Satellite cells are increasingly refractory to activation by nitric oxide and stretch in aged mouse-muscle cultures

Nitric oxide and voluntary exercise together promote quadriceps hypertrophy and increase vascular density in female 18-mo-old mice

Functionally convergent white adipogenic progenitors of different lineages participate in a diffused system supporting tissue regeneration

Neuromuscular junction as an entity of nerve-muscle communication

Sarcolemmal nNOS anchoring reveals a qualitative difference between dystrophin and utrophin

Beyond polymer polarity: how the cytoskeleton builds a polarized cell

Cellular biomechanics in skeletal muscle regeneration

Cellular mechanisms of tissue fibrosis. 4. Structural and functional consequences of skeletal muscle fibrosis

The abuse of exercise

Associations of muscle lipid content with physical function and resistance training outcomes in older adults: altered responses with metformin

Extracellular matrix degradation and remodeling in development and disease

The regulation of Notch signaling in muscle stem cell activation and postnatal myogenesis

Functional properties of human muscle spindles

Tissue damage induces a conserved stress response that initiates quiescent muscle stem cell activation

Wnt signaling in myogenesis

Aberrant repair and fibrosis development in skeletal muscle

The effect of laminin-111 hydrogels on muscle regeneration in a murine model of injury

Revealing enzyme functional architecture via high-throughput microfluidic enzyme kinetics

Dystrophin orchestrates the epigenetic profile of muscle cells via miRNAs

Computational modeling of muscle regeneration and adaptation to advance muscle tissue regeneration strategies

The muscle stem cell niche in health and disease

FGF-2-dependent signaling activated in aged human skeletal muscle promotes intramuscular adipogenesis

Satellite cell of skeletal muscle fibers

Growth of regenerating skeletal muscle fibers in cats

Vascular nitric oxide: effects of exercise training in animals

Effective fiber hypertrophy in satellite cell-depleted skeletal muscle

Fast and slow twitch muscles in man

Initiation and duration of muscle precursor replication after mild and severe injury to skeletal muscle of mice. An autoradiographic study

Regeneration and myogenic cell proliferation correlate with taurine levels in dystrophin-and MyoD-deficient muscles

The effects of altered metabolism (hypothyroidism) on muscle repair in the mdx dystrophic mouse

Continuous myofiber remodeling in uninjured extraocular myofibers: myonuclear turnover and evidence for apoptosis

Activated satellite cells are present in uninjured extraocular muscles of mature mice

Activated satellite cells in extraocular muscles of normal adult monkeys and humans

Contractility of mdx skeletal muscle after denervation and devascularization

MyoD is required for myogenic stem cell function in adult skeletal muscle

Engineering skeletal muscle: Building complexity to achieve functionality

Secretome of adiposederived mesenchymal stem cells promotes skeletal muscle regeneration through synergistic action of extracellular vesicle cargo and soluble proteins

Nitric oxide donors improve prednisone effects on muscular dystrophy in the mdx mouse diaphragm

Redox control of skeletal muscle regeneration

microRNAs in skeletal muscle development

Examining muscle regeneration in zebrafish models of muscle disease

Firearms-related skeletal muscle trauma: pathophysiology and novel approaches for regeneration

Nerve regeneration from one into the other of two rats united in Siamese pairs

The mitotic clock in skeletal muscle regeneration, disease and cell mediated gene therapy

Regeneration versus fibrosis in skeletal muscle

Threedimensional niche stiffness synergizes with Wnt7a to modulate the extent of satellite cell symmetric self-renewal divisions

Early satellite cell communication creates a permissive environment for long-term muscle growth

Bones, Muscles and Vitamin C. I. The effect of a partial deficiency of vitamin C on the repair of bone and muscle in guinea-pigs

The basis of muscle regeneration

Muscle homeostasis and regeneration: from molecular mechanisms to therapeutic opportunities

Interleukin-6 myokine signaling in skeletal muscle: a double-edged sword?

Macrophages decide between regeneration and fibrosis in muscle

Fibro/adipogenic progenitors: a double-edged sword in skeletal muscle regeneration

A new device for measuring muscle strength-the myometer

Muscle specific microRNAs are regulated by endurance exercise in human skeletal muscle

Muscle satellite cells: a validated technique for light microscopic identification and a quantitative study of changes in their population following denervation

Morphological aspects of muscle fiber regeneration

A systems-based investigation into vitamin D and skeletal muscle repair, regeneration, and hypertrophy

Neuromuscular presentations in patients with COVID-19

Nitric oxide in myogenesis and therapeutic muscle repair

Inflammation during post-injury skeletal muscle regeneration

Sarcopenia targeting with autophagy mechanism by exercise

The effect of reinnervation on the distribution of muscle fibre types in the tibialis anterior muscle of the mouse

Aligned nanofibers of decellularized muscle ECM support myogenic activity in primary satellite cells in vitro

In vivo characterization of neural crest-derived fibro/adipogenic progenitor cells as a likely cellular substrate for craniofacial fibrofatty infiltrating disorders

Muscle as an endocrine organ: focus on muscle-derived interleukin-6

Muscles, exercise and obesity: skeletal muscle as a secretory organ

Historical perspectives: plasticity of mammalian skeletal muscle

Cross-talk between skeletal muscle and immune cells: muscle-derived mediators and metabolic implications

Follistatin induction by nitric oxide through cyclic GMP: a tightly regulated signaling pathway that controls myoblast fusion

Form and behavior of adult mammalian skeletal muscle in vitro

Establishing a new animal model for muscle regeneration studies

The nature and biology of basement membranes

Comparative nuclear matrix proteome analysis of skeletal muscle cells in different cellular states

Adult muscle stem cells: exploring the links between systemic and cellular metabolism

Muscle fiber type diversification during exercise and regeneration

Stem cell dynamics in muscle regeneration: Insights from live imaging in different animal models

Macrophages provide a transient muscle stem cell niche via NAMPT secretion

Glial cells maintain synaptic structure and function and promote development of the neuromuscular junction in vivo

Skeletal muscle regeneration and the mitotic clock

Satellie cells, myoblasts, and skeletal muscle regeneration

Epigenetic regulation of adult myogenesis

International consensus statement on the assessment of interprofessional learning outcomes

Emerging development of microfluidics-based approaches to improve studies of muscle cell migration

Traction and attraction: haptotaxis substrates collagen and fibronectin interact with chemotaxis by HGF to regulate myoblast migration in a microfluidic device

A case of Volkmann's contracture treated by shortening the radius and ulna

The value of freedom and exercise after operations

Motor endplate-anatomical, functional, and molecular concepts in the historical perspective

miRNA in the regulation of skeletal muscle adaptation to acute endurance exercise in C57Bl/6J male mice

Malat-1-PRC2-EZH1 interaction supports adaptive oxidative stress dependent epigenome remodeling in skeletal myotubes

Regenerative function of immune system: Modulation of muscle stem cells

Implication of antiinflammatory macrophages in regenerative moto-neuritogenesis: promotion of myoblast migration and neural chemorepellent semaphorin 3A expression in injured muscle

Satellite cells produce neural chemorepellent semaphorin 3A upon muscle injury

Effects of skeletal muscle regeneration on the proliferation potential of satellite cells

Co-administration of ibuprofen and nitric oxide is an effective experimental therapy for muscular dystrophy, with immediate applicability to humans

Ex vivo treatment with nitric oxide increases mesoangioblast therapeutic efficacy in muscular dystrophy

A dual acting compound releasing nitric oxide (NO) and ibuprofen, NCX 320, shows significant therapeutic effects in a mouse model of muscular dystrophy

Ibuprofen plus isosorbide dinitrate treatment in the mdx mice ameliorates dystrophic heart structure

Bioprinting stem cells: building physiological tissues one cell at a time

An electron microscopic study of regeneration and satellite cells in human muscle

Satellite cells and fiber nuclei in muscle regeneration

Mega roles of microRNAs in regulation of skeletal muscle health and disease

Skeletal muscle possesses an epigenetic memory of exercise: role of nucleus type-specific DNA methylation

Delayed but excellent myogenic stem cell response of regenerating geriatric skeletal muscles in mice

Molecular mechanisms underlying maturation and maintenance of the vertebrate neuromuscular junction

3D timelapse analysis of muscle satellite cell motility

Muscle satellite cell proliferation and association: new insights from myofiber time-lapse imaging

Human dystrophin expressing chimeric (DEC) cell therapy ameliorates cardiac, respiratory, and skeletal muscle's function in Duchenne muscular dystrophy

Cardiac protection after systemic transplant of dystrophin expressing chimeric (DEC) cells to the mdx mouse model of Duchenne muscular dystrophy

Acetylation of PAX7 controls muscle stem cell self-renewal and differentiation potential in mice

Age influences the early events of skeletal muscle regeneration: studies of whole muscle grafts transplanted between young (8 weeks) and old (13-21 months) mice

Role of muscle stem cells in sarcopenia

Metabolic activity during the degenerative and early regenerative stages on skeletal muscle

The hallmarks of cancer as ecologically driven phenotypes

Exercise training modulates the nitric oxide synthase profile in skeletal muscle from old rats

Scatter factor/hepatocyte growth factor and its receptor, the c-met tyrosine kinase, can mediate a signal exchange between mesenchyme and epithelia during mouse development

Regulation of cellular chromatin state: insights from quiescence and differentiation

Eph/ephrin interactions modulate muscle satellite cell motility and patterning

Correlation between myofibrillar ATPase activity and myosin heavy chain composition in rabbit muscle fibers

The multiplicity of combinations of myosin light chains and heavy chains in histochemically typed single fibres. Rabbit Tibialis anterior muscle

Nonuniform myosin expression along single fibers of chronically stimulated and contralateral rabbit tibialis anterior muscles

Dynamics of the development of myogenic tissue under conditions of explantation and transplantation

Distinct transcriptional networks in quiescent myoblasts: a role for Wnt signaling in reversible vs. irreversible arrest

Mediators of the resolution of the inflammatory response

Dystrophin-glycoprotein complex: molecular organization and critical roles in skeletal muscle

Modifiers of heart and muscle function: where genetics meets physiology

Mechano-biology of skeletal muscle hypertrophy and regeneration: possible mechanism of stretch-induced activation of resident myogenic stem cells

Mechano-biology of resident myogenic stem cells: Molecular mechanism of stretch-induced activation of satellite cells

Release of hepatocyte growth factor from mechanically stretched skeletal muscle satellite cells and role of pH and nitric oxide

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

Possible implication of satellite cells in regenerative motoneuritogenesis: HGF upregulates neural chemorepellent Sema3A during myogenic differentiation

Slow-myofiber commitment by semaphorin 3A secreted from myogenic stem Cells

A role for calcium-calmodulin in regulating nitric oxide production during skeletal muscle satellite cell activation

Skeletal muscle loading changes its regenerative capacity

Myosin heavy chain isoforms in histochemically defined fiber types of rat muscle

Extracellular matrix structure

Evolving roles of muscleresident fibro-adipogenic progenitors in health, regeneration, neuromuscular disorders, and aging

Mechanical signal transduction in skeletal muscle growth and adaptation

Mechanisms of muscle injury, repair, and regeneration

Macrophages promote muscle membrane repair and muscle fibre growth and regeneration during modified muscle loading in mice in vivo

Satellite cell heterogeneity in skeletal muscle homeostasis

Muscle stem cells exhibit distinct clonal dynamics in response to tissue repair and homeostatic aging

Assessing muscle stem cell clonal complexity during aging

Nitric oxide (NO) and duchenne muscular dystrophy: no way to go?

Paradoxical changes underscore epigenetic reprogramming during adult Zebrafish extraocular muscle regeneration

Mechanisms regulating neuromuscular junction development and function and causes of muscle wasting

Histochemical and ultrastructural changes in senile human skeletal muscle

Regeneration of skeletal muscle

Mesenchymal progenitors distinct from satellite cells contribute to ectopic fat cell formation in skeletal muscle

Anti-inflammatory interventions and skeletal muscle injury: benefit or detriment?

Mechanical overload-induced musclederived extracellular vesicles promote adipose tissue lipolysis

Muscle satellite cell crosstalk with a vascular niche maintains quiescence via VEGF and notch signaling

Subcellular and cellular locations of nitric oxide synthase isoforms as determinants of health and disease

Computational models provide insight into in vivo studies and reveal the complex role of fibrosis in mdx muscle regeneration

Translational systems biology of inflammation

Metformin blunts muscle hypertrophy in response to progressive resistance exercise training in older adults: A randomized, double-blind, placebo-controlled, multicenter trial: the MASTERS trial

Development of a nitric oxide-releasing analogue of the muscle relaxant guaifenesin for skeletal muscle satellite cell myogenesis

EGFR-aurka signaling rescues polarity and regeneration defects in dystrophindeficient muscle stem cells by increasing asymmetric divisions

Review of recent patents and developments in skeletal muscle regeneration

Three-dimensional tissue-engineered human skeletal muscle model of Pompe disease

) c-MET regulates myoblast motility and myocyte fusion during adult skeletal muscle regeneration

Myofibrillar protein synthesis in young and old men

Nucleus type-specific DNA methylomics reveals epigenetic "memory" of prior adaptation in skeletal muscle

Is there a role for microRNAs in exercise immunology? A synopsis of current literature and future developments

The neuromuscular junction: selective remodeling of synaptic regulators at the nerve/muscle interface

Recent athletic injuries and their treatment

A muscle stem cell support group: coordinated cellular responses in muscle regeneration

Single-fiber isolation and maintenance of satellite cell quiescence

Nitric oxide-dependence of satellite stem cell activation and quiescence on normal skeletal muscle fibers

The dynamics of the nitric oxide release-transient from stretched muscle cells

Signaling satellite-cell activation in skeletal muscle: Markers, models, stretch, and potential alternate pathways

C-met expression and mechanical activation of satellite cells on cultured muscle fibers

Cross-talk between notch and hedgehog regulates hepatic stellate cell fate

Matrix metalloproteinase-2 mediates stretch-induced activation of skeletal muscle satellite cells in a nitric oxide-dependent manner

High concentrations of HGF inhibit skeletal muscle satellite cell proliferation in vitro by inducing expression of myostatin: a possible mechanism for reestablishing satellite cell quiescence in vivo

Stem cell aging in skeletal muscle regeneration and disease

Hierarchical signaling transduction of the immune and muscle cell crosstalk in muscle regeneration

Gli1 defines a subset of fibro-adipogenic progenitors that promote skeletal muscle regeneration with less fat accumulation

Hasson P (2021) Fibroblast fusion to the muscle fiber regulates myotendinous junction formation

Decellularized extracellular matrix (ECM) as a model to study fibrotic ECM mechanobiology

Hoxa10 mediates positional memory to govern stem cell function in adult skeletal muscle

Autophagy-dependent regulation of skeletal muscle regeneration and strength by a RHOGEF

ARHGEF3 regulates skeletal muscle regeneration and strength through autophagy

Function of the myogenic regulatory factors Myf5, MyoD, Myogenin and MRF4 in skeletal muscle, satellite cells and regenerative myogenesis

Satellite cell activation and populations on single muscle-fiber cultures from adult zebrafish (Danio rerio)

Acknowledgements Many collaborators, colleagues and students have made this work possible.Author contributions JA is the sole author; after the invitation to write this review, the author conducted all research for the literature review, drafted and edited the manuscript, and composed the figure.Funding The author is grateful for research funding from the Natural Science and Engineering Research Council of Canada (NSERC-RG-PIN-3833-2015).

The author has no relevant financial or non-financial interests to disclose that are relevant to the content of this article. The review was researched and written by the author, alone, after invitation.