Formation of new myotubes occurs exclusively at the multiple innervation zones of an embryonic large muscle

Regulative Mechanism of Guanidinoacetic Acid on Skeletal Muscle Development and Its Application Prospects in Animal Husbandry: A Review

Guanidinoacetic acid is the direct precursor of creatine and its phosphorylated derivative phosphocreatine in the body. It is a safe nutritional supplement that can be used to promote muscle growth and development. Improving the growth performance of livestock and poultry and meat quality is the eternal goal of the animal husbandry, and it is also the common demand of today's society and consumers. A large number of experimental studies have shown that guanidinoacetic acid could improve the growth performance of animals, promote muscle development and improve the health of animals. However, the mechanism of how it affects muscle development needs to be further elucidated. This article discusses the physical and chemical properties of guanidinoacetic acid and its synthesis pathway, explores its mechanism of how it promotes muscle development and growth, and also classifies and summarizes the impact of its application in animal husbandry, providing a scientific basis for this application. In addition, this article also proposes future directions for the development of this substance.

Striated muscle function, regeneration, and repair

As the only striated muscle tissues in the body, skeletal and cardiac muscle share numerous structural and functional characteristics, while exhibiting vastly different size and regenerative potential. Healthy skeletal muscle harbors a robust regenerative response that becomes inadequate after large muscle loss or in degenerative pathologies and aging. In contrast, the mammalian heart loses its regenerative capacity shortly after birth, leaving it susceptible to permanent damage by acute injury or chronic disease. In this review, we compare and contrast the physiology and regenerative potential of native skeletal and cardiac muscles, mechanisms underlying striated muscle dysfunction, and bioengineering strategies to treat muscle disorders. We focus on different sources for cellular therapy, biomaterials to augment the endogenous regenerative response, and progress in engineering and application of mature striated muscle tissues in vitro and in vivo. Finally, we discuss the challenges and perspectives in translating muscle bioengineering strategies to clinical practice.

Hepatocyte Growth Factor and Satellite Cell Activation

Satellite cells are the "currency" for the muscle growth that is critical to meat production in many species, as well as to phenotypic distinctions in development at the level of species or taxa, and for human muscle growth, function and regeneration. Careful research on the activation and behaviour of satellite cells, the stem cells in skeletal muscle, including cross-species comparisons, has potential to reveal the mechanisms underlying pathological conditions in animals and humans, and to anticipate implications of development, evolution and environmental change on muscle function and animal performance.

Targeting the motor end plates in the mouse hindlimb gives access to a greater number of spinal cord motor neurons: an approach to maximize retrograde transport

Lower motor neuron dysfunction is one of the most debilitating neurological conditions and, as such, significantly impacts on the quality of life of affected individuals. Within the last decade, the engineering of mouse models of lower motor neuron diseases has facilitated the development of new therapeutic scenarios aimed at delaying or reversing the progression of these conditions. In this context, motor end plates (MEPs) are highly specialized regions on the skeletal musculature that offer minimally invasive access to the pre-synaptic nerve terminals, henceforth to the spinal cord motor neurons. Transgenic technologies can take advantage of the relationship between the MEP regions on the skeletal muscles and the corresponding motor neurons to shuttle therapeutic genes into specific compartments within the ventral horn of the spinal cord. The first aim of this neuroanatomical investigation was to map the details of the organization of the MEP zones for the main muscles of the mouse hindlimb. The hindlimb was selected for the present work, as it is currently a common target to challenge the efficacy of therapies aimed at alleviating neuromuscular dysfunction. This MEP map was then used to guide series of intramuscular injections of Fluoro-Gold (FG) along the muscles' MEP zones, therefore revealing the distribution of the motor neurons that supply them. Targeting the entire MEP regions with FG increased the somatic availability of the retrograde tracer and, consequently, gave rise to FG-positive motor neurons that are organized into rostro-caudal columns spanning more spinal cord segments than previously reported. The results of this investigation will have positive implications for future studies involving the somatic delivery and retrograde transport of therapeutic transgenes into affected motor neurons. These data will also provide a framework for transgenic technologies aiming at maintaining the integrity of the neuromuscular junction for the treatment of lower motor neuron dysfunctions.

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

Regenerative mechanisms that regulate intramuscular motor innervation are thought to reside in the spatiotemporal expression of axon-guidance molecules. Our previous studies proposed a heretofore unexplored role of resident myogenic stem cell (satellite cell)-derived myoblasts as a key presenter of a secreted neural chemorepellent semaphorin 3A (Sema3A); hepatocyte growth factor (HGF) triggered its expression exclusively at the early-differentiation phase. In order to verify this concept, the present study was designed to clarify a paracrine source of HGF release. In vitro experiments demonstrated that activated anti-inflammatory macrophages (CD206-positive M2) produce HGF and thereby promote myoblast chemoattraction and Sema3A expression. Media from pro-inflammatory macrophage cultures (M1) did not show any significant effect. M2 also enhanced the expression of myoblast-differentiation markers in culture, and infiltrated predominantly at the early-differentiation phase (3-5 days post-injury); M2 were confirmed to produce HGF as monitored by in vivo/ex vivo immunocytochemistry of CD11b/CD206/HGF-positive cells and by HGF in situ hybridization of cardiotoxin- or crush-injured tibialis anterior muscle, respectively. These studies advance our understanding of the stage-specific activation of Sema3A expression signaling. Findings, therefore, encourage the idea that M2 contribute to spatiotemporal up-regulation of extracellular Sema3A concentrations by producing HGF that, in turn, stimulates a burst of Sema3A secretion by myoblasts that are recruited to site of injury. This model may ensure a coordinated delay in re-attachment of motoneuron terminals onto damaged fibers early in muscle regeneration, and thus synchronize the recovery of muscle-fiber integrity and the early resolution of inflammation after injury.

Skeletal muscle degeneration and regeneration in mice and flies

Many aspects of skeletal muscle biology are remarkably similar between mammals and tiny insects, and experimental models of mice and flies (Drosophila) provide powerful tools to understand factors controlling the growth, maintenance, degeneration (atrophy and necrosis), and regeneration of normal and diseased muscles, with potential applications to the human condition. This review compares the limb muscles of mice and the indirect flight muscles of flies, with respect to the mechanisms of adult myofiber formation, homeostasis, atrophy, hypertrophy, and the response to muscle degeneration, with some comment on myogenic precursor cells and common gene regulatory pathways. There is a striking similarity between the species for events related to muscle atrophy and hypertrophy, without contribution of any myoblast fusion. Since the flight muscles of adult flies lack a population of reserve myogenic cells (equivalent to satellite cells), this indicates that such cells are not required for maintenance of normal muscle function. However, since satellite cells are essential in postnatal mammals for myogenesis and regeneration in response to myofiber necrosis, the extent to which such regeneration might be possible in flight muscles of adult flies remains unclear. Common cellular and molecular pathways for both species are outlined related to neuromuscular disorders and to age-related loss of skeletal muscle mass and function (sarcopenia). The commonality of events related to skeletal muscles in these disparate species (with vast differences in size, growth duration, longevity, and muscle activities) emphasizes the combined value and power of these experimental animal models.

Initiation of primary myogenesis in amniote limb muscles

BACKGROUND

Vertebrate muscles are defined and patterned at the stage of primary myotube formation, but there is no clear description of how these cells form in vivo. Of particular interest is whether primary myotubes are "seeded" by a unique myoblast population that differentiates as mononucleated myocytes, similar to the founder myoblasts of insects.

RESULTS

We analyzed the cell populations and processes leading to initiation of primary myogenesis in limb buds of rats and mice. Pax3(+ve) myogenic precursors migrate into the limb bud and initially consolidate into dorsal and ventral muscle masses in the absence of Pax7 expression. Approximately a day later, Pax7(+ve) cells appear in the central aspect of the limb base and subsequently throughout the limb muscle masses. Primary myogenesis is initiated within each muscle mass at a time when only Pax3, and not Pax7, protein can be detected. Primary myotubes form initially as elongate mononucleated myocytes, well before cleavage of the muscle masses has occurred. Multinucleate myotubes appear approximately a day later. A similar process is seen during initiation of chick limb primary myogenesis.

CONCLUSIONS

Primary myotubes of vertebrate limb muscles are initiated by mononucleated myocytes, that appear structurally analogous to the founder myoblasts of insects.

Muscle development and obesity: Is there a relationship?

The formation of skeletal muscle from the epithelial somites involves a series of events triggered by temporally and spatially discrete signals resulting in the generation of muscle fibers which vary in their contractile and metabolic nature. The fiber type composition of muscles varies between individuals and it has now been found that there are differences in fiber type proportions between lean and obese animals and humans. Amongst the possible causes of obesity, it has been suggested that inappropriate prenatal environments may 'program' the fetus and may lead to increased risks for disease in adult life. The characteristics of muscle are both heritable and plastic, giving the tissue some ability to adapt to signals and stimuli both pre and postnatally. Given that muscle is a site of fatty acid oxidation and carbohydrate metabolism and that its development can be changed by prenatal events, it is interesting to examine the possible relationship between muscle development and the risk of obesity.

The innervation and organization of motor units in a series-fibered human muscle: the brachioradialis

We studied the innervation and organization of motor units in the brachioradialis muscle of 25 normal human subjects. We recorded intramuscular EMG signals at points separated by 15 mm along the proximodistal muscle axis during moderate isometric contractions, identified from 27 to 61 (mean 39) individual motor units per subject using EMG decomposition, and estimated the locations of the endplates and distal muscle/tendon junctions from the motor-unit action potential (MUAP) propagation patterns and terminal standing waves. In three subjects all the motor units were innervated in a single endplate zone. In the other 22 subjects, the motor units were innervated in 3-6 (mean 4) distinct endplate zones separated by 15-55 mm along the proximodistal axis. One-third of the motor units had fibers innervated in more than one zone. The more distally innervated motor units had distinct terminal waves indicating tendonous termination, while the more proximal motor units lacked terminal waves, indicating intrafascicular termination. Analysis of blocked MUAP components revealed that 19% of the motor units had at least one doubly innervated fiber, i.e., a fiber innervated in two different endplate zones by two different motoneurons, and thus belonging to two different motor units. These results are consistent with the brachioradialis muscle having a series-fibered architecture consisting of multiple, overlapping bands of muscle fibers in most individuals and a simple parallel-fibered architecture in some individuals.

Structure, innervation, mechanical properties and reflex activation of a striated sphincter in the vestibule of the cat vagina

Vaginal constriction might be important for reproduction in mammals, but existing information is both limited and controversial. This paper shows the structure, mechanical properties, innervation and reflex response of a striated sphincter in the vestibule of the cat vagina. A Foley catheter coupled to a pressure transducer detected in the lumen of the vestibule a pressure wave that was induced by stimulation of the external branch of the motor pudendal nerve. The peak pressure of the wave induced by bilateral stimulation (30.6 cm H2O) was about double of the peak pressure wave induced unilaterally. The tetanus/twitch amplitude ratio was 4.5. The sphincter that produces the increase in vaginal pressure fatigues slowly. Digital, point-to-point summation of unilateral waves was greater than the wave induced bilaterally. Summation of the pressure wave induced by the separate stimulation of the terminal motor branches was also greater than the wave induced by the entire motor nerve. This might reflect multiple innervation of muscle fibres. Single, controlled probing of the vaginal vestibule induced a reflex discharge in the motor nerve. Repetitive probing (10 Hz) induced a motor nerve post-discharge lasting >1 min. The vaginal sphincter is two-half rings of striated muscle fibres in the wall of the vaginal vestibule; the fibres end freely in the dorsal and ventral midlines. Penetration of the vestibule by the penis might trigger sustained contraction of the vaginal sphincter.

Architectural analysis and intraoperative measurements demonstrate the unique design of the multifidus muscle for lumbar spine stability

BACKGROUND

Muscular instability is an important risk factor for lumbar spine injury and chronic low-back pain. Although the lumbar multifidus muscle is considered an important paraspinal muscle, its design features are not completely understood. The purpose of the present study was to determine the architectural properties, in vivo sarcomere length operating range, and passive mechanical properties of the human multifidus muscle. We hypothesized that its architecture would be characterized by short fibers and a large physiological cross-sectional area and that it would operate over a relatively wide range of sarcomere lengths but would have very stiff passive material properties.

METHODS

The lumbar spines of eight cadaver specimens were excised en bloc from T12 to the sacrum. Multifidus muscles were isolated from each vertebral level, permitting the architectural measurements of mass, sarcomere length, normalized fiber length, physiological cross-sectional area, and fiber length-to-muscle length ratio. To determine the sarcomere length operating range of the muscle, sarcomere lengths were measured from intraoperative biopsy specimens that were obtained with the spine in the flexed and extended positions. The material properties of single muscle fibers were obtained from passive stress-strain tests of excised biopsy specimens.

RESULTS

The average muscle mass (and standard error) was 146 +/- 8.7 g, and the average sarcomere length was 2.27 +/- 0.06 microm, yielding an average normalized fiber length of 5.66 +/- 0.65 cm, an average physiological cross-sectional area of 23.9 +/- 3.0 cm(2), and an average fiber length-to-muscle length ratio of 0.21 +/- 0.03. Intraoperative sarcomere length measurements revealed that the muscle operates from 1.98 +/- 0.15 microm in extension to 2.70 +/- 0.11 microm in flexion. Passive mechanical data suggested that the material properties of the muscle are comparable with those of muscles of the arm or leg.

CONCLUSIONS

The architectural design (a high cross-sectional area and a low fiber length-to-muscle length ratio) demonstrates that the multifidus muscle is uniquely designed as a stabilizer to produce large forces. Furthermore, multifidus sarcomeres are positioned on the ascending portion of the length-tension curve, allowing the muscle to become stronger as the spine assumes a forward-leaning posture.

Morphology and innervation of the human cremaster muscle in relation to its function

The electromyographic properties of the cremaster muscle (CM) are quite different from other skeletal muscles. It shows excessive spontaneous discharges, and the motor unit shape and firing frequency of the CM muscle differ from that of limb muscles. In this study, CM of six adult cadavers and six orchiectomy specimens were used to reveal the detailed histology of the muscle and provide an anatomophysiological explanation for these unusual electromyographic properties. Routine histochemical stains revealed the CM was composed of several distinct bundles of smooth and striated muscle fibers within connective tissue. The smooth muscle fibers that were more profuse than previously known and were not arranged in layers, but widely dispersed between striated muscle fibers. Bielschowsky silver staining technique, anti-neurofilament and anti-synaptophysin immunostaining showed the presence of multiple motor end-plates observed as a series of small dots or lines running along the striated muscle fibers and several nerve endings on a single muscle fiber. Myosin immunostaining confirmed the CM is a slow-twitch muscle, and alpha-actin smooth muscle immunostaining confirmed the presence of a large number of smooth muscle fibers. There were also small multipolar neurons forming nerve plexuses between smooth muscle fibers. Anti-GFAP immunostaining confirmed the presence of glial cells similar to astrocytes. In conclusion, the findings of this detailed anatomical study showed the CM, widely known as a striated muscle, contains a large number of smooth muscle fibers, and the spontaneous electromyographic discharges are due to the presence of multiple motor end-plates and dense innervation.

Role for the skeletal muscle action potential in non-Hebbian long-term depression at the amphibian (Bufo marinus) neuromuscular junction

Retrograde signaling from skeletal muscle cells to motor nerve terminals is a recognized mechanism for modulating the strength of neuromuscular transmission. We recently described a form of long-term depression of transmitter release at the mature neuromuscular junction that is dependent on the production of nitric oxide, most likely by the muscle cell (Etherington and Everett 2004 J Physiol (Lond) 559:507-517). We now show that the depression is blocked by treating neuromuscular preparations with mu-conotoxin G111A, an antagonist of skeletal muscle voltage gated sodium channels, indicating that the depression requires postsynaptic action potential firing. Experiments on dually-innervated sartorius muscles revealed that propagation of action potentials generated by low-frequency stimulation of one nerve branch gives rise to nitric-oxide mediated depression at unstimulated nerve terminals located many millimetres away on the same muscle fiber. The non-Hebbian pattern of expression of the depression, as well as its reliance on postsynaptic action potential firing, distinguish it from forms of synaptic depression described at immature neuromuscular synapses and may provide a mechanism for coregulation of the strength of motoneurons innervating the same postsynaptic cell.

Electrophysiological evidence of doubly innervated branched muscle fibers in the human brachioradialis muscle

OBJECTIVE

Motor-unit action potentials (MUAPs) with unstable satellite (late-latency) components are found in EMG signals from the brachioradialis muscles of normal subjects. We analyzed the morphology and blocking behavior of these MUAPs to determine their anatomical origin.

METHODS

EMG signals were recorded from the brachioradialis muscles of 5 normal subjects during moderate-level isometric contractions. MUAP waveforms, discharge patterns, and blocking were determined using computer-aided EMG decomposition.

RESULTS

Twelve MUAPs with unstable satellite potentials were detected, always two together in the same signal. Each MUAP also had a second unstable component associated with its main spike. The blocking behavior of the unstable components depended on how close together the two MUAPs were when they discharged.

CONCLUSIONS

The latencies and blocking behavior indicate that the unstable components came from branched muscle fibers innervated by two different motoneurons. The satellite potentials were due to action potentials that traveled to the branching point along one branch and back along the other. The blockings were due to action-potential collisions when both motoneurons discharged close together in time.

SIGNIFICANCE

Animal studies suggest that branched muscle fibers may be a normal characteristic of series-fibered muscles. This study adds to our understanding of these muscles in humans.

Myofiber length and three-dimensional localization of NMJs in normal and botulinum toxin treated adult extraocular muscles

PURPOSE

The density and three-dimensional localization of neuromuscular junctions (NMJs) of normal and botulinum toxin-treated normal adult rabbit and monkey extraocular muscles (EOMs) were analyzed. To demonstrate average myofiber length, randomly selected individual myofibers were reconstructed and compared with total muscle length.

METHODS

Normal adult rabbit and monkey EOM and normal adult rabbit tibialis anterior were dissected in their entirety, frozen, sectioned longitudinally, and immunostained for NMJ localization. In addition, adult rabbit EOMs were injected with 5 U botulinum toxin, and NMJ density was determined after 2 weeks. NMJ locations for the three groups of EOM were reconstructed, and density of NMJ was determined. Individual myofibers were reconstructed from the orbital and global layers to determine mean fiber length.

RESULTS

NMJs were dispersed throughout the entire length of all EOMs examined from adult rabbits and monkeys and were visualized by alpha-bungarotoxin staining and three-dimensional reconstruction of serial sections. In leg muscle, two relatively tight bands of NMJs were seen. Botulinum toxin significantly increased total NMJ density. Mean fiber lengths were 1.9 and 4.83 mm in the orbital and global layers, respectively, approximately 10% and 24% of the total origin-to-insertion muscle lengths. In addition, individual myofibers continuously changed their intrafascicular relationships over their lengths.

CONCLUSIONS

The density and distribution of NMJs in normal EOMs are more extensive than previously described. Individual myofibers are significantly shorter than the tendon-to-tendon muscle length in both muscle layers. Botulinum toxin results in a doubling of NMJ density. NMJ localization in normal EOMs has ramifications for understanding eye movement control, but it is also important when surgical or pharmacologic intervention is used for the treatment of strabismus, nystagmus, or other eye muscle disorders.

Presence of SERCA and calcineurin during fetal development of porcine skeletal muscle

Mechanisms involved in skeletal myofiber differentiation during fetal development of large animals are poorly understood. Studies in small animals suggest that the calcineurin (Cn) pathway is involved in myofiber differentiation. Neural activity is a prerequisite for Cn activity, implying maintenance of sustained low intracellular Ca(2+) concentrations. To study the role of Cn in fetal myofiber differentiation, we monitored the temporal and spatial distribution of Cn subunits, sarcoplasmic reticulum Ca(2+) ATPase (SERCA), phospholamban (PLB), and myosin heavy chain (MyHC) isoforms in relation to ingrowing nerves in porcine semitendinosus muscle (m. semitendinosus) at 55 and 75 days of gestation (dg) and at term. Immunofluorescence analysis revealed the presence of Cn subunits and SERCA isoforms at all analyzed stages. Cn distribution was not fiber-type specific, but expression became more prominent at term. At 75 dg, differential SERCA2 expression was accompanied by perinuclear PLB in primary fibers. SERCA1 was expressed in all fiber types at all stages. No specific MyHC isoform distribution was seen in relation to neuromuscular contacts, although neuromuscular contacts were present. From these results we speculate that in porcine m. semitendinosus differential SERCA2 expression precedes differential Cn expression. The question whether the Cn pathway is involved in prenatal myofiber differentiation needs further studies.

Intermingling versus clonal coherence during skeletal muscle development: Mosaicism in eGFP/nLacZ-labeled mouse chimeras

To study the relative contributions of clonal coherence vs. myoblast intermingling to the formation of mammalian skeletal muscles, enhanced green fluorescent protein (eGFP) and nLacZ labels were used to analyze neonatal and adult mouse embryo aggregation chimeras. The eGFP marker allowed us to estimate absolute levels of the eGFP transgenic parental contributions as well as variances of levels within a chimeric individual; nLacZ served as a counter-label in neonatal chimeras. In mature chimeric muscle, free intracellular diffusion of eGFP led to an averaging of the eGFP level in individual myofibers. Chimerism, i.e., differing eGFP levels, was evident between corresponding right and left muscles, between adjacent back, body wall, limb (shank, foot), and extraocular muscles and among myofibers within these muscles. Inhomogeneities in muscles far from their somitic origin indicated unbalanced sampling from small founder pools. The results obtained with adult eGFP<-->0 chimeras were corroborated by neonatal chimeras with complementary eGFP<-->nLacZ labels.

Development of a mammalian series-fibered muscle

This study examines the processes by which multiply innervated, serially fibered mammalian muscles are constructed during development. We previously reported that primary myotubes of such a muscle, the guinea pig sternomastoid muscle, span from tendon to tendon and are innervated at each of the muscle's four innervation zones. Secondary myotubes form later, in association with each point of innervation (Duxson and Sheard, Dev. Dyn., 1995; 204:391-405). We now describe the further growth and development of the muscle. Secondary myotubes initially insert onto and grow along the primary myotube. However, as they reach a critical length, they encounter other secondary myotubes growing from serially adjacent innervation zones and may transfer their attachment(s) to these serially positioned secondary myotubes. Other secondary myotubes maintain attachment at one or both ends to their primary myotube. Thus, an interconnected network of primary and secondary myotubes is formed. Patterns of reactivity for cell adhesion molecules suggest that early attachment points between myotubes are the embryonic precursors of adult myomyonal junctions, characterized by the expression of alpha7Bbeta1 integrin. Finally, the results show that secondary myotubes positioned near a tendon are generally longer than those lying in the mid belly of the muscle, and we suggest that the environment surrounding the tendinous zone may somehow stimulate myotube growth.

Increased jitter and blocking in normal muscles due to doubly innervated muscle fibers

Increased jitter and intermittent impulse blocking in electromyographic (EMG) signals are considered evidence of transmission abnormality and are not usually associated with normal muscle. However, motor unit action potentials (MUAPs) that exhibit increased jitter and blocking have recently been shown to occur in the brachioradialis muscles of neurologically healthy subjects. The jitter and blocking result from collisions, refractoriness, and conduction-velocity variability in long muscle fibers that are innervated by two different motoneurons at widely separated endplates. We analyzed MUAPs obtained by decomposing EMG signals from the brachioradialis muscles of four normal subjects. The rate of blocking of some MUAP components was as high as 28%, the jitter between some components exceeded 300 micros (mean consecutive difference), and the mean incidence of irregular MUAPs was 14%. These values would be considered abnormal in many other muscles. Jitter from doubly innervated fibers can be distinguished from other types of pathological jitter because one component exhibits amplitude variability. Clinical neurophysiologists should be aware that increased jitter and blocking do not necessarily indicate pathology in brachioradialis and perhaps other long, parallel-fibered muscles.

Electrophysiological evidence of adult human skeletal muscle fibres with multiple endplates and polyneuronal innervation

Electromyographic (EMG) signals were recorded using intramuscular electrodes at six different sites in the brachioradialis muscles during voluntary isometric contractions in four subjects. The potential waveforms and discharge patterns of up to 12 simultaneously active motor units were identified from each signal using computer-aided decomposition. Out of a total of 301 motor unit potentials identified, 23 potentials exhibited behaviour consistent with having been generated by muscle fibres that were innervated by two different motoneurons at widely separated endplates. These potentials discharged in association with two different motor units, but were blocked or delayed whenever the two motor units discharged within a few milliseconds of one another. The blocking was consistent with a collision or refractoriness when one motoneuron tried to excite the fibre while it was already conducting an action potential initiated by the other motoneuron. The delays were consistent with decreased conduction velocity associated with incomplete recovery of the fibre after a preceding action potential. From the temporal separation between the discharges of the two motoneurons that resulted in blocking, the spatial separation between the endplates was estimated to be between 26 and 44 mm. These findings challenge the classical concept of the motor unit as an anatomically distinct and functionally independent entity. It is suggested that the human brachioradialis muscle may contain both long, polyneuronally innervated fibres and short, serially linked, singly innervated fibres.

Continuous myonuclear addition to single extraocular myofibers in uninjured adult rabbits

Extraocular muscles (EOM) are unique among mammalian skeletal muscles in that they normally express molecules associated with muscle development and regeneration. In this study we show that satellite cells of EOM, unlike those of other skeletal muscles, continually divide in the normal, uninjured adult. Adult EOM contained activated satellite cells positive for the myogenic regulatory factor MyoD. EOM satellite cells did not require a prolonged activation period prior to onset of cell division and differentiation in vitro. EOM satellite cells incorporated bromodeoxyuridine (brdU), a marker for cell division, and with longer postlabeling survival, brdU-labeled nuclei populated EOM myofibers. This was not seen with leg muscle. These findings suggest the possibility that continual division of satellite cells and fusion of their daughter myocytes with existing adult EOM myofibers contribute to the unique sparing or susceptibility of EOM to certain muscle diseases.

Different modes of hypertrophy in skeletal muscle fibers

Skeletal muscles display a remarkable diversity in their arrangement of fibers into fascicles and in their patterns of innervation, depending on functional requirements and species differences. Most human muscle fascicles, despite their great length, consist of fibers that extend continuously from one tendon to the other with a single nerve endplate band. Other mammalian muscles have multiple endplate bands and fibers that do not insert into both tendons but terminate intrafascicularly. We investigated whether these alternate structural features may dictate different modes of cell hypertrophy in two mouse gracilis muscles, in response to expression of a muscle-specific insulin-like growth factor (IGF)-1 transgene (mIGF-1) or to chronic exercise. Both hypertrophic stimuli independently activated GATA-2 expression and increased muscle cross-sectional area in both muscle types, with additive effects in exercising myosin light chain/mIGF transgenic mice, but without increasing fiber number. In singly innervated gracilis posterior muscle, hypertrophy was characterized by a greater average diameter of individual fibers, and centralized nuclei. In contrast, hypertrophic gracilis anterior muscle, which is multiply innervated, contained longer muscle fibers, with no increase in average diameter, or in centralized nuclei. Different modes of muscle hypertrophy in domestic and laboratory animals have important implications for building appropriate models of human neuromuscular disease.

Patterning of skeletal muscle

Recent studies challenge the view that signals provided by motor neurons are required to activate subsynaptic nuclei and induce postsynaptic specializations in developing skeletal muscle. New findings show that acetylcholine receptor genes are expressed and that acetylcholine receptor clusters form preferentially in the prospective synaptic region of muscle independently of motor innervation. These results indicate that developing myotubes are patterned by mechanisms intrinsic to developing muscles and raise the possibility that patterning of muscles may influence the growth pattern of motor axons and the sites where synapses form.

Regulation of the growth of multinucleated muscle cells by an NFATC2-dependent pathway

The nuclear factor of activated T cells (NFAT) family of transcription factors regulates the development and differentiation of several tissue types. Here, we examine the role of NFATC2 in skeletal muscle by analyzing adult NFATC2(-/)- mice. These mice exhibit reduced muscle size due to a decrease in myofiber cross-sectional area, suggesting that growth is blunted. Muscle growth was examined during regeneration after injury, wherein NFATC2-null myofibers form normally but display impaired growth. The growth defect is intrinsic to muscle cells, since the lack of NFATC2 in primary muscle cultures results in reduced cell size and myonuclear number in myotubes. Retroviral-mediated expression of NFATC2 in the mutant cells rescues this cellular phenotype. Myonuclear number is similarly decreased in NFATC2(-/)- mice. Taken together, these results implicate a novel role for NFATC2 in skeletal muscle growth. We demonstrate that during growth of multinucleated muscle cells, myoblasts initially fuse to form myotubes with a limited number of nuclei and that subsequent nuclear addition and increases in myotube size are controlled by a molecular pathway regulated by NFATC2.

Muscle length affects the architecture and pattern of innervation differently in leg muscles of mouse, guinea pig, and rabbit compared to those of human and monkey muscles

The innervation pattern and fascicular anatomy of muscles of different lengths in mouse, guinea pig, rabbit, macaque monkey and human legs were analyzed. Neuromuscular junctions, muscle tendon junctions and ends of intrafascicularly terminating fibers were stained for acetylcholinesterase, and fascicle lengths measured. A high correlation between increasing fascicle length and increasing number of neuromuscular junctions was found, with non-primate (mouse, guinea pig, rabbit) and primate (macaque monkey, human) muscles forming two discrete groups. In non-primates, muscles with a single endplate band, fascicles were always shorter than 35 mm, fixing the limit of fiber length served by one neuromuscular junction. Muscles with fascicles longer than this had multiple discrete bands of motor endplates crossing their width at regular intervals. An increase in muscle length across or within species corresponded to an equivalent, standard increase of 10-12 mm fascicle length per motor endplate band. All human and monkey leg muscles, with the exception of gracilis and sartorius, were singly innervated and all muscle fibers ran the full distance from tendon to tendon. Singly innervated primate muscle fibers were up to 140 mm long whereas the mean distance between endplate bands in the two multiply innervated muscles was also considerably greater than in non-primates. These data indicate that allometric effects of increasing fascicle length, are distinct in common laboratory animals and two primates, when architecture and pattern of innervation are compared.

Motor nerve topology reflects myocyte morphology in hamster retractor and epitrochlearis muscles

Neuromuscular activation is a primary determinant of metabolic demand and oxygen transport. The m. retractor and m. epitrochlearis are model systems for studying metabolic control and oxygen transport; however, the organization of muscle fibers and motor nerves in these muscles is unknown. We tested whether the topology of motor innervation was related to the morphology of muscle fibers in m. retractor and m. epitrochlearis of male hamsters ( approximately 100 g). Respective muscles averaged 47 and 12 mm in length 100 and 35 mg in mass. Staining for acetylcholinesterase revealed neuromuscular junctions arranged in clusters throughout m. retractor and as a central band across m. epitrochlearis, suggesting differences in fiber morphology. For both muscles, complete cross-sections contained approximately 1,700 fibers. Fiber cross-sectional areas were distributed nearly normal in m. epitrochlearis (mean = 1,559 +/- 17 microm(2)) and skewed left (P < 0.05) in m. retractor (mean = 973 +/- 15 microm(2)). Single fiber length (Lf) spanned muscle length (Lm) in m. epitrochlearis, while fibers tapered to terminate within m. retractor (Lf/Lm = 0.43 +/- 0. 02). With myelin staining, a single branch of ulnar nerve projected axons across the midregion of m. epitrochlearis. For m. retractor, the spinal accessory nerve branched to give rise to proximal and distal regions of innervation, with intermingling of axons between nerve branches. Nerve bundle cross-sections stained for acetylcholinesterase indicate that each motor axon projects to an average of 65 muscle fibers in m. epitrochlearis and 100 in m. retractor. Differences in fiber morphology, innervation topology, and neuromuscular organization may contribute to the heterogeneity of metabolic demand and oxygen supply in skeletal muscle.

Examination of intrafascicular muscle fiber terminations: implications for tension delivery in series-fibered muscles

Mammalian skeletal muscles with long fascicle lengths are predominantly composed of short muscle fibers that terminate midbelly with no direct connection to the muscle origin or insertion. The manner in which these short fibers terminate and transmit tension through the muscle to their tendons is poorly understood. We made an extensive morphological study of a series-fibered muscle, the guinea pig sternomastoid, in order to define the full range of structural specializations for tension transmission from short fibers within this muscle. Terminations were examined in single fibers, teased small bundles of fibers, and in sections at both the light and electron microscopic level. In many cases, sites of fiber termination were defined by reactivity for the enzyme acetylcholinesterase, which also marks myotendinous junctions. Additionally, transport of the lipophilic fluorescent dye, DiI, or injection of Lucifer Yellow were used to visualize undisturbed fiber terminations in whole muscles using confocal and fluorescence microscopy. At the light microscopic level, we find that intrafascicularly terminating fibers end about equally often in either a long progressive taper, or in a series of small or larger blunt steps. Combinations of these two morphologies are also seen. However, when analyzed at higher resolution with confocal or electron microscopy, the apparently smooth progressive tapers appear also to be predominantly composed of a series of fine stepped terminations. Stepwise terminations in most cases join face-to-face with complementary endings of neighboring muscle fibers, some via an extended collagenous bridge and others at close interdigitating myomyonal junctions. These muscle-to-muscle junctions show many of the features of myotendinous junctions, including dense subsarcolemmal plaques in regions of myofibrillar termination and we suggest that they serve to pass tension from fiber to fiber along the longitudinal axis of the muscle. In addition, we observe regions of apparent side-to-side adhesion between neighboring fibers at sites where there is no apparent fiber tapering or structural specialization typical of myofibril termination. These sites show acetylcholinesterase reactivity, and large numbers of collagen fibers passing laterally from fiber to fiber. These latter connections seem most likely to be involved in lateral transmission of tension, either from fiber to fiber, or from fiber to endomysium. Overall, our results suggest that tension from intrafascicularly terminating fibers is likely to be passed along the muscle to the tendon using both in-series and in-parallel arrangements. The results are discussed in light of current theories of tension delivery within the series-fibered muscles typical of large, nonprimate mammals.

A molecular mechanism enabling continuous embryonic muscle growth - a balance between proliferation and differentiation

Embryonic muscle growth requires a fine balance between proliferation and differentiation. In this study we have investigated how this balance is achieved during chick development. Removal of ectoderm from trunk somites results in the down-regulation of Pax-3 expression and cell division of myogenic precursors is halted. This initially leads to an up-regulation of MyoD expression and to a burst in terminal differentiation but further muscle growth is arrested. Locally applied bone morphogenetic protein-4 (BMP-4) to somites mimics the effect of the ectoderm and stimulates Pax-3 expression which eventually results in excessive muscle growth in somites. Surprisingly, BMP-4 up-regulates expression of noggin which encodes a BMP-4 antagonist. This suggests that the proliferation enhancing activity of BMP-4 can be limited via up-regulation of noggin and that myogenic cells differentiate, as an intrinsic property, when deprived of BMP-4 influence. In contrast to BMP-4, Sonic hedgehog (Shh) locally applied to somites arrests muscle growth by down-regulation of Pax-3 and immediate up-regulation of MyoD expression. Such premature muscle differentiation in somites at tongue and limb levels prevents myogenic migration and thus tongue and limb muscle are not formed. Therefore, precise limitation of differentiation, executed by proliferative and Pax-3 promoting signals, is indispensable for continuous embryonic muscle growth.

Polysialylated NCAM expression during motor axon outgrowth and myogenesis in the fetal rat

Polysialylation of the neural cell adhesion molecule (NCAM) converts it into an anti-adhesive molecule, attenuating intercellular adhesion and repelling apposed membranes. Previous studies have demonstrated that interaxonal repulsion, or defasciculation, induced by polysialylated NCAM (PSA-NCAM) expressed along outgrowing chick motor axons promotes intramuscular branching and facilitates differential guidance of segregating axonal populations. In the present study, we have examined the expression of PSA-NCAM in a developing mammalian motor system during axonal outgrowth, separation of distinct axonal populations, and intramuscular branching. Furthermore, we provide the first clear demonstration of the spatiotemporal modulation of PSA-NCAM expression on myotubes during each stage of myogenesis. Immunohistochemical labelling was used to compare the spatiotemporal pattern of PSA-NCAM expression with those of total-NCAM, the cell adhesion molecule L1, and growth associated protein (GAP-43) during development of the phrenic nerve and diaphragm of fetal rats (embryonic days, E11-E19). During segregation of phrenic and brachial axonal populations at the brachial plexus (E12.5-E13), PSA-NCAM expression was restricted to phrenics, being absent from brachial motoneurons. Both populations labelled equivalently for NCAM, L1, and GAP-43. We postulate that PSA-NCAM may be a component of the molecular machinery that specifically guides phrenic motoneuron growth at the brachial plexus. During diaphragmatic morphogenesis, PSA-NCAM expression: (i) remained high within the phrenic nerve throughout intramuscular branching; (ii) was transiently up-regulated on myotubes during myotube separation associated with primary and secondary myogenesis; (iii) was restricted to those regions of primary and secondary myotube membranes, which were juxtaposed and about to separate. These data suggest a role for PSA-NCAM in the guidance of specific subsets of mammalian motoneurons and in intramuscular branching, and demonstrate an intimate correlation between PSA-NCAM expression and myotube separation.

The transient existence of 'en passant' nerve terminals in normal embryonic rat skeletal muscle

In a recent examination of labeled normal embryonic rat nerve terminals we have found evidence of synaptic specialisation at non-terminal regions of axons ('en passant' terminals). The specialisation typically consists of a vesicle-filled axonal swelling lying in close apposition to a zone of thickened sarcolemma, with the two structures separated by a basal lamina-filled cleft. Such 'en passant' terminals are never seen in normal mature rat muscle, where all synaptic sites form at the true termination of axonal branches. These terminals thus appear to be a transient feature of developing neuromuscular relations. Analogous structures may arise in mature muscle when axons undergo ultraterminal sprouting in response to paralysis or partial denervation. We discuss our finding in relation to the role such terminals may play in normal development and in partially denervated mature muscle.

Composition of newly forming motor units in prenatal rat intercostal muscle

We have examined the composition of rat intercostal motor units during the period of late gestation, when most muscle fibres are formed, in order to see the pattern of the contacts initially made between single motoneurons and myotubes. At this early stage, the muscle contains two types of myotubes, primary and secondary myotubes, and a major aim was to see whether individual motoneurons preferentially made contact with a particular myotube type. The technique used to define myotubes contacted by a single motoneuron was anterograde labelling of the neuron, followed by electron microscopic detection of labelled terminals and their postsynaptic targets. We find that prenatal motor units are inhomogeneous with respect to their primary/secondary myotube composition. Most individual motoneurons show many permutations of contact with primary myotubes, secondary myotubes, and undifferentiated cells, including single nerve terminals which contact both primary and secondary myotubes. Our results are interpreted in terms of changes to the composition of both the muscle and of the motor units during the final 5 days of gestation. We demonstrate that motoneurons necessarily make their initial contacts on primary myotubes, but that these are surprisingly sparse. As secondary myotubes appear and become innervated, motor units are at first all similar and all heterogeneous. However, primary myotubes are represented more often in motor units than in the muscle as a whole. This probably reflects the relative densities of polyinnervation of primary vs. secondary myotubes. By embryonic day 20, motor units have become divergent in composition, with some dominated by primary myotubes and others by secondaries. We propose that motoneurons initially establish contacts at random on either myotube type, but then begin to express preference for one type or the other and reorganise their periphery. Refining of motor unit composition towards homogeneity in the postnatal period probably involves other elements, such as mutability of muscle fibre and/or motoneuron characteristics as a function of usage and muscle position, perhaps influenced by sensory feedback mechanisms.