Features of the ultrastructural organization of the muscles of skaters in relation to their sport specialization and muscle fiber composition

An electron microscopical and histochemical investigation of bioptates obtained from the external broad femoral muscle of 74 sportsmen-skaters has been performed. Structural adaptation of the muscles to speed loadings and to endurance loadings has been analysed taking into consideration contents of muscle fibers (MF) of various type. Peculiarities in ultrastructural organization of the skeletal MF in the sprinter- and stayer-skaters, who undergo training according to a special program, are described; the character of the muscle changes is also followed in the sportsmen who undergo training according to the program that does not correspond to the contents of their MF. Lesions in the latter up to the necrotic ones are mostly found in the muscles of the stayer-skaters, as well as in the sprinters trained according to the stayer program. Presence of essential destructive alterations of the muscles in the skaters, especially in those whose contents of slow muscles do not correspond to their specialization, demonstrates, evidently, the fact that the given physical loading is not adequate to the functional or potential possibilities of the organism.     

Ultrastructure of the vocal muscle and its morphological differences from the myocardium]

A comparative ultrastructural investigation of the M. vocalis in mammals has been carried out. Morphological differences between the vocal muscle and cardiac tissue are reported; a distinct classification of the M. vocalis according to a typisation of skeletal muscle fibers is presented. In all species investigated (man, dog, cat, guinea-pig and rat) the general ultrastructure of the sarcomeres as well as their mitochondrial content and the innervation pattern allow to classify the M. vocalis as to belong to the "fast twitch (white) skeletal muscle fibers. A single innervation was found with large motor endplates containing numerous synaptic infoldings. Structural specializations known to be characteristic for cardiac tissue, e.g. intercalated discs, T-tubules at the level of the Z-band and nuclei in a midst postion of the muscle cell could not be observed. The m. vocalis, therefore, cannot be considered to have histologically any relationship with cardiac tissue. The vocal muscle is described as a special type of skeletal muscle very similar to the extraocular muscles. The electron microscopic findings are discussed with respect to current theories of phonation. The myoleastic theory of phonation can be favoured according to our ultrastructural results.     

The characteristics of the skeletal muscle fibers of the m. vastus lateralis in highly qualified skaters (a morphometric analysis)

Morphometrical characteristics of the skeletal muscle fibers of the m. vastus lateralis has been presented in the skaters of a high qualification. The muscle biopsies have been obtained by means of the needle biopsy method from 5 sprinters and 10 classical all-round competitors. Longitudinal sections of the peripheral and central parts of the muscle fiber have been investigated, subdividing the fibers in accordance with certain ultrastructural criteria into two types: slow and fast. The area of mitochondria, myofibrils, lipid inclusions has been determined by means of the system of the image analysis ACM-68K (Leitz, BRD). To the defining factors in the skater muscle ultrastructure, influencing the level of sporting achievements, a high parameter of the volumetric density of the whole mitochondrial population (7.6% in the sprinters and 9.2% in the all-around competitors) and an exceptionally high parameter of the sarcoplasmic density (28.5% in the sprinters and 36% in the all-around competitors) should be ascribed. The morphometrical data of the skaters muscle fiber are compared with the muscle parameters (according to the literature data) of the m. vastus lateralis in high qualification sportsmen of other specialization and in nontrained persons.     

Correlation between nerve terminal size and muscle fibers diameter in urodela extrinsic eye muscle.

Extrinsic eye muscles of newts and salamanders were investigated by means of electron microscope. It was possible to distinguish two types of muscle fiber tonic-slow and twitch-fast acting ones. It was shown that mioneural junctions in both types of fibers differ in their ultrastructural organization because of lack of post-synaptic infoldings on the surface of slow tonic fibers. After "cholinesterase" "staining" it was possible to measure the surface of junctional area and correlate it with the diameter of particular muscle fiber. The results show a positive correlation.     

Structure and protein composition of sites of papillary muscle attachment to chordae tendineae in avian hearts

Summary Most cardiac myocytes transmit force across fasciae adherentes, specialized sites of cell-cell adhesion. However, some cardiac myocytes in papillary muscle terminate on collagenous connective tissue in the chordae tendineae. These papillary myotendinous junctions (MTJs) are specialized for force transmission from myocytes to extracellular matrix. In the present study, we compared structural molecules at papillary MTJs to those at fasciae adherentes and skeletal MTJs. By using indirect immunofluorescence, we found that papillary MTJs more closely resemble skeletal MTJs in their molecular composition in that they are enriched in talin, vinculin, integrin, and fibronectin. Zeugmatin and -actinin, both components of fasciae adherentes, are absent from papillary MTJs. Although papillary MTJs and skeletal MTJs display strong similarities in structural protein composition, ultrastructural organization of the two junctions is different. Papillary MTJs display little folding of the junctional membrane and, according to morphological criteria, more closely resemble sites of thin filament-membrane association in smooth muscle than skeletal MTJs. Thus, papillary MTJs display a combination of structural characteristics described previously in skeletal and smooth muscles but exhibit few structural features observed previously in cardiac fasciae adherentes.     

Variable patterns of actin filaments in mammalian skeletal muscle

Summary The ultrastructural organization of myofilaments in skeletal muscle was studied in four mammalian species (mouse, rat, hamster, goat). In all these species, myofibrils showing irregularly distributed arrays of a variable number of actin filaments (from 6 to 11) were observed. The proportion of such myofibrils and the predominant patterns of actin filaments varied from one species to another. These results are in agreement with those previously reported for human skeletal muscle.     

Comparative aspects of cardiac and skeletal muscle sarcoplasmic reticulum.

While differing in numerous physiological and biochemical parameters, mammalian cardiac and skeletal muscles exhibit many common ultrastructural characteristics. General subcellular organization is similar with longitudinal disposition and organization of the myofibrils as well as subcellular organelles such as mitochondria, sarcoplasmic reticulum and transverse tubules. Significant differences are more readily discerned in terms of degree, not only with respect to relative amounts of various organelles, but also in regard to membrane composition. It is these macromolecular variations in membrane components which may, at least in part, provide the basis for differences in overall functional characteristics in the muscles.In cardiac, as well as skeletal muscle, the concentration of Ca²⁺ ions at specific intracellular sites regulates the contractile state of the muscle. The differences in mechanism and sources of Ca²⁺ for contraction in cardiac and skeletal muscle are but a few of the unsolved areas which are now being addressed. We shall focus primarily on research advances involving cardiac and skeletal SR emphasizing the contrasting features related to their functional roles in control of contraction and metabolic events.     

Organization of native and in vitro-reassembled myosin filaments from lobster tonic muscle

Tonic muscle of the crusher claw of the American lobster (Homarus amencanus) was investigated with respect to sarcomeric organization and the capacity for self-assembly of extracted myosin for comparison with the same properties of rabbit muscle. Native myosin filaments in the lobster muscle are much longer than in rabbit skeletal fibers, and differ further in sarcomeric organization in showing an actinto-myosin relationship in which two actin filaments are shared between adjacent myosins in a 12-membered orbital. The self-assembly of lobster myosin into filaments comparable in length and fine structure to the natural filament was achieved in the presence of excess Mg²⁺, a condition not required for rabbit myosin self-assembly. Results of in situ and self-assembly studies indicate a difference in molecular organization between lobster and rabbit myosin filaments and of the inferred presence of regulatory factors in the formation of these ultrastructural elements. These studies represent the groundwork for an investigation of in vitro polymerization of actin in association with the synthetic lobster myosin filament.     

CORRELATION BETWEEN FIBER LENGTH, ULTRASTRUCTURE, AND THE LENGTH-TENSION RELATIONSHIP OF MAMMALIAN SMOOTH MUSCLE

The length-tension relationship was determined for strips of guinea pig taenia coli and correlated with the length and ultrastructural organization of the component fibers. The mean fiber length in "stretched" strips (passive ≥ active tension) was 30% greater than that for fibers in "unstretched" strips (active >> passive tension). In stretched fibers the dense bodies and 100 A diameter myofilaments were consolidated into a mass near the center of fibers in cross-sectional profile. The thick myofilaments were segregated into the periphery of the fiber profiles. In unstretched fibers the dense bodies-100 A diameter filaments and the thick myofilaments were uniformly distributed throughout cross-sectional profiles. A tentative model is proposed to account for the change in fiber length and ultrastructural organization that accompanies stretch. The basic features of the model require the dense bodies to be linked together into a network by the 100 A diameter filaments. The functional consequences of stretching the fibers are discussed in relation to the model proposed for this network.     

The zebrafish myotome contains tonic muscle fibers: Morphological characterization and time course of formation

It is long known that the skeletal muscle of teleost fish contains muscle fibers which are in all probability of a tonic type according to morphological criteria. However, the evidence for the existence of teleost tonic fibers is still confined to a very small number of species, and knowledge concerning their ontogeny and possible functions is even more restricted. A remarkable deficit in this context is that it is not even exactly known whether the zebrafish, which is widely used to study vertebrate developmental biology, has such fibers, or how they arise. The present study demonstrates the existence of tonic fibers in the zebrafish myotome. They are identical with a fiber population previously termed “red muscle rim” fibers. A combined histochemical, immunocytochemical, and ultrastructural approach is used to characterize the morphology and development of these fibers. This study provides a basis for using the zebrafish model system in the future research on the developmental regulation and the functions of tonic fibers. J. Morphol., 2013. © 2012 Wiley Periodicals, Inc.     

Destructive changes in the muscles of sportsmen during dynamic cyclic exercise of different intensities

An electron microscopic investigation of bioptates from the external vast femoral muscle in sportsmen performing cyclic activity of various power has been performed: sprinters to middle distances, short-distance skaters and rowing-sportsmen--work in the zone of submaximal power; skaters to the distance of 3,000-10,000 m and racing skiers to the distance of 5-10 km--work of great power; racing skiers to long distances--prolonged work of moderate power. When working in the zone of the submaximal power, the signs of destruction of contracting structures in the muscle fiber are poorly manifested. When the work is of greater power and of greater duration with moderate power, necrotized fibers are found in the sportsmen's muscles. In muscle fibers with sublethal lesions, increasing autophagic processes are noted. The destructive changes of various degree of manifestation, revealed in muscle fibers predetermine the programme of adaptive changes in the muscles of the sportsmen specialized in different sports.     

Autophagic response to strenuous exercise in mouse skeletal muscle fibers

Strenuous physical exercise induces necrosis of skeletal muscle fibers and increases lysosomal enzyme activities in surviving muscle fibers. This study examines the ultrastructural basis of the stimulation of the lysosomal system in mouse vastus medialis muscle during the appearance and repair of exercise-induced (9 h of running) injuries. Necrotic fibers appeared the day after exercise and an inflammatory response with the replacement of necrotic fibers by phagocytes was highest 2-3 days after exertion. Ultrastructural study of surviving muscle fibers revealed numerous autophagic vacuoles, residual bodies, and spheromembranous structures at the periphery of myofibers, especially in fibers adjacent to necrotic fibers. The autophagic response was most prominent between 2 and 7 days after exertion. Autophagic vacuoles with double or single limiting membranes contained mitochondria at various stages of degradation. Vacuolar and multilamellar structures were also observed in regenerating muscle fibers. The structure of injured skeletal muscle fibers returned to normal within 2 weeks. It is proposed that increased autophagic activity could be related to the breakdown of cellular constituents of surviving muscle fibers to provide structural elements for regenerating muscle fibers.     

Mitochondrial heat shock protein 70, a molecular chaperone for proteins encoded by mitochondrial DNA

The protracted absence of muscle activation initiates complex cellular and molecular reactions aimed at restoring functional neuromuscular transmission and preventing degenerative processes. A central aspect of these reactions is the sprouting of intramuscular nerves in the vicinity of inactivated muscle fibers. Sprouts emerging from terminal nerve branches and nodes of Ranvier can reestablish functional contacts with inactive muscle fibers, and this is an essential restorative process in pathological conditions of the neuromuscular system. Due to their rapid upregulation in inactive skeletal muscle fibers and their ability to induce nerve sprouting in adult muscle, insulin-like growth factors (IGFs) are candidate signaling molecules to promote restorative reactions in the neuromuscular system. In this study we have exploited the high affinity and specificity of IGF-binding protein 4 (IGF-BP4) and IGF-BP5 for IGF1 and IGF2 to determine whether these growth factors are involved in the nerve sprouting reaction in paralyzed skeletal muscle. In tissue culture experiments with sensory- and motoneurons we demonstrate that the neurite promoting activity of IGF1 is blocked by IGF-BP4, and that a similar IGF-BP-sensitive activity is detected in muscle extracts from paralyzed, but not from control muscle. In in vivo experiments, we show that local delivery of IGF-BP4 to Botulinum toxin A-paralyzed skeletal muscle effectively prevents nerve sprouting in that muscle. Our findings indicate that muscle IGFs play an essential role in intramuscular nerve sprouting. In addition, these findings suggest that IGFs are major signaling factors from inactivated muscle to promote local restorative reactions, including interstitial cell proliferation and nerve sprouting.     

Coupled expression of troponin T and troponin I isoforms in single skeletal muscle fibers correlates with contractility

Striated muscle contraction is powered by actin-activated myosin ATPase. This process is regulated by Ca(2+) via the troponin complex. Slow- and fast-twitch fibers of vertebrate skeletal muscle express type I and type II myosin, respectively, and these myosin isoenzymes confer different ATPase activities, contractile velocities, and force. Skeletal muscle troponin has also diverged into fast and slow isoforms, but their functional significance is not fully understood. To investigate the expression of troponin isoforms in mammalian skeletal muscle and their functional relationship to that of the myosin isoforms, we concomitantly studied myosin, troponin T (TnT), and troponin I (TnI) isoform contents and isometric contractile properties in single fibers of rat skeletal muscle. We characterized a large number of Triton X-100-skinned single fibers from soleus, diaphragm, gastrocnemius, and extensor digitorum longus muscles and selected fibers with combinations of a single myosin isoform and a single class (slow or fast) of the TnT and TnI isoforms to investigate their role in determining contractility. Types IIa, IIx, and IIb myosin fibers produced higher isometric force than that of type I fibers. Despite the polyploidy of adult skeletal muscle fibers, the expression of fast or slow isoforms of TnT and TnI is tightly coupled. Fibers containing slow troponin had higher Ca(2+) sensitivity than that of the fast troponin fibers, whereas fibers containing fast troponin showed a higher cooperativity of Ca(2+) activation than that of the slow troponin fibers. These results demonstrate distinct but coordinated regulation of troponin and myosin isoform expression in skeletal muscle and their contribution to the contractile properties of muscle.     

On the unity of cytomembrane system in the skeletal muscle

In situ cytochemical evidence for specific Ca-binding sites in the cytomembrane system of skeletal muscle fibers is reported. High Ca accumulation was found at the junctions between different types of cytomembranes. Such junctions might represent "gate-locks' for intracellular Ca movements. Openings of sarcoplasmic reticulum (SR) in frog muscle fibers and of T-tubules in rat muscle fibers are described. Coated and noncoated caveolae were found in rat muscle fibers. The same positive reaction for TPP-ase was found in trans-Golgi zone, terminal cisternae and subsarcolemmal cisternae. These results suggest the membrane continuity and ontogenetic relationships in the cytomembrane system of skeletal muscle fibers.     

Muscle extracellular matrix applies a transverse stress on fibers with axial strain

It is widely assumed that skeletal muscle contraction is isovolumic. This assumption has been verified at the single fiber and at the myofibril level. Model development and mechanical analyses often exploit this assumption when investigating skeletal muscle and evaluating muscle mechanical properties. This communication describes a method whereby individual muscle fibers and bundles of fibers, which include their constituent extracellular matrix (ECM), were tested to define the change in volume with axial strain. The results demonstrate that fibers are isovolumic, but bundles decrease in volume with strain. The loss of volume implicates a transverse force being applied to the fibers by the ECM. The nature and importance of this transverse force warrant further investigation.     

Age-related appearance of tubular aggregates in the skeletal muscle of almost all male inbred mice

Tubular aggregates (TAs) which have been recently observed in a few mouse myopathies are identical to those described in human diseases. In this study we show that TAs are also found in the skeletal muscle of almost all normal inbred mice strains. In these inbred strains of mice the presence of TAs is shown to be related to both age and sex. Nine different muscles were stained with the modified Gomori trichrome method to reveal the general morphology of the muscles. Anti-SERCA1 ATPase was used to confirm that the TAs were in fact accumulations of sarcoplasmic reticulum and anti-MyHC IIB to demonstrate that these accumulations were found exclusively in the type IIB muscle fibers. An ultrastructural study confirmed the observations revealed by light microscopy that the TAs were derived from the sarcoplasmic reticulum. TAs were never observed in female inbred mice and were only found in type IIB glycolytic muscle fibers of male inbred mice. Therefore when analyzing the effect of genetic knock out and knock in experiments on the muscle phenotype of transgenic mice one should be aware that the presence of these aggregates is a non-specific phenomenon induced by inbreeding.     

On the relationship of ultrastructural and cytochemical features to color in mammalian skeletal muscle

Summary The semitendinosus muscle of the albino rat is divided grossly into two clearly distinguishable parallel longitudinal bands, one red (anterior) and the other white (posterior). By using mitochondrial content as a criterion for distinguishing fiber types, it is demonstrated that the red portion of the muscle is composed predominantly of red (52%) and intermediate (40%) fibers, while the white portion consists primarily of white fibers (82%). Red fibers have the smallest and white fibers have the largest average diameter. Ultrastructural characteristics of the three fiber types resemble closely those previously described for the rat diaphragm. Red fibers are rich in large mitochondria with abundant cristae, and possess the widest Z lines. In red fibers, the H-band region of the sarcoplasmic reticulum consists of an elaborate network of narrow tubules. In white fibers, mitochondria are smaller, less numerous, and have fewer cristae; Z lines are about half as wide as in red fibers. In the H-band region of the sarcoplasmic reticulum there is a more compact arrangement of broad more or less parallel tubules. Intermediate fibers are similar to red fibers except that their diameters are larger; mitochondria are somewhat smaller and cristae are less abundant; the width of the Z lines is close to that of white fibers. The consistent difference in Z line width establishes this dimension as an important criterion for distinguishing fiber types and facilitates ultrastructural identification, especially of the intermediate fiber.The clear relationship between color of the semitendinosus and cytological features of its component fibers supports the use of the terms red, white, and intermediate as simple and valid designations for fiber types in mammalian skeletal muscle. Measurement of the cross-sectional area contributed by each fiber type to the total area indicates that both red and intermediate fibers may contribute to redness in mammalian skeletal muscle.An early portion of this work was carried out with MissSharon Whelan (Mrs.Bernard Weiss). The author acknowledges the important contribution of Mr.Richard Stearns through his skillful work on the photographic illustrations and the technical assistance of MissAnn Campbell and Mrs.Joan Normington. — This study was supported by Grant No. HD 01026-04 from the United States Public Health Service.