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PubMed1990

Changes in myofibrillar components after skeletal muscle necrosis induced by a myotoxin isolated from the venom of the snake Bothrops asper.

The effects of a myotoxic phospholipase A2 isolated from the venom of the crotaline snake Bothrops asper on skeletal muscle myofibrils were studied by histological, ultrastructural, immunohistochemical, and biochemical parameters. Myotoxin induced a rapid and prominent muscle necrosis after intramuscular injection in mice. In this process, myofibrils were affected and three main changes were observed: (A) Initially, they were hypercontracted, eventually forming "clumped," dense masses which alternated with spaces devoid of myofilaments in the cytoplasm. This initial stage is probably due to hypercontraction resulting from a calcium influx after toxin-induced sarcolemmal damage. (B) A second change occurred between 3 and 6 hr, when the clumped or hypercontracted pattern changed to a "hyaline" pattern in which myofilaments were relaxed and had a more uniform distribution in the cellular space. Although there was not a widespread degradation of myofibrillar components at this stage, desmin started to be lost in samples obtained as early as 15 min after toxin injection, and alpha-actinin was almost absent by 7 hr. Thus, it is proposed that this shift may be due to a selective proteolytic degradation of structurally relevant components, particularly alpha-actinin. As a consequence, the mechanical integration of myofilaments is impaired, precluding hypercontraction. (C) Finally, at later time periods (24, 48, and 72 hr), there was widespread degradation of myofibrillar proteins, probably caused by proteases derived from inflammatory cells such as neutrophils and macrophages, whose numbers in necrotic muscle increased markedly at these time periods.

Actinin↗
PubMed1992

Human fetal heart development after mid-term: morphometry and ultrastructural study.

A total of 44 normally developed human fetal hearts aged from 17 to 40 weeks gestation were provided for routine ultrastructural and morphometric studies. For morphometric analysis, the maximal thicknesses of the anterior, lateral and posterior wall of both ventricles and that of interventricular septum were measured and the left-to-right thickness ratios calculated. The cross-sectional areas of both ventricles in the plane of the greatest heart diameter were measured with computerized image analysis system. Data were analyzed using linear regression and one-way analysis of variance. Myofibril formation occurred by attachment of thin filaments into amorphous Z materials which were presented in sarcolemmal plaques, sarcoplasmic condensations, desmosomes and in Z lines. From these Z centers, myofibrils radiated many directions and branched and anastomosed with further development. This pattern of myofibrillar development continued throughout the whole fetal period. A transverse tubule system was clearly evident in later fetal development. It occurred by invagination of sarcolemma into myocardial cells and formation of subsarcolemmal caveolae. Mitochondria, well-developed Golgi complexes, glycogen granules and well-developed microvessels were found throughout the whole fetal period. Binucleated myocytes appeared by 32 weeks gestation and this suggests that myocyte hyperplasia may cease before birth in humans. The growth of both ventricular walls, the interventricular septum and that of both ventricular cross-sectional areas showed linear regression, and the left-to-right wall thickness ratios were nearly constant. Also, there were no differences in morphometric data between the left and right ventricles. In conclusion, development of the myocyte is an ongoing process which may be continued in the post-natal period in humans, and our statistical results do not support the theory of the right ventricular dominance during the fetal period.

Cell Differentiation↗
PubMed1985

Novel thick filament protein of chicken pectoralis muscle: the 86 kd protein. I. Purification and characterization.

A new thick-filament-associated protein, the 86 kd protein, of chicken pectoralis major muscle was isolated from a crude C-protein preparation by a method similar to that used to purify H-protein from rabbit skeletal muscle. However, the protein with an apparent Mr of 86,000 and 370,000 as estimated by gel electrophoresis and gel permeation, respectively, is not related to C-protein and differs from rabbit H-protein by its elution behaviour from hydroxyapatite columns, by its molecular weight, ultraviolet light spectrum, amino acid composition and localization, and by its amount present in myofibrils. The amino acid composition reveals a high content of proline and gel permeation indicates an either highly asymmetric or polymeric structure of the molecule. Antibodies raised in rabbits against the 86 kd protein were demonstrated by double immunodiffusion and immunoblotting experiments to be specific for this protein. They show no cross-reactivity with any other myofibrillar protein of chicken pectoralis muscle, e.g. myosin, M-band proteins, titin or C-protein, nor did they exhibit a significant cross-reactivity with H-protein from rabbit. The 86 kd protein, which has been purified also by antibody affinity chromatography from a freshly prepared Guba-Straub extract of washed myofibrils, is a specific myofibrillar component located within each half of the A-band.

Amino Acids↗
PubMed1985

Proximity of regulatory light chains in scallop myosin.

The distance between the regulatory light chains of the two heads of the scallop myosin molecule was estimated with the aid of two photolabile cross-linkers, benzophenone maleimide and p-azidophenacylbromide. These cross-linkers selectively alkylate thiol groups and have a maximum length of about 9 A. One of the two regulatory light chains of scallop myosin was removed by treatment of myofibrils at 10 degrees C with EDTA and replaced with a foreign regulatory light chain carrying a cross-linker. Cross-linking between the scallop and foreign regulatory light chains was effected by photolysis. This was demonstrated by incubating nitrocellulose transfers of sodium dodecyl sulfate/polyacrylamide gels of the photolyzed hybrid myofibrils with specific antibodies against the different light chains, followed by fluorescein isothiocyanate-125I-labeled secondary antibody. Scallop regulatory light chains cross-linked extensively (20 to 50%) with Mercenaria regulatory light chains (cysteine in position approximately 50) in solutions that induce rigor in skinned fibers (no ATP) and in relaxing solutions (ATP but no Ca2+). Neither the regulatory light chains of chicken skeletal myosin (cysteines 129 and 157) nor those of gizzard myosin (cysteine 108) were cross-linked to scallop regulatory light chains in either medium. These results indicate that the N-terminal portions of the myosin regulatory light chains can approach each other within 9 A or less, while the distance between the C-terminal halves exceeds 9 A, and support the view that the N termini of the regulatory light chains point toward the myosin rod. Since the relative distance between the regulatory light chains of the two myosin heads is not altered between rigor and rest, we suggest that motion of the essential light chains is mainly responsible for the observed difference in the relative positions of the regulatory and essential light chains between conditions of rigor and rest.

Adenosine Triphosphatases↗
PubMed1985

Analysis of the metal-induced conformational change in myosin with a monoclonal antibody to light chain two.

A monoclonal antibody capable of detecting a conformational change in myosin light chain two (LC2) was characterized in detail. The antibody was shown to bind only to myosin LC2 when tested against fast skeletal myosin (chicken pectoralis muscle). With cardiac or slow muscle myosins, the antibody exclusively recognized their first light chains (LC1). Staining of myofibrils by the monoclonal antibody could be observed only after their irreversible denaturation by acetone or ethanol, or after incubation of the myofibrils in divalent metal chelators. This latter effect was shown to be fully reversible. The metal effect was independent of ionic strength although the affinity of the antibody for myosin was depressed at high salt concentrations. Similar metal effects were detected in the binding of antibody to cardiac or slow myosins. Neither the metal nor the ionic strength-related inhibition of antibody binding were detected with denatured myosin. The antibody binding site overlaps one of the alpha-chymotryptic sites in LC2 protected by divalent metals. Electron microscopic observations of myosin-antibody complexes demonstrated that the antibody binding site is located near the head-rod junction of myosin. Since the binding site of this monoclonal antibody has been mapped by recombinant DNA methods to the junction of the first alpha-helical domain with the calcium binding site of LC2, the location of the calcium binding site must also be located near the head-tail junction of myosin. A model for conformational changes at the myosin head-tail junction is proposed to account for the metal-induced blockage of antibody binding and the inhibition of alpha-chymotryptic digestion of LC2.

Animals↗
PubMed1985

Regulatory light chains and scallop myosin. Form of light chain removal or reuptake is dependent on the presence of divalent cations.

Readdition of regulatory light chains to regulatory light chain denuded scallop myofibrils, in the presence of magnesium, results in a negatively co-operative restoration of calcium sensitivity as a function of regulatory light chain content. The form of the stoichiometry curves obtained in the presence of 10 mM-EDTA, by light chain removal from scallop myofibrils at various temperatures, are parabolic in shape, consistent with a random removal process. However, in the presence of EDTA at low temperatures, regulatory light chains are removed in a biphasic manner, indicating that the binding constants of the light chains for each myosin head are not equivalent under these conditions. It is shown here that as the temperature is raised, light chain removal by EDTA approaches that of a random process. The stoichiometry curves obtained in the presence of 10 mM-EDTA may therefore be seen as a composite of both a biphasic removal process (temperatures below 20 degrees C) and a random removal process (temperatures above 20 degrees C), there being a temperature-dependent switch in the myosin molecule between 17 and 23 degrees C that governs the mode of light chain removal. These results indicate that both myosin heads must contain light chains for calcium sensitivity and are consistent with our earlier proposals for head-head co-operativity within the scallop myosin molecule.

Adenosine Triphosphatases↗
PubMed1975

Stages in fibre breakdown in Duchenne muscular dystrophy. An electron-microscopic study.

A scheme is presented which divides the process of fibre breakdown in Duchenne muscular dystrophy into 5 sequential stages. In Stage 1 the fibre appears superficially normal although there are subtle changes in the relative volumes of the fibre components. In Stage 2 localised overcontraction of the myofibrils results in excessive stretching of the sarcomeres in other regions of the fibre. This continuing process results in the formation of contraction clumps in which the contractile filaments form an increasingly homogeneous mass. By the final stage the normal structural features of the fibre have completely disappeared and it is invaded by macrophages. It is suggested that the central process of myofibril clumping results from a localised inability of the sarcomeres to relax which implies a defect in the mitochondria, Z-line or sarcoplasmic reticulum. This is discussed in relation to other workers' findings of a decrease in the calcium-accumulating capacity of the sarcoplasmic reticulum in Duchenne muscular dystrophy.

Basement Membrane↗
PubMed1984

Behavior of sarcotubular system formation in experimentally induced regeneration of muscle fibers.

To examine the behavior of transverse (T)-tubule formation in experimentally-induced regenerating fibers, a local anesthetic, bupivacaine hydrochloride, was injected directly into the rat soleus muscle to cause myonecrosis. The regenerating fibers following necrosis were then examined by electron microscopy using lanthanum nitrate which clearly demonstrated the sarcotubular system. In the early stage of regeneration within 7 days after muscle necrosis, the T-tubules seemed to be composed of markedly proliferated subsarcolemmal caveolae with occasional honeycomb structure formation. Around 10 days, the T-tubules in regenerating fibers were tortuously and irregularly arranged with focal dilatation in diameter, and extended longitudinally along the axis of well organized myofibrils. As the regenerating fibers matured, the sarcotubular system, irregular in course and in shape, gradually became organized into a regular transverse position against the myofibrils, along with a marked decrease in longitudinally arranged tubular components. These morphological findings of the early T-tubule formation seen in the present study were similar to those found in early myogenesis, and in biopsied muscles from cases of polymyositis and progressive muscular dystrophy.

Animals↗
PubMed1978

High resolution scanning electron microscopy of frog sartorius muscle.

A field emission-type scanning electron microscope (SEM) was used to study the three-dimensional ultrastructure of frog sartorius muscles. Various preparative procedures were tested to seeks better specimen preparation for high resolution SEM observation. Procedures should be chosen depending on the information desired. The cell surface and intracellular organization of muscle fibers were best visualized when the tissues were fixed with tannic acid-OsO4 and torn after critical point drying. The basal lamina appeared as a continuous felt-like layer, onto which fine filamentous materials adhered. The true outer surface of the sarcolemma was not seen, whereas the true inner surface was occasionally exposed and exhibited numerous caveolae, membraneous fragments and fine filaments attached to its surface. In freeze-fractured and dried tissues, the cleaved sarcolemma showed numerous apertures of caveolae and T-system tubules. Inside the cell, the myofibrils showed a typical branding pattern of the sarcomere. Thick myofilaments were regularly beaded except for the pseudo-H-zone. Around the myofibrils the sarcoplasmic reticulum and T-system were also clearly observed. The results are discussed with special reference to the usefulness and limitation of the high resolution SEM in studying the ultrastructure of cells and tissues.

Animals↗
PubMed1978

Fine structure of the myotendinous junction of lathyritic rat muscle with special reference to connectin, a muscle elastic protein.

The fine structure of the myotendinous junction of the skeletal muscle of lathyritic rats caused by beta-aminopropionitrile was investigated. In the junction there are finger-like processes of muscle fibers, in which thin filaments were extended from the last Z lines of myofibrils and attached to the sarcolemma of the processes. By the heavy meromyosin decoration technique, these thin filaments were identified as actin filaments. In the lathyritic muscle, the thin filaments were markedly fewer in number and distributed sparsely in the sarcoplasm. The content of connectin, an elastic protein, which is localized in myofibrils and also in sarcolemma was significantly decreased in the lathyritic muscle. A possible relationship between the changes in the fine structure of the myotendinous junction and in the connectin contents is discussed.

Aminopropionitrile↗
PubMed1979

Organ culture of adult amphibian heart: a fine structural analysis.

Pieces of hearts from adult newts were cultured up to 2 months. Within 7 days of culture, approximately 37% of the cardiac explants were attached to the substrate and more than 33% of the attached explants and approximately 15% of the unattached explants established pulsation rates ranging from 3 to 67 beats/min. The control and cultured explants were processed at weekly intervals for electron microscopy. The diameter of the control cardiac muscle cells ranged approximately 3-5 micron. The cell surface was provided with microvilli. The intercellular spaces ranged approximately 150-500 A. The intercalated discs lacked the step-like courses observed in the mammalian cardiac muscle. Sarcoplasmic reticulum was scanty. Desmosomal-dense materials were frequently continuous with the Z-bands of both control and cultured cardiac muscle cells. The transverse tubular system and gap junction were absent in newt ventricles. The functional implications of these characterisitics are discussed. At the end of 1 week of culture, the surfaces of the explants were covered by one or more layers of non-muscle cells, and the core of the explants consisted mostly of cardiac muscle cells. In a few cardiac muscle cells the myofibrillar organization was disrupted, resulting in the distribution of scattered patches of myofibrils and free myofilaments in the sarcoplasm. A small number of intact muscle cells contained a considerable number of dense granules in the sarcoplasm. At 15 days in culture, a large number of muscle cells showed structural features reminiscent of embryonic cardiac muscle cells. These cells possessed patches of myofibrils, scattered myofilaments and scanty sarcoplasmic reticulum along with other cellular organelles and inclusions. Several of these altered cardiac muscle cells contained mitotic figures. The cardiac explants maintained the initial beating rate until the end of 2 months of culture, except for the 11% of the explants which stopped beating. By 3-4 weeks in culture, most of the cardiac muscle cells possessed the altered cell morphology mentioned above. The explants after 60 days in culture became more flattened than the earlier explants. The intact cardiac muscle cells were rare, and the cores of the explants were mostly occupied by the altered cardiac muscle cells. It is evident from our studies that the cardiac muscle cells have undergone dedifferentiation in long-term culture, and that this dedifferentiation process has yet had no effect in the maintenance of contractility of the explants. Furthermore, these dedifferentiated cardiac muscle cells are capable of DNA synthesis and mitosis.

Animals↗
PubMed1980

Rattlesnake shaker muscle: I. A light microscopic and histochemical study.

Rattlesnake body and shaker muscles were studied using light microscopy and histochemistry. Five myofiber types are distinguishable in the body musculature. The majority are large diameter fast twitch fibers with high alkaline-stable ATPase activity and few mitochondria. In the shaker muscle the major fiber differs from all body fibers in that myofibrils do not entirely fill the fibers. The myofibrils branch repeatedly with one another, which leaves large areas of sarcoplasm devoid of filaments and gives the fibers a characteristic mottled appearance. Mitochondria and glycogen deposits are very numerous. Shaker fibers have high alkaline stable ATPase activity and, in addition, stain intensely for NADH-TR and alpha GPD. Myofibers of the shaker muscle are unusual in that they are extremely fast contracting yet are highly fatigue resistant.

Adenosine Triphosphatases↗
PubMed1980

The primary body-wall musculature in the arrow-worm Sagitta setosa (Chaetognatha): an ultrastructural study.

The primary musculature of Sagitta is mainly made up of two kinds of alternating fibers, A and B. These fibers differ markedly in their localization in the muscular tissue, by the development of their SR and their mitochondria, and the shape of their myofibrils. Their contractile apparatus is similar and possesses myofibrils of regular thickness with very short I bands, flanked by invaginations which are large compartments communicating with the extracellular space. This fiber diversity appears and is maintained in the presence of an apparent common innervation. Nerve ending-like structures are scattered in the epidermis against the basement membrane and there are no nerve beneath this. The presence of at least two kinds of fibers in the primary musculature and the presence of the secondary musculature would suggest that the displacements of sagitta may be more complex than is generally admitted. The specializations of the trunk musculature underline the degree of specialization in the Chaetognatha phylum.

Animals↗
PubMed1980

Structures located at the levels of the Z bands in mouse ventricular myocardial cells.

Within ventricular myocardial cells of the mouse, the myoplasmic regions located immediately adjacent to the Z lines of the sarcomeres contain a variety of structures. These include: (1) transversely oriented 10 nm ('intermediate') filaments that apparently contribute to the cytoskeleton of the myocardial cell; (2) the majority of the transverse elements of the T-axial tubular system; (3) specialized segments of the sarcoplasmic reticulum (SR) that are closely apposed to the sarcolemma or T-axial tubules (junctional SR); (4) 'extended junctional SR' ('corbular SR') that exists free of association with the cell membrane; (5) 'Z tubules' of SR that are intimately apposed to the Z line substance; and (6) leptofibrils. In addition, fasciae adherentes supplant Z lines where myofibrils insert into the transverse borders (intercalated discs) of the cells. The concentration of these myocardial components at the level of the Z lines suggests that a particular specialization of structural and physiological activities exists in the Z-level regions of the myoplasm. In particular, it appears that the combination of intermediate filaments, T tubules, and Z-level SR elements forms a series of parallel planar bodies that extend across each myocardial cell to impart transverse rigidity. The movement and compartmentation of calcium ion (Ca2+) would seem especially active near the Z lines of the myofibrils, in view of the preferential location there of Ca2+-sequestering myocardial structures such as T tubules, junctional SR, extended junctional SR and Z tubules.

Animals↗
PubMed1982

Quantitative analyses of ultrastructure and vascularization of the slow muscle fibres of the anchovy.

A quantitative study has been made of the ultrastructure and vascularization of slow fibres in the lateral muscles of the European anchovy (Engraulis encrasicolus). Mitochondria and myofibrils occupy 45.5 and 44.3% of total fibre volume respectively. More than 95% of all myofibrils are adjacent to mitochondria. A total of 51% of the sarcolemma is in direct contact with capillaries with a mean of 12.9 capillaries per fibre. In transverse sections anchovy slow fibres are considerably flattened (long to short axis 12:1) such that the surface to volume ratio is more than twice that of a cylindrical fibre of the same area (1115 micron2). The capillary surface required to supply 1 micron3 of mitochondria is 0.18 micron2 and the maximum distance between any capillary and mitochondrion 8 microns. T-system and sarcoplasmic reticulum occupy 0.43 and 2.7% of fibre volume respectively. Adaptations for increasing the capacity of skeletal muscle for aerobic work are discussed.

Animals↗
PubMed1984

Atypical 'fibrillar' flight muscle in strepsiptera.

The fine structure of the principal and ancillary metathoracic flight muscle fibres in the adult male of a strepsipteran, Elenchus tenuicornis, is described. Power-producing dorsal longitudinal and dorso-ventral flight muscles show features consistent with myoneural asynchrony: myofibrils are large and discrete and are separated by large closely packed mitochondria; the sarcoplasmic reticulum is very reduced but engages with T-system membranes in dyads at the mid-sarcomere H-band level. With respect to other asynchronous insect flight muscles, the fibres of Elenchus are anomalous (i) in the small fibre diameter, (ii) in the variable contour of the myofibrils and (iii) in the absence of tracheolar invagination. The functional significance of these structural features is discussed. Ancillary metathoracic muscles are structurally comparable with other synchronous fibres in possessing an extensive SR compartment. Structural evidence for asynchrony in the flight mechanism of Strepsiptera is considered in the context of the evolution of this mechanism throughout the insect Orders.

Actins↗
PubMed1990

Pathologic changes in the long-term transplanted heart: a morphometric study of myocardial hypertrophy, vascularity, and fibrosis.

Myocyte hypertrophy and myocardial fibrosis have been observed in transplanted human hearts, and both could potentially have an adverse effect on long-term cardiac function. There has been some concern that distant donor heart procurement and cyclosporine treatment increase the risk of these changes, but their incidence and severity have not been documented quantitatively in large numbers of cardiac transplant recipients. We used light microscopic morphometric methods to estimate myocardial collagen volume fraction and myocyte width in right ventricular endomyocardial biopsies from 95 recipients at 3 years posttransplantation, and electron microscopic stereology to estimate myocardial vascularity and myocyte myofibril content in 40 recipients, also at 3 years posttransplantation. We compared those with locally and distantly procured donor hearts (mean ischemic time 160 minutes) and cyclosporine versus noncyclosporine immunosuppression. Controls were pretransplant right ventricular biopsies from 20 donor hearts which were free of heart disease. We found no significant differences in myocardial collagen volume fractions. Myocyte hypertrophy was typical of all the transplant biopsies (mean myocyte width 20.2 microns, SD 3.0 in all transplants versus 11.8 microns, SD 2.2 in controls, P less than 0.001), but distant donor procurement and cyclosporine had no significant effect. There were significant reductions of myofibril volume fraction in the transplants, which raises the possibility of gradual decompensation in some patients. There were no significant differences in myocardial vascularity, although a few patients were well below the control range. We conclude that distant donor heart procurement, with ischemic times averaging less than 3 hours, and cyclosporine treatment are not responsible for significant hypertrophy or fibrosis in most transplants. Hypertrophy is typical of the transplanted heart, and it is possible that associated abnormalities might have an effect on cardiac function in some long-term survivors.

Adult↗
PubMed1980

The synthesis and distribution of desmin and vimentin during myogenesis in vitro.

Electrophoretic and autoradiographic analyses of the incorporation of 35S--methionine into newly synthesized proteins during myogenesis reveal that presumptive chicken myoblasts synthesize primarily one intermediate filament protein: vimentin. Desmin synthesis is initiated at the onset of fusion. Synthesis rates of both filament subunits increase during the first three days in culture, relative to the total protein synthesis rate. The observed increase in the rate of desmin synthesis (at least 10 fold) is significantly greater than that observed for vimentin, and is responsible for a net increase in the cellular desmin content relative to vimentin. Both filament subunits continue to be synthesized through at least 20 days in culture. Immunofluorescent staining using desmin- and vimentin-specific antisera supports the conclusion that desmin is synthesized only in fusing or multinucleate cells. These results indicate that the synthesis of the two filament subunits is not coordinately regulated during myogenesis. The distributions of desmin and vimentin in multinucleate chicken myotubes are indistinguishable, as determined by double immunofluorescence techniques. In early myotubes, both proteins are found in an intricate network of free cytoplasmic filaments. Later in myogenesis, several days after the appearance of alpha--actinin-containing Z line striations, both filament proteins become associated with the Z lines of newly assembled myofibrils, with a corresponding decrease in the number of cytoplasmic filaments. This transition corresponds to the time when the alpha--actinin-containing Z lines become aligned laterally. These data suggest that the two intermediate filament systems, desmin and vimentin, have an important role in the lateral organization and registration of myofibrils and that the synthesis of desmin and assembly of desmin-containing intermediate filaments during myogenesis is directly related to these functions. These results also indicate that the Z disc is assembled in at least two distinct steps during myogenesis.

Actinin↗