The ultrastructure of the striated muscle of the tail of Cercaria chackai Nadakal et al., 1969

The electron microscopic study of the tail of Cercaria chackai reveals that it contains four sets of striated muscle bundles located central to the nonstriated circular and longitudinal muscles. The striated muscle consists of longitudinally oriented lamellar myofibres. Each myofibre contains a single "U" shaped myofibril. The banding pattern is analogous to that of vertebrate striated muscle. The sarcolemma is a simple surface membrane. There are no transverse tubular extensions of sarcolemma. The sarcoplasmic reticulum (SR) is very well developed with cisternae, tubules, and vesicles. SR cisternae form dyadic couplings with the sarcolemma. There is a set of flattened tubules of SR origin traversing the myofibril exactly at the Z region. These tubules are unique to the striated muscle of the cercarian tail and may have functional significance. A diagrammatic reconstruction of the myofibre is presented.     

Specialized contacts between sarcolemma and sarcoplasmic reticulum at the ends of muscle fibers in the diaphragm of the rat

Summary An electron-microscopic study of the myotendinous portion of the diaphragm in the Wistar rat has shown that at the ends of muscle fibers, longitudinally oriented invaginations and peripheral furrows of the sarcolemma establish specialized contacts with individual sacs of the sarcoplasmic reticulum. The construction of these terminal contacts is similar to that of contacts between sarcolemmic T-tubules and terminal cisternae of the sarcoplasmic reticulum, characterized by formation of triads. The contact zones of the sac membrane are undulated and bound to the adjoining sarcolemma via electron-dense profiles of varying forms. Frequently, the terminal contacts and triads are located at the same level within the muscle fiber, at the borderline between A- and I-bands of the sarcomeres. At the ends of muscle fibers combined contacts between peripheral furrows of the sarcolemma, terminal cisternae of the sarcoplasmic reticulum, and T-tubules of the triads are also disclosed. The implications of the terminal contacts for muscle contraction are discussed.     

The membrane systems of the cardiac muscle cell of Tmetonyx cicada O. Fabricius (Crustacea,Amphipoda)

Summary The membrane systems of the cardiac muscle cell of the amphipod Tmetonyx cicada (O. Fabricius) are described. The sarcolemma invaginates and forms a transverse network of tubules at the level of the Z band. Narrow longitudinal tubules branch from the network and connect to another transverse network of tubules at the H band level, where dyadic and triadic junctions are formed with the sarcoplasmic reticulum. Adjacent myofibrils are normally separated by a well developed double layer of the sarcoplasmic reticulum. In areas where the myofibrils closely approach the outer sarcolemma, peripheral couplings have been found at the level of the H band.     

Localization of Mg2+-ATPase in the membranes of the skeletal muscles and myocardium of the frog Rana temporaria

Using differential centrifugation in sucrose density gradient, from muscles of the frog fractions were obtained which contain fragments of sarcolemma, as well as membranes of T-system tubules and sarcoplasmic reticulum. In isolated membrane fractions, studies were made on the activity of cation-stimulated ATPases (Na+, K+-, Ca2+, Mg2+- and Mg2+-ATPases). Enzymic and electrophoretic analyses showed that the highest content of Mg2+-ATPases is typical of the fractions which are located on the surface of 35% sucrose. The data obtained indicate that Mg2+-ATPase is the enzyme which is specific for the membranes of T-system tubules in skeletal muscles of not only birds but amphibians as well. From cardiac muscle of the frog, membrane fraction was isolated which is similar (with respect to its predominant content of Mg2+-ATPase) to the membranes of T-system tubules. It is suggested that the presence of Mg2+-ATPase in these membranes is a common property of phasic striated muscle fibers in all mature vertebrate animals.     

Assessment of pathological changes in fish muscle tissue, forming as a result of exposure to toxic factors in the environment

Our investigation of muscle tissue of fishes, inhabiting the regions with unfavorable ecological conditions (the river Volga), permitted to select four types of degenerative changes in muscle tissue. These alterations are associated with both the phylogenetic status of fish species and ecological dispositions of species. Using different methods of investigation several types of muscle destruction were shown. I. Destruction of myofibrillar apparatus (lysis of protofibrils), with sarcolemma remaining intact. II. Destruction of the myofibrillar apparatus, with sarcolemma, T-system, and sarcoplasmic reticulum being disrupted. III. Invasion of muscle fibers by lymphoid cells and macrophages; with sarcolemma being intact. IV. Lysis of sarcolemma by proteolytic enzymes of lymphoid elements; with muscle fibers being disintegrated. The objects of this study were muscle tissues of 8 fish species (Acipenser gueldenstadti, A. stellatus, A. ruthenus, Lucioprerca lucioperca, Esox lucius, Perca fluviatilis, Tinca tinca, Caprinus carpio). The white muscle degeneration followed the patterns of types I and II, while that of red muscles corresponded to types III and IV. White and red muscles of the Chondrostei fishes (sturgeon, stellate, sterlet) undergo destruction more frequently, than muscles of the Holostei fishes (pike, perch, zander, sazan, tench). Degenerative processes of white and red muscles of fish-eating fishes were more obvious than those of herbivorous fishes.     

Transverse tubular system depolarization reduces tetanic force in rat skeletal muscle fibers by impairing action potential repriming

When muscle fibers are repeatedly stimulated, they may become depolarized and force output decline. Excitation of the transverse tubular system (T-system) is critical for activation, but its role in muscle fatigue is poorly understood. Here, mechanically skinned fibers from rat fast-twitch muscle were used, because the sarcolemma is absent but the T-system retains normal excitability and its properties can be studied in isolation. The T-system membrane was fully polarized by bathing the skinned fiber in an internal solution with 126 mM K⁺ (control solution) or set at partially depolarized levels (approximately –63 and –58 mV) in solutions with 66 or 55 mM K⁺, respectively, and action potentials (APs) were triggered in the sealed T-system by field stimulation. Prolonged depolarization of the T-system reduced tetanic force proportionately more than twitch force, with greater effect at higher stimulation frequency (responses at 20 and 100 Hz reduced to 71 and 62% in 66 mM K⁺ and to 54 and 35% in 55 mM K⁺, respectively). Double-pulse stimulation showed that depolarization increased the repriming period (estimated minimum time before a second AP can be produced) from 4 ms to 7.5 and 15 ms in the 66 and 55 mM K⁺ solutions, respectively. These results demonstrate that T-system depolarization reduces tetanic force by impairing AP repriming, rather than by preventing AP generation per se or by inactivating the T-system voltage sensors. The findings also explain why it is advantageous to reduce the rate of motoneuron stimulation to muscles during repeated or prolonged periods of activity. T-system; muscle fatigue; excitation-contraction coupling     

DIFFERENTIATION OF THE SARCOPLASMIC RETICULUM AND T SYSTEM IN DEVELOPING CHICK SKELETAL MUSCLE IN VITRO

The electron microscope was used to investigate the first 10 days of differentiation of the SR and the T system in skeletal muscle cultured from the breast muscle of 11-day chick embryos. The T-system tubules could be clearly distinguished from the SR in developing muscle cells fixed with glutaraldehyde and osmium tetroxide. Ferritin diffusion confirmed this finding: the ferritin particles were found only in the tubules identified as T system. The proliferation of both membranous systems seemed to start almost simultaneously at the earliest myotube stage. Observations suggested that the new SR membranes developed from the rough-surfaced ER as tubular projections. The SR tubules connected with one another to form a network around the myofibril. The T-system tubules were formed by invagination of the sarcolemma. The early extension of the T system by branching and budding was seen only in subsarcolemmal regions. Subsequently the T-system tubules could be seen deep within the muscle cells. Immediately after invaginating, the T-system tubule formed, along its course, specialized connections with the SR or ER: triadic structures showing various degrees of differentiation. The simultaneous occurrence of myofibril formation and membrane proliferation is considered to be important in understanding the coordinated events resulting in the differentiated myotube.     

Fine structure and differentiation of Ascidian muscle. I. Differentiated caudal musculature of Distaplia occidentalis tadpoles

The structure of the caudal muscle in the tadpole larva of the compound ascidian Distaplia occidentalis has been investigated with light and electron microscopy. The two muscle bands are composed of about 1500 flattened cells arranged in longitudinal rows between the epidermis and the notochord. The muscle cells are mononucleate and contain numerous mitochondria, a small Golgi apparatus, lysosomes, proteid-yolk inclusions, and large amounts of glycogen. The myofibrils and sarcoplasmic reticulum are confined to the peripheral sarcoplasm. Myofibrils are discrete along most of their length but branch near the tapered ends of the muscle cell, producing a Felderstruktur. The myofibrils originate and terminate at specialized intercellular junctional complexes. These myomuscular junctions are normal to the primary axes of the myofibrils and resemble the intercalated disks of vertebrate cardiac muscle. The myofibrils insert at the myomuscular junction near the level of a Z-line. Thin filaments (presumably actin) extend from the terminal Z-line and make contact with the sarcolemma. These thin filaments frequently appear to be continuous with filaments in the extracellular junctional space, but other evidence suggests that the extracellular filaments are not myofilaments. A T-system is absent, but numerous peripheral couplings between the sarcolemma and cisternae of the sarcoplasmic reticulum (SR) are present on all cell surfaces. Cisternae coupled to the sarcolemma are continuous with transverse components of SR which encircle the myofibrils at each I-band and H-band. The transverse component over the I-band consists of anastomosing tubules applied as a single layer to the surface of the myofibril. The transverse component over the H-band is also composed of anastomosing tubules, but the myofibrils are invested by a double or triple layer. Two or three tubules of sarcoplasmic reticulum interconnect consecutive transverse components. Each muscle band is surrounded by a thin external lamina. The external lamina does not parallel the irregular cell contours nor does it penetrate the extracellular space between cells. In contracted muscle, the sarcolemmata at the epidermal and notochordal boundaries indent to the level of each Z-line, and peripheral couplings are located at the base of the indentations. The external lamina and basal lamina of the epidermis are displaced toward the indentations. The location, function, and neuromuscular junctions of larval ascidian caudal muscle are similar to vertebrate somatic striated muscle. Other attributes, including the mononucleate condition, transverse myomuscular junctions, prolific gap junctions, active Golgi apparatus, and incomplete nervous innervation are characteristic of vertebrate cardiac muscle cells.     

"Contractile" calcium and relation between the sarcoplasmic reticulum and the T-system in the myocardium and skeletal muscle

On the basis of published experimental data a consideration of the role of Ca ions in the myocardial and skeletal muscle contractility of the warmblooded was made. It has been shown that during the relaxation period Ca concentration in the cardiomyocytes sarcoplasmic reticulum (SR) must be of the order 10(-4) M and the corresponding concentrative gradient of Ca "SR-myoplasma" of 10(3) is maximally possible at the work of Ca-pump (the greater gradients of Ca concentration are the products of individual gradients). During the contraction period Ca "SR-myoplasma" gradient is lowered at the level 1 divided by 10(1). The SR membrane Ca-pump power is approximately 10(2) times greater than one of the sarcolemmal membranes. It was shown that because of spacely morphological peculiarities of the muscle cell structure out of the whole external Ca (coupled with the sarcolemma) only the part of Ca located at T-system can take part in the contractile act. The peculiarities of the T-system structure and the interrelation of the volumes and areas of T-system and SR permit to introduce a notion about "the coefficient of using external Ca in the contractile act" (UCa) and also enable to explain the differences in cardiomyocytes and skeletal myocytes resistance to a decrease of Ca concentration in the external environment.     

Three-dimensional electron microscopy of the sarcoplasmic reticulum and T-system in embryonic chick skeletal muscle cells in vitro

Summary The formation of the sarcoplasmic reticulum (SR) and the transverse tubular system (T-system) in embryonic chick skeletal muscle cells in vitro was studied by either the critical point drying-physical rupturing or physical rupturing-freeze drying together with rotary shadowing. In these cells, two membranous systems were observed. One was composed of flattened sacs which were either isolated or were connected to each other with slender processes to form mostly longitudinally oriented strands. Initially, these sacs had small granules at their surface and were found mainly under the sarcolemma. Later, they became smooth at their surface, extending throughout the cytoplasm to form irregular and dense networks. At later phases, the networks tended to be disposed at right angle to nascent myofibrils, exhibiting a characteristic honeycomb appearance. From the similarities in thin section images, they were identified as developing SR.The other membranous system were tubules with many enlargements. They were frequently associated with coated vesicles which appeared to take part in the formation, elongation, and anastomosing of developing tubules. These tubules could be impregnated with a tannic acid-glutaraldehyde-potassium ferrocyanide complex and, thus, were identified as T-tubules.Abbreviations CPD critical-point drying - ES exoplasmic surface of the sarcolemma - FD freeze-drying - PR physical rupture - PS protoplasmic surface of the sarcolemma - SR sarcoplasmic reticulum - TAGPF tannic acid-glutaraldehyde-potassium ferrocyanide - T-system transverse tubular system     

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.     

The membrane systems of the cardiac muscle cell of Munida tenuimana G. O. Sars (Crustacea,Decapoda)

The membrane systems of the cardiac muscle cell of Munida tenuimana G. O. Sars are described. The sarcolemma invaginates at the Z level, forming tubules. Narrow tubules branch off in a longitudinal direction from these transverse and radially arranged tubules, forming a narrow transverse collar at the H level where dyadic and triadic junctions are formed with the sarcoplasmic reticulum.     

Fine structure of the homologous tergo-coxal muscles of flying and flightless beetles

The flight-related tergo-coxal muscles of flying and flightless beetles are compared. In the flying beetle, Pachynoda sinuata, the myofibrils and cylindrical and the myofilaments packed in double hexagonal arrays. The sarcomeres are short (2.8 micrometer) and wide with many large, closely packed adjacent mitochondria but the sarcoplasmic reticulum is poorly developed in this fibrillar (asynchronous) muscle. Sarcoplasmic glycogen in rosette form is abundant. In the flightless beetle, Anthia thoracica, the myofibrils are lamellar-like with sarcomeres of 5.3 micrometer. The myosin filaments form a single hexagonal array each thick filament having an orbital of 11 to 12 thin filaments. The width of the Z-line (120 nm) of A. thoracia muscle was twice that of the Z-line of P. sinuata muscle. The sarcoplasmic reticulum and T-system are well-developed in this afibrillar (synchronous) muscle. Few glycogen granules are present. Triangular projections of the sarcolemma occur regularly opposite the Z-lines in A. thoracica and they appear to extend into the Z-lines. Membranous connections joint adjacent Z-lines in A. thoracica and occasionally in P. sinuata.     

About the T-system in the myofibril-free sarcoplasm of the frog muscle fibre

Previous investigations of the T-system in skeletal muscle fibres described the inter-myofibrillar relationships between T-tubules and the sarcoplasmic reticulum. They disregarded the arrangement of the T-system in the myofibril-free sarcoplasm in the area of muscle fibre nuclei. In the present investigation, the T-system was filled by means of lanthanum incubation and the myofibril-free sarcoplasm was ultrastructural examined by means of thin (< or = 100 nm) as well as thick sections (> 300 nm-1 microm) with the electron microscope. The investigation of thick sections revealed that T-tubules meander through this myofibril-free sarcoplasm and tangle up at the poles of muscle fibre nuclei and in the area of fundamental nuclei of the motor end plate. They are, far from myofibrils, in proximity to these nuclei, the Golgi apparatus and mitochondria. On basis of this proximity and their openings at the muscle fibre surface, a contribution at the drainage of metabolic products and at the local calcium control is discussed.     

FORMATION OF ELABORATE NETWORKS OF T-SYSTEM TUBULES IN CULTURED SKELETAL MUSCLE WITH SPECIAL REFERENCE TO THE T-SYSTEM FORMATION

Muscle cells, cultured for 1–28 days from 11-day chick embryo breast muscles, often show elaborate, three-dimensional networks of a membranous system. The network consists of tubular units which are quite regularly arranged. The tubular units composing the network are accessible to ferritin particles suspended in the culture medium; this suggests continuity with the extracellular fluid. These networks can be regarded as a special morphological elaboration of the T-system tubules. Such network formations can be seen much more often in well-developed myotubes. The networks usually exhibit a hexagonal pattern, which is formed of tubular units of a constant diameter. However, some early myotubes contain tetragonal networks, which are composed of spherical pockets with channels of lesser diameter connecting the pockets. Networks are also observed which probably represent a transitional form between these two patterns. Myotubes show many inpocketings of the sarcolemma similar to what are commonly referred to as caveolae or micropinocytotic vesicles. The similarity in configuration and dimension of the tubular units of the network to the caveolae leads to the plausible suggestion that repeated caveola-formation from the sarcolemma or T-system tubule may result in formation of these networks. In this connection, a possible mechanism of the T-system tubule formation is discussed.     

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 mitchondria. 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 fibr es 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 μm²). The capillary surface required to supply l μm³ of mitochondria is 0.18 μm² and the maximum distance between any capillary and mitochondrion 8 μm. 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.     

Myocardial damage during immobilization stress and the protective effect of thyroid hormones

In the experiments performed on 30 white inbred male rats, the stress produces an essential increase of dilated canals in the T-system and sarcoplasmic reticulum, lesions of sarcolemma and nucleolemma, fragmentation and homogenization of mitochondrial cristae; there are many pinocytic vesicles in the capillary walls. Preliminary injection of thyroid hormones in small doses, that do not result in any noticeable influence on the body mass, cardiac contraction rate and thyrotoxin concentration in blood serum, at the stress causes less pronounced changes in ultrastructure of cardiomyocytes, demonstrating as a poor swelling of the T-system elements, preservation of the sarcolemmic integrity at its larger length; besides, large mitochondria with compact arrangement of cristae appear, amount of the sarcoplasmic reticulum elements increases, chromatin margination becomes evident, i. e. a complex of adaptive processes develops. The data presented demonstrate a defensive role of physiological concentrations of the thyroid hormones under the stress.     

The membrane systems of the cardiac muscle cell of Cirolana borealis Lilljeborg (Crustacea,Isopoda)

The membrane systems of the cardiac muscle cell of the isopod Cirolana borealis Lilljeborg are described. The sarcolemma invaginates at the level of the Z band, forming transverse tubules. Narrow tubules branch off in a longitudinal direction from these transverse and radially arranged Tz-tubules forming a transverse collar at each A-I level, where dyadic and triadic junctions are formed with the sarcoplasmic reticulum. Two different orientations of the coupling discs have been detected in the supercontracted sarcomere, and this observation has been discussed. Adjacent myofibrils are separated by a double layer of sarcoplasmic reticulum.     

Ultrastructure of the radula protractor of Busycon canaliculatum

Summary The distribution of the sarcoplasmic reticulum and sarcolemmic tubules in the radula protractor muscle of the whelk, Busycon canaliculatum, has been investigated. The sarcoplasmic reticulum consists of an interconnected system of cisternae and tubular channels. The cisternae are closely associated with the sarcolemma. The tubular channels project from the cisternae into the interior of the cell and run parallel to the long axis of the myofilaments. Parallel tubular channels are interconnected with one another by short branches. This finding of an elaborate sarcoplasmic reticulum supports previous physiological work on this smooth muscle which indicated the presence of an intracellular compartmentalization of calcium ions. There is also an extensive system of tubular invaginations of the sarcolemma which we have termed sarcolemmic tubules. These tubules are 600 Å in diameter and about 0.5 microns in length. There is a substructure associated with the leaflet of the tubular membrane bordering the extracellular space. The sarcolemmic tubules penetrate only half a micron from the surface of the cell and interdigitate with the sarcoplasmic reticulum associated with the sarcolemma. Calculations have shown that the surface area of this smooth muscle cell is more than doubled by the presence of sarcolemmic tubules.     

Electrical properties of frog skeletal muscle fibers interpreted with a mesh model of the tubular system.

This paper presents the construction, derivation, and test of a mesh model for the electrical properties of the transverse tubular system (T-system) in skeletal muscle. We model the irregular system of tubules as a random network of miniature transmission lines, using differential equations to describe the potential between the nodes and difference equations to describe the potential at the nodes. The solution to the equations can be accurately represented in several approximate forms with simple physical and graphical interpretations. All the parameters of the solution are specified by impedance and morphometric measurements. The effect of wide circumferential spacing between T-system openings is analyzed and the resulting restricted mesh model is shown to be approximated by a mesh with an access resistance. The continuous limit of the mesh model is shown to have the same form as the disk model of the T-system, but with a different expression for the tortuosity factor. The physical meaning of the tortuosity factor is examined, and a short derivation of the disk model is presented that gives results identical to the continuous limit of the mesh model. Both the mesh and restricted mesh models are compared with experimental data on the impedance of muscle fibers of the frog sartorius. The derived value for the resistivity of the lumen of the tubules is not too different from that of the bathing solution, the difference probably arising from the sensitivity of this value to errors in the morphometric measurements.