Ultrastructure of muscle cells in Siboglinum fiordicum (Pogonophora)

Two different muscle types are found in the body of Siboglinum fiordicum: body wall muscle and blood vessel muscle. Both are of a myomesothelial type. The myofibrils of the body wall muscle are non-striated and consist of thick and thin myofilaments. Scattered dense bodies and attachment plaques are described. The sarcoplasmic reticulum forms a three-dimensional network in the myofibrils and only peripheral couplings are observed. The thick filaments are of a paramyosin type and have a diameter ranging from 400-1500 A. The blood vessels muscle is non-striated, but sometimes a sarcomere-like organization has been observed. Both thick and thin filaments are present. The thick filaments have a diameter of 250-400 A and lack transverse striations. Dense bodies and attachment of plaques are few. The sparse sarcoplasmic reticulum is restricted to the myofibril periphery where it makes peripheral couplings with sarcolemma. The luminal surface of the vessels is lined by a basal lamina with collagen-like inclusions. No endothelium is found. The body wall muscle and the blood vessel muscle are compared with other muscle types described in invertebrates.     

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.     

ELECTROMECHANICAL COUPLING IN TUBULAR MUSCLE FIBERS : I. The Organization of Tubular Muscle Fibers in the Scorpion Leiurus quinquestriatus

The tubular fibers of the claw-closer muscle of the scorpion have a central core containing nuclei and mitochondria. The myofibrils have the shape of thin lamellae (1 µ) extending radially from the core to the surface membrane (20 µ). The thick myofilaments are organized in a hexagonal array with orbits of 10–13 thin myofilaments. The ratio of thick-to-thin filaments is 1:5. Transverse tubular system (TS) openings are located between lamellated myofibrils. In each sarcomere two TS's are found, one on each side of the H band. The TS is composed of a transverse tubule and tubular pockets (TP). The TP's form diadic contact with the terminal cisternae of the sarcoplasmic reticulum. The TS can be traced from the cell membrane down to the cell core. The surface area of the TS was calculated to be six times that of the outer surface membrane.     

A COMPARATIVE STUDY ON THE FINE STRUCTURE OF THE BASALAR MUSCLE OF THE WING AND THE TIBIAL EXTENSOR MUSCLE OF THE LEG OF THE LEPIDOPTERAN ACHALARUS LYCIADES

Basalar and tibial extensor muscle fibers of Achalarus lyciades were examined with light and electron microscopes. Basalar muscle fibers are 100–150 µ in diameter. T-system membranes and sarcoplasmic reticulum make triadic contacts midway between Z lines and the middle of each sarcomere. The sarcoplasmic reticulum is characterized by a transverse element situated among myofilaments halfway between Z lines in every sarcomere. The morphology of Z lines, hexagonal packing of thin and thick myofilaments, and thin/thick myofilament ratios are similar to those of fast-acting insect muscles. Tibial extensor muscle fibers are 50–100 µ in diameter. Except for a lack of the transverse element, the T system and sarcoplasmic reticulum are similar to those of basalar muscle. Wavy Z lines, lack of a hexagonal packing of myofilaments, and larger thin/thick myofilament ratios are similar to those of other postural muscles of insects. The morphology of basalar and tibial extensor muscle is compared to that of other insect muscle with known functions, and reference is made to the possible contribution of the transverse element of sarcoplasmic reticulum in basalar flight muscle to speed and synchrony in this muscle.     

CORRELATED MORPHOLOGICAL AND PHYSIOLOGICAL STUDIES ON ISOLATED SINGLE MUSCLE FIBERS : I. Fine Structure of the Crayfish Muscle Fiber

Single fibers isolated from walking leg muscles of crayfish have 8- to 10-µ sarcomeres which are divided into A, I, and Z bands. The H zone is poorly defined and no M band is distinguishable. Changes in the width of the I band, accompanied by change in the overlap between thick and thin myofilaments, occur when the length of the sarcomere is changed by stretching or by shortening the fiber. The thick myofilaments (ca. 200 A in diameter) are confined to the A band. The thin myofilaments (ca. 50 A in diameter) are difficult to resolve except in swollen fibers, when they clearly lie between the thick filaments and run to the Z disc. The sarcolemma invaginates at 50 to 200 sites in each sarcomere. The sarcolemmal invaginations (SI) form tubes about 0.2 µ in diameter which run radially into the fiber and have longitudinal side branches. Tubules about 150 A in diameter arise from the SI and from the sarcolemma. The invaginations and tubules are all derived from and are continuous with the plasma membrane, forming the transverse tubular system (TTS), which is analogous with the T system of vertebrate muscle. In the A band region each myofibril is enveloped by a fenestrated membranous covering of sarcoplasmic reticulum (SR). Sacculations of the SR extend over the A-I junctions of the myofibrils, where they make specialized contacts (diads) with the TTS. At the diads the opposing membranes of the TTS and SR are spaced 150 A apart, with a 35-A plate centrally located in the gap. It appears likely that the anion-permselective membrane of the TTS which was described previously is located at the diads, and that this property of the diadic structures therefore may function in excitation-contraction coupling.     

Sarcoplasmic reticulum in the cross-striated adductor muscle of the bay scallop,Aequipecten irridians

Summary The fine structure of the striated adductor muscle of the bay scallop, Aequipecten irridians has been investigated with particular emphasis on the sarcoplasmic reticulum. Each cell of the muscle contains a single myofibril. There is no transverse tubular system in this muscle. The cisternae of the sarcoplasmic reticulum are all interconnected by means of tubular elements. This extensive, interconnected system of flattened cisternae and tubular vesicles is distributed randomly with respect to the sarcomere and is in close association with the sarcolemma.     

The fine structure of fast and slow crustacean muscles

Known phasic and tonic muscle fibers of the crab Cancer magister were studied by electron microscopy. Phasic fibers have sarcomeres about 4.5 µ long, small polygonal myofibrils, and a well-developed sarcoplasmic reticulum. The thick myofilaments, disposed in hexagonal array, are each surrounded by six thin filaments. The tonic fibers have a sarcomere length of about 12 µ, larger myofibrils, a poorly developed sarcoplasmic reticulum, and a disorderly array of myofilaments. Each thick myofilament is surrounded by 10–12 thin filaments. The same morphological type of slow muscle has been found in the crustaceans, Macrocyclops albidus, Cypridopsis vidua, and Balanus cariosus, in each case in an anatomical location consistent with tonic action. A search of the literature indicates that this type of muscle is found in all classes of arthropods and is confined to visceral and postural muscles or specializations of these.     

THE STRUCTURE OF INTERSEGMENTAL MUSCLE FIBERS IN AN INSECT, PERIPLANETA AMERICANA L

The organization of intersegmental muscle fibers associated with the dorsal abdominal sclerites of the cockroach is described. These fibers correspond closely, in the disposition and derivation of the membranes of the transverse tubular system and sarcoplasmic reticulum cisternae, with insect synchronous flight muscle fibers, but differ markedly from these in their fibrillar architecture and mitochondrial content. The mitochondria are small and generally aligned alongside the prominent I bands of the sarcomere, and, in the best-oriented profiles of the A bands, thick filaments are associated with orbitals of twelve thin filaments, a configuration that has also been observed in striated fibers of insect visceral muscle. These structural features of insect muscles are compared and discussed in terms of possible variations in the control of contraction and relaxation, and in the nature of their mechanical role.     

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.     

The organization and fine structure of the muscles of the scolex of the cysticercoid of Hymenolepis microstoma

The organization and fine structure of the muscles of the scolex of the cysticercoid of Hymenolepis microstoma are described. The contractile apparatus consists of thick (175–325 Å diameter × 1.4 μm) and thin (60–80 Å diameter × 1 μm) filaments. The thick filaments are occasionally attached to the thin filaments by cross bridges. The thin filaments are attached to the dense bodies or to a dense zone at the sarcolemma at muscle insertions. In contracted muscle the thick filaments appear as quasi-hexagonal arrays or in lines. Each thick filament is surrounded by an orbit of up to 12 thin filaments, which in turn may be shared by adjacent thick filaments. Thin filaments may be present in quasi-rectangular or hexagonal groupings indicating some low order degree of actin lattice. The fusiform dense bodies (1,500 Å × 900 Å), consisting of up to 25 discrete substructures, are distributed uniformly throughout the myofiber and/or attached to the sarcolemma at attachment plaques. The sarcoplasmic reticulum, consisting of a presumed anastomosing network of tubules is structurally connected to the sarcolemma by periodic deposits of electron opaque material. Sarcoplasmic extensions of the myofiber(s) contain the nucleus, Golgi complexes, rough endoplasmic reticulum, ribosomes, β-glycogen, mitochondria and membrane bound electron dense structures. Upon activation of the metacestode, groups of α-glycogen and enlargement of the rough endoplasmic reticulum were observed. Microtubules which were conspicuously absent from the sarcoplasm of the unactivated worms appeared adjacent to the myofibers in activated worms.     

Ultrastructure and differentiation of the larval esophageal muscle cells of the starfish Pisaster ochraceus

The muscle cells that cause constriction of the starfish larval esophagus (esophageal muscle cells) are one of the first cell types to express their differentiated morphological characteristics during development. Ultrastructurally these muscle cells resemble vertebrate and invertebrate smooth muscles. They contain a nucleus, a Golgi apparatus, contractile myofilaments, hemidesmosome-like structures, and what appears to be a simple sarcoplasmic reticulum. In asteroid embryos, this muscle layer originates during mouth formation when mesenchyme cells migrate from the tips of the coeloms to the esophagus. Once there, they elongate, forming processes. Over the next few days, the processes become filled with arrays of longitudinally arranged thick and thin myofilaments and thin sacs of smooth endoplasmic reticulum. The latter appear between the bundles of contractile filaments and the cell membranes. Contractile activity begins at approximately this time. The cisternae may represent a sarcoplasmic reticulum that is required for contraction. The majority of the esophageal muscle cell processes extend around the circumference of the developing esophagus, but occasional cells may be oriented in other directions. The latter cells are always farther away from the basal lamina and probably have little or no contact with it. Contact with basal lamina may serve to direct the migration of the cells and the orientation of the processes. J. Morphol. 237:1–18, 1998. © 1998 Wiley-Liss, Inc.     

The heart Ultrastructure of lepidopleurus asellus (Spengler) and Tonicella marmorea (Fabricius) (Mollusca: Polyplacophora)

Summary The ultrastructure of the auricles, the ostia, and the ventricle of L. asellus and T. marmorea is described. The heart wall consists of an epicardium, a basement membrane, and an inner loose myocardium. The epicardial cells of the auricle are podocytes. The exposed cell body and the branched processes show pedicles. Ventricular epicardium is flat and simple. The slender, unbranched, mononucleated muscle fibres have a peripheral nucleus located midway along the fibre. Mitochondria are peripherally located, leaving the center to longitudinally running thick and thin myofilaments. Dense bodies and attachment plaques make up the Z-material. Sarcomeres and myofibrils are absent, as are transverse tubules and intercalated disks. The sarcoplasmic reticulum consists of peripheral tubules and subsarcolemmal cisternae, some of which radiate, branch, and run between myofilaments. Couplings are lacking. Ventricular fibres in T. marmorea show nexuses and desmosomes; in L. asellus only nexuses. The muscular ostia are tubular, and muscle fibres resemble those of the ventricle; nexuses are detected in T. marmorea and desmosomes in L. asellus. The only nervous elements observed are some nerve processes, structurally similar to those of other molluscs.Supported by grants from the Norwegian Research Council for Science and Humanities     

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.     

The relation between excitation-contraction coupling and fine structure of a molluscan muscle,the radular retractor of the whelk,Buccinum undatum

The Buccinum radula is of the rachiglossate type with two outer rows of fierce hook-like attack teeth and a medial row of straight sharp-pointed shredding teeth. Individual cells of the radular retractor muscle are 10–12 m in diameter and separated at the closest by gaps of only 40 nm, providing areas of potential electrical contact. The cell membranes are heavily invested with long finger-like invaginations, associated with sarcoplasmic reticular cisternae, and surface caveolae; the latter are associated with the numerous dense body membrane attachment plaques found in this muscle. The radular retractor muscle possesses a significant sarcoplasmic reticulum of peripheral cisternae and deeper vesicles associated with mitochondria. The surface caveolae may result from myofilament force exerted via attachment plaques at the cell membrane, while deeper invaginations may constitute a rudimentary transverse tubular system to relay surface depolarization to associated sarcoplasmic reticular cisternae inducing calcium release to effect excitation-contraction coupling. The radular retractor muscle possesses the usual thick paramyosin and thin actin myofilaments, the latter associated with dense bodies and attachment plaques presumably to transduce force to the cell membrane. The mitochondria are unusually large and packed into dense central clusters surrounded by large deposits of glycogen granules. The nerve endings on the radular retractor muscle fibres show four different types of transmitter vesicle, presumably related to the four kinds of agonist action in this muscle, cholinergic, serotonergic, peptidergic and purinergic. All nerve endings have mixed vesicle populations, clear evidence of co-transmission. In this muscle we see a modification of usual smooth muscle structure to effect fast sustained contractions, an ultrastructural configuration functionally designed for the muscle's central role in the feeding cycle.Abbreviations ABRM anterior byssus retractor muscle - EC coupling excitation-contraction coupling - RP radular protractor muscle - RR radular retractor muscle - SR sarcoplasmic reticulum - T-system transverse tubular system     

SELECTIVE DISRUPTION OF THE SARCOTUBULAR SYSTEM IN FROG SARTORIUS MUSCLE : A Quantitative Study with Exogenous Peroxidase as a Marker

Skeletal muscles which have been soaked for 1 hr in a glycerol-Ringer solution and then returned to normal Ringer solution have a disrupted sarcotubular system. The effect is associated with the return to Ringer's since muscles have normal fine structure while still in glycerol-Ringer's. Karnovsky's peroxidase method was found to be a very reliable marker of extracellular space, filling 98.5% of the tubules in normal muscle. It was interesting to note that only 84.1% of the sarcomeres in normal muscle have transverse tubules. The sarcotubular system was essentially absent from glycerol-treated muscle fibers, only 2 % of the tubular system remaining connected to the extracellular space; the intact remnants were stumps extending only a few micra into the fiber. Thus, glycerol-treated muscle fibers provide a preparation of skeletal muscle with little sarcotubular system. Since the sarcoplasmic reticulum is not destroyed and the sarcolemma and myofilaments are intact in this preparation, of the properties of the sarcolemma may thus be separated from those of the tubular system.     

Cytological differentiation of human fetal skeletal muscle.

The ultrastructural differentiation of several different muscles was investigated in human fetuses ranging in age from 13 weeks to neonatal. At approximately 16 weeks of gestation cell cluster containing both myotubes and satellite cells lie enclosed by a newly formed basal lamina and show evidence of fusion. The development of organelles is evident in myoblasts, proceeds as the cells transform into myofibers, and continues in the neonate. Filament synthesis occurs primarily in the cell periphery where thin filaments appear to align themselves in relations to parallel arrays of ribosome-studded thick filaments: Z line formation follows the appearance of thin filaments. Intermediate filaments, approximately 10-12 nm thick, were also consistently observed in perinuclear regions and distal to filament assembly. Although sarcoplasmic reticulum (SR) development is closely related to fibril formation, connections between Z lines and SR are not consistent, thus supporting the conclusion that SR does not evoke the formation of the Z line. Bristlecoated vesicles appear to be the precursors of elements of the SR, possibly the lateral sacs. Development of the transverse tubules, as invaginations of the sarcolemma, is closely associated with the formation of lateral sacs since the latter occur along the sarcolemma as soon as transverse tubules appear. Cytological differentiation is similar, though not identical, in several different muscles. During the last trimester muscle fibers show some evidence of diversity mainly of variation in Z line width. In gerneral the results suggest that the sequence and stages of human myogenesis are similar to those of other species.     

The pedal muscle of the land snail Megalobulimus oblongus (Gastropoda, Pulmonata): an ultrastructure approach

M. Cristina Faccioni-Heuser, Denise M. Zancan, Christiane Q. Lopes and Matilde Achaval. 1999. The pedal muscle of the land snail Megalobulimus oblongus (Gastropoda, Pulmonata): an ultrastructure approach. — Acta Zoologica (Stockholm) 80: 325–337
The ultrastructure of the pedal muscle of the Megalobulimus oblongus is described. This muscle consists of transverse, longitudinal and oblique bundles ensheathed in collagenous tissue. Each muscle cell is also ensheathed by collagen. The smooth muscle cells contain thin and thick filaments; the thin filaments are attached to dense bodies. These cells contain a simple system of sarcoplasmic reticulum, subsarcolemmal caveolae and mitochondria with dense granules in the matrix, and glycogen. Three types of muscle cells were identified. Type A cells exhibited densely packed myofilaments, abundant glycogen rosettes, numerous mitochondria and sarcoplasmic reticulum profiles. Type B cells exhibited scanty glycogen and mitochondria, few cisternae of sarcoplasmic reticulum and large intermyofibrillar spaces. Type C cells exhibited intermediate characteristics between type A and type B cells. Neither nexus nor desmosomes were observed between the muscle cell membranes. The muscle contains well developed connective tissue and blood vessels. These structures and the distribution of muscle cells are probably involved in the muscular-hydrostat system. The muscle is richly innervated, having neuromuscular junctions with clear and electron-dense synaptic vesicles. The clear vesicles probably contain acetylcholine because the axons to which they are connected arise from acetylcholinesterase positive neurones of the pedal ganglion. The other vesicles may secrete monoamines such as serotonin and/or neuropeptides such as substance P.     

Ultrastructural features of cardiac muscle cells in a tarantula spider

The ultrastructural features of cardiac muscle cells and their innervation were examined in the tarantula spider Eurypelma marxi Simon. The cells are transversely striated and have an A band length of about three microns. H zones are indistinct and M lines are absent. Thick and thin myofilament diameters are approximately 200 and 70 Å respectively. Eight to 12 thin filaments usually surround each thick one. Accumulations of thick and thin myofilaments occur perpendicular to the bulk of the myofilaments in some cells. The Z line is discontinuous and thick filaments may pass through the spaces in the Z line. Extensive systems of sarcoplasmic reticulum and transverse tubules are present; these form numerous dyadic junctions in both A and I band regions. Sarcolemmal invaginations form Z line tubules; lateral extensions of the plasma membrane portion of these invaginations form dyads. Nerve branches of the cardiac ganglion make multiple neuromuscular synapses with at least some of the cardiac muscle cells. Both large granular and small agranular vesicles are present in the presynaptic terminals. Intercalated discs similar to those present in other arthropod hearts occur between the ends of adjacent cardiac muscle cells.     

SARCOPLASMIC RETICULUM OF AN UNUSUALLY FAST-ACTING CRUSTACEAN MUSCLE

The fast-acting, synchronous "remotor" muscle of the lobster second antenna was examined by light and electron microscopy and was found to have a more profuse sarcoplasmic reticulum (SR) than any other muscle known. Myofibrils are widely separated from one another and occupy only about one-fourth of the volume of the muscle; most of the remaining volume is taken up by the SR, which resembles the smooth-surfaced reticulum of steroid-secreting cells. Dense granules (0.03–0.1 µ in diameter) are scattered through the reticulum. T-tubules penetrate into the fibers and form dyads along the A bands of myofibrils; however, ferritin-labeling experiments show that the volume of the T-system is very small compared with that of the SR. Myofibrils are ~0.5 µ x 1.0 µ in cross section and consist of thick filaments, which appear tubular except at the M region, and thin filaments, which are situated midway between neighboring thick filaments. The ratio of thin to thick filaments is 3:1. The extreme development of the SR in this muscle is discussed in relation to the exceedingly short duration of the contraction-relaxation cycle.     

Elektronenmikroskopische Untersuchungen an Spinnenmuskeln

Zusammenfassung Die Pasern aus den Beinmuskeln der Vogelspinne Dugesiella hentzi sind zwischen 100 und 250 m dick und durch tiefe Einfaltungen des Sarcolemms in Untereinheiten gegliedert. Die meist bandförmigen Myofibrillen liegen darin in radiärer Anordnung. A-Bandbreite und Sarcomerenlänge variieren sehr stark (Extremwerte 2,8 und 5,6 bzw. 3,0 und 7,3 m). Ausrichtung und Anordnung der Myofilamente sind wenig exakt. Auf ein Primärfilament (Durchmesser 230–235 Å) kommen durchschnittlich 4–4,5 Sekundärfilamente (70–80 Å).Das sarcoplasmatische Reticulum (SR) ist extensiv und in Form eines unregelmäßigen Netzes aus schlauchartigen Elementen ausgebildet. Im Bereich des A-Bandes erweitern sich einzelne Schläuche zu Cisternen, die mit den Tubuli des Transversalsystems Dyaden bilden. Die SR-Membran zeigt dabei im Dyadenbereich charakteristische Strukturen: punktförmige Membranverdickungen, die ein Muster von großer Regelmäßigkeit bilden. Lage und Zahl der Dyaden sind sehr variabel (Durchschnitt 3–4 pro Sarcomer).

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An electron microscopical study of spider muscles

Summary Four different leg muscles of the tarantula Dugesiella hentzi were investigated electron microscopically. The fibers measure 100 to 250 m in diameter. They are divided into subunits by deep invaginations of the sarcolemma. The myofibrils have the shape of irregular ribbons which are arranged radially within the fiber subunits. The length of the A band as well as the sarcomer length varies from 2.8 to 5.6 and 3.0 to 7.3 n respectively. The myofilaments do not form very regular patterns. The ratio thick filaments (diameter 230 to 235 Å) to thin filaments (70 to 80 Å) is approximately 1 to 4 or 4.5. The sarcoplasmic reticulum (SE) is extensively developed. It consists of an irregular network of tubular elements surrounding the myofibrils and frequently crossing the Z discs. In the A band region some of the SR tubules widen. These cisternae form dyads with the tubules of the transversal system. In the dyads the membrane of the cisternae shows a characteristic structure: i.e. an exact pattern of small, point-like membrane thickenings. The position and the number of the dyads vary widely. Usually there are 3 to 4 in each sarcomer.

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