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.     

Ultrastructure of muscle fibers of an extraocular muscle of the pigeon

An electron microscopic analysis of the internal rectus muscle of the eye of the pigeon permitted identification of three types of muscle fibers: the first type shows the features previously described in vertebrate twitch fibers. The second type has very scarce sarcoplasmic reticulum at the A-band, their myofibrils fuse together at this level; the Z-line is large and the M-line is not present; the thick filaments are more abundant per unit area than in the first type of fibers, their hexagonal array is slightly disrupted and the fibers appear more opaque than the other two fiber types. The third type of fibers has bundles of myofibrils incompletely surrounded by sarcoplasmic reticulum at the A-band; the Z-line is large; the M-line is present and the hexagonal array of the thick filaments is maintained.     

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.     

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.     

Fine structural study of the abdominal muscle receptor organs of the crayfish (Procambarus clarkii). Fast and slow receptor muscles

Receptor muscles of the abdominal muscle receptor organs of the crayfish, Procambarus clarkii, were examined by electron microscopy. Both the fast and the slow receptor strand comprises a single muscle fibre which is divided by invagination of the cell membrane into numerous cytoplasmic processes in its intermediate region (the so-called intercalated tendon). Most of these myofibrillar processes insert in this region, but some of them pass through the intermediate region without interruption and join the other portion of the fibre. Thus the receptor muscles, whilst maintaining cytoplasmic continuity throughout their whole length, are modified in their intermediate regions, becoming fasciculated and providing spaces which are occupied by the connective tissue and the dendrites of the sensory neurone. Clear-cut differences in fine structure are shown between the muscle of the two types of receptor unit. The fast receptor muscle shows the typical features of arthropod fast muscles, including short sarcomere length (on average 3.3 μm), cylindrical myofibrils, well-developed sarcoplasmic reticulum, and regular hexagonal array of the myofilaments. By contrast, the slow receptor muscle fibre is characterized by long sarcomeres (average 6.5 μm) and unique organization of the myofilaments, with very thick ‘thick’ filaments having diameters in the range of 25–36 nm surrounded by about 12 thin filaments.     

Insect visceral muscle. Fine structure of the proctodeal muscle fibres

The fine structure of the longitudinal muscle fibres of the cockroach proctodeum was investigated by electron microscopy. The fibre is separated incompletely into fibrils, the resting sarcomere length is variable: about 5·8 to 7·3 μm, and the A- and I-bandings are not always clear in longitudinal sections. The ratio of thin and thick filaments at the overlapped region is about 4·1:1 when relaxed, and about 9·8:1 when fully contracted. The myofilament array is not well organized.The previously observed prolonged time course of muscle contraction seems to correlate with the present observations on the poorly developed sarcoplasmic reticulum and irregular distribution of transverse tubules. The Z-bands are irregularly aligned and discontinuous in longitudinal sections. The Z-band structure was studied in relation to the supercontractility. It was found that at maximal isotonic contraction (about 25 per cent rest length) the myofilaments pass through the expanded Z-regions.     

Fine Structure of Cardiac Sarcomeres in the Black Widow Spider Latrodectus mactans

Fine structural characteristics of the cardiac muscle and its sarcomere organization in the black widow spider, Latrodectus mactans were examined using transmission electron microscopy. The arrangement of cardiac muscle fibers was quite similar to that of skeletal muscle fibers, but they branched off at the ends and formed multiple connections with adjacent cells. Each cell contained multiple myofibrils and an extensive dyadic sarcotubular system consisting of sarcoplasmic reticulum and T‐tubules. Thin and thick myofilaments were highly organized in regular repetitive arrays and formed contractile sarcomeres. Each repeating band unit of the sarcomere had three apparent striations, but the H‐zone and M‐lines were not prominent. Myofilaments were arranged into distinct sarcomeres defined by adjacent Z‐lines with relatively short lengths of 2.0 μm to 3.3 μm. Cross sections of the A‐band showed hexagon‐like arrangement of thick filaments, but the orbit of thin filaments around each thick filament was different from that seen in other vertebrates. Although each thick filament was surrounded by 12 thin filaments, the filament ratio of thin and thick myofilaments varied from 3:1 to 5:1 because thin filaments were shared by adjacent thick filaments.     

Structure of multinucleated smooth muscle cells of the ascidian Halocynthia roretzi

Summary Cells isolated from ascidian smooth muscle were about 1.5–2 mm in length. Each contained 20–40 nucle in proportion to cell length. The cytoplasm was characterized by the presence of an enormous quantity of glycogen particles, tubular elements of sarcoplasmic reticulum coupled to the cell membrane, and conspicuous contractile elements. Thick and thin filaments had diameters of about 14–16 nm and 6–7 nm, respectively. The population density of the thick filaments was much higher (mean 270/m² filament area) than in vertebrate smooth muscles. The ratio of thick to thin filaments was about 16. All the thick filaments were surrounded by a single row of 5–9 thin filaments forming a rosette, and cross-bridges with periodicities of 14.5 and 29 nm were found between them. The contractile apparatus consisted of numerous myofibrils which were arranged nearly along the cell axis and were separated from each other by a network of 10-nm filaments. The myofibrils further consisted of many irregularly arranged sarcomerelike structures, each of which was comprised of a small group of thick and thin filaments with attached dense bodies.     

THE FINE STRUCTURE OF THE VENTRAL INTERSEGMENTAL ABDOMINAL MUSCLES OF THE INSECT RHODNIUS PROLIXUS DURING THE MOLTING CYCLE : I. Muscle Structure at Molting

Rhodnius prolixus, a South American insect, molts five times in its development to an adult after emerging from the egg. Each molting cycle is triggered with a blood-meal. The ventral intersegmental abdominal muscles of Rhodnius develop during each molting cycle and are functional at molting. The fine structure of these fully developed muscles from fourth stage larval insects is studied. They have the characteristic structure of slow muscles. They have multiple motor nerve endings, and the myofibrils are poorly defined in cross-section. Longitudinal sections show long sarcomeres (8–10 µ), irregular Z-lines, and no apparent H zones. No M line is seen. Transverse sections through the A-band region show that each hexagonally arranged thick filament is surrounded by 12 thin filaments. Two thin filaments are shared by two neighboring thick filaments. The ratio of thin to thick filaments is 6:1. This structure is related to that found in vertebrate skeletal muscle and insect flight muscle.     

The architecture of the accessory reproductive glands of the male desert locust: III. Components of the muscular wall

An electron microscopic study of the sheath enclosing the accessory reproductive glands of the male desert locust has shown that it consists, for the most part, of a single myofibril, and that other tissues (nerve fibres, tracheal elements, and the fat body) are also associated with it to a greater or lesser extent. The myofibril has special features associated with the Z-bands, including the regular infolding and the attachment of the sarcolemma at the Z-bands, and the synapsing of nerve axons at these infoldings, which perhaps facilitate the rapid transmission of nerve impulses into the myofibril. The distribution of the T-systems and sarcoplasmic reticulum (SR) is described, and their relationship to the speed of action of the myofibril is discussed. The myofibril exhibits three distinct bands: the A-, I-, and Z-bands. In the A-band, each thick myofilament is surrounded by 10 to 12 thin filaments. This finding is related to similar findings in other arthropod visceral and slow-acting skeletal muscles. The basement membrane surrounding the glandular epithelium comprises two parts: the inner part, which is structureless and contains neutral mucopolysaccharide; and the outer part which contains numerous collagen-like fibrils and stains for acid mucopolysaccharide. This characteristic is considered in relation to the insertion and function of the myofibril.     

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.     

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.     

The fine structure of muscle in a marine turbellarian

Summary The fine structure of the myofibers of Notoplana acticola as studied by electron microscopy indicates that they are composed of thick myofilaments about 200 Å wide with tapering ends and thin myofilaments about 50 Å wide, arranged alongside each other parallel to the long axis of the cell. There is no orderly transverse arrangement of filaments; instead they appear staggered in the fiber. In cross sections 6 to 10 thin filaments form an orbit around one thick filament with possible cross-linkage between the two types of filaments.Dense bodies are associated with the sarcolemma and with the sarcoplasmic reticulum, and appear to serve as attachments for the thin filaments. Dense bodies are compared to elements forming a fragmented Z-disc.Mitochondria, situated in the periphery or the center of fibers, are associated with granules interpreted as glycogen.The sarcoplasmic reticulum consists of: sacs or cisternae in close proximity to the sarcolemma, longitudinal tubular elements between and parallel to the myofilaments, and a tubular network around the filaments. There is no well-defined sarcolemmal-derived transverse tubule system as described in striated muscles. It is hypothesized that in these muscles, the functional equivalent of the T system may be the area of sarcolemma in contact with the cisternae of the sarcoplasmic reticulum.This work was supported by Grant No. GM 10292 from the U. S. Public Health Service to Professor Richard M. Eakin, Department of Zoology at the University of California, Berkeley, USA, where this investigation was conducted during the author's sabbatical leave of absence from the University of Illinois.I wish to thank Professor Eakin for valuable discussions and for his kind hospitality in extending the facilities of his laboratory and the use of the electron microscope to me, and the John Simon Guggenheim Memorial Foundation for the Fellowship which I held during 1964–65.     

The ultrastructural organization of femoral muscles in Musca domestica (Diptera)

The femoral muscles of Musca domestica are organized as typical synchronous muscles. The nuclei occupy the central region of the fibres. The myofibrils have a lamellar shape with the major diameter oriented radially with respect to the nuclei; they have an extremely regular organization. In cross section each thick filament is surrounded by 10-12 thin filaments. The mitochondria are relatively abundant and are organized in three concentric layers: perinuclear, intermyofibrillar and subsarcolemmal. They have an elongated shape and possess numerous cristae, which are oriented trasversally with respect to the long axis of mitochondria. The S.R. is organized as a complex system of tubules which completely surround the contractile material. The T-system has a characteristic shape, and takes contact with the sarcoplasmic reticulum forming peculiar triadic structures.     

MYOFIBRILLOGENESIS OF THE LATERAL MUSCULATURE IN THE TROUT (SALMO TRUTTA L.)

The developing sarcomeres in the lateral musculature of 60-somite trout embryos have been examined with special reference to the ultrastructure and sequence of events accompanying sarcomere formation. Myogenesis begins at the first somites in the head region and progresses towards the tail of the embryo. The tail somites are composed of undifferentiated presumptive myoblasts, myoblasts and mesenchyme cells. The very tip of the tail contains a mass of undifferentiated cells. Myofibrils with fully developed sarcomeres and well-organized sarcoplasmic reticulum are present in the midbody somites.
Microtubules are found in muscle cells throughout the period preceding the sarcomere assembly. They may represent a cytoskeletal network which contributes directly to the shape of myoblasts.
Thick and thin filaments appear mostly near the periphery of the cell. In successive stages of the sarcomere development "Z-bodies" appear, which then coalesce to form Z-bands. The assembly of the thick and thin filaments into sarcomeres seems to occur at that stage of myogenesis when the "Z-bodies" develop a certain amount of bonding sites for thin filaments, which interact with thick filaments to form A-bands and I-bands.     

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.     

OBSERVATIONS ON THE FINE STRUCTURE OF PHARYNGEAL MUSCLE IN THE PLANARIAN DUGESIA TIGRINA

Pharyngeal muscle of the planarian Dugesia tigrina was studied by electron microscopy after osmium tetroxide fixation. The muscle cell was observed to contain one myofibril or bundle of myofilaments parallel to its longitudinal axis. The myofilaments were of two types, different in size and distribution. No Z lines or myofilament organization into cross or helical striations were seen. Dense bodies were seen as projections from an invagination of the plasma membrane and as dense lines parallel to the myofilaments. The muscle cells are surrounded by a plasma membrane which is structurally associated with dense body projections, with vesicles and cisternae of sarcoplasmic reticulum, and with synaptic nerve endings. The cell has sarcoplasmic projections perpendicular to its long axis; these projections are seen to contain the nucleus or mitochondria and granules. Mitochondria and granules are also seen in a sarcoplasm rim around the fibril. The dense bodies may serve as attachment for thin myofilaments and function in transmission of stimuli from plasma membrane to the interior of the fibril.     

Myoblast differentiation in vitro: morphological differentiation of mononucleated myoblasts.

Phospholipase C from Clostridium perfringens has been shown previously to inhibit the fusion of cultured chick myoblasts without affecting recognition or cell cycle parameters. In this paper we report that the mononucleated myoblasts, in phospholipase C, synthesize thick and thin filaments and organize them into myofibrils, and that T-tubules and sarcoplasmic reticulum differentiate and join in morphologically typical junctions. The structurally differentiated myoblasts can then fuse with one another to form myotubes. We conclude that cell fusion is not necessary for muscle differentiation.     

Ultrastructurally different muscle cell types in Eisenia foetida (Annelida,Oligochaeta)

Muscles in the body wall, intestinal wall, and contractile hemolymphatic vessels (pseudohearts) of an oligochaete anelid (Eisenia foetida) were studied by electron microscopy. The muscle cells in all locations, except for the outer layer of the pseudohearts, are variants of obliquely striated muscle cells. Cells comprising the circular layer of the body wall possess single, peripherally located myofibrils that occupy most of the cytoplasm and surround other cytoplasmic organelles. The nuclei of the cells lie peripherally to the myofibrils. The sarcomeres consist of thin and thick myofilaments that are arranged in parallel arrays. In one plane of view, the filaments appear to be oriented obliquely to Z bands. Thin myofilaments measure 5–6 nm in diameter. Thick myofilaments are fusiform in shape and their width decreases from their centers (40–45 nm) to their tips (23–25 nm). The thin/thick filament ratio in the A bands is 10. The Z bands consist of Z bars alternating with tubules of the sarcoplasmic reticulum. Subsarcolemmal electron-dense plaques are found frequently. The cells forming the longitudinal layer of the body wall musculature are smaller than the cells in the circular layer and their thick filaments are smaller (31–33 nm centrally and 21–23 nm at the tips). Subsarcolemmal plaques are less numerous. The cells forming the heart wall inner layer, the large hemolymphatic vessels, and the intestinal wall are characterized by their large thick myofilaments (50–52 nm centrally and 27–28 nm at the tips) and abundance of mitochondria. The cells forming the outer muscular layer of the pseudohearts are smooth muscle cells. These cells are richer in thick filaments than vertebrate smooth muscle cells. They differ from obliquely striated muscle cells by possessing irregularly distributed electron-dense bodies for filament anchorage rather than sarcomeres and Z bands and by displaying tubules of smooth endoplasmic reticulum among the bundles of myofilaments. © 1995 Wiley-Liss, Inc.     

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.