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.     

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 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.     

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.     

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.     

CARDIAC MUSCLE OF THE HORSESHOE CRAB, LIMULUS POLYPHEMUS : I. Ultrastructure

The fine structure of the cardiac muscle of the horseshoe crab, Limulus polyphemus, has been studied with respect to the organization of its contractile material, and the structure of its organelles and the cell junctions. Longitudinal sections show long sarcomeres (5.37 µ at L_max), wide A bands (2.7 µ), irregular Z lines, no M line, and no apparent H zone. Transverse sections through the S zone of the A band show that each thick filament is ca. 180 A in diameter, is circular in profile with a center of low density, and is surrounded by an orbit of 9–12 thin filaments, each 60 A in diameter. Thick filaments are confined to the A band: thin filaments originate at the Z band, extend through the I band, and pass into the A band between the thick filaments. The sarcolemmal surface area is increased significantly by intercellular clefts. Extending into the fiber from these clefts and from the sarcolemma, T tubules pass into the fiber at the A-I level. Each fibril is enveloped by a profuse membranous covering of sarcoplasmic reticulum (SR). Sacculations of the SR occur at the A-I boundary where they make diadic contact with longitudinal branches of the T system. These branches also extend toward the Z, enlarge at the Z line, and pass into the next sarcomere. Infrequently noted were intercalated discs possessing terminal insertion and desmosome modifications, but lacking close junctions (fasciae occludentes). These structural details are compared with those of mammalian cardiac and invertebrate muscles.     

Mastophorus muris (Nematoda: Spirurina): Ultrastructure of somatic muscle development

Hamada G. S. and Wertheim G. 1978. Mastophorus muris (Nematoda: Spirurina): ultrastructure of somatic muscle development. International Journal for Parasitology8; 405–414. The ultrastructure of the somatic muscle cells of the adult and six developmental stages of Mastophorus were studied. In all stages the cells consisted of a contractile region containing myofibrils separated by dense bands and a noncontractile region with nuclei, mitochondria, glycogen, lipid droplets and vesicles. Two sizes of myofilaments were present. The dense band contained T tubules and sarcoplasmic reticulum, and, in more advanced stages, support filaments, glycogen and dense bodies. The contractile region of the adult muscle cell consisted of several hundred irregularly shaped myofibrils arranged in a random pattern. This pattern of myofibrils was defined as irregular-coelomyarian. The third stage larva had a shallow-coelomyarian myofibril configuration, which changed to coelomyarian in the late third stage through the addition of new myofibrils at the apical contractile border. In the fourth stage larvae, the subdivision of existing myofibrils changed the pattern to irregular-coelomyarian.     

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.     

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.     

Shape and disposition of clefts,tubules, and sarcoplasmic reticulum in long and short sarcomere fibers of crab and crayfish

Summary The disposition of surface invaginations (clefts, Z and T tubules) and of the sarcoplasmic reticulum has been examined by electron microscopy at three accelerating voltages (100, 200 and 1000 kV) and by phase-contrast light microscopy in crustacean muscles infiltrated by the Golgi stain. In long-sarcomere, tonic type fibers, an extensive system of invaginating clefts has been observed, along with both Z and T tubules. Z and T tubules form interconnections with each other, but only T tubules form specific contacts with the sarcoplasmic reticulum, which in these fibers forms an extended and continuously fenestrated network. In short-sarcomere, phasic type fibers, a ladder-like disposition of an abundant T network is found. Z tubules are absent in these fibers. The sarcoplasmic reticulum forms more frequent junctions with flattened areas of T tubules and with clefts, but has less extensive free surfaces than in the long-sarcomere fibers.We wish to dedicate this paper to the late Graham Hoyle, whose lifetime of work and interest in the study of muscle from a comparative point of view has been an inspiration to us.     

Structural organization of two fast,rhythmically active crustacean muscles

Summary The organization of the flagellum abductor muscle and of a scaphognathite levator muscle of the green crab, Carcinus maenas, has been compared quantitatively using light and electron microscopy. These muscles are rhythmically active at relatively high frequencies and for long durations. Fibers of both muscles are interconnected to form fascicles of 50 or more fibers within which there is cytoplasmic continuity. A single muscle is made up of 8–12 fascicles. Individual fibers consist of a peripheral rind of densely packed mitochondria, a thick region of glycogen granules, and myofibrils arranged into scattered central islands. Less than half the volume-density of these muscles is contractile material, the balance being largely mitochondria and glycogen. The fibers within a muscle are structurally similar. They have short sarcomeres (about 2 m), thin to thick filament ratios of about 3:1, and junctions between the sarcoplasmic reticulum and the transverse tubules at the M line. Sarcoplasmic reticulum occupies about 10% of the myofibrillar volume-density. A well developed sarcoplasmic reticulum appears to underlie the capacities of these two muscles for high frequency contraction; extensive mitochondria and glycogen stores should confer fatigue resistance under both aerobic and anaerobic conditions.     

FINE STRUCTURE OF THE MYOEPITHELIUM OF THE ECCRINE SWEAT GLANDS OF MAN

The secretory coils of glutaraldehyde-osmium tetroxide-fixed and Epon-Araldite-embedded eccrine sweat glands from the palms of young men were studied with the electron microscope. The myoepithelial cells lie on the epithelial side of the basement membrane and abut other epithelial elements directly. The irregularly serrated base of the cell has dense thickenings along the plasma membrane which alternate with zones bearing pits; the smooth apical surface lacks dense thickenings, is studded with pits, and conjoined to secretory cells by occasional desmosomes. Masses of myofilaments, 50 A in diameter, fill most of the cell and are associated with irregular dense zones. In cross-section the arrangement of the myofilaments seems identical with that of the I band of striated muscle, and the dense zone has typical Z band structure. A few microtubules and cytoplasmic cores bearing profiles of the endoplasmic reticulum, filamentous mitochondria, and glycogen granules penetrate the fibrillar masses and run parallel to the oriented myofilaments. In the perinuclear zone, Golgi membranes, rough- and smooth-surfaced elements of the endoplasmic reticulum, mitochondria, glycogen, microtubules, lipid, pigment, and dense granules are variable components in the cytoplasm. The interrelationships of the myoepithelial cells with the secretory cells suggest that the former may act as regulators, controlling the flow of metabolites to the secretory epithelium.     

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.     

THE ORGANIZATION OF FLIGHT MUSCLE FIBERS IN THE ODONATA

The cytological organization of flight muscle fibers of Odonata has been investigated. These fibers, in representatives of the Zygoptera and Anisoptera, have been compared and found to be similar, except that, in the former, pairs of lamellar fibrils, rather than single fibrils, alternate with the mitochondria. In each instance, in these synchronous muscles, the actin filaments of the myofibrils are found to lie opposite to and midway between pairs of myosin filaments—a configuration previously reported in asynchronous flight muscle fibers. The disposition of the T system and sarcoplasmic reticulum membranes in glutaraldehyde-fixed anisopteran muscle is described in detail: the T system tubules are shown to be radially continuous across the fiber, and are derived as openmouthed invaginations from the surface cell-membrane. The detailed organization of the dyad junctions between these tubules and the adjoining cisternae of the sarcoplasmic reticulum is described. The accessibility of the T system interior to diffusion exchange with the general extracellular milieu has been investigated by studies on the penetration of ferritin into the fiber: molecules of this marker have been found to diffuse solely along the T system tubules, and their presence in the tubule extremities adjoining the centrally placed nuclei confirms the morphological evidence suggesting that these tubules provide open diffusion channels extending across the radius of the fiber. The possible physiological role of these membrane components and their distribution in synchronous muscles of insects and vertebrates and in asynchronous insect flight muscle are discussed.     

The Striated Musculature of Blood Vessels : I. General Cell Morphology

The musculature of small lung veins, of the thoracic portion of the inferior vena cava, and of other thoracic veins of the mouse have been studied in the electron microscope. Tissues were fixed in 1 per cent osmium tetroxide buffered with veronal, to which either sodium chloride or sucrose had been added. Methacrylate or araldite served as embedding matrices. Phosphotungstic acid or uranyl acetate was used to stain some of the preparations. Thin sections were examined in a Siemens and Halske Elmiskop Ib electron microscope. The entire musculature of the veins examined was of the striated type. It represents a variety of cardiac muscle, characterized by centrally located nuclei, typical mitochondria, and narrow I bands. Many I bands cannot be recognized at all. H and M bands are likewise indistinct. There is a double array of primary and secondary myofilaments. Mitochondria are large and numerous and contain many cristae. The endoplasmic reticulum consists of longitudinal tubules which run through the whole sarcomeres and bypass Z bands, and of transverse tubules which accompany Z bands. Some "triads," located at Z levels, consist of flattened vacuoles flanked by such transverse tubules. Small vesicles located at Z bands, close to the nucleus, and beneath the plasma membrane may represent still other portions of the reticulum.     

Ultrastructure of the heart of the marine mussel,Geukensia demissa

The structure of the heart of Geukensia demissa, a common object of physiological and biochemical investigation, is described by scanning, transmission and freeze-fracture electron microscopy. A single-cell epithelial layer covers the ventricle, but an endothelium is lacking. Myofibers are small (6–7 μm diam.), mononucleate, and tapered. Glycogen is concentrated peripherally. Mitochondria are particularly concentrated under the sarcolemma, near the ends of the nucleus, and in rows between bundles of myofilaments. The myofilaments (6–8nm thin, 30–35 nm thick filament diam.) are loosely arranged into sarcomeres (2–4 μm) by Z bodies. Many of these Z bodies interconnect, and some anchor to the sarcolemma forming attachment plaques. Cells are joined by intercalated discs consisting of fascia adherentes, spot desmosomes, and gap junctions. The gap junctions include intramembrane particles. T tubules are absent. The sarcolemma is coupled to the junctional sarcoplasmic reticulum (JSR) over 357ndash;40% of the cell surface. Tubules extend from the JSR deep into and throughout the cell as an irregularly dispersed network. The SR occupies 1% of the cell volume. A few, small (0.1–1.0 μm) unmyelinated nerves are present, but no neuromuscular junctions were seen. The auricles have fewer and smaller myocytes than the ventricle. The auricles also contain podocytes with pedicels having 20–35 nm slits and containing sieve-like projections. The morphology of the Geukensia heart is similar to that of other bivalves.     

THE SARCOPLASMIC RETICULUM AND ITS ASSOCIATION WITH THE T SYSTEM IN AN INSECT

The fine structure of the sarcoplasmic reticulum and the transverse tubular system of the femoral muscle of the cockroach, Leucophaea maderae, was studied after prefixation in glutaraldehyde, postfixation in osmium tetroxide, and embedding in Epon. The sarcoplasmic reticulum in this muscle reveals features not previously reported. The sarcoplasmic reticulum is abundant, consisting mainly of a fenestrated envelope which surrounds each myofibril at all levels in the sarcomere. This sarcoplasmic reticulum envelope is continuous transversally as well as longitudinally along the myofibrils. Dyadic junctions are formed by a single T system element which contacts the unfenestrated sarcoplasmic reticulum of adjacent myofibrils in an alternating manner at the ends of the A band. At the dyads, regularly spaced thickenings of the sarcoplasmic reticulum membranes bordering the dyadic spaces are noted. These thickenings, however, do not contact the T tubule membrane. Typical dyadic contacts also are seen between the cell surface membrane and sarcoplasmic reticulum. Z line-like material is seen in contact with the membranes of the cell surface and longitudinal branches of the T systems.     

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.     

Fine structure and differentiation of ascidian muscle, 2. Morphometrics and differentiation of the caudal muscle cells of Distaplia occidentalis tadpoles

The locomotor function of the caudal muscle cells of ascidian larvae is identical with that of lower vertebrate somatic striated (skeletal) muscle fibers, but other features, including the presence of transverse myomuscular junctions, an active Golgi apparatus, a single nucleus, and partial innervation, are characteristic of vertebrate myocardial cells. Seven stages in the development of the compound ascidian Distaplia occidentalis were selected for an ultrastructural study of caudal myogenesis. A timetable of development and differentiation was obtained from cultures of isolated embryos in vitro. The myoblasts of the neurulating embryo are yolky, undifferentiated cells. They are arranged in two bands between the epidermis and the notochord in the caudal rudiment and are actively engaged in mitosis. Myoblasts of the caudate embryo continue to divide and rearrange themselves into longitudinal rows so that each cell simultaneously adjoins the epidermis and the notochord. The formation of secretory granules by the Golgi apparatus coincides with the onset of proteid-yolk degradation and the accumulation of glycogen in the ground cytoplasm. Randomly oriented networks of thick and thin myofilaments appear in the peripheral sarcoplasm of the muscle cells of the comma embryo. Bridges interconnect the thick and thin myofilaments (actomyosin bridges) and the thick myofilaments (H-bridges), but no banding patterns are evident. The sarcoplasmic reticulum (SR), derived from evaginations of the nuclear envelope, forms intimate associations (peripheral couplings) with the sarcolemma. Precursory Z-lines are interposed between the networks of myofilaments in the vesiculate embryo, and the nascent myofibrils become predominantly oriented parallel to the long axis of the muscle cell. Muscle cells of the papillate embryo contain a single row of cortical myofibrils. Myofibrils, already spanning the length of the cell, grow only in diameter by the apposition of myofilaments. The formation of transverse myomuscular junctions begins at this stage, but the differentiating junctions are frequently oriented obliquely rather than orthogonally to the primary axes of the myofibrils. With the appearance of H-bands and M-lines, a single perforated sheet of sarcoplasmic reticulum is found centered on the Z-line and embracing the I-band. The sheet of SR establishes peripheral couplings with the sarcolemma. In the prehatching tadpole, a second collar of SR, centered on the M-line and extending laterally to the boundaries with the A-bands, is formed. A single perforated sheet surrounds the myofibril but is discontinuous at the side of the myofibril most distant from the sarcolemma. To produce the intricate architecture of the fully differentiated collar in the swimming tadpole (J. Morph., 138: 349, 1972). the free ends of the sheet must elevate from the surface of the myofibril, recurve, and extend peripherally toward the sarcolemma to establish peripheral couplings. Morphological changes in the nucleus, nucleolus, mitochondria, and Golgi bodies are described, as well as changes in the ground cytoplasmic content of yolk, glycogen, and ribosomes. The volume of the differentiating cells, calculated from the mean cellular dimensions, and analyses of cellular shape are presented, along with schematic diagrams of cells in each stage of caudal myogenesis. In an attempt to quantify the differences observed ultrastructurally, calculations of the cytoplasmic volume occupied by the mqjor classes of organelles are included. Comparison is made with published accounts on differentiating vertebrate somatic striated and cardiac muscles.     

Ultrastructural features of the columellar muscle and contractile protein analyses in different muscle groups of Megalobulimus abbreviatus (Gastropoda, Pulmonata)

In Megalobulimus abbreviatus, the ultrastructural features and the contractile proteins of columellar, pharyngeal and foot retractor muscles were studied. These muscles are formed from muscular fascicles distributed in different planes that are separated by connective tissue rich in collagen fibrils. These cells contain thick and thin filaments, the latter being attached to dense bodies, lysosomes, sarcoplasmic reticulum, caveolae, mitochondria and glycogen granules. Three types of muscle cells were distinguished: T1 cells displayed the largest amount of glycogen and an intermediate number of mitochondria, suggesting the highest anaerobic metabolism; T2 cells had the largest number of mitochondria and less glycogen, which suggests an aerobic metabolism; T3 cells showed intermediate glycogen volumes, suggesting an intermediate anaerobic metabolism. The myofilaments in the pedal muscle contained paramyosin measuring between 40 and 80 nm in diameter. Western Blot muscle analysis showed a 46-kDa band that corresponds to actin and a 220-kDa band that corresponds to myosin filaments. The thick filament used in the electrophoresis showed a protein band of 100 kDa in the muscles, which may correspond to paramyosin.