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

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

The sarcoplasmic reticulum of mouse heart: its divisions, configurations, and distribution

The sarcoplasmic reticulum (SR) is a prominent, highly ramified component of mouse myocardial cells. The use of ferrocyanide-reduced osmium tetroxide (OsFeCN) as a postfixative solution facilitates appreciation of both its extent and three-dimensional architecture. We have found that the individual volume fractions (Vv) of myofibrils, mitochondria, and SR are similar in cells of the right and left ventricular walls. Vv(total SR) is approximately 7%, a value considerably larger than previously reported. We attribute this disparity in large part to the recognition factor which comes into play with OsFeCN-treated tissue. Previous observations pertaining to the stereology of myocardial SR have likely substantially underestimated both volume fraction and surface density of this membrane system, since none to this point has utilized specific staining such as that conferred by the OsFeCN regimen. Our stereological measurements of different depths of the ventricular cell indicate that although considerable differences are found between SR configuration at peripheral and deep cell levels, no significant difference exists between the volume fractions of either the total SR or its individual constituents. Two different stereologic regimens gave close agreement on volume fractions of the various SR segments; the majority (approximately 92%) of the total SR is network SR, whereas the remainder is composed of the various categories of junctional SR (peripheral, apposed to the surface sarcolemma; interior, complexed with the transverse-axial tubular system; corbular, existing free of sarcolemmal contact). In the adult mouse, interior junctional SR greatly preponderates the other types of junctional SR; corbular SR is qualitively assessed to be a far more common component of atrial cells than of ventricular cardiomyocytes.     

Ruthenium-red staining of skeletal and cardiac muscles

Summary The effects of ruthenium red (RR) on amphibian and mammalian skeletal muscles and mammalian myocardium were examined. In skeletal muscle cells, a discrete pattern of staining can be brought about within the lumina of the terminal cisternae (junctional sarcoplasmic reticulum [SR]) by sequential exposure to RR and OsO₄. After prolonged immersion in RR solution, formation of pentalaminar segments (zippering) occurs at various points along the longitudinal (network) SR tubules. Zippering can be elicited in skeletal SR at any stage of preparation prior to postfixation with OsO₄. By means of dispersive X-ray analysis, both ruthenium and osmium were seen to be deposited in skeletal muscle junctional SR, and ruthenium was detected in the myoplasm as well. In skeletal muscles whose T tubules were ruptured by exposure to glycerol, the pattern of SR staining and zippering resulting from ruthenium-osmium treatment was not affected. These findings indicate that RR is capable of passage across the sarcolemma of skeletal muscle and that this passage does not occur solely under conditions in which the plasma membrane is damaged. In contrast, RR does not opacify or modify any region of the SR of cardiac muscle. However, after this treatment, randomly distributed opaque bodies, composed of parallel lamellar structures, appear throughout the myocardial cells. A few of these bodies are associated with lipid droplets, but the rest are of unknown origin. The failure of the SR of cardiac muscle to stain after exposure to ruthenium dye (even though this material enters these cells) suggests that the chemical composition of cardiac SR is significantly different from that of skeletal muscle SR.Supported in part by PHS grant HL-11155 (to N.S.) and American Heart Grant-in-Aid 78-753 (to M.S.F.). The authors are grateful to Drs. David Harder and Lawrence Sellin for their assistance with the preparation of frog skeletal muscle, to Dr. S.K. Jirge for his helpful suggestions and discussions, and particularly to Dr. Kenneth R. Lawless and Ms. Ann Marshall of the Department of Materials Sciences, University of Virginia School of Engineering, and Col. John M. Brady of the United States Army Institute of Dental Research, Walter Reed Army Medical Center, for their help with, and for the use of, the X-ray analysis equipment     

The atrial myocardial cells of mouse heart: a structural and stereological study

Structural and stereological studies of mouse atrial myocardial cells, carried out in the same fashion as our previous investigations on mouse ventricle, demonstrate an extremely well-developed sarcoplasmic reticulum (SR) in atrial cells. The volume fraction (Vv) of the SR exceeds 12% in mouse atrial cells; perimyofibrillar network SR constitutes the major portion. We have confirmed the findings of Bossen et al. (1981, Tissue Cell 13, 71-77) of a difference between atria in terms of coupling density, the right atrium having a significantly lower incidence of interior junctional SR than the left. The SR of mouse atrium comprises a rich variety of specialized segments, including the IJSR, peripheral junctional SR, corbular SR, cisternal SR (including regions similar to fenestrated collars of striated skeletal muscle SR), as well as a peculiar form of extended junctional SR (EJSR). Although less frequent in occurrence than corbular SR, the EJSR seems closely related, since it occurs in multiple clusters at or near the Z-line regions, contains internal granular densities, and bears surface-connected structures resembling junctional processes. Seen in thin sections, mouse atrial EJSR elements are more complex than corbular SR, being larger in diameter and frequently circular in profile. Thick-section and serial-section analyses reveal that bodies of EJSR are in fact hollow spheroids. The transverse-axial tubular system of mouse atrium is rather poorly developed in comparison to its ventricular counterpart. The Golgi apparatus and associated specific atrial granules are prominent cell components. "Focal ellipsoidal deposits" (FEDs) previously described by Page and co-workers (1986, Amer. J. Physiol.) are consistently located adjacent to the Golgi region, but immunocytochemical staining for two different segments of atrial natriuretic peptide reveals no specific reaction in FEDs, whereas the SAGs are densely labeled for both antibodies.     

Immunoelectron microscopical localization of phospholamban in adult canine ventricular muscle

The subcellular distribution of phospholamban in adult canine ventricular myocardial cells was determined by the indirect immunogold-labeling technique. The results presented suggest that phospholamban, like the Ca2+-ATPase, is uniformly distributed in the network sarcoplasmic reticulum but absent from the junctional portion of the junctional sarcoplasmic reticulum. Unlike the Ca2+-ATPase, but like cardiac calsequestrin, phospholamban also appears to be present in the corbular sarcoplasmic reticulum. Comparison of the relative distribution of phospholamban immunolabeling in the sarcoplasmic reticulum with that of the sarcolemma showed that the density of phospholamban in the network sarcoplasmic reticulum was approximately 35-fold higher than that of the cytoplasmic side of the sarcolemma, which in turn was found to be three- to fourfold higher than the density of the background labeling. However, a majority of the specific phospholamban labeling within 30 nm of the cytoplasmic side of the sarcolemma was clustered and present over the sarcoplasmic reticulum in the subsarcolemmal region of the myocardial cells, suggesting that phospholamban is confined to the junctional regions between the sarcolemma and the sarcoplasmic reticulum, but absent from the nonjunctional portion of the sarcolemma. Although the resolution of the immunogold-labeling technique used (60 nm) does not permit one to determine whether the specific labeling within 30 nm of the cytoplasmic side of the sarcolemma is associated with the sarcolemma and/or the junctional sarcoplasmic reticulum, it is likely that the low amount of labeling in this region represents phospholamban associated with sarcoplasmic reticulum. These results suggest that phospholamban is absent from the sarcolemma and confined to the sarcoplasmic reticulum in cardiac muscle.     

The unraveling architecture of the junctional sarcoplasmic reticulum

The sarcoplasmic reticulum (SR) of skeletal muscle controls the contraction-relaxation cycle by raising and lowering the myoplasmic free-Ca²⁺ concentration. The coupling between excitation, i.e., depolarization of sarcolemma and transvers tubule (TT) and Ca²⁺ release from the terminal cisternae (TC) of SR takes place at the triad. The triad junction is formed by a specialized region of the TC, the junctional SR, and the TT. The molecular architecture and protein composition of the junctional SR are under active investigation. Since the junctional SR plays a central role in excitation-contraction coupling and Ca²⁺ release, some of its protein constituents are directly involved in these processes. The biochemical evidence supporting this contention is reviewed in this article.     

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.     

Molecular architecture of Ca2+ signaling control in muscle and heart cells

Ca²⁺ signaling in skeletal and cardiac muscles is a bi-directional process that involves cross-talk between signaling molecules in the sarcolemmal membrane and Ca²⁺ release machinery in the intracellular organelles. Maintenance of a junctional membrane structure between the sarcolemmal membrane and the sarcoplasmic reticulum (SR) provides a framework for the conversion of action potential arrived at the sarcolemma into release of Ca²⁺ from the SR, leading to activation of a variety of physiological processes. Activity-dependent changes in Ca²⁺ storage inside the SR provides a retrograde signal for the activation of store-operated Ca²⁺ channel (SOC) on the sarcolemmal membrane, which plays important roles in the maintenance of Ca²⁺ homeostasis in physiology and pathophysiology. Research progress during the last 30 years had advanced our understanding of the cellular and molecular mechanisms for the control of Ca²⁺ signaling in muscle and cardiovascular physiology. Here we summarize the functions of three key molecules that are located in the junctional membrane complex of skeletal and cardiac muscle cells: junctophilin as a “glue” that physiologically links the SR membrane to the sarcolemmal membrane for formation of the junctional membrane framework, mitsugumin29 as a muscle-specific synaptophysin family protein that contributes to maintain the coordinated Ca²⁺ signaling in skeletal muscle, and TRIC as a novel cation-selective channel located on the SR membrane that provides counter-ion current during the rapid process of Ca²⁺ release from the SR.     

The Ca2+-release channel/ryanodine receptor is localized in junctional and corbular sarcoplasmic reticulum in cardiac muscle

The subcellular distribution of the Ca(2+)-release channel/ryanodine receptor in adult rat papillary myofibers has been determined by immunofluorescence and immunoelectron microscopical studies using affinity purified antibodies against the ryanodine receptor. The receptor is confined to the sarcoplasmic reticulum (SR) where it is localized to interior and peripheral junctional SR and the corbular SR, but it is absent from the network SR where the SR-Ca(2+)-ATPase and phospholamban are densely distributed. Immunofluorescence labeling of sheep Purkinje fibers show that the ryanodine receptor is confined to discrete foci while the SR-Ca(2+)-ATPase is distributed in a continuous network-like structure present at the periphery as well as throughout interior regions of these myofibers. Because Purkinje fibers lack T- tubules, these results indicate that the ryanodine receptor is localized not only to the peripheral junctional SR but also to corbular SR densely distributed in interfibrillar spaces of the I-band regions. We have previously identified both corbular SR and junctional SR in cardiac muscle as potential Ca(2+)-storage/Ca(2+)-release sites by demonstrating that the Ca2+ binding protein calsequestrin and calcium are very densely distributed in these two specialized domains of cardiac SR in situ. The results presented here provide strong evidence in support of the hypothesis that corbular SR is indeed a site of Ca(2+)-induced Ca2+ release via the ryanodine receptor during excitation contraction coupling in cardiac muscle. Furthermore, these results indicate that the function of the cardiac Ca(2+)-release channel/ryanodine receptor is not confined to junctional complexes between SR and the sarcolemma.     

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.     

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.     

Co-localization of nitric oxide synthase I (NOS I) and NMDA receptor subunit 1 (NMDAR-1) at the neuromuscular junction in rat and mouse skeletal muscle

Nitric oxide synthase I (NOS I) has been localized to the skeletal muscle sarcolemma in a variety of vertebrate species including man. It is particularly enriched at neuromuscular junctions. Recently, the N-methyl-d-aspartate (NMDA) receptor subunit 1 (NMDAR-1) has been detected in the postjunctional sarcolemma of rat diaphragm, providing a clue as to the possible source of Ca²⁺ ions that are necessary for NOS I activation. To address this possibility, we studied the distribution of NMDAR-1 and NOS I in mouse and rat skeletal muscles by immunohistochemistry and enzyme histochemistry. NMDAR-1 and NOS I were closely associated at neuromuscular junctions primarily of type II muscle fibers. NOS I was also present in the extrajunctional sarcolemma of this fiber type. Dystrophin, β-dystroglycan, α-sarcoglycan, and spectrin were found normally expressed in both the junctional and extrajunctional sarcolemma of both fiber types. By contrast, in the muscle sarcolemma of MDX mice, dystrophin and dystrophin-associated proteins were reduced or absent. NOS I immunoreactivity was lost from the extrajunctional sarcolemma and barely detectable in the junctional sarcolemma. NOS I activity was clearly demonstrable in the junctional sarcolemma by NADPH diaphorase histochemistry, especially when the two-step method was used. NMDAR-1 was not altered. These data suggest that different mechanisms act to attach NOS I to the junctional versus extrajunctional sarcolemma. It may further be postulated that NMDA receptors are involved not only in the regulation but also sarcolemmal targeting of NOS I at neuromuscular junctions of type II fibers. The evidence that glutamate may function as a messenger molecule at vertebrate neuromuscular junction is discussed.     

T-system formation in cultured rat skeletal tissue

By using a lanthanum-staining technique which enhances the visualization of the plasma membrane and its derivatives we have studied the formation of the T system in rat muscle cells differentiating in vitro. We have found that: (1) T-system formation normally occurs after myoblast fusion and is especially extensive in mature myotubes; myoblasts grown in calcium-deficient medium to prevent fusion show increased number of sarcolemmal caveolae but rare, short T tubules. (2) T-system formation in vitro differs from that displayed by rat muscle cells in vivo in that it precedes and is independent of junctional SR differentiation; the uncoordinated development of T tubules and junctional SR in vitro leads to the formation of ‘inverted’ triads and labyrinthine T-system networks. (3) Coated vesicles are frequently found either free in the cytoplasm or associated with growing T tubules in rat muscle cells differentiating in vitro. A role of coated vesicles in T-system formation is proposed.     

The sarcoplasmic reticulum: an organized patchwork of specialized domains

The sarcoplasmic reticulum (SR) of skeletal muscle cells is a convoluted structure composed of a variety of tubules and cisternae, which share a continuous lumen delimited by a single continuous membrane, branching to form a network that surrounds each myofibril. In this network, some specific domains basically represented by the longitudinal SR and the junctional SR can be distinguished. These domains are mainly dedicated to Ca²⁺ homeostasis in relation to regulation of muscle contraction, with the longitudinal SR representing the sites of Ca²⁺ uptake and storage and the junctional SR representing the sites of Ca²⁺ release. To perform its functions, the SR takes contact with other cellular elements, the sarcolemma, the contractile apparatus and the mitochondria, giving rise to a number of interactions, most of which are still to be defined at the molecular level. This review will describe some of the most recent advancements in understanding the organization of this complex network and its specific domains. Furthermore, we shall address initial evidence on how SR proteins are retained at distinct SR domains.     

FREEZE FRACTURE OF SKELETAL MUSCLE FROM THE TARANTULA SPIDER : Structural Differentiations of Sarcoplasmic Reticulum and Transverse Tubular System Membranes

The structure of the membranes of sarcoplasmic reticulum (SR), tubular (T) system, and sarcolemma has been studied by freeze fracture in leg muscles of the Tarantula spider. Two regions of the sarcoplasmic reticulum can be differentiated by the distribution of particles on the fracture faces: a junctional SR, at the dyads, and a longitudinal SR, elsewhere. The dyads are asymmetric junctions, the disposition of particles in the apposed membranes of SR and T tubules being different from one another and from the regular arrangement of feet in the junctional gap. It is concluded that no channels can be visualized to directly connect SR- and T-system lumina.     

Corbular sarcoplasmic reticulum of rabbit cardiac muscle

The structure of corbular sarcoplasmic reticulum as part of the sarcoplasmic reticulum (SR) in perfusion-fixed rabbit cardiac muscle was studied by thin sections and freeze fracture. In thin sections, processes on the surface of corbular SR have all the anatomical features of junctional processes of junctional SR. By freeze fracture, the E face of corbular SR was particle poor and showed deep pits; the P face was particle rich. The demonstrated structural homology of corbular SR to all forms of junctional SR justifies its inclusion in that group.     

PCC4azal teratocarcinoma stem cell differentiation in culture: II. Morphological characterization

The ultrastructural morphology of the PCC4azal embryonal carcinoma cells and their differentiated counterparts, endoderm-like cells and giant cells, was characterized and compared with that of the cells of embryoid bodies. The ultrastructure of the PCC4azal embryonal carcinoma cells is similar to that of the embryonal carcinoma cells of the embryoid body. These cells are small, with a large nucleus and relatively few cytoplasmic organelles. Gap junctions and modified adherens junctions are formed at some areas of intercellular contact between the embryonal carcinoma cells. The differentiated PCC4azal endoderm-like cells have a more developed cytoplasm, containing an extensive endoplasmic reticulum with large Golgi regions. Most striking is the de novo appearance of epithelial-like junctional complexes which join the apical borders between the endoderm-like cells, thus polarizing the cell monolayer. The zonula occludens junctions of the junctional complex are extensive, consisting of six or more strands of tight junctional ridges. Terminal webs are present in the apical regions that are inserted into the zonula adherens region of the junctional complex. Gap junctions continue to join neighboring cells, and some gap junctions are intercalated within tight junctional ridges. The ultrastructure of the differentiated endodermal cells of the embryoid bodies is very similar to that of the PCC4azal endoderm-like cells. The embryoid body endodermal cells form similar junctional complexes which also contain continuous belts of tight junctions that are intercalated with gap junctions. As the PCC4azal endoderm-like cells are transformed to giant cells, a massive cytoskeleton is formed, consisting of a large complex system of 10-nm filaments, microtubules, and 7-nm microfilaments. The junctional complexes that were present during the endodermal stage are partially disassembled as the giant cells migrate apart. Thus, the differentiation process in this system is characterized by significant and distinctive morphological changes.     

Morphology and fine structure of the heart of the burbot, a cold stenothermal fish

The ventricle of the burbot Lota lota heart comprised 0·148 ± 0·006% of the body mass which is nearly two-fold heavier than the relative ventricular mass ( M _V) of other similarly sized teleosts. The shape of the ventricle is pyramidal and the wall is exclusively composed of spongious muscle without a distinct compact layer. The atrium forms 0·017 ± 0·002% of the body mass. Length, width, sarcolemmal surface area and volume of enzymatically isolated myocytes from burbot ventricle were 147·2 ± 10·2 μm, 6·3 ± 0·4 μm, 2440·8 · 251·5 μm² and 2356·8 ± 316·6 μm³, respectively. The myofibrils were peripherally located and their volume density was remarkably high: 65 ± 2 and 68 ± 3% in ventricle and atrium, respectively ( P >0·05). Although not particularly conspicuous, some nonjunctional and junctional sarcoplasmic reticulum (SR) was present in both atrial and ventricular myocytes. The SR formed peripheral couplings with the sarcolemma and the junctional clefts were frequently occupied by foot processes. These findings suggest that cold-adaptation is achieved by cardiac enlargement, high volume density of myofibrils and well-developed peripheral couplings in the SR in the heart of stenothermal burbot.     

Comparative ultrastructure of the gastrotrich pharynx and the evolution of myoepithelial foreguts in aschelminthes

Summary Structural and ultrastructural data are presented for the myoepithelial pharynges of 20 species of Gastrotricha representing the marine Macrodasyida and marine and freshwater Chaetonotida. A comparative analysis reveals that pharynges with several plesiomorphic characters occur in Chordodasys among the Macrodasyida and Neodasys among the Chaetonotida. The Gastrotricha are systematized based on pharyngeal characters and the system is shown to be concordant with all recent classifications of the group. The plesiomorphic design of the Gastrotrich pharynx is given as: a cylindrical pharyngeal pump composed of monociliated myoglanduloepithelial cells surrounding a circular or oval lumen; radial myofibrils organized into several cross-striated sarcomeres with Z-discs composed of planar aggregations of dense bodies; excitation-contraction coupling is achieved by peripheral couplings of SR with the sarcolemma; apical cell surface with microvilli protruding through the two-layered cuticle; pharynx with at least 3 longitudinal tracts of monociliated sensory cells; nerves as at least 4 basal intraepithelial, longitudinal tracts of neurites. As a model for muscle cell evolution, the investigation postulates a monociliated, cross-striated myoglanduloepithelial cell as the original muscle cell design within the Gastrotricha. Triradiate myoepithelial foreguts occur only in Bryozoa (Ectoprocta), Gastrotricha, Nematoda and Tardigrada. The potential homology of pharyngeal organization of the latter three phyla is discussed. Based on pharyngeal structure, it is concluded that Gastrotricha (Chaetonotida-Paucitubulatina) and Nematoda share several apomorphic characters and share, therefore, a most recent common ancestor. Affinities of Tardigrada with Aschelminthes are considered feasible but currently inconclusive for lack of sufficient comparative ultrastructural data for the Tardigrada.     

Membrane potential and Ca2+ concentration dependence on pressure and vasoactive agents in arterial smooth muscle: A model

Arterial smooth muscle (SM) cells respond autonomously to changes in intravascular pressure, adjusting tension to maintain vessel diameter. The values of membrane potential (V_m) and sarcoplasmic Ca²⁺ concentration (Ca_in) within minutes of a change in pressure are the results of two opposing pathways, both of which use Ca²⁺ as a signal. This works because the two Ca²⁺-signaling pathways are confined to distinct microdomains in which the Ca²⁺ concentrations needed to activate key channels are transiently higher than Ca_in. A mathematical model of an isolated arterial SM cell is presented that incorporates the two types of microdomains. The first type consists of junctions between cisternae of the peripheral sarcoplasmic reticulum (SR), containing ryanodine receptors (RyRs), and the sarcolemma, containing voltage- and Ca²⁺-activated K⁺ (BK) channels. These junctional microdomains promote hyperpolarization, reduced Ca_in, and relaxation. The second type is postulated to form around stretch-activated nonspecific cation channels and neighboring Ca²⁺-activated Cl⁻ channels, and promotes the opposite (depolarization, increased Ca_in, and contraction). The model includes three additional compartments: the sarcoplasm, the central SR lumen, and the peripheral SR lumen. It incorporates 37 protein components. In addition to pressure, the model accommodates inputs of α- and β-adrenergic agonists, ATP, 11,12-epoxyeicosatrienoic acid, and nitric oxide (NO). The parameters of the equations were adjusted to obtain a close fit to reported V_m and Ca_in as functions of pressure, which have been determined in cerebral arteries. The simulations were insensitive to ±10% changes in most of the parameters. The model also simulated the effects of inhibiting RyR, BK, or voltage-activated Ca²⁺ channels on V_m and Ca_in. Deletion of BK β1 subunits is known to increase arterial–SM tension. In the model, deletion of β1 raised Ca_in at all pressures, and these increases were reversed by NO.