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PubMed1997

Ultra-high-resolution scanning electron microscopy of the sarcoplasmic reticulum of the rat atrial myocardial cells.

BACKGROUND: The sarcoplasmic reticulum (SR) of mammalian ventricular and atrial muscles share common features but also differ because T-tubules are rare and extended junctional SR is exclusively seen in the atrium. This scanning electron microscope (SEM) study was undertaken to clarify the three-dimensional organization of the rat atrial SR system. Specific preparations were examined with an ultra-high-resolution SEM. METHODS: Fixed right rat atria were frozen, fractured, and macerated by the aldehyde-osmium-DMSO-osmium method to remove myofibrils and cytoplasmic matrix. Left exposed were mitochondria, SR, and sarcolemma. Dried specimens were then impregnated by osmium-hydrazine and examined without metal coating. RESULTS: In place of conventional T-tubules, a prominent type of sarcotubules, Z-tubules, were found at the Z-line level. Branches from these tubules joined the cisternal SR, which was 100-300 nm in diameter and localized near the Z-line, and formed extensive SR meshworks and polygonal patches. Bulbous swellings, the corbular SR, were also evident. Sarcotubular reticulum completely surrounded each myofibril. The intermyofibrillar SR, especially Z-tubules, joined the peripheral subsarcolemmal SR, which was also arranged as a meshwork and was closely apposed to the sarcolemma. CONCLUSIONS: These SEM observations confirm the organization of the rat atrial SR system and present new, detailed, three-dimensional images of Z-tubules, cisternal SR, extended junctional SR, and peripheral SR, which provide further structural insight.

Animals↗
PubMed1999

Ultrastructural changes in the chick thymus following unilateral vagotomy.

The ultrastructure of the thymus in the chick (Gallus domesticus) was studied after unilateral vagotomy at survival times of 3, 7 and 10 days. Ultrastructural changes in the ipsilateral thymus were observed in axon boutons as well as in myoid and cystic cells in the medulla, especially those situated near the corticomedullary junction. Structural changes in axon boutons ranged from granular degeneration of the axonal cytoskeleton to vacuolation of the axoplasm. Myelin figures of different sizes and configurations and clumping of small agranular vesicles were commonly observed in the axon terminals. Degeneration of myoid cells appeared to peak at 7 days post-vagotomy. Changes ranged from oedematous appearance and intense vacuolation of the peripheral cytoplasm to disorganisation and clumping of myofibrils. In some myoid cells the sarcomeres showed granular degeneration at the I-bands and in others, the myofibrils were completely degenerated such that amorphous material and partially degenerated organelles filled the entire cell. The majority of cystic cells at 3 days post-vagotomy showed a uniform increase in electron density. Numerous electron dense bodies, some displaying concentric lamellation, were observed throughout the expanse of the cytoplasm. At 7 days post-vagotomy, the cytoplasm of some cells gave a "moth-eaten" appearance. Dying cystic cells were encountered at 10 days after vagotomy. Degeneration in the myoid and cystic cells suggests that these cellular components may be the putative targets of the vagal fibres in the chick thymus. The changes in these cells reflect a disturbance in the cell metabolism presumably brought about by the removal of vagal influence.

Animals↗
PubMed2000

Cell death in denervated skeletal muscle is distinct from classical apoptosis.

Denervation of skeletal muscle is followed by the progressive loss of tissue mass and impairment of its functional properties. The purpose of the present study was to investigate the occurrence of cell death and its mechanism in rat skeletal muscle undergoing post-denervation atrophy. We studied the expression of specific markers of apoptosis and necrosis in experimentally denervated tibialis anterior, extensor digitorum longus and soleus muscles of adult rats. Fluorescent staining of nuclear DNA with propidium iodide revealed the presence of nuclei with hypercondensed chromatin and fragmented nuclei typical of apoptotic cells in the muscle tissue 2, 4 and to a lesser extent 7 months after denervation. This finding was supported by electron microscopy of the denervated muscle. We found clear morphological manifestations of muscle cell death, with ultrastructural characteristics very similar if not identical to those considered as nuclear and cytoplasmic markers of apoptosis. With increasing time of denervation, progressive destabilization of the differentiated phenotype of muscle cells was observed. It included disalignment and spatial disorganization of myofibrils as well as their resorption and formation of myofibril-free zones. These changes initially appeared in subsarcolemmal areas around myonuclei, and by 4 months following nerve transection they were spread throughout the sarcoplasm. Despite an increased number of residual bodies and secondary lysosomes in denervated muscle, we did not find any evidence of involvement of autophagocytosis in the resorption of the contractile system. Dead muscle fibers were usually surrounded by a folded intact basal lamina; they had an intact sarcolemma and highly condensed chromatin and sarcoplasm. Folds of the basal lamina around the dead cells resulted from significant shrinkage of cell volume. Macrophages were occasionally found in close proximity to dead myocytes. We detected no manifestations of inflammation in the denervated tissue. Single myocytes expressing traits of the necrotic phenotype were very rare. A search for another marker of apoptosis, nuclear DNA fragmentation, using terminal deoxyribonucleotidyl transferase mediated dUTP nick end labeling (the TUNEL method) in situ, revealed the presence of multiple DNA fragments in cell nuclei in only a very small number of cell nuclei in 2 and 4 month denervated muscle and to less extent in 7 month denervated muscle. Virtually no TUNEL reactivity was found in normal muscle. Double labeling of tissue denervated for 2 and 4 months for genome fragmentation with the TUNEL method and for total nuclear DNA with propidium iodide demonstrated co-localization of the TUNEL-positive fragmented DNA in some of the nuclei containing condensed chromatin and in fragmented nuclei. However, the numbers of nuclei of abnormal morphology containing condensed and/or irregular patterns of chromatin distribution, as revealed by DNA staining and electron microscopy, exceeded by 33-38 times the numbers of nuclei positive for the TUNEL reaction. Thus, we found a discrepancy between the frequences of expression of morphological markers of apoptosis and DNA fragmentation in denervated muscle. This provides evidence that fragmentation of the genomic DNA is not an obligatory event during atrophy and death of muscle cells, or, alternatively, it may occur only for a short period of time during this process. Unlike classical apoptosis described in mammalian thymocytes and lymphoid cells, non-inflammatory death of muscle fibers in denervated muscle occurs a long time after the removal of myotrophic influence of the nerve and is preceded by the progressive imbalance of the state of terminal differentiation. Our results indicate that apoptosis appears to be represented by a number of distinct isotypes in animals belonging to different taxonomic groups and in different cell lineages of the same organism.

Animals↗
PubMed1997

Skeletal muscle fiber degeneration in mdx mice induced by electrical stimulation.

We present an in vitro model in which mouse skeletal muscle fibers undergo degeneration by increasing the current strength of tetanic stimulation. To understand the mechanisms of muscle fiber necrosis in Duchenne muscular dystrophy patients, the process of fiber degeneration was compared between mdx and control mice. The process consisted of four steps, beginning with muscle fiber contraction and extending to onset of myofibril disruption. The four processes were not observed in fibers in Krebs-HEPES (-Ca2+) buffer, nor in the presence of L-type Ca2+ channel blockers. These results suggest that this degenerative phenomenon is regulated by intracellular Ca2+, which moved into fibers mainly through voltage-dependent L-type Ca2+ channels. With the exception of myofibril disruption, mdx mice also exhibited the three other steps, but at a significantly lower current strength than in the fibers in the control mice. We postulate that excess Ca2+ flux occurs in fibers, mainly through abnormal L-type Ca2+ channels, and that the excessively accumulated calcium results in premature degeneration of the fibers by tetanic contraction. This study would provide a clue to investigate and prevent the degeneration processes in Duchenne muscular dystrophy.

Animals↗
PubMed1982

Extracardiac rhabdomyoma: report of different types with light microscopic and ultrastructural studies.

Five extracardiac rhabdomyomas were studied by light and electron microscopy: two of the adult type, two of the fetal type, and one of the female genital tract type. The three types can be distinguished histologically and ultrastructurally. Adult type rhabdomyomas are encapsulated and reveal histologically closely packed, large tumor cells with granular or vacuolated cytoplasm and only scant stroma. Electron microscopically, myofibrils are haphazardly arranged with rodlike Z-band material. Sometimes a sarcomeric arrangement of myofibrils is seen. The tumor cells are packed with mitochondria bearing lamellar inclusions. Fetal type rhabdomyomas contain a mixture of muscle cells in different stages of differentiations; the amount of stroma is increased. The myofilaments are usually disorganized and in only a few cells is rodlike Z-band material found. The cytoplasm reveals few mitochondria. In the rhabdomyoma of the female genital tract, the tumor cells are large and resemble more mature muscle fibers; the stroma is abundant and the myofilaments are usually arranged in an orderly fashion. Generally, the prognosis for rhabdomyomas is good. Metastases are not known. Two of the five rhabdomyomas studied recurred. One rhabdomyoma was found at autopsy. Embryonal rhabdomyosarcoma must be considered in differential diagnosis particularly for fetal type rhabdomyoma.

Abdominal Neoplasms↗
PubMed1981

The ultrastructure of the atrioventricular junctional tissues in the newborn ferret heart.

In this study the structure of the atrioventricular (AV) node and bundle in the newborn ferret heart was examined by light and electron microscopy. At the light microscopic level the AV node could be subdivided into deep and superficial portions. Electron microscopy revealed that both superficial and deep AV nodal cells were characterized by a paucity of myofibrils, desmosomes, fasciae adherentes and gap junctions. Deep V nodal cells, however, had more surface specializations than did superficial AV nodal cells. In both subdivisions the constituent cells were ellipsoid with tapering end-processes. In contrast to the nodal cells, the newborn AV bundle cells were round to ovoid. The AV bundle cells were organized into large fascicles, and there was a high frequency of anastomosing intercommunication between fascicles. These bundle cells had few myofibrils and high incidence of apposed plasma membrane. The present morphological findings support the concept that there are significant postnatal morphological changes that occur in the region of the AV junction. These results are also consistent with findings in other species that AV nodal conduction time is similar in newborns and adults, while conduction through the AV bundle increases with age.

Animals↗
PubMed1988

Structural manifestations of diabetic cardiomyopathy in the rat and its reversal by insulin treatment.

The structural effects of diabetes and subsequent insulin treatment upon the contractile and supporting elements of the rat myocardium were examined at progressive stages of both untreated and treated disease. Diabetes was induced by intravenous injection of alloxan, and tissue was examined after 6, 12, and 26 weeks. Insulin treatment began after 12 weeks of diabetes and tissue from these animals was examined after the same intervals. Within the cardiocytes, diabetes produced a focal yet progressive loss of myofibrils, transverse tubules, and sarcoplasmic reticulum, and separation of the fasciae adherens was evident at the intercalated disk. Mitochondrial damage was not evident. These cytoplasmic alterations were accompanied by intercellular and perivascular deposition of connective tissue, thickening of the endothelial cytoplasm with pinocytotic hyperactivity, and characteristic basal laminar changes. When insulin treatment began after 12 weeks of diabetes, most, but not all, of these changes were reversed, and this reversal was essentially complete within 6-12 weeks. Even with longer periods of insulin treatment, cardiocytes still exhibited scattered areas of myofibril loss and extracellular matrix was retained. In contrast, diabetic changes in the intercalated disk and capillaries, including their basal laminae, were completely and rapidly reversed. It is hypothesized that the structural manifestations of diabetic cardiomyopathy consist of two major components; the first is a short-term, physiologic adaptation to metabolic alterations, while the other represents degenerative changes for which the myocardium has only a limited capacity for repair.

Animals↗
PubMed1991

Studies of hamster cardiac myofibrillogenesis in vivo with antibodies to spectrin, desmin, and alpha-actinin.

The spectrins are a family of cytoskeletal-membrane proteins that have generated much interest in the past decade. In the present study, we utilized immunohistochemical, morphological, and electrophoretic techniques to assess the possible function(s) of spectrin in mammalian cardiac tissue during development. Antibodies generated against alpha-actinin and desmin were also employed to identify myofibrils and intermediate filaments in relation to changes in the distribution of spectrin. Spectrin is localized along the sarcolemma of pre-myofibrillar hamster cardiac myocytes (day 8, postcoitum) and remains associated with the cell membrane throughout development. The staining pattern is somewhat diffuse at first, but eventually the cell margin becomes clearly defined by day 13 postcoitum. A second, more profound change in the distribution of spectrin occurs during the newborn stage, when spectrin begins to appear in the sarcoplasm. It appears as regularly spaced invaginations that are diffuse at first, eventually attaining a position around the Z-bands of adult muscle. The change in the distribution of spectrin coincides temporally with the appearance of T-tubules, which are sarcolemmal invaginations that reside at the Z-bands of adult heart. Thus, spectrin may act as a guidance mechanism for the proper positioning of T-tubules around the Z-discs of mammalian cardiac tissue. Although spectrin does not appear to interact directly with early myofibrils it may assist in the proper alignment of T-tubules and, in doing so, act to stabilize the entire contractile apparatus by enveloping it and attaching it to the sarcolemma.

Actins↗
PubMed2006

Sarcolemmal reorganization in facioscapulohumeral muscular dystrophy.

OBJECTIVE: We examined the sarcolemma of skeletal muscle from patients with facioscapulohumeral muscular dystrophy (FSHD1A) to learn if, as in other murine and human muscular dystrophies, its organization and relationship to nearby contractile structures are altered. METHODS: Unfixed biopsies of control and FSHD deltoid and biceps muscles, snap-frozen at resting length, were cryosectioned, indirectly immunolabeled with fluorescent antibodies to sarcolemmal and myofibrillar markers, and examined with confocal microscopy to localize the immunolabeled proteins. Glutaraldehyde-fixed samples were stained with heavy metals, embedded, thin-sectioned, and examined with electron microscopy to determine the relationship between the sarcolemma and the underlying myofibrils. RESULTS: Confocal microscopy showed that some of the structures at the sarcolemma in FSHD samples were misaligned with respect to the underlying contractile apparatus. Electron microscopy showed a significant increase in the distance between the sarcolemma and the nearest myofibrils, from less than 100 nm in controls to values as high as 550 nm in FSHD. INTERPRETATION: Our results show that the pathophysiology of FSHD includes novel changes in the organization of the sarcolemma and its association with nearby contractile structures and suggest that, as in other muscular dystrophies, the integrity of the sarcolemma may be compromised in FSHD.

Cytoskeleton↗
PubMed1981

Quantitative morphological analysis of interatrial muscle cells in the ferret heart.

Cells located in the interatrial septum of the ferret heart were examined and mean cell volume, surface area, length, width, as well as cell length/width and surface area/volume ratios were obtained. The muscle cells were from two different regions. One region was the area of the middle internodal tract while the other was from the area where the anterior and middle internodal tracts intermingled. Based on the data obtained, at least two different subpopulations of interatrial muscle cells could be defined. The larger cells had a mean cell length of 109.7 micrometer, a mean cell width of 13.1 micrometer, a length/width ratio of 8.61, a mean cell surface area of 5,057.6 micrometer2, a mean cell volume of 5960.8 micrometer3, and a surface area/volume ratio of 0.87. The smaller cells had a mean cell length of 58.0 micrometer, a mean cell width of 12.2 micrometer, a length/width ratio of 4.85, a mean cell surface area of 2494.1 micrometer2, a mean cell volume of 2553.6 micrometer3, and a surface area/volume ratio of 1.00. The large cell population had cells that were myofibril rich and also others that were myofibril poor. These quantitative data indicate that the regions of internodal pathways are not composed of a single specialized cell type, but rather are composed of at least two, if not more, cell types that intermingle with each other.

Animals↗
PubMed1987

Stereological analysis of the muscle-tendon junction in the aging mouse.

Transmission of contractile tension from skeletal muscle fibers to connective tissue elements is thought to occur at the muscle-tendon junctions, specialized regions at the extreme ends of the fibers. Previous stereological studies on adult mouse and chicken fibers have shown that, with reference to equal cross-sectional areas of myofibrils, the muscle-tendon junctions of faster fibers have significantly more surface membrane devoted to force transmission than do those of slower fibers (Trotter et al.: Anat. Rec. 213:16-25, 1985a; Trotter et al.: Anat. Rec. 213:26-32,1985b; Trotter and Baca: Anat. Rec. 218:256-266, 1987). In the present study we have analyzed the muscle-tendon junctions of 30-month-old mice, employing techniques for scanning and transmission electron microscopy and for ultrastructural stereology which are identical to those previously used to study the same muscles in 4-month-old mice. Whereas the principal structural features of the muscle-tendon junctions of fibers from adult and aged mice are indistinguishable, stereological analyses of the fiber-tendon interfaces indicate that, in aging animals, the interfacial ratio (the ratio of the surface area of force-transmitting membrane to the cross-sectional area of force-generating myofibrils) is significantly reduced. While the interfacial ratio of fast-twitch fibers of the adult plantaris is about 14.5, the corresponding ratio in aged animals is about ten. In the predominantly slow-twitch fibers of the adult soleus, the interfacial ratio is about ten at the insertion and about 12.7 at the origin, whereas the corresponding ratios in the aged animals are both about ten.(ABSTRACT TRUNCATED AT 250 WORDS)

Aging↗
PubMed1990

Ultrastructure of the myocardium of the least shrew, Cryptotis parva Say.

The heart of the least shrew, Cryptotis parva Say, is an extremely active organ, capable of achieving rates of 800-1,200 beats/minute. The general features of myocardial cell ultrastructure in this insectivore are much like those of other small mammals; no single striking feature of fine structure is present to which the physiological properties of this heart might necessarily be attributed. Still there exist in these myocardial cells a number of atypical properties. These include 1) mitochondria having a wide variety of sizes and internal configurations 2) a pleiomorphic, highly ramified, small-diameter transverse-axial tubular system (TATS) 3) numerous "labyrinths," which are proliferated components of the TATS, and 4) myofibril-free regions, located both in juxtanuclear and other myoplasmic levels and populated by a concentration of TATS elements and fibrillar structures. Features (2) and (3) are also characteristic of another fast-beating heart, that of the mouse. The sinoatrial and atrioventricular nodal regions, as well as a Purkinje system, have been identified in the least shrew heart, along with sparsely distributed atrial cells whose myofibrils contain proliferated Z-band material. A feature frequently encountered in atrial working muscle cells is the occurrence of close appositions between gap junctions and tubules of sarcoplasmic reticulum; such appositions are also present in other regions of the shrew heart, as are complexes composed of gap junctions and mitochondria.

Animals↗
PubMed1990

Immunofluorescent studies for alpha-actinin in cultured cardiomyopathic hamster heart cells.

Primary cultures of cardiac myocytes from normal and genetically cardiomyopathic (CM) newborn hamsters (strain UM X7.1) were analyzed by indirect immunofluorescent microscopy after 3, 5, 7, and 9 days in culture. The cultures were fixed in cold acetone and immunostained by an indirect method using FITC-labelled anti-alpha-actinin to label the myofibrillar Z bands. Most normal and CM myocytes appeared round in shape after 3 days in culture. Normal cardiac myocytes began to exhibit cytoplasmic projections after 5 days in culture and their myofibrils usually showed parallel arrangements with respect to each other. The cardiac cells from CM hearts showed an obvious myofibril disarray. Moreover, projections formed later than normal. As the size of the cells increased, more and more projections formed in normal hamster myocytes during development. By contrast, most of the cardiomyopathic myocytes showed few projections even as late as 9 days in culture. Hence, the number of projections per cell was much less in cardiomyopathic myocytes than in normal, especially after 7 and 9 days in culture. These results suggest that cardiomyopathic cells have abnormal shapes in culture and, in particular, fail to form projections as in normal cells. Whether this unusual behavior is related to an abnormality of the membranes or cytoskeletal system in cardiomyopathic heart cells or to some other factor requires further study.

Actinin↗
PubMed1993

Cardiac conduction system in the chicken: gross anatomy plus light and electron microscopy.

Twenty-three chicken hearts were used to study the cardiac conduction system by light and electron microscopy. In addition to a sinus node, atrioventricular node (AVN), His bundle, left and right bundle branches (LBB, RBB), the chicken also has an AV Purkinje ring and a special middle bundle branch (MBB). The sinus node lies near the base of the lower portion of the right sinoatrial valve. The AV node is just above the tricuspid valve and anterior to the coronary sinus. The His bundle descends from the anterior and inferior margin of the AV node into the interventricular septum, then dividing into right, left and middle branches some distance below the septal crest. The middle bundle branch turns posteriorly toward the root of the aorta. The AV Purkinje ring originates from the proximal AV node and then encircles the right AV orifice, joining the MBB to form a figure-of-eight loop. The chicken conduction system contains four types of myocytes: 1) The P cell is small and rounded, with a relatively large nucleus and sparse myofibrils. 2) The transitional cell is slender and full of myofibrils. 3) The Purkinje-like cell resembles the typical Purkinje cell, but is smaller and darker. 4) The Purkinje cell is found in the His bundle, its branches, and the periarterial and subendocardial Purkinje network.

Animals↗
PubMed1995

Cellular distribution patterns of lanthanum and morphometry of rat hearts exposed to different degrees of ischemic stress.

BACKGROUND: The element lanthanum (La) can be used as a tracer for verification of membrane permeability. The aim of this study was to establish whether 1) distribution of La in the myocardium of rat hearts depends on the degree of ischemic stress and 2) morphometrically determined cell and mitochondrial swelling correlates with the La distribution. MATERIALS AND METHODS: Isolated beating rat hearts were arrested by coronary perfusion with the cardioplegic solution Custodiol (controls) or by aortic cross clamping followed by exposure to different degrees of ischemic stress. The solutions for perfusion-and postfixation as well as for rinsing contained 1.1% La(NO3)3. Cellular and mitochondrial swelling were determined morphometrically and myocytes exhibiting intracellular La were quantified and stated as percentage of test fields. RESULTS: Immediately after cardiac arrest La was present as precipitates only in a few myocytes adjacent to the outer mitochondrial membrane as seen by cTEM and ESI. In such cells La was also detected by EELS in mitochondrial matrix and myofibrils. Advanced ischemic stress led to an increase of the percentage of myocytes containing detectable intracellular La. After 45 min ischemia at 30 degrees C, myocytes and mitochondria showed a remarkable edema and different intracellular distribution patterns of La. After 90 min of ischemia at 20 degrees C interruptions of sarcolemma could only be detected in a few of the swollen myocytes. Roundish La granules were seen in the myofibrils. The percentage of myocytes containing intracellular La and the extent of cellular and mitochondrial swelling showed a significant correlation. CONCLUSIONS: Patterns of intracellular La distribution depend on the degree of ischemic stress and correspond to the degree of cellular as well as mitochondrial edema. These results point at a direct relation between alterations of membrane permeability and development of edema.

Animals↗
PubMed2003

Anisotropic stretch-induced hypertrophy in neonatal ventricular myocytes micropatterned on deformable elastomers.

Because cell shape and alignment, cell-matrix adhesion, and cell-cell contact can all affect growth, and because mechanical strains in vivo are multiaxial and anisotropic, we developed an in vitro system for engineering aligned, rod-shaped, neonatal cardiac myocyte cultures. Photolithographic and microfluidic techniques were used to micropattern extracellular matrices in parallel lines on deformable silicone elastomers. Confluent, elongated, aligned myocytes were produced by varying the micropattern line width and collagen density. An elliptical cell stretcher applied 2:1 anisotropic strain statically to the elastic substrate, with the axis of greatest stretch (10%) either parallel or transverse to the myofibrils. After 24 h, the principal strain parallel to myocytes did not significantly alter myofibril accumulation or expression of atrial natriuretic factor (ANF), connexin-43 (Cx-43), or N-cadherin (by indirect immunofluorescent antibody labeling and immunoblotting) compared with unstretched controls. In contrast, 10% transverse principal strain resulted in continuous staining of actin filaments (rhodamine phalloidin); increased immunofluorescent labeling of ANF, Cx-43, and N-cadherin; and upregulation of protein signal intensity by western blotting. By using microfabrication and microfluidics to control cell shape and alignment on an elastic substrate, we found greater effects for transverse than for longitudinal stretch in regulating sarcomere organization, hypertrophy, and cell-to-cell junctions.

Actins↗
PubMed2005

Dynamics of Z-band based proteins in developing skeletal muscle cells.

During myofibril formation, Z-bodies, small complexes of alpha-actinin and associated proteins, grow in size, fuse and align to produce Z-bands. To determine if there were changes in protein dynamics during the assembly process, Fluorescence Recovery after Photobleaching was used to measure the exchange of Z-body and Z-band proteins with cytoplasmic pools in cultures of quail myotubes. Myotubes were transfected with plasmids encoding Yellow, Green, or Cyan Fluorescent Protein linked to the Z-band proteins: actin, alpha-actinin, cypher, FATZ, myotilin, and telethonin. Each Z-band protein showed a characteristic recovery rate and mobility. All except telethonin were localized in both Z-bodies and Z-bands. Proteins that were present both early in development in Z-bodies and later in Z-bands had faster exchange rates in Z-bodies. These results suggest that during myofibrillogenesis, molecular interactions develop between the Z-band proteins that decrease their mobility and increase the stability of the Z-bands. A truncated construct of alpha-actinin, which localized in Z-bands in myotubes and exhibited a very low rate of exchange, led to disruption of myofibrils, suggesting the importance of dynamic, intact alpha-actinin molecules for the formation and maintenance of Z-bands. Our experiments reveal the Z-band to be a much more dynamic structure than its appearance in electron micrographs of cross-striated muscle cells might suggest.

Animals↗
PubMed1983

A covalently cross-linked matrix in skeletal muscle fibers.

When skeletal, cardiac, and smooth muscle is exhaustively extracted with a protein-unfolding reagent such as 6 M guanidine HCl and a disulfide-reducing reagent such as 5% beta-mercaptoethanol, a tissue ghost remains intact and retains the characteristic shape and dimensions of the tissue before extraction. In the case of chicken pectoral muscle, the tissue ghost contains 1% of the original muscle proteins. Guanidine HCl extraction followed by collagenase treatment of glycerol-extracted chicken pectoral muscle releases a clean preparation of elongated structures containing 0.2% of the original protein and representing the covalently cross-linked remnants of the muscle fibers. The material of these muscle fiber ghosts extends throughout the interior of the cell. Antibodies raised against the tissue ghosts of smooth muscle cross-react with glycerol extracted skeletal myofibrils, forming a banding pattern which coincides with the banding pattern observed when myofibrils are reacted with antibodies against titin. Titin, a large and soluble protein found in skeletal muscle, cross-reacts with our antigizzard antibody. However, amino acid analysis of the muscle fiber ghosts indicates that titin cannot be the only subunit of the insoluble polymer, but that one or more proteins with a very high glycine and alanine content and a very low basic and acidic amino acid content must also form part of the covalently cross-linked matrix. The possibility is presented that this matrix may be the basis of the superthin 2-3-nm filaments which have been observed in a variety of cell types.

Amino Acids↗