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PubMed1983

Immunofluorescent studies for myosin, actin, tropomyosin and alpha-actinin in cultured cardiomyopathic hamster heart cells.

Primary cultures of cardiac myocytes from newborn normal and genetically cardiomyopathic (strain UM-X7.1) hamsters were analyzed by electron microscopy and immunofluorescent staining for myosin, actin, tropomyosin, and alpha-actinin. Antibody staining of these contractile proteins demonstrates that both normal and cardiomyopathic (CM) myocytes contain prominent myofibrils after 3 days in culture, although the CM myofibrils are disarrayed and not aligned as those in normal cells. The disarray becomes even more pronounced in CM cells after 5 days in culture. The immunofluorescent staining patterns of individual myofibrils in normal and CM cells were similar for myosin, actin, and tropomyosin. However, alpha-actinin staining reveals that the CM myofibrils have abnormally wide and irregularly shaped Z bands. Electron microscopy confirms the irregular Z-band appearance as well as the myofibril disarray. Thus, CM cardiomyocytes clearly show an aberrant pattern of myofibril structure and organization in culture.

Actinin↗
PubMed1992

The vinculin/sarcomeric-alpha-actinin/alpha-actin nexus in cultured cardiac myocytes.

Experiments are described supporting the proposition that the assembly of stress fibers in non-muscle cells and the assembly of myofibrils in cardiac cells share conserved mechanisms. Double staining with a battery of labeled antibodies against membrane-associated proteins, myofibrillar proteins, and stress fiber proteins reveals the following: (a) dissociated, cultured cardiac myocytes reconstitute intercalated discs consisting of adherens junctions (AJs) and desmosomes at sites of cell-cell contact and sub-sarcolemmal adhesion plaques (SAPs) at sites of cell-substrate contact; (b) each AJ or SAP associates proximally with a striated myofibril, and conversely every striated myofibril is capped at either end by an AJ or a SAP; (C) the invariant association between a given myofibril and its SAP is especially prominent at the earliest stages of myofibrillogenesis; nascent myofibrils are capped by oppositely oriented SAPs; (d) the insertion of nascent myofibrils into AJs or into SAPs invariably involves vinculin, alpha-actin, and sarcomeric alpha-actinin (s-alpha-actinin); (e) AJs are positive for A-CAM but negative for talin and integrin; SAPs lack A-CAM but are positive for talin and integrin; (f) in cardiac cells all alpha-actinin-containing structures invariably are positive for the sarcomeric isoform, alpha-actin and related sarcomeric proteins; they lack non-s-alpha-actinin, gamma-actin, and caldesmon; (g) in fibroblasts all alpha-actinin-containing structures are positive for the non-sarcomeric isoform, gamma-actin, and related non-sarcomeric proteins, including caldesmon; and (h) myocytes differ from all other types of adherent cultured cells in that they do not assemble authentic stress fibers; instead they assemble stress fiber-like structures of linearly aligned I-Z-I-like complexes consisting exclusively of sarcomeric proteins.

Actinin↗
PubMed1983

Highly homologous filamin polypeptides have different distributions in avian slow and fast muscle fibers.

The high molecular weight actin-binding protein filamin is located at the periphery of the Z disk in the fast adult chicken pectoral muscle (Gomer, R. H., and E. Lazarides, 1981, Cell, 23: 524-532). In contrast, we have found that in the slow anterior latissimus dorsi (ALD) muscle, filamin was additionally located throughout the l band as judged by immunofluorescence with affinity-purified antibodies on myofibrils and cryosections. The Z line proteins desmin and alpha-actinin, however, had the same distribution in ALD as they do in pectoral muscle. Quantitation of filamin and actin from the two muscle types showed that there was approximately 10 times as much filamin per actin in ALD myofibrils as in pectoral myofibrils. Filamin immunoprecipitated from ALD had an electrophoretic mobility in SDS polyacrylamide gels identical to that of pectoral myofibril filamin and slightly greater than that of chicken gizzard filamin. Two-dimensional peptide maps of filamin immunoprecipitated and labeled with 125I showed that ALD myofibril filamin was virtually identical to pectoral myofibril filamin and was distinct from chicken gizzard filamin.

Actinin↗
PubMed1991

Morphometric evaluation of the contractile apparatus in primary cultures of rabbit cardiac myocytes.

Rabbit cardiac myocytes remain quiescent for more than 1 month when cultured at low density. During this period, myofibrillar volume density declines sixfold as myofibrils are disassembled or degraded and are replaced by actin and alpha-actinin-positive, myosin-negative structures that resemble myofibrils but lack thick filaments. Such structures are termed minute myofibrils. The length of the sarcomeres in these altered myofibrils is significantly less than length values obtained from freshly isolated heart cells or from contracting myocytes. A number of high density cultures develop spontaneous, synchronous contraction during the second week of culture. Myofibrillar volume density is stabilized when beating begins, and no further decline is observed in the succeeding weeks of culture. Such contracting myocytes display myofibrils typical of normal heart with no visible evidence of minute myofibrils. The volume density of the transverse tubular system also declines significantly in both beating and nonbeating myocytes, and its reduction appears more closely correlated with cell spreading than with beating per se. No quantitative changes in volume density of mitochondria or sarcoplasmic reticulum could be documented, but the structural organization of the sarcoplasmic reticulum seems to be greatly influenced by the physiological state of the heart cell. The present observations document the importance of mechanical factors in regulating the integrity of the contractile apparatus in cardiac myocytes and emphasize the utility of the cultured heart cell to directly investigate structure-function relations in individual myocytes.

Animals↗
PubMed1994

Rearrangement of mRNAs for costamere proteins during costamere development in cultured skeletal muscle from chicken.

Mature skeletal myofibrils are surrounded by costameres, ribs of metavinculin, vinculin, intermediate filaments, and other proteins that connect the myofibril to the extracellular matrix. Costameres have recently been shown to be the sites at which the forces generated by the myofibril are transduced laterally into the extracellular matrix. We observed costameres developing in cultured skeletal muscles, grown in micromass culture from cells taken from embryonic chicken leg. We detected proteins by immunofluorescence and mRNA by in situ hybridization. Antibody and probe signals were imaged by laser scanning confocal microscopy. Antibody to vimentin protein is first detected in stripes in register with the Z line of the myofibril, at approximately day 12 after fusion; soon thereafter probe to vimentin mRNA is also detected in the same stripes. Optical sections indicate that vimentin mRNA and protein are very close, no more than 0.1 mm apart and possibly in immediate contact. Antibody to vimentin is detected in stripes only in cells that twitch spontaneously. Antibodies and probes to desmin and vinculin protein and mRNA are next detected in stripes of the same periodicity, at approximately day 17 after fusion. Vinculin protein (but not mRNA) is detected at focal contacts much earlier in development. Controls for bleed through of fluorescence, RNase H sensitivity, hybridization without probe, and binding to the myofibril all gave appropriate results. Probes to glyceraldehyde-3-phosphate dehydrogenase, a glycolytic enzyme, stained diffusely and did not associate with the myofibril. These results show that components of the costamere arrive at the structure in a defined sequence, and that mRNA organization is a conspicuous, precise and temporally controlled aspect of costamere development. These results may have wider implications. In these cells, some mRNAs are positioned with submicrometer precision in space and differentially over time. Particular mRNAs differ in the time and place of such positioning. This implies both that cellular structures provide physical cues for such positioning and that mRNA contains information that interacts with such cues in a message-specific manner. If such precision in mRNA location is found in other somatic cells, it could have significant implications for the ways in which cells generate and maintain cellular structures.

Animals↗
PubMed1995

Ultrastructural and morphometric features of nodal and impulse-conducting cardiac myocytes of the bat Pipistrellus pipistrellus.

Cells of the impulse-generating and conducting tissues of the insect-eating bat Pipistrellus pipistrellus were studied and evaluated using ultrastructural morphometry. Sinoatrial node cells are smaller than working atrial cells and measure about 6.5 microm in diameter. Their mitochondira and myofibril content constitute 23% and 19% of cytoplasmic volume, respectively. Corresponding values for working atrial cells are 23% and 52%. Atrioventricular node cells are 4.2 microm in diameter and contain abundant glycogen in the cytoplasm. The fractional volume of mitochondria in about 24% while that of myofibrils is 7%. Cells of the bundle of His are larger (6-8 microm diameter) and contain more cellular organelles than do nodal cells. Their mitochondria and myofibril contents are 25% and 25%, respectively. Cells in the proximal part of the right bundle branch are slender with diameters averaging 3.4 microm. Mitochondrial content is 23% while myofibrils occupy 20% of the cytoplasmic volume of these cells. Distally located bundle branch cells measure 7-10 microm in diameter with mitochondria and myofibril volumes of 30% and 33%. Subendocardial cells in the ventricular free wall are large reaching 28 microm in diameter (cf. 14-18 microm in working ventricular cells) and have mitochondira and myofibril volume fractions of 32% and 29%, respectively (35% & 40% for working ventricular cells).

Animals↗
PubMed1992

Mutation of the high affinity calcium binding sites in cardiac troponin C.

Fast skeletal and cardiac troponin C (TnC) contain two high affinity Ca2+/Mg2+ binding sites within the C-terminal domain that are thought to be important for association of TnC with the troponin complex of the thin filament. To test directly the function of these high affinity sites in cardiac TnC they were systematically altered by mutagenesis to generate proteins with a single inactive site III or IV (CBM-III and CBM-IV, respectively), or with both sites III and IV inactive (CBM-III-IV). Equilibrium dialysis indicated that the mutated sites did not bind Ca2+ at pCa 4. Both CBM-III and CBM-IV were similar to the wild type protein in their ability to regulate Ca(2+)-dependent contraction in slow skeletal muscle fibers, and Ca(2+)-dependent ATPase activity in fast skeletal and cardiac muscle myofibrils. The mutant CBM-III-IV is capable of regulating contraction in permeabilized slow muscle fibers but only if the fibers are maintained in a contraction solution containing a high concentration of the mutant protein. CBM-III-IV also regulates myofibril ATPase activity in fast skeletal and cardiac myofibrils but only at concentrations 10-100-fold greater than the normal protein. The pCa50 and Hill coefficient values for Ca(2+)-dependent activation of fast skeletal muscle myofibril ATPase activity by the normal protein and all three mutants are essentially the same. Competition between active and inactive forms of cardiac and slow TnC in a functional assay demonstrates that mutation of both sites III and IV greatly reduces the affinity of cardiac and slow TnC for its functionally relevant binding site in the myofibrils. The data indicate that although neither high affinity site is absolutely essential for regulation of muscle contraction in vitro, at least one active C-terminal site is required for tight association of cardiac troponin C with myofibrils. This requirement can be satisfied by either site III or IV.

Adenosine Triphosphatases↗
PubMed1986

Isolation and characterization of a new 40-kilodalton protein from bovine cardiac muscle.

A new protein having a subunit weight of 40,000 has been purified from myosin-extracted bovine cardiac myofibrils. Its amino acid composition and isoelectric point are distinct from actin, eu-actinin, and a variety of sarcoplasmic proteins of similar size. Affinity-purified antibodies made to this protein only react with a single 40-kDa protein band from cardiac myofibrils on immunoblots. The anti-40-kDa protein also shows cross-reactivities with cardiac myofibrils from rabbits, rats, and chickens. Immunofluorescence studies demonstrate that the 40-kDa protein is localized at the Z-bands of cardiac myofibrils and at the intercalated discs. The antibody did not react with skeletal muscle myofibrils by immunofluorescence or immunoblotting. It appears that the 40-kDa protein may play a role in the strong attachments between adjacent myofibrils in cardiac muscle.

Actinin↗
PubMed1998

Ectopic expression of tropomyosin promotes myofibrillogenesis in mutant axolotl hearts.

Expression of tropomyosin protein, an essential component of the thin filament, has been found to be drastically reduced in cardiac mutant hearts of the Mexican axolotl (Ambystoma mexicanum) with no formation of sarcomeric myofibrils. Therefore, this naturally occurring cardiac mutation is an appropriate model to examine the effects of delivering tropomyosin protein or tropomyosin cDNA into the deficient tissue. In this study, we describe the replacement of tropomyosin by using a cationic liposome transfection technique applied to whole hearts in vitro. When mouse alpha-tropomyosin cDNA under the control of a cardiac-specific alpha-myosin heavy chain promoter was transfected into the mutant hearts, tropomyosin expression was enhanced resulting in the formation of well-organized sarcomeric myofibrils. Transfection of a beta-tropomyosin construct under control of the same promoter did not result in enhanced organization of the myofibrils. Transfection of a beta-galactosidase reporter gene did not result in the formation of organized myofibrils or increased tropomyosin expression. These results demonstrate the importance of alpha-tropomyosin to the phenotype of this mutation and to normal myofibril formation. Moreover, we have shown that a crucial contractile protein can be ectopically expressed in cardiac muscle that is deficient in this protein, with the resulting formation of organized sarcomeres.

Ambystoma↗
PubMed1993

Dynamics of actin and assembly of connectin (titin) during myofibrillogenesis in embryonic chick cardiac muscle cells in vitro.

Immunogold electron microscopy of cardiac myocytes microinjected with biotin-labeled actin showed that gold labeling was first found around the A band level of myofibrils at their proximal parts. This observation suggests that polymerization of actin and/or the addition of newly formed actin filaments occurs preferentially in association with myosin filaments to increase the myofibrillar girth. At the distal portions of developing myofibrils, their terminal ends were initially labeled, suggesting that continued reorganization and/or de novo formation of myofibrils occurs at these locations. Soon, gold particles were seen along the termini of growing myofibrils. This appears to indicate that actin subunits are added at the membrane-associated ends of preexisting actin filaments to increase the length of myofibrils. Adhesion plaque proteins, e.g., vinculin, do not appear to play any role in assembling actin monomers at these sites on the inner surface of the sarcolemma. Immunofluorescence and immunoelectron microscopy of cardiomyocytes double-stained with antibodies against two distant domains of connectin (titin) filaments and other sarcomeric proteins showed that these domains of connectin filaments and myosin were synthesized almost simultaneously on large polyribosomes and/or associated immediately after the synthesis of these molecules. Connectin and myosin bands were formed after alpha-actinin striations (Z bands) were seen on preformed I-Z-I-like structures.(ABSTRACT TRUNCATED AT 250 WORDS)

Actinin↗
PubMed1995

Visualization of the transverse cytoskeletal network in insect-flight muscle by scanning-electron microscopy.

Located at the level of the Z-line, the transverse cytoskeletal network of insect-flight muscle interconnects adjacent myofibrils with one another, and interconnects peripheral myofibrils with the cell membrane. This network has been presumed to keep myofibrils in register, or to distribute tension laterally among myofibrils. In this study, we used scanning-electron microscopy to reveal details of the three-dimensional arrangement of this network. The network is seen to interconnect longitudinal elements of the cytoskeletal network which surround each myofibril. The arrangement is not unlike that seen in vertebrate skeletal muscle. Interestingly, the transverse network makes contact with cell components such as dense bodies and mitochondria. Such contacts imply potential roles over and above those noted above. The network may be involved not only in mechanical function, but possibly also in intracellular communication.

Animals↗
PubMed2006

Differential expression of tropomyosin during segmental heart development in Mexican axolotl.

The Mexican axolotl, Ambystoma mexicanum, serves as an intriguing model to investigate myofibril organization and heart development in vertebrates. The axolotl has a homozygous recessive cardiac lethal gene "c" which causes a failure of ventricular myofibril formation and contraction. However, the conus of the heart beats, and has organized myofibrils. Tropomyosin (TM), an essential component of the thin filament, has three known striated muscle isoforms (TPM1alpha, TPM1kappa, and TPM4alpha) in axolotl hearts. However, it is not known whether there are differential expression patterns of these tropomyosin isoforms in various segments of the heart. Also, it is not understood whether these isoforms contribute to myofibril formation in a segment-specific manner. In this study, we have utilized anti-sense oligonucleotides to separately knockdown post-transcriptional expression of TPM1alpha and TPM4alpha. We then evaluated the organization of myofibrils in the conus and ventricle of normal and cardiac mutant hearts using immunohistochemical techniques. We determined that the TPM1alpha isoform, a product of the TPM1 gene, was essential for myofibrillogenesis in the conus, whereas TPM4alpha, the striated muscle isoform of the TPM4 gene, was essential for myofibrillogenesis in the ventricle. Our results support the segmental theory of vertebrate heart development.

Ambystoma mexicanum↗
PubMed2002

Cold acclimation induces proliferation of sarcoplasmic reticulum without increase in Ca2+-ATPase activity in white axial muscle of striped bass (Morone saxatilis).

The effects of acclimation of striped bass to cold (5 degrees C) and warm (25 degrees C) temperatures upon ultrastructural features of white axial skeletal muscle are quantified. Surface density of sarcoplasmic reticulum (SR) increased by almost 30%, and SR volume density increased by about 20% during cold acclimation. Proliferation of SR suggests an increase in available SR surface for re-sequestration of Ca2+ and a decrease in diffusion path length for Ca2+ during cold acclimation. Average cross-sectional areas and cross-sectional perimeters of myofibrils situated in the center of muscle fibers decreased during cold acclimation by approximately 20% and 11%, respectively. Additionally, average major and minor axes of ellipses fit to central myofibrillar cross-sections decreased by approximately 12% and 8%, respectively, during cold acclimation. These measurements define a decrease in average myofibrillar diameter and suggest a decrease in diffusion path length for Ca2+ to and from myofibrillar activation sites. Measurements of peripheral myofibrils that had elongated profiles in cross-sections indicate that maximum profile length of these myofibrils decreases by about 17%. Peripheral myofibrils may break up into smaller myofibrils with more rounded cross-sectional profiles during cold acclimation. SR Ca2+-ATPase of white axial muscle was also measured in unfractionated homogenates and in crude SR-enriched subcellular fractions from cold- and warm-acclimated striped bass. No difference in SR Ca2+-ATPase activity per g wet weight was observed between cold- and warm-acclimated animals. Lack of increase in SR Ca2+-ATPase per g wet weight, despite a significant proliferation of SR, probably results in a decrease in average Ca2+-ATPase pump density within the SR membrane during cold acclimation. Thus, compensation for decreased diffusion coefficient of Ca2+ during cold acclimation appears due to the combined effects of proliferation of SR surface density and a decrease in average myofibrillar diameter.

Acclimatization↗
PubMed1997

Vinculin is an essential component for normal myofibrillar arrangement in fetal mouse cardiac myocytes.

Vinculin is a cytoskeletal protein that is believed to be an essential component in the linkage of cytoskeletal actin filaments to the plasma membrane. To investigate the precise function of vinculin in the development of cardiac myofibrils, antisense oligodeoxynucleotides complementary to vinculin mRNA were used to perturb the expression of the protein during myofibril assembly and arrangement in mouse cardiac myocytes. Fetal (day 18-20 post-conception) mouse cardiac myocytes were isolated by collagenase digestion, separated by Percoll density gradient centrifugation, and plated on aligned collagen gels. By 72 h of culture, mouse myocytes displayed an elongated in vivo-like phenotype in parallel with the aligned fibrils of the collagen gels with polarized arrays of myofibrils. Two different antisense oligonucleotides (20-mer) altered the formation of the tissue-like phenotype of myocytes. These antisense oligonucleotides suppressed vinculin protein expression at 43.5+/-26.8% and 48.7+/-20.9% when compared to myocytes that were not treated. Examination of these myocytes by confocal scanning laser and transmission electron microscopy revealed a disruption of the aligned in vivo-like phenotype, assembly of thick and thin filaments, and formulation of Z-bands. Random sequence 20-mer oligonucleotides used as controls had little detectable effect on vinculin protein expression (94.2+/-14.8%), cell shape, normal alignment or assembly of myofibrils. These results indicate that vinculin is a critical cytoskeletal component, that functions in the determination of cell shape and the arrangement and organization of developing myofibrils.

Animals↗
PubMed1985

A periodic cytoskeletal lattice in striated muscle.

The axial periodicities of electron density in striated muscle fibers extend over four orders of magnitude, ranging from the sarcomere repeat (2000-3000 nm) to a residue repeat in the alpha-helix of structural proteins (0.15 nm). A prevailing idea about the regular arrangement of structures in the contractile apparatus maintains that long-range axial spacings, related to the organization of sarcomere repeats, are essentially independent of the short-range periodicities with molecular dimensions. This is a central theme of the sliding filament hypothesis but is only supported by evidence from measured spacings near the upper and lower limits in the spectrum of dimensions, leaving a wide gap in resolved structural information extending from about 460 down to 50 nm. Several independent morphological methods show an electron-dense cross-striation of low amplitude with a pseudo-period of 230 nm, out of phase with the sarcomere repeat, in myofibrils of frog twitch fibers. Averaged images of embedded muscle fibers indicate that the sarcomere repeat contains five symmetrical pairs of these striations, which are coordinated with discrete repeats of the major molecular periods in the thick and thin filaments, in register within A and I bands. The pseudo-period therefore correlates short-range molecular repeats in the filaments with long-range registry of the sarcomere repeats in myofibrils. This raises the interesting possibility that the 230-nm pseudo-periodicity identifies a replicated axial structure in myofibrils that integrates the organization of the major structural proteins into the sarcomere repeat. The density distribution in sarcomeres of isolated unstained myofibrils also establishes that symmetrical pairs of striations with intrinsically low amplitudes are independently distorted out of uniform register in stretched sarcomeres. This behavior is consistent with the properties of N lines. The out-of-phase arrangement of 230-nm striations in the sarcomere repeat of twitch fibers should produce special diffraction effects in the region of the gap in the spectrum of periodicities recorded from muscle, with maxima at spacings extending from 200 to 80 nm. Correspondence between the diffraction spectrum of myofibril models containing a 230-nm spaced axial pseudo-period and the observed very low-angle X-ray diffraction spacings from living muscle (Huxley and Brown, 1967) suggests that the 230-nm pseudo-periodicity is a regular detectable component of striated muscle, resembling the structure of naturally occurring leptomeric fibrils in extrafusal and intrafusal fibers (Karlson and Andersson-Cedergren, 1968).(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1975

Distribution and dimension of the T-system in different muscle fiber types in the atlantic hagfish (Myxine glutinosa, L.).

Triad density relative to sarcomeres, size of T-system tubules, sarcomere length, muscle fiber diameter in native and fixed states, and size of myofibrils were measured in four straited muscle fiber types in Atlantic hagfishes (Myxine glutinosa, L.) of different sizes. Traids occur at A/I-injunctions in all fiber types. The density of traids relative to sarcomeres is higher in "white" than in "red" muscle fibers. The T-tubules show no sign of branching. The area of the T-system tubules is 3-4 times the surface area in 80 mum "white" muscle fibers and 1-2 times that in 60 mum "red" fibers. The size of myofibrils is similar in "white", "intermediate", and "red" fibers of m. parietalis, and constant through a large span of animal size. In "white" fibers, increase in diameter up to 90 mum is accompanied by an increase in the number of myofibrils, not by an increase in the individual size of the myofibrils. Above 90 mum, "white" fibers grow by increasing the amount of intermyofibrillar space. This is reflected by an extensive shrinkage of the thicker "white" fibers during the preparative procedure for electron microscopy, a shrinkage that is limited only by complete packing of the myofibrils. "Red" fibers shrink much less.

Animals↗
PubMed1993

Remodelling of cardiomyocyte cytoarchitecture visualized by three-dimensional (3D) confocal microscopy.

The break-down and reassembly of myofibrils in long-term cultures of adult rat cardiomyocytes was investigated by a novel combination of confocal laser scanning microscopy and three-dimensional image reconstruction, referred to as FTCS, to visualize the morphological changes these cells undergo in culture. FTCS is discussed as an alternative imaging mode to low-magnification scanning electron microscopy. The three-dimensional shape of the cells are correlated with the assembly state of myofibrils in different stages. Based on immunofluorescence and confocal laser scanning microscopy it was shown that myofibrils are degraded within a few days after plating and that newly assembled myofibrils are predominantly confined to the continuous area in the perinuclear region close to the membrane in contact with the substratum. The localization of myofibrils along the cell's vertical axis has been investigated both by optical sectioning using confocal light microscopy and by physical sectioning followed by transmission electron microscopy. Based on the distribution of myofibrillar proteins we propose a model of myofibrillar growth locating the putative assembly sites to a region concentric around the nuclei. We provide evidence that the cell shape is dominated by the myofibrillar apparatus.

Animals↗
PubMed1978

Histological and ultrastructural studies on the ringbands in human extraocular muscles.

Ringbands are a typical finding in the extraocular muscles of individuals of twenty years and older. The number of these structures increases distinctly after the 60th year of life. In optimally fixed muscles the ringbands are separated from the sarcolemma by a space containing the so-called sarcoplasmic masses. These masses were composed of vaculated mitochondria, dilated tubules of the sarcoplasmic system and, in some fibers, of fragments of myofibrils. Structures from which a neoformation of myofibrils as the origin of the ringbands could be concluded (as proposed by Jonecko, 1967), were never observed in this hypolemmal area. In contrast, the formation of the ringbands seems to be correlated to the ultrastructural alterations of the myofilaments and the looser arrangement of the myofibrils which can first be observed in extraocular muscles of 10-year-old patients and increasingly in older patients (Mühlendyck, 1977). These alterations lead to continous and repeated changes in the orientation of the myofibrils during the extension and contraction processes of the muscle fiber. Thus, the myofibrils undergo a constant mechanical stress which they cannot resist indefinetly, leading to disruption of some of the fibrils. The disrupted free end contracts, deviates almost at a right angle from its former course and, while the sarcolemma forms thread-like folds during the contraction of the muscle fiber, it is coiled around the longitudinal undisrupted fibrillar core. This leads to a disfunction of the muscle fiber and aids in explaining the steady decrease of the excursion of the eye after about 30 years of age.

Adult↗