Small leptomeric organelles in intrafusal muscle fibers of the frog as revealed by electron microscopy

Formation of leptofibrils is associated with remodelling of muscle cells and myofibrillogenesis in the border zone of myocardial infarction

Leptofibrils, or leptomeres, remain the least studied cytoskeletal structures in muscle cells, and their function and mechanism of assembly are still poorly understood. Our ultrastructural study of the surviving cardiac myocytes located in the perinecrotic border zone of the infarcted left ventricle in rats revealed intense formation of leptofibrils and leptofibrillar clusters during 4-15 days following experimental myocardial infarction. In the perinecrotic myocytes, leptofibrils developed predominantly in the subsarcolemmal areas, near disassembled intercalated discs and at the sites of intense myofibrillogenesis in the peripheral zones of the sarcoplasm. We found that the development of these structures occurred before or at the time of assembly of myofibrils. In our material, leptofibrils consisted of longitudinally oriented filamentous bundles inserted in electron dense Z-band-like material and periodically crossed by 3-8 bands of this material with the period of cross-striation of 120-210 nm. The presence of leptofibrils in growing cytoplasmic processes and ruffles developing in the border zone in the areas of lost intercellular contacts indicates their formation de novo during post-infarction period. We observed four major morphological types of localization of these structures: (1) direct contact of one end of leptofibrils with Z bands of nascent, mature or disassembling myofibrils; (2) direct contact with the sarcolemma: (a) multifocal attachment of leptofibrils to the sarcolemma through the lateral surfaces of their minute Z band-like structures; (b) attachment of one or both ends of leptofibrils to the sarcolemma without contacts or in contact with myofibrils; (3) attachment of leptofibrils to subsarcolemmal accumulations of electron dense Z-band material in newly formed fasciae adherentes of the remodeled intercalated disks; (4) clustering and contacts of leptofibrils with one another predominantly at the level of their Z bands. Interestingly, most leptofibrils of all four types were topographically associated with the system of T-tubules, the sarcoplasmic reticulum and subsarcolemmal vesicles. Serial sections through the areas containing leptofibrils indicate their spindle-like or nearly cylindrical shape. Thus, we found that leptofibrils assemble in terminally differentiated cardiac myocytes following destabilization of their differentiated state and partial dedifferentiation induced by myocardial infarction. The results of this study demonstrate that formation of leptofibrils, earlier described mainly in the developing and malignant muscle, is temporally associated with adaptive structural remodelling and the activation of myofibrillogenesis in functionally overloaded cardiac myocytes of adult animals. Our findings suggest that re-expression of some structural characteristics of the embryonic muscle appear to represent one of the mechanisms that underlie adaptive plasticity of the myocardium following injury and under conditions of hyperfunction.

Myosin flares and actin leptomeres as myofibril assembly/disassembly intermediates in sonic muscle fibers

The sonic muscle of type 1 male midshipman fish produces loud and enduring mating calls. Each sonic muscle fiber contains a tubular contractile apparatus with radially arranged myofibrillar plates encased in a desmin-rich cytoskeleton that is anchored to broad Z bands (approximately 1.2 micro m wide). Immunomicroscopy has revealed patches of myosin-rich "flares" emanating from the contractile tubes into the peripheral sarcoplasm along the length of the fibers. These flares contain swirls of thick filaments devoid of associated thin filaments. In other regions of the sarcoplasm at the inner surface of the sarcolemma and near Z bands, abundant ladder-like leptomeres occur with rungs every 160 nm. Leptomeres consist of dense arrays of filaments (approximately 4 nm) with a structure that resembles myofibrillar Z band structure. We propose that flares and leptomeres are distinct filamentous arrays representing site-specific processing of myofibrillar components during the assembly and disassembly of the sarcomere. Recent reports that myosin assembles into filamentous aggregates before incorporating into the A band in the skeletal muscles of vertebrates and Caenorhabditis elegans suggest that sonic fibers utilize a similar pathway. Thus, sonic muscle fibers, with their tubular design and abundant sarcoplasmic space, may provide an attractive muscle model to identify myofibrillar intermediates by structural and molecular techniques.

Ultrastructural study of striated organelles in vestibular sensory cells of human fetuses

The presence of striated organelles in the vestibular receptor of human fetuses of 10 to 22 weeks gestation was investigated. They were very frequent under the cuticular plate of the sensory cells, especially in type-I hair cells. Cross-sections of striated organelles showed that there were twice as many filaments in the dark bands as in the light bands. These filaments were often arranged in broken lines. Striated organelles were frequently associated with microtubules, mitochondria, and endoplasmic reticulum, and were fused with the plasma membrane forming an incomplete ring in the cytoplasm of the cells. We discuss the possibility that in adults, these striated organelles participate in an active mechanism that regulates the transduction of stimuli by means of a feedback control of the apical part of the calyx.

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)

Stromal cells of the fibroadenoma of the human breast. An immunohistochemical and ultrastructural study

Fourteen fibroadenomas of the human breast were examined by light and electron microscopy, and by immunohistochemistry for actin. They were classified into 3 groups according to their stromal patterns; myxoid, fibrous-cellular and sclerotic. Actin immunohistochemistry revealed that the stromal areas were strongly positive in the fibrous-cellular group and weakly positive in the myxoid and sclerotic groups. By electron microscopy the stromal cells in most cases of the myxoid and fibrous-cellular groups were fibroblasts, containing varying amounts of microfilaments, 5-7 nm in diameter (actin type filaments). However, a dense body was not usually present suggesting these stromal cells were variants of myofibroblasts. The amount of microfilaments in fibroblasts was greater in the fibrous-cellular group than in the myxoid group. This was consistent with the results of actin immunohistochemistry. In 3 cases of the fibrous-cellular group peculiar structures simulating Z-lines of striated muscles were noted in some stromal cells. Since no myosin filaments were detected, they were regarded as intermediate structures between Z-lines of striated muscles and dense bodies of smooth muscles. In the sclerotic group, stromal fibroblasts were sparse and had fewer organelles.

The consistent occurrence of a striated organelle (Friedmann body) in the inner hair cells of the normal chinchilla

Striated organelles have consistently been observed in electron micrographs of serial sections from the inner hair cells of normal chinchilla cochleas. The striated organelle is located in the infracuticular plate region. It lines the cuticular plate, and the direction, pattern and periodicity of the striations vary along its length. The striated organelle is seen in close association with the cell membrane, smooth endoplasmic reticulum, microtubules and mitochondria. The striated organelle may play an active role in inner hair cell function, and its proliferation under pathological conditions, as observed by others, may be accompanied by alterations in sensitivity of the inner hair cell to stimuli.

Polypoid sarcoma of the pulmonary trunk: analysis of the literature and report of a case with leptomeric organelles and ultrastructural features of rhabdomyosarcoma

A case of polypoid sarcoma of the pulmonary trunk is described in an 80-year-old woman who had a 21-year history of episodic chest pain and hemoptysis. Ultrastructural examination revealed Z bands, characteristic of rhabdomyosarcoma, and leptomeric organelles. This is the 60th reported case of sarcoma of the pulmonary trunk. Symptoms in reported cases were variable, usually caused by tumor emboli to the lungs or by right ventricular outflow obstruction. Angiography was diagnostic. The tumor was characteristically polypoid, often multicentric, and by definition was fixed to the pulmonary trunk or valves. Non-differentiated sarcoma (often uniquely pleomorphic) was found in 37% of patients, leiomyosarcoma (once confirmed by electron microscopy) in 17%, myxosarcoma in 13%, elements of rhabdomyosarcoma in 8%, fibrosarcoma in 8%, elements of chondrosarcoma in 11%, and 5% were malignant mesenchymomas. The authors suggest that these tumors originate from the undifferentiated tissues of the bulbus cordis and propose the name myenchymoma for the tumor.

Leptomeres in cultured human muscle

Leptomeres, the laminated structures consisting of bundles of very fine filaments separated into bands about 260 nm wide by periodic transverse dense lines 20--80 nm wide, were observed frequently in cultured muscle fibers of 8 patients with acid maltase deficiency, 4 with sporadic, adult-onset idiopathic "autophagic" vacuolar myopathy (that is not acid-maltase deficient) and one with abnormal mitochondria, but in only one of greater than 50 other cultures of normal and denervated human muscle. They were also induced abundantly in cultured normal human muscle by exposure to 0.5 mM DNP.

[Proliferation of the transverse tubular system during a tardive and familial myopathy]

The authors report a case of non-progressive, late-onset, recessive, sex-linked myopathy. Electron microscopy reveals a striking proliferation of the T system. This proliferation might be interpreted as indicating an abortive attempt of muscle to regenerate, which could explain the clinical course. Some basic ultrastructural aspects concerning the T system are reported. The signification of the findings of numerous "zebra bodies" is discussed.

Extraocular muscles: light microscopy and ultrastructural features

Thirty extraocular muscles (EOM) from 20 patients were evaluated by light microscopy (LM), electron microscopy (EM), and enzyme histochemistry (EZH). Twenty-one EOM were obtained from 13 patients with strabismus, 9 EOM from 4 patients undergoing eye surgery for other reasons and from 3 autopsy cases. One mum thick sections revealed marked variation in muscle fibre shape and size and in myofibrillar structure; also noted were small, hypertrophied, whorled, and ringbinden fibres. Dense and granular material in the central portion of some fibres and sarcomere disruption in 2--3 mum sections was observed. EZH revealed the absence of the classical mosaic pattern usually found in skeletal muscles. ATPase studies were inconsistent and did not correlate with the expected reciprocal activity of NAD-H diaphorase, particularly on the large fibres. Ultrastructural features consisted of vacuoles within myofilament bundles, "smearing" of Z bands, and "nemaline rods". Occasional myelin figures and lipid-like droplets were observed in subsarcolemmal spaces, associated with scattered clusters of glycogen granules. Abnormal mitochondria and subsarcolemmal inclusions of dense and granular material were conspicuous. "Leptomeric" profiles, "Zebra bodies", or "striated bodies" were noted in 8 EOM's, and an Hirano body was found in 1. The intramuscular nerves contained structures resembling "Luse bodies" in 7 cases. These observations suggest that EOM from individuals with and without strabismus possess unique structural characteristics suggestive of developmental and morphological disarrangement of contractile elements. Some of these changes might play a role in the pathogenesis of strabismus and in the development of clinical symptoms. These features are significantly different from striated skeletal muscle. Therefore the criteria used in the pathological evaluation and diagnosis of skeletal muscle disorders cannot be unequivocally applied to EOM investigations. These data establish the necessity to determine histological norms, ultrastructural patterns, and develop new enzyme histochemistry criteria for the evaluation of EOM. Only then can an acceptable comparison of EOM and skeletal muscle be made.

Evidence for a structural relationship between successive parallel tubules in the SR network and supernumerary striations of Z line material in purkinje fibers of the chicken, sheep, dog and rhesus monkey heart

The striations and the intervening filaments observed in the present study have been variously designated in the literature as: prodomal pattern, leptomeric myofibril, microladder, leptomeric organelle, leptofibril and zebra body. Electron microscope examinations of Purkinje fibers from the septa, papillaries, trabeculae carneae and small endocardial strands from chicken, sheep, dog and monkey hearts have revealed a close association between densely stained striations of supernumerary Z line material and successive parallel tubules in the network formed by the sarcoplasmic reticulum (SR). The striations appear to be linked together by filaments that somewhat resemble the part of thin filaments attached to Z lines in normal fibrils. The evidence for a close association of striations and SR tubules is derived from a similarity of spacing between striations and successive parallel tubules in the SR network and from a resemblance of striation and SR network patterns. The evidence for a structural relationship between striations and SR tubules is derived from the observation of electron-opaque strands traversing the space between striations and SR tubules.

Fine structure of rat intrafusal muscle fibers. The equatorial region

An ultrastructural study has been undertaken on the equatorial (sensory) region of the rat muscle spindle. Two kinds of intrafusal muscle fibers, a nuclear bag fiber and a nuclear chain fiber, have been identified in this region on the basis of fiber diameter, nuclear disposition, and M-band appearance. The large-diameter nuclear bag fiber contains an aggregation of tightly packed vesicular nuclei, while the small-diameter nuclear chain fiber contains a single row of elongated, well-separated nuclei. Both muscle fibers contain an attenuated peripheral cylinder of myofilaments surrounding a central core of sarcoplasm. Elements of the sarcotubular system, dilatations of the sarcoplasmic reticulum, and the presence of other sarcoplasmic organelles and inclusions are considerably more abundant in the nuclear chain fiber than in the nuclear bag fiber. Leptomeric organelles and membrane-bounded sarcoplasmic granules are present in both intrafusal fiber types and may be situated between the myofibrils or in intimate association with the sarcolemma. The functional significance of some of these structural findings is discussed.