Electron microscopy of the impulse conducting system op the sheep heart

Morphological study of the sinus node and its artery in yak

The sinus node of yak has been studied by the histological methods and transmission electron microscopy. The sinus node artery of yak was also determined by the injection-corrosion casting technique, the angiography, and histological methods. The results showed that the sinus node of yak contained an extensive framework of collagen and two main type cells: pacemaker cells (P cells) and transitional cells (T cells). The P cells had a perinuclear clear zone, contained less myofibrils, and appeared smaller mitochondria than T cells. The T cells were longer and slender than P cells, and had a variety of shapes. At the periphery of sinus node there were many nerve fibers and ganglions. Gap junction did not reveal reaction with anti-connexin43, but it was detected by electron microscopy in the central part of sinus node of yak. The sinus node artery of yak originated from left coronary artery more frequently (98%) than by right (2%). The artery located at the periphery of sinus node. It had an internal elastic membrane throughout its course, and a large nerve bundle was found running in a longitudinal direction.

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.

Differentiation of the atrioventricular conducting system of the heart

The structure and function of the atrioventricular conducting system of the heart, and its relationship to the myocardium, are examined from a developmental point of view. On the basis of information derived from electron micrographic, electrophysiologic, and developmental studies of heart tissue, it is concluded that: (1) The idea of the syncytial nature of the heart lacks a sound anatomic basis. (2) Cytodifferentiation during embryonic cardiogenesis results in the development of at least 2 distinct populations of cells: those comprising the bulk of the myocardium and a second type, the specialized cells of the conductive tissue, which differs in histology, biochemistry, and physiology. (3) The common view of the myocardium as a spontaneously active tissue may require revision, since several lines of evidence appear to indicate that myocardial cells are quiescent until stimulated by an extrinsic source. Under normal circumstances, this stimulus source is the conductive tissue.

SPECIFIC GRANULES IN ATRIAL MUSCLE CELLS

Large populations (up to 600/cell) of spherical, electron-opaque granules ∼0.3 to 0.4 µ in diameter are characteristically found in muscle fibers of mammalian atria. They are absent in muscle fibers of the ventricles. The granules are concentrated in the sarcoplasmic core and occur in lesser numbers in the sarcoplasmic layers between myofibrils and under the plasma membrane. Their intimate association with a central voluminous Golgi complex and the frequent occurrence of material reminiscent of the granular content within the cisternae of the Golgi complex suggest that the latter is involved in the formation of the atrial granules. Atrial granules are larger and more numerous in smaller species (rat, mouse), and generally smaller and less numerous in larger mammals (dog, cat, human); they are absent from the atrial fibers of very young fetuses (rat) but are present in those of newborn animals. A small population of bodies containing glycogen particles and remnants of the endoplasmic reticulum and mitochondria occurs in the sarcoplasmic cores of atrial as well as ventricular muscle fibers in the rat; they contain acid phosphatase and thus appear to be residual bodies of autolytic foci. Their frequency increases with the age of the animal. Typical lipofuscin pigment granules, which are known to contain acid phosphatase and are found in the sarcoplasmic cores in old animals (cat, dog and human), are presumed to arise by progressive aggregation and fusion of small residual bodies.

Effect of several uncouplers of cell-to-cell communication on gap junction morphology in mammalian heart

Electrical conduction in sheep Purkinje fibers has been blocked by three different procedures: (I) 1 mM 2-4-dinitrophenol, (II) 3.5 mM n-Heptan-1-ol (heptanol), and (III) treatment by a hypotonic (120 mOsmoles) Ca2+-free solution for half an hour, followed by return to normal conditions. The gap junction morphology was analyzed quantitatively in freeze-fracture replicas and compared in electrically conducting and nonconducting fibers. It is found that the three uncouplers of cell-to-cell conduction induce consistent and statistically significant alterations of the gap junction structure. The investigated morphological criteria: (a) P-face junctional particle diameter, control value 8.18 +/- 0.70 nm (mean +/- SD), (b) P-face junctional particles center-to-center spacing, control value 10.23 +/- 1.57 nm, and (c) E-face pits spacing, control value 9.45 +/- 0.98 nm, are, respectively, decreased to 7.46 +/- 0.62 nm, 9.25 +/- 1.34 nm and 8.67 +/- 1.13 nm in Purkinje fibers with complete conduction blocks. All three gap junctional dimensions are seen to decline progressively with time from the onset of an uncoupling treatment towards stable minima reached in half an hour. The observed morphological transitions appear related to the electrical uncoupling for the following reasons: partial electrical uncoupling results in values of the gap junctional dimensions that are intermediate between those measured in electrically coupled and uncoupled preparations, and the three morphological indices are seen to increase again towards control values very soon after electrical conduction has been re-established. It is concluded that the junctional channels closure on electrical uncoupling correlates with a measurable (-0.72 +/- 0.01 nm, difference of the means +/- SE) decrease of the junctional particle diameters.

The development of Purkinje fibres and ordinary myocytes in the bovine fetal heart. An ultrastructural study

A comparative ultrastructural study of bovine Purkinje fibres and ordinary myocytes during fetal development has been undertaken. Differences between the two cell types with respect to the intercalated disc, amount of myofibrils, arrangement of mitochondria, amount of glycogen and formation of T-tubules became apparent gradually. In all stages studied an abundance of intermediate filaments was typical for the Purkinje fibres. Myofibrillar M-bands developed at an earlier stage in Purkinje fibres than in ordinary myocytes. Myofilament-polyribosome complexes typical of adult cow Purkinje fibres were not observed in the fetal hearts. Only in late fetal stages were leptofibrils observed in both cell types. We conclude that in the bovine heart Purkinje fibres develop along a different pathway from ordinary myocytes.

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.

Sarcoplasmic reticulum in the conducting fibers of the dog heart

The ultrastructure or sarcoplasmic reticulum (SR) was studied in the conducting fibers of the dog heart. A dense network of sarcoplasmic tubules occurred in well preserved cytoplasmic areas. Some of the tubules extended into large sacs filled with finely filamentous material. The sacs often appeared in subsarcolemmal apposition connected to the sarcolemma by electron dense projections. The flattened cisternae of SR occurred beneath the sarcolemma in the myofibrillar region of the cell. Long flattened cisternae only partially apposed to the sarcolemma were quite common in one specimen. Sarcoplasmic reticulum was continuous throughout the interfibrillar spaces surrounding each myofibril. At the level of the Z-line the SR formed a quasi tubular flattened structure surrounding the Z-line and closely adhering to it.

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.

Computed action potentials for Purkinje fiber membranes with resistance and capacitance in series

The Hodgkin-Huxley equations, as modified by Noble for computation of Purkinje fiber action potentials, have been solved numerically for a membrane whose equivalent circuit contains a constant resistance in series with part of the capacitance. The rates of depolarization and repolarization of the computed action potential have thereby been brought into agreement with measured values. Possible explanations of the frequently observed pre-plateau notch and of fibrillatory activity arise. The effects of a time-dependent K conductance dependent on the second power of the parameter n, instead of the fourth, have also been considered.

Cardiac muscle. A comparative study of Purkinje fibers and ventricular fibers

With light and electron microscopy a comparison has been made of the morphology of ventricular (V) and Purkinje (P) fibers of the hearts of guinea pig, rabbit, cat, dog, goat, and sheep. The criteria, previously established for the rabbit heart, that V fibers are distinguished from P fibers by the respective presence and absence of transverse tubules is shown to be true for all animals studied. No evidence was found of a permanent connection between the sarcoplasmic reticulum and the extracellular space. The sarcoplasmic reticulum (SR) of V fibers formed couplings with the sarcolemma of a transverse tubule (interior coupling) and with the peripheral sarcolemma (peripheral coupling), whereas in P fibers the SR formed only peripheral couplings. The forms of the couplings were identical. The significance, with respect to excitation-contraction coupling, of the difference in the form of the couplings in cardiac versus skeletal muscle is discussed together with the electrophysiological implications of the differing geometries of bundles of P fibers from different animals.

A strand of cardiac muscle. Its ultrastructure and the electrophysiological implications of its geometry

The structure of a small strand of rabbit heart muscle fibers (trabecula carnea), 30-80 micro in diameter, has been examined with light and electron microscopy. By establishing a correlation between the appearance of regions of close fiber contact in light and electron microscopy, the extent and distribution of regions of close apposition of fibers has been evaluated in approximately 200 micro length of a strand. The distribution of possible regions of resistive coupling between fibers has been approximated by a model system of cables. The theoretical linear electrical properties of such a system have been analyzed and the implications of the results of this analysis are discussed. Since this preparation is to be used for correlated studies of the electrical, mechanical, and cytochemical properties of cardiac muscle, a comprehensive study of the morphology of this preparation has been made. The muscle fibers in it are distinguished from those of the rabbit papillary muscle, in that they have no triads and have a kind of mitochondrion not found in papillary muscle. No evidence of a transverse tubular system was found, but junctions of cisternae of the sarcoplasmic reticulum and the sarcolemma, peripheral couplings, were present. The electrophysiological implications of the absence of transverse tubules are discussed. The cisternae of the couplings showed periodic tubular extensions toward the sarcolemma. A regularly spaced array of Z line-like material was observed, suggesting a possible mechanism for sarcomere growth.

Comparative ultrastructure of the sinus node in man and dog

Ultrastructure of the sinus node was studied in human and canine hearts and found to be similar. The principal cell of the sinus node is a small round pale cell with randomly distributed sarcosomes and sparse myofibrils. These cells have been designated as P cells. They occur in elongated clusters and make contact with each other in all directions but do not make direct contact with ordinary working myocardium. Transitional cells, which have features intermediate between P cells and working myocardium, serve as the connections between P cells and the rest of the heart. Each P cell is surrounded by a plasma membrane and groups of them are bound by a basement membrane. The only specialized junction between P cells is the desmosome, which occurs singly and seldom, with most contact being between apposing plasma membranes. P cells exhibit unusually active pinocytosis but have only a sparsely developed sarcoplasmic reticulum. The sarcosomes of P cells are much simpler in internal structure than are those of adjacent working myocardium obtained at the same time in the same heart in the same way, by direct perfusion with glutaraldehyde into the beating sinus node. Correlation of the electron microscopic appearance of the sinus node with that based on light microscopy is discussed, and some of the possible functional significances of the fine structure are considered.

Membrane capacity of the cardiac Purkinje fibre

1. The basis for the relatively high membrane capacitance of the cardiac Purkinje fibre has been investigated.2. The capacitance measured by analysis of the cable response to a current step (square wave) was compared in the same fibres to the capacitance calculated from the foot of the propagated action potential. The square wave value for capacitance was 12.8 +/- 1.3 muF/cm(2) and that from the foot of the action potential, 2.4 +/- 0.5 muF/cm(2).3. The capacitative filling at the beginning of a voltage clamp in short Purkinje fibres was measured. The current-time course deviated from that predicted by a model membrane containing resistance and capacitance in parallel.4. The results obtained by both methods are consistent with two components to the membrane capacitance, with part in parallel with the membrane resistance (2.4 muF/cm(2)) and part (7 muF/cm(2)) in series with a resistor (300 Omega cm(2)).5. The value of the series resistor could be increased by decreasing the conductivity of the extracellular fluid.6. The possible anatomical basis for these findings is discussed.7. Implications of this model on the shape of the Purkinje fibre action potential and on the electrical triggering of contraction are considered.

An attempt to infer the electrophysiological functions of some intracellular structures in cardiac cells by an electronic analogue

A circuit which simulates the electrical conduction characteristics of the neuron has been modified by the addition of a feedback loop to simulate the electrical properties of some of the "specialized" tissues of the mammalian heart. It is suggested that there is similar electrical feedback in the muscle cells which is responsible for their electrical properties, and possible relationships between the feedback and observed structures are discussed.