The structure of the atrioventricular node in the heart of the female laying ostrich (Struthio camelus).

The electrical impulse for cardiac contraction is generated in the Sinoatrial node (SA node), subsequently spreads to the Atrioventricular node (AV node) and continues in the Atrioventricular bundle (AV bundle). The AV node may not always be present in different avian species and seems to differ in location and contents between species. In this study, the anatomy and histology of the AV node were studied five female adult ostriches (Struthio camelus). Routine paraffin sectioning and transmission electron microscopic method were performed. The study showed that in the ostrich, the AV node is located in the endocardium of the atrial surface of the right atrioventricular valve adjacent to the fibrous ring. The parenchyma of the AV node is formed by small specialized muscle fibres that are spread within a loose connective tissue network. The AV node is not covered by a connective tissue sheath and some arterioles are present. Nerve fibres are seen related to the node. Ultrastructurally, they stain lighter and contain fewer organized myofibrils than usual myocardial cells. The myofibril bundles run parallel to one another and have interspersed mitochondria, which display distinct cristae. The cells have a large euchromatic nucleus with a clear perinuclear area, and they connected by desmosomes. The ostrich is, thus, one of the birds that have the AV node, whose position varies from the other birds.

Spectroscopic studies of the human heart conduction system ex vivo: implication for optical visualization.

Fluorescence excitation and emission spectra of the heart tissues specimens have been measured ex vivo with the aim of finding out the optical differences characteristic for the human heart conduction system (the His bundle) and ventricular myocardium. The optimal conditions enhancing the spectral differences between the His bundle and myocardium were found by recording the fluorescence signal in the range from 420 nm to 465 nm under the excitation at wavelengths starting from 320 nm to 370 nm. In addition, the spectral differences between the His bundle and the connective tissue, which is often present in the heart, could be displayed by comparing the ratios of fluorescence intensities being measured at above 460 nm under the preferred excitation of elastin and collagen. The left and right branches of the His bundle were visualized ex vivo in the interventricular septum of the human heart under illumination at 366 nm.

[Ultrastructure of the heart conduction system in diphtheria]. / Ul'trastruktura provodiashchei sistemy serdtsa pri difterii.

Cytoarchitectonics and ultrastructure of the node and conducting myocardiocytes of the conducting system of the heart and surrounding blood capillaries and nervous fibers are described. In diphtheria, metabolic and destructive changes were found in the node myocytes, conducting myocardiocytes and surrounding structures. The most noticeable changes were seen in the left pedicle of the atrioventricular fascicle and later others components of the conducting heart system. These changes are more pronounced in clear cells of conducting myocardiocytes as compared to the node myocytes.

The elusive extracellular AV nodal potential: studies from the canine heart, ex vivo.

Various forms of extracellular recordings from the AV node (AVN) have been reported. However, lack of consistent validation have precluded the use of such recordings in experimental and clinical studies. In 14 Langendorff perfused dog hearts, the triangle of Koch (TOK) was exposed and an octapolar electrode catheter (2 mm rings, 2 mm spacing) was inserted under the endocardium so that the bipolar pairs recorded electrograms from the apex to the base of the TOK. All recording were filtered between 0.05 and 250 Hz, except for a His bundle (Hb) recording (30-250 Hz) made from another bipolar electrode catheter placed in the aortic root. Transmembrane action potentials (AP) were recorded close to the sites of extracellular electrograms. Pin electrodes at the periphery of the bath were arranged to register two ECG leads from the volume conductor. During recovery of electrical activity 11 of 14 preparations developed a junctional rhythm that initially manifested only an AV nodal extracellular and corresponding intracellular AV nodal potentials followed gradually by conduction to the Hb and ventricles but no retrograde atrial activation; 3 preparations initially produced Hb rhythms based on extracellular and transmembrane AP recordings from the AVN and Hb. The amplitude and duration of the AVN extracellular potentials (average: 97 +/- 26 microV and 92 +/- 25 msec, respectively) during AVN rhythms, significantly differed from those during atrial pacing (262 +/- 185 microV and 78 +/- 26 msec, p < 0.05). Histologic sections of the sites underlying the electrodes recording AVN potentials showed AVN tissue throughout. We conclude that extracellular AV nodal potentials are independent waveforms with specific qualitative and quantitative characteristics that distinguish them from adjacent atrial, transitional, Hb or ventricular potentials.

LocaLisa: new technique for real-time 3-dimensional localization of regular intracardiac electrodes.

BACKGROUND: Estimation of the 3-dimensional (3D) position of ablation electrodes from fluoroscopic images is inadequate if a systematic lesion pattern is required in the treatment of complex arrhythmogenic substrates. METHODS AND RESULTS: We developed a new technique for online 3D localization of intracardiac electrodes. Regular catheter electrodes are used as sensors for a high-frequency transthoracic electrical field, which is applied via standard skin electrodes. We investigated localization accuracy within the right atrium, right ventricle, and left ventricle by comparing measured and true interelectrode distances of a decapolar catheter. Long-term stability was analyzed by localization of the most proximal His bundle before and after slow pathway ablation. Electrogram recordings were unaffected by the applied electrical field. Localization data from 3 catheter positions, widely distributed within the right atrium, right ventricle, or left ventricle, were analyzed in 10 patients per group. The relationship between measured and true electrode positions was highly linear, with an average correlation coefficient of 0.996, 0.997, and 0.999 for the right atrium, right ventricle, and left ventricle, respectively. Localization accuracy was better than 2 mm, with an additional scaling error of 8% to 14%. After 2 hours, localization of the proximal His bundle was reproducible within 1.4+/-1.1 mm. CONCLUSIONS: This new technique enables accurate and reproducible real-time localization of electrode positions in cardiac mapping and ablation procedures. Its application does not distort the quality of electrograms and can be applied to any electrode catheter.

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).

Immunoelectron microscopic localization of HNK-1 in the embryonic rat heart.

To confirm the role of HNK-1 in conduction tissue, the ultrastructural localization of monoclonal antibody HNK-1 was analyzed in developing rat hearts at embryonal day 14.5 by immunoelectron microscopic labeling procedures with post-embedding immunogold staining. Tissue sections in different planes containing the sino-atrial (SA) node, atrio-ventricular (AV) node and His bundle were used to demonstrate HNK-1. Immunogold labeling was detected on the cell surfaces and in the extracellular matrices of cells that had features common to conduction tissue cells. Non-specialized contractile myocytes were not labeled by this antibody. Furthermore, immunogold labeling was more prominent in wide intracellular spaces than in narrow intercellular spaces, and rarely observed in cell-cell contact regions. The cell surfaces and extracellular matrices of mesenchymal cells in the endocardial cushion, which contacts the His bundle, were also positive, suggesting the involvement of tract formation to the AV node. These findings may indicate that HNK-1 plays an important role in cell-cell adhesion processes both temporally and spatially in the developing conduction tissue. It was concluded, therefore, that HNK-1 is a suitable marker of the embryonic heart conduction system and might be useful in analyzing anomalous conduction systems, as in congenital heart disease.

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.

The atrioventricular node of the golden hamster (Mesocricetus auratus): a light and electron microscopic study including immunocytochemistry.

The atrioventricular (AV) node of the golden hamster is situated unusually high in the interatrial septum when compared to other species such as the rat. 2 main cell types, characterized by electron-lucent or electron-dense cytoplasm respectively, are found in the node; although both types contain numerous myofilaments these are irregularly arranged and sarcomeric banding is poor. A third variety comparising transitional cells, with features intermediate between the main nodal cells and general atrial myocardial cells, are found at the periphery of the node. Similar electron-lucent and electron-dense cells are also found in the bundle and the mean diameter of bundle cells increases as one passes from the node to the bundle bifurcation. In the node, specific heart granules (SHG) identified by ANP-28 immunoreactivity are found only in transitional cells and even here they are very sparse, unlike general atrial myocytes in which they are plentiful. Numerous nerve varicosities are present throughout the node and bundle and 5-hydroxydopamine (5-OHDA) labelling demonstrates that most of them have features of either noradrenergic or cholinergic terminals; a few non-cholinergic, non-adrenergic varicosities are also present.

Morphological study of sarcoplasmic reticulum in the atrioventricular node and bundle cells in guinea pig hearts.

The osmium-ferrocyanide method for staining of the sarcoplasmic reticulum (SR) was used for a morphological investigation of the various components of the SR in the atrioventricular node and bundle (AVNB) cells of guinea pig hearts. On the basis of light microscopic observations, the AVNB tissue in guinea pig hearts can be divided into five regions: atrionodal junction, midnode, proximal bundle, distal bundle, and bundle branches. Electron microscopic observations revealed two types of junctional SR (j-SR) saccules in the cells from all the regions of AVNB tissue. One is similar to that seen in the working cardiac cells, i.e., flattened saccules with junctional granules. The second type is dilated and contains electron-dense granular material throughout its lumen. The flattened type is seen more often than the dilated type in atrionodal junctional cells and midnode cells, whereas the dilated type occurs more often in distal bundle cells and bundle branch cells. In most cells from the atrionodal junction and midnode regions, the j-SR saccules are apposed more often to sarcolemmal areas associated with nonspecialized regions of intercellular junctions than to other sarcolemmal areas. This distribution was not found in the distal bundle and bundle branch cells. Free SR tubules around the myofilament bundles are poorly developed in the midnode cells, generally in accord with the extent of development of myofibrils. Z-tubules are found in cells from all regions but are poorly developed in midnode cells. Corbular SR vesicles are found in cells from all the regions of AVNB tissues but are rare in midnode cells. Thus, each of the regions in the AVNB tissue has a different, characteristic distribution of SR components. Because of their possible relationship to the regulation of the intracellular concentrations of calcium, these differences in SR morphology may contribute to the diverse physiological properties of the different regions of the AV node and bundle.

The distribution of sympathetic nerve fibres in the AV node and AV bundle of the bovine heart.

The sympathetic nervous system has important effects on the properties of the heart, including the conduction of the impulse. However, it is not known how this nervous system is distributed in the atrioventricular (AV) bundle, which together with the AV node constitutes the only conduction pathway between the atria and ventricles in normal hearts. Therefore, in the present study the adrenergic innervation in the bovine AV node/AV bundle was examined by use of the glyoxylic acid induced method for histofluorescence demonstration of catecholamines. Acetylcholinesterase (AChE) histochemistry was also used. It was found that the AChE-positive nerve fascicles in these regions partly contain sympathetic nerve fibres, that sympathetic nerve fibres occur in the proximity of some of the ganglionic cells that occur outside the AV node/AV bundle, that the arteries supplying AV bundle tissue as well as AV nodal tissue have perivascular plexuses of sympathetic nerve fibres, and that there is a substantial number of sympathetic nerve fibres outside Purkinje fibre bundle surfaces. The observations give new insight into the question of the distribution of the sympathetic nerves in the AV bundle in relation to the distribution of these nerves in the AV node. Possible functional implications of the observations are discussed.

Functional morphology of the conduction system and the myocardium in the sheep heart as revealed by scanning and transmission electron microscopic analyses.

The pacemaker, Purkinje system and myocardium of the sheep heart were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In the case of SEM, the heart tissues were subjected to chemical digestion procedures. The nodal cells in both the sinoatrial (SA) node and atrioventricular (AV) node were small in size and contained few nexuses with poor sarcoplasmic reticulum and myofibrillar development. These nodal cells were spindle-shaped and their ends often showed ramifications. In addition, the strands of nodal cells in the central part of the AV node were considerably compact and connected with neighboring strands to form a complicated three dimensional architecture. The muscle cells in the common bundle and Purkinje system were cuboidal or oval in shape and were broader and shorter than the working cardiac muscle cells. They had abundant nexuses, but exhibited poor sarcoplasmic reticulum and myofibrillar development. Three-dimensionally, the Purkinje strands formed a delicate network resembling a fishing-net. The atrial and ventricular myocardium consisted of long cylindrical muscle cells which often bifurcated and connected with neighboring cells. These cells had abundant nexuses, rich sarcoplasmic reticulum and well-developed myofibrils. This report discusses such morphological findings in correlation with their physiological properties.

The structure of the atrioventricular conducting system in the avian heart.

The atrioventricular conduction system in three avian species has been studied by light and electron microscopy. A morphologically definable atrioventricular node was not found in any of these. The atrioventricular bundle is a well-defined structure, the proximal portion of which is in direct continuity with the atrioventricular ring, located in the arterial sheet of the muscular valve of the right atrioventricular opening. In the zone of transition between atrioventricular ring and bundle the compactness of the bundle is loosened, but the fibers do not establish continuity with the atrial fibers. The ring consists of Purkinje-like fibers, 10-15 microns in diameter, and (peripherally) small 3-5-microns-diameter junctional fibers which are in continuity with the common atrial fibers. In the muscular atrioventricular valve the fibers of the ring are insulated from the ventricular myocardium by a connective tissue sheet of the annulus fibrosus. It is suggested that in the avian heart the atrioventricular ring may fulfill a role similar to that of the atrioventricular node of mammals.

Morphologic alterations in the long Q-T syndrome. Light and electron microscopic observations in the conduction system and in sympathetic trunks.

Post-mortem examinations of 2 patients with long Q-T syndrome revealed marked focal fibrosis and lipomatosis of the conduction system as well as focal round cell ganglionitis of both sympathetic trunks. The patients, young women of different nationality displaying syncopal attacks and a long Q-T interval in the ECG died suddenly of ventricular arrhythmias. A family study of one of them revealed Q-T prolongation in 4 generations. The ultrastructure of the conductive tissue and the ventricular myocardium showed no specific alterations indicative of a primary metabolic defect. Ganglionitis of the sympathetic trunks has not yet been reported in the long Q-T syndrome. Although the etiology of the inflammatory changes is uncertain a chronic viral infection, noninfectious toxic alterations or an autoimmunopathy are among the plausible causes discussed.

Scanning electron microscopy of the canine atrioventricular bundle and moderator band.

The atrioventricular (AV) bundle and the moderator band in the canine heart were examined by scanning electron microscopy. The AV bundle and the moderator band both were comprised of large, cylindrically shaped cells. These cells were highly organized into bundles, with minimal lateral communication between bundles. There was extensive cell-to-cell communication between cells within a bundle. The end branching of individual cells was prominent, with some interbundle communication. These results are discussed in relationship to the electrophysiologic properties of the AV bundle and the conduction velocity.

Scanning electron microscopy of the human myocardium.

Owing to the impossibility of fixation, in the human being, by means of perfusion in vivo and the rare availability of fresh comparatively intact hearts resected at transplantation, the possibility was explored of using post-mortem material in instances where the autopsies were performed within 2 hours of demise of the patients. Four cases were examined: 2 normal adult hearts and 2 hearts of hypertensive patients in the stage of decompensation. The hearts were fixed by means of perfusion with 2.5% glutaraldehyde in phosphate buffer, pH 7.4. Contiguous blocks were taken for light microscopy (LM) and scanning electron microscopy (SEM) from the left ventricular free wall of all 4 cases and of the impulse conducting system of the heart in the 2 normal hearts. The SEM material was processed by the osmium-thiocarbohydrazide-osmium method (O-T-O). There was good correlation between the LM and SEM findings. The left ventricular blocks were sectioned in the transverse axis and SEM showed a step-wise transection of the myofibrils. Z and M bands, mitochondria and myofilaments were identified a ultrastructural magnifications. The difference in vascularity between normal and hypertensive myocardium and the presence of para-arterial fibrosis in the latter were demonstrated by SEM. The SEM study of the impulse conducting system of the heart included the sinoatrial (SA) and atrioventricular (AV) nodes and the penetrating portion of the atrioventricular (AV) bundle of His. Characteristic pacemaker or "P" cells were identified in the nodes.

Ultrastructure of the human atrioventricular conduction tissues.

The ultrastructure of the human atrioventricular conduction tissue has been studied by obtaining material from recipient hearts at transplant operations. The hearts were dissected immediately after surgical removal in order to expose the conduction system, and tissue samples were taken directly from the atrioventricular node, the penetrating bundle, the branching bundle, and both bundle branches. Examination with the electron microscope showed that the entire atrioventricular system throughout its length was composed of a spectrum of cells which ranged widely in size and in myofibril content from slim cells resembling cardiac muscle and packed with myofibrils to wide 'empty' cells containing relatively few myofibrils. The cells were polymorphic, and many branched with the branches varying greatly in width. Transverse junctions between cells or between their branches were made by intercalated discs. Lateral connections between cells were extremely rare; they were made by desmosomes only. Nerves were present throughout the axis. The striking features of the atrioventricular conduction system as a whole were firstly that the constituent cells were so widely heterogeneous as to defy any classification into cell types, and secondly that totally dissimilar cells established direct continuity by means of intercalated discs.

[Cells of the conduction pathways of the human bundles of His. Histoenzymologic and ultrastructural study]. / Les cellules des voies de conduction dans le faisceau de His humain. Etude histoenzymologique et ultrastructurale.

A human His bundle was studied two hours after death by histoenzymological techniques and electron microscopy. The pathway had a much higher cholinesterase activity than the working myocardium, due to its richness in nerve endings: this was confirmed by electron microscopy which also distinguished "common" contractile cells (working cells) from P type and "intermediary" cells; those were by far the most common, presenting an ultrastructure identical to that of the Purkinje cells, classically described in the bundle branches alone. These findings and the unique longitudinal architectural organisation of the His bundle, confirm the studies of JAMES and may explain the rapidity of conduction in this structure.

[Electron microscopic study of the innervation of the heart conduction system in rats]. / Elektronno-mikroskopicheskoe issledovanie innervatsii provodiashchei sistemy serdtsa krysy.

A systemic quantitative electron microscopic analysis on innervation of the sinus node, the atrioventricular node, the bundle of His and its pedicles within the interventricular septum has been performed in intact hearts of mature rats. The data have been obtained on the size of nonmyelinated and myelinated nerve fibres, efferent and afferent terminals within different parts of the cardiac conductive system, their interconnection with specialized cardiomyocytes have been described. Application of certain methods for electron microscopic investigation on the innervation of mammalian cardiac conductive system has been discussed.