The ultrastructure of calf heart conducting fibers with special reference to nexuses and their distribution

Insights into the interstitium of ventricular myocardium: interstitial Cajal-like cells (ICLC)

We have previously described interstitial Cajal-like cells (ICLC) in human atrial myocardium. Several complementary approaches were used to verify the existence of ICLC in the interstitium of rat or human ventricular myocardium: primary cell cultures, vital stainings (e.g.: methylene blue), traditional stainings (including silver impregnation), phase contrast and non-conventional light microscopy (Epon-embedded semithin sections), transmission electron microscopy (TEM) (serial ultrathin sections), stereology, immunohistochemistry (IHC) and immunofluorescence (IF) with molecular probes. Cardiomyocytes occupy about 75% of rat ventricular myocardium volume. ICLC represent approximately 32% of the number of interstitial cells and the ratio cardiomyocytes/ICLC is about 70/1. In the interstitium, ICLC establish close contacts with nerve fibers, myocytes, blood capillaries and with immunoreactive cells (stromal synapses). ICLC show characteristic cytoplasmic processes, frequently two or three, which are very long (tens up to hundreds of microm), very thin (0.1-0.5 microm thick), with uneven caliber, having dilations, resulting in a moniliform aspect. Gap junctions between such processes can be found. Usually, the dilations are occupied by mitochondria (as revealed by Janus green B and MitoTracker Green FM) and elements of endoplasmic reticulum. Characteristically, some prolongations are flat, with a veil-like appearance, forming a labyrinthic system. ICLC display caveolae (about 1 caveola/ 1 microm cell membrane length, or 2-4% of the relative cytoplasmic volume). Mitochondria and endoplasmic reticulum (rough and smooth) occupy 5-10% and 1-2% of cytoplasmic volume, respectively. IHC revealed positive staining for CD34, EGFR and vimentin and, only in a few cases for CD117. IHC was negative for: desmin, CD57, tau, chymase, tryptase and CD13. IF showed that ventricular ICLC expressed connexin 43. We may speculate that possible ICLC roles might be: intercellular signaling (neurons, myocytes, capillaries etc.) and/or chemomechanical sensors. For pathology, it seems attractive to think that ICLC might participate in the process of cardiac repair/remodeling, arrhythmogenesis and, eventually, sudden death.

Gap junction channels in the cardiovascular system: pharmacological and physiological modulation

Intercellular communication provides the basis for the intact functioning of tissue and for various organs and tissue types in an organism to work together. It is the crucial difference between isolated cells and intact tissue. Cells communicate in various ways with each other; these include the release of chemical transmitters, hormones and mediators as well as direct electrical and chemical intercellular communication via gap junction channels. The gap junction coupling is important for the organization of the tissue as an electrical syncytium and for accurate development. Pharmacological modulation of these channels could be important in the fields of arrhythmogenesis, vasomotion and cell differentiation. In this review, Stefan Dhein outlines the structure, synthesis and function of gap junction channels. Since their physiology and pharmacology are best investigated in the cardiovascular system, the second part of the article focuses on the role of gap junctions in the heart and vasculature, with special emphasis on the regulation of the channels by physiological stimuli such as ions, pH mediators and transjunctional voltage as well as their pharmacological modulation.

The spatial distribution and relative abundance of gap-junctional connexin40 and connexin43 correlate to functional properties of components of the cardiac atrioventricular conduction system

Electrical coupling between heart muscle cells is mediated by specialised regions of sarcolemmal interaction termed gap junctions. In previous work, we have demonstrated that connexin42, a recently identified gap-junctional protein, is present in the specialised conduction tissues of the avian heart. In the present study, the spatial distribution of the mammalian homologue of this protein, connexin40, was examined using immunofluorescence, confocal scanning laser microscopy and quantitative digital image analysis in order to determine whether a parallel distribution occurs in rat. Connexin40 was detected by immunofluorescence in all main components of the atrioventricular conduction system including the atrioventricular node, atrioventricular bundle, and Purkinje fibres. Quantitation revealed that levels of connexin40 immunofluorescence increased along the axis of atrioventricular conduction, rising over 10-fold between atrioventricular node and atrioventricular bundle and a further 10-fold between atrioventricular bundle and Purkinje fibres. Connexin40 and connexin43, the principal gap-junctional protein of the mammalian heart, were co-localised within atrioventricular nodal tissues and Purkinje fibres. By applying a novel photobleach/double-labelling protocol, it was demonstrated that connexin40 and connexin43 are co-localised in precisely the same Purkinje fibre myocytes. A model, integrating data on the spatial distribution and relative abundance of connexin40 and connexin43 in the heart, proposes how myocyte-type-specific patterns of connexin isform expression account for the electrical continuity of cardiac atrioventricular conduction.

Cardiac gap junctions and gap junction-associated vesicles: ultrastructural comparison of in situ negative staining with conventional positive staining

By comparing in situ negative staining of mammalian heart muscle using La(NO3)3 with conventional positive staining by uranium and lead salts, we showed that 1) the membrane area of rat cardiac gap junctions (GJs) at the intercalated disks is threefold to fourfold greater than previously thought; 2) connexon arrays of cardiac GJ are subdivided into microdomains by connexon-free aisles; 3) profiles of GJ-associated vesicles (GJAVs) of plasmalemmal origin (which are present extracellularly and sharply localized at three extracellular sites) are paired to form GJs with each other and with myocyte plasmalemma; 4) some GJAVs contain arrays of assembled connexons; and 5) myocytes contain intracytoplasmic complexes lying within cylindrical or cigar-shaped membranes and consisting of GJs and multiple vesicles apparently dissociating from these GJs.

Intercalated discs of mammalian heart: a review of structure and function

Intercalated discs are exceptionally complex entities, and possess considerable functional significance in terms of the workings of the myocardium. Examination of different species and heart regions indicates that the original histological term has become out-moded; it is likely, however, that all such complexes will continue to fall under the generic heading of 'intercalated discs'. The membranes of the intercalated discs establish specific associations with a variety of intracellular and extracellular structures, as well as with numerous types of proteins and glycoproteins. Characterization of discs and their components has already brought together a large number of research disciplines, including microscopy, cytochemistry, morphometry, cell isolation and culture, cell fractionation, cryogenics, immunology, biochemistry, and electrophysiology. The continued dissection of substance and function of intercalated discs will depend on such interdisciplinary approaches. The intercalated disc component which continues to attract the greatest amount of interest is the so-called gap junction. All indications thus far point to a great deal of inherent lability in the architecture of the gap junction. There is thus considerable potential for the creation of artefact while preserving and observing gap junctions, and this problem will doubtless continue to hamper the understanding of their functions. A question of special interest concerns whether the gap junctions of intercalated discs are required for transfer of electrical excitation between cells, or maintain cell-to-cell adhesion, or in fact subserve both electrical and structural phenomena. Two schools of thought exist with respect to cell-to-cell coupling in the heart. One proposes that low-resistance junctions in the discs mediate electrical coupling, whereas the other supports the possibility of coupling across ordinary high-resistance membranes. Thus the intercalated discs continue to be a source of controversy, just as they have been since they were originally discovered in heart muscle over a century ago.

Differentiation of the atrioventricular node, the atrioventricular bundle and the bundle branches in the bovine heart: an immunohistochemical and enzyme histochemical study

The previous observations of differences between different cardiac regions (ventricular myocardium, atrial myocardium, Purkinje fibre system) with respect to the maturation of the M-line region and the establishment of mature metabolic characteristics, have been extended. It was found that M-line maturation proceeds differently also between different regions of the conduction system. The M-line proteins, myomesin and MM-creatine kinase, were detected earlier, by means of immunohistochemistry, in the AV bundle and bundle branch cells than in the AV node cells. Also, a difference was observed in large foetuses. Striations in the AV node were less evident than in the AV bundle and the bundle branches in sections incubated with antibodies against myomesin as well as against MM-creatine kinase. Using enzyme histochemistry it was observed that the differences in metabolic properties between the AV node, the AV bundle and the bundle branches on the one hand, and the ordinary myocardium on the other, of adult hearts, are not established at the early stages. No clear difference in activity of succinate dehydrogenase was seen between the conduction tissues and the ordinary myocardium in the foetal hearts, while the conduction tissues showed a lower activity in the adult hearts. Furthermore, the pattern of activity of mitochondrial glycerol-3-phosphate dehydrogenase between the conduction tissues and the atrial and ventricular myocardium was quite different in early foetal stages compared with the adult stage.

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.

Histiocytoid cardiomyopathy in infancy: a new hypothesis?

A case of histiocytoid cardiomyopathy in a ten-month-old boy is reported. Histologic and ultrastructural examinations led us to consider the possibility that the disease, rather than being a focal lesion of the common myocardium as it was heretofore considered, might be regarded as a diffuse lesion on the specific myocardium.

A scanning electron microscopic study of the atrioventricular bundle of the ferret

Scanning electron microscopy was used to examine the three-dimensional morphology of atrioventricular (AV) bundle cells in the ferret heart. These cells are organized into fascicles with extensive intercellular contact between cells within a fascicle and less contact between cells of different fascicles. The cell surface shows conspicuous ridges and depressions, as well as smooth regions. The distribution of myofibrils within an AV bundle cell is not uniform. Mitochondria of these cells are oval to round, and randomly distributed throughout the sarcoplasm. Junctions between cells within a fascicle are often complex with processes at the end of one cell interdigitating with those of another. In the proximal AV bundle, located above the anulus fibrosus, a small number of large cells are interspersed among the AV bundle cells.

The plasma membrane of leading pacemaker cells in the rabbit sinus node. A qualitative and quantitative ultrastructural analysis

We studied the fine structure of the plasma membrane of electrophysiologically identified leading pacemaker cells from the rabbit sinus node, using both ultrathin sections of fixed tissue and replicas of freeze-cleaved material. We found that differences exist between sinus node and working myocardial membranes, but these are only quantitative. The caveolae or sarcolemmal invaginations are present in very large numbers; they increase the surface area of the plasma membrane by about 100%. The small macular nexuses that are present represent 0.2% of the membrane surface area. Nexuses are therefore about 10 times less numerous in leading sinus node cells than in working myocardium cells. A single equivalent electrical representation of the sinus node shows, nevertheless, that an appreciable electrical coupling may be expected.

The atrioventricular node and bundle in the ferret heart: a light and quantitative electron microscopic study

The cells of the atrioventricular (AV) junction in the ferret heart were examined using light microscopy, a wax-model reconstruction and quantitative electron microscopy to determine their organization and characteristics. A series of subdivisions of the specialized tissues of the AV junction was apparent at both the light and electron microscopic levels. A transitional zone was observed interposed between the atrial muscle cells and the AV node. The AV node consisted of a coronary sinus portion, a superficial portion and a deep portion. The AV bundle had a segment above the anulus fibrosus, a segment which penetrated the right fibrous trigone, a non-branching segment below the anulus fibrosus and a branched segment. At the ultrastructural level the AV junctional conduction tissues had fewer irregularly oriented myofibrils than did working atrial myocardial cells. T-tubules, present in atrial muscle cells, were not observed in the modified muscle cells of the AV node and bundle. Conventional intercalated discs also were not observed between the cells of the AV node or the AV bundle. Atrial myocardial cells had the highest percentage of the plasma membrane occupied by desmosomes, fasciae adherentes and gap junctions. The AV bundle cells had the highest percentage of appositional surface membrane and a relatively large fraction of plasma membrane occupied by gap junctions. Cells of the superficial portion of the AV node had the smallest percentage of the plasma membrane composed of gap junctions, desmosomes or fasciae adherentes, as well as the smallest fraction of the cell membrane apposed to adjacent cells. The stereological data indicate that the most useful distinguishing characteristic between atrial muscle cells and conduction cells was that a smaller percentage of the conduction cell sarcoplasm was occupied by mitochondria and myofibrils. The most useful characteristics that could be used to differentiate between the regions of the AV junctional conduction tissues were the amounts and types of surface membrane specializations in the respective cell types.

The ventricular conduction system - ultrastructure and function

The ultrastructure and function of the ventricular conducting system in animals are briefly reviewed. As the ventricular conducting cells are resistant to anoxia, an adequate ultrastructural preservation has been obtained in human autopsy material. In a case of cardiomyopathy, abnormalities in the conducting cells are related to current theories of abberant cardiac rhythm. We propose that further knowledge of the human conduction system can be obtained by extended studies of this kind.