On the impulse conducting system of the monkey heart (Macaca mulatta). II. The atrio-ventricular node and bundle

T-tubule profiles in Purkinje fibres of mammalian myocardium

Purkinje (P)-fibres are cardiac myocytes that are specialized for fast conduction of the electrical signal. P-fibres are usually defined as having the following identifying features: lack of T tubules; frequent lateral cell junctions; deep indentations at the intercalated discs level; the CX40 isoforms of gap junction proteins and, in large mammals, paucity of myofibrils and abundance of glycogen. We have examined the ultrastructure of P-fibres in free running P-strands from right and left ventricles of small (mouse and rat) intermediate (rabbit) and large (dog) size mammals focusing on presence and distribution of the T tubules. In contrast with previous studies, we find that P-fibres do have T tubules which form normal dyadic associations with the sarcoplasmic reticulum and that the frequency of tubules varies with the size of the animal. Profiles of T tubules and dyads are present over short segments of individual P-cells flanked by totally T tubule-free segments. It is thought that lack of T tubules in P-cells is necessary to reduce capacitance and thus accelerate action potential spread. This may not be as important in a small heart.

Stereology of the myocardium in Leontopithecus (Lesson, 1840) callitrichidae - primates

Rare morphological features of the Leontopithecus cardiovascular system have been reported in the literature. The samples analyzed in this study came from 33 specimens of Leontopithecus from the collection of the Center of Primatology of Rio de Janeiro-FEEMA (CPRJ-FEEMA). Morphometry and stereological data were obtained from all animals. Adult body weights of L. rosalia were the lowest, the greatest being those of L. chrysopygus caissara; body weights of L. chrysomelas and L. c. chrysopygus were similar and in between those of the two former species. Cardiomyocytes (left ventricular myocardium) were bigger in adults than in infants. The myocardium of L. rosalia showed focal fibrosis, fatty vacuoles, and hyalinization. In L. chrysomelas the myocardium showed areas of fibrosis and presence of mononuclear cells. Fibrosis and areas of congestion were observed in L. c. chrysopygus; areas of disorganization and vascular congestion were found in L. c. caissara. In L. rosalia infants, a greater density of vessels per myocardial area and a greater length density of vessels were observed as compared with those of L. chrysomelas. In adults, L. chrysomelas showed greater density of connective tissue in the myocardium than L. c. chrysopygus and L. c. caissara did. In L. rosalia, cardiomyocyte nuclei had a greater area density than those of the other forms of Leontopithecus. These characteristics may explain the faster development of L. rosalia infants as compared with that of L. chrysomelas and L. c. chrysopygus kept under the same handling conditions at the CPRJ-FEEMA.

Stereology of the myocardium in two species of Callithrix (Callitrichidae, primates)

The majority of studies on cardiac morphology have concentrated on Old World monkeys. Ten marmoset hearts of the genus Callithrix were studied (5 hearts of C. jacchus and 5 of C. penicillata), dissected and fixed in a 10% buffered formaldehyde solution, pH 7.2. Unbiased stereological estimates were obtained from isotropic uniform random sections of the myocardium. For stereological quantification the myocardium was regarded as consisting of cardiac myocytes and interstitium. The volume density (Vv) was determined by point counting. We used the disector method to obtain the numerical density of the cardiac myocytes (Nv[nuclei]). Myocardial stereological differences between the two species of marmoset were not statistically significant. We can therefore determine the pooled Vv[myocyte] and Nv[nuclei] as 68.6% and 41.6% (10(3)/mm3) respectively. The values found for Vv[myocyte] and Nv[nuclei] in the marmoset are respectively about 23.0 and 92.0% greater than those of the baboon, and respectively 57.3 and 45.5% greater than those in man. In contrast, the mean myocyte volume in the marmoset is not significantly different to that of man but is almost 36.0% less than that of the baboon.

Immunohistochemical delineation of the conduction system. II: The atrioventricular node and Purkinje fibers

Using an antibody that reacts specifically with the myocytes of the conduction system of the bovine heart, we have studied the atrioventricular node and the spatial distribution of the Purkinje fibers in the bovine heart. This study was complemented by studying the distribution of the gap junction protein connexin43 in these areas in the bovine heart and in the human heart. The large Purkinje fibers in the bovine heart are arranged in a two-dimensional network underneath the endocardium. At discrete sites, these fibers branch to the Purkinje fibers situated between the muscle bundles of the ventricular mass. These intramural Purkinje fibers are arranged in sheets that form a complex three-dimensional network of lamellas. Contacts with the ventricular myocytes are found throughout the myocardial wall, with the exception of a subepicardial layer of 2-mm thickness, ie, 10% to 15% of the wall thickness. The spatial arrangement of the Purkinje fibers correlates well with data on electrophysiology. Connexin43 was not detected in the myocytes of the atrioventricular node, whereas in the Purkinje fibers of the atrioventricular bundle and of the bundle branches, abundant expression of connexin43 was found in both humans and cows. In the bovine Purkinje fibers, a remarkable subcellular distribution of connexin43 is found: it occupies the entire plasma membrane facing other Purkinje cells but not that facing the surrounding connective tissue. The structural differences in architecture of the ventricular conduction system in humans and cows seems not to result in substantial differences in conduction velocities. However, the Purkinje fiber network in the bovine heart may explain the efficient ventricular excitation, as reflected by the relatively short QRS complex compared with that in the human heart, where intramural Purkinje fibers are not found.

Immunohistochemical identification of Purkinje fibers and transitional cells in a terminal portion of the impulse-conducting system of porcine heart

The ultrastructure of porcine ventricular tissue was studied by electron microscopy and immunocytochemical techniques. Electron-dense specific granules were found in both Purkinje fibers and transitional cells in the ventricular walls, and were positively stained by the immunogold staining method using an antiserum against atrial natriuretic polypeptide (ANP). This suggests that both the Purkinje fibers and transitional cells display the same specific granules as atrial cardiocytes containing ANP. These results demonstrate that Purkinje fibers and two types of transitional cells, in addition to the ordinary ventricular cardiocytes, can be identified in porcine ventricular wall tissue.

Immunohistochemistry and immunocytochemistry of atrial natriuretic polypeptide in porcine heart

Specific granules in porcine hearts were observed in atrial cardiocytes, Purkinje fibers, and transitional cells of the ventricle. These granule-containing cells were immunohistochemically stained by applying the avidin-biotin-peroxidase complex method using an antiserum against alpha-human atrial natriuretic polypeptide (ANP). Immunoelectron microscopy of sections stained using the immunogold method indicated that these specific granules are storage sites of ANP. Furthermore, an impulse-conducting system consisting of immunoreactive cells was clearly distinguishable from nonimmunoreactive ventricular cardiocytes. We conclude that specific-granule-containing cells, i.e., ANP-producing cells, are located in both the atrial walls and the ventricular impulse-conducting system. The presence of ANP may be correlated with impulse conduction.

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.

Immunohistochemical study of atrial natriuretic polypeptides in the embryonic, fetal and neonatal rat heart

An immunohistochemical study of atrial natriuretic polypeptides was carried out on embryonic, fetal and neonatal rat hearts, using an antiserum raised against alpha-human atrial natriuretic polypeptide (alpha-hANP). Weakly immunoreactive cells were seen in both atrial and ventricular walls at 11 days post coitum (pc). After this stage, the immunoreactive cells became more intensely stained in both atrial and ventricular walls. The immunoreactivity during the prenatal period was stronger in the superficial cell layer beneath the endocardium, than in the deep cell layer of the atrial wall. The cells in the trabecular meshwork also had an apparent, but weak, immunoreactivity, which showed a greater intensity in the left ventricle than in the right one. It is suggested that these immunoreactive cells in the ventricle may differentiate, in situ, into the cells of the impulse-conducting system during the further development of the heart.

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.

Function of the atrioventricular node considered on the basis of observed histology and fine structure

From the hearts of 20 young dogs, the region of the atrioventricular (AV) node was studied in vitro utilizing direct perfusion of the AV node artery. Intracellular impalement with microelectrodes provided records of local transmembrane action potentials in all 20 dogs. These were correlated with serial section histologic studies in 7 of the 20 dogs to characterize a smaller region that served as an anatomic guide for electron microscopic examination in 4 other dog hearts. This report describes the variety of specific cells found, including their intracellular content and organization, as well as the nature of their intercellular junctions. On the basis of these findings, AV nodal cells were arbitrarily divided into two types, transitional cells and P cells, although three somewhat different groups of transitional cells were identified. The first group, found principally at the outer margin of the AV node, has long and slender cells that exhibit large profiles of gap junctions or nexuses. The second and third groups of transitional cells, which constitute most of the body of the AV node, are oblong or oval and contain fewer and smaller gap junctions. P cells of the AV node resemble those more abundantly present in the sinus node; they are found principally at the junction of the AV node and His bundle. On the basis of these fine structural features and the histologic organization and transmembrane action potentials observed, clinical and experimental aspects of the local electrophysiologic events are discussed.

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.

A scanning electron microscopic study of the ferret atrioventricular node

The atrioventricular (AV) node and surrounding transitional zone in the ferret heart were examined with scanning electron microscopy. This permitted the direct visualization of the three-dimensional cell shape, as well as intercellular relationships. Transitional cells were roughly cylindrical with extensively branching end processes. These cells were apposed to many adjacent transitional cells. The superficial AV nodal cells were smaller than transitional cells and were fusiform in shape. Most of the cell contacts between superficial AV nodal cells were between overlapping end processes, and there was very little branching of these cells. The deep AV nodal cells were similar to the superficial AV nodal cells, but were slightly larger and also had more cell contacts with adjacent cells. The possible significance of cell sizes and shape and intercellular relationships as they relate to atrioventricular impulse propagation and AV nodal delay are discussed.

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 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.