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.     

Observations on the development of the human atrioventricular node and bundle

This light microscopic study of the cardiac junctional tissues was based on 27 human embryos, fetuses and postnatal hearts. Evidence was presented that superficial and deep portions of the postnatal AV node were derived from two cellular primordia in the posterior wall of the common atrium at the 6-mm stage. The small right primordia was associated with the right venous valve and gave rise to the loosely organized superficial AV node that extended posteriorly to the coronary sinus ostium. A larger left primordia formed the more compact deep subdivision of the AV node located against the anulus fibrosus. In most postnatal hearts the two subdivisions are partially or completely fused to form the adult AV node. Failure of the nodal primordia to fuse during cardiogenesis may result in two separate nodal cell aggregates above the anulus. The present observations provide a rational explanation for the two AV nodal masses described in the literature and an additional specimen that is illustrated in this communication. An AV bundle was first identified in a 13-mm embryo and appeared to be derived from large clear cells of the posterior AV canal. At 25 mm the bundle formed a broad band across the top of the IV septum and continued into both ventricles. At this stage multiple cell strands penetrated the endocardial cushion to connect the AV bundle to the two nodal primordia. Failure of normal fusion between the AV node primordia and AV bundle can result in a variety of junctional anomalies including congenital heart block.     

The ultrastructure of the atrioventricular junctional tissues in the newborn ferret heart

In this study the structure of the atrioventricular (AV) node and bundle in the newborn ferret heart was examined by light and electron microscopy. At the light microscopic level the AV node could be subdivided into deep and superficial portions. Electron microscopy revealed that both superficial and deep AV nodal cells were characterized by a paucity of myofibrils, desmosomes, fasciae adherentes and gap junctions. Deep AV nodal cells, however, had more surface specializations than did superficial AV nodal cells. In both subdivisions the constituent cells were ellipsoid with tapering end-processes. In contrast to the nodal cells, the newborn AV bundle cells were round to ovoid. The AV bundle cells were organized into large fascicles, and there was a high frequency of anastomosing intercommunication between fascicles. These bundle cells had few myofibrils and a high incidence of apposed plasma membrane. The present morphological findings support the concept that there are significant postnatal morphological changes that occur in the region of the AV junction. These results are also consistent with findings in other species that AV nodal conduction time is similar in newborns and adults, while conduction through the AV bundle increases with age.     

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 size and shape and intercellular relationships as they relate to atrioventricular impulse propagation and AV nodal delay are discussed.     

人体心脏房室结、房室环及主动脉后结的巨微解剖

目的解剖分离人体心脏房室结和房室环的结构,阐述它们的形态特征及相互关系。方法通过体视显微镜解剖12例人体心脏的房室结、主动脉后结及房室环,再进行组织学观察,并绘图演示它们的结构关系。结果在二尖瓣环和三尖瓣环靠近冠状窦前缘处分别暴露了左、右房室环(12/12),直径分别为(0.69±0.12)mm、(0.78±0.13)mm。此处的左、右房室环穿行在房室隔内的心房肌与心室肌之间的间隙中,向房室结方向延伸。主动脉后结在主动脉根后方的房间隔中被探查到(7/9),它的后上方的房间隔间隙中有肌纤维与其相连,它的前下方分出左、右房室环,并且此处的左环比右环粗。在中心纤维体后方的心内膜下的深部,主动脉后结与房室结之间有直接的心肌组织连接通路,这条通路有别于另两条通路(左、右房室环)。结论主动脉后结和房室环可通过体视显微镜解剖暴露,主动脉后结与房室结之间有3条通路。     

Morphology of the atrioventricular node,bundle and proximal bundle branches: A study employing computerized reconstruction

The morphology of the human atrioventricular node, atrioventricular bundle and bundle branches is described. A block of tissue bounded by the ostium of the coronary sinus, the pars membranacea, the septal leaflet of the tricuspid valve and the atrial and ventricular septa is removed. This block is then sectioned serially from the right endocardial surface in the frontal plane of the heart. Sectioning in this way produces fewer sections than from techniques previously described. Outlines of the atrioventricular node, atrioventricular bundle and proximal bundle branches are digitally registered and stored in a computer. Three dimensional reconstructions of the structures are then generated by computer and displayed on an oscilloscope so that the entire three dimensional image can be rotated in any plane. Stereoscopic image pairs are produced to assist perception of the shape of the atrioventricular node, bundle and branching patterns of the bundles. This technique is unique in that it describes a method from which a relatively small number of histologic sectionsare generated permitting not only a complete histologic examination, but also a study of the morphology of the area.     

Morphology of the atrioventricular node, bundle and proximal bundle branches: a study employing computerized reconstruction.

The morphology of the human atrioventricular node, atrioventricular bundle and bundle branches is described. A block of tissue bounded by the ostium of the coronary sinus, the pars membranacea, the septal leaflet of the tricuspid valve and the atrial and ventricular septa is removed. The block is then sectioned serially from the right endocardial surface in the frontal plane of the heart. Sectioning in this way produces fewer sections than from techniques previously described. Outlines of the atrioventricular node, atrioventricular bundle and proximal bundle branches are digitally registered and stored in a computer. Three dimensional reconstructions of the structures are then generated by computer and displayed on an oscilloscope so that the entire three dimensional image can be rotated in any plane. Stereoscopic image pairs are produced to assist perception of the shape of the atrioventricular node, bundle and branching patterns of the bundles. This technique is unique in that it describes a method from which a relatively small number of histologic sections are generated permitting not only a complete histologic examination, but also a study of the morphology of the area.     

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 junctonal granules. The second type is dilated and contains electrondense 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.     

Study of the development of the atrioventricular conduction system as a consequence of observing an extra atrioventricular node in the normal heart of a human fetus

We have observed an extra atrioventricular node in the normal heart of a human fetus. It is located in the septal wall of the right atrium, subendocardially, and just where Todaro's tendon leaves this wall to go toward the inferior vena cava valve. In its trajectory, this tendon gives way to a remarkable prominence in the cavity of the right atrium: the sinus band. In order to explain the embryogenesis of this extra atrioventricular node, we have studied the normal development of the atrioventricular specific system and have concluded that the atrioventricular node is formed from a growth and displacement toward the atrium of the primitive atrioventricular specific material, which originates from the myocardium of the posterior wall of the atrioventricular canal. Likewise, during its development, the atrioventricular node keeps in close proximity with the Todaro's tendon. In our view, this accounts for the embryogenesis of the extra atrioventricular node, since a fragment of the atrioventricular node can remain cranial to Todaro's tendon and be displaced by it in a craniodorsal direction. This fragment would then lead to the formation of an extra atrioventricular node like the one present in the heart of the fetus we have examined.     

Intercellular coupling in the atrioventricular node and other tissues of the rabbit heart.

1. A fluorescent tracer dye, sodium fluorescein (mol.wt. 332), was used to assess the relative degree of intercellular coupling in various tissues of the rabbit heart. 2. Dye was injected intracellularly by micro-iontophoresis. Subsequent movement into contiguous cells was monitored by video microscopy. From these data the permeability of the intercellular boundaries was computed. 3. The values of boundary permeability were consistent with those expected from previous studies with tracers whose molecular weights bracketed that of fluorescein. 4. In the atrium, ventricle, Purkinje strands and His bundle, the relative magnitude of the boundary permeability correlated reasonably well with the relative profusity of gap junctional area on the intercalated disk, the latter estimated from published data. 5. The rate of passage of dye between N cells of the atrioventricular, AV, node was at least three orders of magnitude lower than between cells of the other tissues studied; this result is consistent with published reports indicating few gap junctions between cells within the region of slow conduction. 6. Quantitative considerations based on these data indicate that N cells may not be sufficiently well coupled to permit impulse propagation through the AV node by intercellular current flow, alone.     

Complete heart block in the 18p--syndrome. Congenital calcification of the atrioventricular node

Complete heart block exists when the atria and ventricles beat completely independently of each other. It is heterogeneous with respect to pathogenesis. Occasionally, complete heart block may be symptomatic in infancy. Rarely, it is associated with genetic syndromes. Cardiac abnormalities are unusual in the 18p-syndrome. We describe a female stillborn infant who had 18p-syndrome with hydrops fetalis and complete heart block secondary to atrioventricular node calcification.     

Cystic tumor of the atrioventricular node: an unexpected finding in an explanted heart

SummaryWe report herein a unique case of cystic tumor of atrioventricular (AV) node (CTAVN), which, to our knowledge, is the first of its kind diagnosed in an explanted heart specimen and only the fourth diagnosed antemortem. Often, this rare tumor can only be diagnosed by careful gross examination and adequate sampling of AV node region. It is an important differential diagnosis in young patients with syncopal attacks and varying degrees of heart blocks.ContextCTAVN is a rare, benign tumor. Most cases have been reported in young females (mean age, 38 years). Patients typically present with conduction abnormalities including complete heart block leading to sudden cardiac death. Most cases have been identified at autopsy; no cases to our knowledge have been reported in an explanted heart.DesignA 19 year-old female presented to the cardiac transplant clinic for evaluation of severe congestive heart failure felt to be secondary to postpartum cardiomyopathy. The patient's history was significant for congenital heart block requiring placement of a permanent pacemaker at 12 years of age. At the time of this presentation, electrocardiogram revealed second-degree AV block, and two-dimensional echocardiogram showed lipomatous hypertrophy of the interatrial septum. Seven months later, orthotopic cardiac transplantation was performed.ResultsOn gross examination, the explanted heart weighed 500 g and had biventricular dilatation. Histologic sections of left and right ventricle revealed myocyte hypertrophy and interstitial fibrosis consistent with dilated cardiomyopathy. Sections from the AV node showed a lesion with morphological features of CTAVN. It was composed of cysts of varying sizes lined by transitional, cuboidal and squamous epithelium. Some cysts were filled with proteinaceous debris that were periodic acid Schiff-positive and diastase resistant.ConclusionsCTAVN occurs exclusively in the area of the AV node, tricuspid valve, and inferior atrial septum. These lesions are now believed to be endodermal in origin, although mesothelial origin was earlier proposed. We report herein a case of CTAVN, the first of its kind diagnosed in an explanted heart specimen and only the fourth diagnosed antemortem.     

Pathological aspects of radiofrequency catheter ablation of the canine atrioventricular node and bundle of His. With special reference to chronic incomplete atrioventricular block.

Radiofrequency catheter ablation of the atrioventricular (AV) node or bundle of His was performed in 12 adult mongrel dogs. The aim was to create chronic incomplete AV block (first- and second-degree AV block) and to examine the histopathology of the ablated lesions. However, the late electrophysiological results (2-4 weeks follow-up) were various: normal in 2 dogs, mild PR prolongation (less than 50%) in 2 dogs, first-degree AV block (PR prolongation greater than or equal to 50%) in 2 dogs, second-degree AV block in 2 dogs, complete AV block in 4 dogs. The maximally ablated area (%) of the atrioventricular conduction system in serial histologic sections from dogs with these conditions was 69%, 75%, 89.5%, 95% and 99.5%, respectively. The number of intact conduction cells at the maximally ablated site varied from 6 to 30 in the four cases of incomplete AV block. The mean ablated volume (%) of either the AV node or penetrating His bundle correlated roughly with the degree of AV block. The ablated lesions were well demarcated and almost replaced by dense fibrous tissue at 4 weeks. Interruption (3 dogs) or thinning (1 dog) of the endocardial elastic lamellae was detected, in association with endocardial thickening (mean 913 microns). Endocardial thrombi were found in 3 dogs (2 fresh, 1 organized). We conclude that radiofrequency catheter ablation does not cause severe complicated lesions. Several possible conditions for creating chronic incomplete AV block are discussed.     

Characteristics of single cells isolated from the atrioventricular node of the adult guinea-pig heart

To date, data regarding the cellular electrophysiology of the atrioventricular node (AVN) have derived from AVN cells isolated from the rabbit heart. The aim of this study was to characterise for the first time the electrophysiological properties of single cells isolated from the AVN of the guinea-pig heart. Cells were isolated from the AVN region by a combination of enzymatic and mechanical dispersion. Mean (+/-SEM) cell dimensions were 110.8+/-2.3 microm in length by 11.4+/-1.3 micro m in width (n=76 cells). Electrophysiological recordings were made using the whole-cell patch-clamp technique at 37 degrees C. Mean cell capacitance was 25.1+/-0.9 pF (n=43) and mean cell input resistance was 1,377+/-178 Muomega (n=21). Spontaneously active cells exhibited action potentials characteristic of cardiac pacemaker tissue. Under whole-cell voltage clamp, the mean 'zero current' potential was -39.7+/-4.1 mV (n=21). Voltage commands of 500 ms duration to a range of test potentials from a holding potential of -40 mV revealed a number of distinct current components. At potentials positive to -40 mV an early inward current was observed that exhibited a bell-shaped current voltage (I-V) relation, typical of L-type calcium current. A delayed outward current that increased progressively with test potential magnitude was also observed. Analysis of the outward current 'tails' on repolarisation to -40 mV showed this to be comprised of two components with half-maximal activation voltages of -17.2+/-1.8 mV, and +27.1+/-3.6 mV (n=13). 'Transient outward' current appeared absent from guinea-pig AVN cells. Hyperpolarising test pulses activated net inward current, for which three components could be observed: a barium-sensitive (100 microM Ba(2+)) inwardly rectifying current evident at the start of the voltage command and prominent at potentials negative to the diastolic potential range; a time-dependent, hyperpolarisation-activated current, and a residual background current. On repolarisation to -40 mV, a large inward current spike typical of cardiac Na current was observed in some cells. Notably, the following electrophysiological characteristics of guinea-pig AVN cells are distinct from those characteristics previously reported for the rabbit AVN: (1) an absence of transient outward current, (2) the presence of two components of delayed outward current, and (3) the presence of a Ba(2+)-sensitive inwardly rectifying current at negative voltages. The guinea-pig AVN isolated cell preparation may be valuable in providing additional insights into the cellular electrophysiology of this important region of the heart.     

Histological organization of the right and left atrioventricular valves of the chicken heart and their relationship to the atrioventricular purkinje ring and the middle bundle branch

In the avian heart the right and left atrioventricular (AV) valves not only exhibit their own special anatomical characteristics, but they also are in close proximity to the conduction system. The right AV valve is a single, spiral plane of myocardium, in remarkable contrast to the fibrous structure characteristic of the mammalian tricuspid valve. A ring of Purkinje tissue encircles the avian right AV orifice and connects to the muscular valve. The chicken has no crista supraventricularis, its right AV valve serving that function as well as opening and closing the right AV orifice. The left AV valve consists of three leaflets instead of the two typical of mammalian hearts. Its anterior and posterior leaflets are small; its large aortic (medial) leaflet merges with the bases of both the left and noncoronary cusps of the aortic valve by fibrous tissue, resembling that of the mammalian heart. However, unlike in mammals, there is a slim cylinder of continuous myocardium coursing parallel to this fibrous junction. This unusual arc of myocardium in the chicken serves to complete an entire subaortic ring of myocardium and is thus potentially capable of constricting the outflow tract of the chicken's left ventricle. The middle bundle branch connects with both the muscle arch and the AV Purkinje ring. Thus the myocardium in or near both AV valves (and the left ventricular outflow tract) in the chicken heart is so arranged that it may receive direct early activation from the conduction system. ©1993 Wiley-Liss, Inc.