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.     

The development of the atrioventricular node and bundle in the ferret heart

The development of the atrioventricular (AV) node and bundle in the ferret heart was examined at the light microscopic level. The AV node develops from two primordia which were first observed in the posterior wall of the common atrium during the stage when the single heart tube convolutes. During septation of the heart, the AV nodal primordia eventually fuse and come to lie at the base of the interatrial septum. The right AV nodal primordium is located below the attachment of the right venous valve to the interatrial septum. The left AV nodal primordium maintains a position anterior to the prospective ostium of the coronary sinus. At 16 days of gestation, large pale cells were seen in the dorsal AV canal. By 21 days of gestation these AV canal cells have been replaced by AV bundle cells. At this time the bundle is continuous with both nodal primordia. At birth the AV bundle is continuous mainly with the component of the AV node that is derived from the right AV node primordium. The anulus fibrosus begins to undergo the greatest developmental change after the AV node and bundle attain their final position in the AV junction. However, the anulus does not completely separate the atria from the ventricles during the later stages of development nor at birth, so that accessory AV pathways are present in the newborn ferret heart. Both the AV node and the AV bundle also demonstrated continuity with the myocardial cells of the interventricular septum in the neonatal heart. During development there was no evidence that rings of specialized tissues at the junctions of the cardiac chambers give rise to any component of the cardiac conduction system.     

Cytoarchitecture and intercalated disks of the working myocardium and the conduction system in the mammalian heart

Working and specialized cardiac myocytes and their intercalated disks (ID) in the mammalian heart were examined by transmission and scanning electron microscopy. The NaOH/ultrasonication treatment of cardiac tissues resulted in the digestion of collagen fibers and separation of intercellular junctions. Auricular and ventricular myocytes were cylindrical in shape, bifurcated, and connected end-to-end at the ID. The ID in the working myocardium showed a stair-like profile, consisting of steps (plicate segments) and corresponding risers (interplicate segments). The ventricular myocytes had many steps and risers. The steps were filled with numerous finger-like microprojections, including desmosomes, fasciae adherentes, and small gap junctions. The risers showed the smooth surface, including desmosomes and large gap junctions. The cell strands of the sinoatrial node were oriented linearly, while those of the atrioventricular node formed a reticular network. The ID in both nodal cells was underdeveloped, having few microprojections. Myocytes in the His bundle and its branches were arranged in parallel, and Purkinje cell strands formed reticular networks. The ID in the His-Purkinje system was irregular in appearance, and the microprojections were larger in size and smaller in number than those of working myocytes. There were few microprojections in the sheep Purkinje cells. The gap junctions in the conduction system were few or small in size in the nodal tissue, but large in the His-Purkinje system.     

Anatomy of the ferret heart: An animal model for cardiac research

The hearts of 38 black-footed ferrets (Mustela nigripes) were studied with the use of physiologic, microdissection, vascular injection and histologic methods. These animals had a mean heart rate of 265 per minute, a heart weight of 3.7–5.2 gm, and a mean aortic pressure of 139.5 mm Hg. The predominant left coronary artery supplied usually both the SA and AV nodes, as well as the AV bundle, bundle branches and most of the ventricular myocardium. The cells of a well differentiated cardiac conduction system increase in cytoplasmic diameter from the SA node to the distal bundle branches. A cartilaginous right fibrous trigone and thick anulus fibrosus form useful landmarks for delineating AV node and AV bundle relationships. Small size, discrete nodal masses and a unique coronary arterial pattern make this heart an ideal model for histochemical, ultrastructural, electrophysiologic and pathologic circulation research.     

The AV junction region of the heart: a comprehensive study correlating gross anatomy and direct three-dimensional analysis. Part I. Architecture and topography.

There is little detailed knowledge of the architecture of the AV junction region, the cytoarchitecture of the AV node or of its atrial connections. In the present study, the gross anatomy and topography of intracardiac structures in 21 adult canine hearts were photographically compared in whole and dissected hearts and tissue blocks and serial histologic sections made in three orthogonal planes. There are seven major new findings: 1) A coronary sinus fossa exists at the crux of the heart. It separates the right medial atrial wall (MAW) superoposterior region from the left atrium, its floor is the coronary sinus, and it carries the medial atrionodal bundle and proximal AV bundle on its right wall. 2) The posterior MAW forms two isolated bridges of myocardium as it surrounds the coronary sinus ostium, is isolated from the sinus venarum with crista terminalis and interatrial septum-by the floor of the inferior vena cava, and the narrow bridges link the posterior atrial wall to the mid MAW. 3) The tendon of Todaro has both epicardial and endocardial exposures, terminates in the superoposterior MAW and its medial aspect is adjacent sequentially to the medial atrionodal bundle and proximal AV bundle. 4) Only ordinary myocardium contacts the anulus fibrosus. 5) The ventricular septum's shoulder is humped shape posteriorly, is completely overlaid by anular myocardium and the medial leaflet and is joined by struts of papillary muscle. 6) The membranous septum joins the anterior ventricular septum to the crista supraventricularis, forms part of the posterior noncoronary and right aortic valve sinus walls and encases the right bundle branch. 7) The specialized conduction tissues, the superior, medial and lateral atrionodal bundles, the proximal AV bundle, AV node, distal AV bundle and right bundle branch are subjacent to MAW epicardium outside the right atrium, share regular intracardiac relationships with topographic landmarks and the medial atrionodal bundle, terminal superior atrionodal bundle, the proximal AV bundle and AV node are aligned to the medial leg of Koch's triangle. Thus, atrial myocardium of the AV junction region is that of the MAW. The floor of the inferior vena cava forms a natural barrier to impulse transmission along the full extent of the posterior MAW. The specialized tissues are outside of the MAW. Anatomic landmarks form reliable topographic landmarks for the specialized AV junction region tissues. A knowledge of the association of the specialized conduction tissues with specific regions of the MAW is useful in localizing the tissues and along with the coronary sinus fossa provides several extracardiac approaches.     

The AV junction region of the heart: A comprehensive study correlating gross anatomy and direct three‐dimensional analysis. Part I. Architecture and topography

There is little detailed knowledge of the architecture of the AV junction region, the cytoarchitecture of the AV node or of its atrial connections. In the present study, the gross anatomy and topography of intracardiac structures in 21 adult canine hearts were photographically compared in whole and dissected hearts and tissue blocks and serial histologic sections made in three orthogonal planes. There are seven major new findings: 1) A coronary sinus fossa exists at the crux of the heart. It separates the right medial atrial wall (MAW) superoposterior region from the left atrium, its floor is the coronary sinus, and it carries the medial atrionodal bundle and proximal AV bundle on its right wall. 2) The posterior MAW forms two isolated bridges of myocardium as it surrounds the coronary sinus ostium, is isolated from the sinus venarum with crista terminalis and interatrial septum—by the floor of the inferior vena cava, and the narrow bridges link the posterior atrial wall to the mid MAW. 3) The tendon of Todaro has both epicardial and endocardial exposures, terminates in the superoposterior MAW and its medial aspect is adjacent sequentially to the medial atrionodal bundle and proximal AV bundle. 4) Only ordinary myocardium contacts the anulus fibrosus. 5) The ventricular septum's shoulder is humped shape posteriorly, is completely overlaid by anular myocardium and the medial leaflet and is joined by struts of papillary muscle. 6) The membranous septum joins the anterior ventricular septum to the crista supraventricularis, forms part of the posterior noncoronary and right aortic valve sinus walls and encases the right bundle branch. 7) The specialized conduction tissues, the superior, medial and lateral atrionodal bundles, the proximal AV bundle, AV node, distal AV bundle and right bundle branch are subjacent to MAW epicardium outside the right atrium, share regular intracardiac relationships with topographic landmarks and the medial atrionodal bundle, terminal superior atrionodal bundle, the proximal AV bundle and AV node are aligned to the medial leg of Koch's triangle. Thus, atrial myocardium of the AV junction region is that of the MAW. The floor of the inferior vena cava forms a natural barrier to impulse transmission along the full extent of the posterior MAW. The specialized tissues are outside of the MAW. Anatomic landmarks form reliable topographic landmarks for the specialized AV junction region tissues. A knowledge of the association of the specialized conduction tissues with specific regions of the MAW is useful in localizing the tissues and along with the coronary sinus fossa provides several extracardiac approaches. Anat Rec 256:49–63, 1999. © 1999 Wiley‐Liss, Inc.     

Atrioventricular node and purkinje fibers of the guinea pig heart

The ultrastructure of the atrioventricular node and Purkinje fibers of the guinea pig heart was studied. The atrioventricular node consists of slender, irregularly shaped muscle cells which are arranged in a complex network with frequent interdigitation. Nodal cells contact with each other by means of many desmosomes (maculae adherentes) and rare intercalated discs that contain short nexal regions (gap junctions). The Purkinje fibers are much larger and show less cellular interdigitation than the fibers of the atrioventricular node. Desmosomes are infrequent but there are many intercalated discs with rather a long nexus between the Purkinje fibers. The T-system is not present in the atrioventricular node or in the Purkinje fibers. The most pronounced morphological differences between the atrioventricular node and Purkinje fibers are the size of the cells and the mode of intercellular connection, which may be related to different conduction velocities in these two regions.     

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.     

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. © 1993 Wiley-Liss, Inc.     

Intercellular junctions in the full term human placenta

Summary Intercellular junctions within the villous stroma and the cytotrophoblastic layer of the human full term placenta were investigated using thin sectioning and freeze-fracturing. Numerous maculae adherentes (desmosomes) were found between the cytotrophoblast cells and the syncytiotrophoblast. This junction type was also seen connecting adjacent cytotrophoblast cells. Large gap junctions were frequently observed in contact areas of perikarya or at processes of adjacent fibroblasts. They often exhibited a peculiar pattern of their particles on the P-face of the membrane. Small rows of junctional particles were found on the P-faces of interconnected smooth muscle cells and gap junctions frequently bridged myoendothelial and interendothelial contact zones.The significance of the junctional complexes is discussed in relation to functional systems within the villous stroma of the human full term placenta.     

Morphology and electrophysiology of the mammalian atrioventricular node

The AV node of those mammalian species in which it has been thoroughly investigated (rabbit, ferret, and humans) consists of various cell types: transitional cells, midnodal (or typical nodal cells), lower nodal cells, and cells of the AV bundle. There are at least two inputs to the AV node, a posterior one via the crista terminalis and an anterior one via the interatrial septum, where atrial fibers gradually merge with transitional cells. The role of a possible third input from the left atrium has not been investigated. Since the transition from atrial fibers to nodal fibers is gradual, it is very difficult to define the "beginning" of the AV node, and gross measurements of AV nodal length may be misleading. Histologically, the "end" of the AV node is equally difficult to define. At the site where macroscopically the AV node ends, at the point where the AV bundle penetrates into the membranous septum, typical nodal cells intermingle with His bundle cells. A conspicuous feature, found in all species studied, is the paucity of junctional complexes, most marked in the midnodal area. The functional counterpart of this is an increased coupling resistance between nodal cells. An electrophysiological classification of the AV nodal area, based on transmembrane action potential characteristics during various imposed atrial rhythms (rapid pacing, trains of premature impulses), into AN (including ANCO and ANL), N, and NH zones has been described by various authors for the rabbit heart. In those studies in which activation patterns, transmembrane potential characteristics, and histology have been compared, a good correlation has been found between AN and transitional cells, N cells and the area where transitional cells and cells of the beginning of the AV bundle merge with midnodal cells, and NH cells and cells of the AV bundle. Dead-end pathways correspond to the posterior extension of the bundle of lower nodal cells and to anterior overlay fibers. During propagation of a normal sinus beat, activation of the AN zone accounts for at least 25% of conduction time from atrium to His bundle, the small N zone being the main source of AV nodal delay. Cycle length-dependent conduction delay is localized in the N zone. Conduction block of premature atrial impulses can occur both in the N zone and in the AN zone, depending on the degree of prematurity. Several factors determining AV nodal conduction delay have been identified.(ABSTRACT TRUNCATED AT 400 WORDS)     

Electron microscopic studies on the myotomes of larval lamprey,Lampetra japonica

Myotomes of the caudal one-third of the body of 26-day-old larval lampreys, Lampetra japonica, were studied by electron microscopy. Each myotome consists of horizontally stacked muscle lamellae. The myotomes are covered laterally by a single layer of flattened cells called here “lateral cells,” and the other aspect is covered by an external lamina. The myotomes are mid-segmentally innervated. Each muscle lamella usually contains two single cortical layers of myofibrils along the dorsal and ventral sarcolemma with a nucleus and mitochondria interposed between two layers. Numerous peripheral couplings are observed with relatively less developed triads. There are no membrane specializations to connect adjacent muscle lamellae within a myotome. Intermyotomal junctions are, however, noted between tips of cytoplasmic processes of muscle lamellae of adjoining myotomes. They resemble tight or gap junctions. No myofibrils are present in these cytoplasmic processes. Myotendinous junctions, with “terminal couplings” (Nakao, 1975), are seen under development at the myoseptal ends of muscle lamellae. Lateral cells contain only ordinary organelles and no special structures such as myofibrils are found in the cytoplasm. They are connected to each other and to muscle lamellae by primitive desmosomes. They generally have no external lamina investment.     

Ultrastructure of the atrioventricular junctional area in the heart of Molossus molossus Pallas 1766 (Chiroptera: Molossidae)

The atrioventricular junctional area (AVJA) consists of a group of structures that connects the atrial and ventricular myocardium. Five hearts of an insect-eating bat were studied in light and transmission electron microscopy. In M. molossus, the AVJA consists in a mass of muscle fibers intermingled with variable amount of connective tissue and blood vessels surrounded by the adjacent myocardium and the attachment of the right atrioventricular and aortic valves in the fibrous skeleton. In light microscopy, conducting cells of the AV node and bundle can be distinguished from working cells: smaller size, paler staining reaction and the presence of e sheath of connective tissue surrounding each cell (largely composition by type I collagen fibers). Three cell types are observed in the AVJA. Nodal cells are irregular with few cytoplasmic organelles and several slender sarcolemmal modifications. Myofibrils are sparse and not clearly observable. Transitional cells are spindle-shaped and grouped together into bundles. The cytoplasm, poor in glycogen, has scarce electron-density and myofibrils organized into sarcomeres. Caveolae is observed randomly distributed at the periphery of the cell. The AV bundle cells are elongated with clusters of myofibrils organized in the periphery and a glycogen free area around the nucleus. Ventricular cells are bigger than the atrial ones and show well-developed myofibrils in alternated rows with mitochondria. Lipid droplets are seen near mitochondria and glycogen granules. Intercalated discs and T-tubules are found in working cells but not in conducting ones. The fibrous skeleton has collagen fibers intercalated with fibroblasts.     

Intracytoplasmic Junctions in Cardiac Muscle Cells

Junctional structures formed by two parts of the plasma membrane of the same cardiac muscle cell were observed in ventricular myocardium of: a) patients with neoplasms, aortic valvular disease or idiopathic hypertrophic subaortic stenosis and b) dogs subjected to prolonged normothermic anoxic cardiac arrest. Most of these structures had features of desmosomes; other, more complex structures had components with features of desmosomes, fasciae adherentes and nexuses, and, therefore, resembled intercalated discs. These intracytoplasmic junctions were localized to: a) the peripheral cytoplasm at the sides or ends of cells, b) narrow invaginations of plasma membranes, c) narrow zones of deep, broad plasmalemmal invaginations and d) narrow branches of T tubules. In patients with idiopathic hypertrophic subaortic stenosis or aortic valvular disease and in the dogs subjected to anoxic cardiac arrest, intracytoplasmic junctions were observed in hypertrophied or degenerated muscle cells which were located in areas of fibrosis and which showed loss of contact with adjacent cells. In patients with neoplasms, intracyto-plasmic junctions were found in degenerated cells which were located in areas of interstitial edema and which also showed loss of contact with adjacent cells. Our observations suggest that remodeling of cell surfaces following loss of intercellular contact is the most likely mechanism of formation of intracytoplasmic junctions.     

Cellular architecture of the sheep myocardium and heart conduction system as revealed with silver-impregnation method]

Morphological studies were carried out to delineate the cellular architecture of the myocardium and heart conduction system in adult sheep using the silver-impregnation method. Reticular fibers were heavily stained in a deep black color, while collagen fibers were less intensely silver-stained in a brownish color. Individual working myocardial cells were ensheathed by thin reticular fibers and showed a polygonal form, and were connected with adjacent cells mainly end-to-end and sometimes side-to-side. Thick collagen septa were distributed between masses of myocardial cells. The nodal cells in both the sinoatrial (SA) node and atrioventricular (AV) node were spindle shaped, smaller in size than myocardial cells, and surrounded by thin reticular fibers. The bundles of SA node cells were linearly oriented, while those of AV node cells formed a reticular pattern. The cells in the His bundle and Purkinje system were oval in shape, larger in size than myocardial cells and formed the strands composed of 4-8 cells. Each stand was surrounded by thick reticular fibers. These cells had both end-to-end and side-to-side contacts. Purkinje cells were followed by transitional cells which were in contact with myocardial cells. On the other hand, the reticular fibers and collagen fibers showed characteristic structures at the different portion of heart. The present study discusses the topographical relationship between cardiac muscle cells containing the conduction system and the connective tissue sheaths surrounding muscle cells.     

A method of study of the pathology of the conduction system for electrocardiographic and his bundle electrogram correlations

There have been advances in electrophysiology which have necessitated a more thorough semi-quantitative analysis of the entire conduction system to yield data useful for correlation purposes. Thus an attempt is made to modify and expand our previous method of studying the conduction system pathologically. This method thus includes the study of the sinoatrial (SA) node and its approaches, the atrial preferential pathways, the approaches to the atrioventricular (AV) node, the AV node, the penetrating and branching portions of the AV bundle, the bundle branches, the peripheral Purkinje nets, and the remainder of the atrial and ventricular myocardium. The SA node and its approaches are studied in a longitudinal manner. This gives a better insight into the pathologic changes than does a study in the transverse direction. The approaches to the AV node, bundle and bundle branches are studied in an oblique manner, rather than horizontally apicalward, or from the posterior to the anterior septal region. The horizontal manner does not give sufficient sampling of the AV node and bundle unless complete serial sections are made. Sectioning from the posterior to the anterior septal wall makes difficult an evaluation of the right bundle branch. In conduction system correlation with Wolff-Parkinson-White and Lown-Ganong-Levine syndromes complete serial sectioning of both AV rims is advisable. Where complete serial sectioning is impossible in large adult hearts, retaining every fifth section may be permissable. In the study of congenitally abnormal hearts, it is advisable to embed the entire heart as a unit. If that is impossible because of the size of the heart, then very careful judicious planning of the fashioning of the blocks is necessary, so that displaced SA nodes, and anterior AV nodes and bundles are not overlooked.     

HISTOPATHOLOGICAL STUDY OF THE CONDUCTION SYSTEM OF THE HEART IN DUCHENNE PROGRESSIVE MUSCULAR DYSTROPHY

The cardiac conduction systems including sinoatrial (SA) node, atrioventricular (AV) node, atrioventricular(His) bundle, and peripheral conduction system (left and right bundle branch, and Purkinje fiber) of 23 patients with Duchenne progressive musculr dystrophy(DMD) were studied with light microscope. Infiltration of fat tissue and mild fibrosis were occasional findings in SA and AV nodes. Degeneration of the conduction muscle fiber was hardly noted in SA node, AV node, and His bundle. Only the peripheral conduction system (Purkinje fiber) showed significant degenerations such as eosinophilic, necrotic and vacuolar changes with fibrosis. These necrobiotic changes resembled hyaline and vacuolar skeletal and cardiac muscular degenerations in DMD and were assumed to have occurred on the basis of the structural and constitutional characteristics of the peripheral conduction fiber as a striated muscle fiber. The vascular changes and amyloid deposit suggesting precocious aging in the conduction systems were not observed.     

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 μm in diameter, and (peripherally) small 3–5-μm-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.     

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.     

Immunolabelling patterns of gap junction connexins in the developing and mature rat heart

Summary The distribution of gap junctions in prenatal, postnatal, and adult rat hearts was studied by laser scanning confocal microscopy, using antiserum raised to a peptide (HJ) matching part of the sequence of connexin43 (a cardiac gap junction protein). Using digital reconstruction of optically-sectioned tissue volumes, a highly sensitive detection of immunolabelled gap junctions was achieved. The distribution of positive anti-HJ immunolabelling was regionalised in the prenatal heart from its first detection at 10 days post-coitus. High levels of immunopositive staining occurred in the trabeculae of the embryonic ventricles. Other zones of the early myocardium including early central conduction tissues had no detectable signal. The prenatal outflow tract, interventricular septum and a narrow zone of myocardium subjacent to the epicardial free wall also had low levels of immunopositive signal. During postnatal growth and in the adult rat heart, a marked distinction emerged between the central conducting tissues of the atria and ventricles. Whilst small immunostained gap junctions became detectable within the atrioventricular node on the atrial side of the junction, between the interatrial and interventricular septa, no immunolabelling was found within the ventricular branching bundle. This difference between the atrioventricular node and branching bundle is consistent with potential functional distinctions between these two structures, and is not consistent with the recent proposal that the His bundle and its branches act as an extended atrioventricular node in smaller mammals such as the rat. Ventricular Purkinje fibres, distal to the branching bundle, showed high levels of anti-HJ immunostaining. Organisation of gap junctions into intercalated disks within the ventricle proceeded late into the adolescent stages of heart growth. The distribution of a second connexin protein, MP70, not previously characterised in the heart, was studied using monoclonal antibodies. MP70 was transiently immunolabelled in the heart during the postnatal period, but only within valves. Previously, this protein has been reported only in the eye lens. MP70-containing gap junctions may represent a specialisation in avascular tissues, since blood vessels are not present in either the eye lens or the cusps of heart valves.