Distribution of atrial and nodal cells within the rabbit sinoatrial node: models of sinoatrial transition.

BACKGROUND: In the sinoatrial node (SAN) the course of the action potential gradually changes from the primary pacemaker region toward the atrium. It is not known whether this gradient results from different intrinsic characteristics of the nodal cells, from an increasing electrotonic interaction with the atrium, or from both. Therefore we have characterized the immunohistochemical, morphological, and electrophysiological correlates of this functional gradient. METHODS AND RESULTS: The distribution of rabbit nodal myocytes in the SAN has been studied by immunohistochemistry. After cell isolation, the electrophysiological characteristics of different nodal cell types were measured. (1) The staining pattern of a neurofilament protein coincides with the electrophysiologically mapped pacemaker region in the SAN. (2) Enzymatic digestion of the SAN reveals three morphologically different nodal cell types and one atrial type. Of each nodal cell type, neurofilament-positive as well as neurofilament-negative myocytes are found. Atrial cells are all neurofilament-negative. (3) In contrast to previous findings, we observed atrial cells in the very center of the SAN. The relative number of atrial cells gradually increases from the central pacemaker area toward the atrium. (4) Differences in electrophysiological characteristics between individual nodal cells are not associated with differences in cell type. CONCLUSIONS: (1) The expression of neurofilaments can be used to delineate the nodal area in the intact SAN but is not sufficiently sensitive for characterizing all individual isolated nodal cells. (2) A fundamentally different organization of the SAN is presented: The gradual increase in density of atrial cells from the dominant area toward the crista terminalis in the SAN causes a gradual increase of atrial electrotonic influence that may be an important cause of the gradual transition of the nodal to the atrial type of action potential.

Expression of the smooth-muscle proteins alpha-smooth-muscle actin and calponin, and of the intermediate filament protein desmin are parameters of cardiomyocyte maturation in the prenatal rat heart.

BACKGROUND: Coexpression of alpha- and beta-myosin heavy chain (MHC) is a characteristic of the primary myocardial tube. To establish if the smooth-muscle proteins alpha-smooth-muscle actin (alpha-SMA) and calponin, and the intermediate filament protein, desmin, contribute to the specific functional properties of these early cardiomyocytes, we studied their spatiotemporal expression pattern. METHODS: Sections of prenatal and neonatal Wistar rats were stained with antibodies against alpha- and beta-MHC, alpha-SMA, calponin, and desmin. RESULTS: The expression of alpha-SMA and calponin in embryonic cardiomyocytes increases to reach its highest level at ED14. Subsequently, these proteins gradually disappear, beginning in the interventricular septum (IVS) and followed successively by the compact myocardium of the left ventricle, the right ventricle, and the central atrium. Expression of alpha-SMA persists longer in the ventricular conduction system, making it a convenient marker for the ventricular conduction system of the fetal rat. Desmin becomes expressed one day later than alpha-SMA, but also reaches its peak at ED14, suggesting that a relatively high concentration is required to form mature sarcomeres. CONCLUSIONS: The results indicate that alpha-SMA, calponin, and desmin are involved in the myofibrillar development in rat heart. The presence of spatiotemporal differences in the expression of these proteins reveals regional differences in the developmental timing of cardiomyocyte maturation. The maturation process extends from the compact myocardium in the IVS to the left and right ventricular free walls, whereas the atrioventricular junction, the ventricular trabeculae, and developing ventricular conduction system show a relatively slow maturation. Smooth-muscle proteins may contribute to the slow shortening speed that is characteristic of the embryonic myocardium.

The development of the atrioventricular junction in the human heart.

The histogenesis of the separation between atrial and ventricular myocardium at the atrioventricular junction in the developing human heart has been investigated immunohistochemically by using monoclonal antibodies specific for atrioventricular cushion tissue, mesenchymal cells, atrial and ventricular myocardium, and myocardium of the primary ring. It was found that the insulation between the muscle masses of atrium and ventricle is established by the fusion of the tissues of the atrioventricular sulcus (located at the epicardial side of the junctional myocardium) with those of the atrioventricular cushions (located at the endocardial side of the junctional myocardium). This process takes place at the ventricular margin of the myocardium of the atrioventricular canal. The separation of atrial and ventricular myocardium starts at approximately 7 weeks of development in the anteromedial portion of the right atrioventricular junction and is largely completed around the 12th week of development. The only remaining myocardial continuity between atrial and ventricular myocardium is the atrioventricular axis of conduction. Our findings show that the nonmuscular part of the developing leaflets of the atrioventricular valves derives from the atrioventricular cushions and that the tissues of the atrioventricular groove do not contribute to the development of these leaflets.

Dystrophin expression in the developing conduction system of the human heart.

Duchenne muscular dystrophy (DMD) is frequently associated with myocardial involvement. Dystrophin, the DMD protein, is found at the plasmamembrane of striated muscle fibers. Although dystrophin is missing in most or all muscle fibers of DMD patients, cardiac muscle is not as severely affected as skeletal muscle. Therefore it is of great importance to study the expression of dystrophin in normal cardiac muscle. We performed immunohistochemical studies and examined cardiac muscle of fetuses of 8 to 13 weeks of development on dystrophin expression. At these stages dystrophin is observed in the myocytes of the developing ventricular conduction system and in the atrial cardiomyocytes. Dystrophin was absent from the heart of a 12-week-old DMD fetus.