Comparative ultrastructure of cellular junctions in working myocardium and the conduction system under normal and pathologic conditions

Abstract

The comparative ultrastructure of muscle cell junctions in the working myocardium and conduction system is reviewed. In both kinds of muscles, the cell junction may be divided into four types of regions: the nexuses, the desmosomes, the fasciae adherentes and the undifferentiated regions. The nexus, originally interpreted as a fused membrane, has now been redefined as a gap junction. In the cell junctions (intercalated disks) of working muscle, the transverse segments are chiefly occupied by large fasciae adherentes, the longitudinal segments are provided with large nexuses, and the undifferentiated regions are much less extensive. In addition to their distinctive intercellular arrangement, the sinus node and arterioventricular node are characterized by cell junctions in which nexuses and fasciae adherentes are infrequent and of small size, and the undifferentiated regions are often extensive. The cell junctions of the Purkinje fibers are characteristic in the Artiodactyla because of polyhedral interconnections of large cells, small and infrequent fasciae adherentes, and large and frequent nexuses and undifferentiated regions. In other animal species, the Purkinje cell junctions rather resemble the intercalated disks of working muscle, but instead of crossing the fiber transversely stepwise, their disks often course obliquely to the fiber axis. Several patterns of dissociation of the intercalated disks which take place in some heart diseases and experimental conditions are also reviewed. The functional implication of the ultrastructural difference in muscle cell junctions between working myocardium and conduction system and the dehiscence processes of the intercalated disks are discussed.