Ultrastructure of the human atrioventricular conduction tissues.

The ultrastructure of the human atrioventricular conduction tissue has been studied by obtaining material from recipient hearts at transplant operations. The hearts were dissected immediately after surgical removal in order to expose the conduction system, and tissue samples were taken directly from the atrioventricular node, the penetrating bundle, the branching bundle, and both bundle branches. Examination with the electron microscope showed that the entire atrioventricular system throughout its length was composed of a spectrum of cells which ranged widely in size and in myofibril content from slim cells resembling cardiac muscle and packed with myofibrils to wide 'empty' cells containing relatively few myofibrils. The cells were polymorphic, and many branched with the branches varying greatly in width. Transverse junctions between cells or between their branches were made by intercalated discs. Lateral connections between cells were extremely rare; they were made by desmosomes only. Nerves were present throughout the axis. The striking features of the atrioventricular conduction system as a whole were firstly that the constituent cells were so widely heterogeneous as to defy any classification into cell types, and secondly that totally dissimilar cells established direct continuity by means of intercalated discs.