Fine Structure of the His Bundle

The fine structure of the His bundle is described on the basis of its light and electron microscopic appearance. Electron microscopy was performed on one human and two canine hearts, and light microscopy on over 400 human and 60 canine hearts. The His bundle was identified by its light microscopic appearance. There were no significant differences in the fine structure of human and canine His bundles. In both, the principal cell was a typical Purkinje cell containing few myofibrils and a large perinuclear clear zone; these cells are shorter and broader than working myocardial cells, and their intercellular junctions (which are obliquely rather than transversely oriented) contain a high proportion of nexus formations. Both the human and canine His bundles are partitioned by fine collagen septa, which are longitudinally oriented with comparatively few crossover connections. The general organization of the His bundle is thus into multiple strands of Purkinje cells, and these strands are largely separated from one another by collagen. This longitudinal separation of Purkinje strands by collagen, plus the specialized nature of intercellular junctions within each strand, form an anatomic basis for suspecting longitudinal separation of conduction within the normal His bundle. Some electrophysiologic implications of these findings are discussed.