Ultrastructure of the sinus node was studied in human and canine hearts and found to be similar. The principal cell of the sinus node is a small round pale cell with randomly distributed sarcosomes and sparse myofibrils. These cells have been designated as P cells. They occur in elongated clusters and make contact with each other in all directions but do not make direct contact with ordinary working myocardium. Transitional cells, which have features intermediate between P cells and working myocardium, serve as the connections between P cells and the rest of the heart. Each P cell is surrounded by a plasma membrane and groups of them are bound by a basement membrane. The only specialized junction between P cells is the desmosome, which occurs singly and seldom, with most contact being between apposing plasma membranes. P cells exhibit unusually active pinocytosis but have only a sparsely developed sarcoplasmic reticulum. The sarcosomes of P cells are much simpler in internal structure than are those of adjacent working myocardium obtained at the same time in the same heart in the same way, by direct perfusion with glutaraldehyde into the beating sinus node. Correlation of the electron microscopic appearance of the sinus node with that based on light microscopy is discussed, and some of the possible functional significances of the fine structure are considered.

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.

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