Ultrastructure of the human atrioventricular node.

Ultrastructure of the A-V node was studied in two human hearts. Fine details of intranodal cellular organization were then resynthesized on the basis of light microscopic examinations of over 250 human hearts. There are four different types of cells and no anatomic syncytium in the human A-V node: (1) a simple rounded cell identical to P cells of the sinus node, (2) a slender transitional cell which is by far the most numerous type in the A-V node, (3) Purkinje cells, and (4) ordinary working myocardium; both of the latter are almost exclusively at the nodal margins. The possible significance of these ultrastructural features relative to certain functions of the A-V node, such as pacemaking and delay of conduction, is discussed.