Developmental transitions in cardiac conduction.

The proper sequence of electrical activation of the mature four-chambered heart requires specialized conduction pathways including the His-Purkinje system and a nearly complete separation of the atrial and ventricular myocardium. We tracked the emergence of the structure of the mature His-Purkinje system in the developing chicken embryo with anti-polysialylated neural cell adhesion molecule (PSA-NCAM) and the HNK1 antibody against a sulfated carbohydrate epitope. The function of the His-Purkinje system was assayed using extracellular electrodes and high-resolution voltage-sensitive two-dimensional optical mapping. The appearance of the mature form of the His-Purkinje system delineated by the markers coincided with the onset of the mature electrophysiological pattern of ventricular activation. These data suggest that, at the completion of ventricular septation, the His-Purkinje system undergoes critical structural and functional transitions that impact on the global pattern of conduction and contraction of the developing four-chambered heart.