Location of pacemaker in chick embryo heart at the time of initiation of heartbeat

Abstract

Electrograms and intracellular action potentials were recorded of the hearts of chick embryos during the 1st week of incubation. All studies were carried out with the embryos in situ and with spontaneously beating hearts. It has been possible to show that the pacemaker even before initiation of the heartbeat (8 somite embryos) is already located in the sinoatrial portion of the heart, i.e., there is no pacemaker shift from ventricle to sinus venosus during the early phases of cardiac development, as is often stated. The (sino) atrial conduction time has been found remarkably constant at 90-100 msec throughout development. Short bursts of sinoatrial ectopic beats often occurred on impaling a sinoatrial cell. Such tachycardia never resulted from impalement of a ventricular cell. Ventricular action potentials rarely exhibited pre-potentials and then only in the younger specimens.