Cardia bifida and the development of pacemaker function in the early chick heart

Abstract

Double-hearted or “cardia bifida” embryos were produced by making a mid-line cut through the tissues of the anterior intestinal portal of the 1–3 somite chick embryo, at a time when the foregut is a shallow crescent and the cardiac primordia have not yet fused in the bulboventricular region. Each lateral primordium in the operated chicks forms a separately beating tubular heart. From the time of initiation of heart beat (at 10–11 somites) to the stage of 18–19 somites, the left heart beats faster than does the right in the great majority of embryos. This observation is in consonance with the concept of “left dominance” as applied to early embryonic development. As development progresses, however, the left heart declines in rate while the right remains stable or increases in rate. By the 21–24 somite stage, the right heart usually has a faster rate than the left. It is at 19–20 somites that the sinus venosus is just forming in the heart. It is concluded that: (1) material from the left cardiac primordium is predominant in many respects over that from the right during early embryonic development and acts as pacemaker for the entire embryonic heart up to the stage of 18–19 somites. (2) At about the time the sinus venosus forms, this dominance relation reverses, material from the right primordium taking over the pacemaker function. Whether this change represents a collapse of some left-right gradient of metabolic activity, or the overriding of such a gradient by specific sinoatrial cells from the right side, is not known. It is suggested that the present results provide evidence that the definitive sinoatrial pacemaker of the adult heart is derived wholly or in the main from the right cardiac primordium.