Development of the cardiac conduction tissue in human embryos using HNK-1 antigen expression - Possible relevance for understanding of abnormal atrial automaticity

Background-Abnormal atrial automaticity in young patients with structurally normal hearts is often located around the pulmonary veins and in sinus ven osus-related parts of the right atrium. We hypothesize that these ectopic p acemaker sites correspond to areas of embryonic myocardium with an early ph enotypic differentiation, as indicated by differences in antigen expression during normal cardiac development. Methods and Results-In human embryos ranging in age from 42 to 54 days of g estation, the development of the cardiac conduction system was studied with the use of HNK-1 immunohistochemistry. HNK-1 stains the developing atriove ntricular conduction system, ie, the bundle branches, His bundle, right atr ioventricular ring, and retroaortic ring. In addition, the myocardium aroun d the common pulmonary vein showed transient HNK-1 antigen expression. In t he right atrium, 3 HNK-1-positive connections were demonstrated between the sinoatrial node and the right atrioventricular ring. An anterior tract thr ough the septum spurium connects the sinoatrial node with the anterior righ t atrioventricular ring, and 2 posterior tracts connect the sinoatrial node with the posterior right atrioventricular ring through the right venous va lve (future crista terminalis) and sinus septum, encircling the coronary si nus. The medioposterior part of the right atrioventricular ring connected t o the His bundle and the medioanterior part form 2 node-like structures. Conclusions-In patients with abnormal atrial automaticity, the distribution of left and right atrial pacemaker foci correspond to areas of the embryon ic myocardium that temporarily express the I-INK-1 antigen.