Development of the myocardium of the atrioventricular canal and the vestibular spine in the human heart

To establish the morphogenetic mechanisms underlying formation and separati on of the atrioventricular connections, we studied the remodeling of the my ocardium of the atrioventricular canal and the extracardiac mesenchymal tis sue of the Vestibular spine in human embryonic hearts from 4.5 to 10 weeks of development. Septation of the atrioventricular junction is brought about by downgrowth of the primary atrial septum, fusion of the endocardial cush ions, and forward expansion of the vestibular spine between atrial septum a nd cushions. The vestibular spine subsequently myocardializes to form the v entral rim of the oval fossa. The connection of the atrioventricular canal with the atria expands evenly. In contrast, the expression patterns of crea tine kinase M and GlN2, markers for the atrioventricular and interventricul ar junctions, respectively, show that the junction of the canal with the ri ght ventricle forms by local growth in the inner curvature of the heart. Gr owth of the caudal portion of the muscular ventricular septum to make conta ct with the inferior endocardial cushion occurs only after the canal has ex panded rightward. The atrioventricular node develops from that part of the canal myocardium that retains its continuity with the ventricular myocardiu m.