I. Embryonic development of the heart Changes in the shape of the heart: The heart originates from the mesenchymal stroma at the head of the embryo, i.e., the germinal plate. The heart begins to form a pair of parallel ducts (cardiac canals) with a pericardial cavity on its dorsal side. At this time, the long axis of the cardiac canals is parallel to the body axis. As the embryo grows, the head extends forward and the cardiac canals reach posteriorly to the ventral side of the pharynx. At the beginning of the fourth week, the right and left cardiac tubes are combined into a single tube, which serves as the primordium of the endocardium. The mesodermal dirty layer surrounding the tube lumen thickens to form the myocardium and epicardium. The single heart tube is divided into four local expansions, from head to tail, the bulb, ventricle, atrium and venous sinus in that order. Kong Qingfeng, Department of Ultrasound, Jining First People’s Hospital Evolution and separation within the heart: ① Separation of the atrioventricular canal: The atrioventricular canal is the narrow part between the original atrium and the original ventricle. In the fourth week, a mesenchymal hyperplasia center, called endocardial cushion, appears here on the dorsal caudal and ventral cephalic sides of the mesenchymal ring. The endocardial cushions gradually protrude into the atrioventricular canal, and their free edges grow toward the opposite side. In the sixth week, the two dorsoventral endocardial cushions fuse in the midline, separating the AV canal into two canals on the left and right sides. The endocardium at the mouth of the canals forms folds, whose arches face the atria and form the AV valves, namely the tricuspid and mitral valves. ② Separation of the atria: Initially, a sickle-shaped septum, called the first interatrial diaphragm, occurs at the midline of the atrial parietal wall. It extends toward the endocardial cushion, and when it reaches the endocardial cushion, a hole is temporarily left between the free edge of the sickle septum and the endocardial cushion, called the first interatrial foramen, which serves as a channel to communicate with the right and left atria. The first interatrial foramen is completely closed at the sixth week. At the same time, the upper part of the first interatrial diaphragm absorbs its own perforation to allow the right and left atria to communicate there, and this hole is called the second interatrial foramen. After the formation of the first interatrial diaphragm, a similar sickle-shaped septum occurs to its right, called the second interatrial diaphragm. The second interatrial diaphragm grows from the anterior to the superior aspect of the atria toward the posterior to the inferior aspect, i.e., toward the entrance of the inferior vena cava, gradually covering the second interatrial foramen. The lower edge of the second interatrial diaphragm extends anteriorly and posteriorly with a foot attached to the endocardial cushion, and its free edge encloses an ovoid hole below the second interatrial diaphragm, called the foramen ovale. The tissue of the first atrial diaphragm is thin and is attached to the left side of the foramen ovale in a membrane-like fashion, resembling a living flap over the foramen ovale. When the pressure in the right atrium is high, blood can pass through the oval orifice, push open the first atrial diaphragm, and penetrate into the left atrium through the second atrial orifice, thus ensuring that the fetus receives fresh maternal blood from the umbilical vein, and the right atrium flows into the left atrium through the oval orifice to supply the fetus with blood transport throughout the body. However, after birth, the pressure in the left atrium exceeds that in the right atrium, and the valve of the foramen ovale is attached to the atrial diaphragm to prevent blood shunting. During the formation of the atrial diaphragm, the venous sinus also gradually moves right into the right atrium and expands to become part of the right atrium. The main branches of the venous sinus open directly into the right atrium, forming the entrance to the superior and inferior vena cava and the coronary sinus. (iii) Separation of the ventricles: The bottom wall of the primitive ventricle grows toward the endocardial cushion, separating the ventricle into left and right halves, but a hole remains above it, called the interventricular foramen. At the end of the seventh week, this hole is closed by the membrane formed by the convergence of the myocardial septum, conus septum and atrioventricular endocardial cushions, forming the membranous septum, which is the membranous part of the septum of the adult heart. (4) Separation of the arterial trunk: At the same time as the ventricular septum occurs, a spiral septum occurs in the arterial trunk (equivalent to the distal end of the cardiac bulb), which is connected to the ventricular septum and separates the ascending aorta from the pulmonary arterial trunk. The former opens into the left ventricle and the latter into the right ventricle. If the septum is not properly separated in evolution, it can cause post-stenotic inversion of the aorta after the beginning of the pulmonary artery. Second, the development of great vessels Six pairs of arterial arches emanate from the arterial trunk extending from the cardiac bulb, bypassing both sides of the pharynx toward the dorsal side and synthesizing two longitudinal aorta, which in turn synthesizes a descending aorta on the caudal side. The first, second, and fifth pairs of arterial arches degenerate and disappear, the third pair becomes the carotid arch, the fourth pair becomes the innominate and right subclavian arteries on the right side, and on the left side divides into the final aortic arch, which emanates from the left subclavian artery and is connected to the descending aorta. When the arterial trunk splits longitudinally into the ascending aorta and the pulmonary trunk, the 6th pair of arterial arches connects with the pulmonary arteries to form the left and right pulmonary arteries. On the left side, the 6th pair of arterial arches retains its connection with the dorsal aorta’s, while forming the ductus arteriosus (the arterial ligament in adults).