Cardiac embryology

Cardiac embryology

Cardiovascular system is the first system to develop in the embryo. Cardiac progenitor cells develop into cardiac myoblasts. These surround the cardiogenic field formed by the coalescence of the blood islands in the mesoderm. The primitive heart tube is formed from the cardiogenic filed. The cranial part of the primitive heart tube connects to the dorsal aorta while the caudal end connects to the venous system. The primitive heart tube has three layers – the inner layer of endothelium which forms the endocardium, the middle layer of myocardium and outer layer visceral pericardium.

Cardiac loop

The heart tube elongates and forms the cardiac loop at 23 days. The cranial end bends ventrally, caudally and to the right. Caudal end bends dorsally, cranially and towards the left. The cardiac loop is fully formed by 28 days. Bulbus cordis is formed from the proximal part of the heart tube. Conus cordis is the middle segment and the distal part is truncus arteriosus.

Septation of the heart

The septa form from tissue masses known as endocardial cushions. Parts of the atrial and ventricular septum, atrioventricular valves and aortopulmonary septum are formed from the endocardial cushions. Septum primum develops from the roof of the common atrium by the end of fourth week of development. Initially its growth stops short of the endocardial cushions, leaving the ostium primum. Later when the septum primum fuses with the endocardial cushions, ostium primum closes. By this time, perforations develop in the central part of the septum primum which coalesce to form the ostium secundum. A new septum called septum secundum develops from the upper part of right atrium which closes the ostium secundum partially. The lower part of the septum secundum is crescentic and remaining part of the ostium secundum is the foramen ovale. The septum primum gradually disappears, leaving the lower portion as the valve of the foramen ovale. Foramen ovale allows right to left shunt across the interatrial septum in fetal life. After birth, when lungs start functioning, pulmonary venous return increases the left atrial pressure leading to closure of the valvular foramen ovale. But probe patency of foramen ovale may remain up to 20% of adults.

Of the four endocardial cushions (superior, inferior and two lateral), the superior and inferior fuse to divide the atrioventricular canal into two. The atrioventricular valves are formed by thinning of the endocardial cushions. Truncal swelling within the truncus rotate and fuse to form the aorticopulmonary septum. It is the rotation which causes the spiral nature of the septum and the special relationship of aorta and pulmonary artery. Division of the conus cordis also proceeds on similar lines leading the formation of the right and left ventricular outflow tracts. After completion of the septation, semilunar valves develop by thinning of the tubercles on the truncal swellings.

Muscular ventricular septum develops from the muscular ventricular wall while the membranous septum develops from the endocardial cushions.