Heart Development - Anatomy & Physiology
Overview of Developmental Anatomy of the Heart
The primordium of the heart forms in the cardiogenic plate located at the cranial end of the embryo. Angiogenic cell clusters which lie in a horseshoe shape configuration in the plate coalesce to form two endocardial tubes. These tubes are then forced into the thoracic region due to cephalic and lateral foldings where they fuse together forming a single endocardial tube. The tube is then subdivided into primordial heart chambers starting caudally at the inflow end: the sinus venosus, primitive atria, ventricle and bulbus cordis. The heart tube grows rapidly forcing it to bend upon itself resulting in the bulboventricular loop. Septa begin to grow in the atria, ventricle and bulbus cordis forming right and left atria, right and left ventricles and two great vessels - the pulmonary artery and the aorta. The processes involved in forming a simple heart, such as that in a fish, or a multi-chambered heart, as seen in oxygen-breathing vertebrates, follow similar pathways.
Cardiac Tube Formation
Within the cardiogenic plate, angiogenic cell clusters give rise to a horseshoe-shaped structure known as the endocardial tube. The lateral limbs of the horseshoe-shaped construction form the left and right endocardial tubes. The wall of the tubular heart is composed of loosely organised myocardium that is several cells thick and is separated from the endocardium by a layer called the cardiac jelly. Many of the major intra-embryonic blood vessels including the dorsal aortae are formed at the same time as the endocardial tubes and extra-embryonic vessels. Mesodermal cells proliferate in a position cranial to the cardiogenic plate and form the septum transversium. Two major blood vessels which form ventral to the neural tube become the left and right dorsal aortae. In the mesenchyme adjacent to the truncus arteriosus, another series of paired aortic arch arteries develop that join the dilated end of the truncus arteriosus with the dorsal aortae. Branches of the dorsal aortae, the intersegmental arteries, supply the developing somites. Additional branches supply the yolk sac through the vitelline arteries and the umbilical arteries supply the allantois. On each side of the developing embryo, the cranial and caudal cardinal veins fuse forming the common cardinal veins which enter the sinus venosus. At this stage of morphogenesis, the developing mammalian cardiovascular system and the fully formed circulatory system of the fish are very similar to each other.
Atrial Septation
Septation of the tubular heart begins in the atrioventricular canal. Two masses of cardiac mesenchymal tissue, known as endocardial cushions, extend towards each other and fuse. The fused endocardial cushions form the septum intermedium, which divides the common atrioventricular canal into left and right atrioventricular canals.
During proliferation of the endocardial cushions the initial atrial septum forms inside the common atrial chamber as a crescent-shaped ridge called the septum primum. The septum primum emerges from the dorsal wall of the common foetal atrium and extends towards the endocardial cushions. The septum primum becomes progressively smaller as the septum primum advances and disappears altogether when the septum primum fuses with the endocardial cushions. Before closure of the foramen primum, however, apoptosis in the central part of the septum primum results in the formation of a new foramen between the left and right atria, the foramen secundum. A second membrane, the septum secundum, arises from the dorsal wall of the right atrium and extends towards the septum intermedium. The opening which persists between the free edge of the septum secundum and the foramen secundum is known as the foramen ovale.
Ventricular Septation
The first sign of ventricular septation is the bulboventricular septum, which separates the dilated portion of the bulbus cordis (primordial right ventricle) from the embryonic ventricle (primordial left ventricle). The bulboventricular septum exists as a muscular fold and forms the primitive interventricular septum. Continued growth results in enlargement of the ventricles and subsequent lengthening of the interventricular septum. The interventricular foramen finally closes as a consequence of differential cellular proliferation.