Heart Development - Anatomy & Physiology

()Map CARDIORESPIRATORY SYSTEM (Map) HEART

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. These angiogenic cells are derived from the lateral plate Mesoderm The Angiogenic cell clusters lie in a horseshoe shape configuration in the plate and 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, mesoderm derived angiogenic cells cluster, giving 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 mesodermal cells anterior to this form the septum trnsversum. The embryonic disc undergoes a process of folding and the endocarcial tubes and septum tranversum are rotated 180 degrees. The endocardial tubes are brought together and fuse to form the cardiac tube. The wall of the tube differentiates into the endocardium and the myocardium. The myocardium secretes an extracellular matrix known as caridac jelly, this causes areas of the tube to distend forming distinct regions, the Bulbus cordis, the Primitive ventricle, the Atrioventricular sulcus, the Primitive atria and the Sinus venosus.

At this point blood enters the caridac tube at the posterior end. The bulbis cordis loops to the right and forms two new bulges, the conotruncus and the right ventricle. This causes the primitive atrium to be displaced dorsally. The ventricles expand while the conotruncus divides into two further bulges, the truncus areteriosus (anterior) and the conus cordis (posterior). The atria expand dorsally and also on either side of the truncus arteriosis, this gives rise to the formation of two chambers, the left and right atria. The sinus venosus is located on the dorsal aspect of the interconnected atria. The sinus venosus is composed of a left and right horn, the majority of the left horn is lost, meaning that the communication between the sinus venosus and the atria, the sinoatrial opening, is confined to the right atria. This means the left side is devoid of blood vessels . The developing heart combats this by sprouting blood vessles that connect the left atrium to the lungs. The sprout splits into four channels, providing two vessels per lung.

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. The septum is shifted to the right allowing the left side of the heart to become bigger than the right

As the endocardial cushions proliferate the initial atrial septum forms in the common atrial chamber. The Septum Primum emerges from the roof of the common atrium and extends down through the chamber, eventaully fusing with the Septum Intermedium to create a seperate left and right atria. Programmed cell death occurs and a large hole is created in the septum primun allowing blood to flow between the left and right. A muscular sheet known as the Septum Secundum then grows on the right of the Septum Primum. This occludes the majority of the hole in the Septum Primum, only the small Foramen Ovale is left. The window in the Septum Primum is slightly offset from the window in the septum Secundum and this creates a valve like effect so that blood can only flow in one direction between the atriae.

Ventricular Septation

Expansion of the ventricular myocardium results in the formation of the interventricualr groove at the interface of the right and left ventricles. As growth continues the muscles push together forming a fold, this is the primitive interventricular septum. Continued growth results in enlargement of the ventricles and subsequent lengthening of the interventricular septum. The Interventricualr septum extends towards the Septum Intermedium but does not fuse, leaving a gap called the interventricualr foramen. The interventricular foramen finally closes as a consequence of differential cellular proliferation.

Further information on development of the heart can be found here

The fate of the aortic arches are described [here]

Ventricular Outflow Septation

At this stage the ventricles have one common outflow tract, the anterior side of the tract being the Truncus arteriosis and the postierior side being the Conus cordis. A pair of subendothelial swellings appear on either side of the tract and eventually fuse to form a setpum, dividing the outflow tract into an aortic trunk and a pulmonary trunk. The septum takes on a spiral form so that blood from the left ventricle is directed into the aortic trunks and blood from the right is directed into the pulmonary trunk. Initially the septum is composed only of endocardial cells but this is later supplemented by neural crest cells.

The Development of the Semilunar Valves

The semiluanr valves prevent blood from flowing from the aortic and pulmonary trunks back into the ventricles. They form after septation of the outflow tract occurs. The endocardium swells in three places at the origins of the aortic and pulmonary trunks, these swellings grow towards each other but never fuse. The swellings form shelf like projections which are then remolled to form three thin walled cusps, which prevent back flow.