Heart Development - Anatomy & Physiology
Overview of Developmental Anatomy of the Heart
The formation of the mammalian heart is a fairly complex process. It begins when angiogenic mesodermal cells in the cardiogenic plate coalesce to form the endocardial tubes. The endocardial tubes then fuse to form a single duct, the cardiac tube. This undergoes a process of distension, folding and septation and a four chambered, dual circuit pump is formed . The simple heart seen in fish or amphibians forms via the same path but development ceases at an earlier stage.
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 transversum. 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 cardiac 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. Four masses of cardiac mesenchymal tissue, known as endocardial cushions, extend towards each other, the dorsal (inferior) and the ventral (superior) cushions grow more than the left and right cushions and eventually fuse. The fused endocardial cushions form the Septum Intermedium, which divides the common atrioventricular canal into left and right atrioventricular canals. The myocardium secretes factors which causes an epithelial to mesenchymal change in the endocardium. These mesenchymal cells go on to form the bicuspid and tricuspid atrioventricular valves.
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 semilunar 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.
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From Pathology
Normal Development
The heart develops from a simple endothelial tube which acquires a mesodermal layer (forms myocardium and epicardium). As the heart grows a flexure forms so that the original cranial portion becomes the truncus arteriosus and the ventricles and the oringinal caudal part of the tube will form the sinus venosus and the atria.
The atria are separated by a growth from the dorsal wall, the septum primum and ventral wall, the septum secundum. The ventricles are separated by a septum growing from the apex.
Congenital abnormalities can be broadly classified into 3 groups:
- Abnormalities of chamber development forming shunts.
- Abnormalities of great vessels forming shunts or anomalies.
- Abnormalities of valve formation resulting in dysplasias.