Partitioning of the Heart
During the embryonic period, the heart tube is partitioned into a right atrium and ventricle, to collect deoxygenated blood from all over the body and to pump it to the lungs for oxygenation; and a left atrium and ventricle to collect oxygenated blood from the lungs and to pump it to the organs and tissues.
The main processes that lead to subdivision of the heart tube into a four chambered pump involve:
- Division of Atrioventricular Canal
- Interatrial & Interventricular Septae
- Division of Conus & Truncus Arteriosus
Partitioning of the heart is accomplished by the growth of septa within the common atrium, ventricle and conus arteriosus and the fusion of these with the endocardial cushions and the endocardial cushions with each other.
During fetal life, two shunts are in place to bypass the pulmonary circuit, since the lungs are developing and non functional. Instead, fetal blood is oxygenated in the placenta and delivered directly to the fetus by the umbilical vein, bypassing the right side of the heart until after birth.
Division of Atrioventricular Canal
This animation depicts separation of the common atrioventricular canal into right and left atrioventricular canals.
Cardiac jelly forms endocardial cushions that surround the common AV-canal. These will ultimately form elements of the cardiac skeleton, including the cusps of the atrioventricular and semilunar valves.
Growth and fusion of the anterior and posterior endocardial cushions separates the common AV-canal into right and left AV-canals as the atria and ventricles expand around them.
The right AV-canal connects right atrium to the right ventricle and will be guarded by the tricuspid valve (right AV valve). The left AV canal connects left atrium and ventricle and is guarded by the bicuspid (mitral) valve (left AV valve.
Fig. 1: Animated gif image showing division of the common atrioventricular canals into right and left atrioventricular canals by the endocardial cushions.
Fig. 2: Right and left AV canals will be guarded by the tricuspid and bicuspid valves eventually. Eventually completion of the interventricular septum will form separate outflow channels for the right and left ventricles.
Partitioning of Common Atrium
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Initially, the heart tube is one continuous channel, with a common atrium, common ventricle and a narrow atrioventricular canal connecting them.
Endocardial cushions, derived from cardiac jelly, narrow the atrioventricular canal and will eventually form the cardiac skeleton.While cardiac jelly forms the matrix of the endocardial cushions, cells that form the cardiac skeleton, including the valve cusps, their annular attachments and chordae tendinae, are derived from the endocardium.
Fig. 3a: Around week 4, the common atrium and common ventricle are separated by the endocardial cushions and communicate via the atrioventricular canal.
Heart Tube at 4 Weeks of Development -
By 5 weeks of development, septum primum begins to grow down from the roof of the common atrium. As it grows toward the endocardial cushions, it narrows the space between the right and left sides of the common atrium, forming the ostium primum.
Ostium primum closes when the septum primum fuses with the endocardial cushions.
Fig. 3b: At week 5, the endocardial cushions have divided the common AV canal into right and left AV canals. Septum premium begins to grow downward from the roof of the common atrium.
Heart Tube at 5 Weeks of Development -
Before ostium primum can be obliterated, a new opening develops in the upper end of septum primum. This is the ostium secundum, or "second opening", and occurs around day 33 of development.
Fig. 3c: At 5.5 weeks septum premium fuses with endocardial cushions to close ostium premium as ostium secundum opens. The interventricular foramen is narrowing because of growth of the interventricular septum.
Heart Tube at 5.5 Weeks of Development -
Around day 35, a second septum, septum secundum, begins to grow downward from the roof of the right atrium, to the right of septum premium.
It never reaches the endocardial cushions, leaving a crescent shaped opening called the foramen ovale.
Fig. 3d: At 6 weeks septum secundum grows as a crescent shaped membrane tp cover osmium secundum and form the foramen oavle.
Heart Tube at 6 Weeks of Development -
Foramen ovale forms a one way "flutter" valve that permits shunting of blood from the right atrium to the left atrium during development. The purpose of the shunt is to bypass the fetal lungs which are not functional until after birth. In utero, oxygenation of the fetal blood occurs in the placenta. This oxygenated blood reaches the fetus through the umbilical vein.
After shunting through the liver it enters the heart through the inferior vena cava (IVC).
Fig. 3d: At 7 weeks foramen vale makes a one way valve for shunting oxygenated blood from right to left atrium.
Heart Tube at 7 Weeks of Development
Foramen Ovale
Blood that enters the right atrium by way of the IVC is shunted through foramen ovale into the left atrium. From there is passes into the left ventricle, which pumps it to the body of the fetus through the aorta.
During fetal life, higher pressure in the right atrium favors the function of the foramen ovale as a one way shunt.
At birth, higher pressure shifts to the left atrium and septum primum is pushed over the foramen ovale, closing it.
This becomes the fossa ovalis that can be observed in the interatrial wall of the right atrium.
Fig. 4: Oxygenated blood enters right atrium and due to higher pressure is shunted through foramen ovale to the left atrium and into left ventricle.
Fig. 5: Oxygenated blood enters right atrium and due to higher pressure is shunted through foramen ovale to the left atrium and into left ventricle.
Development of Interventricular Septum
Right and left ventricles initially communicate widely with one another but are eventually separated by the development of an interventricular septum.
The muscular part of the interventricular septum grows upward from the ventricular wall, narrowing the gap between the two ventricles. The endocardial cushions contribute to the membranous part of the inter ventricular septum.
Completion of the inter ventricular septum occurs roughly around the same time as the development of the aorticopulmonary septum.
Fig. 6: Initially the right and left ventricles communicate widely. The interventricular septum has a muscular component that grows upward from the floor of the common ventricle. Completion of the interventricular septum is by fusion with the aorticopulmonary septum.
Division of Truncus Arteriosus
At the beginning of week 7, neural crest cells, derived from the region of the rhombencephalon (hindbrain) contribute to the formation a a pair of conotruncal ridges (blue and purple) that will form the spiral aorticopulmonary septum within the conus & trunks arteriosus.
The conotruncal ridges fuse with each other and the endocardial cushions to divide the truncus arteriosus into two channels:
- pulmonary trunk for outflow from the right ventricle to the lungs
- aorta for outflow from the left ventricle to the body tissues
GIF Animation
Fig. 7: The spiral of the aorticopulmonary septum divides the trunks arteriosus into a right ventricular outflow channel into the pulmonary trunk and a left ventricular outflow channel into the aorta.
The red line represents blood flow from left atrium and ventricle into the aorta. The blue line represents blood flow from the right atrium and ventricle into the pulmonary trunk.
Clinical Correlates
Cardiac Anomalies
Cardiac anomalies make up the largest category of human birth defects. They are found in approximately 1% of all newborns. While about 2% of these anomalies are attributed to environmental factors only, the majority are multifactorial resulting from complex interactions between genetic and environmental causes.
Atrial Septal Defect
Atrial septal defect is seen in approximately 6.4/10,000 births and has a couple of different causes.
At birth, pressure changes caused by opening of the alveoli in the lungs and establishment of the pulmonary circulation, pressure increases in the left atrium relative to the right atrium and the foramen ovale is closed.
If the foramen ovale does not close either due to excessive diameter of the ostium secundum or failure of the septum primum to completely cover foramen ovale an interatrial septal defect may be present..
Since the pressure in the left atrium is now greater than in the right atrium, oxygenated blood is pushed through the opening into the right atrium, causing some oxygenated blood to pass through the lungs twice.
Depending of the degree of severity, the condition may go unnoticed for many years.
(Image provided by Centers for Disease Control and Prevention, National Center on Birth Defects and Developmental Disabilities)
Fig. 8: Image showing an interatrial septal defect allowing oxygenated blood to flow from the left to the right atrium.
Case courtesy of randers, Radiopaedia.org. From the case rID: 48742
Fig. 9: Doppler ultrasound showing passage of oxygenated blood from the left atrium (LA) into the right atrium (RA) through an interatrial septal defect or patent foramen ovale. (LV = left ventricle; RV = right ventricle.
Failure of Endocardial Cushions to Fuse
The endocardial cushions are important in separation of the common atrioventricular outflow into right and left atrioventricular canals.
Anterior and posterior endocardial cushions fuse in the middle of the common AV canal to divide it into right and left AV canals. Failure of these to develop results in atrial and ventricular septal defects and mixing of oxygenated and oxygenated blood in the heart.
(Image provided by Centers for Disease Control and Prevention, National Center on Birth Defects and Developmental Disabilities)
Fig. 10: Image showing a common atrioventricular canal due to failure of the endocardial cushions to divide the common AV canal.
(Image provided by Centers for Disease Control and Prevention, National Center on Birth Defects and Developmental Disabilities)
Fig. 11: Image showing interventricular septal defects that can occur in either the membranous or muscular part of the interventricular septum.
Interventricular Septal Defect
Interventricular septal defects are one of the most common cardiac anomalies with an incidence of about 12 in 10,000 births. The may involve the muscular part of the septum, in which case they often resolve spontaneously as the child grows or they may may involve the membranous part of the interventricular septum, in which case they may be part of a more complex deficit that involves the aorticopulmonary septum as well.
Tetralogy of Fallot
Tetralogy of Fallot is the most common anomaly of the conotruncal region. It is generally thought to arise as a result of anterior displacement of the conotruncal ridges, leading to an unequal division of the conus arteriosus.
This leads to a narrowing of the pulmonary outflow tract, (pulmonary stenosis). The membranous portion of the inter ventricular septum fails to develop, producing an interventricular septal defect. The aorta, which overrides the muscular portion of the interventricular septum draws blood from both ventricles, forcing the right ventricle to work harder and therefore hypertrophy.
(Image provided by Centers for Disease Control and Prevention, National Center on Birth Defects and Developmental Disabilities)
Fig. 12: Image showing tetralogy of Fallot, the most common anomaly of the conotruncal region of the heart.
(Image provided by Centers for Disease Control and Prevention, National Center on Birth Defects and Developmental Disabilities)
Fig. 13: Image showing transposition of the great vessels, with right ventricular outflow going into aorta and left ventricular outflow going into pulmonary trunk.
Transposition of the Great Vessels
The endocardial cushions are important in separation of the common atrioventricular outflow into right and left atrioventricular canals.
Anterior and posterior endocardial cushions fuse in the middle of the common AV canal to divide it into right and left AV canals. Failure of these to develop results in atrial and ventricular septal defects and mixing of oxygenated and oxygenated blood in the heart.