Heart Development Quiz

Heart Development

• Heart development is the first major system to function in the embryo.
• The heart and vascular system appears in the middle of week 3.
• Heart development is critical as the rapidly growing embryo cannot meet its nutritional and oxygen requirements through diffusion alone.
• Heart tissues are derived from mesoderm with contributions from neural crest cells.

Formation and Positioning of the Heart Tube

• The cardiogenic area is initially located anterior to the future mouth and forebrain.
• The forebrain grows past the heart to the thoracic position.
• The paired cardiac tubes are brought together as a single tube via lateral folding of the embryo.

Formation of Cardiac Loop

• The heart tube undergoes a complex looping process to form the characteristic S-shape.
• The looping process begins around day 22 and is complete by day 24.
• This process establishes the correct positioning of the heart within the chest cavity.

Anomaly of Cardiac Looping

• If the looping of the heart tube fails to occur properly, it can lead to a variety of congenital heart defects.
• For example, one type of congenital heart defect includes the ventricles being transposed: the ventricles would share positions.

Development of Sinus Venosus

• The sinus venosus is a quadrangular space that precedes the atria.
• It receives all blood from the embryo, including from the left and right sinus horns.
• The left sinus horn is incorporated into the left atrium, while the right sinus horn is incorporated into the right atrium.
• This process establishes the unique venous drainage patterns of the heart.

Endocardial Cushions

• Growing masses of endocardium that project into the lumen of the heart, contributing to the division of the heart canal.

Septal Formation in Common Atrium

• Septum primum is a sickle-shaped structure that grows from the roof of the common atrium towards the endocardial cushions in the atrioventricular canal.
• Ostium primum is the opening between the lower rim of septum primum and the atrioventricular opening.
• Ostium secundum arises from merging perforations within the septum primum, allowing blood flow between the atria.
• Septum secundum is a crescent-shaped fold formed by the incorporation of the sinus horn in the right atrium.
• The opening left by the septum secundum is called the oval foramen, which closes after birth, forming the fossa ovalis.

Congenital Heart Anomalies

• Congenital heart anomalies can be classified based on the onset of cyanosis and the direction of the shunt.
• Acyanotic anomalies involve a left-to-right shunt and late onset of cyanosis.
• Examples of acyanotic anomalies include atrial septal defects (ASD), ventricular septal defects (VSD), patent ductus arteriosus (PDA), atrioventricular canal defects, and coarctation of the aorta.
• Cyanotic anomalies involve a right-to-left shunt and early onset of cyanosis.
• Examples of cyanotic anomalies include persistent truncus arteriosus, transposition of great vessels, tricuspid atresia, tetralogy of Fallot, and Ebstein's anomaly.

Atrial Septal Defects

• Atrial septal defects affect 6.4/10,000 births, with a 2:1 male predominance.
• Ostium secundum defect is a common acyanotic anomaly, characterized by a large opening between the atria due to excessive cell death and resorption of septum primum, and inadequate development of septum secundum.

Septal Formation in the Atrioventricular Canal

• The atrioventricular canal divides into the right and left atrioventricular canals through a process involving the endocardial cushions.
• The endocardial cushions merge and fuse, forming the interventricular septum.

Atrioventricular Valves

• Atrioventricular valves, bicuspid and tricuspid, form from local proliferations of mesenchymal tissue that become fibrous.
• Muscular cords attaching valves degenerate and are replaced by dense connective tissue.
• Valves attach to papillary muscles by chordae tendineae.

Ebstein Anomaly

• This anomaly affects 1/200,000 births and is characterized by cyanosis.
• It involves displacement of the tricuspid valve towards the apex of the right ventricle, leading to an expanded right atrium and small right ventricle, along with abnormal valve leaflets.

Septal Formation in the Outflow Tract

• Cardiac neural crest cells migrate contributing to the outflow tract development.
• Disruptions in this process can lead to tetralogy of Fallot, pulmonary stenosis, persistent truncus arteriosus, and transposition of great vessels.
• Conotruncal ridges grow and divide the outflow tract into the aortic and pulmonary channels.

Endocardial Cushions

• Growing masses of endocardium that project into the lumen of the heart, forming the atrioventricular canal.
• Contribute to the development of cardiac septa.

Cardiac Septa Formation

• The septum primum is a sickle-shaped crest that grows from the roof of the common atrium towards the endocardial cushions.
• The ostium primum is an opening between the lower rim of the septum primum and the atrioventricular opening.
• Perforations merge in the septum primum to form the ostium secundum, which allows blood flow between the atria.
• The septum secundum is a crescent-shaped fold that forms from the incorporation of the sinus horn into the right atrium.

Foramen Ovale

• Opening left by the septum secundum, closes off after birth, leaving behind a depression called the fossa ovalis.

Atrial Septal Defect (ASD)

• Common congenital heart anomaly with a frequency of 6.4 per 10,000 births, with a 2:1 male to female ratio.
• Most ASDs are ostium secundum defects, which are acyanotic.
• These defects are characterized by a large opening between the atria.
• Caused by excessive cell death and resorption of the septum primum, and inadequate development of the septum secundum.

Ebstein Anomaly

• Rare congenital heart anomaly, occurring in about 1 in 200,000 births.
• Results in malformation of the tricuspid valve, which is displaced towards the apex of the right ventricle.
• Leads to an expanded right atrium and a small right ventricle with abnormal valve leaflets, causing cyanosis in affected individuals.

Conotruncal Ridge Formation

• Cardiac neural crest cells migrate to contribute to the outflow tract, its disruption leads to:
  • Tetralogy of Fallot
  • Pulmonary stenosis
  • Persistent truncus arteriosus
  • Transposition of great vessels
• Conotruncal ridges grow and divide the outflow tract into the aortic and pulmonary channels.

Atrioventricular Valve Formation

• The bicuspid and tricuspid valves develop from local proliferation of mesenchyme, which becomes fibrous.
• The muscular cords attaching the valves degenerate and are replaced by dense connective tissue.
• The valves attach to the papillary muscles via chordae tendineae.

Septal Defects

• Abnormalities in the formation of the septum primum or septum secundum can result in atrial septal defects (ASD).
• A common type of ASD is the ostium secundum type, attributed to excessive resorption of the septum primum and insufficient development of the septum secundum.

Tetralogy of Fallot

• Caused by disruption of the cardiac neural crest cell migration during outflow tract development, leading to a combination of four heart defects, including:
  • Ventricular septal defect (VSD)
  • Pulmonary stenosis
  • Overriding aorta – a condition where the aorta arises from both ventricles instead of just the left ventricle.
  • Right ventricular hypertrophy - the right ventricle becomes enlarged due to increased workload.
• Individuals with Tetralogy of Fallot exhibit cyanosis due to reduced blood flow to the lungs, resulting in reduced oxygenation.

Transposition of Great Vessels

• Occurs when the aorta and the pulmonary artery are reversed, leading to parallel circulation with no mixing of oxygenated and deoxygenated blood
• This defect results in cyanosis and requires surgical correction.
• The failure of the aorticopulmonary septum to spiral during formation results in this anomaly.

Persistent Truncus Arteriosus

• The truncus arteriosus, the common outflow tract of the heart, fails to divide into the pulmonary artery and aorta.
• Results in a single vessel that carries both oxygenated and deoxygenated blood.
• Causes cyanosis and requires surgical repair in newborns.

Coarctation of the Aorta

• A narrowing of the aorta below the origin of the left subclavian artery.
• Occurs in 3.2 out of 10,000 births.
• Caused by intimal proliferations.
• Has preductal and postductal types.
• Characterized by high upper limb blood pressure and low lower limb blood pressure.

Transposition of the Great Vessels

• Occurs in 4.8 out of 10,000 births.
• Cyanotic heart defect.
• Results from the conotruncal septum failing to take a spiral course and going straight down.
• Causes the aorta to originate from the right ventricle, and the pulmonary artery from the left ventricle.
• Usually associated with a defect in the membranous interventricular septum and patent ductus arteriosus.

Ventricular Septal Defects

• Occurs in 12 out of 10,000 births.
• Acyanotic heart defect.
• Occurs in the membranous or muscular portions of the ventricular septum.
• Associated with abnormal partitioning in the conotruncal region.

Tetralogy of Fallot

• Occurs in 9.6 out of 10,000 births.
• Cyanotic heart defect.
• Results from unequal division of the conus due to anterior displacement of the conotruncal septum.
• Associated with patent ductus arteriosus and pulmonary stenosis.

Persistent Truncus Arteriosus

• Occurs in 0.8 out of 10,000 births.
• Cyanotic heart defect.
• Occurs when the conotruncal ridges fail to form, resulting in no division of the outflow tract.
• Accompanied by a defective interventricular septum.

Fetal Circulation Pathways

• Oxygenated Blood: Umbilical vein → ductus venosus → inferior vena cava → right atrium → oval foramen → left atrium → left ventricle → aorta → body tissues.
• Deoxygenated Blood: Body tissues → superior vena cava/inferior vena cava → right atrium → right ventricle → pulmonary trunk → ductus arteriosus → descending aorta → umbilical arteries.

Circulatory Changes After Birth

• Closure of the umbilical arteries shortly after birth.
• Closure of the umbilical vein and ductus venosus shortly after the umbilical arteries.
• Closure of the ductus arteriosus immediately after birth.
• Closure of the oval foramen at the first breath, completing by one year of age.

Septal Formation in the Outflow Tract

• The septum is formed by the fusion of the truncus ridges and the aorticopulmonary septum.

Septal Formation in the Ventricles

• The medial walls of the ventricles expand to form the muscular interventricular septum.
• The membranous part of the interventricular septum closes the interventricular foramen.
• The semilunar valves develop from small tubercles on the main truncus swellings.
• The upper surface of the tubercles is hollowed out to form the valves.
• Neural crest cells influence the development of the semilunar valves.

This quiz explores the process of heart development in embryos, highlighting the formation, positioning, and looping of the heart tube. Understand the critical stages of cardiac formation and the importance of the heart in meeting the nutritional and oxygen needs of the growing embryo.