Development of the Heart

Question 1

From what tissue does the heart develop?

Answer 1

The cardiogenic mesoderm (originally above the end of the neural tube in development)

Question 2

What is mesenchyme?

Answer 2

A loosely organized, mainly mesodermal embryonic tissue which develops into connective and skeletal tissues, including blood and lymph.

Question 3

Describe the formation of the endocardial tubes in heart development

Answer 3

Angiogenic clusters in the carcinogenic mesoderm join to form the left and right endocardial tubes; each tube has a cranial, dorsal aorta (outflow tract) and a caudal, vitelloumbilical vein (the inflow tract)

Question 4

Describe the development of the primitive heart tube in embryological heart development

Answer 4

> At day 21 the endocardial tubes fuse to form the primitive heart tube, and here the ventricular primordial lies above the ‘atria’
The mesoderm from the foregut forms the external layer of this primitive gut tube and this is the primitive myocardium
The vitelloumbilical vein is replaced by the sinus venous which acts as the inflow tract
The primitive ventricle expands to become the left ventricle
The interventricular sulcus separates the primitive ventricle from the bulbs cordis

Question 5

At what day does the embryological heart start beating?

Answer 5

At day 22

Question 6

Describe heart looping in embryological development of the heart.

Answer 6

The primitive heart lengthens, but because the ends are fixed in place, this forces the heart to bulge and twist within the pericardial sac which leads to heart looping at around day 22

Question 7

Describe septum formation in cardiac development. At what time-scale does this occur?

Answer 7

At about 28 days gestation the atrioventricular canal becomes divided; there is growth of the dorsal and ventral endocardial cushions which grow and fuse together. At this point there is no communication between the atria and right ventricle, there is communication between the common atria into the left ventricle and then into the right ventricle and therefore when the inter ventricular septum forms the AV canal has to shift to the right in order to allow direct communication between both atria and ventricles.

Question 8

Describe atrial partitioning in cardiac development

Answer 8

At the same time as septum formation, the septum primum begins to grow from the dorsal atrial wall towards the endocardial cushions where they fuse. Perforations of the upper portion of the septum primum occur as a result of apoptosis (this is the ostium secundum). After the fusion of the septum primum, this stimulates the growth of a strong, muscular septum just to the right of the septum primum known as the septum secundum.

The incomplete partition of the atrium formed by the septum secundum forms the foramen ovale and the remaining portion of the septum primum acts as the valve of the foramen ovale to prevent left to right atrial flow

Question 9

Describe ventricle formation in cardiac development

Answer 9

At 28 days of gestation, the two ventricles begin to expand and new myocardium is added to the outside

Question 10

Describe the partitioning of the outflow tract (aorta and pulmonary artery formation) in cardiac development

Answer 10

a. A septum forms from two pairs of swellings in the walls of the outflow tract (bulbar and truncal ridges) whereby two spiral mesodermal ridges grow from the inner walsl of the truncus arteriosus and bulbus cordis and twist around each other ad fuse to form a spiral aortico-pulmonary septum.
b. The septum fuses in the midline and also with the top of the muscular ventriclar septum.

Question 11

What does the bulbus cordis give rise to?

Answer 11

Alongside the primitive ventricle it gives rise to the ventricles of the heart

Question 12

What are endocardial cushions?

Answer 12

A subset of cells in the development of the heart that play a vital role in the proper formation of the heart septa.

Question 13

What is the septum primum?

Answer 13

Works to separate the primitive atrium into right and left chambers

Question 14

What is the ostium primum?

Answer 14

This is a small gap found below the septum primum before it fuses with the endocardial cushion and closes the ostium primum off completely

Question 15

What is the ostium secundum?

Answer 15

A perforation in the superior part of the septum primum due to apoptosis (this forms the foramen ovale hole)

Question 16

What is the septum secundum?

Answer 16

This septum grows downward from the upper wall of the atrium immediately to the right of the primary septum and ostium secundum. Shortly after birth it fuses with the septum primum, and closes the foramen ovale.

Question 17

What is the foramen ovale formed from?

Answer 17

A gap in the septum secundum

Question 18

What is the truncus arteriosus?

Answer 18

An arterial trunk that originates from both ventricles of the heart that later divides into the aorta and the pulmonary trunk.

Question 19

Outline the stages involved in the development of the heart

Answer 19

1) Formation of endocardial tubes from angiogenic cluster
2) Formation of primitive heart tube from endocardial tube fusion
3) Heart looping
4) Septum formation
5) Atrial partitioning
6) Ventricular formation
7) Partitioning of the outflow tract (aorta and pulmonary trunk form)

Question 20

What is the ductus arteriosus?

Answer 20

Connects the pulmonary artery (right ventricle) to the proximal descending aorta (left ventricle), and as there is increased pulmonary vascular resistance (to reduce blood flow to the fluid-filled lungs) this allows the blood from the ventricle to bypass the lungs and directly enter the aorta. This protects the lungs from circulatory overload and allows the right ventricle to strengthen

Question 21

What is the ductus venosus?

Answer 21

A continuation of the umbilical vein and it connects the umbilical vein to the inferior vena cava. The flow along this structure is regulated by a sphincter and it conducts highly oxygenated blood

Question 22

Describe foetal circulation

Answer 22

There is high pulmonary pressure and therefore blood shunts from the right to the left atrium via the foramen ovale, and blood that leaves in the pulmonary artery can enter the descending aorta via the ductus arteriosus. The ductus venosus connects the umbilical vein to the IVC.

Question 23

What happens to the umbilical vein after birth?

Answer 23

Becomes the ligamentum teres and the surrounding mesentery becomes the falciform ligament

Question 24

What happens to the ductus venosus after birth?

Answer 24

It constricts so that all of the blood has to pass through the hepatic sinusoids

Question 25

What happens to the foramen ovale after birth?

Answer 25

The foramen ovale shuts and becomes the fossa ovalis

Question 26

What happens to the ductus arteriosus after birth?

Answer 26

The ductus arteriosus becomes the ligamentum arteriosum as the pulmonary vascular resistance falls below systemic pressure, therefore the blood flow through the ductus arteriosus is diminished, and this increase in pO2 and decrease in blood flow leads to closure mediated by bradykinin (prostaglandin E2 may have a role in re-opening the ductus arteriosus)

Question 27

What happens to the umbilical arteries after birth?

Answer 27

The umbilical arteries constrict, however, some parts do remain patent and supply the bladder.

Question 28

What are cyanotic heart lesions? Give three examples

Answer 28

Where blood from the right side of the heart goes to join the left (deoxygenated goes to oxygenated region).

Examples: tetralogy of ballot, transposition of the great vessels and persistent truncus arteriosus

Question 29

What are acyanotic heart lesions? Give four examples

Answer 29

Where blood from the left side of the heart goes to join the right (oxygenated goes to deoxygenated region)

Examples: atrial septal defect, ventricular septal defect, persistent ductus arteriousus and coarctation of the aorta

Question 30

Describe tetralogy of fallot?

Answer 30

This is a condition whereby there are four lesions involved:

1. There is an ‘over-riding’ aorta that arises directly above the septal defect
2. There is right ventricular hypertrophy due to high right ventricular pressure (as a result of other issues)
3. There is a ventricular septal defect
4. There is a narrow right ventricular outflow due to an infundibular stenosis

Question 31

Describe persistent truncus arteriosus

Answer 31

Cyanotic heart lesion where there is only a single artery that arises from the heart and therefore supplies both the pulmonary artery and aorta with mixed blood, and this is usually as a result of a large ventricular septal defect below the truncal valve

Question 32

Describe transposition of the great vessels

Answer 32

This is a cyanotic heart lesion where the conotruncal septum fails to follow the spiral course, and instead runs straight down and therefore the right ventricle feeds into the aorta and the left ventricle feeds into the pulmonary artery. Cathetrisation of the fossa ovals is done to increase mixing of blood

Question 33

Describe atrial septal defects

Answer 33

In adult life this will cause left–>right shunting which is often asymptomatic but symptoms may occur on exertion

Question 34

Describe ventricular septal defects

Answer 34

These are the most common form of cardiac defect where most defects are in the membranous septum, and 10% are in the muscular septum

Question 35

Describe patent ductus arteriosus

Answer 35

CThe ductus arteriosus connects the descending aorta to the main pulmonary trunk, and ordinarily in the neonate closure results in fibrosis which leads to the formation of the ligamentum arteriosum instead. Although a small PDA doesn’t increase the risk of heart failure, it does increase the risk of obtaining bacterial endocarditis. Current treatment involves the administration of a prostaglandin inhibitor (such as ibuprogen) but if it still fails t oclose then surgical closure with a clip is performed.

Question 36

Describe coarctation of the aorta

Answer 36

Constriction of the aorta can occur above or below the ductus arteriosus. If the constriction is pre-ductal then the ductus arteriosus will persist and continue to allow blood flow, however in post-ductal constriction, collateral circulation must be established for adequate perfusion of the body and legs.

Question 37

What tissue is the inter ventricular septum?

Answer 37

Trabeculated muscle