Development of the heart and its conduction system

Question 1

What are the 3 germ layers the embryo has already formed shortly after gastrulation?

Answer 1

The endoderm, ectoderm and mesoderm

Question 2

What are the 2 ‘fields’ the heart forms from, which make a simple crescent shape in the flat disc of the embryo?

Answer 2

The first heart field and the second heart field.

Question 3

From what germ layer do the FHF and SHF originate?

Answer 3

The mesoderm

Question 4

What does the simple crescent develop into, and what does this consist of?

Answer 4

The simple crescent develops into a tube with the outflow tract at the top and a venous pole at the bottom, connected by the primary heart tube.

Question 5

What happens to the primary heart tube as it develops?

Answer 5

It balloons out to form the right and left ventricles and atria

Question 6

What happens to proepicardial cells of the proepicardial organ?

Answer 6

They migrate from the proepicardial organ to surround the heart and give rise to the coronary vasculature

Question 7

What are the cells above the outflow tract?

Answer 7

CNCCs- cardiac neural crest cells- which migrate to the circumpharyngeal ridge to contribute to the 3rd, 4th and 6th pharyngeal arches

Question 8

Where does the proepicardial region arise?

Answer 8

As an outgrowth of the coelomic mesothelium at the ventrocaudal base of the developing heart

Question 9

In what weeks post-conception does heart development occur?

Answer 9

Weeks 3-10

Question 10

What type of cells derive the cardiac crescent?

Answer 10

Mesodermal cells

Question 11

What lines the lumen of the crescent-shaped tube of myocardium that is the origin of the heart?

Answer 11

A single cell layer of endocardium

Question 12

What will the medial part of the crescent become?

Answer 12

The medial part of the crescent expands to form the left ventricle.

Question 13

What controls the direction of looping of the heart tube as it elongates and bends?

Answer 13

Genetics

Question 14

What is dextrocardia?

Answer 14

A rare congenital heart condition in which your heart points towards the right side of your chest instead of the left side

Question 15

How does the heart tube elongate?

Answer 15

Cells from the second heart field are added at both the outflow and inflow ends

Question 16

What cells make outflow cushions

Answer 16

Neural crest cells

Question 17

Where do epicardium cells come from?

Answer 17

The pro-epicardial organ

Question 18

What’s the name of the process by which the tube is divided into chambers?

Answer 18

Partitioning

Question 19

What does the single layer of epithelium lining the myocardium of the simple heart tube secrete?

Answer 19

Cardiac jelly (mesenchymal tissue)

Question 20

Where is cardiac jelly found?

Answer 20

The space between the myocardium and the endocardium

Question 21

In the outflow tract region, neural crest cells migrate into the cardiac jelly to do what 3 things?

Answer 21

Separate the pulmonary artery and aorta
Form outflow cushions
Form valves in the heart

Question 22

Describe the surfaces of the 4 chambers of the heart once they’ve formed

Answer 22

The atria have a smooth surface, while the ventricular chambers have a honeycombed appearance known as trabeculation

Question 23

What is atrioventricular septation?

Answer 23

The forming of a septum between the atria and ventricles

Question 24

What is atrial septation?

Answer 24

The forming of a septum between the left and right atria

Question 25

What is ventricular septation?

Answer 25

The forming of a septum between the left and right ventricles

Question 26

What is outflow tract septation?

Answer 26

The forming of a septum to separate the pulmonary artery from the aorta

Question 27

What are the 2 things that can divide the heart?

Answer 27

Cushions | Muscular walls

Question 28

What are the 2 types of cushions?

Answer 28

Atrioventricular cushions | Outflow tube cushions

Question 29

What are 3 possible consequences of cushion formation problems?

Answer 29

Valve formation defects Stenosis Atresia defects

Question 30

How does AV cushion formation occur?

Answer 30

Cardiac jelly is secreted by the myocardium. Endocardial cells undergo epithelial-mesenchymal transition. Epithelial cells separate from their neighbours, become migratory and migrate from the epithelial layer to the cardiac jelly layer and populate the jelly

Question 31

What septa are formed from muscular walls?

Answer 31

The primary and secondary interatrial septa and the interventricular septum

Question 32

What is pulmonary atresia?

Answer 32

A congenital heart defect that's normally diagnosed soon after birth and involves incorrect formation of the pulmonary valve

Question 33

Name a condition in which AVSDs are common

Answer 33

Down's syndrome

Question 34

When the primary inertial septum grows down to fuse with the AV cushions, what happens to the trailing edge?

Answer 34

The trailing edge of the primary septum breaks down to form a hole called ostium secundum or foramen ovale, which allows blood to continue to flow from the right to left atrium

Question 35

What does the septum secundum grow down to form?

Answer 35

A flap valve which allows communication between the right and left sides of the heart in foetal circulation

Question 36

What causes the flap valve to close and foramen ovale to seal shut after birth?

Answer 36

An increase in left atrial pressure

Question 37

Name 3 atrial septal defects

Answer 37

Persistent foramen ovale Ostium primum ASD Ostium secundum ASD

Question 38

When does an ostium primum ASD occur?

Answer 38

When the atrial septum near the AV valves has a communication between the 2 atria, causing a left to right shunt

Question 39

What causes an ostium secundum ASD?

Answer 39

When there's a hole in the centre of the atrial septum that allows blood to mix and the heart works less efficiently as a result

Question 40

Why does ASD lead to left to right shunting?

Answer 40

The pressure is higher in the left side of the heart than the right, leading to an acyanotic defect

Question 41

What % of ventricular septal defects are perimembranous?

Answer 41

75%

Question 42

What are 25% of ventricular septal defects?

Answer 42

Muscular

Question 43

Are VSDs cyanotic or acyanotic?

Answer 43

VSDs are acyanotic

Question 44

What 2 conditions does a left-to-right shunt cause?

Answer 44

Pulmonary hypertension and right heart hypertrophy

Question 45

Describe septation of the outflow tract in 3 steps

Answer 45

A single tube is separated into the aorta and pulmonary artery. 2 cushions spiral through the trunks arteriosus. There's a complex remodelling at each end of the tube

Question 46

Name 3 initial properties of pharyngeal arch arteries

Answer 46

Paired Symmetrical Formed sequentially

Question 47

What's the order of formation of the pairs of pharyngeal arch arteries?

Answer 47

The 1st pair, then the 2nd pair, then the 3rd pair, then the 4th pair, then the 6th pair

Question 48

Which pairs contribute to the heart?

Answer 48

Pairs 3, 4 and 6 form part of the aortic arch

Question 49

Describe the formation of the outflow tract in 4 steps

Answer 49

Spiralling cushions separate the aorta and pulmonary artery Neural crest cells contribute to the AP septum Complex remodelling of the aortic arches Proximal cushions fuse with the IVS and AVS to separate the 2 ventricles

Question 50

What can unequal division of the OFT cause?

Answer 50

Atrial or pulmonary stenosis

Question 51

What can happen without spiralling of cushions through the common arterial trunk?

Answer 51

Transposition of the great arteries

Question 52

What is transposition of the great arteries?

Answer 52

The aorta is connected to the right side and the pulmonary artery to the left side, which is a cyanotic defect that Cana be lethal

Question 53

What are the 4 characteristic features of tetralogy of Fallot?

Answer 53

VSD Pulmonary stenosis Right ventricular hypertrophy Overriding aorta

Question 54

What does tetralogy of Fallot cause?

Answer 54

Right to left shunt, which leads to cyanosis.

Question 55

How can cyanosis be recovered from in a patient with tetralogy of Fallot?

Answer 55

Squatting, as this increases peripheral arterial resistance and causes a left to right shunt which increases pulmonary blood flow

Question 56

What tissue does conduction tissue originate from?

Answer 56

Myocardium (not nerves)

Question 57

What causes variations in conduction properties?

Answer 57

Differences in ion channel and connexin expression