Development of Heart & Vessels

Question 1

What are the stages of Heart development & the potential defects that can occur?

Answer 1

1. Formation of 4 chambered heart (situs inversus)
2. Cardiac Looping (dextrocardia)
3. Division of atrioventricular canal into L & R channels
   (ventricular septal defect)
4. Formation of atrial septa (atrial septal defect)
5. Formation of conotruncal cushions &divisions of
   outflow tract (Tetralogy of Fallot)

Question 2

What is the first sign of fertilisation observed within the embryo?

Answer 2

Embryo has 2 pronuclei

Question 3

Describe the development of a human embryo

Answer 3

Developing embryo contains 6-8 cells 3 days after fertilisation
5 days after its called a blastocyst and contains ~100 cells

Question 4

What are the 3 structural divisions of a developing emrbyo?

Answer 4

Ectoderm
Mesoderm
Endoderm

Question 5

What is the ectoderm?

Answer 5

skin (epidermis), brain, spinal cord, CNS, sensory organs

Question 6

What does the mesoderm eventually form?

Answer 6

heart, skeletal muscle, kidneys, urogenital, connective tissues etc.

Question 7

What is the endoderm?

Answer 7

lining of GI tract and Lungs, pancreas, Liver etc.

Question 8

What occurs during gastrulation?

Answer 8

primary germ layers are formed
- (endoderm, ectoderm and mesoderm)

organised into their proper locations

Question 9

When does the formation of the heart tube begin to occur?

Answer 9

after 18 days

Question 10

Where does the heart initially begin to develop from?

Answer 10

The heart develops from the primitive streak via the blood islands in the Splanchnic mesoderm

Question 11

Explain how the heart tube develops during the 20th - 22nd days of gastrulation

Answer 11

20 days
- blood islands slowly form together into 2 tubes called
cardiogenic cords

21 days
- the tubes fuse down the middle to form one heart tube

22 days
- further development gives rise to a tube with
asymmetric openings forming an arterial & venous end
- beginning of the asymmetry in bulge towards left

Question 12

What is the situs inversus?

Answer 12

A congenital condition where major visceral organs are reversed / mirrored from their original positions

Question 13

When does Cardiac looping begin to occur?

Answer 13

after 22 days

Question 14

How does cardiac looping occur during the 22nd - 24th days?

Answer 14

22 days
- cells at each end of the heart proliferate making a
longer tube
- entire structure is in a confined space so folds up on
itself

23 - 24 days
- primitive atrium loops up above and behind the primitive
ventricle.
- looping process brings the primitive area of the heart
into the proper spatial relationship for development

At 35 days
atria and ventricles in correct positions and tubes for circulation are ready

Question 15

What is dextrocardia?

Answer 15

rare congenital heart condition where the heart points to the right side of the chest rather than the left as normal

Question 16

What is the role of the septum primum?

Answer 16

Division of the atrioventricular canal

Septum primum grows along the midsagittal plane, separating the atria for a temporary space called the Foramen Primum

Question 17

How does the division of the atrioventricular canal occur?

Answer 17

The posterior & anterior endocardial tissues fuse, dividing the atria from the ventricles

The left side of the atrium grows pulmonary veins sending growing veins to the lungs

A ventricular septum eventually grows upwards, forming a division between the 2 ventricles. - attaches to atrial and ventricular division

Question 18

What is the consequence of ventricular septal defect?

Answer 18

During ventricular contraction, some blood form left ventricle leaks into right ventricle, passing through lungs and re enters the left ventricle via the pulmonary veins and left atrium.

Question 19

Approximately how big is the ventricular septal defect?

Answer 19

the extent of the opening can vary from pin size to complete absence of the ventricular septum
- viewed via ECG (mid-oesophegal 4 chamber view)

Question 20

What are the 2 main effects of the ventricular septum defect?

Answer 20

1. systemic circulation doesn’t receive all the blood being
   pumped out the left ventricle
2. leakage of blood into right ventricle elevates right
   ventricular pressure& volume causing pulmonary
   hypertension

Question 21

What can cause ventricular septum defect?

Answer 21

one cause is the failure of the endocardial cushions to provide an anchor point for the developing ventricular septum

Question 22

What occurs during after 40 days of gastrulation?

Answer 22

Formation of the atrial septa

Question 23

What is the first step of the atrial septum formation?

Answer 23

initially the temporary hole in the septum primum (foramen primum) allows right to left shunt for foetal blood as there’s no pulmonary circulation.

this closes before a second hole, foramen secundum opens

Question 24

What occurs at 40 days of heart development?

Answer 24

a second more muscular and robust septum grows down from the roof of the atria just lateral to the septum primum.
it obscures the place the foramen primum was

Question 25

How does the atrial septum form at 43 days?

Answer 25

Septum secundum grows but leaves a space for the foramen ovale to allow the right to left shunt of blood

the foramen secundum is partially obscured by the septum secundum but the foramen ovale remains providing an alternative left to right shunt closing the foramen primum

the ventricular septum grows up to fuse with the now fused endocardial cushion

Question 26

Summarise the formation of the atrial septa

Answer 26

1. septum primum grows - (atrial septa)
2. foramen primum is a hole in the septum primum
3. foramen primum closes, foramen secundum opens
4. septum secundum grows, foramen secundum isn’t
   completely obliterated as it later becomes foramen
   ovale

Question 27

What is the result of the atrial septa formation?

Answer 27

Results in the septum primum and the more muscular, septum secundum
foramen ovale (hole) permitting right to left shunt of blood (as still no pulmonary circulation)

Question 28

How does the foramen ovale close at birth?

Answer 28

At birth, the lungs become functional
- pulmonary vascular pressure decreases
- left atrial pressure > right atrial pressure
=> forces septum primum against septum secundum
-> closes foramen ovale

(in time the septum eventually fuse)

Question 29

What disorders can occur due to incorrect atrial septum formation?

Answer 29

a patent foramen ovale is a common septal defect present (to some extent < 25% of adults)

Question 30

How does the division of the outflow tract arise?

Answer 30

Separation of the ventricles results from the union of the:

• conotruncal septum
• endocardial cushions
• ventricular septum

Question 31

Explain how conotruncal septum, endocardial cushions and the ventricular septum unite

Answer 31

The conotruncal septum grows as a spiral down the conus arterious - meets with the endocardial cushions & ventricular septum

collectively they form the final full separation of the right and left sides

Question 32

What is the consequence of incorrect division of the outflow tract?

Answer 32

If the continual septum is misaligned
- differently sized aorta & pulmonary arteries form
- won’t meet with endocardial cushions correctly
=> gives rise to septal defects (tetralogy of Fallot)

Question 33

What are the consequences of the Tetralogy of fallot?

Answer 33

Pulmonary stenosis

• spiral valve grown off centre
• large aorta & small pulmonary artery
• restricts blood to lungs

Overriding aorta

• aortic opening positioned over ventricular septal defect
• blood from both sides of the heart enter aorta
• some deoxygenated blood pumped around the body

Ventricular septal defect
- during systole, blood from LV leaks into RV, passes
through lungs & re enters LV via pulmonary veins & LA
- increases pressure in pulmonary circulation
- volume overload in LV

Right ventricular hypertrophy
- right ventricular wall increases in size to deal with
obstruction in pulmonary artery

Question 34

Which congenital autosomal syndrome are heart development defects a common occurrence?

Answer 34

along with ventricular and atrial septal defects, tetralogy of fallot is common in Down’s Syndrome

Question 35

What is the ductus arteriosus?

Answer 35

a blood vessel connecting the main pulmonary artery to the proximal descending aorta

Question 36

What is the role of the ductus arteriosus in fetus’?

Answer 36

It allows most blood from the right ventricle to bypass foetal fluid filled, non functioning lungs
upon closure at birth, it becomes the ligmentum arteriosum

Question 37

What is the role of the Foramen Ovale in foetal hearts?

Answer 37

The foramen ovale allows blood to enter the LA from the RA
Along with the ductus arteriosus, it enables blood to bypass pulmonary circulation. At birth the foramen ovale closes and later forms the Fossa Ovalis