development of the heart and blood vessels

Question 1

What are the 5 main stages of heart development?

Answer 1

1. Formation of the four chambered heart tube
2. Cardiac looping
3. Division of the atrioventricular canal into the left and right channels
4. Formation of atrial septa
5. Formation of the conotruncal cushions and division of the outflow tract

Question 2

Name a heart defect that can occur during the formation of the 4 chambered heart tube formation

Answer 2

Situs inversus

All the major organs are mirrored from their normal position due to a mistake in the notochord

Question 3

Name a heart defect that can occur during cardiac looping

Answer 3

Dextrocardia
ONLY the heart is mirrored from its normal position: heart points towards right rather than left due to a mistake in the cardiac tube

Question 4

Name a heart defect that can occur during the division of the AV canal into the left and right channels

Answer 4

Ventricular septal defects

Question 5

Name a heart defect that can occur during the formation of the atrial septa

Answer 5

Atrial septal defect (foramen ovale)

Question 6

Name a heart defect that can occur during the formation of the conotruncal cushions and division of the outflow tract

Answer 6

Tetralogy of fallot

Question 7

Describe briefly the formation of the human embryo

Answer 7

• First sign of fertilisation: the fertilised egg has 2 pronuclei (one egg, one sperm)
  
  • 3 days after fertilisation: the developing embryo contains 6-8 cells
  • 5 days after fertilisation: known as a blastocyst and has 100 cells

Question 8

What is the morula?

Answer 8

Embryo developed after 4 days

Solid ball of cells formed as the zygote undergoes cleavage

Question 9

What is the early blastocyst?

Answer 9

Embryo developed after 6 days
Hollow ball of cells with a fluid filled cavity
THESE CELLS ARE TOTIPOTENT

Question 10

What is the late blastocyst?

Answer 10

Pre-embryo with the embryonic disk, two layers of cells that become the embryo proper

Question 11

What is the gastrula?

Answer 11

Embryo with 3 primary germ layers:

• Ectoderm
• Mesoderm
• Endoderm

Question 12

What does the ectoderm go on to become in the human body?

Answer 12

Skin and nervous system

- Epidermis (skin)
- Brain 
- Spinal cord (CNS)
- Sensory organs

Question 13

What does the mesoderm go on to become in the human body?

Answer 13

Rest of the organs (minus intestinal organs)

• Heart
• Skeletal muscle
• Kidneys
• Urogenital organs
• Connective tissue

Question 14

What does the endoderm go on to become in the human body?

Answer 14

Intestinal organs

• Lining of the GI tract
• Lining of the lungs
• Pancreas
• Liver

Question 15

Describe the formation of the heart tubes

Answer 15

→ Heart forms from the primitive streak via the blood islands in the mesoderm (18 days)
→ The blood islands slowly form together into 2 tubes called cardiogenic cords (20 days)
→ The cardiogenic cords fuse down the middle to make ONE heart tube (21 days)
→ Further development gives rise to a tube with asymmetric openings; forming an arterial and venous end
- Notochord send signals at this point to ensure that the left bulges more compared to right
(22 days)

Question 16

What is the function of the notochord?

Answer 16

• Cartilage rod that supports the embryonic structure
• It is the source of signalling molecules; helps ensure the body have a dorsal and ventral surface as well as a left and a right

Question 17

Describe the cardiac looping from 22-35 days

Answer 17

22 days
The cells at each end of the heart proliferate, making the tube longer. The entire structure is in a confined space; therefore the tube begins to fold upon itself

23/24 days
The primitive atrium loops up above and behind the primitive ventricle. The looping process brings the primitive areas of the heart into the proper spatial positions for development

24 days
• The primitive atrium (viewed from side) is growing out of the back and becoming 2 atria
• The future right ventricle grows downwards at the front

35 days
The heart is beating at this stage and the foetal heart can be seen on the ultrasound. However, the four interior chambers of the heart are not yet divided

Question 18

Describe the division of the AV canal at 30 days

Answer 18

• The septum primum grows along the midsagittal plane, separating the atria, except for a temporary space called the foramen primum
  
  • The left side of the atrium grows pulmonary veins, sending growing veins to the developing lungs
  • The posterior and anterior endocardial cushions fuse, dividing the atria from the ventricles

A ventricular septum will eventually grow upwards to form a division between the 2 ventricles. It will attach to the division between the atria and the ventricles

Question 19

What causes a ventricular septum defect?

Answer 19

One cause is the failure of the endocardial cushions to provide an anchor point for the developing ventricular septum. This hole produced can be the size of a pin OR completely absent

Question 20

What can happen to the blood flow in the heart if a patient has a ventricular septum defect?

Answer 20

During ventricular contraction, some of the blood from the left ventricle could leak into the right ventricle. This blood will then go to the lungs and re-enter the left ventricle via the pulmonary veins and left atrium

Question 21

What effect does a ventricular septum defect have on the patient?

Answer 21

TWO main effects:

1. Systemic circulation doesn’t receive all the blood being pumped by the left ventricle
2. Leakage of blood into the right ventricle elevates right ventricular pressure and volume, causing pulmonary hypertension

Question 22

Describe the formation of the atrial septa across 40-43 days

Answer 22

40 days
• The anterior and posterior endocardial cushions fuse, forming the right and left AV canals
• The temporary hole in the septum primum called the foramen primum allows the blood to be shunt from the right to the left atrium, bypassing pulmonary circulation
• Septum secondum (more muscular) grows down the roof of the atria, lateral to the septum primum. It obstructs the place where the foramen primum was. The foramen secondum is partially obscured by the septum secondum, but the foramen ovale still remains, providing an alternative left to right shunt.

43 days
• Septum secondum grows; leaving the foramen ovale exposed to allow the right—>left shunt of blood
• Foramen secondum is partially obscured by the septum secondum, but the foramen ovale remains providing an alternative left –> right shunt as the foramen primum is closed
The ventricular septum grows upwards to fuse with the now fused endocardial cushions

Question 23

What happens to the foramen ovale when we are born?

Answer 23

At birth, the lungs become functional
This means that the pulmonary vascular pressure decreases and the left atrial pressure exceeds that of the right, forcing the septum primum against the septum secondum; functionally closing the foramen ovale.
Overtime the septa fuse, forming the fossa ovalis

Question 24

Describe the division of the outflow tract

Answer 24

At this stage there is no separation between the ventricles, so all the blood from the heart is leaving via ONE vessel.
Then..ONE vessel / 2 = right vessel (pulmonary artery)+ left vessel (aorta)

Question 25

Separation of the ventricles results from the union of which 3 things?

Answer 25

1. Conotruncal septum
   
   2. Endocardial cushions
   3. Ventricular septum
      The conotruncal septum grows as a spiral down the conus arteriosus; meeting with the endocardial tissues and the ventricular septum to allow the formation of the right and left sides

Question 26

What is tetrology of fallot?

Answer 26

Defect that occurs during division of outflow tract
If the conotruncal septum is misaligned, you get differently sized aorta and pulmonary artery OR it wont meet with the endocardial tissue correctly giving rise to a septal defect called tetrology of fallot

Question 27

What 4 outcomes can occur due to tetrology of fallot?

Answer 27

1. Pulmonary stenosis
   When the spiral valve has grown off centre = large aorta + small pulmonary artery
   Small pulmonary artery = restricted blood flow to the lungs
   
   2. Overriding aorta
      Aortic opening is positioned over a ventricular septal defect allowing blood from both sides of the heart to enter the aorta; this means that some deoxygenated blood is pumped around the body
   3. Ventricular septal defect
      During systole blood from the left ventricle leaks into the right ventricle. This blood will then go to the lungs and re-enter the left ventricle via the pulmonary veins and left atrium. Increases pressure in pulmonary circulation and also causes volume overload in left ventricle
   4. Right ventricular hypertrophy
      Right ventricular wall increases in size to deal with obstruction in pulmonary artery

Question 28

What is the ductus arteriosus?

Answer 28

It is a blood vessel connecting the main pulmonary artery to the proximal descending aorta. It allows most of the blood from the right ventricle to bypass the foetal fluid filled non-functioning lungs. Upon closure at birth, it becomes the ligamentum arteriosum

Question 29

Name the 2 main changes that occur when a foetal heart develops into a newborn heart

Answer 29

1. Foramen ovale closes

2. Ductus arteriosus closes