Session 3 ILOs - Development of the heart

Question 1

Describe the formation and looping of the primitive heart tube

Answer 1

Formation:
- The primitive heart tube is formed by folding of the embryo laterally and there is a merging of the paired cardiac tube with an inner endothelial lining and an outer myocardial layer

Looping:
- The heart tube continues to elongate as cells are added to its cranial end and the tube begins to bends so that the cephalic portion (ventricles) bend ventrally, caudally and to the right. Whereas the caudal portion (atria) shift dorsocranially and to the left = creates the cardiac loop

Folding takes the pericardium with it. It also forms the transverse and oblique sinuses

Question 2

Name the regions of the developing heart (heart tube)

Answer 2

In the heart tube:

• Truncus arteriosis
• Primative ventricles
• Primative atria
• Sinus venosus

Question 3

Describe the process of atrial septation

Answer 3

Atrial septation:

• Endocardial cushions grow from the dorsal and ventral walls of the atrio-ventricular canal
• The septum premium develops coming down from the roof of the primitive atria - with a small gap at the bottom before reaching the endocardial cushion forming the osteum premum
• Apoptosis forms a second hole the osteum secondum (allows shunting of blood between atria) and the osteum premum is lost as the septum premum continues to grow
• The septum secondum grows down from the roof of the primitive atria and overlaps with the osteum secondum
• The opening left by the septum secundum is called the oval foramen (foramen ovale)

Question 4

Understand the principles of the foetal circulation

- Fate of the foetal shunts

Answer 4

After birth, when lung circulation begins and pressure in the left atrium increases, the valve of the oval foramen is pressed against the septum secundum, obliterating the oval foramen and separating the right and left atria.

Question 5

Describe in brief the development of the great vessels

Answer 5

The aortic arches, arise from the aortic sac, the most distal part of the truncus arteriosus.

Division of the truncus arteriosus by the spiral septum divides the outflow channel of the heart into the ventral aorta and the pulmonary trunk (the spiral septum joins with the inter ventricular septum)

The aorta and pulmonary trunk do twist around each other so that the pulmonary trunk is anterior to the aorta

Question 6

Describe the common types of congenital malformation of the heart and great vessels

Answer 6

Patent foramen ovale:

• Where the septum premum and the septum secondum fail to fuse together after birth so have communication between LA and RA
• Can be asymptomatic if small, but in adults it can also allow thrombi formed in the venous system to cross from the RA to LA and go into systemic circulation and because a stroke (otherwise would have caused a pulmonary embolism)

Hypoplastic left heart:

• Too narrow of a pathway between LA and RA
• Inadequate movement of blood from RA to LA in foetus, so the left ventricle degenerates and the right ventricle becomes hypertrophied as it’s the main ventricle supplying both pulmonary and systemic circuits

Patent ductus arteriosus:

• Failure of the ductus arteriolus to close
• Leads to mixing of blood from the pulmonary and systemic circuits of blood from the aorta into the pulmonary artery (due to pressure gradient)

Transposition of the great arteries:

• The spiral septum continues spiralling an extra 180 degrees
• The pulmonary artery comes from the left ventricle and the aorta comes from the right ventricle
• Very problematic upon birth, where the pulmonary and systemic circuits are completely separate
• Needs surgery after birth (prostaglandins keep ductus arteriosus open)

Tetralogy of Fallot:

• Main issue is that the spiral septum forms 2 uneven great vessels (aorta is bigger than the pulmonary artery)
• Causes 4 things: ventricular septal defect, over riding aorta that causes pulmonary stenosis and therefore RV hypertrophy

Question 7

Describe the process of ventricular septation

Answer 7

Ventricular septation:

• Endocardial cushion forms in the centre
• Thick muscular component grows up from the muscular wall and there is a small gap
• A thin membranous component grows down to meet the thick muscular component
• Together, they form the interventricular septum