Development of the Heart & its Conduction System

Question 1

Where does the Cardiac crescent come from?

What occurs once the cardiac crescent forms?

LOOK AT DIAGRAM!

Answer 1

Mesodermal cells

1. Tube formation - wrapping muscle (mesoderm) around endothelium to form Heart Tube.
2. Attachment - attaching tube to developing vessels (arteries and veins)
3. Elongation - adding of cells to both ends of tube
4. Bending, Looping - while neural crest cells migrate to outflow region, making outflow cushions
5. Septation - form heart chambers

Question 2

What occurs during Septation?

What are the 2 types of septation?

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Answer 2

• Its the dividing of the heart chambers, so there’s Atrioventricular (AV) septation, Atrial septation, Intraventricular septation, and Outflow tract (OFT) septation (divides the pulmonary artery from aorta).
• 1. Cushions - for AV and OFT:
     Allow for valve formation; defects like stenosis can occur here.

2. Muscular walls - for atrial and intraventricular septa:
   Defects here can cause holes to form.

Question 3

How do the primary Inter-atrial septum and septum secundum for the 2 atria?

Identify the pulmonary vein and venous sinus orifices in the picture

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Answer 3

Primary inter-atrial septum grows downwards to fuse with the AV cushions - back edge of the septum breaks down to allow blood to continue to flow from right to left atrium.

Septum secundum grows downwards to form the flap valve and, after birth, the left trial pressure increases to close the valve and seal off both atria.

Question 4

What defects can occur during Atrial septation?

What are the types of ASDs that can occur?

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What do ASDs cause?

What type of defect is it?

Answer 4

• Atrial septal defects (ASD)
• • Persistent Foramen Ovale (PFO) - where the flap valve doesn’t seal completely, and is a risk factor for stroke.
• Ostium primum defect (OPD) - hole at the top
• Ostium secundum defect (OSD) - hole at the bottom
• Shunting from left to right atrium due to pressure difference.
• ACYANOTIC - levels of oxygen in body ISN’T affected.

Question 5

How is the AV septum formed?

What defects can occur here?

Answer 5

• By the union of the dorsal and ventral AV cushions

- Atrioventricular septum defects (AVSD) - commonly present in those with Down’s Syndrome.

Question 6

How is the Intraventricular septum formed?

What are the defects that can occur here?

What can they cause?

What type of defect is it?

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Answer 6

• It grows from the wall and grows towards the AV cushion, and is initially a thickened trabecula.

The superior part (75%) of the IV septum is membranous, and the inferior part (25%) is muscular.

• Ventricular Septal Defects (VSD) - can be small/medium/large
• Shunting from left to right ventricle due to pressure difference.
• ACYANOTIC - levels of oxygen in body ISN’T affected.

Question 7

How does OFT septation occur?

What is the Ductus Arteriosus and what is its role?

LOOK AT DIAGRAM!

Answer 7

• Divides the pulmonary arteries from the aorta:
  1. Single tube is separated into the aorta and pulmonary artery - must be attached to the correct ventricle.
  2. Two cushions spiral through the truncus arteriosus - the vessel that both ventricles are connected to.
  3. To form the OFT, the spiralling cushions separate the aorta and pulmonary artery, and the neural crest cells contribute to the AP (aortopulmonary) septum.
  4. Remodelling of the aortic arches then occurs to make the great vessels.
• Results in left to right shunting and is Acyanotic. It allow for foetal circulation to bypass the lungs. After birth, it should become a ligament, called Ligamentum Arteriosum.

Question 8

What are the 2 defects that can occur during this process?

LOOK AT DIAGRAM!

Answer 8

1. Transposition of the Great Arteries (TGA) - where aorta and pulmonary artery are connected to the wrong ventricles:
   Is CYANOTIC, and is lethal without there being a VSD, ASD, or PDA due to shunting. Surgery can be carried out to correct a TGA by switching the arteries/atria.
2. Tetralogy of Fallot (ToF) - has 4 main characteristics:
   LARGE VSD, PULMONARY STENOSIS, RV HYPERTROPHY, OVERRIDING AORTA. This creates a right to left shunt and the defect is CYANOTIC.

Question 9

Describe the origin of the Cardiac Conduction system

What are the 2 layers of the conduction system?

Answer 9

• Conduction tissue is a SPECIALISED MYOCRADIUM (not nerves).

There are variations in the conduction properties caused by differences in ion channels and gap junction expression. The conduction system differentiates by progressive, localised recruitment from the heart tube myocardium.

• Fibro-fatty layer at the AV junction, and the Fibrous sheath wrapping around the ventricular bundle branches.