Cardiovascular - Embryology

Question 1

In cardiovascular embryology, what is another name for the inflow tract and outflow tracts?

Answer 1

Inflow - sinus venosus

Question 2

In CVS embryological development, what “cardiac marker” is secreted by the “first heart field”?

a. Nkx2.5
b. Tbx5
c. IL-2
d. IL-4

Answer 2

a. Nkx2.5

Nkx2.5(+) lineage cells possess the potential to differentiate into various cardiomyocyte cell types.
·

Nkx2.5 is essential in cardiac development, and mutations in Csx (which encodes Nkx2-5) cause various congenital heart diseases.
·

Nkx2.5 is the putative cardiomyocyte specification gene.

Question 3

What happens in the first heart field in embryology under the influence of the cardiac marker Nkx2.5?

Answer 3

Precursor cells of the First Heart Field differentiate and the two sides of the heart
zip together to form a heart tube.
This heart tube consist of a primitive inflow tract (sinus venosus), atrium, ventricle and outflow tract.

Question 4

After the straight heart tube is formed by the “zipping up” of the two endocardial tubes in the First Heart Field, what happens next?

Answer 4

Differentiation starts to take place in the Second Heart Field, which expresses transcription factor Tbx1 (T-box 1). The Tbx1-induced differentiation results in remodelling of the straight heart tube into the four-chambered heart and major vessels.

Genes in the T-box family play important roles in the formation of tissues and organs during embryonic development.
The T-box 1 protein appears to be necessary for the normal development of large arteries, muscles and bones of the face and neck, and glands such as the thymus and parathyroid.

The straight heart tube undergoes remodelling to form a four-chambered heart in stages:

1. The single atrium is divided in two to form a three-chambered heart: two atrium & one ventricle
2. Single ventricle is divided into left & right
3. Pulmonary & aortic circulations are divided from the single outflow tract.

Question 5

What critical process in heart development is dependent on the formation and fusion of four endocardial cushions?

a. Septation of the atria
b. Septation of the ventricles
c. Septation of the outflow tract
d. Alignment of atria with ventricles

Answer 5

d. Alignment of atria with ventricles.

The fusion of the dorsal, ventral , right & left endocardial cushions form the left and right atrioventricular valves and thus align the right atrium with the right venticle and left atrium with left ventricle.

Question 6

How is the septation of the atria achieved?

Answer 6

Two successive growths of tissue from top of the heart down toward the endocardial cushions.

1. The septum primum grows down to divide left from right, stopping before it reaches the endocardial cushions.
2. The gap left by the septum primum is called the ostium primum. This is transitory - eventually ostium primum closes as the septum primum and the endocardial cushions merge.
3. Another hole opens up in the septum primum called the ostium secondum.
4. The septum secundum then grows down immediately adjacent to the septum primum, overlapping the ostium secondum.
5. The septum secondum doesn’t grow all the way down to the endocardial cushions, but it forms a flap over the ostium secondum that opens to allow oxygenated blood from the placenta, draining into the right the right atrium from the inferior vena cava, to be shunted across to the left atrium, down into the left ventricle & out the aorta.

This is the foramen ovale.

Question 7

How are the ventricles septated?

Answer 7

Growth of muscle interventricular septum.

Mesenchymal cap merges with atrioventricular (endocardial) cushion in the cranial portion, dividing left and right

NB separation of atria different to separation of ventricles

Question 8

How is the outflow tract divided into pulmonary arteries & aorta ie., pulmonary & aortic circulations?

Answer 8

The outflow tract contains endocardial cushions that develop in the same way as those in the AV canal.

However, neural crest cells are responsible for septation of the OFT into aortic and pulmonary trunks.

Neural crest cells from hindbrain migrate as a continuous sheet of cells, becoming mesenchymal, giving rise to parts of the heart’s OFT.

Aortic Arches 1, 2 & 5 are lost.

The remaining 3rd, 4th & 6th aortic arch arteries give rise to:

• *3rd – left & right carotid arteries** (common & internal)
• *4th – aorta & right subclavian artery**
• *6th – left & right pulmonary arteries & ductus arteriosus**