Development of the Heart and Foetal Circulation

Question 1

When do the cells of the heart first appear?

Answer 1

During gastrulation the cells of the primary heart field are specified

Question 2

How is cell fate determined in the Primary Heart Field?

Answer 2

Position within the heart field determines cell fate. The primary heart field divided into the first and second heart fields based on time of cell differentiation. The first heart field differentiates at the start of embryonic life

Question 3

How is the primitive heart tube formed?

Answer 3

Precursor cells within the furst heart field differentiate and the tow sids of the heart move down (towards tail end/caudally) and zip together to form a heart tube which conists of a primitve inflow tract (sinus venosus), ventricle and outflow tract

Question 4

How is this primitive heart tube remodelled to produce the four chambered heart?

Answer 4

The heart tube changes shape via looping going from a very primitve one chambered heart to a more complex yet still ismple two chambered heart with one of each component (atrium,ventricle etc.)
The atrium is then split into two to give a 3 chambered heart and finally the left and right ventricles and the aortic and pulmonary circulations are divided.

Question 5

What is the purpose of the second heart field?

Answer 5

Important for later stages in development of the heart and contributes to the right ventricle and outflow tract

Question 6

How are the key steps in heart development achieved?

Answer 6

Division of the atria, ventricles and outflow tract is dependent upon four key events -
Septation of the atria
Septation of the Ventricles
Septation of the Outflow tract
Alignement of the atria with the appropriate ventricle

Question 7

How does alignment of the atria with the correct ventricle occur?

Answer 7

Dependent upon the formation of endocarial cushions (derived from cardiac jelly/ECM) which fuse to define the left and right AV canals
These endocardial cushions grow pushing endocardial cells towards each other until they fuse, dividing left from right and atria from ventricle
These endocardial cushions also give rise to the valves of the developing heart

Question 8

What are the two septa and what are their roles?

Answer 8

The septum primium grows down to the endocardial cushion and fuses to divide left from right whereas the seprtum secundum grows down immediatey adjacent to the septum primium but doesnt go as far as the cushion (passes ostium)

Question 9

What is the ostium primum?

Answer 9

Hole that initially allows communication between the RA and LA - present when the septum primum grows down but not the secundum

Question 10

How is the ostium secundum formed?

Answer 10

The ostium primum closes and the ostium secundum is formed (communication between the atria is maintained)
The septum secundum grows down and covers the ostium secundum but still allows communication between the two atria

Question 11

How are the ventricles seperated during development?

Answer 11

By the growth of a muscular septum which grows upwards and contains as mesenchymal cap that when it reaches the AV cushion fuses thw two together

Question 12

How are the outflow tract and pulmonary trunks septated?

Answer 12

The outflow tract contains endocardial cushions that develop in the same way as tose in the AV canal however neural crest cells are responsible for the septation of the outflow tract into aortic and pulmonary trunks

Question 13

How re tge paired aortic arch arteries remodelled into specific portions of the cranial circulation?

Answer 13

Aortic arches 1,2 and 5 are lost
while the 3rd 4th and 6th remain and give rise to:
3rd = Left and right carotid arteries (common and internal)
4th = Aorta and right subclavian artery
6th - Left and right pulmonary arteries and ductus arteriosus

Question 14

What is the ductus venous?

Answer 14

A foetal blood vessel connecting the umbilical vein to the inferior vena cava
Half the blood from the umbilical vein is directed into the IVC, the other half goes into the liver and then enters the IVC through the hepatic veins

Question 15

How is blood flow regulated in the foetus?

Answer 15

Sphincter mechanism:
Sphincter closes = blood cannot flow into ductus venosus and therefore goes into the liver

Question 16

What is the Ductus Arteriosus?

Answer 16

Foetal blood vessel which connects the pulmonary trunk to the descending aorta
A small amount of blood passes from the RV through the pulmonary arteries. Only 10% of it is needed for the lungs, the rest is re-routed directly into the aorta through the DA thus protecting the foetal lungs from circulatory overload.
DA allows the right ventricle to exercise at full capacity and aids muscle development for birth

Question 17

What is the Foramen Ovale?

Answer 17

An opening that connects the Right to Left atrium, to protect the developing lungs.
Blood enters the RA at high pressure since it comes directly from the umbilical vein and has mostly bypassed the capillary beds if the liver. High pressure exit through the right side would take blood into the pulmonary trunk and the collapsed lungs. Most of the blood goes to the LA via the foramen ovale then the LV and aorta not through the right ventricle to the lungs