embryology of the heart

Question 1

gastrulation

Answer 1

mass movement and invagination of the blastula to form three layers – ectoderm, mesoderm (middle layer) and endoderm.

Question 2

ectoderm

Answer 2

(outside) – skin, nervous system, neural crest (which contributes to cardiac outflow, coronary arteries)

Question 3

mesoderm

Answer 3

middle – all types of muscle, most of cardiovascular system, kidneys, blood, bone

Question 4

endoderm

Answer 4

gastrointestinal tract (inc liver, pancreas, but not smooth muscle), endocrine organs

Question 5

FHF (day 15)

Answer 5

future left ventricle (1st field)

Question 6

SHF (day 15)

Answer 6

outflow tract, future right ventricle, atria (2nd field)

Question 7

between day 15 -50

Answer 7

The first heart field generates a scaffold which is added to by the second heart field and cardiac neural crest

Question 8

transcription factor

Answer 8

type of protein which when expressed ‘turns on/off’ many other gene(s) expression: master regulators of complex processes
(Some) cardiac transcription factors:
•	Nkx2.5 (tinman in fly)
•	GATA
•	Hand 
•	Tbx

Question 9

1. formation of the primitive heart tube

Answer 9

• during the 3rd week of development, the heart is formed from cells that form a horseshoe shaped region called the cardiogenic region
• by day 19, two endocardial tubes form. These two tubes will fuse to form a single, primitive heart tube
• day 21: as the embryo undergoes lateral folding, the two endocardial tubes have fused to form a single heart tube
• (overexpression of Nkx2.5 (by injecting RNA) increases heart size)
Preventing GATA4 transcription induces cardia bifida – failure of the endocardial tube to fuse

Question 10

2. Cardiac looping

Answer 10

• The primitive ventricle moves to the embryo’s left side
• The bulbis cordis moves inferiorly, anteriorly and to the embryo’s right
• The primitive atrium and sinus venosus move superiorly and posteriorly.
• The primitive ventricle moves to the embryo’s left side
• The bulbis cordis moves inferiorly, anteriorly and to the embryo’s right
• The sinus venosus is now posterior to the primitive atrium

Question 11

which way is left?

Answer 11

• During development, the node secretes nodal, which circulates to the left due to ciliary movement
• A cascade of transcription factors (e.g. lefty, Pitx2, Fog-1) transduce looping
• (preventing Fog-1 transcription prevents cardiac looping)

Question 12

3. cardiac septation - endocardial cushion formation

Answer 12

• At this stage in heart development, there is one common atrium and one common ventricle
• Blood first enters the atrium, passes through the atrioventricular canal and into the ventricle, then exits the heart through the truncus arteriosus
• Masses of tissue called endocardial cushions grow from the sides of the atrioventricular canal to partition it into two separate openings
• As the endocardial cushions grow together, the atrioventricular canal also is being repositioned to the right of the heart
• The superior and inferior endocardial cushions fuse, forming two separate openings that now are called the right and left atrioventricular canals
• These canals become the left and right atrioventricular openings of the heart