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When does the cardiogenic field appear?

week 3


Describe the formation of the cardiogenic field.

1. Cells in splanchnopleuric mesoderm (primitive heart field) differentiate into myeloblasts and blood islands.
2. These unite to form a horseshoe-shaped endothelial-lined tube at cephalic end of embryo = cardiogenic field.


When does embryo folding occur?

week 4


What are the effects of embryo folding on formation of the primitive heart?

1. Cephalocaudal folding (to form C shape) - causes pericardial cavity to move from cranial end to thorax.

2. Lateral folding - causes caudal regions of paired cardiac tube to merge (except at caudal-most ends) - forms single primitive heart tube.


Describe the formation of the primitive heart tube.

1. Lateral folding of embryo causes caudal regions of paired cardiogenic field tube to merge (except at caudal-most ends).
2. Central part of horseshoe simultaneously expands to form future outflow tract and ventricular regions.
3. Sulci (contrictions) appear in primitive heart tube to form primitive heart chambers.


Name the different chambers of the primitive heart tube.

All The Best Vaginas Are Shaved:
- Aortic arch/roots (outflow portion)
- Truncus arteriosus
- Bulbis cordis
- primitive Ventricle
- primitive Aorta
- Sinus venosus (inflow portion)


When does cardiac looping occur?

Week 4: days 23-28


Why does cardiac looping occur?

Heart tube elongates, causing cardiac looping due to limited space in pericardial cavity


In which directions do the cephalic and caudal portions of the primitive heart tubes move during cardiac looping?

- cephalic portion moves ventrally, caudally and right (VCR)
- caudal portion moves dorsally, cranially and left


Why is cardiac looping important and what passageway does this form?

i) Places inflow and outflow portions in correct orientation relative to each other - creates transverse pericardial sinus as inflow portion (sinus venosus and primitive atrium) now behind outflow portion (aortic root and truncus arteriosus).
ii) Primitive atrium now above primitive ventricle.


When does interatrial septation and dev. of atria occur?

End of 4th wk and 5th wk (days 27-37).


What are endocardial cushions?

Migration and proliferation of neural crest cells


Describe the process of interatrial septation.

1. Superior and inferior endocardial cushions form and fuse in centre of atrioventricular canal.
2. Crescent-shaped wedge of tissue grows down from atrial roof to endocardial cushions, forming 2 atria - septum primum. Leaves gap between itself and cushions - ostium primum.
3. Apoptosis in upper part of septum primum creates 2nd gap - ostium secundum. Forms just as ostium primum closes.
4. Second septum forms to the right - septum secundum. Leaves gap between itself and cushions - foramen ovale.


What is the function of the ostium secundum and foramen ovale?

Allow staggered R to L shunt of blood from RA to LA (due to higher pressure in RA) - flap valve is formed, preventing backflow of blood.


What are the 2 components of the ventricular septum?

- Muscular portion: forms most of the septum
- Membranous portion


Describe the process of interventricular septation.

1. Muscular portion of septum grows upwards from base of ventricles towards fused endocardial cushions.
2. Leaves small gap - primary interventricular foramen.
3. Membranous portion formed by connective tissue growing down from endocardial cushions - meets and fuses with muscular portion to close gap.


In which part of the interventricular septum do defects usually occur?

In membranous portion as thinner and develops later.


Describe the formation of the aorta and pulmonary artery.

1. Mesenchymal proliferation in bulbus cordis and truncus arteriosus forms 2 bulbar ridges on opposite walls of outflow tract.
2. Ridges fuse in midline and twist 180 degrees as they grow.
3, Forms spiral septum that separates aorta and pulmonary artery.


What do the arterial systems of the body develop from?

Bilaterally symmetrical systems of arched vessels exiting the primitive heart - undergo extensive remodelling to create major arteries leaving the heart.


What is the 4th arch of the primitive arterial system remodeled into?

- Right arch: right subclavian vein (proximal part)
- Left arch: arch of aorta


What is the 6th arch of the primitive arterial system remodeled into?

Remodelled into 'pulmonary arch'
- Right arch: right pulmonary artery
- Left arch: left pulmonary artery and ductus arteriosus


With which cranial nerve is the is the 6th arch of aorta associated with?

recurrent laryngeal nerve


Which factors influence the course of the recurrent laryngeal nerve?

i) Caudal shift of developing heart and expansion of developing neck region.
ii) Need for foetal shunt between PT and aorta.

As heart descends caudally into thorax, nerve hooks around 6th aortic arch and also shifts caudally into thorax:
- left laryngeal nerve descends to T5 and hooks around aortic arch as becomes caught by ductus arteriosus
- right laryngeal nerve descends to T1/T2 and hooks around right subclavian artery


Why is a hoarse voice a symptom of heart problems?

After being hooked by 6th aortic arch, recurrent laryngeal nerves loop back up to innervate the larynx.


Where do oxygenation and CO2 removal occur in the foetus and what is the consequence of this for foetal circulation?

In foetus, lungs aren't functioning so oxygenation and CO2 removal occur at placenta.
Means shunts are required to maintain foetal life, that must be reversible at birth.


Describe the mature circulatory system.

- Oxygenated blood moves from lungs to heart... pumped around body.
- Deoxygenated blood collected from body... moves into heart... is pumped into lungs for reoxygenation and CO2.


Describe the foetal circulatory system.

- Oxygenate blood arrives at umbilicus and travels up umbilical vein through abdomen.
- Must bypass liver so is shunted into inferior vena cava and RA.
- Shunted into LA to bypass lungs.
- LV pumps it to rest of body, esp. brain.


Why must oxygenated blood be shunted around the liver?

Is highly metabolically active so would deplete all oxygenated blood.


Why is a shunt required between pulmonary artery and aorta?

A small amount of blood is required in RV to allow muscle dev. but a shunt is required from pulmonary artery to prevent blood movement into lungs.


What are the 3 foetal shunts and what are their functions?

1. Ductus venosus - from umbilical vein to IVC to bypass liver
2. Foramen ovale - from RA to LA to bypass RV and lungs
3. Ductus arteriosus - from pulmonary trunk to aorta to bypass lungs