4. CVS Embryology

Question 1

Describe the formation of the Primitive Heart Tube

Answer 1

1. BEFORE the embryo folds, their are a pair of blood islands containing small blood vessels within the mesoderm on either side of the notocord
2. These become two separate endocardial tubes
3. As the embryo folds, tubes meet at midline forming the primitive heart tube

Question 2

When does the formation of the primitive heart tube begin and end?

Answer 2

1. Starts at 19 days
2. Ends at 22 days

Question 3

What are the 6 sectons of the primitive heart tube?

Answer 3

1. Aortic roots
2. Truncus Arteriosus
3. Bulbus cordis
4. Primitive ventricle
5. primtive atrium
6. Sinus venosus

Question 4

Describe looping of primitive heart tube

Answer 4

1. Pericardial sac drives size of looping
2. Tube elongates, runs out of room
3. Bulbus cordis folds down, Primitive atrium folds up

Note: ventricle enlarges more than atrium

Question 5

Describe, in brief, the development of the great vessels

Answer 5

1. Early arterial system = bilaterally symmetrical system of arched vessels
2. degeneration of some of the vessels as we don’t need them
3. RIGHT 4th arch remodels to give proximal part of subclavian artery. LEFT 4th arch = arch of aorta
4. RIGHT 6th arch = R pulmonary artery, LEFT 6th arch = L pulmonary artery and ductus arteriosus (shunt that bypasses lungs)

Question 6

What are important relations to the development of the great vessels?

Answer 6

1. Left recurrent laryngeal nerve gets hooked around ductus arteriosus (shunt) which then after birth degenerates so is hooked around arch of aorta
2. Right recurrent laryngeal nerve gets hooked around right subclavian artery

Question 7

Describe fetal circulation

Answer 7

1. Oxygenated blood from mother in placenta
2. BY PASS LIVER (ductus venosus)
3. IVC –> RA
4. a little bit of blood goes from RA to RV to PT (so that RA forms) but then SHUNTED from PT to Aorta via ductus arteriosus
5. most blood shunted from RA –> LA via foramen ovale to by pass RV
6. Aorta
7. Body
8. deoxygenated blood back to placenta and exchange of nutrients with mother

Question 8

Describe how the foramen ovale is built

Answer 8

1. Septum primum forms = wedge of tissue from top down to endocardial tissue shelf. There is a hole in this tissue called the ‘ostium primum’
2. Ostium primum obliterates and a hole forms above it within the septum primum called ‘ostium secundum’
3. Once complete, a second wall built called ‘Septum secundum’. within it is a hole called the foramen ovale
4. As pressure in right atrium > left atrium, leaves pushed apart so blood can move from right to left side of heart through foramen ovale and ostium secundum

Note: should not line up as after birth, pressure in LA > RA so want two leaves to push together obliterating shunt

Question 9

How are the shunts obliterated?

Answer 9

1. LA pressure > RA pressure as respiration begins so septum primum pushed against septum secundum obliterating it.
2. Ductus arteriosus contracts due to pressure too
3. Ductus venosus is obliterated as placental support removed

Question 10

Describe how the interventricular septum is formed

Answer 10

1. Muscular portion grow up from floor toward endocardial cushions
2. Leaves a small gap (called the primary interventricular foramen)
3. Membranous ventricular septum then forms in this foramen up to the endocardial cushions

Note: membranous portion can be underdeveloped = L –> R shunt

Question 11

How are the outflow tracts separated?

Answer 11

1. Endocardial cushions appear in truncus arteriosus (TA) forming a scaffold on which another septum can form
2. Divides TA into two discrete channels which appear slightly ofset from each other
3. Together they grow upward in a spiral manner into the lumen of the TA ‘spiral septum’.

Question 12

Describe transposition of the great vessels

Answer 12

1. Normally the aorta arises from the LV and PT from RV
2. In this problem, it is switched
3. Therefore baby = cyanotic as ox blood goes to lungs and deox around body
4. Arterial switch surgery needs to be carried out

Question 13

Describe tetralogy of fallot

Answer 13

1. Large ventricular septal defect
2. Overriding aorta (straddling two ventricles)
3. Therefore Right ventricular outflow tract obstruction (pulmonary stenosis)
4. Right ventricular hypertrophy

Could be due to abnormal septation of truncus arteriosus (skewed to one side?)

The neural crest cells control this - derived from neural ectoderm (v. sensitive to alcohol)

Question 14

Describe patent ductus arteriosus

Answer 14

1. Remains open after birth
2. Blood will shunt from left to right (oxygenated blood will go into deox side)
3. can be tied shut/plugged

Question 15

Describe atrial septal defect

Answer 15

1. ostium secundum and foramen ovale holes can align, septum primum resorbed, septum primum too short so stays open
2. = L–>R shunt post birth

Question 16

Describe ventricular septal defect

Answer 16

1. oxygenated blood from LV to RV therefore
2. LV volume overload
3. pulmonary venous congestion
4. eventual pulmonary hypertension

Question 17

Describe atrio-ventricular septal defect

Answer 17

1. Mixing of blood

Question 18

Describe tricuspid atresia

Answer 18

1. No blood inlet into RV
2. RA pressure > LA pressure
3. R to L atrial shunt

Question 19

Describe pulmonary atresia

Answer 19

1. no blood inlet into PT, no RV outflow
2. Pressure in RA > LA
3. Right to left shunt