Development of the heart and great vessels

Question 1

Where do heart tubes develop from?

Answer 1

Lateral plate mesoderm at the cranial end of the embryonic disc

Question 2

Overview of week 4 of development

Answer 2

Cephalic and lateral fold of the embryonic disc results in fusion to form a single primitive heart tube

Question 3

What happens between weeks 5 and 8?

Answer 3

Primitive heart tube folds, remodels and separates, resulting in separation of pulmonary and systemic circulation

Question 4

Days 19-22

Answer 4

• Day 19: primitive heart tubes develop in cardiogenic region
• Days 20-21: folding results in fusion of heart tubes
• Day 22: heart tube begins to beat
• Day 24: heart begins to circulate

Question 5

Vasculogenesis

Answer 5

• Arterial and venous system will subsequently undergo complex remodelling process
• Failure to do this results in vascular abnormalities
• Lungs and oesophagus from primitive foregut
• Paired dorsal aorta - precursor of arteries
• Right paired dorsal aorta will involute which results in left aortic arch
• Involution of L aortic arch and failure of involution of right results in a right sided aortic arch - ductus arteriosus stretched - dysphagia
• When neither aorta involutes forms vascular ring which encircles trachea and oesophagus and constricts them

Question 6

Day 23-28

Answer 6

• Day 21: series of constrictions and expansions appear in heart tube
• At this point, at venous end is sinus venosus. Each horn receives 3 veins - umbilical vein, ophlomesenteric vein, common cardinal vein
• Cranial to sinus venosus is primitive atrium, separated from primitive ventricle by atrioventricular sulcus
• Day 23: heart tube starts to elongate and fold. Primitive ventricle displaced to left and bulbus cordis displaced to right
• Reversal of direction of folding leads to abnormalities of lateralisation
• Midportion of bulbus cordis forms outflow tracts
• Distal portion of bulbus cordis forms roof of aorta and pulmonary artery

Question 7

What is dextrocardia with situs solutes?

Answer 7

Atrial and ventricular septal defect
Single ventricle
Transposition of great arteries

Question 8

Atrial remodelling week 4-5

Answer 8

• Sinus venosus forms vena cava and becomes incorporated into right atrium
• Vitelline from yolk sack, umbilical from placenta, common cardinal vein from trunk and head
• 4-5 week reconfiguration of venous system shifts venous return to right - communication between sinus venosus and atrium becomes narrowed and shifts to the right
• Umbilical and l vitelline obliterates
• Common cardinal obliterates
• Right sinus horn enlarges and incorporated into atrium to form sinus venarum - opening of vena cava
• Left horn forms oblique vein of atrium
• Primitive pulmonary veins develop from left atrium and become incorporated into left atrium
• Single pulmonary vein into l atrium = opening of 4 branches into l atrium
• Primitive l atrium forms trabeculated part whilst incorporated veins form smooth walled part
• Smooth walled part of right atrium is sinus venarium - from right sinus horns
• R atrium forms trabeculated part - pectinate muscles
• Pectinate muscles and sinus venarium demarkated by crista terminalis
• L atrial appendage by l atrium
• Smooth part by pulmnonary veins

Question 9

Atrial septation and formation of atrioventricular canal (wk 4+):

Answer 9

• At end of wk 4, endocardial cushions form dividing atrioventricular canal into r and l atrioventricular orifices
• Forms basis for separation of pulmonary and systemic circulations
• The septum primum and then septum secondum form, dividing atria
• Osteum primum closed by growth of endocardial cushions
• Osteum secondum develops by cell death
• Septum secondum doesn’t completely block off osteum secundum
• Opening left by septum secondum is foramen ovale
• Valve of ovale forale pressed against septum secondum, forming fossa ovalis
• Failure to form atrial septa results in atrioseptal defect
• Failure of fusion of septum primum and secundum after birth leads to patent foramen ovale

Question 10

How does atrioventricular canal defect form?

Answer 10

Failure of formation of endocardial cushions

Question 11

Transition to post-natal circulation

Answer 11

• At birth, pulmonary vascular resistance falls and blood circulates through lungs
• Patent ductus arteriosus and foramen ovale normally close forming ligamentum arteriosum and fossa ovalis respectively
• Failure of closure of foramen ovale = patent foramen ovale
• Failure of closure of ductus arteriosus = patent ductus arteriosus
• PDA sometimes need for tetralogy of fallot
• PDA forms pulmonary hypertension and heart failure

Question 12

What is the tetralogy of Fallot?

Answer 12

Pulmonary stenosis
VSD
Overriding aorta
Right ventricular hypertrophy

Question 13

How do outflow tracts form?

Answer 13

• Truncoconal swellings grow from walls of common ventricular outflow tract
• Spiral and fuse to form pulmonary trunk and aortic root
• Truncus arteriosus forms aortic and pulmonary trunks by opposing swellings that form aortopulmonary and truncus septum
• Tissues of truncus and conus swellings populated by neural crest cells - influence development of outflow tracts
• Outflow tracts form aortic root (away from l ventricle) and pulmonary trunk (away from r ventricle)

Question 14

How does persistent trunks arteriosus form?

Answer 14

Failure of formation of cornotruncal septa

May be cyanosed, associated with heart failure

Question 15

How do the great vessels transpose?

Answer 15

Failure of formation of conotruncal septa - aorta from right ventricle

Question 16

Valve on LHS

Answer 16

2 cusps (posterior and anterior) 
Bicuspid/mitral valve

Question 17

Valve on RHS

Answer 17

3 cusps

tricuspid valve

Question 18

What is a ventriculoseptal defect?

Answer 18

Failure of formation of ventricular septum

Question 19

What does the common primitive ventricle form?

Answer 19

Trabeculated part of left ventricle

Question 20

How is the trabeculated par of right ventricle formed?

Answer 20

Bulbus cordis

Question 21

How does the atrioventricular foramen close?

Answer 21

Outgrowth from atrioventricular cushions

Question 22

What causes primitive common ventricle to separate?

Answer 22

Bulboventricular sulcus, forming intraventricular septum - grows towards atrioventricular cushions which have now fused to form septum intermedium (common canal into l and r atrioventricular canals)

Question 23

How does L ventricle form?

Answer 23

Common primitive ventricle trabeculated

Question 24

How does ventricular septum form?

Answer 24

• Atrioventricular canal is repositioned to right, aligning the right AV canal with right A and V
• Left ventricle aligned with truncus arteriosus
• Transverse expansion of common primitive atrium on each side of the bulbus cordis leads to shift in conal truncal portion of heart tube
• Bulbus cordis to more medial position
• Common primitive ventricle trabeculated to form L ventricle
• Junction between common primitive ventricle and bulbus cordis marked by bulboventricular sulcus
• Septation of primitive common ventricle contributed by bulboventricular sulcus, forming intraventricular septum - grows towards atrioventricular cushions which have now fused to form septum intermedium (common canal into l and r atrioventricular canals)
• Septum doesn’t reach cushions
• Atrioventricular foramen closed by outgrowth from atrioventricular cushions - fuses interventricular septum to form membraneous part of septum (contribution from neural crest cells)
• Trabeculated part of r ventricle formed from bulbus cordis
• Trabeculated part of l ventricle formed from common primitive ventricle