Development of the heart (and its Conduction system) Flashcards
Outline the expansion of the primary heart tube
-The cardiac mesoderm forms a tube, consisting of myocardium on the outside and the lumen of the tube is lined by endocardium.
The medial (middle) part of the crescent expands (this will become the left ventricle)
Endocardial tube attaches to developing aortic arches.
Outline the steps in the elongation of the heart tube
-Cells from the second heart field are added at both the inflow and outflow (anterior and posterior)
-The ends of the tube are fixed, so the heart must bend as it elongates
-The direction of looping is genetically controlled to shift left.
Briefly say a summary of early heart development
1) Tube formation- muscle wrapped around endothelium
2) T attached to arteries at one end, veins at the other
3)Tube elongates by addition at each end of cells from the SHF (second heart field)
4) Tube bends, loops and coils
5) Neural crest cells migrate into outflow regions to make outflow cushions
How are primitive chambers formed?
Primitive chambers are formed by expansion of the tube
How are ventricles characterised?
Ventricles are characterised by the presence of trabeculae (honeycomb)
Give the 4 types of separation of the heart tube
1) Atrioventricular septation:
-divides atria from ventricles
-divided by AV cushions
2) Atrial septation:
-divides L from R atrium
-divided by interatrial septum
3) Ventricular septation:
-divides L from R ventricle
-divided by interventricular septum
4) Outflow tract septation:
-divides pulmonary artery from the aorta
What are two types of septation that divide the heart tube
1) Cushions:
-AV and OFT cushions
-Valve formation, stenosis and atresia defects
2) Muscular walls :
-1 and 2 degree inter atrial septa, interventricular septum
-defects give rise to holes
Describe the process of AV cushion formation
AV cushion formation:
-Cardiac jelly (ECM) secreted by myocardium
-Endocardial cells undergo epithelial- mesenchymal transition and populate the jelly
-Neural crest cells contribute significantly to OFT cushions
Explain atrioventricular septal defects
AVSDs :
-occurs due to improper formation of walls or valves during heart formation
-occurs at the common AV junction
-AVSDs are rare in the general population but are common in down syndrome (roughly 40%)
Explain how atrial septation- the primary interatrial septum occurs
This is the first separation between the atria.
-the left atrium with the orifice (hole) of the pulmonary vein.
-right atrium with the orifice of the systematic venous sinus.
-Primary interatrial septum (wall between the atria) is growing to divide the left and right atrium.
Explain atrial septation using the foramen ovale and septum secundum
-primary interatrial septum grows down to fuse with the AV cushions
-The trailing edge of the primary septum breaks down to allow blood to continue to flow from right to left atrium.
-septum secundum grows down to form a flap valve
-left atrial pressure increases post birth which causes the valve to close
Give 3 examples of atrial septal defects
1) Persistent foramen ovale:
-flap valve doesnt seal completely
-common (10%) o the population
-possible risk factor for stroke and in divers
2) Ostium primum defect
3) Ostium secundum defect
What can persistent holes in the interatrial septum lead to?
Persistent holes in the interatrial septum lead to various malformations.
Explain ASD left to right shunting
Due to a hole present between the atria, the pressure on the left side of the heart is higher than the right side of the heart. The flow of blood occurs in a left to right direction due to pressure differences.
-Defect is acyanotic (normal levels of haemoglobin in systemic circulation)
Explain interventricular septum formation
Interventricular septum formation is the process of forming a wall between the left and right ventricles.
-The IVS grows from the wall, towards the AV cushion. Initially as a thickened trabeculum.
Explain features of ventricular septal defects
Features of ventricular septal defects:
-75% perimembranous
-25% muscular
Types of ventricular septal defects:
-Small (small L>R shunt)
-Medium (moderate L>R shunt)
Large (large L>R shunt)- surgically removed in childhood
Explain VSD with resultant left to right shunting
-pressure on the left ventricle is higher than the right ventricle
-defect is acyanotic
Explain septation of the outflow tract (aorta and pulmonary trunk)
-a single tube is separated into the aorta and pulmonary artery
-these must be attached to the left and right ventricles
-two cushions spiral through thee turn us arteriousus
-complex remodelling at each end
Explain formation of the outflow tract (OFT)
1) spiralling cushions separate the aorta and pulmonary artery
2) neural crest cells contribute to the AP septum
3) Complex remodelling of the aortic arches
4) Proximal cushions fuse with the IVS
Describe patent ductus arteriosus with resultant left to right shunting
-defect is acyanotic
-allows fetal circulation to bypass lungs
-normally closes after birth and becomes the ligamentum arteriosum.
What is patent ductus arteriosus
Patent Ductus Arteriosus (PDA) is a congenital heart defect where the ductus arteriosus, a blood vessel that connects the pulmonary artery to the aorta, remains open (patent) after birth.
Explain OFT (outflow) defects
-There are several types of OFT defects
-Different types result in a common truncus
-OFT defects are associated with neural crest defects and some genetic conditions
-unequal division of the OFT can lead to aortic or pulmonary stenosis.
Explain transposition of the great arteries
-Aorta connected to the right ventricle
-pulmonary trunk connected to left ventricle
-arterial trunks usually side by side
-probably caused by abnormal outflow tract cushions
-lethal without VSD, ASD, or PDA.
-cyanotic
Explain tetralogy of fallot
This is a defect that involves 4 key changes in the heart:
- VSD (ventricular septal defect (large))
-pulmonary stenosis
-right ventricle hypertrophy
-overriding aorta
explain tetralogy of fallot in terms of shunting and what this lead to
There is higher pressure in the right side of the heart in comparison to the left side of the heart.
Therefore there is a right >left shunt as blood flows this way
This can lead to cyanosis
is tetralogy of fallot cyanotic or acynotic?
tetralogy of fallot is cyanotic
What can cause a left to right shunt in tetralogy of fallot?
Squatting increases peripheral arterial resistance and causes a left to right shunt.
Provide a summary of CHD effects
Acyanotic (left to right shunt):
-VSD
-ASD
-AVSD
Holes in septum’s are usually acyanotic (pressure is higher on left than right)
Cyanotic (right to left shunt):
-TGA
-ToF
-Truncus arteriosus
-TAPVC (total anomalous pulmonary venous connection)
Explain the origins of the cardiac conduction system
-Conduction tissue is specialised myocardium.
-Variations in conduction properties caused by differences in ion channels and cone in (gap junctions) expression.
-conduction system differentiates by progressive, localised recruitment from heart tube myocardium.
-electrical insulation layers:
1) fibro-fatty layer at the AV junction
2) Ventricular bundle branches wrapped in fibrous sheath.
