Development of Heart & Vessels Flashcards
What are the stages of Heart development & the potential defects that can occur?
- Formation of 4 chambered heart (situs inversus)
- Cardiac Looping (dextrocardia)
- Division of atrioventricular canal into L & R channels
(ventricular septal defect) - Formation of atrial septa (atrial septal defect)
- Formation of conotruncal cushions &divisions of
outflow tract (Tetralogy of Fallot)
What is the first sign of fertilisation observed within the embryo?
Embryo has 2 pronuclei
Describe the development of a human embryo
Developing embryo contains 6-8 cells 3 days after fertilisation
5 days after its called a blastocyst and contains ~100 cells
What are the 3 structural divisions of a developing emrbyo?
Ectoderm
Mesoderm
Endoderm
What is the ectoderm?
skin (epidermis), brain, spinal cord, CNS, sensory organs
What does the mesoderm eventually form?
heart, skeletal muscle, kidneys, urogenital, connective tissues etc.
What is the endoderm?
lining of GI tract and Lungs, pancreas, Liver etc.
What occurs during gastrulation?
primary germ layers are formed
- (endoderm, ectoderm and mesoderm)
organised into their proper locations
When does the formation of the heart tube begin to occur?
after 18 days
Where does the heart initially begin to develop from?
The heart develops from the primitive streak via the blood islands in the Splanchnic mesoderm
Explain how the heart tube develops during the 20th - 22nd days of gastrulation
20 days
- blood islands slowly form together into 2 tubes called
cardiogenic cords
21 days
- the tubes fuse down the middle to form one heart tube
22 days
- further development gives rise to a tube with
asymmetric openings forming an arterial & venous end
- beginning of the asymmetry in bulge towards left
What is the situs inversus?
A congenital condition where major visceral organs are reversed / mirrored from their original positions
When does Cardiac looping begin to occur?
after 22 days
How does cardiac looping occur during the 22nd - 24th days?
22 days
- cells at each end of the heart proliferate making a
longer tube
- entire structure is in a confined space so folds up on
itself
23 - 24 days
- primitive atrium loops up above and behind the primitive
ventricle.
- looping process brings the primitive area of the heart
into the proper spatial relationship for development
At 35 days
atria and ventricles in correct positions and tubes for circulation are ready
What is dextrocardia?
rare congenital heart condition where the heart points to the right side of the chest rather than the left as normal
What is the role of the septum primum?
Division of the atrioventricular canal
Septum primum grows along the midsagittal plane, separating the atria for a temporary space called the Foramen Primum
How does the division of the atrioventricular canal occur?
The posterior & anterior endocardial tissues fuse, dividing the atria from the ventricles
The left side of the atrium grows pulmonary veins sending growing veins to the lungs
A ventricular septum eventually grows upwards, forming a division between the 2 ventricles. - attaches to atrial and ventricular division
What is the consequence of ventricular septal defect?
During ventricular contraction, some blood form left ventricle leaks into right ventricle, passing through lungs and re enters the left ventricle via the pulmonary veins and left atrium.
Approximately how big is the ventricular septal defect?
the extent of the opening can vary from pin size to complete absence of the ventricular septum
- viewed via ECG (mid-oesophegal 4 chamber view)
What are the 2 main effects of the ventricular septum defect?
- systemic circulation doesn’t receive all the blood being
pumped out the left ventricle - leakage of blood into right ventricle elevates right
ventricular pressure& volume causing pulmonary
hypertension
What can cause ventricular septum defect?
one cause is the failure of the endocardial cushions to provide an anchor point for the developing ventricular septum
What occurs during after 40 days of gastrulation?
Formation of the atrial septa
What is the first step of the atrial septum formation?
initially the temporary hole in the septum primum (foramen primum) allows right to left shunt for foetal blood as there’s no pulmonary circulation.
this closes before a second hole, foramen secundum opens
What occurs at 40 days of heart development?
a second more muscular and robust septum grows down from the roof of the atria just lateral to the septum primum.
it obscures the place the foramen primum was
How does the atrial septum form at 43 days?
Septum secundum grows but leaves a space for the foramen ovale to allow the right to left shunt of blood
the foramen secundum is partially obscured by the septum secundum but the foramen ovale remains providing an alternative left to right shunt closing the foramen primum
the ventricular septum grows up to fuse with the now fused endocardial cushion
Summarise the formation of the atrial septa
- septum primum grows - (atrial septa)
- foramen primum is a hole in the septum primum
- foramen primum closes, foramen secundum opens
- septum secundum grows, foramen secundum isn’t
completely obliterated as it later becomes foramen
ovale
What is the result of the atrial septa formation?
Results in the septum primum and the more muscular, septum secundum foramen ovale (hole) permitting right to left shunt of blood (as still no pulmonary circulation)
How does the foramen ovale close at birth?
At birth, the lungs become functional
- pulmonary vascular pressure decreases
- left atrial pressure > right atrial pressure
=> forces septum primum against septum secundum
-> closes foramen ovale
(in time the septum eventually fuse)
What disorders can occur due to incorrect atrial septum formation?
a patent foramen ovale is a common septal defect present (to some extent < 25% of adults)
How does the division of the outflow tract arise?
Separation of the ventricles results from the union of the:
- conotruncal septum
- endocardial cushions
- ventricular septum
Explain how conotruncal septum, endocardial cushions and the ventricular septum unite
The conotruncal septum grows as a spiral down the conus arterious - meets with the endocardial cushions & ventricular septum
collectively they form the final full separation of the right and left sides
What is the consequence of incorrect division of the outflow tract?
If the continual septum is misaligned
- differently sized aorta & pulmonary arteries form
- won’t meet with endocardial cushions correctly
=> gives rise to septal defects (tetralogy of Fallot)
What are the consequences of the Tetralogy of fallot?
Pulmonary stenosis
- spiral valve grown off centre
- large aorta & small pulmonary artery
- restricts blood to lungs
Overriding aorta
- aortic opening positioned over ventricular septal defect
- blood from both sides of the heart enter aorta
- some deoxygenated blood pumped around the body
Ventricular septal defect
- during systole, blood from LV leaks into RV, passes
through lungs & re enters LV via pulmonary veins & LA
- increases pressure in pulmonary circulation
- volume overload in LV
Right ventricular hypertrophy
- right ventricular wall increases in size to deal with
obstruction in pulmonary artery
Which congenital autosomal syndrome are heart development defects a common occurrence?
along with ventricular and atrial septal defects, tetralogy of fallot is common in Down’s Syndrome
What is the ductus arteriosus?
a blood vessel connecting the main pulmonary artery to the proximal descending aorta
What is the role of the ductus arteriosus in fetus’?
It allows most blood from the right ventricle to bypass foetal fluid filled, non functioning lungs
upon closure at birth, it becomes the ligmentum arteriosum
What is the role of the Foramen Ovale in foetal hearts?
The foramen ovale allows blood to enter the LA from the RA
Along with the ductus arteriosus, it enables blood to bypass pulmonary circulation. At birth the foramen ovale closes and later forms the Fossa Ovalis
