Embryology of the CVS 1 - Heart Flashcards
What are the 4 main stages of heart devolopment
1) Formation of heart tube
2) Looping of the heart tube
3) Atrial & Ventricular septation
4) Development of the outflow tract
What structure forms before the heart tube?
Bilateral heart primordia
Two tubes with 4 smaller bridge tubes linking them
Which mesoderm gives rise to the cardiovascualr system?
Lateral plate splanchnic mesoderm
In the 3rd week, all the mesoderms etc have formed and the CVS can begin development
Describe how the embryo forms the 2 primitive heart tubes
Angiogenic cells collect in the splanchnic mesoderm
They then move towards to midline
Once at the midline, they coalesce and form the 2 primitive heart tubes
Why does the CVS develop & start working so early on?
Embryo is growing
Can not satisfy nutritional needs through diffusion
Creation of the cardiogenic field is a key precursor step to the formation of the cvs
What is the cardiogenic field?
Primitive heart + blood vessels in the embryo
In the formation of the cardiogenic field:
What areas first develop blood vessels?
The walls of the yolk sac, allantois, connecting stalk and the chorion
Initially, these are not actually linked to the heart
In formation of the cardiogenic field:
How are the newly formed blood vessels ‘linked’ to the tubular heart?
1) Angioblastic cords form in the cardiogenic mesoderm (3rd week)
2) The cords fold into a tube (canalize) to form heart tubes
3) Heart tubes link up with the blood vessels to form ‘primordial CVS’
The pericardium forms alongside the primitive heart
What is the pericardium derived from?
Intra-embryonic coelom
Initially, the pericardium does not contain the heart tube, but is instead the Pericardial cavity
Describe how the 2 structures join each other
Cephalic/cranial folding of the embryo kind of shoves the 2 together
PC moves from dorsal to ventral
Cardiac tube moves from ventral to dorsal
The pericardium is multilayered
How does each layer form?
Fibrous pericardium + Parietal layer of serous pericardium = SOMATIC mesoderm
Visceral (inner) layer of serous pericardium = SPLANCHNIC mesoderm
2 important events happen on the 22nd day
What are they?
1) Cranial/cephalic folding begins
2) The 2 Heart tubes start fusing
The fused primitive heart has 2 ends
What are they?
Cranial end (arterial)
Caudal end (venous)
Describe the parts of the fused primitive heart
Truncus arteriosus (L&R horns)
Bulbus cordis
Ventricle
Atrium
Sinus venosus (L&R horns)
Describe the structure & function of the sinus venosus
2 horns
Each horn has 3 veins, receiving venous blood from:
- Common cardinal vein
(body of embryo) - Umbilical vein
(placenta) - Vitelline vein
(yolk sac)
The truncus arteriosus leads into what blood vessels?
Left and right dorsal aortae
Describe the structure thing that leaves the Truncus arteriosus
Aortic sac with left and right horns
Each horn splits into 6 arches (the top being the 1st)
Aortic arches terminate in left or right dorsal aorta
Describe the formation of the cardiac (bulboventricular) loop?
Day 22:
- Bulbus cordis & Ventricle expand rapidly and bulge out
Day 23:
- Heart tube droops and loops to the right side
Day 24:
- BC & Ventricle are have sunk down past the atrium
Abnormal cardiac looping causes ______
Dextrocardia
Describe how abnormal cardiac looping happens?
Heart tube loops to the left instead of the right
This means it ends up lying facing the right (= dextro)
Dextrocardia can be associated with what major positional abnormality?
Situs transversus
all your viscera face the wrong way
When does partitioning of the heart occur?
27th & 37th days
What are the 2 stages of heart partitioning?
27th = Endocardial cushion formation
37th = septum formation
What does endocardial cushion formation do?
Separates the atrium and ventricle
Forms L&R atrioventricular canals
What does septum formation do?
Separates right atrium from the left atrium
Separates the left Vent from the right Vent
(Forms the two sides of the heart)
Problems with heart partitioning can cause what health problems?
atrial septal defect (ASD)
& ventricular septal defect (VSD)
Describe the process of endocardial cushion formation
Superior & inferior endocardial cushions grow across the common atrioventricular canal
Meet & fuse in the middle
leave 2 gaps, which are the L&R AV canals
Describe the process of atrial partitioning
Starts at end of 4th week
1) Septum primum grows down from top of atrium. Gap between SP and endocardial cushion = foramen primum
2) Cell death in the SP creates a hole at the top = foramen secundum
3) Foramen primum fuses shut & Septum Secundum begins to grow in the right atrium
4) Septum secundum grows down, Septum primum grows up. The gap between them = Foramen Ovale
5) Top part of SP and bottom part of SS degenerate. Foramen ovale becomes small
Why does the foramen ovale remain open until after birth?
1) When in utero, the baby does not use it’s lungs ∴ no pulmonary circulation
Oval foramen allows blood flow from right atrium to left atrium ∴ skips pulmonary circulation
Once born, baby begins to breathe ∴ hole closes
2) Also prevents blood flow in wrong direction
What is the name given to the opening between the septum primum and the endocardial cushion?
Ostium primum
What changes occur in the atrium post birth?
Foramen ovale closes:
- Sept. primum and Sept. secundum fuse together^
- OVAL FOSSA remnant of Foramen Ovale (fossil = fossa)
What is the epidemiology of Atrial Septal defect (ASD)?
One of the most common congenital heart anomalies
More common in females
Common form - patent foramen ovale
What are the 4 clinically significant types of ASD?
Atrial septal defect:
1) Foramen secundum defect
2) Endocardial cushion defect with foramen primum defect
3) Sinus venosus defect
4) Common atrium
The first two types are more common
What happens after the atria have split?
Ventricles split
probably overlaps but hey ho
Describe the process of ventricular septation
Muscular ventricular septum forms. Opening is called interventricular foramen
Bottom of spiral aorticopulmonary septum fuses with MVS to form Membranous Interventricular Septum, closing interventricular foramen. (Aorticopulmonary septum divides bulbis cordis and truncus arteriosus into aorta and pulmonary trunk)
Growth of endocardial cushions also contributes to membranous portion of the interventricular septum
Describe how the bulbis cordis and truncus arteriosus are split
During 5th week of development…
Aorticopulmonary septum (mad spiral thing) goes up the middle of the BC & TA and creates two sections
Two sections:
- Aorta
- Pulmonary trunk
As time goes on, these sort of separate and form two distinct shaped tubes
What congenital heart condition is causes by ventricular septation not reading the textbook
Ventricular septal defect (VSD)
What type of VSD is most common?
Membranous type is most common
Initially, the heart does not have any nodes n shit
What acts as the primitive pacemakers?
primitive atrium and then sinus venosus
How does the conducting system of the heart develop?
SA node (pacemaker) develops during 5th week
AV node and bundle (bundle of His) develops from cells of AV canal and sinus venosus
What conditions are linked to abnormalities in the development of the conducting system?
Cot death or sudden infant death syndrome (SIDS)
Cause – abnormalities of conducting tissue
Overall, what is the fate of the Aortic sac in the primitive heart tube?
Aortic arches
Overall, what is the fate of the Bulbis cordis in the primitive heart tube?
Right ventricle
Part of the outflow tract
Overall, what is the fate of the primitive ventricle in the primitive heart tube?
Left ventricle
Overall, what is the fate of the primitive atrium in the heart tube?
Forms parts of the right and left atria
What is the fate of the sinus venosus on the primitive heart tube?
Superior vena cava
Right atrium
What are the main causes of congenital heart disease?
Rubella infection in pregnancy (PDA)
Maternal alcohol abuse (septal defects)
Maternal drug treatment and radiation
Genetic - 8%
Chromosomal – 2% (Down’s and Turner’s syndrome)
It is multifactorial though
What is the link between ASD/VSD and congenital heart disease?
VSD and ASD account for ~ 30% of
congenital heart disease: VSD for
20% and ASD for 10%.
What are the causes of transposition of great vessels?
1) Failure of aorticopulmonary septum to take a spiral course
2) Defective migration of neural crest cells
Why is transposition of great vessels dangerous?
Permits exchange of pulmonary and systemic blood
(oxygenated & deoxygenated blood mixes up)
Common cause of cyanotic disease in newborns
Associated with ASD & VSD
What is Fallot?
Congenital heart condition made up of several heart defects:
- Pulmonary stenosis (obstruction of right ventricular outflow)
- Ventricular septal defect (VSD)
- Dextroposition of aorta (“overriding” aorta)
- Right ventricular hypertrophy
What causes Fallot
Unequal division of the conus due to anterior displacement of aorticopulmonary septum
