Cardiac Embryology Flashcards
What happens to the heart in development?
Starts of a midline structure that rotates to the left so left-sided structures sit posteriorly + right-sided structures sit anteriorly (retained embryonic naming)
Where do the blood vessels of the heart develop?
Extra-embryonic mesoderm (forms part of placenta + umbilical cord)
What do the umbilical arteries and veins communicate with?
The developing fetal circulation + developing placenta
At how many weeks is blood present in the fetal vessels?
End of week 3
When does the cardiogenic mesoderm develop?
Mid week 3 (day 18)
How does the heart develop in the cardiogenic mesoderm?
- Paired angioblastic cords develop
- Cord canalise to form paired endocardial tubes
- Heart tubes fuse during lateral folding (late week 3)
- Blood flow starts (week 4)
- Neural crest cells involved in process too
Where is the developing heart initially situated?
Above the head
How does the heart end up in its adult position?
Longitudinal folding + reversal moves the brain, mouth + heart into the adult position (~day 19)
What is the septum transversum?
Pulled underneath the heart during longitudinal folding forming part of the fibrous pericardium + part of the diaphragm
What is the system that the developing single arterial outflow and 2 ventral venous inputs pump into?
Aortic/pharyngeal arches
What does the heart grow into?
The pericardial cavity; becomes completely surrounded by it
What is the point of reflection?
Where the visceral pericardium folds back on itself + is continuous with the parietal pericardium
Does the heart have a mesentery?
It loses it early on in development
Dilations appear in the heart tube, from top to bottom what are these dilations called and what do they form?
- Aortic sac
- TA -> initial part of aorta + PT
- BC -> outflow region/tracts of both ventricles
- V -> ventricles
- Atria -> rough wall of atria
- SV -> smooth wall of right atria
- R + L horn of sinus -> vena cavae + coronary sinus respectively
How does the heart develop into left and right?
Septa grows out of endocardial cushion + forms to divide chambers + outflow tracts
Where do the pulmonary veins grow out of? What happens to them?
Grow out of LA + then branch
Proximal part gets absorbed into atrial wall making the smooth part so there is only 4 PVs
What vascular shunts exist in the embryonic heart?
RA -> LA (valve of foramen ovale)
PT -> aorta (DA)
= R -> L shunt
Why do vascular shunts exist in the embryonic heart?
To bypass the lungs as they are not fully developed so do not want to put all circulating volume through them
Why does heart tube folding occur?
Fixed positions of aortic arches + venous inputs so when heart starts to rapidly grow it will naturally fold
What occurs at day 22?
BC grows rapidly + moves infero-anteriorly + right
What occurs at day 24?
Ventricle moves left + atria move postero-superiorly
How can you get dextrocardia?
If the BC folds to the left + the ventricle folds to the right instead
If you remove the truncus arteriosus at day 27, what are you able to visualize?
The four chambered structure: atrial chambers connected to ventricular chambers via the atrioventricular (AV) canal
What forms in the atrioventricular (AV) canal?
Bicuspid/mitral + tricuspid valves
What happens to the atrioventricular canals (AV) during development?
Endocardial cushion grows across canal from anterior to posterior + meet in the middle to separate L + R AV canals
What does the endocardial cushion form?
Septa
Explain the process of atrial septation.
- Septum primum grows towards endocardial cushion
- Does not grow right up to cushion but leaves a hole called the foramen primum
- Foramen secundum hole appears at the top of the septum
- Septum primum meets the cushion
- Septum secundum starts to grow on the right
- Leaves a hole called the foramen ovale
How does the right to left atrial shunt work in the developing embryo?
Pressure-gradient driven
Septum primum = floppy acting as a valve cusp
Septum secundum = muscular/stiff
In utero R sided pressure higher than L so septum primum opening allowing blood shunt from RA to LA through foramen ovale
When does atrial septation occur?
Mid week 4 - week 5
Why is right sided atrial pressure higher than left in utero?
Lungs are fluid filled so there is high pulmonary vascular resistance
What happens to the blood shunt postnatally? Why?
LA pressure exceeds RA pressure so septum primum pushed shut covering the foramen ovale so there is no more hole or shunt allowing greater pulmonary blood flow
More blood in LA too
Why is left sided atrial pressure higher than right postnatally?
Lungs drained + functional meaning there is low pulmonary vascular resistance
What process during breath causes the shunt from right atrium to left atrium to shut off?
When the baby takes a breath and cries
What should happen to the tissues of the atrial septa after several months postnatally?
They should fuse
What is a probe-patent foramen ovale? How is it detected?
Fusion of atrial septa tissue does not occur
Micro-bubble test (US)
What problems can a probe-patent foramen ovale cause?
Valsalva manoeuvre transiently increases RA pressure over LA opening the foramen ovale -> emboli can pass from R to L increasing possibility of TIA + stroke
What is an atrial septal defect (ASD)?
LA to RA shunt after birth
Do atrial septal defects (ASD) cause the baby any problems?
Small ASDs are not cyanotic so may be asymptomatic
May close during growth
Where can atrial septal defects (ASD) occur?
- Foramen/septum secundum
- Foramen/septum primum
- Endocardial cushion
When does ventricular septation occur?
Weeks 5-7
What is the ventricular septum formed from?
Muscle
Membrane
Endocardial cushion
What does the ventricular septum separate?
RV + LV
Aorta + PT
How is the ventricular septum formed?
- Muscular part grows up toward endocardial cushion from ventricle floor in a crescent-shape
- Remains incomplete in cranial (upper) membranous region
- Completed in cranial region by membranous down-growth of truncal + bulbar ridges (derived from neural crest mesenchyme) in TA + BC down towards endocardial cushion
- Septum spirals 180o so aorta is anterior to PT
- Fusion of ridges separates blood flow
Why is the membranous part of the septum the most likely congenital ventricular septal defect?
Forms from aorticopulmonary septum + endocardial cushion so relying on 4 bits of tissue joining together so most likely to go wrong at some point
What are truncus arteriosus defects?
Failure of bulbar ridge formation causing persistent TA so you get a VSD with an overriding TA -> cyanotic condition
What are the 2 truncus arteriosus (TA) defects?
- Failure of bulbar ridge formation -> persistent TA = VSD with overriding TA - cyanotic condition
- Failure of equal division -> pulmonary/aortic stenosis often w/ VSD with larger vessels overriding the VSD - pulmonary stenosis may cause cyanosis
What is transposition of great vessels?
2 isolated circulations perhaps due to failed development of conus arteriosus or malformation of aorticopulmonary septum
= cyanotic condition
What 2 things can be beneficial for babies for transposition of great vessels?
- DA shunt remaining open
2. ASD/VSD
What is the tetralogy of fallot?
- Pulmonary stenosis
- RV hypertrophy
- Over-riding aorta
- VSD
= cyanotic condition
When does congenital cyanosis related to heart defects occur in babies?
When deoxygenated blood enters the arterial circulation OR when insufficient oxygenation takes place
If a baby has a patent ductus arteriosis (DA), is this a cyanotic condition? Why?
No
Oxygenated blood is being put into the deoxygenated system
