Heart Development Flashcards
What is the function of the foetal circulation?
Carry nutrients and gases around the body
Allow exchange at the placenta
What are the 3 foetal blood circuits that need to form in the embryo?
- Vitelline circuit
- Umbilical circuit
- Common cardinal circuit
What two structures does the vitelline circuit innervate?
Yolk sac and embryo
The liver develops at the site where the vitelline veins empty into the sinus venosus
What two structures does the umbilican circuit innervate?
Embryo and placenta
What two structures does the common cardinal circuit innervate?
Anterior cardinal veins drain in the head and arms
Posterior cardinal veins drain the trunk and legs
What are the main steps of heart formation?
- Heart forming mesoderm migrates to the anterior end of the embryo
- The cells in the mesoderm destined to be heart cells form a crescent-shaped tube at the cranial border of the disc
- The two ends of the tube meet in the midline under the gut to form one tube
- The heart tube loops
- The atria and ventricles are gradually separated by septation
How many days into development do heart-forming cells appear?
17 days
How many days into development does the heart begin to beat?
23 days
When does septation begin?
27 days
When does septation end?
37 days
What are the two heart fields that contribute to heart formation?
Primary heart field - develops into the left and right atria and left ventricle
Second heart field - becomes the right ventricles and the outflow tract
What does the cardiac crescent give rise to?
The primary heart tube
Describe the components of the primary heart tube
On each side of the heart tube there is:
- an inlet/venous end (sinus venosus)
- an atrial region
- a ventricle region
The two sides have a common conotruncus
Which are the two planes in which the heart tube folds
Anterioposteriorly
Bilaterally
Describe the anterioposterior folding of the heart tube
Folding of the head brings the region of the heart tube under the head and foregut
Describe the bilateral folding of the heart tube
Folding of the gut ventrally brings the two endocardial tubes together and fuse to form a single midline heart tube under the completed foregut
What are the steps of the division of the heart into its many compartments?
- Division of the atrioventricular canal
- Division of the atria
- Division of the ventricles
- Partitioning of the outflow tract
Describe the division of the atrioventricular canal
The atrioventricular canal separates the atria and ventricles by the formation of thickenings in the canal wall called endocardial cushions
The superior and inferior endocardial cushions meet in the midline, dividing the atrioventricular canal into right and left canals
The two canals initially empty into the future left ventricle
Connective tissue from the endocardial cushions invades the myocardium to form the annulus fibrosus which will insulate the atria from the ventricles - except at the passage of the atrioventricular bundle
Describe the division of the atria
- Septum primum grows down from the roof of the primitive atrium towards the AV cushions forming the foramen primum
- The septum primum meets the endocardial cushions, closing the foramen primum
- Apoptosis in the wall of the septum primum near the room of the atrium forms the foramen secundum, allowing blood to flow from the right to the left atrium
- A second, more rigid septum secundum begins to grow to the right of the septum primum
- The space between the edge of the septum secundum and the endocardial cushion is called the foramen ovale
What is the foramen ovale?
The space between the edge of the septum secundum and the endocardial cushion
What happens to the foramen ovale following birth?
Functional closure of the foramen ovale occurs because of pressure differences between the two atria
The tissues of the two septa also grow together such that they become anatomically fused
What is the function of the foramen ovale in the fetal circulation?
During embryonic development, oxygenated blood from the placenta delivered via the inferior vena cava is shunted from the right atrium to the left atrium via the foramen ovale
The septum secundum forms a valve across the foramen secundum
This ensures the direction of the blood is unidirectional
Describe the division of the ventricles
- The atrioventricular canal shifts from the primitive centricle on the left towards the developing interventricular septum in the middle
- The right ventricle expands in the outlet region
- Both ventricles expand caudally
- The muscular septum develops
Describe the partitioning of the outflow tract
- Neural crest cells migrate to the truncus arteriosuss= and conus cordis, transform into mesenchymal tissue and proliferate to form ridges
- The two truncoconal ridges grow forward toward each other, fuse first at the tuncoconal transition and then zip distally and proximally
- As the truncoconal ridges grow toward the ventricles, they contribute to the interventricular septum
- The ridges spiral around one another
- The ridges fuse
Features of the fetal heart at 12 weeks
Foramen ovale and ductus arteriosus maintain the foetal circulation
The ventricles are almost fully separated
The interatrial shunt is fully functional
The outflow tract is fully divided
What is the ductus arteriosus?
Connection between the pulmonary artery and aorta
What is the function of the ductus arteriosus?
Shunts blood away from the lungs and into the aorta
Describe fetal circulation
- Umbilical vein carries oxygenated blood from the placenta to the inferior vena cava
- Oxygenated blood is shunted past the liver and into the inferior vena cava by the ductus venosus
- The inferior vena cava enters the right atrium
- The blood passes through the foramen ovale into the left atrium, down to the left ventricle and into the aorta
- The first major branches to the aorta take the blood into the developing brain
- Deoxygenated blood from the head, arms and thorax passes through the superior vena cava and enters the right atrium
- The blood passes through the right ventricle and into the pulmonary artery
- The ductus arteriosus causes the blood to bypass the lungs as it enters the aorta instead of the pulmonary artery
- Deoxygenated blood returns to the placenta in umbilical arteries
What are the remnants of the fetal atrium in the adult heart?
Smooth walled parts = sinus venosus
Rough walled parts = pectinate muscle
Describe the function of the early heart tube
Early heart tube contracts slowly and unidirectionally from the caudal end
Parts of the heart that develop from the early tube retain this slower pace and forms the pacemaker system
The myocardium of the chambers conduct more quickly
What is the inlet of the early heart tube?
Where all the embryonic veins drain
Caudal
What is the conotruncus of the early heart tube?
Where are the arteries arise - arterial end
Rostral
Why does a patent interatrial foramen form?
Foramen secundum is too large
Septum secundum is too short
Foramen primum does not close
Which cell populations contribute to heart development?
Precardiac mesoderm
Cardial neural crest cells
What does the precardiac mesoderm give rise to?
The endocardium lining the heart
The epicardium covering the heart
The primary heart field
The secondary heart field
What does the primary heart tube form?
The left ventricle and the primary atria
What does the secondary heart tube form?
Right ventricle
Atria
Outflow tract
What do the cardiac neural crest cells form?
Outflow tract
Pro-epicardial cells of epicardial layer and coronary blood vessels
Describe what happens to the heart following childbirth
The rise in the left atrial pressure pushes the septum primum against the septum secundum and closes the foramen ovale
The change in blood oxygen levels and blood flow closes the ductus venosus and ductus arteriosus
