Cardiac embryology Flashcards
What are the two pericardia of the heart?
Fibrous and serous pericardium
Which two layers is serous pericardium composed of?
Parietal layer and visceral layer
What are the three layers of the heart wall?
Endocardium, myocardium and epicardium
What are the two heart chambers?
1) Atria
Right and left atrium
2) Ventricles
Right and left ventricle
What are the two heart valves?
1) Atrioventricular valves
Right atrioventricular (tricuspid) and left atrioventricular (bicuspid/mitral)
2) Semilunar valves
Pulmonary and aortic vales
What are the two septa of the heart?
1) Atrial/interatrial septum
2) Ventricular/interventricular septum
What is the flow of blood circulation?
Vena cava –> right atrium –> right ventricle –> pulmonary arteries –> lungs –> pulmonary veins –> left atrium –> left ventricle –> aorta
What is pulmonary circulation?
When blood goes from the heart to the lungs and back to the heart.
What is systemic circulation?
When blood goes from the heart to the rest of the body and returns to the heart.
When does the cardiovascular system appear?
Middle of the third week
Why does the cardiovascular system appear?
When the embryo is no longer able to satisfy its nutritional requirements by diffusion alone.
What is the first organ to function in human embryos?
The heart
When does the heart first start functioning?
Begins beating as early as 21st day. Starts pumping blood by the 24th to 25th day.
What is the structure of the first morphologically identifiable embryonic heart?
A single tube composed of contractile myocardium surrounding an inner endocardial (endothelial) tube, with an intervening extracellular matrix.
Where are cardiac progenitor cells of the first heart field derived from?
Intraembryonic mesoderm
Where do cardiac progenitor cells emerge from?
Cranial third of the primitive streak during early gastrulation
Which direction do cardiac progenitor cells migrate in?
Cranial-lateral
What happens to cardiac progenitor cells during their migration?
They become localised on either side of the primitive streak. Eventually become localised within the cranial part of the visceral layer of the lateral plate of mesoderm on both sides of the embryo. Then extend to the developing head fold to form a cardiac crescent.
Where are cardiac progenitor cells in the second heart field?
Present medial to the first heart field.
What do cardiac progenitor cells in the first heart field form?
Atria, left ventricle and part of the right ventricle
What do cardiac progenitor cells in the second heart field form?
Part of right ventricle, outflow tract and contribution to the formation of atria.
What happens as the lateral body folds move medially?
- Right and left sides of the first heart field move medially
- Two limbs of the first heart field fuse at the midline, caudal to the head fold and ventral to the foregut
- Vascular elements called the endocardial tubes develops within each limb of the first heart field
- The cells of the endocardial tubes coalesce into a single tube as the limbs of the first heart field join to make the primary heart tube.
What happens during the process of craniocaudal folding?
Cranialmost portion of the first heart field is pulled ventrally and caudally to lie ventral to the newly forming foregut endoderm. This brings the developing heart tube into the thoracic region late in the third week.
What is the endocardium derived from?
Cardiac progenitor cells
What is the myocardium derived from?
Splanchnic mesoderm containing myocardial progenitors by the 21st to 22nd day.
What is cardiac jelly?
A thick layer of acellular extracellular matrix deposited mainly by the developing myocardium. Separates the myocardium from the endocardial tube.
What is the epicardium?
Visceral lining of the pericardial cavity covering the heart.
How is epicardium formed?
Formed later by a population of mesodermal cells that are independently derived from splanchnic mesoderm migrating onto the outer surface of the myocardium.
When does the heart tube begin to bend?
Begins to bend on day 23 and creates the cardiac loop which is complete by day 28.
What are the two bends that occur in the cardiac loop?
Cranial/cephalic portion of the tube bends ventrally, caudally and to the right. Caudal (atrial) portion shifts dorsally, cranially and to the left.
What are the 5 subdivisions of the cardiac loop?
1) Atrial portion
2) Atrioventricular junction
3) Early embryonic ventricle
4) Bulboventricular sulcus
5) Bulbus cordis
What does the atrial portion form?
Primitive left and right atria.
What does the atrioventricular junction form?
Atrioventricular canal.
What does the early embryonic ventricle form?
Primitive left ventricle.
What does the bulboventricular sulcus form?
Primary interventricular foramen.
What are the three components of the bulbus cordis?
A) Proximal third: trabeculated part of the right ventricle
B) Middle third: conus cordis (will form the outflow tracts of both ventricles)
C) Distal third: truncus arteriosus (will form the roots and proximal portion of the aorta and pulmonary artery)
When and from where does the sinus venosus receive venous blood from?
Right and left sinus horns in the middle of the fourth week.
What are the three veins that provide blood to the left and right sinus horns?
1) Vitelline (omphalomesenteric) vein
2) Umbilical vein
3) Common cardinal vein
When do left-to-right shunts of blood happen?
During the fourth and fifth weeks
What is the result of left-to-right shunts?
Causes the shift of the entrance of the sinus venosus to the right
Why does the left sinus horn lose its importance in the left-to-right shunt?
1) Obliteration of the right umbilical vein during the fifth week
2) Obliteration of the left vitelline vein during the fifth week
3) Obliteration of the left common cardinal vein
When is the cardiac septa formed?
Between the 27th and 37th days of development.
What are the two methods of septa formation?
Active septation and passive septation
What does active septation involve?
Involves actively growing masses of tissue (endocardial cushions) that approach each other until they fuse, dividing the lumen into two separate canals.
Where are endocardial cushions developed?
A) Atrioventricular region: assists in the formation of:
* atrial septum
* ventricular septum
* atrioventricular canals and valves
B) Conotruncal region: assist in formation of aortic and pulmonary channels
What does passive septation involve?
Relies on a narrow strip of tissue in the wall of the atrium or ventricle failing to grow while areas on each side of it expand rapidly. Never completely separates the two cavities.
When does the septum primum appear?
End of the fourth week.
How does the septum primum appear?
Passive growth of a sickle-shaped crest from the roof of the common atrium into the lumen.
What is the ostium primum?
Opening between the lower rim of the septum primum and the endocardial cushions.
What is the ostium secundum?
Appears just before the closure of ostium primum.
What does the foramen ovale stem from?
Appearance of septum secundum.
What is the probe patency?
A narrow oblique cleft between the two atria.
When do the primitive ventricles expand?
End of fourth week
What closes the interventricular foramen?
Outgrowth of tissue from the anterior endocardial cushion along the top of the muscular interventricular septum. Formation of membranous part of the interventricular septum.
When do the four atrioventricular endocardial cushions appear?
End of the fourth week
Where are the four endocardial cushions found?
(1,2) one on each side
(3) one at the dorsal (superior) border of the atrioventricular canal
(4) one at the ventral (inferior) border of the atrioventricular canal
When do pairs of opposing ridges (swellings or cushions) in the truncus appear?
Fifth week.
What are the two swellings?
Right superior truncus swelling and left inferior truncus swelling.
What does the division of the truncus arteriosus by the aorticopulomary septum do?
Divides the outflow channel of the heart into aorta and pulmonary trunk.
Where may mixing occur in fetal circulation?
1) Liver
2) Inferior vena cava
3) Right atrium
4) Left atrium
5) The entrance of the ductus arteriosus into the descending aorta
How may mixing occur in the liver?
By mixture with a small amount of blood returning from the portal system.
How may mixing occur in the right atrium?
By mixture with blood from the head and upper limbs.
How may mixing occur in the left atrium?
By mixture with blood returning from the lungs.
What are the four key circulatory changes at birth?
1) Closure of the umbilical arteries
2) Closure of the umbilical vein and ductus venosus
3) Closure of ductus arteriosus
4) Closure of oval foramen
What does the closure of umbilical arteries involve?
Distal parts of the umbilical arteries: form the medial umbilical ligaments
Proximal portions of the umbilical artiers: remain open as the superior vesical arteries
What does closure of the umbilical vein and ductus venosus involve?
Umbilical vein: forms the ligamentum teres hepatis
Ductus venosus: forms the ligamentum venosum
What does closure of the ductus arteriosus form?
Ligamentum arteriosum.
What is dextrocardia?
Condition where the heart lies on R side of the thorax instead of the L.
How does dextrocardia occur?
When the heart loops to the L. Defect may be induced during gastrulation when laterality is established or slightly later when cardiac looping occurs.
What is Atrial Septal Defect (ASD)?
ASD primary type will develop when ostium primum does not fuse properly. This then defects the ostium secundum as there is excessive resorption of the septum primum. This results in failed development of the septum secundum.
What is Ventricular Septal Defect (VSD)?
This defect is encountered frequently and may involve the muscular and membranous portion of septum. (when there is a hole in the heart that divides the ventricles)
What is Tetralogy of Fallot (ToF)?
4 components:
Pulmonary stenosis, overriding aorta, interventricular septal defect, hypertrophy of right ventricle.
Physiological shunts present before birth remain functional - no reversal of pressure happens following birth.
How does ToF manifest?
Newborn babies with ToF are at first acyanotic. With increase in R ventricular obstruction in the first few weeks of life, cyanosis occurs. After prolonged physical resting, an activity e.g breastfeeding can trigger an attack-like condition with sudden paleness and loss of consciousness.
