Development of the Heart 2

Question 1

When does formation of the heart begin?

What are the 3 layers that it is composed of?

Answer 1

• formation of the heart begins in week 3 with the first contraction occurring on day 22
• the heart is composed of 3 layers:

Epicardium:

• this is the visceral layer of the pericardium that is derived from the visceral mesoderm

Myocardium:

• this is derived from visceral mesoderm overlying the heart tube

Endocardium:

• this is derived from the heart tube

Question 2

What does vasculogenesis lead to the formation of by day 18?

Why does this occur?

Answer 2

• the endoderm induces cells of the overlying visceral/splanchnic mesoderm to differentiate into angioblasts
• the angioblasts differentiate into endothelial cells and form tubes through vasculogenesis
• this results in the formation of endocardial tubes

Question 3

How is the primitive heart tube formed?

What does the visceral mesoderm surrounding this structure form?

Answer 3

• the endocardial tubes fuse during lateral embryonic folding to form the primitive heart tube
• the visceral mesoderm surrounding the primitive heart tube differentiates to form the myocardium
• the myocardium secretes a thick layer of extracellular matrix - the cardiac jelly

Question 4

What is the importance of craniocaudal folding in heart development?

Answer 4

• craniocaudal folding brings the developing heart tube into the thorax

Question 5

What are the 3 layers of the primary heart tube?

Where is cardiac jelly found?

Answer 5

Endocardium:

• this forms the internal endothelial lining of the heart

Myocardium:

• this is the muscular wall of the heart

Epicardium:

• this covers the outside of the heart tube and is responsible for the formation of coronary arteries

Cardiac jelly:

• this is gelatinous connective tissue that separates the myocardium and heart tube endocardium

Question 6

What structures are connected to the caudal and cranial regions of the heart tube in a 4 week embryo?

Answer 6

Caudal region:

• 3 paired veins drain into the tubular heart via the right and left horn of the sinus venosus

1. umbilical vein
2. vitelline vein
3. common cardinal vein

Cranial region:

• this region connects to 2 dorsal aortae

Question 7

How can the heart tube be divided into 5 dilatations?

Answer 7

• differential growth of the heart tube leads to the appearance of 5 dilatations:

1. truncus arteriosus
2. conus arteriosus
3. ventricle
4. atrium
5. sinus venosus

• the truncus arteriosus and conus arteriosus together make up the bulbus cordis (outflow tract)

Question 8

When does the heart tube start to fold?

How do the bulbus cordis, primitive ventricle and primitive atrium move during this process?

Answer 8

• the heart tube starts to fold on day 23 in preparation for dividing into 4 chambers
• the bulbus cordis moves caudally, ventrally** and **to the right
• the primitive ventricle is displaced before moving back to the midline
• the primitive atrium is displaced cranially** and **dorsally

Question 9

When is the sinus venosus obliterated?

What are the remnants of this structure?

Answer 9

• the sinus venosus largely degenerates by week 5
• the right horn remains as part of the wall of the right atrium
• the left horn contributes to venous drainage of the heart by forming the oblique vein of the left atrium and the coronary sinus

Question 10

What is the name of the part of the right atrium formed by the sinus venosus?

What is the origin of the rest of the right atrium and how can these 2 regions be distinguished from one another?

Answer 10

• the right horn of the sinus venosus forms the smooth-walled part of the right atrium - the sinus venarum
• the majority of the right atrial wall is derived from the primitive atrium and appears rough / trabeculated
• there is a clear border between the trabeculated part of the right atrium and the sinus venarum called the crista terminalis

Question 11

What is the crista terminalis?

Answer 11

• the crista terminalis marks the border between the sinus venarum (smooth part) and the trabeculated part of the right atrium

Question 12

What are the 2 origins of the ventricular wall?

What structures are formed and how can these origins be distinguished from each other?

Answer 12

• the majority of the ventricular wall is formed by the primitive ventricle with a small contribution from the conus arteriosus
• the conus arteriosus forms the smooth walls of the left and right ventricles

this is the aortic vestibule in the left ventricle, which leads into the aorta

and the conus arteriosus in the right ventricle, which leads into the pulmonary trunk

• the rest of the ventricle wall is trabeculated (rough) and formed from the primitive ventricle - this is the trabeculae carneae

Question 13

How do the right and left atria undergo further differentiation up to week 5 of development?

Answer 13

Right atrium:

• the right atrium enlarges by incorporating the right horn of the sinus venosus

Left atrium:

• an outgrowth of the left atrial wall forms a single pulmonary vein
• this pulmonary vein branches into left and right veins, which then bifurcate to form 4 pulmonary veins
• in week 5, the 4 pulmonary veins are incorporated into the wall of the left atrium by the process of intussusception

Question 14

When does septation of the primitive atrium begin?

How does this process take place?

Answer 14

• septation involves dividing the primitive atrium into right and left atria and it begins at the end of week 4
• a crescent-shaped outgrowth appears from the dorsal wall - this is the septum primum
• the septum primum extends and the diminishing connection between the right and left sides of the primitive atrium is the foramen primum

Question 15

How is the atrioventricular septum formed following extension of the septum primum?

Answer 15

• the septum primum extends into the atrium and the endothelium lining the boundary between the atrium and ventricle expands to form dorsal and ventral endocardial cushions
• the endocardial cushions will fuse in the midline to form the atrioventricular septum
• there are 2 channels communicating between the future left atrium and ventricle and future right atrium and ventricle

Question 16

How is the foramen primum (ostium primum) obliterated and what forms in its place?

Answer 16

• by week 6, the septum primum has fused with the atrioventricular septum to obliterate the ostium primum
• apoptosis occurs in the upper part of the septum primum to form the foramen secundum
• at the same time, a crescent-shaped projection forms from the dorsal wall of the atrium - the septum secundum

Question 17

What is the opening that is present in the septum secundum and what is its significance?

Answer 17

• the septum secundum does not form a complete partition
• the opening left in the septum secundum is the foramen ovale
• the foramen ovale allows blood to flow from the right atrium to the left atrium to bypass the lungs

Question 18

What are the 2 origins of the interventricular septum and when do they develop?

Answer 18

Muscular part:

• at the end of week 4, the muscular part of the septum projects from the floor of the primitive ventricle towards the endocardial cushions
• this leaves an interventricular foramen

Membranous part:

• during week 7, the membranous part projects inferiorly from the endocardial cushion
• this closes the interventricular foramen

Question 19

How is the truncus arteriosus septated into the aorta and pulmonary trunk?

Answer 19

• the truncus arteriosus is divided into 2 channels by endocardial swellings called conotruncal ridges
• the conotruncal ridges fuse to form the conotruncal septum
• the conotruncal septum separates the outflow of the right and left ventricles and fuses with the interventricular septum
• the conotruncal swellings do not fuse in a straight line, but spiral round each other, allowing the RV to connect with the pulmonary trunk and LV with the aorta

Question 20

What cell type is found within the conotruncal swellings?

Answer 20

neural crest cells

• these separate from the neural tube as the neural folds elevate and fuse
• they migrate laterally and ventrally and differentiate into a variety of structures throughout the body

Question 21

Why is foetal circulation different to adult circulation?

Answer 21

• the mother’s circulation is oxygenating and detoxifying the blood, so foetal circulation shunts blood away from the lungs and liver

Question 22

Describe the foetal circulation after oxygenated blood enters through the umbilical vein

Answer 22

• some oxygenated blood enters the liver and the rest enters the ductus venosus to bypass the liver and enter the inferior vena cava
• the IVC enters the right atrium
• most blood passes into the left atrium via the foramen ovale, followed by the left ventricle and aorta
• blood entering the right atrium from the SVC is poorly oxygenated
• this passes through the ductus arteriosus and reduces the oxygenation of the blood in the aorta

Question 23

What does the ductus venosus become in the adult?

Answer 23

• termination of umbilical circulation causes the ductus venosus to close and degenerate
• it becomes the ligamentum venosum in the adult

Question 24

What changes take place after the baby takes its first breath?

Answer 24

• the first breath causes pulmonary arteries to dilate and changes the pressures in the atria
• blood returning from the lungs increases the pressure in the left atrium
• the increased pressure pushes the flimsy septum primum against the sturdy septum secundum, closing the foramen ovale
• the foramen ovale becomes the fossa ovalis in the adult
• the first breath also causes changes in oxygen saturation in the ductus arteriosus which causes it to constrict
• the ductus arteriosus becomes the ligamentum arteriosum in the adult