Development of the Heart 2 Flashcards
When does formation of the heart begin?
What are the 3 layers that it is composed of?
- 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
What does vasculogenesis lead to the formation of by day 18?
Why does this occur?
- 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
How is the primitive heart tube formed?
What does the visceral mesoderm surrounding this structure form?
- 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
What is the importance of craniocaudal folding in heart development?
- craniocaudal folding brings the developing heart tube into the thorax
What are the 3 layers of the primary heart tube?
Where is cardiac jelly found?
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
What structures are connected to the caudal and cranial regions of the heart tube in a 4 week embryo?
Caudal region:
- 3 paired veins drain into the tubular heart via the right and left horn of the sinus venosus
- umbilical vein
- vitelline vein
- common cardinal vein
Cranial region:
- this region connects to 2 dorsal aortae
How can the heart tube be divided into 5 dilatations?
- differential growth of the heart tube leads to the appearance of 5 dilatations:
- truncus arteriosus
- conus arteriosus
- ventricle
- atrium
- sinus venosus
- the truncus arteriosus and conus arteriosus together make up the bulbus cordis (outflow tract)
When does the heart tube start to fold?
How do the bulbus cordis, primitive ventricle and primitive atrium move during this process?
- 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
When is the sinus venosus obliterated?
What are the remnants of this structure?
- 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
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?
- 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
What is the crista terminalis?
- the crista terminalis marks the border between the sinus venarum (smooth part) and the trabeculated part of the right atrium
What are the 2 origins of the ventricular wall?
What structures are formed and how can these origins be distinguished from each other?
- 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
How do the right and left atria undergo further differentiation up to week 5 of development?
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
When does septation of the primitive atrium begin?
How does this process take place?
- 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
How is the atrioventricular septum formed following extension of the septum primum?
- 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