B4.046 Prework 2: Chamber Partitioning and Blood Vessel Formation

Question 1

weeks 4-7

Answer 1

atrial and ventricular septation occurs

Question 2

how is early septation initiated

Answer 2

endocardial cushion formation
myocardium deposits a unique ECM between the endocardium and itself
endocardial cells in certain regions undergo an epithelial to mesenchymal transformation
mesenchymal cells (migrating endocardial cushion cells) aid in formation of the AV canal

Question 3

when does partitioning of the AV canal occur?

Answer 3

day 23-35; weeks 4-5

Question 4

in what 2 regions do endocardium derived cushion tissue form?

Answer 4

AV region and outflow tract region

outflow tract region also populated by neural crest cells

Question 5

what forms from the fusion of opposing cushion tissues?

Answer 5

2 AV canals
outlets of both ventricles (aorta and pulmonary trunk)
membranous portion of IV septum

Question 6

what happens in the first phase of atrial septation at day 28?

Answer 6

septum primum grows down from common atrium roof towards the endocardial cushions in the AV canal
foramen primum is the hole between the R and L atrium at this point

Question 7

what happens in the second phase of atrial septation in the 5th week?

Answer 7

programmed cell death creates new openings in the septum primum near the superior edge
this new hole is the foramen secundum
endocardial cushions grow along septum primum and close foramen primum

Question 8

what happens in the third phase of atrial septation, also in the 5th week?

Answer 8

septum secundum forms to the right of the septum primum
septum secundum grows down and fuses with an intermediate septum by the end of the 6th week, but a foramen ovale persists throughout embryonic development

Question 9

what is the function of the foramen ovale in the septum secundum and the foramen secundum in the septum primum?

Answer 9

allows blood to flow from RA to LA, but not L to R
acts as a one way inter-atrial valve, allowing oxygenated venous blood from placenta to bypass the fetal lungs and be pumped to rest of the embryo

Question 10

what happens in the final stage of atrial septation by the 6th week?

Answer 10

septum primum meets and fuses with septum intermedium
foramen secundum is obliterated and R and L atria are now separated
blood flows through foramen ovale when its valve is open, but not when it is closed

Question 11

when does the muscular IV septum form?

Answer 11

during the 5th week

medial walls of the expanding ventricles becomes apposed

Question 12

describe the formation of the membranous portion of the IV septum on day 37

Answer 12

outgrowth of endocardial cushions toward muscular IV septum closes IV foramen
neural crest cell derivatives contribute to membranous part of IV septum
outflow tract septation also contributes to membranous IV septum

Question 13

when do the AV valves form?

Answer 13

5th to 8th weeks

Question 14

how do the AV valves form

Answer 14

dependent on AV cushions and ventricular myocardium
valves are connective tissue covered by endocardium
valves connected to thick trabeculae in the ventricle wall via chordae tendinae; start to function

Question 15

describe the septation of the outflow tract during the 5th week

Answer 15

opposing ridges form from the walls of the truncus arteriosus and bulbus cordis
ridges form a spiral within the outflow while growing towards aortic sac
ridges (conotruncal swellings) form aorticopulmonary septum in week 8
neural crest cells contribute to ridges

Question 16

defects due to abnormal partitioning of the outflow tract

Answer 16

transposition of great arteries
persistent truncus arteriosus (single outflow)
pulmonary stenosis

Question 17

what forms the aortic and pulm valves?

Answer 17

outflow cushion and neural crest cells

Question 18

where do SA pacemaker cells reside initially/ where do they end up?

Answer 18

initially at caudal end of left heart tube
as sinus venosus becomes incorporated into RA, pacemaker cells come to reside near the junction of the superior vena cava and RA forming the SA node

Question 19

where are the AV node and bundle of His cells derived from?

Answer 19

1. cells in the left wall of the sinus venosus

2. cells from the AV canal

Question 20

where does the AV node end up?

Answer 20

base of interatrial septum in the right atrium, near the opening of the coronary sinus

Question 21

first major blood vessels

Answer 21

dorsal aortae

cardinal veins

Question 22

coronary artery formation

Answer 22

arise from epicardium following epithelial to mesenchymal transformation
neural crest cells contribute to smooth muscle cells along proximal segments of coronary arteries
coronary arteries grow to invade the aorta to establish blood flow to the heart muscle

Question 23

vitelline veins

Answer 23

drain blood from yolk sac
functional in early development
become portal venous system

Question 24

umbilical vein

Answer 24

carries blood from placenta to embryo

Question 25

cardinal veins

Answer 25

drain the body of the embryo

form superior vena cava

Question 26

vitelline arteries

Answer 26

supply yolk sac

eventually fuse and form vessels to gut

Question 27

umbilical arteries

Answer 27

course to placenta

Question 28

right horn of the sinus venosus

Answer 28

becomes incorporated into the right atrium

Question 29

left horn of the sinus venosus

Answer 29

becomes the coronary sinus

major venous drainage for blood that has perfused the myocardium