Heart Development

Question 1

formation of the heart primordium

Answer 1

• cardiac precursors in the primitive streak migrate out both sides of the streak and forward in the lateral plate (splanchnic) mesoderm
• cardiac muscle cell precursors in the precardiac masses begin to differentiate and aggregate into left and right heart tubes
• the tubes are brought together and fuse into one tube
• heart starts beating at day 18

Question 2

cardiac looping

Answer 2

•brings inflow tract cranially so that inflow and outflow are next to each other

Question 3

ventricular inversion

Answer 3

• in situs solitus
• the heart loops to the left instead of the right
• right atrium connects to the left ventricle and vice versa
• with concomitant transposition of the great vessels (congenitally corrected transposition of the great arteries - ccTGA) the circulation is functional after birth but not optimized due to differences between the muscle strengths of the ventricles

Question 4

situs inversus

Answer 4

•complete left looping can occur in combination with reversed looping of the gut to produce normal but reversed viscera, in which the heart (and gut) is a fully functional mirror image of normal

Question 5

sinus venosus

Answer 5

•has 2 horns
•each horn receives blood from three sources
-umbilical vein (placenta)
-vitelline vein (gut tube —> portal vein)
-common cardinal vein form the anterior and posterior cardinal veins –> IVC)
•from right to left these inputs form the Superior Vena Cava, the smooth part of the right atrium between the IVC and SVC and the Coronary Sinus
•don’t worry about the formation of the IVC - too complicated

Question 6

primitive atrium

Answer 6

• above the sinus venosus
• will eventually enlarge and be spilt by a septum and the muscular walls will form the trabeculated parts of the left and right atria

Question 7

atrioventricular canal

Answer 7

•the constriction between the primitive atrium and the ventricle is the site where the AV valves will form

Question 8

primitive ventricle

Answer 8

• early in development, only a single ventricle is needed to expel blood from the heart tube
• this muscular ventricle will eventually form the trabeculated parts of (mostly) the left and right ventricle
• will be divided by a septum

Question 9

outflow tract

Answer 9

•connects between the single ventricle and the aortic arch
•will divide into subregions
-bulbus cordis (smooth walled parts of the ventricles)
-truncus arteriosis (ascending aorta and pulmonary trunk)

Question 10

arch vessels

Answer 10

• I, II (V) and part of VI degenerate
• III forms carotids
• IV forms brachiocephalic, subclavian, arch of the aorta
• VI pulmonary arteries and ductus arteriosus

Question 11

pulmonary veins

Answer 11

•grow into left atrium from the pulmonary mesenchyme by de novo angiogenesis can can (rarely) attach inappropriately

Question 12

septum intermedium

Answer 12

• between atria and ventricle

* fusion of cardiac cushions

Question 13

septum primum

Answer 13

• first wall, atrial septation
• grows towards toward septum intermedium at the AV canal
• the lower border of the crescent outlines an opening between the two atria called the ostium primum

Question 14

ostium secundum

Answer 14

• just before the septum primum grows enough to close the ostium primum, a second hole develops in the superior part of the septum primum
• localized apoptosis, maintains an opening between atria

Question 15

septum secundum

Answer 15

• second wall between atria
• does not reach the septum intermedium and leaves an opening at its lower edge that creates a flap valve with the ostium secundum called the patent foramen ovale (failure of septa to overlap can leave a PFO or, if really big, ASD)

Question 16

fossa ovalis

Answer 16

• oval pit
• the thin part of the atrial septum where the septum primum is exposed by the foramen ovale creates a visible postnatal depression

Question 17

interventricular septum

Answer 17

• mostly muscle tissue - fusion and growth of trabeculae carnae from the base of the primitive ventricle
• extends towards the septum intermedium but does not contact it, leaving an interventricular foramen

Question 18

membranous septum

Answer 18

• when the outflow tract is fully divided into the great vessels, the interventricular foramen is closed by connective tissue extending from the septum intermedium and the bulbar cushions dividing the outflow tract
• most common sight of VSD

Question 19

division of the outflow tract

Answer 19

• separation into aorta and pulmonary trunk
• spiraling
• failure to do so leads to a variety of defects

Question 20

ductus arteriosis

Answer 20

• temporary shunt that diverts right ventricular blood into the aorta
• normally closes at birth by contraction initiated by prostaglandin E2 removal (placental origin)
• becomes the ligamentum arteriosum

Question 21

endocardium

Answer 21

• endothelial cells line the heart tube and are continuous with the endothelial cells of the blood vessels
• mesoderm - angioblasts

Question 22

myocardium

Answer 22

• precursors arise in the anterior ventral splanchnic mesoderm
• the early heart has both contractile myocytes and myoblasts - by late in development, all are beating cells
• some form SA node, some form AV node

Question 23

cardiac jelly

Answer 23

• the largest concentration of extracellular matrix in the early embryo is in the interval between the endocardium and the myocardium (in an adult it is called sub-endocardial connective tissue)
• proteoglycans and glycoproteins
• wall strength to early heart, growth factors
• later in fetal life, myocardial cells secrete an interstitial matrix of collagen fibers
• after birth. cardiac extracellular matrix is largely produced by fibroblasts formed from the epicardium

Question 24

epicardium

Answer 24

• begins as single cell layer which moves onto the heart surface from near the liver and migrates over the surface of the myocardium
• undergo epithelial-mesenchymal cell transformation to form the precursors of coronary vessels and the mural capillaries
• will also form the fibroblasts of the heart and the smooth muscle cells of the blood vessels

Question 25

neural crest

Answer 25

• during looping, neural crest cells enter the heart tube from the pharyngeal arches (4 mostly, some from 3 and 6), migrate into the outflow tract and continue down to the junction with the ventricle (superior at this stage!)
• participate in the division of the outflow tract into the aorta and pulmonary trunk via the spiral septum