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2. Heart Development
· Endocardial tubes fuse to form heart tube (21 days)
· Heart begins to beat (22 days)
· Heart folding (21-22 days), folding (23-28 days)
a) D (right) loop, L (left) loop
b) Bulboventricular loop --- future ventricles
c) Cellular differentiation
d) Bulbus cordis - conus cordis--- RVOT
e) Truncus arteriosus --- great vessels

3. Atrial Septation
· Septum primum forms from roof of atrium
a) Ostium primum - closed by fusion of septum to endocardial cushion
b) Ostium secundum - coalescence of fenestrations
· A-V canal divided by endocardial cushions
· Septum secundum grows down from roof of atrium
a) Fuses with endocardial cushions
b) Overlaps ostium secundum
c) Foramen ovale remains open until after birth

4. Ventricular Septation and A-V Valves
· Muscular interventricular septum forms
· Fusion of ventricular septum with endocardial cushion must await partition of truncus arteriosus
· Undermining of myocardium forms valve leaflets
· Papillary muscles and chordae tendinea derived from ventricular myocardium

5. Clinical Correlates - Septal Defects
· Atrial septal defect
a) Ostium secundum = excess resorption of septum primum or inadequate development of septum secundum (foramen ovale defect)
b) Ostium primum = septum primum fails to fuse with endocardial cushion (low defect with semilunar shape, right above the AV valves)
· Ventricular septal defect
a) Failure of membranous portion to develop from extension of endocardial cushion to fuse with truncoconal septum
b) Malalignment
c) Muscular defect = resorption of septum

6. Truncoconal Septation · Bulbar-truncal ridges form truncoconal or aorticopulmonary septum
· Streaming of blood flow may account for spiral configuration of truncoconal septum
· Bulbar-truncal ridges fuse to divide truncus arteriosus (aorta and pulmonary artery)
· Fused bulbar-truncal ridges extend to fuse with endocardial cushion and muscular septum to partition ventricular septum · Semilunar valves derived from truncoconal swellings

7. Clinical Correlates - Truncoconal Septation
· Truncus arteriosus = defective fusion of bulbotruncal ridges
· Transposition of Great Arteries = failure of truncoconal spiral
· Tetralogy of Fallot = unequal division of conus cordis
· Semilunar valve stenosis = failure of development of truncoconal swellings or unequal partition

8. Aortic Arch Derivatives
· Truncus arteriosus
a) Proximal ascending aorta
b) Main pulmonary artery
· Aortic sac
a) Ascending aorta, 1/2 arch
b) Brachiocephalic artery

9. Aortic Arch Derivatives Part II
· Aortic arches
a) 1, 2, 5, R6 disappear
b) 3 => carotid arteries
c) 4 => mid arch, R proximal subclavian artery
d) 6 => RPA and ductus arteriosus
· Dorsal aorta
a) Left => descending aorta
b) Right => R distal subclavian, distal disappears
c) Internal carotid arteries

10. Clinical Correlates - Aortic Arch Derivatives
· Coarctation of the Aorta = probably related to ductus incorporation into the aortic wall
· Fetal blood flow and resorption of the dorsal aorta may also play a role
· Double aortic arch = failure of right dorsal arch to disappear
· Abnormal origin R subclavian artery = R4 arch and R dorsal aorta disappear, leaving 7 intersegmental artery originating as fourth branch of aorta behind esophagus

11. Fetal Circulation
· Three shunts permit most of the blood to bypass liver and lungs
a) Ductus venosus --- Ligamentum teres, venosum
b) Foramen ovale -- Fossa ovalis
c) Ductus arteriosus -- Ligamentum arteriosus
· Shunts close after birth and become ligamentous