Developmental Anatomy

Developmental Anatomy

1. Basic Principles
A. Cardiovascular system is first functional system in embryo
B. Blood circulation by 3 weeks (21 days)
C. Heart develops 3-8 weeks
D. Critical period for anomalies 3-6 weeks

2. Heart Development
A. Endocardial tubes fuse to form heart tube (21 days)
B. Heart begins to beat (22 days)
C. Heart folding - 21-22 days, folding - 23-28 days
1) D loop, L loop
2) Bulboventricular loop --- future ventricles
3) Cellular differentiation
4) Bulbus cordis - conus cordis --- RVOT
5) Truncus arteriosus --- great vessels

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

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

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

6. Truncoconal Septation
A. Bulbar-truncal ridges form truncoconal or aorticopulmonary septum
B. Streaming of blood flow may account for spiral configuration of truncoconal septum
C. Bulbar-truncal ridges fuse to divide truncus arteriosus (aorta and pulmonary artery)
D. 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
A. Truncus arteriosus = defective fusion of bulbotruncal ridges
B. Transposition of Great Arteries = failure of truncoconal spiral
C. Tetralogy of Fallot = unequal division of conus cordis
D. Semilunar valve stenosis = failure of development of truncoconal swellings or unequal partition

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

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

10. Clinical Correlates - Aortic Arch Derivatives
A. Coarctation of the Aorta = probably related to ductus incorporation into the aortic wall
B. Fetal blood flow and resorption of the dorsal aorta may also play a role
C. Double aortic arch = failure of right dorsal arch to disappear
D. 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
A. Three shunts permit most of the blood to bypass liver and lungs
1) Ductus venosus --- Ligamentum teres, venosum
2) Foramen ovale -- Fossa ovalis
3) Ductus arteriosus -- Ligamentum arteriosus
B. Shunts close after birth and become ligamentous