1. Introduction and Primary Heart Field
- The heart is the first functional organ to develop, derived from lateral plate mesoderm.
- Day 18-19: Progenitor heart cells migrate to the cranial end of the embryo, forming the Primary Heart Field (PHF) (crescent-shaped).
- Day 20: Angiogenic clusters coalesce to form two endocardial tubes.
- Day 22: Lateral folding of the embryo fuses these tubes into a single primitive heart tube.
2. The Primitive Heart Tube
Consists of five distinct Dilatations (Cranial to Caudal):
- Truncus Arteriosus: Forms proximal aorta and pulmonary artery.
- Bulbus Cordis: Forms right ventricle and outflow tracts.
- Primitive Ventricle: Forms the left ventricle.
- Primitive Atrium: Forms the trabeculated parts of both atria.
- Sinus Venosus: Forms the smooth part of right atrium (sinus venarum), SA node, and coronary sinus.
3. Cardiac Looping
- Timing: Occurs between Day 23 and 28.
- The heart tube elongates and bends because the bulbus cordis and ventricle grow faster than the pericardial cavity.
- D-looping: The bulboventricular segment loops to the right.
- Clinical Significance: Abnormal looping (L-looping) leads to L-TGA (Congenitally Corrected Transposition).
4. Atrial and Ventricular Septation
- Atrial Septation (Day 27-37):
- Septum Primum grows toward endocardial cushions; ostium primum then ostium secundum form.
- Septum Secundum grows to the right of septum primum, forming the Foramen Ovale.
- Ventricular Septation:
- Muscular septum grows upward from the apex.
- Membranous septum forms last by the fusion of the inferior endocardial cushion and the conotruncal ridges.
- Endocardial Cushions: Essential for dividing the AV canal into right and left orifices.
5. Conotruncal Septation
- Neural Crest Cells migrate to form the cono-truncal ridges.
- These ridges undergo a 180-degree spiral to form the aorticopulmonary septum.
- Defects here result in TOF, Persistent Truncus Arteriosus, or TGA.
6. Summary Table: Adult Derivatives
| Embryonic Structure | Adult Structure |
|---|---|
| Bulbus Cordis | Right Ventricle & Outflow tracts (Conus cordis) |
| Left Horn of Sinus Venosus | Coronary Sinus |
| Right Horn of Sinus Venosus | Smooth part of Right Atrium |
| Truncus Arteriosus | Ascending Aorta and Pulmonary Trunk |