Human Heart Development: Embryonic Timeline from Tube to Four Chambers

The heart is the first functional organ. Development begins at ~day 18–19 post-fertilization (gestational week 3).

Day 18–19: Cardiogenic mesoderm forms bilateral cardiogenic cords → fuse at midline → cardiac crescent (primary heart field).
Day 20–21: Linear heart tube forms; begins spontaneous contraction (primitive peristalsis).
Day 22–23: Cardiac looping (D-loop) — tube bends rightward, creating spatial arrangement: primitive ventricle (left), atrium (right), sinus venosus (posterior).
Day 24–28: Chamber specification — ballooning of atrial and ventricular chambers; secondary heart field adds outflow tract (conotruncus) and right ventricle/inflow.
Day 28–35: Septation — simultaneous partitioning:
- Atrial septum: septum primum + secundum → foramen ovale.
- Ventricular septum: muscular + membranous (AV canal + conotruncal).
- Outflow tract: aorticopulmonary septum (neural crest-derived).
Day 35–49: Valvulogenesis — endocardial cushions → AV valves (tricuspid, mitral); conotruncal cushions → semilunar valves (aortic, pulmonary).
Week 8–10: Coronary vasculature — epicardial-derived; connects to aortic sinuses.
Week 10 (end embryonic period): Four-chambered heart structurally complete; fetal circulation pattern established (foramen ovale, ductus arteriosus).
Fetal period: Maturation, hypertrophy, conduction system refinement.

Days 20–50: Highest teratogen sensitivity (thalidomide, retinoids, alcohol, rubella, diabetes, lithium).
Neural crest migration (weeks 4–7) crucial for outflow tract — defects → TOF, TGA, truncus, VSD.

Congenital heart defects (CHD): 1% live births; most originate in septation/valvulogenesis.
Fetal echo (18–22 wks) detects major CHD.
Postnatal transition: lung inflation → ↓ PVR → ↑ pulmonary flow → functional closure of foramen ovale/ductus.