19.5 Development of the Heart

Embryonic Heart Development and Fetal Heart Anatomy

The heart is the first functional organ in the embryo. It begins as a simple tube and transforms into a four-chambered pump within the first eight weeks of development. Understanding this process helps explain congenital heart defects, which are among the most common birth defects.

Stages of Embryonic Heart Development

Heart development follows a specific sequence of events. Each stage builds on the last, so problems at any point can cascade into structural defects.

1. Heart tube formation (Day 22): Paired heart tubes arise from cardiogenic mesoderm and fuse along the midline to form a single heart tube. This tube is the earliest recognizable heart structure.

2. Looping (Days 23–28): The heart tube elongates and bends into a C-shape, then an S-shape. This looping step is what establishes left-right asymmetry. Embryonic folding also helps position the developing heart within the thorax.

3. Septation (Days 28–56): Septa form to divide the heart into four chambers:

   • The atrial septum divides the primitive atrium into left and right atria
   • The interventricular septum divides the primitive ventricle into left and right ventricles
   • The atrioventricular septum separates the atria from the ventricles

4. Valve formation (Days 35–56): Endocardial cushions, supported by an extracellular matrix called cardiac jelly, develop into the heart valves and contribute to septal tissue:

   • Atrioventricular cushions form the tricuspid and mitral (bicuspid) valves
   • Outflow tract cushions form the aortic and pulmonary semilunar valves

5. Outflow tract division (Days 28–56): The aorticopulmonary septum spirals through the truncus arteriosus, splitting it into the ascending aorta and pulmonary trunk. This spiral explains why these two great vessels wrap around each other in the adult heart.

6. Coronary vasculature development (Day 56 through birth): Coronary arteries and veins form from epicardial cells. The coronary arteries connect to the aorta, and the coronary veins drain into the right atrium via the coronary sinus.

Regions of the Fetal and Adult Heart

The primitive heart tube has distinct regions, each of which gives rise to specific adult structures:

Two fetal-specific structures are also worth knowing:

• Foramen ovale: An opening between the right and left atria that allows oxygenated blood from the placenta to bypass the non-functional fetal lungs. It closes after birth.
• Ductus arteriosus: A short vessel connecting the pulmonary artery to the aorta, diverting most blood away from the lungs. It also closes after birth.

Transformation of Fetal to Adult Cardiac Anatomy

The transition from fetal to adult heart anatomy involves completing septation, finalizing valve structures, and closing fetal shunts.

Atrial and ventricular septa formation:

1. Septum primum grows down from the roof of the primitive atrium, followed by septum secundum. These two septa overlap to create a flap-valve mechanism (the foramen ovale) that allows right-to-left shunting in the fetus but seals shut after birth.
2. The interventricular septum forms in two parts: a thick muscular portion grows upward from the floor of the ventricle, and a thin membranous portion closes the remaining gap from above. Both must fuse completely to prevent a ventricular septal defect (VSD), one of the most common congenital heart defects.

Valve formation from endocardial cushions:

• Tricuspid and mitral valves regulate flow between the atria and ventricles
• Aortic and pulmonary valves regulate flow into the great arteries

Outflow tract septation:

• The aorticopulmonary septum divides the truncus arteriosus into the ascending aorta and pulmonary trunk, establishing separate systemic and pulmonary circuits

Closure of fetal shunts:

• The foramen ovale closes when left atrial pressure rises after the newborn's first breaths, pressing the septum primum against the septum secundum. The sealed remnant is called the fossa ovalis.
• The ductus arteriosus constricts in response to rising blood oxygen levels and becomes the ligamentum arteriosum, a fibrous cord connecting the aorta and pulmonary trunk.

Coronary circulation development:

• Coronary arteries and veins, derived from epicardial cells, supply the heart muscle (myocardium) with oxygenated blood and return deoxygenated blood to the right atrium.

Cardiac Neural Crest and Fetal Circulation

Cardiac neural crest cells migrate from the neural tube into the developing heart and are critical for outflow tract septation and formation of the great vessels. Defects in neural crest migration can lead to conditions like persistent truncus arteriosus (failure of the outflow tract to divide) or tetralogy of Fallot.

Fetal circulation depends on the placenta for gas exchange rather than the lungs. The foramen ovale and ductus arteriosus work together to shunt most blood away from the high-resistance pulmonary circuit. Once the baby takes its first breaths, pulmonary resistance drops, blood flows through the lungs, and both shunts close. This shift establishes the separate pulmonary and systemic circulations that define the adult cardiovascular system.