19.5 Development of the Heart

The heart's journey from a simple tube to a complex four-chambered organ is a marvel of embryonic development. This process, occurring in the first eight weeks of pregnancy, involves intricate folding, looping, and septation to create distinct chambers and vessels.

After birth, the fetal heart undergoes crucial changes to adapt to life outside the womb. Fetal shunts close, allowing for separate pulmonary and systemic circulations. The coronary system develops fully, ensuring proper blood supply to the heart muscle itself.

Embryonic Heart Development and Fetal Heart Anatomy

Stages of embryonic heart development

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• Cardiogenic mesoderm forms heart tubes
  • Primitive heart tubes arise from cardiogenic mesoderm and fuse to form a single heart tube (day 22)
• Heart tube elongates and loops
  • Elongation and looping of the heart tube establishes left-right asymmetry (day 23-28)
  • Embryonic folding contributes to the positioning of the developing heart
• Septa form to divide heart into four chambers
  • Atrial septum develops to divide the primitive atrium into left and right atria
  • Interventricular septum forms to divide the primitive ventricle into left and right ventricles
  • Atrioventricular septum separates the atria from the ventricles (day 28-56)
• Endocardial cushions develop into valves and septa
  • Tricuspid and mitral valves form between the atria and ventricles from atrioventricular cushions
  • Aortic and pulmonary valves develop at the bases of the great arteries from outflow tract cushions (day 35-56)
  • Cardiac jelly, an extracellular matrix, plays a crucial role in cushion formation
• Outflow tract divides into aorta and pulmonary artery
  • Aorticopulmonary septum separates the truncus arteriosus into the ascending aorta and pulmonary trunk (day 28-56)
• Coronary vasculature develops from epicardium
  • Coronary arteries and veins form from epicardial cells and connect to the aorta and right atrium, respectively (day 56-birth)

Regions of fetal and adult heart

• Primitive atrium
  • Develops into the left and right atria, which receive blood from the lungs and body, respectively
• Primitive ventricle
  • Develops into the left and right ventricles, which pump blood to the body and lungs, respectively
• Bulbus cordis
  • Develops into the outflow tracts, including the ascending aorta and pulmonary artery
• Sinus venosus
  • Contributes to the formation of the right atrium and sinoatrial node, the heart's pacemaker
• Truncus arteriosus
  • Divides into the ascending aorta, which supplies the body, and the pulmonary trunk, which supplies the lungs
• Foramen ovale
  • Fetal shunt between the right and left atria that allows oxygenated blood to bypass the lungs; closes after birth
• Ductus arteriosus
  • Fetal shunt between the pulmonary artery and aorta that diverts blood away from the lungs; closes after birth

Transformation of fetal to adult cardiac anatomy

• Atrial and ventricular septa formation
  1. Septum primum and septum secundum fuse to divide the primitive atrium into left and right atria
  2. Muscular and membranous portions of the interventricular septum fuse to divide the primitive ventricle into left and right ventricles
• Valve formation from endocardial cushions
  • Tricuspid and mitral valves develop from atrioventricular cushions and regulate blood flow between the atria and ventricles
  • Aortic and pulmonary valves develop from outflow tract cushions and regulate blood flow into the aorta and pulmonary artery
• Outflow tract septation
  • Aorticopulmonary septum divides the truncus arteriosus into the ascending aorta and pulmonary artery, separating systemic and pulmonary circulation
• Closure of fetal shunts
  • Foramen ovale closes to separate the left and right atria, preventing mixing of oxygenated and deoxygenated blood
  • Ductus arteriosus constricts and becomes the ligamentum arteriosum, a fibrous remnant connecting the aorta and pulmonary artery
• Coronary circulation development
  • Coronary arteries and veins form from epicardial cells and connect to the aorta and right atrium, respectively, to supply the heart muscle with oxygenated blood

Cardiac Neural Crest and Fetal Circulation

• Cardiac neural crest cells contribute to the development of the outflow tract and great vessels
• Fetal circulation relies on specialized shunts to bypass the non-functional lungs
• Septation of the heart chambers and great vessels is crucial for establishing separate pulmonary and systemic circulations