Embryonic Development of the Cardiovascular System

Early Development and Significance of the Cardiovascular System

• Initial Functional System: The cardiovascular system is the first functional system to develop in the embryo.
• Nutritional Requirements:
  • During early phases, the embryo relies on diffusion from fluid secreted by uterine glands.
  • As the embryo rapidly increases in size and complexity, diffusion becomes insufficient.
  • There is an urgent requirement for a circulatory system to support continued development.
• Embryological Origins:
  • Mesoderm: Differentiates into mesenchyme (pluripotent cells).
  • Angioblastic Tissue (Hemangioblasts): Differentiates from mesenchyme.
  • Cardiovascular System: Both blood and blood vessels originate from these tissues.
• Angiogenesis: The name given to the process of blood vessel development.

Anatomy of the Cardiogenic Field and Early Folding

• Cardiogenic Field:
  • A horseshoe-shaped structure located around the anterior and lateral portions of the neural plate.
  • An area of blood-forming cavities in the visceral mesoderm.
  • These cavities coalesce to form the primitive heart, blood vessels, and blood cells.
• Embryonic Folding:
  • Antero-posterior (Cranio-caudal) Folding: This movement brings the cardiac tube to a caudo-ventral position.
  • Heart Placement: The cardiogenic tube is moved ventral to the dorsal aortae.
• Fusion Events:
  • The caudal portion of the cardiogenic tube fuses with the cranial portion of the vitelline veins.
  • The caudal portions of the dorsal aortae fuse.
  • The two sides of the cardiac tube fuse into a single primitive heart tube.
• Associated Structures: During the folding process, other structures such as the septum transversum, primitive gut, brain vesicles, and the pericardial cavity are positioned relative to the developing heart.

The Five Regions of the Primitive Heart Tube and Their Derivatives

The primitive heart tube undergoes looping and partitioning to form recognizable adult structures. The development progresses from a single tube around day $18$ to a multi-chambered heart by day $56$.

• 1. Truncus Arteriosus:
  • Refers to the most cranial part of the tube.
  • Divides into the outflow tracts of the aorta and the pulmonary trunk.
• 2. Bulbus Cordis:
  • Develops into the right ventricle, the conus cordis, and the truncus arteriosus.
• 3. Primitive Ventricle:
  • Develops into most of the left ventricle.
• 4. Primitive Atrium:
  • Develops into the anterior portions of the left and right atria.
• 5. Sinus Venosus:
  • The most caudal part of the tube.
  • Develops into the posterior portion of the right atrium, the sinoatrial (SA) node, and the coronary sinus.

Comparative Cardiac Anatomy

• Fish: Possess a $2$-chambered heart consisting of one atrium and one ventricle, pumping blood through the gills to the body.
• Amphibians: Possess a $3$-chambered heart (two atria, one ventricle). This results in mixed oxygenated and deoxygenated blood.
• Reptiles (specifically Turtles): Possess a $3$-chambered heart that is septated, indicating a partial division of the ventricle.
• Birds and Mammals: Possess a $4$-chambered heart, fully separating oxygenated and deoxygenated blood circuits.

Partitioning of the Atrio-Ventricular Canal and Atria

• Role of Endocardial Cushions: These structures fuse in the center of the heart to divide the common canal into right and left atrioventricular (AV) canals.
• Septum Primum (Primary Atrial Septum):
  • Grows toward the endocardial cushions.
  • Ostium Primum: The initial opening between the lower edge of the septum primum and the endocardial cushions, allowing right-to-left blood flow.
  • Ostium Secundum: Formed by perforations in the upper part of the septum primum before the ostium primum fully closes.
• Septum Secundum (Secondary Atrial Septum):
  • Grows to the right of the septum primum.
  • It creates the permanent opening known as the foramen ovale.
• Valve Mechanism: The remaining part of the septum primum acts as a one-way flap valve for the foramen ovale, preventing blood from returning to the right atrium.

Partitioning of the Ventricles and Great Vessels

• Interventricular Septum:
  • Composed of a muscular portion and a membranous portion.
  • The muscular portion grows from the apex upward, while the membranous portion completes the separation near the endocardial cushions.
• Aortico-pulmonary Septum:
  • The truncus arteriosus is divided into the ascending aorta and the pulmonary trunk by a spiral septum.
  • The Spiral Advantage: The spiral formation ensures the right ventricle connects to the pulmonary trunk and the left ventricle connects to the aorta.
  • Transposition of the Great Vessels: If this septum develops straight instead of spiraling, deoxygenated blood from the right ventricle enters the aorta, and oxygenated blood from the left ventricle goes to the lungs. This condition is incompatible with life.

Fetal Circulation and Shunts

Fetal circulation is adapted for the in utero aqueous environment, where oxygenation occurs at the placenta rather than the lungs.

• 1. Ductus Venosus:
  • Connects the umbilical vein to the inferior vena cava.
  • Enables oxygenated blood from the placenta to bypass the liver.
• 2. Foramen Ovale:
  • An opening in the heart allowing blood to pass from the right atrium to the left atrium.
  • Bypasses the non-functional fetal lungs.
• 3. Ductus Arteriosus:
  • A vessel connecting the pulmonary artery directly to the aorta.
  • Bypasses the lungs.

Postnatal Transitions and Anatomical Closures

• The Transition at Birth:
  • Lungs expand with the first breath.
  • A sudden drop in blood pressure occurs in the pulmonary circulation.
• Closure of the Foramen Ovale:
  • Physiological Closure: The pressure drop in the right side and pressure increase in the left side pushes the primary septum (septum primum) against the secondary septum (septum secundum).
  • Anatomical Closure: The two septa eventually fuse together.
• Adult Remnants:
  • Ductus Venosus becomes the Ligamentum venosum (found in the liver).
  • Foramen Ovale becomes the Fossa ovalis (a depression in the right atrium).
  • Ductus Arteriosus becomes the Ligamentum arteriosum (near the adult heart).