Anatomy and Embryological Development of the Human Heart and Blood Vessels

Early Development of Heart and Blood Vessels

• Initial Embryonic State: The embryo begins as a relatively flat trilaminar disc consisting of three primary germ layers:
  • Ectoderm: The top layer (depicted in green).
  • Mesoderm: The middle layer (pinkish).
  • Endoderm: The bottom layer (blue).
• Neural Folding: The central ectoderm thickens and lifts to form neural folds, which represent the initial development of the central nervous system.
• Lateral Plate Mesoderm Splitting: The mesoderm on either side of the midline splits into two distinct layers:
  • Somatic Mesoderm: Associated with the overlying ectoderm; it migrates to form the limb buds and eventually gives rise to all bones, cartilage, and ligaments.
  • Splanchnic Mesoderm: Associated with the underlying endoderm; this layer is specifically responsible for the development of the heart tube.
• Pericardial Coelom: The empty space created between the somatic and splanchnic layers is the pericardial coelom. This is the primitive body cavity that will eventually evolve into the fluid-filled pericardial cavity surrounding the heart.

Progression of Folding and Heart Tube Formation

• Step 1 & 2: Flat Disc and Coelom Formation: The process begins with the flat disc undergoing early neural grooving and the splitting of the lateral plate mesoderm to form the pericardial coelom.
• Step 3: Lateral Folding and Foregut Pocket:
  • The sides of the embryonic disc curl downward and inward toward the midline (lateral folding).
  • The endoderm pinches inward to create the foregut pocket, the precursor to the digestive tract.
  • Pericardial Mesoderm: The cardiac-destined mesoderm is pulled inward toward the midline by this folding motion.
• Step 4: Angiogenic Clusters and the AIP:
  • Heart-Forming Regions: Bilateral regions develop in the splanchnic mesoderm, containing angiogenic clusters that form the endocardium (inner lining of the heart tubes).
  • The AIP (Anterior Intestinal Portal): This is the opening located directly beneath the developing gut tube.
• Step 5: Midline Fusion:
  • Continued lateral folding forces the two separate heart-forming regions to meet at the ventral (front) midline, just below the gut.
  • The two endothelial tubes fuse into a single, central heart tube surrounded by the primitive pericardial cavity.
  • The endodermal tube completely pinches off to form a closed foregut cavity.
• Step 6: Completion (Day $21$-$22$):
  • A single, definitive central heart tube is fully fused and located ventrally to the gut.
  • The neural folds meet to close the neural tube.
  • The pericardial coelom fully expands to surround the heart, completing the ventral body wall closure.

Primitive Heart Tube Dilations and Adult Derivatives

The primitive heart tube consists of several embryonic dilations that correspond to specific adult structures:

• Truncus Arteriosus (T):
  • Adult Structure: Aorta and Pulmonary trunk.
  • Note: It becomes the pulmonary trunk and the aortic arch.
• Bulbus Cordis (B):
  • Distal $\frac{1}{3}$: Truncus arteriosus (forms ascending aorta and pulmonary trunk).
  • Middle $\frac{1}{3}$: Conus (forms the outflow tracts of the right and left ventricles, specifically the conus arteriosus in the right and aortic vestibule in the left).
  • Proximal $\frac{1}{3}$: Bulbo-ventricular sulcus (forms the trabeculated part of the primitive right ventricle).
• Primitive Ventricle (PV):
  • Adult Structure: Trabeculated part of the left ventricle (LV).
• Primitive Atrium (PA):
  • Adult Structure: Trabeculated parts of the right atrium and left atrium.
  • Note: The primitive atrium is partitioned to form separate right and left atria.
• Sinus Venosus (SV):
  • Body and Right Horn: Absorbed into the right atrium to form the smooth part (sinus venarum).
  • Left Horn: Retrogresses; the remnants form the coronary sinus and the oblique vein of the left atrium.
  • Venous Inflow (3 Tracts): Common cardinal veins, Umbilical veins, and Vitelline veins.
    • Right common cardinal vein forms part of the superior vena cava.
    • Right vitelline vein forms part of the inferior vena cava.
    • Right umbilical vein disappears.

Crucial Landmarks and Cardiac Looping

• Primary Fold (PF): Also known as the bulboventricular sulcus. This external groove separates the bulbus cordis from the primitive ventricle. Internally, it marks the site where the primitive interventricular septum will develop.
• Atrioventricular Ring (AVR): A transitional collar isolating the primitive atria from the primitive ventricle. It outlines the atrioventricular canal, which is later divided by endocardial cushions into the mitral and tricuspid valves.
• Cardiac Looping Process:
  • Timeframe: Begins on Day $23$ and is complete by Day $28$.
  • Mechanism: The linear heart tube bends and folds to become asymmetrical.
  • D-looping (Normal): The heart bends toward the right side.
    • The Bulbus Cordis and Ventricle move ventrally (forward), caudally (downward), and to the right.
    • The Atria and Sinus Venosus move dorsally (backward) and cranially (upward).
  • Dextrocardia (L-looping): When the heart loops to the left instead of the right, resulting in a mirror-image heart on the right side of the chest. This may occur alone or as part of Situs Inversus (all internal organs flipped). It is primarily caused by mutations in dynein proteins.

Partitioning of the Atrioventricular Canal

• Endocardial Cushions: Toward the end of the $4^{th}$ week, dorsal and ventral AV endocardial cushions form on the walls of the AV canal.
• Composition: Developed from specialized extracellular matrix (cardiac jelly) and neural crest cells.
• Fusion: The cushions approach each other and fuse to form the septum intermedium.
• Result: This divides the single AV canal into separate right and left AV orifices/canals, which eventually function as the AV valves (mitral and tricuspid).

Partitioning of the Primordial Atrium

• Septum Primum (approx. 27 days / $5^{th}$ week): A thin septum grows from the atrial roof toward the septum intermedium.
• Ostium Primum: The temporary gap between the septum primum and the cushions.
• Ostium Secundum: Before the ostium primum closes, the upper part of the septum primum undergoes apoptosis, creating fenestrations that merge to form the ostium secundum.
• Septum Secundum: A thick septum that develops to the right of the septum primum. It grows downward but stops by the end of the $7^{th}$ week.
• Foramen Ovale: The gap left by the septum secundum. It acts as a one-way valve allowing a right-to-left shunt, essential for bypassing the unventilated, fluid-filled fetal lungs.

Partitioning of the Primordial Ventricle

• Interventricular (IV) Septum Components: The IV septum consists of three distinct parts:
  1. Muscular Part: Develops from the floor of the primitive ventricle near its apex as a median muscular ridge growing upward toward the AV cushions. Its upper margin is concave.
  2. Bulbar Part: Develops from the right and left bulbar ridges in the distal part of the bulbus cordis. These ridges grow and fuse together.
  3. Membranous Part: Closes the remaining Interventricular (IV) foramen (the crescent-shaped gap between the muscular septum and bulbar ridges). It is derived from three sources: the right bulbar ridge, the left bulbar ridge, and the AV cushions.
• Timeline for Closure: The IV foramen usually closes by the end of the $7^{th}$ week via the fusion of tissue from the bulbar ridges and an extension from the right side of the endocardial cushion, aided by neural crest cells.
• Ventricular Septal Defect (VSD):
  • The most common congenital heart anomaly.
  • Defects are most frequent in the membranous part of the interventricular septum.