Primitive Heart Development

Primitive Heart Tube Formation

• Angiogenic cell clusters develop beside the developing pericardial cavities.
• Lateral folding brings endocardial heart tubes to the midline, where they fuse.
• The dorsal mesocardium breaks down, suspending the heart tube in the pericardial cavity.

Primitive Heart Tube Parts & Adult Derivatives

• Sinus Venosus: Coronary sinus
• Endocardial Cushions: Membranous ventricular septum
• Bulbus Cordis: Ventricles
• Septum Secundum: Atrial septum
• Truncus Arteriosus: Proximal part of aorta

Atrioventricular Canal Formation

• A wedge of endocardial tissue forms, directing blood flow.

Atrial Septum Formation

• Two-phase process (pre- and post-birth).
• Septum Primum: Grows from the atrium roof; foramen primum is the opening below. Develops perforations as it grows.
• Reaches endocardial cushions, some portions disappear, creating foramen secundum.
• Septum Secundum: Begins to grow on the right side, overlaps septum primum.
• Foramen ovale is the path between the two septa.

Foramen Ovale's Fetal Role

• Allows blood flow between atria before birth.

Aorta and Pulmonary Trunk Formation

• Aorticopulmonary Septum: Forms in a spiral, dividing bulbus cordis and truncus arteriosus into aorta and pulmonary trunk.
• Neural crest cells are involved.

Interventricular Septum Formation

• Two parts: Muscular and membranous.
• Muscular: Outgrowth of ventricular floor; cells from bulbus cordis and primitive ventricle proliferate.
• Membranous: Right and left bulbar/truncal ridges and endocardial cushions fuse with muscular septum, creating separate blood flows.

Primitive Heart Tube Blood Flow and Changes

• Open communication between ventricles until week 7 (interventricular foramen).
• Changes occur when heart septa form.

Atrial and Ventricular Septal Defects (Etiology)

• Ventricular Septal Defects (VSD):
  • Membranous (common) or muscular (rare).
  • Normally, blood flows left-to-right with normal blood pressure.
  • However, increased oxygenated blood can cause issues elsewhere, the heart works harder, leading to hypertension.
  • Uncorrected defects lead to heart failure; small defects may self-correct.
  • VSDs often result from aorticopulmonary septum issues.

Great Vessel Anomalies

• Persistent Truncus Arteriosus: Truncus arteriosus fails to divide.
• Transposition of Great Vessels: Aorticopulmonary septum grows straight instead of spiraling.
• Overriding Aorta/Pulmonary Trunk: Unequal truncus division; one artery is larger; misplaced aorticopulmonary septum.
• Tetralogy of Fallot: Four defects occurring together: overriding aorta, VSD, pulmonary stenosis, RV hypertrophy.

Embryonic Vessel Formation

• Extraembryonic vessels: From yolk sac extraembryonic mesoderm, forming a network of angioblastic cells/blood islands. Blood islands become angioblastic cords, then tubes.
• Embryonic vessels: From splanchnic mesoderm, forming blood islands that infiltrate all layers. Blood islands become cords, then vessels.

Blood Cell Sources

• Yolk sac initially supplies blood cells.
• Later, liver, spleen, thymus, and eventually bone marrow are sources.

Embryonic Aortic Arch System Changes

• Arches 1 and 2 largely regress; arch 5 disappears.
• Arch 3 supplies head; arches 4 and 6 supply trunk.
• Arch 3: forms internal and common carotids.
• Arch 4: forms parts of aortic arch and subclavian artery.
• Arch 6: forms parts of pulmonary arteries and ductus arteriosus.

Ductus Arteriosus Origin and Role

• From the distal portion of the 6th arch.
• Important for fetal arterial compensation.

Major Arterial Derivatives of Dorsal Aorta

• Intersegmental arteries (vertebral, intercostal, etc.).
• Ventral branches (vitelline and umbilical arteries).
• Lateral branches (renal, suprarenal, gonadal).

Arterial Anomalies (PDA, Double Aortic Arch, Aortic Coarctation)

• Patent Ductus Arteriosus (PDA): Failure of DA to close; higher left pressure shunts blood to pulmonary arteries.
• Coarctation of the aorta: Localized narrowing of the aorta near the DA.
• Double Aortic Arch: Abnormal persistence of right dorsal aorta.

Embryonic Venous Systems

• Umbilical: Oxygenated blood from placenta.
• Vitelline: Drains yolk sac, then GI tract.
• Cardinal: Three pairs carrying blood from the embryo; anterior and posterior cardinals drain into common cardinals.

Vitelline and Umbilical Vein Transformations

• Vitelline veins: Form a plexus around the gut, contributes to portal vein system.
• Umbilical veins: Bypass liver and connect with vitelline veins. Right umbilical vein degenerates. Left umbilical vein forms anastomosis with ductus venosus.

Ductus Venosus Development and Fetal Role

• Forms anastomosis with left umbilical vein.
• Shunts half the oxygenated blood from umbilical vein to right atrium via inferior vena cava.

Embryonic Veins Contributing to Inferior Vena Cava

• Superior right vitelline vein forms part of the inferior vena cava.

Fetal Circulation (Umbilical Vein to...)

• Umbilical vein, ductus venosus, inferior vena cava, right atrium, foramen ovale, left atrium, left ventricle, aorta, systemic circulation, umbilical arteries.

Fetal vs. Postnatal Circulation Differences

• After birth, ductus arteriosus constricts; blood goes to the lungs.
• Foramen ovale closes (fossa ovalis); inferior vena cava carries deoxygenated blood.
• Ductus venosus degenerates (ligamentum venosum).