Development of the Heart

Introduction

• Goal: Discuss development of the heart in sections.
• Steps: Development of heart tube, Formation of pericardial cavity.

Embryonic Orientation

• Cranial aspect: Head of the embryo.
• Caudal aspect: Tail of the embryo.
• Primitive streak and node: Epiblast cells move through converting hypoblast to endoderm.
• Mesoderm formation: Placed between ectoderm and endoderm.

Formation of Heart Tube

Three Germ Layers

• Ectoderm (outer layer)
• Mesoderm (middle layer)
• Endoderm (inner layer)
• Focus on mesoderm: Three components:
  • Paraxial mesoderm (adjacent to notochord)
  • Intermediate mesoderm
  • Lateral plate mesoderm: Has somatic and splanchnic layers.

Development Steps

1. Initiation by VEGF: Vascular Endothelial Growth Factor (VEGF) stimulates lateral plate mesoderm differentiation.
   • Causes formation of angioblasts (heart tube and blood vessels) and hemocytoblasts (blood cells).
2. Formation of Heart Tubes and Pericardial Cavities.
   • Heart tubes: Develop from the mesoderm.
   • Pericardial cavities: Surround heart tubes.
3. Lateral Folding: Heart tubes fuse into one heart tube.
4. Suspension by Dorsal Mesocardium: Connects heart tube to the pericardial cavity.

Heart Tube Layers

• Endocardium: Inner layer from angioblasts.
• Myocardium: Outer layer from cardiac myocytes; secretes cardiac jelly.

Sagittal Section and Heart Tube Formation

• Start: Heart develops in the head; moves down during folding.
• Cranial-caudal folding: Heart tube moves from head into thorax.
• Heart and Pericardial Cavity Formation: Same process as cross-sectional view.
• Proper Positioning: Heart placed in pericardial cavity during cranial-caudal folding.

Heart Tube Detailed Structure

• Top to Bottom Segments:
  • Aortic sac -> Truncus arteriosus -> Bulbus cordis -> Primitive ventricle -> Primitive atria -> Sinus venosus.
• Blood Flow Path: Blood enters sinus venosus, follows through successive segments, exits via dorsal aortic.

Cardiac Looping and Finalization

• Cardiac Looping: Truncus arteriosus and bulbus cordis move downward and right; primitive ventricle moves left, atria pulled back.
• Dextrocardia and Situs Inversus: If looping proteins (dyneins) malfunction, results in reversed organ positions.
• Formation of Valves (Tricuspid, Mitral): From endocardial cushions and septum intermedium.

Atrial and Ventricular Septation

Interatrial Septum Formation

• Septum primum and ostium primum: First septum descends.
• Ostium secundum: Secondary hole after ostium primum closes.
• Septum secundum and foramen ovale: Ensures fetal blood flow bypasses lungs.
• Patent Foramen Ovale: If it doesn’t close, risk of paradoxical embolus.

Ventricular Septum Formation

• Muscular Part: Arises from apex.
• Membranous part: Descends to close gap; defects lead to VSD (Ventricular Septal Defect).

Inflow Tracts to Right Atrium

Vein Degeneration and Shifting

• Left horn gives way to coronary sinus.
• Right horn shifts, contributing structures:
  • Superior vena cava (right common cardinal vein).
  • Inferior vena cava (right vitelline vein).

Outflow Tracts: Aortic and Pulmonary Arteries

Formation of Aorticopulmonary Septum

• Neural crest cells form ridges that fuse into septum.
• Corkscrew motion: Creates pulmonary trunk and aortic arch from blood flow channels.

Formation of Semilunar Valves

• Formation of Endocardial Cushions: Anterior, posterior, right, and left.
• Rotation Process: Facilitates separation and proper valvular formations.
• Positioning: After rotation, aorta (posterior and right) and pulmonary trunk (anterior and left).

Summary

• Formed right & left atria, right & left ventricles, AV canals, inflow and outflow tracts.
• Cardiac Looping and Proper Valving Ensured Heart Functionality.