Development of the Heart

Introduction

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Goals for the Lecture

• Main focus: Development of the heart.
• Will be covered in sections:
  1. Development of the heart tube.
  2. Development of the pericardial cavity.

Embryonic Orientation

• Superior view/top of the embryo.
  • Cranial aspect (head).
  • Caudal aspect (tail).
• Gastrulation process recap.
  • Epiblast cells moving through the primitive streak.
  • Conversion into three layers: ectoderm, mesoderm, endoderm.
• Mesoderm accumulation in the cranial aspect.
• Heart initially develops in the head, migrates to the thorax.

Heart Tube Development Views

• Cross-sectional view.
• Sagittal section view.
  • Sagittal view: Movement from head to thorax.

Cross-Sectional View

• Post-gastrulation layers:
  • Ectoderm (top).
  • Mesoderm (middle).
  • Endoderm (bottom).
• Mesoderm components:
  • Paraxial mesoderm.
  • Intermediate mesoderm.
  • Lateral plate mesoderm.
    • Somatic (outer) and splanchnic (inner) layers.
• Heart tube development triggered by VEGF (vascular endothelial growth factor) from the endoderm.
  • Differentiation into angioblasts (blood vessels and heart tube) and hemocytoblasts (blood cells).

Formation of Heart Tubes and Pericardial Cavities

• Lateral plate mesoderm stimulated by VEGF.
• Formation of heart tubes and pericardial cavities.
• Lateral folding fuses two heart tubes and pericardial cavities into one each.
• Dorsal mesocardium connects heart tube to pericardial cavity posterior wall.

Heart Tube Structure

• Inner layer: Endocardium (from endothelial cells/angioblasts).
• Outer layer: Myocardium (cardiac myocytes creating cardiac jelly).

Sagittal Section and Cranial-Caudal Folding

• Cardiogenic mesoderm area differentiates under VEGF influence.
• Cranial-caudal folding moves heart tube from head to neck to thorax.

Heart Tube and Pericardial Cavity Connection

• Heart tube and pericardial cavity formation and fusion.
• Migration and cranial-caudal folding embed heart tube into pericardial cavity.

Denoting Heart Tube Sections

• Top to bottom sections of the heart tube:
  • Truncus arteriosus (top).
  • Bulbus cordis.
  • Primitive ventricle.
  • Primitive atrium.
  • Sinus venosus (bottom).

Fate of Heart Tube Sections

• Truncus arteriosus → Pulmonary artery and aorta.
• Bulbus cordis → Right ventricle and right/left outflow tracts.
• Primitive ventricle → Left ventricle.
• Primitive atrium → Left and right atria.
• Sinus venosus → Coronary sinus, superior vena cava, inferior vena cava.

Cardiac Looping

• Importance of cardiac looping and dyneins.
  • Proper looping essential for heart formation.
  • Issues with dyneins → Dextrocardia or situs inversus.
• Cardiac looping steps:
  1. Truncus arteriosus and bulbus cordis move down and to the right.
  2. Primitive ventricle moves left of midline.
  3. Primitive atrium moves backward and upward over bulbus cordis and primitive ventricle.

Formation of Septations and Valves

• Forming AV canals and separating atria and ventricles.
  • Development of septum primum, ostium primum, septum secundum, and foramen ovale.
  • Formation of muscular and membranous portions of the interventricular septum.

Inflow Tracts of Right Atrium

• Breakdown and specialization of veins feeding into sinus venosus.
  • Common cardinal veins, umbilical veins, vitelline veins.
  • Formation of coronary sinus, superior vena cava, inferior vena cava.

Forming Outflow Tracts and Semilunar Valves

• Formation of aortic and pulmonary trunks from truncus arteriosus and bulbus cordis.
• Neural crest cells forming endocardial ridges.
  • Truncal and bulbar ridges formation and rotation.
• Aorticopulmonary septum formation.
  • Separation of aorta and pulmonary trunk.
• Formation of semilunar valves. -- Aortic semilunar valve (left ventricular outflow tract). --Pulmonary semilunar valve (right ventricular outflow tract).

Final Heart Structure

• Completed four-chambered heart.
• Specific valves: Tricuspid, mitral, aortic, and pulmonary semilunar valves.

Conclusion

• Summary of heart development stages.
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