Lecture Notes: Development of the Heart

Introduction

• Discussed the development of the heart in stages.
• Mentioned the importance of gastrulation in setting the stage for heart development.
• Heart development starts in the cranial aspect of the embryo and then moves into the thorax.

Heart Tube Development

Formation

• Heart Tube: Begins with the formation of a singular heart tube.
• Pericardial Cavity: Develops around the heart tube.

Folding Processes

• Cranial and Caudal Folding: Facilitates the movement of the heart from the cranial region to the thoracic cavity.
• Lateral Folding: Causes the fusion of bilateral heart tubes into a singular heart tube.

Layers and Mesoderm

• Gastrulation: Produces three germ layers: ectoderm, mesoderm, and endoderm.
• Mesoderm Differentiation: Paraxial, intermediate, and lateral plate mesoderm.
  • Lateral Plate Mesoderm: Splits into somatic and splanchnic layers.
  • Splanchnic Layer: Forms the heart tube under the influence of VEGF (vascular endothelial growth factor).

Heart Tube and Pericardial Structure

• VEGF: Stimulates mesoderm differentiation into angioblasts (form blood vessels/heart tube) and hemocytoblasts (form blood cells).
• Heart tubes and pericardial cavities form from lateral plate mesoderm.
• Dorsal Mesocardium: Connects heart tube to the pericardial cavity.
• Heart Tube Layers:
  • Endocardium: Inner layer, formed from angioblasts.
  • Myocardium: Outer layer, formed from cardiac myocytes, secretes cardiac jelly between endocardium and myocardium.

Segmentation and Looping

Cardiac Looping

• Dyneins: Proteins crucial for cardiac looping; absence leads to conditions like dextrocardia or situs inversus.
• Segments of Heart Tube:
  • Truncus Arteriosus: Forms aorta and pulmonary artery.
  • Bulbus Cordis: Becomes the right ventricle and outflow tracts.
  • Primitive Ventricle: Forms the left ventricle.
  • Primitive Atria: Becomes left and right atrium.
  • Sinus Venosus: Forms inflow tracts.

Formation of Right and Left Atrioventricular Canals

• Endocardial Cushions: Form from neural crest cells and contribute to septum intermedium.
• AV Canals: Right and left AV canals form, eventually developing into the mitral and tricuspid valves.

Development of Atria and Ventricles

Separation of Atria

• Septum Primum: Grows towards septum intermedium, leaving ostium primum.
• Ostium Secundum: Formed after septum primum fuses with septum intermedium.
• Septum Secundum: Develops to partially cover ostium secundum, leaving foramen ovale.

Separation of Ventricles

• Muscular Interventricular Septum: Forms from the apex, moving upwards.
• Membranous Interventricular Septum: Completes the separation, formed from septum intermedium.

Inflow Tract Formation

• Sinus Venosus Absorption: Into the primitive atria.
• Left Horn: Forms the coronary sinus.
• Right Horn: Forms the superior and inferior vena cava.

Outflow Tract and Valves

Aorticopulmonary Septum

• Neural Crest Cells: Form ridges leading to aorticopulmonary septum.
• Truncal and Bulbar Ridges: Fuse to form the septum separating the aortic arch from the pulmonary trunk.
• Corkscrew Fashion: Septum forms in a spiral, aiding in proper outflow tract separation.

Semilunar Valves

• Formation: From endocardial cushions at the bulbous cordis and conus cordis junction.
• Aortic and Pulmonary Valves: Formed and positioned through rotation.

Conclusion

• Completion of heart development includes separation into chambers and formation of inflow and outflow tracts.
• Importance of the processes for preventing congenital heart defects.
• Encouragement to support the educational content.