Development of the Vascular System

Introduction

• Overview of the development of the vascular system
• Lecture objectives:
  1. Discuss the development of blood vessels (arteries and veins) from the trilaminar disc in the embryo.
  2. Explain the growth of new blood vessels from existing ones (angiogenesis).

Key Concepts

Structure of the Trilaminar Disc

• Cross-section of the embryo around the third week:
  • Ectoderm (top layer)
  • Mesoderm (middle layer, focus of lecture)
  • Endoderm (bottom layer)
• Additional structures:
  • Neural tube (deep red colour)
  • Notochord (black structure)

Layers of the Mesoderm

1. Paraxial Mesoderm
   • Forms somites
2. Intermediate Mesoderm
   • Develops into gonads and the urinary system
3. Lateral Plate Mesoderm
   • Crucial for the development of the cardiovascular system
   • Divisions:
     • Somatic Layer (runs with ectoderm)
     • Splanchnic Layer (runs with endoderm)

Process of Vascular Development

Vasculogenesis and Angiogenesis

• Vasculogenesis: Formation of blood vessels from mesoderm
  • Involves proliferation and differentiation of mesodermal cells:
    • Angioblasts: Form endothelial cells of blood vessels
    • Hemocytoblasts: Develop into blood cells (red cells, white cells, platelets)
• Angiogenesis: Sprouting of new blood vessels from existing ones
  • Involves formation of connections between initial blood vessels

Formation of Blood Vessels

• Canalization from the formation of primordial tubes in mesoderm leads to the formation of blood vessels.
• Differentiation of the cardiac tube and dorsal aorta is essential for vascular development.

Development of the Arterial System

Formation of the Cardiac Tube

• The cardiac tube is formed by the fusion of two cardiac tubes happening during lateral folding.
• Formation of dorsal aorta occurs behind the cardiac tube.

Aortic Arches

• Aortic Sac: Connects to the dorsal aorta and forms a series of aortic arches
• Six aortic arches form during development with specific derivatives in adults:
  1. First Arch: Maxillary artery
  2. Second Arch: Hyoid artery (occasionally stapedial artery)
  3. Third Arch: Part of common carotids and internal carotids
  4. Fourth Arch:
     • Right: Right subclavian artery
     • Left: Aortic arch
  5. Sixth Arch:
     • Right: Right pulmonary artery
     • Left: Left pulmonary artery and ductus arteriosus

Development of the Descending Aorta

• The descending aorta branches into:
  • Dorsolateral branches (intercostal and lumbar arteries)
  • Lateral branches (adrenal, renal, gonadal arteries)
  • Vitelline arteries (supply the gut)

Development of the Venous System

Overview

• The venous system is more complex due to the dominance of right-sided structures.

Umbilical and Vitelline Veins

• Vitelline System:
  • Forms the hepatic portal vein and part of the inferior vena cava in the liver.
• Umbilical System:
  • Mainly contributes to the formation of the ductus venosus, which joins the inferior vena cava.

Cardinal Veins

• Anterior Cardinal Veins:
  • Right and left anterior cardinal veins form the superior vena cava and brachiocephalic veins.
• Posterior Cardinal Veins:
  • Break down and form the supracardinal, subcardinal, and sacrocardinal veins.
  • Supracardinal Veins: Form the azygos system.
  • Subcardinal Veins: Form renal veins and parts of the inferior vena cava.
  • Sacrocardinal Veins: Contribute to the lumbar part of the inferior vena cava.

Conclusion

• Overview of vascular system development completes an understanding of embryonic vascular structures and relationships.