Histology of the Cardiovascular System

Basic Structure of the Circulatory System

• The circulatory system consists of tube-like blood vessels, including the heart, which is a tube folded on itself during embryonic development.
• Layers of Blood Vessels (Tunics):
  1. Tunica Intima (Interna):
     • Inner layer made of a single layer of flattened epithelial cells known as endothelial cells.
     • Endothelial cells originate from the endoderm.
     • Lies on a basement membrane and sub-endothelial connective tissue.
  2. Tunica Media:
     • Intermediate layer predominantly made of smooth muscle fibers.
     • Some arteries have elastic fibers, especially large ones.
     • In the heart, this layer is the myocardium, made of cardiac muscle fibers.
  3. Tunica Adventitia (Externa):
     • Outer layer formed of connective tissue.
     • Contains vasa vasorum in large arteries to supply blood to the vessel walls.
     • In the heart, this is the coronary circulation.

Detailed Histology Slides

Muscular or Distributing Arteries

• Stained to show elastic fibers; composed of three distinct layers:
  • Tunica Intima: Simple squamous epithelium on sub-endothelial connective tissue.
  • Internal Elastic Lamina: Separates Tunica Intima from Media; made of elastic fibers.
  • Tunica Media: Mainly smooth muscle fibers in muscular arteries; controls artery caliber to distribute blood.
  • External Elastic Lamina: Separates Tunica Media from Adventitia.

Elastic or Conducting Arteries

• Large arteries such as the aorta.
• Tunica Media: Composed of multiple elastic laminas, providing elasticity for continuous blood flow during diastole.

Arterioles

• Small arteries less than 0.5mm in diameter.
• Structure similar to larger arteries but with fewer muscle fiber layers (1-5 layers).
• May have internal elastic lamina, but no external elastic lamina.

Veins and Venules

• Veins have a thinner wall relative to lumen size compared to arteries.
• Tunica Media is thinner than Tunica Adventitia.
• Often contain valves to prevent backflow of blood, aiding venous return.

Functional Correlations

• Morphology and Function:
  • Elastic fibers in large arteries assist with the pulsatile flow of blood.
  • Veins' thin walls allow them to be compressed to push blood back toward the heart.

Cardiac Muscle Histology

• Cardiac Muscle Fibers: Features
  • Involuntary, striated, centrally located nucleus (sometimes two).
  • Branched fibers connected by intercalated discs.
  • Intercalated discs contain gap junctions and desmosomes.

Heart Wall Structure

• Endocardium:
  • Includes endothelial cells and sub-endothelial connective tissue.
  • Contains Purkinje fibers (conducting system of the heart) with less contractile elements.
• Myocardium:
  • Main muscular layer of the heart.
• Epicardium:
  • Outer layer similar to Tunica Adventitia.

Capillaries

• Composed only of the Tunica Intima (endothelial cells on a basement membrane).
• Facilitate exchange between blood and interstitial fluid.

Note: Morphology of vessels is closely related to their function, adapting to the pressures they face and their roles in the circulatory system.