Microscopic anatomy

Red pulp

Red pulp is composed of splenic cords and sinuses (sinusoids).

• Splenic cords: a reticular meshwork filled with blood (open circulation system) that filters the blood from damaged erythrocytes
• Spleen sinusoids: long vessels with a fenestrated ring-like (“barrel hoop”) basement membrane that prevent old or malformed RBCs/platelets from reentering venous circulation
  • Open circulation: Blood empties from sheathed capillaries into the splenic cords and then enters the sinusoids through slits in the vessel wall.
  • Closed circulation: Blood empties from sheathed capillaries of the red pulp directly into the sinusoids.
• Macrophages: found in the cords and around the sinusoids
  • Phagocytosis of damaged RBCs/platelets that do not reenter circulation
  • Capture of viruses and opsonized pathogens that enter the red pulp
• Blood flow: splenic artery → arterioles → red pulp (cords → sinusoids) → venules → splenic vein → portal circulation

White pulp

• Periarteriolar lymphatic sheath
  • Surrounds the arterioles
  • Dense lymphoid tissue containing T lymphocytes
• Splenic follicles
  • Main component of white pulp
  • Close to periarteriolar lymphatic sheath
  • Contain B lymphocytes
• Splenic marginal zone
  • Located between the red pulp and white pulp
  • Contains antigen-presenting cells (APCs): macrophages and specialized B cells (marginal zone B cells)