MH-084 MHS-212 MHS-293 Spleen

Spleen
(H&E)

The spleen is an encapsulated organ that filters blood and immunologically monitors blood.

• Capsule - dense connective tissue enclosing the organ.
  • Trabeculae - connective tissue that extends inward from the capsule through which blood vessels enter the pulp.
• White Pulp - composed of lymphatic tissue. It appears basophilic due to the large number of nuclei.
  • Splenic Nodules (#1, #2 and #3) - clusters of B lymphocytes located on central arterioles. They usually contain a germinal center of activated B lymphocytes.
  • Central Arterioles (#1 and #2) - branches of trabecular arteries coated by PALS and adjacent to nodules.
  • PALS (Periarteriolar Lymphatic Sheath; #1 and #2) - cylindrical mass of mature T lymphocytes that surrounds central arterioles.
• Marginal Zone - region between white and red pulp where macrophages, dendritic cells, and lymphocytes interact.
• Red Pulp - filters and degrades red blood cells (RBCs). It appears eosinophilic due to the large number of RBCs.
  • Splenic Sinusoids - vascular spaces lined by specialized endothelial cells that filter RBCs.
  • Splenic Cords (of Billroth) - loose connective tissue that contains macrophages, plasma cells, and lymphocytes.
  • Pulp Arterioles (#1 and #2) - they are not surrounded by lymphocytes like central arterioles in white pulp.
  • Sheathed Arterioles (#1 and #2) - regions of pulp arterioles surrounded by macrophages.

Spleen
(Azan)

Spleen stained with azan to distinguish the stroma (or connective tissue components) from the parenchyma. Collagen fibers (blue) are restricted to the capsule, trabeculae and blood vessels.

• Capsule - dense connective tissue enclosing the organ
  • Trabeculae - connective tissue that extends inward from the capsule through which blood vessels enter the pulp
• White Pulp - composed of lymphatic tissue
  • Splenic Nodules (#1 and #2) - clusters of B lymphocytes located on central arterioles
  • Central Arterioles (#1 and #2) - branches of trabecular arteries coated by PALS and adjacent to nodules
• Red Pulp - filters and degrades red blood cells (RBCs)
  • Splenic Sinusoids - vascular spaces lined by specialized endothelial cells that filter RBCs
  • Splenic Cords (of Billroth) - loose connective tissue that contains macrophages, plasma cells, and lymphocytes
  • Pulp Arterioles (#1 and #2) - they are not surrounded by lymphocytes like central arterioles in white pulp

Spleen
(silver)

The spleen is one of the organs that has a framework (or stroma) of reticular fibers.

Reticular fibers are composed of thin and delicately woven strands of type III collagen. They do not stain with hematoxylin & eosin (H&E), but are specifically stained by silver. This renders them black and makes them easily distinguishable from type I collagen fibers that are stained red/brown.

• White Pulp - composed of lymphatic tissue
  • Splenic Nodules - loosely surround by reticular fibers
  • Central Arterioles - adjacent to nodules and surrounded by a layer of reticular fibers
• Red Pulp - filters and degrades red blood cells (RBCs)
  • Splenic Cords (of Billroth) - loose connective tissue supported by a meshwork of reticular fibers
  • Splenic Sinusoids - vascular spaces between splenic cords are not surrounded by a layer of reticular fibers (unlike blood vessels)
  • Pulp Arterioles (#1 and #2) - surrounded by a layer of reticular fibers

The reticular fibers form attachment sites for lymphocytes and other immune cells.