MH 076-077-078 Lymph Node

Lymph Node
(H&E)

Lymph nodes are small organs interposed along lymphatic vessels that immunologically monitor lymph.

• Capsule - dense connective tissue enclosing the node.
  • Subcapsular Sinus - space underneath the capsule that receives lymph from afferent lymphatic vessels.
  • Trabeculae - connective tissue that extends inward from the capsule.
  • Trabecular Sinuses - spaces alongside trabeculae in which lymph flows from the subcapsular sinus into the cortex.
• Cortex - outer region of the node.
  • Outer Cortex - region adjacent to the capsule.
    • Nodules - spherical clusters of B lymphocytes. The lighter center is the germinal center where B lymphocytes differentiate into plasma cells.
  • Inner Cortex (paracortex) – region between the outer cortex and the medulla that is free of nodules.
    • Thymus-dependent Cortex - this region contains most of the T lymphocytes in a node.
    • High Endothelial Venules (HEV; longitudinal and cross-section) - venules with unusually tall endothelial cells through which lymphocytes in the blood enter the node.
• Medulla - inner part of the node.
  • Medullary Cords - contain antibody secreting plasma cells, macrophages, and B lymphocytes.
  • Medullary Sinuses - intervening spaces in which lymph flows before exiting the node via efferent lymphatic vessels.

Lymph Node
(Azan)

Lymph node 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 node.
  • Subcapsular Sinus - space underneath the capsule that receives lymph from afferent lymphatic vessels.
  • Trabeculae - connective tissue that extends inward from the capsule.
  • Trabecular Sinuses - spaces alongside trabeculae in which lymph flows from the subcapsular sinus into the cortex.
• Cortex - outer region of the node.
  • Lymphatic Nodule - spherical clusters of B lymphocytes in the outer cortex.
  • High Endothelial Venules (HEV; #1,#2) - venules with unusually tall endothelial cells through which lymphocytes in the blood enter the node. HEVs are found in the inner cortex.
• Medulla - inner part of the node. The cells are arranged in medullary cords surrounded by medullary sinuses.

Lymph Node
(Silver)

Lymph nodes is one of the tissues that has a framework (or stroma) of reticular fibers.

Reticular fibers are composed of thin and delicately woven strands of type III collagen. They are not visible with hematoxylin & eosin (H&E) or azan, 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.

• Capsule - contains a mixture of collagen fibers (red) and reticular fibers (black).
  • Trabeculae - contain a mixture of collagen fibers (red) and reticular fibers (black).
• Cortex - reticular fibers form an irregular, anastomosing network in the outer region of the node. Nodules are enclosed by reticular fibers.
• High Endothelial Venules (HEV; longitudinal and cross-section) - venules with unusually tall endothelial cells through which lymphocytes in the blood enter the node.
• Medulla - reticular fibers form networks to support the medullary cords throughout the inner part of node.