Primary (central) lymphoid organs

Bone marrow

• made up of: myeloid tissue

Types:

• red bone marrow: hemopoietic

• yellow bone marrow: fatty tissue

Microscopically consists of:

• Stroma: loose reticular CT

  • fat cells, histocytes, fibroblasts, blood vessels, intracellular matrix

• Parenchyma: hemopoietic tissue

  • precursors of all formed elements of blood

Thymus

• made up of: epithelial tissue (endoderm)

• located: in the thoracic cavity (superior and anterior mediastinum)

Microscopically consists of:

• Stroma:

  • capsule

  • septa (incomplete): incomplete thymic lobules

  • absent reticular network

• Parenchyma:

  • Cortex: dark and peripheral

    • many immature T-lymphocytes

    • epithelial (nurse) cells

    • few finely-branched epithelial reticular cells

    • macrophages

  • Medulla: pale and central

    • few mature T-lymphocytes

    • many finely-branched epithelial reticular cells

    • macrophages and dendritic cells

    • Hassall’s corpuscles

Secondary (peripheral) lymphoid organs

Lymph nodes

Microscopically consists of:

• Stroma:

  • capsule

  • septa: extend into the substance of node

  • reticular network (reticular fibers & reticular cells)

• Parenchyma:

  • Cortex (regular compartments): superficial layer deep to capsule

    • Lymph nodules/follicles: mainly inactivated naive B cells

      • primary: without germinal center

      • secondary: with germinal center

    • Cortical lymph sinuses: vessel-like spaces containing lymph & macrophages

  • Deep cortex or Paracortical zone: ill-defined middle region

    • T-lymphocytes that do not form nodules

  • Inner medulla (irregular compartments): inner most region

    • medullary cords:

      • lymphocytes

      • macrophages

      • plasma cells

    • medullary sinuses: vessel-like spaces containing lymph & macrophages

Spleen

• Stroma:

  • capsule: surrounds the organ; thick & fibromuscular

  • septa: incomplete & extend into the substance of organ

  • reticular network (reticular fibers & reticular cells)

• Parenchyma:

  • red pulp:

  • white pulp

MALT