Lymphoid Organs

Primary (central) lymphoid organs

{{Bone marrow{{

• made up of: myeloid tissue

Types:

• red bone marrow: hemopoietic
• yellow bone marrow: fatty tissue

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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)

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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

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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