Histology

• the lymphatic system is a network of vessels that transports lymph
• the spleen filters blood to remove old red blood cells and pathogens
• the thymus gland produces T-cells which fight infection
• The cells of the lymphatic system are split up into two groups, migratory cells (lymphocytes) and fixed cells (reticular cells and epithelial reticular cells
• Lymphocytes are white blood cells that circulate through the body's tissues via the lymphatic system.
• Reticular cells form a meshwork throughout the lymphoid organs and support the development of lymphocytes.
• Epithelial reticular cells line the walls of lymphatic vessels and produce proteins involved in immune responses.
• The cells of the immune system are organized into diffuse lymphatic tissue and nodular lymphatic tissue.
• Reticular cells are of
• Mesenchyme origin
• Reticular cells have long cytoplasmic processes with numerous branching extensions
• Fibroblasts differentiate to become reticular cells
• Lymph nodes are oval or bean-shaped structures located along major lymphatic vessels.
• Epithelial reticular cells are of
• Endodermal origin
• epithelial reticular cells are stellate shaped cells
• They DO Not produce reticular fibers
• Reticular Fibers are produced by fibroblast/fibrocytes
• Lymphatic tissue form the basis of the immune tissue of the body and are organized into diffuse unorganized lymphocytic infiltration and lymphatic nodules.
• Diffuse unorganized lymphocytic infiltration: lymphocytes are not arranged in any particular pattern but they are scattered in a haphazard manner.

  
• Nodular lymphatic tissue: consists of

  • primary lymphatic nodules
  • secondary lymphatic nodules
• Secondary Lymphatic Nodes: located at sites where antigens may enter the body such as skin (popliteal node), mucous membranes (tonsils)
• Mesenchyme is composed of undifferentiated mesenchymal stem cells that can differentiate to various cell types depending on their location within the embryo.
• Primary Lymphatic Nodes: found throughout the body, especially near blood vessels that drain from internal organs
• Spleen is an organ of the lymphatic system which filters blood to remove old red cells and foreign particles
• Primary Lymphatic Nodes: found along the pathway of lymph vessels, e.g., axillary nodes, inguinal nodes, cervical nodes, etc.
• Connective Tissue Proper: contains extracellular matrix, which includes collagen fibers, elastic fibers, reticular fibers, ground substance, and blood vessels.
• aggregated lymphatic nodules: are not surrounded by a capsule and consist of many nodes
• the primary lymphatic organs include

  1. bone marrow
  2. thymus
  3. Bursa of Fabricius (in birds)
• secondary lymphatic organs:

  1. lymph node
  2. tonsils
  3. hemal node
  4. spleen
• The stroma is made up of:

  • capsule
  • septa/trabeculae
  • epithelial reticular cells
• The parenchyma is made up of t-lymphocytes (macrophages)

  t-lymphocytes are of mesenchymal origin and are only found in secondary lymphatic organs
• the cortex is the region where t-cell maturation occurs
• cortex:

  1. epithelial reticular cells
  2. population of T-cell precursors = thymocytes
  3. macrophages
• medullary cords contain B-cells, plasma cells, macrophages, and dendritic cells
• Medulla:

  1. epithelial reticular cells
  2. differentiate t-lymphocytes
  3. thymus corpuscles - Hassall's corpuscles
• Hassall corpuscle: an eosinophilic structure ( found only inside the medulla) on or several calcified/degenerated large central cells are surrounded by flat keratinized cells
• This barrier exists in the cortex only

  Blood-thymus barrier
• Epithelial reticular cells are euchromatic (very pale) rounded cells
• The blood thymus barrier consists of:

  1. thymic capillaries have a continuous epithelium
  2. tight junctions between endothelial cells
  3. very thick basal lamina
  4. a sheath of epithelial reticular cel processes
  5. macrophages - which may phagocytose molecules that escape from the capillary