Category:Chronic Inflammation

Types

Granulomatous Inflammation

• Granulomatous inflammation is usually caused by organisms of low virulence but great persistence, or by implanted foreign bodies.
• Classically appears as a granuloma.
  • NOT a tumour, despite the suffix "-oma".
  • A circumscribed sphere of chronic inflammatory cells enveloped by poorly organised attempts at encapsulation by local connective tissue.
• The differences between a granuloma and an abscess must be appreciated.
  • The fibrous envelope is never as well developed as that of an abscess.
  • The inner contents are never as completely fluid as in an abscess.

Structure of a Granuloma

Central core

• The central core which contains the agent.
• The agent may be visible with H&E staining in section, e.g.

• • Actinobacillus lignieresii
    • The cause of "Wooden tongue" in cattle.
    • Appears as a granule, with a central core of the bacterial colony surrounded by radiating eosinophilic "clubs".
      • Clubs are considered to be formed from degenerating collagen and antigen-antibody complexes.
  • Actinomyces bovis
    • The cause of "Lumpy Jaw" in cattle.
    • Forms granules containing bacteria and "clubs".
  • Fungal hyphae
  • Parasitic larvae
  • Foreign bodies
• The agent might not be visible without being selectively stained.
  • E.g. Mycobacterium tuberculosis and Brucella abortus.
    • Stain using an acid-fast stain (Ziehl-Neelsen), or a modification.
    • These organisms are intracellular in the macrophages.

Chronic Inflammatory Cells

• Outside the core is a substantial number of chronic inflammatory cells.
  • Mainly macrophages.
    • Often appear as epithelioid cells.
  • Lymphocytes
  • Plasma cells.
• Neutrophils and necrotic remnants of cells can be quite prominent in the granulomas of Actinobacillus and Actinomyces species.
• Eosinophils are prominent in parasitic granulomas.
• A scattered and variable number of Giant cells are often seen, but not always in every granuloma.

Outer Envelope

• The final layer is an outer envelope of incomplete fibrous tissue.
• Giant cells can also be seen in this area.

Gross Appearance of Granulomas

• The cut surface of granulomas varies considerably;

• • Tuberculous granulomas tend to have solid whitish cores which are often calcified.
    • Grate on the knife when cut through.
  • Parasitic granulomas are often greenish in colour due to the substantial numbers of eosinophils.
    • Older ones are also often calcified.
  • Actinobacillus and Actinomyces species often have liquefied cores due to the necrosis and neutrophils.
    • I.e. they are purulent.
    • May discharge to the surface along sinus tracts.
    • The central core of bacteria and ‘clubs’ may appear as yellowish granules in this pus.
      • Often called "sulphur granules".

Granulation Tissue

Granulation tissue (Courtesty of BioMed Archive)

• Is completlely different to granulomatous inflammation, despite the similarity in name!
• Occurs on the surface of the skin where large areas of the epithelium have been lost.
• Makes up the lining of sinus tracts discharging from deeper lesions.
• Takes its name from the gross appearance of the small vessels which appear at the surface.
  • Look like red granules.
  • These vessels supply inflammatory cells, mainly neutrophils, to the infected surface.
• The most frequent example in domestic animals is the formation of excessive granulation tissue on the legs of horses with poorly healing wounds.
  • "Proud flesh"
• Ulcers and open wounds may heal by granulation.

Lymphocytic Inflammation

• Lymphocytic inflammation is a diffuse chronic ongoing inflammation.
• Seen in:
  1. Diseases of the central nervous system.
     • Lymphocytes appear microscopically as several layers of cells around blood vessels in the perivascular space.
     • They indicate that there is damage to the nervous tissue further in.
       • Should alert to the possibility of viral infection, which is a common cause of central nervous system disease.
         • E.g. louping ill.
  2. The gut.
     • An excessive number of lymphocytes diffusely infiltrating the lamina propria, often in conjunction with plasma cells, indicate an ongoing non-specific chronic enteritis.
  3. The respiratory tract.
     • Peribronchial and peribronchiolar cuffing may occur to the point of actual lymphoid follicle formation in these areas.
       • Follicles are sometimes large enough to cause partial occlusion of the airways.
     • A feature of some chronic lung diseases.
       • Ee.g. Mycoplasmosis in swine and calves.