Mineralisation - Pathology

Calcification

• Calcification is the deposition of calcium in tissues other than bone and teeth.
• Where sufficient amounts are deposited to be seen grossly, it imparts a whitish colour to the area.
• There are two types of calcification:
  1. Dystrophic
  2. Metastatic (Hypercalcaemia)

Dystrophic

• Occurs in damaged tissue.
  • Dead tissue is alkaline - favours the deposition of calcium.
• Deposition is usually irreversible.
• NOT related to elevated levels of calcium in the blood.
• Commonly seen in:
  • Caseous necrosis in tuberculous granulomas.
  • Dead migrating parasites in various tissues
  • In part of Vit.E/selenium deficiency in livestock
    • Calcium is deposited in the necrotic tissue.

Gross Appearance

• Calcium salts have dull grey white chalky and granular appearance in tissue.
• Granules are hard.
  • Feel gritty if cut or palpated.

Histological Appearance

• Stains as irregular blue-purple clumps (sometimes shattered) with H&E.
  • Can be confused with bacteria.
• Von Kossa is a special stain that colours calcium black.

Metastatic (Hypercalcaemia)

• Metastatic calcification is calcification of normal tissue without previous damage.
• Is a result of hypercalcaemia.
• Calcium salts are laid down on basement membranes and elastic fibres, for example
  • Blood vessel walls
  • Pleura
  • Mucous membranes of the lungs, kidneys, and stomach.
  • Endocardium
  • Tendons and ligaments of joints.
• Causes of hypercalcaemia
  1. Primary Hyperparathyroidism
     • A rare condition
     • An adenoma of the parathyroid gland produces parathyroid hormone.
       • Stimulates osteoclasts to resorb calcium from bone, producing hypercalcaemia.
  2. Secondary Hyperparathyroidism
     • Renal hyperthyroidism
       • Occurs in chronic kidney failure.
       • Kidneys retain phosphate.
         • Disturbs the calcium:phosphate ratio.
           • Stimulates the parathyroid gland to resorb calcium from bone.
       • Calcium resorption from bone is compounded by lack of activated Vit. D.
         • This is due to the kidney damage.
     • Nutritional hyperthyroidism
       • The parathyroid stimulation here is due to excessive phosphates in the diet
         • E.g. a grain diet.
         • Upsets the calcium:phosphate ratio.
     • Hypervitaminosis D
       • Due to
         • Therapy
         • Ingestion of some plants which produce substances mimicking Vit. D .
       • Results in increased calcium absorption from the gut, and hence hypercalcaemia.
     • Tumours which produce hormone like activity
       • E. g.
         • Some lymphosarcomas
         • Anal gland apocrine malignant tumour
         • Some carcinomas

Other Conditions in Which Calcium is Deposited

Calcinosis circumscripta

• A condition of dogs
  • Particularly affects young German Shepherd Dogs.
• Nodules of a pasty calcium material are laid down in the dermis.
  • Especially laid down near bony prominences.
• Aetiology is unknown,
  • May be related to damage of sweat glands.
    • These are often in the vicinity of the deposits.

Calcinosis cutis

• In Cushing’s Disease (hyperadrenocortism).
• Calcium is diffusely deposited on the collagen and elastin of the dermis .

Ossification

• A metaplastic change in soft tissues.

Extracellular crystals

Ethylene Glycol Poisoning

• Accidental ethylene glycol (antifreeze) poisoning is not uncommon in small animals.
• Ethylene glycol is metabolised in the liver to oxalate.
  • Oxalate crystals precipitates in the renal tubules.
    • Can cause severe traumatic damage.

Gout

• Gout occurs with excessive retention of uric acid/urates and their deposition in tissue.
• Fairly common in man.
  • The crystals are deposited in joint tissue, particularly those of the toe.
    • Sometimes occurs in the tarsal joints of birds.
• In birds, other tissues are more commonly affected.
  • The serous membranes of the body cavities.
    • Look like they have been dipped in flour.
  • The substance of the kidney
    • The star-shaped crystals may block the tubular lumen.
      • Their sharp edges may also damage the tubular epithelium and underlying basement membrane.
        • Provokes a host cellular response.
• The exact pathogenesis in birds is unknown.
  • Birds excrete semisolid urates through the kidneys.
    • This may predispose them to blockages.
  • Other suggested predispositions include
    • Vitamin A deficiency
    • Prior kidney damage due to an inflammatory process
    • Abnormal protein metabolism
• Tortoises are also susceptible to gout.