Dystrophic vs. Metastatic Calcification

Quick Overview of the Table

Pathogenesis - Detailed Explanation

1. Dystrophic Calcification - Pathogenesis

• Normally, living cells actively pump calcium OUT of the cytoplasm (intracellular Ca²+ is kept very low ~0.1 µmol).
• When a cell is injured or dies (necrosis - coagulative, caseous, or liquefactive), the cell membrane loses integrity and the calcium pump fails.
• Calcium floods into the dying cell from the extracellular fluid (where Ca²+ concentration is ~1.3 mmol - over 10,000x higher).

• Necrotic and degenerative cells release phosphate-containing compounds from their damaged organelles (especially mitochondria and cell membranes containing phospholipids).
• These intracellular phosphates accumulate locally at the site of cell death.

• The locally released phosphates have an increased affinity/binding to the dead and necrotic tissue debris.
• Phosphate binds to calcium and forms calcium phosphate precipitates (primarily hydroxyapatite crystals, the same mineral in bone).
• Single necrotic cells can act as "seed crystals" that get encrusted with mineral, sometimes forming psammoma bodies (concentric lamellated calcifications).

• The initial micro-deposits act as a nidus, and more calcium-phosphate layers are deposited over time.
• The result is visible as white, gritty, chalky deposits at the necrotic site.

• Calcific aortic stenosis (damaged valve cusps develop chalk-white calcium deposits - as shown in the Robbins image above)
• Atherosclerotic plaques
• Caseous necrosis of tuberculosis (lymph nodes can turn literally "to stone")
• Areas of fat necrosis (e.g., post-pancreatitis)

Serum calcium is NORMAL - the problem is purely local at the site of tissue injury.

2. Metastatic Calcification - Pathogenesis

1. Hyperparathyroidism - excess PTH causes bone resorption -> calcium released into blood. Also seen with PTH-related protein secreted by malignant tumors.
2. Bone destruction - from primary bone marrow tumors (multiple myeloma), diffuse skeletal metastases (e.g., breast cancer), Paget disease, or immobilization.
3. Vitamin D disorders - vitamin D intoxication, sarcoidosis (macrophages activate vitamin D precursor), Williams syndrome.
4. Renal failure - phosphate retention leads to secondary hyperparathyroidism.

• When serum calcium rises, the calcium x phosphate solubility product is exceeded.
• Calcium phosphate precipitates out at tissues that have an alkaline (high pH) microenvironment - because calcium phosphate is LESS soluble in alkaline conditions.
• Tissues that excrete acid (like gastric mucosa, renal tubules) have an alkaline compartment on their interstitial/serosal side, predisposing them to calcium deposition.

• Lungs (alveolar septa, vessel walls) - very commonly affected; can cause respiratory compromise
• Stomach (gastric mucosa)
• Blood vessels (systemic arteries, pulmonary veins)
• Cornea (band keratopathy)
• Kidneys (nephrocalcinosis - can cause renal failure over time)

• Calcium salts appear identical to dystrophic calcification microscopically: basophilic, amorphous or crystalline (hydroxyapatite) deposits.
• The key difference is that the surrounding tissue is NORMAL.

Memory Hook