Pathological calcification

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Pathological Calcification

Pathological calcification is the abnormal tissue deposition of calcium salts (principally calcium phosphate/hydroxyapatite, along with smaller amounts of iron, magnesium, and other mineral salts) in locations where they do not normally occur. There are two distinct forms based on mechanism and serum calcium status.

1. Dystrophic Calcification

Definition: Calcium deposition occurring in dead or dying tissue despite normal serum calcium levels and no systemic disturbance of calcium metabolism.

Sites

Areas of necrosis - coagulative, caseous, liquefactive, or enzymatic fat necrosis
Atheromatous plaques of advanced atherosclerosis (nearly always present)
Aging/damaged heart valves - causes calcific aortic stenosis and mitral annular calcification (Fig. 2.34 below)
Tuberculous lymph nodes - may be entirely converted to stone
Old infarcts, scarred tissue, thrombi

Dystrophic calcification of the aortic valve - irregular yellowish-white masses of calcium piled behind thickened, fibrotic semilunar cusps in calcific aortic stenosis

Dystrophic calcification of the aortic valve: irregular masses of calcium piled behind thickened, fibrotic cusps in calcific aortic stenosis. (Robbins, Cotran & Kumar)

Pathogenesis

Initiated by two processes:

Extracellular route: Membrane-bound vesicles (derived from injured/dying cells) concentrate calcium via affinity for membrane phospholipids. Phosphatases in the vesicle membrane generate phosphate ions, which combine with calcium to form crystals. These crystals are then propagated, accumulating larger deposits.
Intracellular route: Dying cells lose the ability to regulate cytosolic calcium; calcium accumulates in mitochondria of dying cells, forming initial mineral deposits.

Morphology (H&E)

Macroscopic: Fine white granules or clumps, gritty on sectioning
Microscopic: Basophilic, amorphous granular/clumped deposits - can be intracellular or extracellular
Over time, heterotopic bone may form at the site
Single necrotic cells can become seed crystals encrusted with mineral, forming lamellated concentric rings called psammoma bodies (resemble grains of sand)
- Psammoma bodies are characteristic of: papillary thyroid carcinoma, papillary serous carcinoma of ovary/endometrium, meningioma, mesothelioma
In asbestosis, calcium and iron salts gather on asbestos spicules forming asbestos bodies (beaded dumbbell shapes)

Serum calcium: Normal

Clinical significance

May be an incidental marker of previous cell injury (e.g., healed tuberculosis)
Can cause serious organ dysfunction:
- Calcific aortic stenosis - severely compromised valve motion
- Atherosclerosis - contributes to plaque rigidity and instability

2. Metastatic Calcification

Definition: Calcium deposition in otherwise normal tissues, almost always secondary to hypercalcemia.

Causes of Hypercalcemia (4 principal causes)

Cause	Examples
1. Excess PTH secretion	Primary hyperparathyroidism (parathyroid adenoma/hyperplasia); ectopic PTH-related protein (PTHrP) from malignant tumors
2. Bone resorption/destruction	Multiple myeloma, leukemia, diffuse skeletal metastases; Paget disease; immobilization
3. Vitamin D-related disorders	Hypervitaminosis D; sarcoidosis (macrophages convert vitamin D precursor to active form via 1-alpha hydroxylase)
4. Renal failure	Phosphate retention → secondary hyperparathyroidism

Additional causes: milk-alkali syndrome, thiazide diuretics, adrenal insufficiency.

Preferred Sites

Metastatic calcification can occur in any normal tissue, but preferentially affects tissues with an alkaline intracellular/interstitial pH (because alkalinity favors calcium phosphate precipitation):

Gastric mucosa (secretes acid - interstitium is alkaline)
Kidneys (renal tubules excrete acid - interstitium alkaline; leads to nephrocalcinosis)
Lungs (CO2 excretion creates alkaline alveolar septa) - particularly alveolar septa, bronchiole walls, and pulmonary vessels
Systemic arteries and pulmonary veins

Morphology

Morphologically identical to dystrophic calcification: basophilic granular/amorphous or hydroxyapatite crystalline deposits
Kidneys: nephrocalcinosis - visible on imaging, can lead to renal damage
Lungs: patchy opacities on chest X-ray; HRCT is more sensitive; 99mTc-MDP bone scan shows increased uptake

Clinical Features

Usually causes no clinical dysfunction
Massive pulmonary involvement → respiratory compromise, restrictive deficits
Massive renal involvement (nephrocalcinosis) → renal damage
Clinical manifestations are most common in chronic renal failure patients on hemodialysis

Serum calcium: Elevated (hypercalcemia)

Comparison Table

Feature	Dystrophic	Metastatic
Tissue	Dead/dying/abnormal	Normal
Serum calcium	Normal	Elevated (hypercalcemia)
Mechanism	Local tissue damage + vesicle nucleation	Systemic calcium dysregulation
Sites	Atheromas, valves, necrosis foci	Kidneys, lungs, gastric mucosa, vessels
Psammoma bodies	Yes (in necrotic foci)	No
Clinical significance	Valve disease, atherosclerosis	Nephrocalcinosis, pulmonary compromise

Special Histological Structures

Psammoma bodies: Concentrically lamellated calcified deposits. Occur in: papillary thyroid carcinoma, serous ovarian/endometrial carcinoma, meningioma, mesothelioma. Each layer represents a calcified necrotic cell acting as a nidus.
Asbestos bodies (ferruginous bodies): Calcium + iron deposited on asbestos fibers → exotic beaded dumbbell shapes. Seen in asbestosis.

Sources: Robbins, Cotran & Kumar Pathologic Basis of Disease, p. 59-60, 75-76; Robbins & Kumar Basic Pathology, p. 37-38; Fishman's Pulmonary Diseases and Disorders, p. 1100

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