Intracellular Accumulations & Pathologic Calcification

How and why substances accumulate

Four routes. Cells accumulate material in four ways, and each route has a prototype:

• Overproduction or underexport of a normal substance: triglyceride in hepatocytes (steatosis).
• Misfolded or poorly transported protein: mutant α1-antitrypsin trapped in the hepatocyte ER, where it forms PAS-positive, diastase-resistant globules.
• Missing enzyme: undegraded substrates filling lysosomes in lysosomal storage diseases.
• Indigestible exogenous material: inhaled carbon or tattoo ink held in macrophages.

Some deposits are harmless markers of past events (lipofuscin, anthracotic pigment); others injure the cell directly (iron in hemochromatosis, copper in Wilson disease).

Pigments

Lipofuscin. A yellow-brown, finely granular pigment that sits around the nucleus. It is built from lipid-peroxidation debris (oxidized membrane lipids complexed with protein) that lysosomes could not digest, left behind in residual bodies. It accumulates with age and atrophy, especially in heart, liver, and neurons; a small, lipofuscin-rich organ is described as brown atrophy. Lipofuscin is a marker of accumulated free-radical injury, not a cause of dysfunction, and it does not stain with Prussian blue.

Hemosiderin. Stored iron in the form of coarse, golden-brown granules of aggregated, partly degraded ferritin. The Prussian blue (Perls) reaction turns it blue. Locally it marks any old hemorrhage. In chronic left heart failure or mitral stenosis, congested alveolar capillaries leak red cells, and alveolar macrophages that digest them fill with hemosiderin (heart-failure cells).

Systemic iron overload. In hemosiderosis from repeated transfusions or chronic hemolysis, iron first fills macrophages of the liver, spleen, marrow, and lymph nodes, usually without organ injury, although massive loads eventually reach parenchymal cells. Hereditary hemochromatosis (HFE C282Y) increases intestinal absorption and loads parenchymal cells directly: hepatocytes (cirrhosis), pancreatic islets (diabetes), myocardium (cardiomyopathy), and skin (bronze color).

Bilirubin. The iron-free product of heme catabolism. It stains tissues yellow in jaundice, forms green-brown bile plugs in canaliculi in cholestasis, and, when unconjugated and very high in a newborn, deposits in the basal ganglia as kernicterus.

Melanin. A brown-black pigment made in melanocytes when tyrosinase oxidizes tyrosine; the Fontana-Masson silver stain confirms it. Exogenous pigments differ: inhaled carbon (anthracosis) blackens the lungs and hilar lymph nodes, and tattoo ink stays in dermal macrophages for life.

Lipid and protein accumulations

Steatosis. Hepatocytes store triglyceride whenever fatty acid delivery or synthesis outpaces export as VLDL. Alcohol raises the NADH/NAD⁺ ratio, which favors triglyceride synthesis (more glycerol-3-phosphate and fatty acid synthesis) and slows β-oxidation. Obesity, insulin resistance, and starvation flood the liver with fatty acids from adipose tissue. In protein deficiency (kwashiorkor), too little apoprotein (apoB-100) is made to package VLDL, so fat is trapped. Microscopically, macrovesicular fat forms clear vacuoles that push the nucleus to the edge; microvesicular fat (Reye syndrome, acute fatty liver of pregnancy, valproate) leaves the nucleus central.

Foam cells and xanthomas. Macrophages and intimal smooth muscle cells that ingest oxidized LDL through scavenger receptors become foam cells, the core of fatty streaks and atheromas. Clusters of foamy macrophages in the skin or tendons form xanthomas; Achilles tendon xanthomas in a young adult suggest familial hypercholesterolemia (LDL-receptor defect). Cholesterol-laden macrophages in gallbladder mucosa produce cholesterolosis (strawberry gallbladder).

Russell bodies. Round, homogeneous eosinophilic globules of immunoglobulin that distend the RER of plasma cells in chronic inflammation and plasma cell neoplasms such as myeloma; a plasma cell packed with them is a Mott cell.

Mallory-Denk bodies. Ropy, eosinophilic cytoplasmic tangles of ubiquitinated keratin 8/18 intermediate filaments in ballooned hepatocytes. They are typical of alcoholic hepatitis but also appear in MASH, Wilson disease, and chronic cholestatic disease.

Pathologic calcification

Dystrophic calcification. Calcium phosphate (hydroxyapatite) deposits in dead or damaged tissue while serum calcium and phosphate remain normal. Crystals nucleate on membrane phospholipids of dying cells and their vesicles, where phosphatases generate phosphate, and then propagate. Examples: atheromas; aging or bicuspid aortic valves (calcific aortic stenosis); caseous tuberculous granulomas and lymph nodes; fat necrosis in pancreatitis or breast trauma; old infarcts and thrombi; and a retained dead fetus.

Psammoma bodies. Concentric, laminated calcified spherules (psammos is Greek for sand) that form as single tumor cells, often at papilla tips, die and calcify layer by layer, a dystrophic process. They are classic in papillary thyroid carcinoma, serous papillary carcinoma of the ovary and endometrium, meningioma, and malignant mesothelioma. Medullary thyroid carcinoma, in contrast, contains calcitonin-derived amyloid.

Metastatic calcification. Calcium deposits in normal tissue because serum calcium or phosphate is high. Causes: primary hyperparathyroidism; CKD with phosphate retention and secondary hyperparathyroidism; vitamin D toxicity; milk-alkali syndrome; sarcoidosis, where granuloma macrophages express 1α-hydroxylase and make calcitriol; and extensive bone destruction (myeloma, skeletal metastases). Deposits favor the interstitium of tissues that secrete acid and therefore have an alkaline internal compartment: kidney (nephrocalcinosis, beginning in tubular basement membranes), lung (alveolar walls), gastric mucosa, and systemic arteries and pulmonary veins.

Most metastatic deposits are silent, but massive nephrocalcinosis impairs renal function and extensive lung calcification can compromise breathing. Dystrophic calcification never causes hypercalcemia.

Special stains

Recognizing calcium. On H&E, calcium salts are basophilic, granular to clumped, deep blue-purple deposits. The von Kossa silver method turns calcium phosphate black, and Alizarin red S stains calcium red-orange.