Extracellular accumulations of calcium and uric acid salts
Deposition of calcium in ECM
The deposition of calcium in the ECM can be a product of local causes, such as necrosis, as is the case in atherosclerotic plaques and damaged heart valves, or systemic causes, such as in hypercalcemia, caused by problems such as renal failure, destruction of bone and hyperparathyroidism.
Remember: dystrophic calcification occurs in dead or damaged tissue with normal serum calcium (caseous tuberculosis, atheromas, damaged heart valves, fat necrosis, psammoma bodies). Metastatic calcification occurs in normal tissues because of hypercalcaemia (hyperparathyroidism, bone-destroying tumours, vitamin D excess, renal failure) – mainly in the lungs, kidneys, gastric mucosa and blood vessels.
Macroscopically calcium deposits are visible as fine white granules while microscopically we see extracellular basophilic deposits.
Deposition of urates in ECM
Deposition of urates in the ECM is commonly associated with conditions such as Gout and Lesch-Nyhan syndrome.
Excess uric acid builds up and is deposited in the joints, the build up can be a product of an enzymopathy, excess consumption of animal meat, or super-activity of the enzymes responsible for purine production (phosphoribosyl pyrophosphate synthetase, PRPP), in effect, we either have too much uric acid, or are unable to remove it, so we dump it in the joints.
H&E, gouty tophus: pale, feathery urate deposits (the crystals dissolve during routine processing) ringed by macrophages and foreign-body giant cells.
Image: “Gouty tophus - intermed mag” by Nephron, CC BY-SA 3.0, via Wikimedia Commons.
Remember: gout crystals are needle-shaped monosodium urate, negatively birefringent under polarised light. A tophus is a mass of urate crystals surrounded by macrophages and foreign-body giant cells, typically in the joints, ear helix and kidneys.