Calcification

Pathological calcification is abnormal deposition of calcium salts plus small quantities of other minerals. Macroscopically, calcification is white or pale grey, and it produces a hard or gritty feeling to palpation. Calcium is opaque to X-rays and can be seen radiologically. Histologically, calcium is basophilic with haematoxylin stain (Fig. 3.11.1); it also takes up other dyes. Heavily calcified tissues cannot be processed in the laboratory using routine paraffin wax methods so the tissue must be embedded in a hard medium such as acrylic or epoxy resin, or decalcified so it becomes soft. In long-standing calcification, osseous metaplasia can produce heterotopic bone within the calcified tissue.

Fig. 3.11.1 Mucin with dystrophic calcification. The calcium is the dark blue material in the upper part of the figure. The mucin was produced by a mucinous carcinoma of the appendix and is the pale blue material surrounding the calcification. H&E stain.

Pathological calcification can be described as dystrophic or metastatic. Dystrophic calcification occurs in abnormal tissues in the presence of normal serum calcium. Most commonly, it is encountered in necrotic tissue, for example caseous necrosis, the necrotic cores of atheromatous plaques, and areas of necrosis in cancers and old infarcts. Other places in which calcification may be found are old thrombi and abnormal heart valves. Metastatic calcification occurs in normal tissues as a result of hypercalcaemia. The calcium salts can be deposited in many different tissues, especially in the walls of arteries. If the kidneys are affected (nephrocalcinosis), renal failure can occur.

The chemical reactions causing the precipitation of calcium salts in the tissues are not well understood, but recent evidence suggests nanobacteria may have a role. Nanobacteria are extremely small Gram-negative bacteria that accumulate calcium phosphate on their surfaces. They have been associated with biomineralization in several different circumstances, including kidney stones and tumour calcification.