Scanning electron microscopy images of sections of COM papillary calculi from patient 1. (a, b) A type I calculus, in which COM columnar crystals started to develop in the concave zone in close contact with papillary tissue. (c, d) A type II calculus with a hydroxyapatite core (arrow) in or near the concave zone. (e, f) A type III calculus that developed on the papilla tip, with its concave zone containing hydroxyapatite, calcified tissue and calcified tubules (arrow). (g, h) A type IV calculus in which the core, situated near the concave zone (arrow), was formed by the intergrowth of COM crystals and organic matter.