Bone metaplasia of urothelial mucosa: an unusual biological phenomenon causing kidney stones.

A kidney stone was analyzed by infrared spectrometry, by petrography using ground slices, and by undecalcified histology. Infrared spectrometry showed that the stone was composed of calcium oxalate monohydrate (whewellite), calcium oxalate dihydrate (weddellite), and calcium phosphocarbonate (apatite). Optical microscopy of the ground sections revealed tissue fragments within the stone containing small spaces resembling empty osteocyte lacunae. Histological analysis of methacrylate-embedded sections confirmed the presence of an area of heterotopic ossification. Urothelial mucosa is potentially capable of inducing bone formation and in this case the bony tissue had apparently functioned as a nucleation site for the calculus.