Stone formation in patients with idiopathic hypercalciuria. Initial calcium phosphate (apatite) crystallization was found in the basement membrane of the thin limbs of the loop of Henle (A) with subsequent extension to the vasa recta (B), then to the interstitial tissue surrounding the terminal (inner medullary) collecting ducts (C), and finally, in the most severe cases, extension to the papillae (D). Erosion of this solid phase into the urinary space (E), which is supersaturated with respect to calcium oxalate (CaOx), may have promoted heterogeneous nucleation and formation of calcium oxalate kidney stones (F).