Calcium phosphate/calcium oxalate crystal association in urinary stones: implications for heterogeneous nucleation of calcium oxalate

PURPOSE: Most stones contain more than one type of crystals, and some combinations, such as calcium phosphate/calcium oxalate, are more common than others. Epitaxy between the crystals has been suggested to play a role in growth of such stones. The specific aim of this study is to investigate the involvement of calcium phosphate in crystallization of calcium oxalate. MATERIALS AND METHODS: Twenty calcium oxalate stones or stone fragments were examined using various microscopic techniques, including scanning, transmission and back-scattered electron microscopy. Similarly, calcium oxalate stones induced on a plastic foreign body implanted inside urinary bladders of laboratory rats were also investigated. Examination of the interface between calcium phosphate and calcium oxalate crystals was emphasized. RESULTS: Close association between crystals of calcium phosphate and calcium oxalate were found in both the human and rat stones. All crystals examined were associated with an organic matrix on the surface and contained copious amounts of organic material within the crystalline entities. Interface between the crystals also appeared to be occupied by organic matrix. CONCLUSIONS: Results of this and other studies from our laboratory indicate that epitaxy between various crystals, even though theoretically possible, appears unlikely in vivo. The appearance of specific crystalline combinations in stones is probably a result of the urinary environment being conducive for crystallization of those components. Heterogeneous nucleation of calcium oxalate is most probably induced by biological elements, including membranous cellular degradation products.