Crystallization of Hierarchical Ammonium Urate: Insight into the Formation of Cetacean Renal Stones

Among a wide variety of pathological crystalline materials in nephrolithiasis, uric acid and its salt form are the most abundant organic constituents. Even though rarely found in humans, ammonium urate (NH₄HU) is the most prevalent kidney stone reported in managed bottlenose dolphins. In this study, we investigate the physicochemical properties of NH₄HU crystals associated with dolphin kidney stones, which exhibit distinct hierarchical structures. We present a method of crystallizing NH₄HU without common impurities, which allowed for the first measurement of NH₄HU crystal solubility over a broad range of temperatures. Parametric evaluations of NH₄HU crystallization are reported, wherein we assess the effects of supersaturation and solution alkalinity on crystal phase behavior, the kinetics of urate crystallization, and the size and morphology of synthetic NH₄HU crystals. In vitro bulk crystallization assays from aqueous growth solutions containing supersaturated ionic species (NH⁴⁺ and urate^1-) were performed to prepare NH₄HU crystals with characteristic spheroidal superstructures. Notably, synthesized samples adopt a morphology consisting of spicule-like NH₄HU crystals protruding from the exterior surface of hierarchical structures, analogous to images of actual dolphin kidney stones. Moreover, the characterization of extracted tissues from dolphin kidneys reveals crystallites embedded within the membrane, which suggests the spicule-like morphology incurs an undesirable retention of stones. Collectively, these findings of ammonium urate crystallization may shed additional light onto the factors governing the pathological formation of cetacean renal stones.