Mechanism of calcium oxalate monohydrate kidney stones formation: Layered spherulitic growth

The morphology of calcium oxalate monohydrate (COM) kidney stones is studied using polarized light microscopy and X-ray diffraction. We show that polycrystalline structure of COM stones exhibits spherulitic texture where the arrangement of crystallites indicates that their fast growth direction is perpendicular to the corresponding radius of spherulite, resulting in the layered morphology. This is in contrast to "normal" spherulites, where the crystal growth process leads to the formation of a radiating array of fiber crystallites. We demonstrate that COM stones consist of spherulitic domains. The domains have the shape of comparatively narrow randomly distorted cones in which the crystallites form strong texture, so that their crystallographic axes have almost the same directions and the [100] crystallographic planes are nearly perpendicular to the radial direction of the domain. However, the order among the domains does not exist. Deviations of their radial directions from the corresponding radial directions of the whole stone are not large as a rule, while the other crystallographic directions of the domains are randomly distributed. A model of layered spherulitic growth explaining the observed morphology is proposed. The model suggests that every domain is formed by means of a continuous crystallization process periodically inhibited by precipitation of organic material so that alternating organic and polycrystalline layers appear. Fine crystalline channels remaining in organic layers connect neighboring crystalline layers and maintain, thereby, the coherence of crystal structure all over the domain. Preformed COM microcrystals occasionally adsorbed from urine on the surface of the growing organic layer serve as seeds for new spherulitic domains. The results are important for understanding the general principles of biomineralization, and spherulitic crystallization and could lead to the development of new strategies for preventing kidney stone formation.