Hydroxyapatite induction and secondary aggregation of calcium oxalate, two important processes in calcium stone formation.

Stone formation has often been ascribed to crystal aggregation and fixed particle growth on kidney calcifications. In this paper, the influence of hydroxyapatite (HAP) and of preformed calcium oxalate (CaOx) aggregates on CaOx crystallization was studied in freshly voided urine. Crystallization was induced by different oxalate loads and precipitates were analyzed by the spectrophotometric measurement of sedimentation time (ST), which decreases with increasing particle size. The fact that the ST of aggregates (STA) is significantly lower than the ST of other particles demonstrates that STA is a useful indicator for aggregation. At relatively low oxalate loads the addition of HAP to urine increased STA by a factor of 4.3 (P < 0.001). After a second oxalate load, STA decreased by 56% (P < 0.001), indicating secondary growth of the preexisting aggregates. HAP induced and primary CaOx aggregation occurred at low pH at which a high ionic calcium concentration (Ca2+) was measured. In urine, crystals are coated by macromolecules creating a negative surface potential with a consecutive accumulation of cations such as Ca2+. This Ca2+ accumulation could be responsible for the enhancement of aggregation by preexisting particles, which seems to be important for stone formation and which can otherwise hardly be explained in the presence of coated crystals.