Scanning electron microscopy and energy dispersive spectroscopy of Randall’s plaque stones: an unexpected finding of monosodium urate crystals

Abstract Randall’s plaques (RP) are located at the papillary tip, originating in the basement membranes of the thin loops of Henle, vasa recta and collecting ducts, and are associated with kidney stone retention. Disruption of the papillary epithelial layer exposes interstitial RP to calyceal urine, enabling calcium oxalate monohydrate (COM) overgrowth and papillary RP stone formation. This study aimed to analyze the surface and internal structures of RP stones using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Stones were collected from patients during percutaneous nephrolithotomy, ureteroscopy or both. Eighteen stones from nine patients were examined by stereoscopic microscopy, micro computed tomography (micro CT), SEM and EDS. Seven RP stones were sectioned for internal structure analysis. SEM revealed mineralized tubules potentially originating from thin loops, collecting ducts, ducts of Bellini, or vasa recta. These were frequently covered by collagen fibrils, and some were filled with dense or particulate mineral. Calcium phosphate (CaP) apatite was observed in various crystallized phases within RP regions. In three of the seven sectioned RP stones, monosodium urate monohydrate crystals were intercalated with RP, confirmed by EDS. Our multimodal imaging approach provides new insights into RP composition. This study suggests that sodium urate may precede RP formation in a subset of cases, potentially due to early, unexpected urinary pH shifts. Further studies are needed to validate this hypothesis and advance our understanding of RP stone pathophysiology, informing better diagnostic and therapeutic strategies for kidney stone disease.