Human jackstone arms show a protein-rich, X-ray lucent core, suggesting that proteins drive their rapid and linear growth.

Jackstone calculi, having arms that extend out from the body of the stone, were first described over a century ago, but this morphology of stones has been little studied. We examined 98 jackstones from 50 different patient specimens using micro-computed tomography (micro CT) and infrared (IR) spectroscopy. Micro CT showed that jackstone arms consisted of an X-ray lucent core within each arm. This X-ray lucent core frequently showed sporadic, thin layers of apatite arranged transversely to the axis of the arm. The shells of the jackstones were always composed of calcium oxalate (CaOx), and with the monohydrate form the majority or sole mineral. Study of layering in the shell regions by micro CT showed that growth lines extended from the body of the stone out onto jack arms and that the thickness of the shell covering of jack arms often thinned with distance from the stone body, suggesting that the arms grew at a faster radial rate than did the stone body. Histological cross-sections of decalcified jackstone arms showed the core to be more highly autofluorescent than was the CaOx shell, and immunohistochemistry showed the core to be enriched in Tamm-Horsfall protein. We hypothesize that the protein-rich core of a jack arm might preferentially bind more protein from the urine and resist deposition of CaOx, such that the arm grows in a linear manner and at a faster rate than the bulk of the stone. This hypothesis thus predicts an enrichment of certain urine proteins in the core of the jack arm, a theory that is testable by appropriate analysis.