There are intriguing reports to support Randall's, and our, hypothesis that stones originate from a fixed plaque composed of HA. Earlier microscopic studies of stone structure demonstrated the presence of concavities on small stones compatible with a point of mural attachment and indicated that apatite may be present at the attachment point (Rosenow 1940; Prien 1949). In an early x-ray diffraction and crystallographic study of 10,000 urinary calculi, Herring noted that HA was frequently found as the nucleus of calcium oxalate monohydrate, usually as a small discoid plaque which was felt to resemble Randall's plaque (Herring 1962). Later, Chambers performed an electron probe analysis of 115 small renal calculi. Of 92 predominantly calcium oxalate stones, he was able to identify small central areas of HA, usually 10-200 microns in diameter, in 70 (Chambers, Hodgkinson et al. 1972). Using scanning electron microscopy and x-ray dispersive energy, Cifuentes Delatte found that 63 of 87 passed calcium oxalate stones had evidence of plaque (Cifuentes Delatte, Minon-Cifuentes et al. 1985). Observation of uncalcified tubular lumens found in conjunction with these plaques suggested an interstitial papillary tip origin of this material (Cifuentes Delatte, Minon-Cifuentes et al. 1987).