Endoscopic evidence of calculus attachment to Randall's plaque

A formal test of the hypothesis that idiopathic calcium oxalate stones grow on Randall's plaque

OBJECTIVE

To confirm that more than half of all idiopathic calcium oxalate (CaOx) stones grow on interstitial plaque, as CaOx stones can grow attached to interstitial apatite plaque but whether this is the usual mechanism of stone formation is uncertain.

PATIENTS AND METHODS

In nine idiopathic CaOx stone formers (ICSF) undergoing percutaneous nephrolithotomy or ureteroscopy all accessible renal papillae were endoscopically imaged using a digital endoscope. All stones were removed intact, and recorded by the operating surgeon as being attached or unattached; for all attached stones the surgeon determined if the site of attachment was to plaque. This determination was further verified by reviewing the intraoperative video record, and only instances where plaque was reliably seen on video were used for analysis. Surgical observations were further validated by a combination of microcomputed tomographic analysis and papillary biopsy. The results were analysed statistically using fixed-sample testing and group sequential sampling.

RESULTS

The nine patients had a total of 115 stones, primarily CaOx; 90 stones were attached. Of these, 81 were attached to plaque; surgeons could not visualize the site of attachment with sufficient clarity to judge in the other nine cases. Based on these data, the final point estimate for the number of stones attached to plaque was 0.754 (95% confidence interval 0.575-0.933; P = 0.013).

CONCLUSIONS

In ICSF most stones grow attached to papillae, on plaque, so growth on plaque is the main mechanism for stone formation in this very common group of patients.

Protein content of human apatite and brushite kidney stones: significant correlation with morphologic measures

Apatite and brushite kidney stones share calcium and phosphate as their main inorganic components. We tested the hypothesis that these stone types differ in the amount of proteins present in the stones. Intact stones were intensively analyzed by microcomputed tomography (micro CT) for both morphology (including the volume of voids, i.e., space devoid of X-ray dense material) and mineral type. To extract all proteins present in kidney stones in soluble form we developed a three-step extraction procedure using the ground stone powder. Apatite stones had significantly higher levels of total protein content and void volume compared to brushite stones. The void volume was highly correlated with the total protein contents in all stones (r2 = 0.61, P < 0.0001), and brushite stones contained significantly fewer void regions and proteins than did apatite stones (3.2 +/- 4.5% voids for brushite vs. 10.8 +/- 11.2% for apatite, P < 0.005; 4.1 +/- 1.6% protein for brushite vs. 6.0 +/- 2.4% for apatite, P < 0.03). Morphological observations other than void volume did not correlate with protein content of stones, and neither did the presence or absence of minor mineral components. Our results show that protein content of brushite and apatite stones is higher than that was previously thought, and also suggest that micro CT-visible void regions are related to the presence of protein.

Evidence for increased postprandial distal nephron calcium delivery in hypercalciuric stone-forming patients

A main mechanism of idiopathic hypercalciuria (IH) in calcium stone-forming patients (IHSF) is postprandial reduction of renal tubule calcium reabsorption that cannot be explained by selective reduction of serum parathyroid hormone levels; the nephron site(s) responsible are not as yet defined. Using fourteen 1-h measurements of the clearances of sodium, calcium, and endogenous lithium during a three-meal day in the University of Chicago General Clinical Research Center, we found reduced postprandial proximal tubule reabsorption of sodium and calcium in IHSF vs. normal subjects. The increased distal sodium delivery is matched by increased distal reabsorption so that urine sodium excretions do not differ, but distal calcium reabsorption does not increase enough to match increased calcium delivery, so hypercalciuria results. In fact, urine calcium excretion and overall renal fractional calcium reabsorption both are high in IHSF vs. normal when adjusted for distal calcium delivery, strongly suggesting a distal as well as proximal reduction of calcium reabsorption. The combination of reduced proximal tubule and distal nephron calcium reabsorption in IHSF is a new finding and indicates that IH involves a complex, presumably genetic, variation of nephron function. The increased calcium delivery into the later nephron may play a role in stone formation via deposition of papillary interstitial apatite plaque.

[Calcium oxalate stone formation. New pathogenetic aspects of an old disease]

Calcium oxalate (CaOx) urolithiasis is a very common disorder. Surprisingly, the pathogenetic mechanisms leading to CaOx stone formation have been largely unknown so far. The long-accepted simple explanation by an exceeding of the solubility product of lithogenic substances in the urine cannot sufficiently describe the complex processes. Deviating from the hypothesis that proclaims that the initial crystal deposition takes place in the lumens of renal tubules, new insights suggest a primary plaque formation in the interstitial space of the renal papilla. Initially, calcium phosphate (CaPh) crystals and organic matrix are deposited along the basement membranes of the thin loops of Henle and extend further in the interstitial space to the urothelium, constituting the so-called Randall's plaques that can be regularly found during endoscopy of CaOx-stone-forming patients. These CaPh crystals seem to be the origin for the development of future CaOx stones, which form by the attachment of further matrix molecules and CaOx from the urine to the plaque. The driving forces, the exact pathogenetic mechanisms, and the involved matrix molecules remain largely unknown. Possibly, completely different pathomechanisms lead to the common clinical diagnosis of"CaOx stone former."

Renal intratubular crystals and hyaluronan staining occur in stone formers with bypass surgery but not with idiopathic calcium oxalate stones

Whether idiopathic calcium oxalate (CaOx) stone formers form inner medullary collecting duct (IMCD) crystal deposits bears on pathogenetic mechanisms of stone formation. In prior work, using light and transmission electron microscopy, we have found no IMCD crystal deposits. Here, we searched serial sections of papillary biopsies from a prior study of 15 idiopathic calcium oxalate stone formers, 4 intestinal bypass patients with CaOx stones, and 4 non-stone-forming subjects, and biopsies from an additional hitherto unreported 15 idiopathic calcium oxalate stone formers and 1 bypass patient using polarized light oil immersion optics, for deposits overlooked in our original study. We found no IMCD deposits in any of 1,500 serial sections from the 30 idiopathic calcium oxalate stone formers, nor in 87 additional sections from a frozen idiopathic calcium oxalate stone former biopsy sample processed without exposure to aqueous solutions. Among 4 of the 5 bypass patients but in none of the 30 idiopathic calcium oxalate stone formers or 4 normal stone formers, we found tiny birefringent thin crystalline overlays on scattered IMCD cell membranes. We also found IMCD lumen deposits in two bypass patients that contained mixed birefringent and nonbirefringent crystals, presumably CaOx and apatite. In the bypass patients, we observed focal apical IMCD cell hyaluronan staining, which was absent in idiopathic calcium oxalate stone formers. The absence of any IMCD deposits in 1,500 serial sections of biopsies from 30 idiopathic calcium oxalate stone formers allows us to place the upper limit on the probability of their occurrence at approximately 0.002 and place the lower limit of their size at the resolution of the optics (<0.2 mu). The tiny deposits in bypass patients may be the initial crystal lesion.

A new world revealed: early experience with digital ureteroscopy

PURPOSE

The latest digital ureteroscope, the DUR-D (Gyrus ACMI Inc., Southborough, Massachusetts) offers image quality that greatly exceeds current analog image capabilities. The purpose of this report was to document the applicability of this device in studying the earliest stages of stone formation.

MATERIALS AND METHODS

Symptomatic patients with nephrolithiasis (less than 1 cm in diameter) were prospectively enrolled and their renal papilla digitally mapped prior to stone removal. Recovered stones were photographed and analyzed using micro-computerized tomography. If the procedure could not be completed with the DUR-D, a conventional ureteroscope was used. Minors, pregnant patients and those with systemic disorders were excluded.

RESULTS

Eight patients (10 renal units), 2 cystine and 6 calcium oxalate stone formers, were studied with a mean age of 50.1 years. Excellent images were collected for all papilla except in 2 cases; a proximal ureteral stricture and acute angulation of the lower pole collecting system, respectively. There were no mechanical device failures. Of the 10 renal units 7 had stents postoperatively and there were no patient complications.

CONCLUSIONS

The ease of use and high quality images of digital ureterorenoscopy will allow the documentation of the earliest stages of calcium oxalate stone formation and, thereby, advance our understanding of the pathogenesis of calcium oxalate stone formation.

Pathogenesis of renal calculi

Recent reports suggest that kidney stone disease prevalence is increasing. Despite significant treatment advances, the inciting factor and sequence of events leading to kidney stone formation remain elusive; however, recent efforts to understand the pathogenesis of nephrolithiasis have led to a delineation of the human surgical anatomy, histopathology, and metabolic factors in a variety of kidney stone formers. This article reviews the fundamental concepts of calculus formation, and the leading theories of stone pathogenesis, focusing on recent data from human papillary and renal cortical biopsies in stone formers that provide evidence for the role of Randall's plaque in kidney stone disease pathogenesis. These data suggest there are individual stone-forming phenotypes with unique surgical anatomy, histology, and metabolic profiles.

Calcium oxalate calculi found attached to the renal papilla: Preliminary evidence for early mechanisms in stone formation

BACKGROUND

Calculi are commonly found attached to the renal papilla in calcium oxalate (CaOx) stone formers, but the mechanisms by which stones form in this manner are not well established.

MATERIALS AND METHODS

Data are presented from three attached stones collected from different patients. Stone morphology and composition were determined using micro computed tomography (CT) and infrared microspectrometry.

RESULTS

One of the stones was composed of CaOx with a peripheral region of apatite, such as might have come from a Randall's plaque. Another stone was covered with large CaOx crystals but contained at least two layers of apatite, with no apatite regions exposed at the surface. The third stone contained CaOx with inclusions of apatite and more apatite on its surface, along with a substantial volume of poorly mineralized material that could not be identified.

CONCLUSIONS

The complexity of these stones and their differing morphologies do not by themselves allow inference of the mechanism of stone formation. Future work will require the careful documentation of attached stones on the papilla, as well as study of the papilla after the stone has been removed, before it can be determined whether such diverse CaOx stones originate from the same or different underlying etiologies.

The role of Randall's plaques in the pathogenesis of calcium stones

PURPOSE

Knowledge of the inciting lesion in kidney stone formation has remained rudimentary until quite recently. Randall theorized that areas of apatite plaque on the renal papillae would be an ideal site for an overgrowth of calcium oxalate to develop into a calculus. We reviewed in vivo data that have further defined the role of Randall's plaques in stone disease.

MATERIALS AND METHODS

We examined a set of literature that tested 2 hypotheses, that is 1) Randall's plaques are a specialized disease that begins as apatite in a unique region of the kidney due to local driving forces and anatomy, and 2) stones that arise from causes different from common calcium oxalate stones do not necessarily arise on plaque.

RESULTS

Intraoperative papillary and cortical biopsy specimens obtained during percutaneous nephrolithotomy from the kidneys of 3 types of stone formers (idiopathic calcium stone formers, patients with stones due to bariatric procedures and brushite stone formers) showed unique histopathological findings.

CONCLUSIONS

The metabolic and surgical pathological findings in 3 distinct groups of stone formers demonstrate that the histology of the renal papillae from a stone former is particular to the clinical setting.

Mechanism of Formation of Human Calcium Oxalate Renal Stones on Randall's Plaque

Although calcium oxalate (CaOx) renal stones are known to grow attached to renal papillae, and specifically to regions of papillae that contain Randall's plaque (interstitial apatite deposits), the mechanisms of stone overgrowth on plaque are not known. To investigate the problem, we have obtained biopsy specimens from two stone patients that included an attached stone along with its tissue base and have studied the ultrastructural features of the attachment point using light and transmission electron microscopy, Fourier transform infrared spectroscopy (mu-FTIR), and immunohistochemical analysis. The epithelium is disrupted at the attachment site. The denuded plaque that borders on the urinary space attracts an envelope of ribbon-like laminates of crystal and organic matrix arising from urine ions and molecules. Into the matrix of this ribbon grow amorphous apatite crystals that merge with and give way to the usual small apatite crystals imbedded in stone matrix; eventually CaOx crystals admix with apatite and become the predominant solid phase. Over time, urine calcium and oxalate ions gradually overgrow on the large crystals forming the attached stone.

Henle loop basement membrane as initial site for Randall plaque formation

Since the early description of Randall plaques in 1937, studies of the pathogenesis of stone formation mainly focused on the chemistry involving salt precipitation and crystallization, rather than tubular and interstitial medullar mechanisms of calcium concentration and supersaturation. In 2003, Bushinsky published a suggestive and inspiring sequence of events aimed to show that the basement membrane of the thin limb of the loop of Henle can be the first site of nucleation, as recently shown by the impressive work by Evan et al. The aim of this minireview is to verify the consistency of the Evan and Bushinsky theory with the current literature in the field.