What can the microstructure of stones tell us?

Kidney stone growth through the lens of Raman mapping

Bulk composition of kidney stones, often analyzed with infrared spectroscopy, plays an essential role in determining the course of treatment for kidney stone disease. Though bulk analysis of kidney stones can hint at the general causes of stone formation, it is necessary to understand kidney stone microstructure to further advance potential treatments that rely on in vivo dissolution of stones rather than surgery. The utility of Raman microscopy is demonstrated for the purpose of studying kidney stone microstructure with chemical maps at ≤ 1 µm scales collected for calcium oxalate, calcium phosphate, uric acid, and struvite stones. Observed microstructures are discussed with respect to kidney stone growth and dissolution with emphasis placed on < 5 µm features that would be difficult to identify using alternative techniques including micro computed tomography. These features include thin concentric rings of calcium oxalate monohydrate within uric acid stones and increased frequency of calcium oxalate crystals within regions of elongated crystal growth in a brushite stone. We relate these observations to potential concerns of clinical significance including dissolution of uric acid by raising urine pH and the higher rates of brushite stone recurrence compared to other non-infectious kidney stones.

The impact of crystal phase transition on the hardness and structure of kidney stones

Calcium oxalate kidney stones, the most prevalent type of kidney stones, undergo a multi-step process of crystal nucleation, growth, aggregation, and secondary transition. The secondary transition has been rather overlooked, and thus, the effects on the disease and the underlying mechanism remain unclear. Here, we show, by periodic micro-CT images of human kidney stones in an ex vivo incubation experiment, that the growth of porous aggregates of calcium oxalate dihydrate (COD) crystals triggers the hardening of the kidney stones that causes difficulty in lithotripsy of kidney stone disease in the secondary transition. This hardening was caused by the internal nucleation and growth of precise calcium oxalate monohydrate (COM) crystals from isolated urine in which the calcium oxalate concentrations decreased by the growth of COD in closed grain boundaries of COD aggregate kidney stones. Reducing the calcium oxalate concentrations in urine is regarded as a typical approach for avoiding the recurrence. However, our results revealed that the decrease of the concentrations in closed microenvironments conversely promotes the transition of the COD aggregates into hard COM aggregates. We anticipate that the suppression of the secondary transition has the potential to manage the deterioration of kidney stone disease.

Mineralogy, geochemistry, and micromorphology of human kidney stones (urolithiasis) from Mersin, the southern Turkey

This study describes the primary characteristics of the selected kidney stones surgically removed from the patients at the Mersin University Hospital in the southern Turkey and interprets their formation via petrographic, geochemical, XRD, SEM-EDX, and ICP-MS/OES analyses. The analytical results revealed that the kidney stones are composed of the minerals whewellite, struvite, hydroxyapatite, and uric acid alone or in different combinations. The samples occur in staghorn, bean-shaped composite, and individual rounded particle shapes, which are controlled by the shape of the nucleus and the site of stone formation. The cross-section of the samples shows concentric growth layers due to variations in saturation, characterizing the metastable phase. Kidney stone formation includes two main stages: (i) nucleation and (ii) aggregation and/or growth. Nucleation was either Randall plaque of hydroxyapatite in tissue on the surface of the papilla or a coating of whewellite on the plaque, or crystallization as free particles in the urine. Subsequently, aggregation or growth occurs by precipitation of stone-forming materials around the plaque or coating carried into the urine, or around the nucleus formed in situ in the urine. Urinary supersaturation is the main driving force of crystallization processes; and is controlled by many factors including bacterially induced supersaturation.

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.

Collagen fibrils and cell nuclei are entrapped within Randall's plaques but not in CaOx matrix overgrowth: A microscopic inquiry into Randall's plaque stone pathogenesis

Calcium oxalate (CaOx) stones can grow attached to the renal papillary calcification known as Randall's plaque. Although stone growth on Randall's plaque is a common phenomenon, this mechanism of stone formation is still poorly understood. The objective of this study was to investigate the microenvironment of mature Randall's plaque, explore its molecular composition and differentiate plaque from CaOx overgrowth using multimodal imaging on demineralized stone sections. Fluorescence imaging showed consistent differences in autofluorescence patterns between Randall's plaque and calcium oxalate overgrowth regions. Second harmonic generation imaging established the presence of collagen only in regions of decalcified Randall's plaque but not in regions of CaOx overgrowth matrix. Surprisingly, in these stone sections we observed cell nuclei with preserved morphology within regions of mature Randall's plaque. These conserved cells had variable expression of vimentin and CD45. The presence of nuclei in mature plaque indicates that mineralization is not necessarily associated with cell death. The markers identified suggest that some of the entrapped cells may be undergoing dedifferentiation or could emanate from a mesenchymal or immune origin. We propose that entrapped cells may play an important role in the growth and maintenance of Randall's plaque. Further characterization of these cells and thorough analyses of the mineralized stone forming renal papilla will be fundamental in understanding the pathogenesis of Randall's plaque and CaOx stone formation.

Multicolor imaging of calcium-binding proteins in human kidney stones for elucidating the effects of proteins on crystal growth

The pathogenesis of kidney stone formation includes multi-step processes involving complex interactions between mineral components and protein matrix. Calcium-binding proteins in kidney stones have great influences on the stone formation. The spatial distributions of these proteins in kidney stones are essential for evaluating the in vivo effects of proteins on the stone formation, although the actual distribution of these proteins is still unclear. We reveal micro-scale distributions of three different proteins, namely osteopontin (OPN), renal prothrombin fragment 1 (RPTF-1), and calgranulin A (Cal-A), in human kidney stones retaining original mineral phases and textures: calcium oxalate monohydrate (COM) and calcium oxalate dihydrate (COD). OPN and RPTF-1 were distributed inside of both COM and COD crystals, whereas Cal-A was distributed outside of crystals. OPN and RPTF-1 showed homogeneous distributions in COM crystals with mosaic texture, and periodically distributions parallel to specific crystal faces in COD crystals. The unique distributions of these proteins enable us to interpret the different in vivo effects of each protein on CaOx crystal growth based on their physico-chemical properties and the complex physical environment changes of each protein. This method will further allow us to elucidate in vivo effects of different proteins on kidney stone formation.

Using micro computed tomographic imaging for analyzing kidney stones

Stone analysis is a critical part of the clinical characterization of urolithiasis. This article reviews the strengths and limitations of micro CT in the analysis of stones. Using micro CT alone in a series of 757 stone specimens, micro CT identified the 458 majority calcium oxalate specimens with a sensitivity of 99.6% and specificity of 95.3%. Micro CT alone was also successful in identifying majority apatite, brushite, uric acid, and struvite stones. For some minor minerals-such as apatite in calcium oxalate or calcium salts in uric acid stones-micro CT enables the detection of minute quantities well below 1%. The addition of a standard for calibrating X-ray attenuation values improves the ability of micro CT to identify common stone minerals. The three-dimensional nature of micro CT also allows for the visualization of surface features in stones, which is valuable for the study of stone formation.

Structural and chemical heterogeneities of primary hyperoxaluria kidney stones from pediatric patients

OBJECTIVE

Calcium oxalate stones are the most common type among stone-forming patients and in some cases result from predisposed genetic conditions. In this work, we examined the differences in structure and chemical composition between oxalate stones from patients from three groups: 1) pediatric patients that were genetically predisposed (primary hyperoxaluria) to form stones (PPH); 2) control pediatric patients that did not have such genetic predisposition (PN-PH); 3) adult patients that formed oxalate stones without the genetic predisposition (A-CaOx). A variety of instrumental analyses were conducted to identify physicochemical properties of stones characteristic of predisposed pediatric (PPH), pediatric hyperoxaluria (PN-PH), and adult (A-CaOx) patient populations.

METHODS

Genetic variants of 16 stone-forming patients were determined using whole-exome gene sequencing. Components of stones from PPH (n = 6), PN-PH (n = 5), and A-CaOx (n = 5) groups were identified using Fourier transform infrared (FTIR) spectroscopy. Stone morphology and density were evaluated using high resolution X-ray computed tomography (micro-XCT). Stone microstructure and elemental composition were mapped with scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopy, respectively.

RESULTS

Calcium oxalate bipyramidal crystals were found on stones from all groups. Stones from PPH patients with PH types I and II were composed of calcium oxalate monohydrate (COM) with relatively uniform mineral density (1224 ± 277 mg/cc) and distinct smooth surfaces. By contrast, micro-spherical calcium phosphate particles were found only on PN-PH stones, which also showed a broader range of mineral densities (1266 ± 342 mg/cc). Stones from the PN-PH group also contained phosphorus (P), which was absent in NP-PH stones. A-CaOx stones were of significantly lower mineral density (645 ± 237 mg/cc) than pediatric stones and were more heterogeneous in their elemental composition.

CONCLUSION

Unique structural and compositional characteristics were identified in stones from pediatric patients with primary hyperoxaluria. These include the absence of phosphorus, a narrower mineral density distribution, and a uniform elemental composition compared to stones from pediatric patients without the genetic predisposition. Thus, characterization of stones at the macro- and micro-scales in combination with genetic testing of patients can provide insights and accurate diagnosis to develop a treatment plan for effective patient care.

Multimodal imaging reveals a unique autofluorescence signature of Randall's plaque

Kidney stones frequently develop as an overgrowth on Randall's plaque (RP) which is formed in the papillary interstitium. The organic composition of RP is distinct from stone matrix in that RP contains fibrillar collagen; RP in tissue has also been shown to have two proteins that are also found in stones, but otherwise the molecular constituents of RP are unstudied. We hypothesized that RP contains unique organic molecules that can be differentiated from the stone overgrowth by fluorescence. To test this, we used micro-CT-guided polishing to expose the interior of kidney stones for multimodal imaging with multiphoton, confocal and infrared microscopy. We detected a blue autofluorescence signature unique to RP, the specificity of which was also confirmed in papillary tissue from patients with stone disease. High-resolution mineral mapping of the stone also showed a transition from the apatite within RP to the calcium oxalate in the overgrowth, demonstrating the molecular and spatial transition from the tissue to the urine. This work provides a systematic and practical approach to uncover specific fluorescence signatures which correlate with mineral type, verifies previous observations regarding mineral overgrowth onto RP and identifies a novel autofluorescence signature of RP demonstrating RP's unique molecular composition.

SEM-EDX linear scanning: a new tool for morpho-compositional analysis of growth bands in urinary stones

The genesis and growth of calculi are imprinted in their structure, so the pathogenesis of lithiasis could potentially be read via proper analytical techniques. In this study, electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX) is used to obtain a description of the morphology and compositional structure of a single bladder stone. This technique establishes the chemical and crystalline architecture of the urolith to assess the effect of the chemical environment on its growth. Scanning electron microscopy-backscattered electrons (SEM-BSE) images clearly show that the stone has a multilayered structure. These layers and Liesegang ring-like structures are characterized by one predominant chemical component but also by slighter compositional changes. The mean crystalline components are determined by X-ray diffraction (DRX), infrared spectroscopy (FT-IR), and Raman analysis (RMN). Elemental analysis along a radial trajectory of the calculus by EDX linear scanning (EDX-LS) also reveals the compositional structure of the layers and the spatial distribution of the main chemical components. EDX-LS data processing reveals concentration profiles that clearly show morpho-compositional growth bands, which correspond to precipitation waves and urinary concentration peaks. The width of the growth bands is independent of the radial position, layer, and element analyzed. We conclude that the bands observed are a consequence of slight changes in the biochemical composition of the urine and consequently reflect a short-term biological cycle of the renal system. This non-specific growth rate suggests that stone formation is a kinetically controlled phenomenon in which promoters of crystal cluster aggregation may have played a key role.

Endoscopic observations as a tool to define underlying pathology in kidney stone formers

PURPOSE

Advancements in endoscopy offer the possibility of inspection of intrarenal anatomy and pathology. The aim of the study was to evaluate renal papillary appearance in kidney stone formers and to correlate papillary findings with stone type and patient metabolic data.

MATERIALS AND METHODS

A consecutive cohort of 46 kidney stone formers undergoing retrograde intrarenal surgery was enrolled. During surgery, renal papillae were characterized in the domains of ductal Plugging (DP), surface Pitting, Loss of papillary contour, and Amount of Randall's plaque (RP, PPLA scoring). Stone material was analyzed using micro-CT and infrared spectroscopy, and blood and urine were collected for metabolic evaluation.

RESULTS

In all patients, renal papillae had changes in at least one of the domains of the PPLA score. Examining the total population, it was evident that patients with predominantly plugging (DP > 0) all had very low RP scores. There were no significant trends between mean PPLA scores and urinary analytes for the total group.

CONCLUSION

Efforts to prevent renal stone formation have so far been insufficient in majority of patients. Digital endoscopy reveals that kidney stone formers have different and distinct papillary morphologies that seem to be linked to specific stone-forming pathways. Since renal papillary abnormalities may be easily identified during endoscopy, this may in the future prove to be an important method for tailoring prevention strategies in kidney stone patients.

Morphological characteristics and microstructure of kidney stones using synchrotron radiation μCT reveal the mechanism of crystal growth and aggregation in mixed stones

Understanding the mechanisms of kidney stone formation, development patterns and associated pathological features are gaining importance due to an increase in the prevalence of the disease and diversity in the presentation of the stone composition. Based on the microstructural characteristics of kidney stones, it may be possible to explain the differences in the pathogenesis of pure and mixed types of stones. In this study, the microstructure and distribution of mineral components of kidney stones of different mineralogy (pure and mixed types) were analyzed. The intact stones removed from patients were investigated using synchrotron radiation X-ray computed microtomography (SR-μCT) and the tomography slice images were reconstructed representing the density and structure distribution at various elevation planes. Infrared (IR) spectroscopes, X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to confirm the bulk mineral composition in the thin section stones. Observations revealed differences in the micro-morphology of the kidney stones with similar composition in the internal 3-D structure. Calcium oxalate monohydrate stones showed well-organised layering patterns, while uric acid stones showed lower absorption signals with homogenous inner structure. Distinct mineral phases in the mixed types were identified based on the differential absorption rates. The 3-D quantitative analysis of internal porosity and spatial variation between nine different types of stones were compared. The diversity among the microstructure of similar and different types of stones shows that the stone formation is complex and may be governed by both physiological and micro-environmental factors. These factors may predispose a few towards crystal aggregation and stone growth, while, in others the crystals may not establish stable attachment and/or growth.

Raman chemical imaging, a new tool in kidney stone structure analysis: Case-study and comparison to Fourier Transform Infrared spectroscopy

BACKGROUND AND OBJECTIVES

The kidney stone's structure might provide clinical information in addition to the stone composition. The Raman chemical imaging is a technology used for the production of two-dimension maps of the constituents' distribution in samples. We aimed at determining the use of Raman chemical imaging in urinary stone analysis.

MATERIAL AND METHODS

Fourteen calculi were analyzed by Raman chemical imaging using a confocal Raman microspectrophotometer. They were selected according to their heterogeneous composition and morphology. Raman chemical imaging was performed on the whole section of stones. Once acquired, the data were baseline corrected and analyzed by MCR-ALS. Results were then compared to the spectra obtained by Fourier Transform Infrared spectroscopy.

RESULTS

Raman chemical imaging succeeded in identifying almost all the chemical components of each sample, including monohydrate and dihydrate calcium oxalate, anhydrous and dihydrate uric acid, apatite, struvite, brushite, and rare chemicals like whitlockite, ammonium urate and drugs. However, proteins couldn't be detected because of the huge autofluorescence background and the small concentration of these poor Raman scatterers. Carbapatite and calcium oxalate were correctly detected even when they represented less than 5 percent of the whole stones. Moreover, Raman chemical imaging provided the distribution of components within the stones: nuclei were accurately identified, as well as thin layers of other components. Conversion of dihydrate to monohydrate calcium oxalate was correctly observed in the centre of one sample. The calcium oxalate monohydrate had different Raman spectra according to its localization.

CONCLUSION

Raman chemical imaging showed a good accuracy in comparison with infrared spectroscopy in identifying components of kidney stones. This analysis was also useful in determining the organization of components within stones, which help locating constituents in low quantity, such as nuclei. However, this analysis is time-consuming, making it more suitable for research studies rather than routine analysis.

Anatomic variations and stone formation

PURPOSE OF REVIEW

The pathophysiological mechanisms in kidney stone formation are insufficiently understood. In order to achieve a better understanding of the complexity of stone formation, studies evaluating anatomical variations in the renal papillae have been performed. This review intends to illuminate recent findings. Moreover, new techniques to improve the understanding and interpretation of crystallization mechanisms are reviewed.

RECENT FINDINGS

Due to improvements of digital ureteroscopes, detailed endoscopic mapping of renal papillae is now possible. Connections between papillary morphology and histopathological changes in different subsets of stone formers have been documented. The formation of kidney stones seems to take place in relation to Randall's plaques, Ducts of Bellini or by free formation. Additionally, theories of kidney stone formation because of vascular injury or inflammatory events in the papillae have been suggested.

SUMMARY

Novel techniques including improved digital endoscopic visualization, microcomputed tomography (CT), electron microscopy and energy dispersive compositional analyses of kidney stones seem essential in the search for effective and reliable methods to understand stone forming processes, which ultimately should result in effective measures for more personalized stone prevention strategies in the future.

Electron probe micro-analysis reveals the complexity of mineral deposition mechanisms in urinary stones

Urinary stones are complex mineralogical formations in the urinary system often impairing the kidney function. Several studies have attempted to understand the mechanisms of stone formation and growth; however, it remains to be fully explored. Here, we present a detailed investigation on the morphological and mineralogical characterizations of urinary stones. Structural properties of different types of urinary stones were done by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and field-emission scanning electron microscope (FE-SEM) analyses. X-ray maps of major and the trace elements were obtained using electron microprobe (EPMA) technique. Basic metabolic panel and urinary parameters of the patients were used for comparing mineral compositions among stone types. The study included five major types of stones identified based on the FTIR spectra. FTIR and XRD helped in identifying the major components of these stones. FE-SEM images revealed distinct microstructure and morphology of the stones among the stone types. EPMA analysis showed the presence of many metals other than calcium and certain non-metals within the urinary stone matrix at measurable levels, sometimes with distinct distribution patterns. The study demonstrates the characteristic micro-structure, morphology, distribution, and composition of elements in different stone types. Findings of the study provide scope for understanding the complex mechanisms involved in the urolithogenesis and association of trace elements in it.

Papillary Ductal Plugging is a Mechanism for Early Stone Retention in Brushite Stone Disease

PURPOSE

Mechanisms of early stone retention in the kidney are under studied and poorly understood. To date attachment via Randall's plaque is the only widely accepted theory in this regard, which is best described in idiopathic calcium oxalate stone formers. Brushite stone formers are known to have distinct papillary morphology relative to calcium oxalate stone formers. As such we sought to determine whether stone attachment mechanisms in such patients may be similarly unique.

MATERIALS AND METHODS

Patients undergoing percutaneous and or ureteroscopic procedures for stone removal consented to endoscopic renal papillary examination and individual stone collection. Each removed stone was processed using micro computerized tomography to assess the 3-dimensional microstructure and the minerals contained, and search for common structural features indicative of novel mechanisms of early growth and attachment to renal tissue.

RESULTS

A total of 25 intact brushite stones were removed from 8 patients and analyzed. Video confirmed attachment of 13 of the 25 stones with the remainder believed to have been accidently dislodged during the procedure. Microscopic examination by light and computerized tomography failed to show evidence of Randall's plaque associated with any stone containing brushite. Conversely each brushite stone demonstrated microstructural evidence of having grown attached to a ductal plug formed of apatite.

CONCLUSIONS

Three-dimensional analysis of small brushite stones suggests overgrowth on ductal apatite plugs as a mechanism of early stone growth and retention. Such findings represent what is to our knowledge the initial supporting evidence for a novel mechanism of stone formation which has previously been hypothesized but never verified.