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In the present study, we exposed rats to a crystal-inducing diet (CID) consisting of vitamin D3 and 0.5% ethylene glycol (EG), and we investigated histologically the kidney damage induced by the deposition of calcium oxalate (CaOx) crystals. After 28 days, 50 % of the animals had renal CaOx crystals, of which 60% also had small papillary stones. Most crystals were present in the cortex. The occurrence of these crystals coincided with morphological and cytochemical changes: glomerular damage, tubular dilatation and necrosis, and an enlargement of the interstitium. The number of epithelial and interstitial cells positive for the proliferating cell nuclear antigen (PCNA) was increased. Tamm-Horsfall protein (THP) was not only demonstrable in the thick ascending limb of the loop of Henle (TAL), but also frequently in glomeruli, in the proximal tubular epithelium, and in the papilla. In the lumen of the tubular system, it was associated with urinary casts. Reflection contrast microscopy (RCM) showed that the crystals were coated with a thin layer of THP. In spite of the high urinary oxalate concentrations, the above described cellular changes were not observed in CID-fed rats without renal crystals. We conclude, therefore, that in the kidney, the retained CaOx crystals rather than the urinary oxalate ions are responsible for the observed morphological and immunocytochemical changes.

Lectin reactivity in epithelial apical cell coats of normal rat kidneys was compared to that from animals subjected to crystal inducing diets (CID). The aim was to see whether the absence of lectin reactivity in cell coats is related to intratubular calcium oxalate crystal retention. In normal rat kidneys, after a pre-embedding procedure, it was observed that at the ultrastructural level, reactivity was present but that the lectin specificity for the various parts of the nephron might have to be reconsidered. There was heterogeneity between the epithelial cells with respect to the presence of coat material in the tubular cell apices. Tubular epithelial cell apices from CID rats showed no obvious changes in lectin reactivity pattern. Lectin reactivity was present at the periphery of intratubular crystals but undetectable at true crystal attachment sites or reduced at cell apices in the vicinity of recently attached crystals or agglomerates. After a post-embedding reaction procedure, wheat-germ agglutinin (WGA)-lectin reactivity confirmed the presence of coat material in the cleft between cell apex and retained crystal at crystal-attachment sites. The WGA/Au-10 nm reaction products were also seen inside epithelial cells. WGA/Au-10 nm reaction products mark a crystal matrix component inside intratubular and re-tained crystals. A similar matrix was also marked by an α-osteopontin ( αOPN/Au-10 nm) reaction product.

Crystal retention is studied in a rat-model system as a possible mechanism for the etiology of human nephrolithiasis. A crystal-inducing diet (CID) of ethylene glycol plus NH4Cl in their drinking-water is offered to healthy rats to generate intratubular crystals. Subsequently, the fate of retained crystals is investigated by allowing the rats a tissue recovery/crystalluria phase for three, five and ten days, respectively, on normal drinking water. The process of exotubulosis is observed in cortex and medulla of aldehyde-fixed kidneys after three days recovery. After five days, crystals are predominantly seen there in the interstitium. After ten days, cortex and medulla are virtually free of crystals. However, in the papillary regions after five and ten days recovery, three types of calcium oxalate monohydrate (COM) crystals are present: (1) free in the calycine space, (2) sub-epithelially located surrounded by interstitial cells within, and (3) covered by macrophage-like cells, outside the original papillary surface. After a CID plus three days recovery, a further thirty-seven days extra oxalate challenge with solely 0.3 vol% ethylene glycol induced intratubular and interstitial oxalate crystals. In the papillary region, large sub-epithelial crystals are seen. However, no crystals are seen in kidneys from rats given solely (0.5 or 0.8 vol.%) ethylene glycol for thirty days. An oxalate re-challenge retards crystal removal.

In kidneys of healthy rats submitted to a crystal-inducing diet (CID) with ethylene glycol (EG) and NH4Cl, the fate of retained crystals in the papillar region is studied during a recovery period of one, five or ten days, as model system for human nephrolithiasis. Scanning electron microscopy (SEM) shows, at papillary tips bulging into the calycine space, crystal masses covered either by the epithelium or a thin fibrous veil, or by unidentified mobile cuboidal cells. After CID plus one or five days recovery, small sub-epithelial swellings are seen of large sub-epithelial crystals at or around the papillary tip. After CID plus ten days, massive sub-surface crystal-containing micrometer-sized stones are seen in which the presence of calcium is confirmed by X-ray microanalysis. The papillary tip of rats after a re-challenge with an oxalate load from 0.1 vol% EG for twelve or forty-two days shows minor lesions. But a re-challenge with 0.3 vol% EG for thirty-seven days induces large sub-epithelial papillary millimeter-sized stones. The Von Kossa section staining converts the crystals into a black precipitate, but large peri-tubular or peri-vascular calcium deposits are absent. A new hypothesis about the etiology of an inductive calcium oxalate monohydrate nephrolithiasis is formulated which differs from the one proposed by Randall based on his deductive human kidney studies.

In a rat-model system, tubular crystal retention as a possible mechanism for the etiology of nephrolithiasis in man, was studied by conventional transmission electron microscopy. The animals were supplied for nine days with a crystal-inducing diet, with ethylene glycol plus NH4Cl in their drinking-water. After this induction period, a two day regime with fresh drinking-water was included, to allow crystals to be removed by spontaneous crystalluria. After aldehyde fixation of the rat kidneys, large crystals were seen inside the tubular lumen. The crystals were attached to cell surfaces and covered by neighboring epithelial cells. Some crystals were overgrown by several epithelial cells and underwent a process of so-called exotubulosis, resulting in free or cell-surrounded crystals in the interstitium, and possibly in crystals in Giant cells. To investigate the fate of the retained crystals, some animals were additionally exposed to a low-oxalate challenge from drinking water containing 0.1 volume per cent of ethylene glycol for 12 or 30 days, respectively. It was assumed that this would interfere with the retained intratubular or interstitial crystals, and allow the crystals to grow into mini-stones. This was not observed. After the oxalate challenge, no crystals were found to be retained in the tubules (free or covered by cells). Interstitial crystals were observed, but it remains to be demonstrated whether such crystals actually grow into mini-stones or that they are removed by the sterile inflammation process observed.