Kidneys from 7 mutant Southdown sheep with congenital hyperbilirubinemia, aged from 1 to 5 years were examined. Renal biopsies were taken from another mutant at 3 months and 12 months of age. At 3 months, lesions consisted of thin radial bands of myxomatous tissue in the medullary rays and atrophy of the adjacent collecting tubules. By 1 year collagen had replaced myxomatous tissue. Grossly, the kidneys were normal until 2 years when they became red and gray mottled and stained with bilirubin. The capsules stripped readily to reveal fine granular surfaces in sheep over 2 years of age. On the cut surface of the cortex were 0.5 mm wide radial gray streaks of fibrous tissue. Progressive fibrosis in sheep 2 to 5 years old resulted in a thinning of the cortex. With increasing fibrosis, the number of cystic tubules increased progressively. Protein casts and hyaline droplet degeneration were numerous in sheep over 2 years of age. Plasma cells and lymphocytes were frequently seen in the fibrous bands, and bile pigment was visible in the macrophages in the fibrous tissue and in the epithelium of the proximal tubule cells. Polyuria, low specific gravity urine and reduced effective renal plasma flow and glomerular filtration rates resulted from the replacement of specialized proximal tubule cells by low cuboidal cells, fibrous tissue separating the capillaries from the loops of Henle, destruction of glomeruli and segregation of glomeruli in fibrous bands. The kidney lesions may be determined by the same gene responsible for the hepatic excretion defect for bilirubin.

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