Bilirubin-associated renal papillary necrosis in the homozygous Gunn rat: light-and electron-microscopic observations.

Homozygous members of the mutant Gunn strain of Wistar rats suffer an inherited autosomal recessive deficiency of glucuronyl transferase resulting in unconjugated hyperbilirubinaemia and jaundice. Unconjugated bilirubin visibly accumulated in the interstitium of the renal papillary tip. The deposit is followed by necrosis, first of intersitital cells, loops of Henle and vasa recta and later of collecting ducts. The necrotic, acellular papillary tip eventually separates. Cortical scars correlate with established papillary necrosis. The bilirubin occurs in two ultrastructural forms, crystalline and fibrillar. The deposit is found initially in the interstitium and around but not within basement-membranes. It is accompanied by cytoplasmic adaptive or degenerative changes and loss of cells by desquamation and coagulative necrosis. The lesion can be regarded as a chronic, progressive papillary necrosis similar in its evolution to the papillary necrosis of analgesic nephropathy.