Spontaneous papillary necrosis in the heterozygous Gunn rat

Spontaneous papillary necrosis develops in aging heterozygous non-jaundiced Gunn rats. The lesion is situated in the subapical or mid papilla and in its earliest stages is manifest by the appearance of amorphous material in the interstitial space. This is seen in plastic-embedded sections taken from rats 6 months old. In its later stages, the accumulation of amorphous material is accompanied by loss of interstitial cells and cyst formation, but there is no associated inflammatory reaction. The largest lesions are found in the oldest rats, but even in these animals the macroscopic appearance of the papilla is normal. Similar papillary changes were not found in albino or homozygous Gunn rats, but in aging albino rats there was loss of papillary interstitial cells without accumulation of amorphous material.