Intrarenal urinary extravasation with formation of venous polyps containing Tamm-Horsfall protein

Collections of periodic acid-Schiff-positive material are found in the intrarenal veins and in the renal interstitium in certain cases of hydronephrosis and unexplained unilateral hematuria. Using a fluorescent antibody to Tamm-Horsfall protein, a protein secreted by the ascending limbs and distal convoluted tubules, we have demonstrated that the venous and interstitial masses represent urinary filtrate, which probably enters the kidney parenchyma through forniceal or tubular ruptures.