RESUMO
Ten infants under 6 months old underwent surgery for obstruction of the ureteropelvic junction. Craniocaudal light microscopy showed subdivision of the resected ureteropelvic junction into three portions: prestenotic, stenotic, and poststenotic. The prestenotic portion was characterized by dilatation of the ureteral lumen, flattening of its mucosal folds and thinning of the urothelium; the stenotic tract showed partial or total loss of the epithelium and fibrosis of the mucosal and fibromuscular coats. No modifications were detected in the poststenotic portion. We advance the hypothesis that a primary epithelial break might cause urine to spread inside the ureteral wall and consequently the mastocytes to migrate and degranulate within the mucosal and fibromuscular coats. The histamine and prostaglandins produced by the mastocytes could induce prolonged muscular spasm, in turn responsible for increasing the intrapelvic pressure and so causing enlargement of the epithelial break. A connective tissue reaction of the ureteral wall would thus occur, which should be considered a secondary event leading to fibrotic stenosis of the ureteropelvic junction.