Purpose: We evaluated the collagen-to-smooth muscle tissue matrix ratio and percentage of elastin in the renal pelvis, ureteropelvic junction (UPJ) and ureter, and compared these findings with the degree of obstruction, patient age and post-pyeloplasty renal recovery. Materials and Methods: We analyzed histological sections from 75 patients with UPJ obstruction. Nine patients were excluded owing to bilateral UPJ obstruction and an improper specimen. We divided the specimen obtained from pyeloplasty into 3 parts, namely the renal pelvis above the obstruction, the obstructed UPJ portion and the ureter below the obstruction. To examine the collagen and smooth muscle, sections were stained using Masson's trichrome, and elastic van Giesson stain was used for elastin, smooth muscle and collagen. Collagen, smooth muscle and elastin populations were identified, and the tissue matrix ratio and percentage of elastin were calculated by color image analysis. Results: In patients with lower ratios of collagen-to-smooth muscle in the UPJ proper hydronephrosis was more improved postoperatively (p = 0.049). In patients with a lower percentage of elastin in the renal pelvis, UPJ and ureter hydronephrosis was more improved postoperatively (p <0.0001). Conclusions: Because the UPJ portion was resected during pyeloplasty, the renal pelvis and the ureter remaining after pyeloplasty are likely to be related to improved hydronephrosis. A higher percentage of elastin in the renal pelvis and ureter contributes to inelasticity and low compliance, and results in a slower recovery from hydronephrosis after pyeloplasty.
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