Microanatomy and morphometry of the hydronephrotic "obstructed" renal pelvis in asymptomatic infants.

There has been limited histological study of the obstructed ureteropelvic junction in patients less than 1 year old. We present our prospective studies on the histomorphometry of the ureteropelvic junction complexes of 35 infants, of whom 23 underwent pyeloplasty for obstruction and 12 were age matched autopsy normals. Qualitatively, the major abnormalities of the pelvic microanatomy in the hydronephrotic obstructed kidneys included a lamina muscularis that was significantly thicker than normal and the presence of collagen fibers between muscle fascicles. Additionally, variable amounts of elastin were present in the adventitia and lamina muscularis. At the ureteropelvic junction the abnormalities were an increase in the number of inner longitudinal muscle bundles, collagen between muscle bundles (p less than 0.016) and elastin in the adventitia. Quantitatively, for the pelvis the lamina muscularis was significantly thicker in 18 obstructed kidneys compared to 7 controls (1,075 +/- 79 mu. versus 420 +/- 63 mu., mean +/- standard error of mean, p less than 0.001). The percentage area density of smooth muscle in the obstructed versus normals was increased (45.1% versus 35.2%, p less than 0.039). The percentage area density of collagen in the obstructed specimens versus normal showed an upward trend (38.6% versus 27.6%), which was not significant. In conclusion, the obstructed ureteropelvic junction complexes of kidneys in asymptomatic infants show significant qualitative and quantitative differences from normal.