RESPONSE OF THE NEWBORN URETEROPELVIC JUNCTION COMPLEX TO INDUCED ANDLATER REVERSED PARTIAL URETERAL OBSTRUCTION IN THE RABBIT MODEL

The clinical controversy regarding the timing of surgery for asymptoma tic newborns with obstructed hydronephrosis was addressed using a mode l of reversible partial ureteral obstruction in the newborn rabbit. Th e histomorphometric changes in the ureteropelvic junction complex (for example, pelvis, ureteropelvic junction and upper ureter) and kidney in 44 normal cases were determined and compared with the effects of 47 cases of ongoing partial obstruction and timed reversal of partial ob struction at 1 week in 9 cases, at 2 weeks in 10 or at 4 weeks in 10 ( end of the study at age 8 weeks). After partial obstruction hydronephr osis appeared by 1 week postoperatively. There were progressive increa ses in the thickness of the lamina muscularis and mass index of smooth muscle and collagen (all p <0.001). However, since the per cent surfa ce area of smooth muscle did not change significantly in comparison to normal, there was disproportionately more collagen. For reversals at 1 week the muscle and collagen in the lamina muscularis were not signi ficantly different from normal. For reversals at 2 weeks the mass inde x of collagen was greater than normal (p <0.05) and reversal at 1 week (p <0.05). For reversals at 4 weeks the lamina muscularis was thicker , and the mass index of collagen and muscle was greater than the earli er reversal groups and normal (all p <0.05). In conclusion, partial ur eteral obstruction causes progressive thickening of the lamina muscula ris by collagen and muscle with a disproportionately greater increase in collagen than muscle. The earlier the obstruction can be reversed, the more normal is the ureteropelvic junction complex histology. The f unctional significance of these changes needs to be determined.