Histological, histochemical and electron microscopic techniques have been used to compare dilated and normal calibre segments of ureter and renal pelvis in cases of idiopathic hydronephrosis and primary obstructive megaureter. In both conditions a marked increase in collagen and elastic tissue occurs in the wall of the distended segment and this infiltration extends throughout the proximal dilated ureter and renal pelvis.

Evidence is presented to show that in the dilated segment, smooth muscle cells are directly involved in the synthesis of connective tissue elements. These findings support the view that the primary anomaly in idiopathic hydronephrosis and primary obstructive megaureter can be attributed to a malfunction of smooth muscle cells in the ureter and renal pelvis. However, the possibility remains that the changes in smooth muscle are secondary and have been induced by obstruction and distension caused in these pathological conditions by unknown aetiological factors.