Fused kidneys: morphologic study and theory of embryogenesis.

A study was made of 50 fused kidneys; 41 postmortem specimens and 9 living patients' records were reviewed. Three patterns of renal fusion were identified--midline fused horseshoe kidney [15], laterally fused horseshoe kidney [21], and crossed renal ectopia [14]. These morphologic renal patterns were correlated with coexisting abnormalities of the urinary tract and other organ systems. A high incidence of hydronephrosis of varied etiology, vertebral, and anorectal defects was found in the laterally fused horseshoe and crossed renal ectopic groups. A theory of embryogenesis was developed to explain the asymmetric patterns of renal fusion exemplified by laterally fused and crossed fused kidneys. It was based on the effects that the lateral flexion and rotation of the hind end in the 4- to 5-mm embryo exert on the relative positions of the nephrogenic cords and wolffian ducts.