In this report, we use genetically controlled tissue-specific diphtheria toxin expression to disrupt the WD during embryogenesis to assess directly the impact of cell death on urogenital development. We found that various levels of cell death in the WD result in congenital urogenital defects ranging from agenesis to milder malformation of multiple urogenital organs. These results provide support for the hypothesis that a single factor, cell death, can be the cause of a wide spectrum of congenital anomalies of the kidney and urinary tract.
The severe agenesis and malformation of multiple urogenital organs in the mutants emphasize an essential role of the WD in urogenital development. Phenotypic differences observed in the 2 lines may result from subtle variations, beyond current experimental detection limits, in the timing and completeness of Cre expression. Regardless of the origins, such variations led to differences in the level apoptosis in targeted cell populations (–) and subsequently in phenotypic outcomes.
In mutants from both lines variations in apoptosis in the WD would lead to the cumulative effect of shortening of the WD at its caudal end to different degrees (). The WDs in most HC1;ROSADTA/+ mutants fail to extend to the UB budding site, resulting in renal agenesis. Caudal truncation of the WD in HC2;ROSADTA/+ mutants is less severe but more variable. When the WD extends beyond the UB budding site but fails to reach the cloaca, nephrogenesis can occur in the absence of ureter-bladder connection, resulting in ureteral fistulas. If the WD manages to reach the cloaca, nephrogenesis and ureter-bladder connection can occur, although frequently accompanied by renal hypoplasia, hydroureter, and hydronephrosis ( and data not shown). These later defects may result from the ongoing DTA-induced apoptosis in the ureteral bud derivative and the subsequent negative effects on MM differentiation. Thus, quantitative differences in apoptosis and caudal truncation can translate into qualitatively distinct outcomes (renal agenesis, urogenital fistulas, or other urinary tract defects) in affected individuals.
In humans, fistulas of the genitourinary tract can involve multiple structures, such as kidney, ureter, bladder, urethra and the reproductive ducts. These fistulae can be congenital or secondary to infection, inflammatory disease, neoplasms, trauma or surgery.18–21
Congenital ureteral fistulae connected to the male or the female reproductive tracts are also called ureteral ectopia and their etiology is unclear in most patients.22–25
In many male HC2;ROSADTA/+
mutants, the CND was missing while the ureteral bud reached the MM to initiate nephrogenesis. In such mutants, the UB failed to separate from the WD, resulting in aberrant connection of their derivatives, the ureter and the male reproductive tract. Thus, the transient CND is critical for the separation of the UB and the WD during ureter maturation. The developmental sequence involving the vertical migration of the UB along the CND, lateral migration of the ureteral bud away from the WD and the degeneration of the CND26
appears to be the only pathway for the separation of the ureter and WD derivatives.
After emerging at E12.5 near the anterior end of the urogenital ridge, the MD normally follows the WD toward the cloaca area.27–30
The directional change in MD elongation to follow the ureter in the event of WD caudal truncation () suggests that the ureter retains the guidance ability of the WD for MD development. While previous studies suggest that the WD does not contribute cells to the MD,27
the presence of the ureter-oviduct/uterus fistulas raises the possibility that direct cell-cell exchange between the WD and the MD becomes possible under pathological conditions. Apoptosis in some of the WD cells may compromise the integrity of the WD, providing the tightly associated MD entry points and establishing the fistulas between WD and MD derivatives.