Background: Congenital and acquired diseases of the urinary bladder are prevalent and account for significant morbidity in pediatric patients. Despite this, the early embryologic events leading to the formation of a differentiated urinary bladder are largely unknown. We present data elucidating the epithelial differentiation of the urinary bladder in a mouse model.
Methods: Timed matings were performed and e11.5 through e17 CD-1 mice embryos were harvested and fixed in 10% buffered formalin. Embryos were dehydrated in serial sucrose in formalin solutions, embedded in OCT media and sectioned with a Leica CM1900 cryostat at 12 μm. Immunohistochemistry (IHC) using an indirect double labeling approach with antibodies specific for uroplakin (UP) III and smooth muscle α-actin (SMAA) was used to stain embryos sectioned in axial and sagittal orientations. Sections were examined with a Zeiss Axioscope fluorescence microscope. Sections and whole mount embryos were used for non-radioactive in situ hybridization using digoxigenin-labeled cRNA probes specific for uroplakin Ia, Ib, II, IIIa and IIIb. Sections and embryos were photographed with an MZ15 Leica dissecting microscope.
.
Results: UP-III protein expression is first noted by IHC at e13. In earlier embryonic stages (e13.5-e14.5) UP-III is seen at the dome and dorsal aspect of the bladder (Figure 1A) but is absent in the distal bladder and ventral wall. A similar pattern is seen for SMAA. Newborn embryos (P0, figure1B), however, express UPIII throughout the bladder lumen (figure 1B). RNA in-situ analyses for the different uroplakin genes (Ia, Ib, II, IIIa, IIIB) reveals different mRNA expression patterns depending on the uroplakin subtype analyzed. Partial data to this effect is summarized in figure 2.
Conclusion: The results from both IHC and RNA in situ analysis examining uroplakin expression patterns suggests that the bladder develops from a cephalad to caudad and dorso to ventral direction.