Immunohistochemical analysis of Sonic hedgehog signalling in normal human urinary tract development

Studies of mouse mutants have demonstrated that Sonic hedgehog (SHH) signalling has a functional role in morphogenesis and differentiation at multiple sites within the forming urinary tract, and urinary tract malformations have been reported in humans with mutations that disrupt SHH signalling. However, there is only strikingly sparse and fragmentary information about the expression of SHH and associated signalling genes in normal human urinary tract development. We used immunohistochemistry to demonstrate that SHH protein was localised in distinct urinary tract epithelia in developing normal humans, in the urothelium of the nascent bladder and in kidney medullary collecting ducts. The expression patterns of the SHH-transducing proteins Patched (PTCH) and Smoothened (SMO) were consistent with long-range paracrine signalling associated with detrusor smooth muscle differentiation in the urogenital sinus. In the developing kidney, SHH and PTCH were expressed in epithelia of the collecting system between 16-26 weeks--surprisingly, SMO was not detected. Analysis of cell proliferation and Cyclin B1 immunohistochemistry at 26 weeks, as compared with a 28 week sample in which SHH expression was down-regulated, was consistent with the idea that SHH and PTCH might influence medullary collecting duct growth by regulating the subcellular localisation of Cyclin B1 independently of SMO. Collectively, these descriptive results generate new hypotheses regarding SHH signal transduction in human urinary tract development and help to explain the varied urinary tract malformation phenotypes noted in individuals with mutations in the SHH pathway.