The cloacal outgrowth orchestrates co-development of the bladder and umbilical arteries

The urinary bladder is an innovation of eutherian mammals to manage liquid wastes. However, bladder development and its embryonic function remain poorly understood. Here, we report that in both humans and mice the bladder emerges from the cloaca as the rostral outgrowth, which is flanked by the umbilical arteries. Arrest of the outgrowth, as seen in Shh-null mouse embryos, causes bladder agenesis and unexpected severe defect of the umbilical arteries, suggesting that the outgrowth epithelial signal Shh coordinates development of both the bladder and umbilical arteries. Wnt2 signaling molecule is restricted to the rostral outgrowth mesenchyme; and its expression is dramatically down-regulated in Shh-null mutants. Furthermore, Wnt2-null mutants develop small bladder and single umbilical artery defects. Findings here uncover an unexpected co-development mechanism of the bladder and umbilical arteries orchestrated by the cloacal outgrowth signals. The co-development paradigm offers a conceptual framework to understand evolution and development of the bladder. Significance StatementThe urinary bladder is found in the eutherian mammals but largely absent from other vertebrates. Innovation of the bladder has been viewed as a necessity to manage liquid metabolic wastes on dry land. In this study, we report that the bladder emerges from the rostral outgrowth of the cloaca. We also report an unexpected developmental link between the bladder and umbilical arteries and, demonstrate that their co-development is orchestrated by the outgrowth epithelial signal Shh and mesenchymal signal Wnt2. Findings here highlight a possibility that, in addition to managing metabolic wastes, normal bladder development is required for growth of mammalian embryos by supporting development of the umbilical arteries.