Mammalian kidneys contain a cortex where blood filtration occurs and a medullary region where elongated tubular epithelia concentrate the urine. This organisation is crucial for renal function, but what regulates the formation of the cortico-medullary axis? One key regulator in the developing mouse kidney, suggest Jing Yu, Andrew McMahon and colleagues, is Wnt7b (see ). In the absence of Wnt7b, they report, cortical epithelial development is normal but the medullary zone fails to form and urine is not concentrated normally. Their analysis of cell division planes in the collecting duct epithelium of the emerging medullary zone in normal and Wnt7b mutant mice reveals that Wnt7b regulation of the cell cleavage plane contributes to the establishment of a cortico-medullary axis. Finally, they show that Wnt7b mediates axis establishment by activating the canonical Wnt signalling pathway in the interstitial mesenchyme. Together, these results indicate that Wnt7b plays a pivotal role in the development of the tissue architecture that is required for normal kidney function.
