Rosa26-Tomato females were crossed with Hoxb7Cre;EF1aCrb3-GFP/+ males and kidneys were dissected out at E13.5 (Madisen et al., 2010 (link)). Kidneys were plated on a thin Matrigel layer (1:4 in medium; BD Biosciences, Franklin Lakes, NJ) in Lab-Tek Chambered #1.0 Borosilicate Coverglass System dishes (Nalge Nunc International, Rochester, NY). Additional medium was added to surround half of the kidney, allowing growth at the air/media interface. Four kidneys were cultured and two of them were imaged at 20× magnification. Pictures were taken every 20 min for 20 h using a Nikon A1 laser scanning confocal microscope with a Tokai Hit stage Top incubator (Tokai Hit Co., Shizuoka, Japan).