For frozen sections the tissue was cryoproserved in Optimal Cutting Temperature (OCT) compound (VWR Chemicals, Lutterworth, UK #361603E) and sectioned at 5 μm thickness. Horizontal whole mounts were prepared as described previously (Driskell et al., 2012 (link), 2013 (link)). The following antibodies were used: Prom1 (1:50) (eBioscience, Hatfield, Ireland, UK, #14-1331-82), Keratin 14 (1:1000) (Covance, Cambridge, UK, #PRB-155P), Corin (1:100) (R&D systems, Minneapolis, MN, #AF2209), β-catenin (1:500) (Cell Signalling, Buckingham, UK, #8814S) and Ki67 (1:200) (Dako, Ely, UK, #M7249 clone TEC-3). All microscopy was performed on a Leica SP5 or Nikon A1 confocal microscope and images were analyzed using Image J (NIH).