In vitro cultivation of sorted epithelial cells in Matrigel was done as previously described [38] with minor modifications. Selenium (Gibco, 51300, 1:100), Pen Strep (Gibco, 15070, 1:100), amphotericin B (Gibco, 15290, 1:1000) and 10 mM SB431542 (Sigma-Aldrich, S4317). Culture medium was changed every other day until day 8. SB431542 was then removed and the cultures were maintained for an additional four days before fixing overnight at 4uC with 4% PFA in PBS. The Matrigel was removed by freezing-thawing and rinsing with fresh PBS. The content of the Transwell inserts was embedded in OCT (Shandon Cryomatrix) and 10 mm sections were prepared by cryostat (Leica CM1950).