Endodermal buds and distal lung mesenchyme were isolated from E12.5 lungs as previously described [35, 36] . Briefly, isolated distal lung tips were treated for approximately 5 min with Trypsin 0.5% Pancreatin 2.5% in HBSS 16 (Gibco, 14170) on ice. Mesenchyme was removed using 26G needles and the epithelial buds as well as the isolated mesenchyme were placed in a (1:1) mixture of Growth Factor Reduced Matrigel (BD Biosciences, 356230) and DMEM: F12 (GiBCO, 11039) 10% FCS (Gibco, 10500), Pen Strep (Gibco, 15070, 1:100) and incubated for 8 hours at 37uC, 5% CO 2 .