To validate the functional significance of BMP signaling for the development of the vascular network of the developing lung, ex-vivo cultures of E12 lung explants were treated with the respective BMP-or TGFb/Activin-receptor inhibitors LDN193189 and SB431542. Treatment of lung explants for 24 hours with SB431542 led to a substantial increase in eGFP, Id1, and FGF10 mRNA expression, whereas treatment with LDN193189 led to a decrease ineGFP, Id1 and FGF10 mRNA expression and an increase, probably compensatory, in BMP4 and VEGFa mRNA expression ( Figure 3B and D). Remarkably, even 8 hours of treatment with the SB431542 led to an increase in the density of the CD31 pos network and a more pronounced increase in the eGFP pos network, whereas, similar treatment with LDN193189 led to reduction in the density and connectivity of the eGFP pos and CD31 pos cells ( Figure 3C) .