(A) Tamoxifen intragastric (i.g.) administration and analysis scheme. (B–D) The deletion efficiency of Tie2 was examined by Western blot analysis (B) and immunostaining (C, D) for PECAM-1 (green) and Tie2 (red) of tissues from Tek deleted and littermate control mice at P7. Note that Tie2 expression is downregulated in tip endothelial cells (asterisk in C and D) and also low in newly formed retinal arteries (arrowhead in C and D) when compared with veins (arrow in C and D). (E, F) Quantification of the vascularization index (ratio of vascularized area to total retina area normalized against the littermate controls) after ubiquitous (E, Tek−/iUCKO: 61.95 ± 10.79, n = 12; Control: 100.0 ± 10.94, n = 12; p<0.0001) or EC-specific deletion (F, Tek-/iECKO: 80.69 ± 11.13, n = 5; Control: 100.0 ± 3.77, n = 7; p<0.0015). (G–J) Visualization of retinal blood vessels (G, H, P7) by immunostaining for PECAM-1 (green) and EphB4 (red). Arrows point to veins (v) and arrowheads to arteries (a). Quantification of blood vessel density in the distal retinal venous (I, X 104 μm2/grid; Tek−/iUCKO: 16.91 ± 0.77, n = 6; Control: 14.07 ± 1.54, n = 8; p=0.0014) and arterial segments (J, X 104 μm2/grid; Tek−/iUCKO: 10.45 ± 2.47, n = 6; Control: 10.16 ± 0.76, n = 8; p=0.7571) in Tek mutants compared with control mice. (K, L) Analysis of Dok-2 and ERK1/2 phosphorylation. Total Dok-2, ERK1/2 and beta-actin were used as loading controls. The bands were quantified and normalized against the control group (L). Scale bar: 200 μm in C, D, G; 100 μm in H.