Tip cell–enriched genes include proteases. (A) The proteases up-regulated in DLL4^+/− retinas by microarray analysis include uPAR, members of the ADAMTS family and Cathepsin S. Fold change in DLL4^+/− compared with wild-type is shown in red. (B) Whole mount ISH for plaur (blue) on retinas counterstained for isolectinB4 (green). Plaur shows preferential expression in tip cells (white arrows). Some stalk cells also express PLAUR (red arrows). (C-D) Immunohistochemistry confirms an enhanced expression of uPAR in tip cells (white arrows) and some uPAR positive stalk cells (red arrows). (E-G) Antibody staining shows that Caveolin-1 (blue) and Integrinβ1 (red) are also expressed by IsolectinB4 positive tip cells (green). The pictures represent 1 of 3 independent experiments with similar results. Scale bars: (B-D) 50 μm; (E-G) 5 μm. Images of antibody stainings were acquired on a Leica TCS SP5 confocal microscope using a 63×/1.4 NA objective. Images of ISH were acquired on an Olympus BX50 microscope using a 20×/0.05 NA objective and captured using a Coolsnap camera (Roper) through IPLab software version 3.2.4 (BioVision Technologies). Vessels were visualized with IsolectinB4 directly conjugated to AlexaFluor488 and the ISH signal by bright-field microscopy. Single channels were converted to grayscale (ISH in blue) in Photoshop CS2 (Adobe Systems) and merged in ImageJ (National Institutes of Health).