PECAM/flk-1 double-label immunofluorescent antibody staining of VEGF mutant ES cultures.

Day 5 (A-C) or day 6 (D-F) null (−/−) VEGF mutant cultures were fixed and stained with rat antimouse PECAM (green) and rabbit antimouse flk-1 (red) antibodies, along with the appropriate secondary antibodies. Images were photographed with the appropriate FITC (C, F) or rhodamine (B, E) filters or with a double exposure using both filters sequentially (A, D). The arrows in A to C point to a clump of PECAM+ cells that partially stained for flk-1. The structure in the upper left is patent vasculature. Arrowheads in D to F point to clumps of PECAM+ cells that had no detectable staining for flk-1. The structure to the left is patent vasculature. Magnification, 50×.