Fig. 4. Ang-2-mediated endothelial-cell-monolayer destabilization in explanted fragments of human umbilical vein. Fragments of umbilical vein were cultured overnight and exposed to different combinations of cytokines for 4 hours. Samples were then fixed, embedded and processed for CD34 immunohistochemistry. Control umbilical-cord fragments have an intact CD34-positive surface monolayer (A). Treatment with exogenous Ang-2 leads to vascular destabilization, as evidenced by detachment of the endothelial cells (B, higher magnification in D). Ang-1 (C), sTie-2 (E) and VEGF (F) are all able to rescue vascular destabilization elicited by exogenous Ang-2.