ICAM-2 mediates angiogenesis in vitro: tube formation on Matrigel. (A) Primary murine cardiac endothelial cells (MCECs) isolated from control (WT) or ICAM-2^–/– mice were seeded on Matrigel. In each experiment, 6 age- and sex-matched mice/group were used. The figure shows 1 representative of 3 separate experiments, each performed in triplicate. Left panels show WT; right panels, ICAM-2^–/–. In WT, cells were spreading, elongating, and making contacts with neighboring cells at 3 hours, and an interconnecting tube network was formed over 4 to 5 hours. At all time points studied, the branches and tubes formed by ICAM-2^–/– cells were much less developed compared with control. Scale bar equals 100 μm. Images were acquired using a Leitz Labovert inverted microscope (Leica Microsystems, Milton Keynes, United Kingdom) fitted with a 10 × phase-contrast objective lens (Leitz-Phaco 10 ×/0.25 NA). Images were captured with a digital camera model DP50-CU (Olympus) using Viewfinder Lite (v. 1) software (Olympus). Image processing was carried out using Adobe Photoshop CS (Adobe Systems). (B) Branches from each cell were counted from 3 representative 100 × field/well. Error bars indicate mean ± SEM. **P < .01; ***P < .001 analysis of variance (ANOVA).