Long-term coculturing with platelets induces EVT transformation. Human EVTs isolated at 6 weeks of gestation were cultured for 48 hours in the presence of human PBMCs or platelets that were plated in the collagen type I–coated upper chamber (A). Scale bars indicate 50 μm. Original magnification × 240. At the end of the coculture, EVTs were trypsinized and the cell surface expression of integrin α1 (B) and integrin α5 (C) was examined by flow cytometry. The EVTs became round (A, right), and their integrin α1 expression was enhanced in the presence of platelets (B). The filled curves are control mAb. These morphologic changes were also confirmed by length-width ratio (D). *P < .05; **P < .01.