Fig. 9. Recruitment of VW-EPC by tumor cells. HITA rings cocultured with cells of the human prostate cancer cell line DU-145 (A,B) show an extensive capillary outgrowth (arrows) into the lumen (A) and outside the rings (B) after 24 hours in culture. By contrast, no sprouts are seen in HITA rings cultured without tumor cells (C,D). Tumor cells (arrowheads) remain rounded and are not involved in the capillary outgrowth. Immunohistochemical studies on paraffin wax sections of several human organs show that CD34⁺ cells (arrows) are present in the wall of large and mid-sized arteries (E) and veins (F), as demonstrated here exemplarily on sections of human prostate. By contrast, CD31 immunostaining is only detectable in mature endothelial cells lining the lumina of arteries (G) and veins (H).