MC recruitment to tumor vessels depends on a putative gradient of PDGFB. (a) PDGFB, secreted by an EC tube, has higher concentration near the tube, as shown in the small graphs. (b) Normally, MCs, which express PDGFR-β, sense the PDGFB concentration gradient and move in to closely contact the ECs. (The basement membrane is not shown.) (c) If MCs do not express PDGFR-β, then they are not recruited. (d) Modifying PDGFB to remove its matrix retention eliminates the PDGFB concentration gradient and results in poor EC-MC contact. (e) If tumor cells express PDGFB, the PDGFB concentration in the tumor overwhelms the peri-EC gradient and EC-MC contact remains poor. (f) Perivascular cells expressing GFP under the control of the VEGF promoter surround vessels in the tumor interior. The image was obtained using a two-photon microscope. Orange, vessel lumens; green, MCs. Panel f is reproduced with permission from Nature Medicine (8).