Adherence and aggregation of platelets occurs in tumor vessels but not in normal vessels after treatment with rapamycin. (A) The figure shows the total area of platelet aggregates related to tumor tissue area within a 30-second observation period, as an indicator for an ongoing coagulation process. Platelet aggregates were quantified in 6 to 8 random areas within the tumor (rapamycin R1-R4, ○; control C1-C3, ▵; n = 3-4) or the peritumoral tissue after 6 days of treatment. (B-I) Representative photomicrographs of the dorsal skinfold chambers are shown to illustrate the platelet endothelial interaction in the different groups. (B) In control animals platelets do not stick to tumor vessels; asterisk/arrow indicates flowing platelets. (C) In the peritumoral tissue of rapamycin-treated hamsters no plugging of vessels was observed. (D-G) Thrombus formation in the dorsal skinfold chamber tumors of rapamycin-treated hamsters are shown by sequential photomicrographs; +/arrow indicates adherent platelets (H-I). The overlay of identical sequences in trans- and fluorescent epi-illumination technique shows that thrombus formation is responsible for the observed loss of FCD in tumors. Total magnification for panels B-1, 630×, 20× Leitz Wetzlar long-distance objective lens, numerical aperture 0.32. Images were acquired with Image J software.