Murine vessels were functional in xenografted tumors. (A, B) Paraffin sections of first passage xenografts of the mesothelioma (A) and colon (B) tumors labeled for msCD34 show that murine vessels often contained red blood cells (arrows). (C-F) A patient head and neck tumor was engrafted into a cohort of mice and the growth of xenografts was monitored beginning 19 days after implantation when tumors became palpable. Functional analysis of vessels was carried out on tumors of different sizes. Animals were injected with Dylight-594 labeled tomato lectin (red) and frozen sections were prepared. Representative sections stained for msCD31 visualized with a Dylite 488 (green) secondary antibody are shown: volumes were (C) 19 mm3, (D) 75 mm3, (E) 190 mm3 and (F) 600 mm3. In each case, msCD31(+) vessels (green arrows) were seen in the tumors, both centrally and in the periphery. Perfused vessels which were msCD31(+) were seen in the periphery of tumors (D-F; yellow arrows). Non-perfused vessels were also observed in the central portions of these tumors. (bars = 100 μ).