Human CD34⁺ progenitor cells home to peripheral tumor vasculature. (A) Nude mice with GFP-N202 breast carcinomas under dorsal skinfold transparent chambers are injected with labeled progenitor cells. (B) Real-time video microscopy: GFP tumors in transparent chambers. Magnification, ×10. (C) Tumor vasculature is visible in the transparent chambers. Magnification, ×100. (D) Two images each of peripheral and central tumor vascular beds at 15 minutes after injection in animals injected with CMTMR-labeled CD34⁺ cells (arrowheads). Magnification, ×200. Blood vessels are observed as dark channels (arrows). (E) Average number of CMTMR CD34⁺ cells in the tumor center, tumor periphery, and fat-pad per ×200 field ± SEM. *P < 0.0019. (F) Five micron cryosections of the same tumors from animals injected with CD34⁺ cells were fixed in acetone, then incubated with rat anti-mouse CD31 and goat anti-rat FITC antibodies and photographed. Magnification, ×200. Arrowheads indicate CD34⁺ cells. Arrows indicate CD31⁺ blood vessels. Scale bar: 50 μm. (G) Average number of CD34⁺ cells per ×200 microscopic field in 5 μm cryosections. **P < 0.015. (H) Cryosections from F observed at ×630 magnification indicate CD34⁺ cells (arrows) are found adhering to the blood vessel wall or within tissues near blood vessels (arrowheads).