Vascular Tie2 expression is increased in a time-dependent manner after myelosuppression.Tie2-LacZ mice received a single tail-vein injection of 250 mg/kg 5-FU. Groups of mice were killed before the injection and 4, 7, 10, 14, and 24 days thereafter. Femurs were collected and prepared for histologic analysis by an X-Gal staining procedure. Paraffin sections were counterstained with NFR. Representative LacZ-NFR–stained sections at steady state and after 4, 7, 10, 14, and 24 days are shown. Regenerating neovessels consist of LacZ-positive endothelial cells, which are easily identifiable by their blue stain (yellow arrows). On day 4 after injection, these intensely LacZ-stained neovessels emerge at different sizes and shapes throughout the reconstituting BM (n = 15). The × 400 magnification clearly demonstrates that perivascular hematopoietic cells are not stained, but that LacZ positivity is limited to endothelial cells and their surroundings due to limited diffusion of the blue dye. By day 24, the only LacZ-expressing cells in the femur can be found in the epiphyseal region, consisting of few spindle-shaped endosteal cells (black arrows).