Increase in surface area of Tie2-positive vessels after myelosuppression precedes increase in platelet levels. LacZ/NFR-stained BM sections were obtained at different time points after injection of 5-FU as described. The LacZ-positive BM surface area was measured by quantification of the blue chroma pixels compared with the total number of pixels in the digital image. The surface area demarcated by LacZ-positive vessels was normalized to the percentage of total BM hematopoietic surface area. At steady state, there was a minimal number of LacZ-positive cells (0.02% of total pixels were blue). At 10 days after myelosuppression, Tie2 expression was found to be maximal (6.8% of depicted BM surface area pixels were blue), which was followed closely by large increases in peripheral platelet counts (n = 4). All results are shown as mean ± SEM (n = 6, P < .01 at day 10).