Secondary tumors contain slowly dividing CD44+/Flk-1+ cells. (a) Cryosection of secondary tumor nodule in intestine (differential contrast). The white box shows an area that is further characterized in panel b. Scale bar = 100 μm. (b) Cryosection as in panel a used for immunocytochemistry. Note that Vybrant CM DiI-labeled cells (left panel [red]) express CD44 (right panel [red]) and Flk-1 (right panel [green]). Scale bar = 20 μm. (c) Another area of the secondary tumor model at higher magnification showing co-distribution of Vybrant-CM DiI and CD44. Preservation of Vybrant CM-DiI staining indicates slow cell division. Scale bar = 5 μm.