Tissue distribution of human CFU-C transplanted into sheep fetuses (vertical axis reflects the mean CFU-C recovered/tissue as percent of CFU-C recovered in the 3 tissues tested). Left panel shows mean data from animals that received anti–VLA-4 for the first 4 days posttransplantation. Note the persistence of human cells in circulation and the low levels in BM. Higher levels in liver likely reflect the increased blood volume of this tissue; right panel shows data from fetuses that received anti–VLA-4 later, between days 21 to 24. Note the release of human cells from BM to circulation (*data are from 1 fetus; see also Table 2).