Time course of CA formation on AFT024. The formation of stromal-dependent CA derived from purified fetal liver stem cells was studied in AFT024 cocultures. Characteristic clusters of ≥50 cells were scored as CA over 28 days of culture. Results are expressed as the mean number of CAs/1,000 input stem cells from 3 separate fetal liver purifications (300 to 600 cells/well in 12-well trays). Error bars represent the SEM. The frequency of CA after 28 days is approximately 1 for every 20 input stem cells.