Human erythroid progenitors require Epo, SCF plus Dex for long-term expansion. (A) After precultivation for 18 days (in the absence of lipids), cumulative cell numbers (Figure 2) were determined from cultures kept for 8 more days in either the absence of Epo (▪), or SCF (○), or Dex plus glucocorticoid antagonist ZK (•), or without any factors (▵) by daily counting in an electronic cell counter. Control (□) cultures grown in the presence of all factors. Cross symbol indicates termination of cultures due to excessive cell death. (B) Erythroblasts from cultures in panel A were analyzed for the percentage of apoptotic cells by TUNEL staining and flow cytometry (see “Materials and methods”); symbols are as described for panel A.