Long-term expansion of human erythroid progenitors. (A) Cell size and proliferation kinetics were determined in the presence (from day 0; = standard medium composition for all other experiments if not stated otherwise) or absence of cholesterol-rich lipids by daily measurements in an electronic cell counter. For clarity, values are shown for every third day only; for the full data set, see supplemental Figure S1. Cumulative cell numbers were calculated as described in “Materials and methods”; error bars indicate SD of mean, n = 5. Cultures without lipid were terminated at day 30 due to excessive cell death (cross symbol). Insets on the right show size distribution of cultures grown with or without lipids; a narrow size range corresponds to healthy cultures. Data are also shown in Table 1. (B) DNA content of cells (+ lipid) was monitored by flow cytometry after DAPI staining and the percentage of cells in particular cell cycle phases determined as described in “Materials and methods.”