By day 2 from the time of plating, each CFU-E colony will contain between 8 (minimum) and 64 hemoglobinized cells most of which are in their end-stage of erythroid differentiation. It is possible to see a small spectrum of hemoglobinization level and possibly cell size, indicating that some cells in the colony have achieved the end-stage faster than others. Cell number in a colony is important because pro-erythroblast stage is also Epo-responsive (expresses Epo receptor), but the proliferative capacity of these cells is not as high, thus yielding a colony with fewer than 8 cells. Likewise, an earlier stage of erythroid differentiation may also yield colonies in Epo-only medium, but these colonies would likely be smaller and/or not hemoglobinized, since the stages before the CFU-E stage (MEP and BFU-E) require other factors (IL-3 etc) and more time for growth that will also delay the terminal differentiation and hemoglobinization.