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The doubling time of hematopoietic cells is very dependent on the cell source, culture conditions and the stage of differentiation. CD34+ cells from cord blood tend to proliferate faster than those isolated from bone marrow. True HSCs (typically <1% of the CD34+ cells) are quiescent and will have a lag period that can last 1 or 2 days before they undergo their first division, doubling every 24 hours or so after that, in optimal culture conditions. Erythroid progenitors can double even faster. It depends on your culture conditions as to whether it is possible to decrease the doubling time of your cells. You could try adding more or different cytokines, but be aware that most culture conditions that strongly promote the proliferation of HSCs will also induce their differentiation.