The great Greek fabulist Aesop traditionally penned a tale known by its concluding moral: Slow and steady wins the race. It appears that this classic lesson may also apply to the transplantation of hematopoietic stem cells (HSCs).Classical hematopoietic cell transplantation (HCT) is done utilizing a mixture of cells obtained from either the bone marrow, mobilized peripheral blood, or the blood of an umbilical cord (UCB), of which only a very small fraction represents CD34~+ cells. More recently, application of CD34-selection technology and stringent T-cell depletion has allowed us to utilize haploidentical family members as HSC donors without overwhelming graft-versus-host disease (GVHD) [1]. However, while transplant clinicians continue to use CD34-positivity as a marker of "stem cells," biologists have made incredible progress at determining the hematopoietic hierarchy that makes up the CD34~+ fraction of cells. At the earliest step sit true HSCs (Lin-CD34~+CD38-CD90~+CD45RA~-) with long-term self-renewal properties, while the entire line of multipotent progenitors, as well as both common myeloid and common lymphoid progenitors retain CD34-positivity [2].
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