Transplantation of Stem Cells from Bone Marrow and Blood

Abstract

Hematopoietic stem cells have two important functions: they give rise to all blood cell lineages and they recapitulate themselves. These processes are thought to occur through asymmetric division, in which one daughter cell is identical to the parent and the other daughter is committed to further differentiation into mature blood cells. True hematopoietic stem cells occur rarely in the bone marrow, they divide infrequently, and the forces that regulate their proliferation and differentiation are unknown. Stem cells are identified only with difficulty; they are best defined by their expression of the CD34 antigen and their failure to express any protein that is present on cells committed to develop into a specific cell lineage. Although all stem cells express CD34, only a small fraction of CD34⁺ cells are pluripotent stem cells. The majority of CD34⁺ cells are progenitors that have already been lineage committed.