Hematopoietic stem cells can be CD34+ or CD34-

Until recently, it was thought that the most primitive HSC have a fixed phe notype within a hierarchical differentiation system, and that changes in en graftment and renewal potential occur in a stepwise fashion linked with dif ferentiation. In this review. we summarize the data from several different species and different animal models of hematopoietic stern cell function. T aking into account all of the published data, it becomes clear that the hem atopoietic stem cell compartment contains more than one phenotypically iden tifiable population capable of self-renewal and long term pluripotent engra ftment. It is clear that some stem cells express CD34, and others do not. T he exact phenotypic progression between these cells needs to be further def ined, because different in vivo and ex vivo manipulations may shift the ste m cells from one phenotype to another. and this can complicate interpretati on of experimental transplant data.