• CD34;
  • Hematopoietic stem cells;
  • Developmental changes

Abstract: For more than a decade it was believed that hematopoietic stem cells express CD34. However, this dogma was recently challenged by the observation that stem cells of normal adult mice are CD34. In order to clarify the controversy, we carried out systematic examination of stem cells by using C57BL/6 mice that are congenic for Ly-5. As reported previously, stem cells in the normal adult mice were CD34. However, stem cells stimulated in vivo by 5-flourouracil injection or in vitro by a combination of interleukin-11 and steel factor were CD34+. The activated CD34+ stem cells reverted to CD34 when the recipients' marrow achieved steady state. The majority of G-CSF-mobilized stem cells also were CD34+ and reverted to CD34 under steady-state conditions. Most recently, we examined the developmental changes of stem cell CD34 expression. In order to gain information on the total population of stem cells we prepared CD34+ and CD34 populations of mononuclear cells without prior enrichment and studied their engrafting potentials. All stem cells from perinatal to 5-week-old mice were CD34+. In 7-week-old mice CD34 stem cells began to emerge, and the majority of the stem cells were CD34 in the 10- and 20-week-old mice. An estimated 20% of the adult stem cells expressed CD34. These observations provide insight into the current controversy regarding CD34 expression by adult hematopoietic stem cells.