Developmental changes of CD34 expression by murine hematopoietic stem cells.

It has been reported that fetal murine hematopoietic stem cells are CD34(+), whereas adult stem cells are CD34(-). We sought to delineate the developmental changes of CD34 expression by hematopoietic stem cells and carried out systematic analysis of long-term engrafting cells in the bone marrow and/or blood of perinatal, juvenile, and adult mice. To obtain information on the total population of stem cells, we prepared CD34(+) and CD34(-) populations of mononuclear cells without prior enrichment and assayed their long-term reconstituting abilities by transplantation into lethally irradiated Ly-5 congenic mice. 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. Approximately 20% of adult stem cells expressed CD34. Developmental changes of CD34 expression from the positive to the negative state takes place between 7 and 10 weeks of age for the majority of murine stem cells. Approximately 20% of adult stem cells remain CD34(+). These observations provide insight into the current controversy regarding CD34 expression by adult hematopoietic stem cells and suggest that the majority of stem cells in human umbilical cord blood and bone marrow of young children are CD34(+).