Role of Flk-1 in mouse hematopoietic stem cells.

It was reported that human hematopoietic stem cells in bone marrow were restricted to the CD34(+)KDR(+) cell fraction. We found that expression levels of Flk-1, a mouse homologue of KDR, were low or undetectable in mouse Lin(-)c-Kit(+)Sca-1(+)CD34(low/-) cells as well as Hoechst33342(-) cells (side population), which have long-term reconstitution capacity. Furthermore, neither Flk-1(+)CD34(low/-) cells nor Flk-1(+)CD34(+) cells had long-term reconstitution capacity in mouse. Taken together with other observations using Flk-1-deficient mice, these results indicate that Flk-1 is essential for the development of hematopoietic stem cells in embryo but not for the function of hematopoietic stem cells in adult mouse bone marrow.