Stem and progenitor activity of PU.1^–/– fetal liver cells. (A) Morphology of E14.5 fetal liver cells of PU.1^+/+ and PU.1^–/– embryos (May-Giemsa staining, 600×). (B) The numbers of specific types of colonies derived from purified HSCs (left columns) and MEPs (right columns) from PU.1^+/+ and PU.1^–/– fetal livers. Note that in PU.1^–/– cultures, there were no mature granulocytic and monocytic components that were replaced by immature myeloblastic cells. (C) Analysis of reconstitution activity of PU.1^–/– fetal liver HSCs. Twelve weeks after injection of high doses (1000 cells) of PU.1^–/– HSCs (Ly5.2⁺) into Ly5.1⁺ congenic lethally irradiated hosts, a minor population of donor-derived cells (Ly5.2⁺) of HSC phenotype (Lin^–Sca-1⁺c-Kit⁺) was detected. Control experiments using 100 PU.1^–/+ fetal liver HSCs (Ly5.2⁺) as a donor demonstrated that nearly all of the recipient bone marrow cells were of donor origin. (D) Colony assay of purified secondary PU.1^–/– HSCs. They displayed colony-forming activity almost equal to primary PU.1^–/– HSCs (B). (E) A mixed colony derived from single secondary PU.1^–/– HSC expressed the Ly5.2 donor maker, and contained erythroblasts and megakaryocytes and immature blastic myelomonocytic cells but not mature granulocytes or macrophages (May-Giemsa staining, 1000×).