THE PURIFICATION AND CHARACTERIZATION OF FETAL LIVER HEMATOPOIETIC STEM-CELLS

Thy-1(lo)Sca-1(+)Lin(-)Mac-1(+)CD4(-) cells have been isolated from th e livers of C57BL-Thy-1.1 fetuses. This population appears to be an es sentially pure population of hematopoietic stem cells (HSC), in that i njection of only six cells into lethally irradiated adult recipients y ields a limit dilution frequency of donor cell-reconstituted mice. Six ty-seven to 77% of clones in this population exhibit long-term multili neage progenitor activity. This population appears to include all long -term multilineage reconstituting progenitors in the fetal liver. A hi gh proportion of cells are in cycle, and the absolute number of cells in this population doubles daily in the fetal liver until 14.5 days po stcoitum. At 15.5 days post-coitum, the frequency of this population f alls dramatically. Long-term reconstituting HSC clones from the fetal liver give rise to higher levels of reconstitution in lethally irradia ted mice than long-term reconstituting HSC from the bone marrow. The p recise phenotypic and functional characteristics of HSC vary according to tissue and time during ontogeny.