Direct evidence for the commitment of hematopoietic stem cells to T, B and myeloid lineages in murine fetal liver

We established an experimental system in vitro to examine the developmental capacity of individual hematopoietic progenitors to generate T, B and myeloid (M) cells. By using this system we analyzed the process of lineage commitment of hematopoietic progenitors in murine fetal liver (FL). It is known that small numbers of B and M cells, in addition to T cells, are generated in a co-culture of hematopoietic progenitors and a deoxyguanosine-treated fetal thymus (FT) lobe. We tried to enhance the growth of B and M cells by the addition of IL-7, IL-3 and stem cell factor into the co-culture. This cytokine-supplemented FT organ culture was used to examine the developmental capacity of individual hematopoietic progenitors in FL. Single cells of lineage marker (Lin)^-c-kit⁺Sca-1⁺ (Sca-1⁺) and Lin^-c-kit⁺Sca-^1- (Sca-^1-) populations from the FL harvested at day 12 of gestation were cultured for 10 days, and the phenotypes of cells generated in each lobe were analyzed with a flow cytometer. All progenitors in the Sca-^1- population were shown to be committed to generate only T, B or M cells. On the other hand, multipotent progenitors, which are capable of generating T, B and M cells, as well as unipotent progenitors committed to the T, B or M lineage were found in the Sca-1⁺ population. Bipotent progenitors generating M and T cells and those generating M and B cells were also found in the Sca-1⁺ population, which probably represent progenitors in the process of commitment. However, no bipotent progenitors generating T and B cells were detected.