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Fetal liver (FL) becomes a major organ of hematopoiesis at mouse embryonic day (E) 11 and E12, when definitive hematopoietic stem cells, originating from the aorta-gonads-mesonephros region, colonize the hepatic tissue. Unipotent B-cell progenitors are very rare in FL by day 12, whereas erythropoiesis prevails. We have studied hematopoiesis in FL from different gestational ages, with special emphasis on B lymphopoiesis. The mRNA levels of selected liver-specific genes, hematopoietic lineage-specific genes, and genes for selected cytokines/hormones as well as for their receptors were evaluated by real-time polymerase chain reaction in FL from E12.5, E14.5, and E17.5, adult liver and adult bone marrow (BM). The level of B lineage-related gene expression in FL was very low at E12.5. There was also a significantly lower fraction of B220+ and CD19+ B cells in E12.5 FL compared with E17.5 FL. To analyze whether these differences reflect different stem cell potentials occurring during FL development, 10(6) or 5 x 10(6) of FL cells collected from embryos at E12.5 or E17.5 and those from adult BM were transplanted into sublethally irradiated (3- or 6-Gy) congenic mice. Short-term and long-term repopulation of B and T cells and granulocyte/macrophage lineages from donor FL or adult BM cells were evaluated in competition to adult hematopoiesis of sublethally irradiated recipients. In short-term repopulation, the transplantation of E12.5 FL cells resulted in a lower blood chimerism compared with that of E17.5 FL cells. However, the proportion of B lymphopoiesis exerted by E12.5 FL cells was not different from that of E17.5 FL or adult BM. This study demonstrates that E12.5 FL contains hematopoietic stem cells with fully developed B-cell repopulating capacity and that the developmental period of fetal hematopoiesis between E12.5 and E17.5 is not an obligatory phase for the adult B lymphopoiesis.

Murine hematopoiesis has been analyzed by many authors, and available data allow for quantitative evaluation of this dynamic process. In this study, the capacity of several populations of the bone marrow clonogenic cells (progenitors) to produce blood cells was compared with their actual production. The cell cycle progression rate was directly measured in the following types of hematopoietic progenitors: day 8 colony-forming units-spleen, GM-colony-forming cells, BFU-E, and CFU-E in normal mice. The cell cycle progression rates of the individual progenitors, together with their numbers in the whole hematopoietic tissue, were used to calculate the absolute numbers produced daily in each population. The data reviewed from literature were analyzed in parallel. The capacity of the progenitors to produce mature blood cells were derived from the daily production of progenitors multiplied by their clonogenic potential. This theoretical capacity to produce blood cells was compared to the actual blood cell production determined from the turnover of circulating blood elements. The comparison strongly suggested an intensive cell death rate occurring at the early stages of differentiation and its decline as the hematopoietic cells become more differentiated and mature.

Hematopoietic cells, detected as spleen colony forming units (CFU-S), vary in their proliferation rate, and this can be investigated in vivo by determining the fraction of CFU-S synthesizing DNA. A large number of measurements of CFU-S number and the fraction synthesizing DNA has been obtained under standardized conditions in normal mice and after a single dose of arabinosyl cytosine (ara-C). These data are analyzed with respect to the correlation between the number of CFU-S and their DNA-synthesizing fraction since, in the past, it has been maintained that the CFU-S proliferation rate responds sensitively to changes in CFU-S numbers. The present analysis does not support this traditional view--it instead demonstrates that natural variations in CFU-S numbers occurring in the same range as those following ara-C do not trigger CFU-S proliferation. On the other hand, administration of ara-C triggers most of the surviving CFU-S into the cell cycle, while decreasing CFU-S numbers by only about 40%. Data are also presented showing changes of CFU-S numbers and their DNA-synthesizing fraction in mice given a single dose of cyclophosphamide (CY). CY reduces CFU-S numbers, but the fraction synthesizing DNA can be either increased or decreased depending on time after treatment. Both these experimental situations argue against a close relationship between CFU-S numbers and their proliferation rate and suggest a different or a more complex regulation.