In Vivo Fate Tracing Studies Using the SCL Stem Cell Enhancer: Embryonic Hematopoietic Stem Cells Significantly Contribute to Adult Hematopoiesis.

Evidence for the direct lineage relationship between embryonic and adult hematopoietic stem cells (HSCs) in the mouse is primarily indirect. In order to study this relationship in a direct manner we expressed the tamoxifen-inducible Cre-ER^T-recombinase under the control of the SCL-stem-cell-enhancer in transgenic mice (HSC-SCL-Cre-ER^T). To determine functionality, HSC-SCL-Cre-ER^T transgenics were bred with the Cre-reporter-mice ROSA26R and R26R-EYFP. Flow-cytometric and transplantation studies revealed tamoxifen-dependent recombination occurring in more than 90% of adult long-term HSCs, whereas the targeted proportion within mature progenitor populations was significantly lower. Moreover, the transgene was able to irreversibly tag embryonic HSCs on days 10 and 11 of gestation. These cells contributed to bone marrow hematopoiesis five months later. In order to investigate whether the de novo HSC-generation is completed during embryogenesis, HSC-SCL-Cre-ER^T marked fetal liver cells were transplanted into adult recipients. Strikingly, the proportion of marked cells within the transplanted and the in vivo-remaining HSC-compartment was not different, implying that no further HSC-generation occurred during late fetal and neonatal stages of development. These data demonstrate for the first time the direct lineage relationship between mid-gestation embryonic and adult HSCs in the mouse. Additionally, the HSC-SCL-Cre-ER^T mice will provide a valuable tool to achieve temporally controlled genetic manipulation of HSCs.