| Summary: | To characterize the extent and timing of changes in hematopoietic stem cell (HSC) properties during ontogeny, experimental strategies were developed to allow quantitative assessment of their proliferative activity, self-renewal potential and differentiation behaviour in vivo. All HSCs in the fetal liver [i.e. foetal liver] were found to be cycling and following their transplantation into irradiated adult hosts, they rapidly generated daughter HSCs and produced large numbers of granulopoietic progeny. In contrast, adult HSCs, which are predominantly quiescent, regenerated new HSCs more slowly and produced fewer granulopoietic progeny. They also showed a coordinated change in expression of several transcription factors that regulate HSC functions. Interestingly, HSCs retained a fetal phenotype with respect to all these features until 3 weeks after birth and then, within one week, acquired an adult HSC phenotype. Additional studies of serially transplanted HSCs indicated that this switch also took place within the same time frame in adult mice reconstituted with fetal or 3-week post-natal HSCs, suggesting the switch is intrinsically programmed. To further investigate the mechanism of this switch, an in vitro model suitable for monitoring the survival, proliferation and self-renewal activity of highly purified fetal liver HSCs was developed. Using this model, I found that the cell cycle transit time of optimally stimulated fetal HSCs and adult HSCs is the same, but with lower Steel factor requirements for fetal HSCs. This suggested that the fetal-to-adult switch involves a decreased response to c-Kit activation. Interestingly, the self-renewal behaviour of fetal HSCs expressing a defective form of c-Kit mimicked adult +/+ HSCs, both in vitro and in vivo, but showed no difference in cycling activity, suggesting that Steel factor responsiveness specifically regulates HSC self-renewal responsiveness in vivo. Future studies of changes in gene expression during the switch, including analyses of c-Kit-defective HSCs as well as normal HSCs, may help to link the observed changes in Steel factor responsiveness to the molecular mechanisms that control changes in HSC self-renewal and cycling control during ontogeny. === Medicine, Faculty of === Medical Genetics, Department of === Graduate
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