Developmental programming of hematopoietic stem cell dormancy by evasion of Notch signaling

Dormant hematopoietic stem cells (HSCs) are a rare subset of deeply quiescent, metabolically inactive cells that preserve lifelong hematopoietic regeneration. In mice, these cells were thought to arise exclusively after birth in the BM, while fetal liver HSCs were considered uniformly proliferative and rapidly expanding. Here, using H2B-GFP label-retention strategies, we identify a previously unrecognized population of low-dividing HSCs that is specified during embryogenesis and persists into adulthood. We show that this dormant state is established in specific fetal HSCs that evade the activation of Notch signaling within the fetal liver niche, rather than through global loss of pathway activity. Genetic and transient perturbation of Notch signaling reveals a narrow developmental window in during which reduced Notch exposure promotes entry into a durable dormancy program, whereas irreversible pathway disruption compromises long-term function. Notably, transient Notch during fetal development, but not in adult BM, induces HSC quiescence while enhancing long-term repopulation capacity. Together, these findings demonstrate that HSC dormancy is developmentally programmed before birth and identify evasion of Notch signaling as a key mechanism establishing this state, fundamentally revising current models of how long-term regenerative potential is specified.