Hematopoietic stem cells (HSCs) reside in the bone marrow (BM), are quiescent, can self-renew, and generate all lineages of the hematopoietic system. A coherent picture of how steady-state function and homeostatic responses of HSCs are regulated has not emerged yet, however. Furthermore, although progress has been made in this area, reliable renewal or even maintenance of HSCs in vitro remains challenging, but would have major translational implications. A particular gap is our understanding of the organellar cell biology of HSCs. One organelle of which role in HSCs is unclear is the mitochondrion. Preferential use of glycolysis in stem cells suggests that mitochondrial respiration is more dispensable for HSCs than for progenitors. These findings raise the question whether mitochondria play roles in HSCs that are not directly related to ATP production, such as intermediary metabolism, epigenetics, apoptosis and intracellular calcium handling. Mitochondria dynamics, the fusion and fission of mitochondria, play a central role in the coordination of mitochondrial function. The fusion machinery consists of two partially redundant and interacting outer membrane GTPases, mitofusins (MFN) 1 and 2, and the inner membrane GTPase, OPA1. We observed that mitofusins show redundant and non- redundant as well as cell-intrinsic and cell-extrinsic roles that profoundly and specifically impact HSCs. We also found that HSCs are endowed with elevated mitochondrial mass and attenuated mitophagy, and that all mitofusin mutants with severe phenotypes also had reduced mitochondrial mass in HSCs, but not in mature hematopoietic cells. The goal of this proposal is to elucidate the underlying mechanisms and harness mitochondrial dynamics to achieve more efficient maintenance of HSC in vitro.