Here, using two-photon phosphorescence lifetime microscopy, the local oxygen tension in the bone marrow of live mice is found to be quite low, with spatiotemporal variations depending on the blood vessel type, distance to the endosteum, and changes in cellularity after stress. Low oxygen tension (hypoxia) is commonly thought to be a shared niche characteristic in maintaining quiescence in many stem cell types. However, local oxygen concentration, for example in the bone marrow, has never been measured directly. Charles Lin and colleagues have now developed a method based on two-photon microscopy to measure the absolute local oxygen tension (pO2) in the marrow of live animals. Using this method, they found that while vascular density is high throughout the bone marrow, overall oxygenation is quite low and there is heterogeneity in local pO2 with respect to vessel type and location. For example, surprisingly, the endosteal region is not the region of the lowest pO2. After radiation or chemotherapy, bone marrow pO2 becomes elevated and transplanted haematopoietic stem/progenitor cells do not seek out regions with the lowest pO2 for homing. Characterization of how the microenvironment, or niche, regulates stem cell activity is central to understanding stem cell biology and to developing strategies for the therapeutic manipulation of stem cells1. Low oxygen tension (hypoxia) is commonly thought to be a shared niche characteristic in maintaining quiescence in multiple stem cell types2,3,4. However, support for the existence of a hypoxic niche has largely come from indirect evidence such as proteomic analysis5, expression of hypoxia inducible factor-1α (Hif-1α) and related genes6, and staining with surrogate hypoxic markers (for example, pimonidazole)6,7,8. Here we perform direct in vivo measurements of local oxygen tension ( ) in the bone marrow of live mice. Using two-photon phosphorescence lifetime microscopy, we determined the absolute of the bone marrow to be quite low (<32 mm Hg) despite very high vascular density. We further uncovered heterogeneities in local , with the lowest (∼9.9 mm Hg, or 1.3%) found in deeper peri-sinusoidal regions. The endosteal region, by contrast, is less hypoxic as it is perfused with small arteries that are often positive for the marker nestin. These values change markedly after radiation and chemotherapy, pointing to the role of stress in altering the stem cell metabolic microenvironment.

A single hematopoietic stem cell (HSC) is capable of reconstituting hematopoiesis and maintaining homeostasis by balancing self-renewal and cell differentiation. The mechanisms of HSC division balance, however, are not yet defined. Here we demonstrate, by characterizing at the single-cell level a purified and minimally heterogeneous murine Tie2⁺ HSC population, that these top hierarchical HSCs preferentially undergo symmetric divisions. The induction of mitophagy, a quality control process in mitochondria, plays an essential role in self-renewing expansion of Tie2⁺ HSCs. Activation of the PPAR (peroxisome proliferator–activated receptor)–fatty acid oxidation pathway promotes expansion of Tie2⁺ HSCs through enhanced Parkin recruitment in mitochondria. These metabolic pathways are conserved in human TIE2⁺ HSCs. Our data thus identify mitophagy as a key mechanism of HSC expansion and suggest potential methods of cell-fate manipulation through metabolic pathways.

Achieving intravital optical imaging with diffraction-limited spatial resolution of deep-brain structures represents an important step toward the goal of understanding the mammalian central nervous system^1–4. Advances in wavefront-shaping methods and computational power have recently allowed for a novel approach to high-resolution imaging, utilizing deterministic light propagation through optically complex media and, of particular importance for this work, multimode optical fibers (MMFs)^5–7. We report a compact and highly optimized approach for minimally invasive in vivo brain imaging applications. The volume of tissue lesion was reduced by more than 100-fold, while preserving diffraction-limited imaging performance utilizing wavefront control of light propagation through a single 50-μm-core MMF. Here, we demonstrated high-resolution fluorescence imaging of subcellular neuronal structures, dendrites and synaptic specializations, in deep-brain regions of living mice, as well as monitored stimulus-driven functional Ca²⁺ responses. These results represent a major breakthrough in the compromise between high-resolution imaging and tissue damage, heralding new possibilities for deep-brain imaging in vivo.