This study directly measures local oxygen tension (pO2) in the bone marrow (BM) of live mice using two-photon phosphorescence lifetime microscopy (2PLM). The researchers found that the absolute pO2 in the BM is quite low (<32 mmHg) despite high vascular density. They also observed heterogeneities in local pO2, with the lowest pO2 (~9.9 mmHg, or 1.3%) found in deeper peri-sinusoidal regions. The endosteal region, which is perfused with small arteries positive for the marker nestin, is less hypoxic. These pO2 values change significantly after radiation and chemotherapy, indicating that stress can alter the stem cell metabolic microenvironment. The study highlights the importance of understanding the balance between oxygen supply and consumption in the BM and the role of different vascular niches in hematopoietic stem cell (HSC) regulation.This study directly measures local oxygen tension (pO2) in the bone marrow (BM) of live mice using two-photon phosphorescence lifetime microscopy (2PLM). The researchers found that the absolute pO2 in the BM is quite low (<32 mmHg) despite high vascular density. They also observed heterogeneities in local pO2, with the lowest pO2 (~9.9 mmHg, or 1.3%) found in deeper peri-sinusoidal regions. The endosteal region, which is perfused with small arteries positive for the marker nestin, is less hypoxic. These pO2 values change significantly after radiation and chemotherapy, indicating that stress can alter the stem cell metabolic microenvironment. The study highlights the importance of understanding the balance between oxygen supply and consumption in the BM and the role of different vascular niches in hematopoietic stem cell (HSC) regulation.