Did you know that physiological normoxia generally falls in the 2-9% O2 (14.4-64.8 mm Hg) range for most adult cells in vivo? 3 remarkable exceptions are thymus, kidney medulla and most importantly bone marrow which can exist at 1% O2 (7.2 mm Hg). On the other hand, stem and progenitor cells are frequent residents of hypoxic niches and low O2 regulates their differentiation. Conclusion?
Although most cells are maintained in culture conditions at 21% O2, this is unlikely to be optimal for maintaining their normal proliferative or developmental state. The derivation of novel stem and undifferentiated cell populations should therefore be enhanced by culture in the range of 3–5% O2.
More on this very important and usually neglected oxyphsiological angle on stem cells, development and culture in the very uptodate review: The role of oxygen availability in embryonic development and stem cell function by Simon@Keith in Nat Rev Mol Cell Biol. 2008 Apr;9(4):285-96.
Some stem cells (such as those in the endosteal bone marrow compartment) occupy extremely low O2 microenvironments (<0.5% O2) (a). Other stem cells (such as those described as perivascular SLAM+ (signalling lymphocyte activation molecule) stem cells) occupy relatively well-oxygenated environments because they are in close proximity to blood vessel endothelial cells (b). However, it should be noted that although stem cells can be perivascular, the vessels might be associated with venous structures and might therefore be relatively hypoxic. pO2, oxygen partial pressure.