Re: What's normoxy in cell culture?

According to Metzen, et al.¹, the partial pressure of oxygen at the bottom of a culture dish in an air / 5% CO2 atmosphere at 37° C should be around 120 mm Hg. In the article, the authors measured the partial pressure of the media right around the cultured cells and found that for highly metabolic cells, the actual values were below their detection range (<50 mm Hg). On the other hand, less active cells' pericellular partial pressures were between 70 mm Hg and 110 mm Hg. The second half of your question is more interesting, because the values determined above are viewed by many as hypoxic, or "below normal oxygen levels," since 20% oxygen should yield a partial pressure around 140 mm Hg. In fact, the normal partial pressure of oxygen in our own cells growing in our own bodies is less than 40 mm Hg, which translates to around 5% oxygen. This becomes problematic, because there are many scientific articles that cite 5% oxygen as hypoxic in cell cultures. In reality, cells are quite able to adapt to different resting partial pressures of oxygen: in another article, Khanna, et al.², took brain cells, which live at happily at 35 mm Hg (around 5%) oxygen in the brain, and grew them in culture at various oxygen pressures. Cells grown in 20% oxygen underwent a hypoxic response when moved to 5% oxygen, while brain cells maintained at 5% oxygen in culture behaved as if they were normoxic. Moreover, cells grown at 30% oxygen became hypoxic when moved to 20% oxygen. They demonstrated that "normoxia" in cell culture is the oxygen concentration at which the cells are initially grown, since those are the conditions to which they initially adapt.

1. Metzen E, Wolff M, Fandrey J, and Jelkmann W (1995) "Pericellular pO₂ and O₂ consumption in monolayer cell cultures" Respir Physiol 100(2): 101-106

2. Khanna S, Roy S, Maurer M, Ratan RR, and Sen CK (2006) "Oxygen-sensitive reset of hypoxia-inducible factor transactivation response: prolyl hydroxylases tune the biological normoxic set point" Free Radic Biol Med 40(12): 2147- 2154