No cell in the body ever sees 21% oxygen, so why are we still culturing them under ambient air?
For over a century tissue culture has been performed under what is considered normoxic (21% oxygen) conditions. However, the reason a vast majority of peer reviewed publications describe using 21% oxygen is not scientific but only historic. There is absolutely no scientific evidence that justifies performing these experiments at atmospheric oxygen levels. In fact extensive evidence supports the statement that cells cultured under appropriate physiological conditions express in vivo regulator genes that are silent at normoxic concentrations (e.g. HIF, Hypoxia Inducible Factor). Malaria parasites, lymphocytes, embryos and stem cells to name a few, all require hypoxic environments for optimal cell function.
It is only in the latter half of the 20th century that a few researchers designed in vitro experiments reflecting physiological levels of oxygen in situ. That is in vitro environments that reflect in vivo conditions, (i.e. hypoxic environments, 0.1-10% O2).