Oxygen is essential for life and a common substrate for cell cultures. There's no doubt stem cells, like all other eukaryotic cells, require some oxygen to thrive. What is often overlooked, however, is that oxygen is also quite toxic at high concentrations. Bearing in mind that, in our tissues, O2 tension is much lower than in the ambient air, you might wonder if you could get your pluripotent cells to grow faster in lower oxygen concentrations than, say, your average aerobic bacterial culture.
It does, in fact, seem to be the case, that stem cell cultures, in general, do much better at lower oxygen tensions than ambient, which is 21%. Arterial blood has an O2 tension of about 12%, while, on average, many tissues have O2 tensions around 3%. In embryos this number is even lower. Enhanced stem cell proliferation has been reported for culture oxygen tensions ranging from 1-10%.
Lower oxygen tension means lower production of reactive oxygen species, and less stress on the culture. Although the exact influence of oxygen tension is not completely understood (some cultures do better on higher O2 concentrations), stringent control of oxygen during bioprocessing is important. This can be complicated, especially as the fermentation unit gets bigger, but it is an important factor for ensuring the best yield, and maybe even better quality product with not only optimal proliferation but proper differentiation into the desired cell type.