Mammalian Cell Culture - Osmolality

by Oliver Yu

Osmolality ("osmo") is a measure of how "salty" the media is and significantly impacts the cellular environment of cell culture.

CHO cells in a low osmo environment are bursting at the proverbial seams and conversely in high osmo environments are shriveled like Ahnold's balls on steroids.

High osmolality can cause delayed cell growth or accelerate cell death (depending on where the culture is).

Osmolality is not a controlled parameter per se. It is designed to be somewhere between 270 - 330 mOsm/kg (mammals have interstitial osmo of 290 mOsm) for the typical media.

Once the media is inoculated, the cell culture pH control strategy will increase the osmo when alkali is demanded; this is due to the sodium (Na+) ion of the carbonate/bicarbonate pH control strategy.


Also, additional media or glucose added to the cultures have high osmolality themselves and will raise the culture osmo.

How it happens in the real world.

Generally, osmolality increases during the course of cell culture. The addition of base to offset the acidic forces of the CO2 evolution increases the osmolality of the culture. In fed batch cultures, the nutrient-packed batch feed will increase the osmolality of the culture.

While there are no formal osmolality controls for the cell culture, there are typically osmolality specifications for the media and batch feed. Typically media targets final (after initial QS, media powder, peptones, base addition, final QS) osmolality near 290 to 300 mOsm/kg so that the cell culture has a fighting chance at staying within biological range for the cells.