I have been culturing HepG2 cells for years. About a 5-6 weeks ago the cell growth rate slowed and the cells showed large amounts of vacuoles within the cytoplasm. Nothing has changed with my set-up, from media to the incubator. I just split a flask that has been growing since 7/11 and there was only about 1/4 of the plate with cells. The CO2 reads 4.9% on the incubator readout. Perhaps I should check this as I have a digital CO2 analyzer. Again, I note that the vacuoles are extremely prominent. Any ideas from anyone?