First of all, the morphology of your cells still looks fine. In general, fungi and yeasts do not show a rapid toxic effect in culture. Therefore, you may be suspicious of these two. However, since you have been saying it "for a few months", I am not sure.

The photos look like yeast contamination. Does your medium contain phenol red? Have you observed any ph increase? I recommend that you take samples from the serum and media you use and control by planting in agar or liquid medium.

Dear Paulina, happy to connect you with our Field Application Scientist for Stem Cells, to discuss this. Please feel free to contact me at [email protected]

these dont look like stem cells, maybe MSCs. The culture is P6 but prior to freezing there could be another 50-100 passages. Also, media could be missing some essential component. For example, L-glutamine may be old, add glutamax. If pH of the media is not changing, this is not yeast. Seems like a culture, which is slowly dying. bFGF maybe old, add bFGF (or try fresh serum, if your protocol allows serum). Passage at low density and establish subclones. Without knowing the history or/and culturing methods, it is very hard to say. This could be mycoplasma contamination, check for mycoplasma contamination