Strange round dots in IPS culture with MEF feeder - (Jun/27/2013 )

Hey,

I'm growing iPS cells on a MEF feeder layer, and recently we've started to see these dark round things popping up. I have been trying to figure out what they are but have had no luck so far. We think we see them before any colonies are in the culture, which leads me to believe they are related to the mefs, or the media, or something else. Anyone seen anything like this before?

Thanks!

They could be dead cells or yeasts, they are too large for bacteria, and probably too big for yeast too, but there are some strains that would be about that size.