Empty bubbles in the 3T3-L1 cells - (Nov/19/2010 )

Hi, anyone came across with those empty bubbles in 3T3-L1 cells during differentiation? Is it because the cells has passed too many generation? Is this also happens in other types of cells? I am looking for some theoretical explaination. Thanks.
Happy thanksgiving and have a wonderful turkey:)

It's a little hard to tell from the picture, but it looks like the cells are just a bit over confluent and starting to senesce. Try splitting your cells and see what happens.

I don't think 3t3 cells have a hayflick limit, so I doubt it is because they have been passaged too many times.

I had handled this cell line for only one year two years ago. So I can't remember very clearly. I could also see these empty bubbles in my hand, and they did seem to be increasing with the passage number. Actually I could begin to see some empty bubbles even before differentiation and with a low passage number (eg P3), but in general differentiation was OK even in cells with some empty bubbles. Usually I would discard 3T3-L1 cells older than P12 as they don't differentiate well (only 70-80%) after that.
Sorry that I don't have any explanation and I think I am still new to life science research. Hope I can learn more from you guys during the discussion.

Thanks for the replies. If it is a symbol of senescence, is it also appear in other cells?