Dear Liz:

The compound that is designed for that type of experiment is called Cell
Trakker Green, from Molecular Probes. You incubate that with your living
cells (30 mins, incubator), then they are labeled with a fluorescent
compound that is green, non-toxic and that stays within the cells for
their lifespan. It also stays within the cells after fixation.
(fluorescein optics on your microscope) I have labeled cells and then
transferred them to another co-culture and kept them going for one week
before fixing and I was able to still detect labeled cells. I am not
sure about how well they will last for 3 weeks. But it was the best
thing I could find for the type of thing you are describing. It is also
used for transplantation experiments. 

Good luck!
Sarah
Univ. Cincinnati


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