In response to the question of Ulrich Seydel on long-term labeling of live
cells: there are several options.  You may either like to label cells with
the new lipophilic tracers from Molecular Probes (catalog numbers 7777 and
7778) or with a novel green cell tracing dye (number 7016).
 
The lipophilic tracers can be used with regular cell culture media (no
special "loading-media" needed), do not transfer from labeled to unlabeled
cells and are well retained after cell fixation with formaldhyde.  They seem
to stain all cellular membranes (just like the PKH dyes).
 
Alternatively one can use the # 7016 dye.  This dye behaves close to FDA,
but is much better retained in cells (even after fixation).
 
Tracing labeled cells for an extended period of time (3 to 5 days) always
faces the fact that the cells synthesize new macromolecules (including the
ones initially labeled) and "dilute" any label by cell division.  Decay of
fluorescence over time is related to cell division and indicates that the
label used is non-toxic to the cells.
 
If you need more information about these dyes you can either visit the
Molecular Probes Home Page on the WWW or call them at (541) 465-8300.
 
Hope this helps
 
Martin Poot
 
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