> First, why aren't dermal cells regularly contributing to the top
> layer (meaning, how does it really work)?
The stem cells that give rise to the epidermis are attached to a
sort of mat called a basal lamina, and their progeny move out towards
the skin surface as you describe. The dermis is below this lamina.
Actually the skin is much more complicated than this. It's best to
consult a graduate-level cell biology textbook such as Alberts.
> And second, assuming that the researchers knew this, why did they
> pick a rarely-dividing cell type to do this type of experiment with?
Mainly, I believe, for the same reason that CR studies are mostly done
on lab mice that have for several decades been inadvertantly bred for
rapid reproduction and hence may live unnaturally short lives anyway.
To wit, that they are easy and cheap(ish) to culture. I couldn't agree
more with your implicit suggestion that cells with a more respectable
turnover rate, such as liver cells, might be a lot more informative
about the potential relevance to lifespan, yet I know of no such work.
But I suppose that when one is purely investigating the relationship
between telomeres and replicative senescence, it doesn't matter so much
whether the cells are being asked to do biologically implausible things.
Aubrey de Grey