Well, one answer is that they are MEFs -- mouse embryo fibroblasts -- since that's what I started with. Only the cells pictured are pretty clearly not regular fibroblasts; they look more like neurons or macrophages of some kind. MEFs are a mixed population, consisting of whatever grows out of a dissociated (minced) mouse embryo minus the head, so there are some early neural and immune cells in the mix. The cells pictured are what remains after selection with either G418 or puromycin -- I was making stably transfected cells, and this is one of the control plates.

So what I'm wondering is, would a brief period of exposure to a selective agent like puro be a good way to isolate naturally resistant populations, and what would those populations contain? (Of course, whatever these things are, the most likely explanation for their resistance is terminal differentiation, so they're simply dying more slowly -- I haven't tried taking away the selective agent to see whether they will multiply.)

Any ideas, lazyweb?