As cell biologist, when working in labs I have in the past witnessed several “X-files” in a flask of cells along my way: all cells of an established cell line are supposed to have the same genetic background and to respond similarly to the same stimuli… but sometimes this is simply not what happens!
I would not risk saying that it all comes down to epigenetics, but much of this “unexpected” behaviour is governed at mRNA level independently of the genetic background. “Cell personality” is then defined at several levels: genetic background, epigenetics, mRNA modulation and protein/lipid content.
It’s a sophisticated web of positive/negative signals that governs cell fate and behavior. But how do cells cope with this complicated issue?