into a tissue depleted of its normal cells can, through
proliferation and differentiation, reconstitute that tissue.
By this analysis, the stem cell concept finished passing
through the stages of broad abstraction and precise
operational definition by the 1960s. Yet, despite no small
amount of effort since then, a subsequent stage of
evolution - in which stem cell is redefined in terms of
physical characteristics - has yet to take place. It is
perhaps curious that, after 45 years, we have been unable
to place the general notion of ‘stemness’ on a purely
molecular footing. Of course, the fact that a goal has not
been achieved after a long time does not mean that the
answer in not around the corner. But it does give one
cause to wonder whether something we are doing needs
to change, either the question we are asking or the way we
are approaching it.