The second problem with defining stem cells in terms of
potency is that traditional notions of what a cell type is,
and how cells move between types, seem to be eroding,
thanks to recent work by stem cell researchers and an
influx of ideas from systems biology. The traditional view
of cell differentiation as a set of irreversible, deterministic
transitions from one stable state to another is giving way to
a view in which cell states are quasi-stable points on an
‘energy landscape’ along which cells move in response to
both stochastic variation and external signals (see, for
example, [4-6]). By this view, potency is not really a
property that a cell has independent of its environment. A
good analogy likens cell types to metabolites in metabolic
networks. Arrow diagrams showing pathways by which
glucose can be converted into hundreds of other substances
make nice wall charts, but if one wants to say what glucose
will actually become in any real situation, one needs to
know enzyme levels and activities, levels of other metabolites,
temperature, pH and so forth.