The ground state of pluripotency.

Pluripotency is defined as the capacity of individual cells to initiate all lineages of the mature organism in response to signals from the embryo or cell culture environment. A pluripotent cell has no predetermined programme; it is a blank slate. This is the foundation of mammalian development and of ES (embryonic stem) cell biology. What are the design principles of this naïve cell state? How is pluripotency acquired and maintained? Suppressing activation of ERKs (extracellular-signal-regulated kinases) is critical to establishing and sustaining ES cells. Inhibition of GSK3 (glycogen synthase kinase 3) reinforces this effect. We review the effect of selective kinase inhibitors on pluripotent cells and consider how these effects are mediated. We propose that ES cells represent a ground state, meaning a basal proliferative state that is free of epigenetic restriction and has minimal requirements for extrinsic stimuli. The stability of this state is reflected in the homogeneity of ES cell populations cultured in the presence of small-molecule inhibitors of MEK (mitogen-activated protein kinase/ERK kinase) and GSK3.