Many mammal cells, such as this neuron, remain permanently or semipermanently in G 0.
The G0 phase (G sub 0) is a period in the cell cycle where cells exist in a quiescent state. G0 is sometimes referred to as a "post-mitotic" state since cells in G0 are in a non-dividing phase outside of the cell cycle. Cells enter the G0 phase from a cell cycle checkpoint in the G1 phase, such as the restriction point (animal cells) or the start point (yeast). This usually occurs in response to a lack of growth factors or nutrients. During the G0 phase, the cell cycle machinery is dismantled and cyclins and cyclin-dependent kinases disappear. Cells then remain in the G0 phase until there is a reason for them to divide. Some cell types in mature organisms, such as parenchymal cells of the liver and kidney, enter the G0 phase semi-permanently and can only be induced to begin dividing again under very specific circumstances; other types, such as epithelial cells, continue to divide throughout an organism's life.
Although many cells in the G0 phase may die along with the organism, this does not mean that cells that enter the G0 phase are destined to die; this is often simply a consequence of the cell lacking any stimulation to re-enter in the cell cycle.
However, the term "post-mitotic" is sometimes used to refer not only to quiescent cells but also to senescent cells. Cellular senescence is a state that occurs in response to DNA damage or degradation that would make a cell's progeny nonviable; it is often a biochemical alternative to the self-destruction of such a damaged cell by apoptosis.
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