G0 phase
The G0 phase (G zero) is a period in the cell cycle where cells exist in a quiescent state. Cells enter the G0 phase from a G1 phase checkpoint, such as the restriction point (animal cells) or the start point (yeast) usually 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. Some cells remain in the G0 phase until there is a reason for them to divide. For instance, epidermal fibroblasts remain in the G0 phase until stimulated by growth factors generated in response to a wound. 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 simply a consequence of the cell lacking any stimulation to re-enter in the cell-cycle.
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