The G0 phase (referred to the G zero phase) or resting phase is a period in the cell cycle in which cells exist in a quiescent state. G0 phase is viewed as either an extended G1 phase, where the cell is neither dividing nor preparing to divide, or a distinct quiescent stage that occurs outside of the cell cycle. Some types of cells, such as nerve and heart muscle cells, become quiescent when they reach maturity (i.e., when they are terminally differentiated) but continue to perform their main functions for the rest of the organism's life. Multinucleated muscle cells that do not undergo cytokinesis are also often considered to be in the G0 stage. On occasion, a distinction in terms is made between a G0 cell and a 'quiescent' cell (e.g., heart muscle cells and neurons), which will never enter the G1 phase, whereas other G0 cells may.
In relation to 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 be induced to begin dividing again only under very specific circumstances. Other types of cells, such as epithelial cells, continue to divide throughout an organism's life and rarely enter G0.
Distinction from senescent cells 
Although many cells in the G0 phase may die along with the organism, not all cells that enter the G0 phase are destined to die; this is often simply a consequence of the cell's lacking any stimulation to re-enter in the cell cycle.
Cellular senescence is distinct from quiescence because it is a state that occurs in response to DNA damage or degradation that would make a cell's progeny nonviable. Senescence then, unlike quiescence, is often a biochemical alternative to the self-destruction of such a damaged cell by apoptosis. Furthermore, quiescence is reversible whereas senescence is not.
- Re: Are the cells in the G0 (g zero) phase of mitosis really suspended? Erin Cram, Grad student, Molecular and Cellular Biology, University of CA, Berkeley. 1999. MadScience Network. Question ID 942142089.Cb.
See also