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.
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