How is the quiescent state regulated?

Recent studies have shown that a combination of intrinsic and extrinsic mechanisms controls the quiescent state, including cell cycle and transcriptional regulators, contact with the extracellular matrix, and metabolic and systemic cues. Cell cycle inhibitors such as p27 and p21 play a role in maintaining quiescence. They inhibit the activity of CDK-cyclin complexes, preventing the cell cycle progression from G0 to G1 phase. Growth factor signaling pathways such as IGF and FGF pathways help regulate quiescence. Activation of these pathways by extracellular signals trigger signaling cascades that lead to cell cycle arrest. The DNA damage response pathway activates checkpoints and triggers cell cycle arrest to prevent DNA damage.