Parenchymal cells in relation to cell cycle (G0–Resting phase; G1, G2–Gaps; S–Synthesis phase; M–Mitosis phase). The inner circle shown with green line represents cell cycle for labile cells; circle shown with yellow-orange line represents cell cycle for stable cells; and the circle shown with red line represents cell cycle for permanent cells. Compare them with traffic signals—green stands for ‘go’ applies here to dividing labile cells; yellow-orange signal for ‘ready to go’ applies here to stable cells which can be stimulated to enter cell cycle; and red signal for ‘stop’ here means non-dividing permanent cells.