Final answer:
Cells in the resting phase (G0) may temporarily pause and then restart the cell cycle, remain in G0 permanently if they are differentiated like nerve and cardiac muscle cells, or enter senescence where they cease to divide due to damage or aging.
Step-by-step explanation:
After a cell enters the resting phase (G0), it may take one of three possible routes:
The cell could remain in G0 temporarily and then re-enter the cell cycle, starting with the G1 phase, in response to external signals such as the availability of nutrients or growth factors. This is common in cells that only divide when necessary.
Some cells may enter G0 and never leave this stage because they are terminally differentiated, meaning they have reached a state where they no longer need to divide. Mature cardiac muscle and nerve cells are common examples of cells that stay in G0 permanently.
Alternatively, cells in G0 could be in a state of senescence, a permanent arrest due to factors like DNA damage or aging, effectively marking the end of the cell's ability to proliferate.