Do all cells enter the G0 phase?

No, all cells do not enter the G0 phase, for several reasons. Cells enter the G0 phase when they no longer need to divide. This may be part of their differentiation process. For example, nerve and cardiac cells remain in the G0 phase forever because once they reach maturity, they never divide again. Other cells enter the G0 phase because they lack the necessary nutrients and energy to divide. These cells may only temporarily stay in the G0 phase and re-enter the cell cycle (G1 phase) in response to stimulation by mitogens, induced by normal cell turnover or injury.