Explanation

Some cells in the adult animals do not appear to exhibit division ($e.g.,$ heart cells, and many other cells divide only occasionally $e.g.,$ when there is need to replace cells that have been lost due to injury or cell death. These cells that do not divide further and exit $G_{1}$-phase to enter an inactive stage called quiescent stage ($G_{0}$) of the cell cycle. Cells in this stage remains metabolically active but no longer proliferate