External regulators direct cells to start or stop the cell cycle. They act as signals from outside the cell, influencing the cell's decision to divide or not.

Here's a breakdown:

* Growth factors: These are proteins that stimulate cell division. They bind to receptors on the cell surface, triggering a cascade of events leading to the activation of proteins that drive the cell cycle forward.

* Other external signals: Besides growth factors, other signals like hormones or cell-to-cell contact can also act as external regulators. They can either promote or inhibit cell division depending on the specific signal and its effect on the cell.

Essentially, external regulators act as the "go" or "stop" signals for the cell cycle, ensuring that cells divide only when appropriate and necessary.