An animal cell depends on multiple extracellular signals. Every cell type displays a set of receptor proteins that enables it to respond to a specific set of signal molecules produced by other cells. These signal molecules work in combinations to regulate the behavior of the cell. As shown here, cells may require multiple signals (blue arrows) to survive, additional signals (red arrows) to divide, and still other signals (green arrows) to differentiate. If deprived of survival signals, most cells undergo a form of cell suicide known as programmed cell death, or apoptosis.  Fair Use and Copyright info