Final answer:
Apoptosis, or programmed cell death, can be triggered by internal and external signals. Internal signals include abnormalities detected by internal checkpoints, while external signals include the binding of cellular receptors to the extracellular matrix or the recognition of self proteins by immune cells.
Step-by-step explanation:
Apoptosis, or programmed cell death, can be initiated by various signals. Internal signals that can trigger apoptosis include abnormalities detected by internal checkpoints that monitor a cell's health. External signals, such as the binding of cellular receptors to the extracellular matrix or the recognition of self proteins by immune cells, can also lead to apoptosis. For example, if a cell moves away from the extracellular matrix, the signaling ceases and the cell undergoes apoptosis.