<P> Because apoptosis can not stop once it has begun , it is a highly regulated process . Apoptosis can be initiated through one of two pathways . In the intrinsic pathway the cell kills itself because it senses cell stress , while in the extrinsic pathway the cell kills itself because of signals from other cells . Both pathways induce cell death by activating caspases , which are proteases , or enzymes that degrade proteins . The two pathways both activate initiator caspases , which then activate executioner caspases , which then kill the cell by degrading proteins indiscriminately . </P>
where do the signals for apoptosis come from
from other cells