TRIGGERS. “Programmed cell death induction may depend essentially on cell-lineage information, such as in the nematode Caenorhabditis elegans, on the activation of gene transcription, such as in the fruitfly Drosophila melanogaster, or, in a more stochastic way, on a combination of cell-lineage information, intercellular signaling, transcription factor activation and cytoplasmic second messengers, such as in mammals.”
http://www.nature.com/cdd/journal/v9/n4/full/4400950a.html