Fig 2.

Relationships among suicidal cell, scavenger cell and live cell in a tissue of living animal. A: As best studied with macrophages, a scavenger's surface receptor directly recognizes an “eat-me” molecule on the surface of an apoptotic cell, or uses a “bridging” molecule to tether the “eat-me” molecule. After engulfment, the macrophage's lysosomal enzymes continue the killing procedure if the cell is still alive (in this case apoptosis is actually a combination of suicide and euthanasia), and then decompose the corpse. Although in vitro studies suggest that lysosomal enzymes may elicit caspase-independent apoptosis, in living animals how these enzymes act before and after the cell is engulfed, and whether or not they coordinate with scavenger's enzymes, have not yet been fully characterized. B: Apoptosis in living animals is a game involving three players, i.e. the suicidal cell, the scavenger and many surrounding live cells. Each of the three not only executes a highly programmed series of actions, i.e. suicide, corpse removal and cell regeneration, respectively, but also coordinates with the other two players to maintain the tissue homoeostasis. In contrast, cell death in a Petri dish is one-player game, i.e. it is the suicidal cell's own business, because the cell has no need to discuss with the live cells about the regeneration issue, and with scavengers about the corpse it will leave behind. (Oppositely oriented double-arrows indicate mutual communications between the two players, whereas question marks indicate the current lack of strong evidence.)