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Cell suicide for beginners
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Martin RaffThe ability to commit suicide is a fundamental property of animal cells. This overview considers recent progress in understanding the nature of the suicide process and how it is controlled.
Cell death is an invariable part of animal development, and it often continues into adulthood. In a mature
human, for example, millions of cells die every minute; we remain the same size only because, by an unknown mechanism, cell division exactly balances cell death. It seems peculiarly wasteful for so many cells to die, especially as most of them are perfectly healthy at the time of their demise. What purpose does this carnage serve?
In some cases the answer is clear1. During development, cell death helps sculpt parts of the body, carving out cavities or separating digits, for example; it also eliminates structures that once served a function, but are no longer needed, such as the tail of the tadpole when the tadpole becomes a frog. Throughout vertebrate life, cell death eliminates most newly formed lymphocytes (the principal cells of the immune system), including those that are useless or potentially dangerous. It also helps to control cell numbers by removing excess cells: neutrophils (a type of white blood cell), for instance, are produced continuously in the bone marrow, but the vast majority die there within a few days without ever functioning2. This apparently futile cycle of production and destruction presumably serves to maintain a ready supply of cells that can be mobilized to fight infection wherever it occurs in the body. Compared with the life of the organism, cells are apparently cheap.
Based on the characteristic way cells look when they die in different circumstances, it was proposed in 1972 that normal cell deaths, as well as some pathological ones, are suicides3. That is, the cells activate an intracellular death programme and kill themselves in a controlled way a process now known as programmed cell death, or apoptosis. Apoptotic cells shrink and are rapidly eaten by neighbouring cells, before there is any leakage of their contents. Because they are eaten and digested so quickly, there are usually few dead cells to be seen, even when large numbers of cells have died. This is probably why...