Apoptosis is a process leading to the death of a cell. The cell itself, surrounding cells, or the immune system bring about the death of a particular cell in order to protect the rest of the organism, or to promote its development. It differs from traumatic cell death or necrosis in that it is orderly and generally occurs without harm to surrounding tissue.
Apoptosis operates to eliminate old or defective cells, redundant cells, or diseased ones. In the young, extra cells are carved away from the limbs of the embryo to sculpt fingers and toes, for example, by this process. Apoptosis helps maintain homeostasis, balance, in an organism. When a certain number of cells are born, a certain number must die.
It also works to prevent the spread of infection, by killing cells that are invaded by viruses, although man viruses have evolved defenses against apoptosis. In certain circumstances, it kills cells damaged by ionizing radiation or chemicals. It also prevents cancer by killing cells that might otherwise become tumors. Immortality is one of the hallmarks of a cancer cell.
A number of different mechanisms can begin apoptosis. Cell signals can induce it, if the sell itself detects a defect. Intracellular signals can also begin the process. Intracellular signals may arise from glucocorticoids, heat, radiation, viruses, hypoxia, or other damage. Extra cellular signals can induce it. These kinds of signals can include toxins, hormones, nitric oxide, and cytokines. Cytokines are small proteins released by cells that change the interactions between cells. Some cytokines are interleukins, lymphokines, the interferons, and tumor necrosis factor, TNF.
Tumor necrosis factor is a cytokine that promotes the death of cells in order to prevent the growth of tumors. Defects in the TNF mechanism, however, may lead to autoimmune disease.
One pathway that destroys cells does so by destroying the mitochondria within the cells that allow them to process oxygen. Without oxygen, the cell dies. Another pathway induces cell death by destroying the membrane that protects the cell. Another way to kill a cell is to cut up its DNA into useless strips.
The various systems that induce apoptosis are complex, and a lot can go wrong. Without sufficient apoptosis, a baby may be born with fingers still joined, for example, or with still graver defects. The worst effect of insufficient programmed cell death in adults might be the spread of cancer. Excessive cell death however, may cause atrophy.
Carl Vogt first described apoptosis in 1842. In 1885 Walther Flemming came out with a more precise description of programmed cell death. In 1965 John Foxton Ross Kerr clearly distinguished apoptosis from traumatic cell death using an electron microscope.
Now, apoptosis is an area of serious inquiry for researchers fighting cancer. Perhaps someday, many cancers will be induced to eliminate themselves before they become a problem. This is possible because, in reality, many cancers already do this in people's bodies, thanks to apoptosis.
The term apoptosis is from Greek, and means falling away. It was originally taken to mean falling the way leaves fall off trees, or the way petals drop from an overblown flower. In apoptosis, the organism discards what it does not need or want, and lives on.