Like all living things, the various types of cells in plants, animals, and the many different cell types in humans must eventually die. Cell death occurs in one of two ways. Cells can be killed by the effects of physical, biological, or chemical injury. Additionally, cells are induced to kill themselves. Cell suicide is also referred to as apoptosis (from the Greek words apo, meaning from, and ptosis, meaning to fall or to drop).
Cell death is important in disease and the aging process. Cellular suicide is also necessary in the fetal development of some organs and tissues.
Cell death that results from injury can be caused by mechanical damage such as tearing, or can be due to physical stresses such as heat. A third-degree sunburn, for example, results in the death of many skin cells. Exposure to toxic chemicals such as acids, corrosive bases, metabolic poisons, and other chemicals is also lethal to many types of cells. Excessive drinking of alcohol (ethanol) causes death of liver cells in humans.
Substances that dehydrate cells can also cause cell death. If the environment outside of a cell contains more salt than the interior of the cell, water flows out of the cell in an attempt to dilute the outside environment. The loss of water can disrupt the functioning of the cell to the point of death. This is called plasmolysis. Conversely, if the interior of a cell is saltier than the exterior environment, water flows into the cell. The cell can swell and burst. This phenomenon is called plasmoptisis.
Some diseases and infections cause chemical cell death. For example, infection of the upper respiratory cells with viruses that causes the common cold kills cells during the viral life cycle.
Causes of chemical or mechanical cell death are varied. Some agents act on the membrane that surrounds cells. The membrane can be dissolved or damaged. Other agents disrupt enzymes that the cell requires to sustain life. Still other agents can disrupt the genetic material inside the cell.
The process of programmed cell death, apoptosis, or suicide, is a necessary part of the functioning of an individual cell and, in multi-celled organisms such as humans, of the whole organism. For example, reabsorption of a tadpole's tail during the change from tadpole to frog involves apoptosis. Sloughing of uterine cells in women at the start of menstruation is due to apoptosis of the cells lining the uterine wall. Additionally, apoptosis of extraneous cells during development of a human fetus produces the distinct fingers and toes.
Apoptosis is also important as a means of dealing with threats to an organism. For example, the human immune system contains cells that can stimulate apoptosis of other cells that have been infected with a virus. Similarly, cells with damaged genetic material undergo cell death. Thus, apoptosis helps the entire organism function efficiently by eliminating cells that threaten the whole organism.
Programmed cell death occurs either by the withdrawal of a chemical signal that is required to continue living, or by exposure to a chemical signal that begins the death process. Once stimulated to die, apoptotic cells shrink, develop irregular cell surfaces, and show disintegration of genetic material within their nuclei. Eventually, these cells break into small membrane wrapped fragments that are engulfed by nearby cells. The apoptosis process is complex, and involves interactions between numbers of different biochemical compounds. This helps ensure that apoptosis does not initiate by accident, and that the process is limited only to specifically targeted cells.
Molecular biologists Sydney Brenner, Robert Horvitz, and John Sulston were awarded the 2002 Nobel Prize in Physiology or Medicine for their pioneering studies on the genetic regulation of programmed cell death. Their studies, which were carried out in the 1980s using a nematode worm as the model system, has since been shown to have relevance to the process of cell death in humans.
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Zhivotovsky, B. "From the Nematode and Mammals Back to the Pine Tree: On the Diversity and Evolution of Programmed Cell Death." Cell Death and Differentiation 9 (September 2002): 867–70.