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Cellular Suicide: An Essential Part of Life
By Stephanie Dutchen and Kirstie Saltsman
Posted February 23, 2011
At the cellular level, death is essential for life.
Apoptosis, sometimes called "cellular suicide," is a normal, programmed process of cellular selfdestruction. Even though it involves cell death, apoptosis serves a healthy and protective role in
our bodies. The work of many researchers funded by the National Institutes of Health has taught
us that apoptosis helps shape our physical features and organs before birth and rids our bodies
of unneeded or potentially harmful cells. Without apoptosis, we wouldn't have distinct fingers
and toes or the brain cell connections to understand the words in this article.
Apoptosis also plays a critical role during viral infections, killing off invaded cells before they spill
over with virus particles. This act of self-sacrifice hampers the spread of viruses and can save
the whole organism.
Many of the genes involved in
regulating apoptosis were discovered in
the tiny roundworm C. elegans in the
1970s by Massachusetts Institute of
Technology geneticist H. Robert Horvitz and then—graduate student Hilary Ellis. Horvitz's
research, which won a Nobel Prize in physiology or medicine in 2002, proved that apoptosis is
directed from within—by our very own genes. Credit: Ewa M. Davison.
Cells come equipped with the instructions and instruments necessary for apoptosis. They keep
these tools, called proteases, carefully tucked away like a set of sheathed knives, until some
signal—either from within or outside the cell—triggers their release. This initiates a cascade of
carefully coordinated events that culminate in the efficient, pain-free excision of cells.
During apoptosis, the cell shrinks and pulls away from its neighbors. Then the surface of the cell
appears to boil, with fragments breaking away and escaping like bubbles from a pot of hot
water. The DNA in the cell's nucleus condenses and breaks into evenly sized fragments. Soon
the nucleus itself disintegrates, followed by the entire cell. A cellular cleanup crew made of
phagocytic cells—immune cells that engulf and dispose of dead cells and debris—arrives on the
scene to mop up the remains.
A Second Way to Die
Apoptosis is one of two ways cells can die. The second, necrosis, is an unplanned response to
an overwhelming stress such as a traumatic injury or exposure to poison. It's what happens to
heart cells during a heart attack, cells in severely frostbitten fingers and toes, and lung cells
undergoing a bout of pneumonia. While apoptosis neatly removes select cells, necrosis
destroys without strategy.
Accordingly, the process of necrosis is much messier. Having lost the ability to control the flow
of liquid in and out, cells experiencing necrosis swell up and eventually burst, releasing their
contents into the surrounding tissue. While phagocytes still swoop in to clear the area, the
chemicals involved in necrosis cause the area to become inflamed and sensitive. Think of the
redness and pain in your finger after you accidentally touch a hot stove.
Life in Balance
A balance between mitosis and apoptosis is critical to human health. As apoptosis destroys
unwanted cells, mitosis (cell division) makes new cells. While they may seem to be at odds,
apoptosis and mitosis work together to keep us healthy. For example, our skin and hair cells are
renewed via a continuous cycle of apoptosis and mitosis. So are the cells lining our intestines.
Because new cells replace old, worn-out ones, our tissues remain healthy.
As you can well imagine, loss of the balance between apoptosis and mitosis can have
hazardous consequences. If apoptosis is triggered when it shouldn't be, our bodies squander
perfectly good cells. Scientists believe that too much apoptosis is at least partly to blame for
some neurodegenerative diseases, such as Alzheimer's, Parkinson's and Lou Gehrig's, and it
may play a role in the fatal body-wide infection known as sepsis. On the other hand, unchecked
mitosis can lead to cancer.