What is Apoptosis?
Apoptosis, named after the ancient Greek words for “falling off,” is controlled cellular death. This process occurs in all multicellular organs, including humans. Whereas traumatic cell death wreaks havoc on biological processes, apoptosis is a vital part of the body’s cellular life cycle.
Cellular apoptosis helps shape bodily structures during growth. Apoptosis, for example, dissolves the webbing between the fingers and toes of a human fetus. Apoptosis also helps isolate infected or damaged cells, cutting them off from the body to limit damage. Apoptosis clears out the old to make way for the new.
Cells “commit suicide” during apoptosis, as opposed to dying from outside factors during necrosis. Cells die in droves each day, with a human adult losing between 50 to 70 billion cells per day to apoptosis, according to Molecular Biology of the Cell (Alberts). The body replaces these cells as they die with new healthy cells at a rate of approximately one million cells per second. Many diseases can upset this delicate balance, including AIDS, leukemia and cancer. Scientific studies such as the 2000 study Apoptosis and Alzheimer’s Disease (Behl) implicate both runaway apoptosis and necrosis in both Alzheimer’s and Parkinson’s diseases.
Apoptosis begins when the cell receives the appropriate stimulus. These stimuli are tightly regulated as once apoptosis begins, the cell will die. Once stimulus is received, enzymes called caspases begin degrading the cell’s components and RNA. The cell shrinks and its membrane shows irregular buds called blebs. The membrane continues deforming into long protrusions of various shapes, before the cell breaks apart entirely and dies. As the cells die they exhibit certain molecules marking them for disposal by macrophages: cells which engulf and digest cellular debris. Macrophages, when working properly, remove dead or dying cells without provoking an inflammatory response. Macrophages aren’t the only cells which can clean up apoptotic cells, but they are the cells most often used for that purpose.
The key difference between apoptosis and necrosis is the cell death’s origin. Apoptosis is planned, spurred on by the body’s natural processes. Necrosis occurs because of outside damage or infection. Apoptosis is a natural, beneficial bodily process while necrosis is always detrimental. Unlike apoptosis, necrosis causes inflammation and decreased blood flow at the affected site, eventually killing entire tissues.
While apoptosis is a necessary biological process, it can go awry and harm the body. Some cancers can disrupt apoptosis, such as the lung cancer NCI-460 detailed in the 2003 study Predominant Suppression of Apoptosome by Inhibitor of Apoptosis Protein in Non-Small Cell Lung Cancer H460 Cells (Yang). The cancer causes certain proteins which limit apoptosis to proliferate wildly, causing defective cells to replicate. The damaged cells pass their problems to their progeny, replacing healthy cells with defective ones.
Other diseases, such as HIV, Alzheimer’s and Parkinson’s can cause hyperactive apoptosis, which causes too many healthy cells to self-destruct. Some cancer treatments including chemotherapy and radiation therapy can also trigger premature or excessive apoptosis.