Apoptosis

Apoptosis is a process in which cells self-destruct upon receiving a certain trigger. Also known as programmed cell death or cellular suicide, apoptosis occurs when a cell is no longer needed or when it poses a threat to an organism's health. When apoptosis takes place, the affected cell breaks down, digests its own cellular components, and is eventually absorbed and disposed of by other cells. In addition to helping maintain a healthy bodily environment by keeping the growth of cells in check and removing malfunctioning cells, apoptosis plays a key role in embryonic development. Generally, the rate of apoptosis is closely balanced with the rate of mitosis, or the creation of new cells. When it occurs too frequently or not frequently enough, however, apoptosis can contribute to the process of diseases such as cancer. The study of apoptosis, which dates back as far as the mid-nineteenth century, remains one of the most active realms of research in modern neuroscience.

Background

The concept of apoptosis was first observed by Carl Christoph Vogt, a German scientist who studied the development of tadpoles in the 1840s. Vogt's breakthrough observations were largely forgotten, however, until biologist Alfred Glücksmann wrote "Cell Deaths in Normal Vertebrate Ontogeny," a 1951 paper in which he describes apoptosis as it occurs during embryonic development. The term apoptosis, taken from an ancient Greek word meaning "falling off," was formally coined in 1972 by John F.R. Kerr, who, along with several other scientists, wrote a landmark paper in which he refined the idea of programmed cell death by asserting that cell death that occurs during embryonic development is the same as cellular suicide triggered by hormones or toxins. The study of apoptosis increased dramatically after the 1988 discovery of Bcl-2, a protein that plays a key role in the process of programmed cell death. This renewed interest subsequently led scientists to the definitive conclusion that apoptosis and programmed cell death were in fact the same process in 1992. Although many questions still remain as to how apoptosis works and the mechanisms and chemicals involved in the process, research into programmed cell death continues.

It is important to note that apoptosis is one of two distinct forms of cell death. The other type of cell death is called necrosis. While apoptosis is an orderly, naturally occurring process, cell death by necrosis takes place as the result of an external force such as bodily injury, poisoning, or a disruption in blood supply. Unlike cells that undergo apoptosis, cells that experience necrosis die outside of any programmed process. When cells die in such an uncontrolled manner, there can be serious bodily consequences. Necrotic cells gradually swell up until they burst. The rupture of necrotic cells in turn leads to the expulsion of chemicals that can cause inflammation and increased sensitivity. Necrosis can also spread to other nearby cells and cause further injury. Apoptosis, by comparison, is a carefully controlled process that typically occurs only in single cells and generally does not result in any pain or injury to the organism.

Overview

Apoptosis is a process during which a cell undergoes a series of biological changes that ultimately lead to its death. When the process of apoptosis is triggered, the affected cell begins to shrivel and move away from other nearby cells. Next, special enzymes called caspases start to break down the cytoskeleton, which gives the cell its shape. This, in turn, spurs the release of special enzymes called nucleases that begin breaking down the cell's DNA. Soon after, the cell starts to experience a phenomenon called blebbing, which causes bubble-like blobs to appear on the cell's surface as if the cell were beginning to boil. At the same time, the cytoplasm, or the material within the cell outside the nucleus, begins to undergo rapid condensation. This condensation causes the cell to become denser and leads the organelles, or the structures within the cell, to become compacted. As the process continues, the cell's DNA also condenses and is broken down into small fragments. Eventually, the entire cell disintegrates into small pieces known as apoptotic bodies that are enclosed in membranes to protect nearby cells. These bodies are subsequently engulfed by phagocytic cells that break them down into sugars, proteins, fats, and nucleic acids—all of which are the base units of life. These units, and any surviving organelles, are later recycled for use in new cells.

Apoptosis is an essential natural function that plays a key role in the development and health of any multicellular organism. In the early stages of human life, apoptosis is critical to the process of embryogenesis, or the development of the embryo into a fully formed fetus. As the embryo matures, it usually grows to include more cells than are necessary to achieve full fetal development. Apoptosis helps to ensure that any unnecessary cells are killed off before embryogenesis is completed. This is especially important to the development of the brain. As it is formed, many more cells are created within the brain than are needed. The cells that do not form synaptic connections are killed off by apoptosis to ensure that the remaining cells function properly. Apoptosis that occurs during fetal development also has some more visible effects. While in the womb, fetuses typically have a sort of webbing between their fingers and toes. More often than not, this webbing disappears before birth because of apoptosis. In children and adults, apoptosis helps to promote good health by removing potentially dangerous cells and limiting the spread of viral infections. In adult women, apoptosis is also responsible for triggering the menstrual cycle.

The health effects of apoptosis can vary depending on the rate at which it occurs. Ideally, the rate of cell death through apoptosis is carefully balanced with the rate of new cell creation through mitosis. When this balance is upset, there can be serious consequences. If the rate of apoptosis is too high, an excess number of cells may be killed off. Researchers suggest that this type of "hyper-apoptosis" may contribute to the development of neurodegenerative conditions such as Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis (ALS). When the rate of apoptosis is too slow, the resulting unchecked growth of cells can lead to cancer.

Bibliography

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