- Apoptosis is a form of programmed cell death, or “cellular suicide.” It is different from necrosis, in which cells die due to injury.
- Apoptosis is an orderly process in which the cell’s contents are packaged into small packets of membrane for “garbage collection” by immune cells.
- Apoptosis removes cells during development, eliminates potentially cancerous and virus-infected cells, and maintains balance in the body.
Video: Overview of apoptosis
Khan Academy video wrapperhttps://www.khanacademy.org/embed_video?slug=apoptosis&internal_video_only=1
You may think of it as a bad thing for cells in your body to die. In many cases, that’s true: it’s not good for cells to die because of an injury (for example, from a scrape or a harmful chemical). However, it’s also important that some cells of our bodies do die – not randomly, but in a carefully controlled way.For example, have you ever wondered how your fingers formed? It turns out that the cells between your developing fingers were instructed to die long ago, while you were still an embryo. If they hadn’t done so, you would have webbed hands, or perhaps just paddles of tissue with no fingers at all.The cells between your embryonic fingers died in a process called apoptosis, a common form of programmed cell death. In programmed cell death, cells undergo “cellular suicide” when they receive certain cues. Apoptosis involves the death of a cell, but it benefits the organism as a whole (for instance, by letting fingers develop or eliminating potential cancer cells). In this article, we’ll take a closer look at apoptosis, seeing when it happens and why it’s important.
Apoptosis vs. necrosis
Broadly speaking, there are two ways that cells die in a multicellular organism such as yourself:
- They are killed by things that harm them (such as toxic chemicals or physical injury), a process called necrosis.
- They are triggered to undergo programmed cell death. The best-understood form of programmed cell death is apoptosis.
Necrosis and apoptosis occur under different circumstances and involve different steps. Simply put, necrosis is messy and causes an immune response of inflammation, while apoptosis is tidy and splits the cell into little parcels that can be taken up and recycled by other cells.
Necrosis (the messy way)
When cells are damaged by harmful factors (such as injury or toxic chemicals), they usually “spill their guts” as they die. Because the damaged cell’s plasma membrane can no longer control the passage of ions and water, the cell swells up, and its contents leak out through holes in the plasma membrane. This often causes inflammation in the tissue surrounding the dead cell.
Apoptosis (the tidy way)
Cells that undergo apoptosis go through a different and much more orderly process. They shrink and develop bubble-like protrusions (technical name: “blebs”) on their surface. The DNA in the nucleus gets chopped up into small pieces, and some organelles of the cell, such as the endoplasmic reticulum, break down into fragments. In the end, the entire cell splits up into small chunks, each neatly enclosed in a package of membrane.What happens to the chunks? They release signals that attract debris-eating (phagocytic) immune cells, such as macrophages. Also, the fragments of the dying cell display a lipid molecule called phosphatidylserine on their surface. Phosphatidylserine is usually hidden on the inside of the membrane, and when it is on the outside, it lets the phagocytes bind and “eat” the cell fragments.
Why do cells undergo apoptosis?
Many cells in the human body have the built-in ability to undergo apoptosis (in the same way that they have the built-in ability to copy their DNA or break down fuels). Basically, apoptosis is a general and convenient way to remove cells that should no longer be part of the organism.
- Some cells need to be “deleted” during development – for instance, to whittle an intricate structure like a hand out of a larger block of tissue.
- Some cells are abnormal and could hurt the rest of the organism if they survive, such as cells with viral infections or DNA damage.
- Cells in an adult organism may be eliminated to maintain balance – to make way for new cells or remove cells needed only for temporary tasks.
Apoptosis is part of development
In many organisms, programmed cell death is a normal part of development. In some cases, apoptosis during development occurs in a very predictable way: in the worm C. elegans, 131131131 cells will die by apoptosis as the worm develops from a single cell to an adult (and we know exactly which ones they are)^11start superscript, 1, end superscript!Apoptosis also plays a key role in human development. For instance, as we saw in the introduction, your hand started out as a paddle-like block of tissue when you were an embryo. The block was “carved” into fingers by apoptosis of the cells in between the developing fingers.
Microscope images from a scientific paper, showing a developing mouse paw. The cells between the developing digits are stained by a marker that indicates apoptotic cells.Image modified from “Duplication of digit 4 is preceded by reduced apoptosis and expanded chondrogenesis in the posterior limb mesenchyme,” by Farin et al. ^22squared, CC BY 4.0.This process occurs in all sorts of vertebrate species that have finger- or toe-like digits, and less apoptosis results in more webbing between the digits. Sometimes, if a small mistake happens during finger or toe development, apoptosis may be incomplete (leading, for instance, to fused toes).Other examples of apoptosis during normal development include the loss of a tadpole’s tail as it turns into a frog, and the removal of unneeded neurons in as neural circuits in the brain are “wired.”
Apoptosis can eliminate infected or cancerous cells
In some cases, a cell can pose a threat to the rest of the body if it survives. For instance, this may be the case for cells with DNA damage, pre-cancerous cells, and cells infected by viruses. If these cells undergo apoptosis, the threat to the rest of the organism (such as cancer or spread of a viral infection) is removed.When a cell’s DNA is damaged, it will typically detect the damage and try to repair it. If the damage is beyond repair, the cell will normally send itself into apoptosis, ensuring that it will not pass on its damaged DNA. When cells have DNA damage but fail to undergo apoptosis, they may be on the road to cancer.